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The annual course of drought and heat resistance in heath plants from an oceanic environment
Flora, Bd. 159, S. 105-·123 (1970) Department of Botany, the University of Glasgow, Scotland.
The annual c_ourse of drought and heat resistance m heath plants from an oceanic environment By PETER BANNISTER With 12 figures (Received June 9, 1969)
Many field studies of the drought and heat resistance of plants have been made in regions where there are marked annual fluctuations in both temperature and rainfall (see LEVITT (1956) and TROSHIN (1967) for a fuller consideration of resistance phenomena). However there is very little information from the oceanic regions of Britain which are characterised by a high annual rainfall (with a minimum in spring and early summer) and no great annual fluctuations in temperature (Fig. 1). Despite this mild climate, heath plants (and in particular species such as Calluna vulgaris1) which have a fairly extensive continental distribution) have been shown to suffer large water deficits during the late winter and early spring (BANNISTER 1964). The heat resistance of British heath plants does not appear to have been examined, although (LANGE 1961) has examined the heat resistance of Erica tetralix in Germany. I should like to gratefully acknowledge the technical assistance of Mrs. J. A. THORNHILL who carried out most of the resistance determinations and was also responsible for much of the primary calculation of results. I should also like to acknowledge the receipt of a grant from the British Council (Younger Research Workers Interchange Scheme) in 1966 which allowed me to visit the University of Innsbruck and the University of Giittingen. In these two universities the helpful discussions with Professor A. PISEK, Professor W. LARCHER, Professor 0. L. LANGE and others encouraged me to embark on the studies outlined in this paper.
A. Materials and methods of resistance determination
1. Plant material Four species were selected for examination; two species, Erica tetralix and E. cinerea, with a markedly oceanic distribution and two further species, V accinium myrtil!us and Cal!una vulf garis, with a more continental and northern distribution. All were collected from an area ot moorland near Strathblane, some 10 km to the north of Glasgow. Material was collected as enshoots (about 6 em in length) and enclosed in polythene tubes for transportation to the labora1) The nomenclature of species follows CLAPHAM, TuTIN & WARBURG (1962).
106
P. BANNISTER oC lfo.rimum femperolure fdai(y mean)
20
°C lfinimum lemperolurefdailr mean) 12 8
Sunshine (lola/}
mm
Rainfall f lola/)
200
0
Fig. 1. The annual course of various meteorological parameters in 1966-1967 (8). (Monthly averages or totals). Data for Glasgow Airport. The black circles indicate long-term average values. tory. Monthly collections were made over the period from February, 1967 to February, 1968. The determinations of heat and drought resistance were usually separated by intervals of-one week.
2. Resistance determinations Shoot samples were allowed to become fully saturated with water prior to the onset of resistance determinations. The method is essentially the same as that used by BANNISTER (1964) to determine the percentage relative water content1 ) of shoots from the field. On suitable (dry) days, field measurements of relative water content were made. Shoot water contents have also been expressed as percentage water saturation deficits (water deficits) as in STOCKER (1929) the water deficit is merely the difference between the percentage relative water content and 100. Heat resistance was determined after the method of LANGE (1961) and shoots were immersed in a water-bath at predetermined temperatures for periods of 30 minutes. After exposure the samples were drained and blotted before their return to polythene tubes where they were again allowed to take up water through their cut ends. In the determination of drought resistance the shoot samples were exposed to different periods of desiccation (1 / 2 , 11/2 , 21 / 2 , 31 / 2 , 41 / 2 , 51 / 2 , 61 / 2 , 71 / 2 hours) in a glass-fronted incubator that was maintained at a temperature of between 28 °C and 30 °C. Samples were weighed before and after exposure; the exposed samples were returned to polythene tubes after weighing and allowed to take up water. These weighings allowed the calculation of shoot water deficits. 1) The use of the term 'relative turgidity' has been abandoned as it can be misleading ( cf. WALTER 1963) and the determination of shoot relative water content is much more com-
parable to the method of STOCKER (1929) than to discing techniques. Dry weight changes are not detectable, water is taken up through the cut ends of shoots and so infiltration does not occur, and changes in water content due to growth do not normally constitute a serious error in the determination.
107
The annual course of drought etc.
In a limited number of experiments the effect of water deficits upon the heat resistance of shoots was determined. Samples were allowed to desiccate for varying periods of time and then exposed in a glass vessel submerged in a water-bath. The temperature of the water bath or the internal temperature of the vessel was maintained at a predetermined range of temperatures during these experiments.
3. Assessment of damage In the determinations of both drought and heat resistance the samples were examined after 24 hours of resaturation. The resaturated weight was determined and expressed as a percentage of the original saturated weight (cf. RYCHNOVSKA-SouDKOVA (1963)) and the electrical conductivity of a water extract of the samples was made. The measurement of resaturation was discontinued in the heat resistance determinations as it was found that heat-treated samples, irrespective of the amount of damage sustained, always took in amounts of water that were sufficient to cause their complete or over-saturation. Similar results were obtained when samples were exposed to heat in air (Tablel). After a period of one week, samples were divided into damaged and undamaged material and damage was expressed as percentage of the fresh weight of the shoot. This method was employed on account of the difficulty of making assessment in terms of leaf areas. In Vaccinium the damage to leaves, stems and buds was determined separately. Table 1 Mean levels of resaturation for heat treated samples (% of initial saturated water content)
Erica cinerea E. tetra!ix Ca!!una vulgaris Vaccinium myrtil!us
exposed in water undamaged damaged
exposed in air undamaged
damaged
106.3 104.1 113.2 106.8
104.2 105.1
112.1 115.8
104.5 105.8 114.5 108.5
4. Expression of results Heat and desiccation resistance have been expressed as the temperature or water deficit required to induce 20% damage in the samples (KAPPEN (1964)). In Vaccinium the 20% point for stems was approximately equal to the 50% point for buds and the 90% point for leaves; accordingly these two levels have been used for these organs. This critical point has been estimated from both handdrawn curves and by the statistical method of probit analysis (FINNEY (1952)). There is good agreement, in all four species, between the results of the two methods and no evidence for a significant deviation from a regression coefficient of 1.0. There was less scatter of points in the determinations of heat resistance (a mean correlation coefficient of 0.9638 as compared with 0.9459 in the drought resistance determinations) and there is a closer agreement between the visual and probit estimates in this instance. Thus there would appear to be little value in adoption of the more sophisticated and laborious method of probit analysis in determining the temperature or water deficit at the critical level of damage. Unfortunately the utilisation of a standard desiccation treatment resulted in shoots of T' accinium and Cal! una escaping damage at certain times of year. The amount of material that escaped damage, averaged over the eight exposures of the standard desiccation treatment, gave
108
P.
BANNISTER
a more generally applicable index of resistance. The average levels of deficit and resaturation at each monthly determination have also been used in the interpretation of the drought resistance of each species. Initially it was hoped that the measurement of either resaturation or conductivity made after 24 hours would provide a rapid method of estimating damage to the tissues. Figure 2 shows the generalised relationship between damage and recovery over the annual cycle of determinations for all four species; the curves are derived from pro bit' analyses of all available data. There is a somewhat different relationship for all four species and the level of recovery at 20% damage varies from 75% in the stems of Vaccinium to an estimate of over 100% in the leaves of the same species. However if the data for bud damage are taken as representative of Vaccinium then the narrow range from 86-92% includes the point at which all four species experience 20% damage. These curves represent averages for the whole year; each monthly relationship does not necessarily conform to the annual pattern for the species and the differences in each monthly relationship do not necessarily show the same seasonal changes in the four species under consideration. A less amenable situation exists in the relationships between damage and conductivity. In the heat experiments large changes in the level of damage cause only small changes in conductivity whereas in desiccation experiments a similar change in damage occasions a larger change in conductivity (Figs. 3 & 4). This could be due to mechanical damage, caused by shrinkage during desiccation, allowing an easier escape of solutes from the drought-damaged tissues or by the immobilisation of solutes as a result of heating. Thus the conductivity at 100% damage may
damage 100
80
I I
I I
'·
20
resalurolion /%) Fig. 2. The relationship between damage and resaturation in drought experiments. Solid lines - Vaccinium (upper curve for leaves, middle for buds and lowerfor stems); dotted line -Er-ica tetralix; long dashes- Calluna; shorter dashes- Erica cinerea.
109
The annual course of drought etc.
vary with the type of treatment as well as its duration and intensity; furthermore the amount of tissue involved and, very strikingly, the season of the year all contribute to the variability. Various workers (e. g. FLINT et al (1967)) have used measures which essentially express the increase in conductance due to damage in the experimental sample as a percentage of the total increase in conductance of a completely damaged (usually heat killed) sample. The full statement of index of injury as used by FLINT et al (1967) goes a long way towards eliminating some of the errors mentioned above, but fails in two important respects. Firstly if 100% injury can occur at a conductance below that possible when a sample is killed by some other means, then indices of less than 100% damage could be obtained for material that was completely dead or values of more than 100% could be obtained for material that was not completely killed (depending upon which dead sample was used as the control). The use of the index of injury in this department (A. PoL WART) has led to such errors. Secondly the relationship between damage and conductivity (or, more exactly, the logarithm of conductance) is of a sigmoid type and any estimate of damage based on proportionalities would be accurate at only three points (0, 50, 100 %) ; below 50% there would be an overestimate of damage and above an underestimate. Luckily the most commonly used critical point is that at 50% damage in the samples, and, in the event of an accurate determination of the point of complete damage, is likely to be accurately assessed. In view of these complications and the availability of values for directly observed damage, the conductivity values have not been used in the interpretation of the results of this paper. damage(%)
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log speci/ic conductivity (mlcromhos/cm)
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4 Fig. 3. The relationship between damage and the logarithm of specific conduction in drought experiments. (Symbols as in Fig. 2). Fig. 4. The relationship between damage and the logarithm of specific conductivity in heat experiments. (Symbols as in Fig. 2).
P.
110
BANNISTER
B. Results
1. Heat Resistance The annual course of heat resistance is similar in all instances (Fig. 5) and there is a strong correlation between the levels of resistance in all four species and between the measurements for different organs in Vaccinium (values of the correlation coefficient varying from a minimum of 0.6989 to a maximum of0.9338). The general pattern is that of a winter maximum falling to a minimum value in May - June and thereafter a rise to the next winter maximum. There are some small deviations from this general pattern. In Calluna and Erica tetralix the resistance of the young growth declines during the first part of the growing season. There is some evidence of an increase in heat resistance in June (which was warm and dry) and also in August (when the preceding weather was wet). The heat resistance of the current growth of Erica tetralix is appreciably lower than that of the previous season; a comparable effect can be seen in the observations of LANGE (1961) and in the course of drought resistance in this species (Figs. 9 & 10). The annual course of heat resistance would appear to be inversely related to the annual course of temperature and to be more reasonably related to the state Critical temperature (°C J
a
~5
'/
dale
JIJ/IIY Y Jll7!Yill!I I ll1ll I II
dale IIlillY Y Yf Yf!Yill!II Ill!!I II
Fig. 5. The annual course of heat resistance. a. Erica cinerea; b. E. tetralix; c. Calluna vulgaris; d. V accinium myrtillus. Dotted lines indicate growth current in the 1966-1967 season (a-c). In (d) the solid line indicates the resistance of stems; the dashed line refers to buds and the dotted line to leaves.
The annual course of drought etc.
111
of growth of the plants. However, it should be possible to use the annual course of heat resistance as a 'baseline' against which to measure the fluctuations in other resistances. In February 1967 and January 1968, the effect of water deficits upon the level of heat resistance was determined in shoots of Calluna and V accinium. In 1967 the temperature control was based on the temperature of the water-bath whilst in 1968 the control was based on the internal temperature of the glass containers. If the absolute temperatures are ignored, then the pattern of the results can be seen to be essentially the same in both years (Fig. 6). A small water deficit ( < 20 %) is sufficient to cause an appreciable increase in heat resistance (measured in this instance as the temperature causing 50% damage), whilst further increases have only a slight effect. These results may be compared with those of KAPPEN (1966) who demonstrated that large deficits (>40 %) led to a dramatic increase in heat resistance. Deficits as large as 60% were induced in Calluna in my experiments, but there is no evidence to suggest a rise of heat resistance comparable to that observed by KAPPEN. It is possible that the induction of larger deficits would have led to such increases; similarly, if KAPPEN had examined the effects of small deficits, he might have been able to demonstrate the initial rise in heat resistance that has been shown Heal resistance ("C) Sfrr--a.---.---.---.---,---r-,
0
'
0
!968
---------------0
SJ
0
wafer defictl (%) !0
20
Fig. 6. The relationship between heat resistance and water deficits in Calluna and Vaccinium. Open circles - Calluna; filled circles - V accinium.
112
P.
BANNISTER
in the experiments with Calluna and Vaccinium. It is, however, evident that the presence of water deficits, comparable to those found in the field, is sufficient to cause an increase in the heat resistance of the shoot. The heat resistance of the shoots is related to the reciprocal of the water deficit; the annual course of heat resistance is related to the reciprocal of the prevailing saturated water content of the shoot in a similar manner. 2. Drought resistance Drought resistance may be considered as a generic term which describes the ability of a plant to survive extreme water stress (cf. LEVITT, SuLLIVAN & KRULL, 1960). Three parameters are considered here as important in determining or describing this resistance to drought. a) The ability to resist the induction of large water deficits, both in the laboratory and in the field ('drought avoidance' of LEVITT et al., 1960). b) The ability to endure and recover from a given level of internal water deficit (the 'critical water deficit' of HoFLER et al., 1941). This is termed 'desiccation resistance' (cf. KAPPEN (1964) in the text as distinct from 'drought resistance' which is meant to have a more general meaning. c) The damage suffered by a standard desiccation treatment. This is the resultant of the two previous parameters (comparable in some respects to the 'drought tolerance' of LEVITT et al., 1960). a) Deficits in the laboratory and in the field Conditions in the drought chamber were kept as uniform as possible, but the absence of a precise control could possibly account for some of the differences in the levels of water deficit induced in the shoots throughout the year. However, despite the fact that species were exposed simultaneously, there is no overall similarity between reactions of the four species; moreover, there is a correlation between the response of the plants in the field and that in the laboratory. The correlation is strongest in Erica cinerea (correlation coefficient of 0.8870) and becomes significant in Calluna and Vaccinium when values for December 1967 and January 1968 are excluded, although the correlation for V accinium remains poor. These observations suggest that the deficits induced in the laboratory were related to the physiological condition of the plant in the field rather than to fluctuations in the evaporating power of the air in the drought cabinet. In Erica tetralix and E. cinerea the largest water deficits, both from the field and the laboratory, are in the winter and at the beginning of the growing season; field observations for Calluna and, to a lesser extent, Vaccinium show a similar pattern. However, in the laboratory Calluna and Vaccinium are highly resistant to the formation of deficits in January and February of the second winter; if these last
The annual course of drought etc.
113
two values are excluded, then the pattern of deficits in Calluna is quite similar to that of Erica cinerea and E. tetralix. In V accinium, the leafless stems are highly resistant to the induction of water deficits, although in April (before the expansion of the leaves) larger deficits were induced and in October (before the complete shedding of leaves) there was an appreciable decrease in the size of the induced deficit. The increased resistance to the induction of deficits in the second winter as compared with the first is marked in Calluna and V accinium but not detectable in Erica tetralix It is possible that this increased resistance is due to the 'hardening' of the plants during the growing season, as the summer of 1967 was somewhat drier, warmer, and sunnier than the summer o£1966; however it seems more likely that the resistance to the formation of deficits was induced by conditions prevailing in the field in the second winter. The period immediately before both the December and January readings was both dry and unusually cold. Large deficits were induced in the field in Calluna and Vaccinium (Fig. 7) and this, coupled with a probable increase in frost resistance which would be associated with the mobilisation of osmotically active compounds [cf. THREN(1934)], would lead to an increase in the resistance to the formation of deficits. It is interesting to note that two oceanic species of Erica showed only a slight response to what were essentially 'continental' winter conditions. The calculation of correlation coefficients (r) underlines the similarity of reaction of Erica cinerea and E. tetralix, both in the laboratory (r = 0. 7916) and in the field (r = 0.6093). Erica cinerea shows some similarity of response to Calluna [r = wafer deficil (%) 0 r---------.
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~
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dale
BO I!l!l!Y Y 'If l1/l'l!IIJI I XIIIII
dale I! l!liY Y 'lll1/l'l!IIJIII!17U
Fig. 7. The annual course of water deficits from the field (upper lines and induced in the laboratory (lower lines). a . .En:ca cinerea; b . .E. tetralix; c. Calluna vulgaris; d. Vaccinium myrtillus. 8 Flora, Bd. 159
114
P.
BANNISTER
0.5267 (lab.), r = 0.5926 (field)]; this is not surprising as the material of these two species was collected from a limited area where both occurred together. Calluna shows a similarity to the field behaviour of Vaccinium (r = 0.5524) (the two plants were collected from different areas of the moor) whilst there is some similarity in the laboratory response of Erica tetralix and Calluna (r = 0.5675). The annual course of change in the ratio of saturated water content to dry weight shows a very similar pattern in all species; but with a slight earlier rise to a summer maximum in Vaccinium (Fig. 12). b) Recovery from induced deficits In V accinium and Calluna the pattern of recovery from the deficits induced in the laboratory was very similar (r = 0.9181), but whilst in Vaccinium there is a strong correlation between the pattern of deficits and recovery, there is no such correlation in Calluna. Recovery is best in winter and poorest in summer. In Erica tetralix and E. cinerea the differences between winter and summer are less marked; the two species of Erica show some similarity of pattern (r = 0.6657). The recovery of Erica tetralix is much more dependent upon the the initial deficit than is that of E. cinerea (Fig. 8), and is best in winter and poorest during the growing season. In Erica cinerea the recovery is poorest in spring and autumn but average in the middle of the growing season; there is a suggestion of a similar pattern in E. tetralix. recovety f% of origifiCll saluroled waler confenl/ ,-------------,
100
a
b
d
date 70 lll!I lY Y YI Y!lFJ!!I I I!.Ilf1
!! III lY Y Ff YllF/Il/I I II Ill!
Fig. 8. Recovery from induced water deficits (mean values for a standard desiccation treatment 1967-1968. a. Erica cinerea; b. E. tetralix; c. Calluna vulgaris; d. V accinium myrtillus. The dotted line indicate the recovery expected on the basis of the relationship between deficit and reco very. Stippled areas indicate a greater recovery than expectation.
115
The annual course of drought etc.
c) 'Desiccation resistance' (Fig. 9) and Average damage (Fig.10) The deficit required to produce a given level (20% except in the leaves (50%) and stems (90 %) of V accinium) of damage corresponds to the generally accepted definition of desiccation resistance (KAPPEN 1964). Here two opposing trends are seen; in Erica tetralix and E. cinerea there is an increase in resistance from the beginning of the growing season, rising to a summer maximum and falling in the autumn before rising to a maximum in the second winter, whilst in V accinium and Calluna there is a decline in resistance in the early part of the growing season which is followed by a rise to the winter level. The results for the second winter are incomplete in these last two species as the material escaped damage in the desiccation treatment; however in all cases it seems that the level of resistance is greater in the second winter than in the first. The resistance of V accinium leaves follows that of buds except that in the autumn there is a sudden decline in desiccation resistance. At this time of year even a small deficit is sufficient to cause extensive leaf-fall. On account of the lack of information on the winter resistance of the plants given by the previous measures, the average amount of damage (averaged over the whole desiccation treatment) has been used (Fig. 10). The scale of damage has been crtlical water defti:i/ f%)
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dale ll'::'lll:'::JY:':Y:':Y£-=.lll:::'YII!'==':IJ:~I;';Il;;'Ill~I
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Fig. 9. Annual course of 'desiccation resistance'. 1967-1968. a. Erica cinerea; b. E. fetralix; c. Calluna vulgaris; d. V accinium myrtillus. Dashed lines (a-c) indicate 11:rowth eurrent in the 1966-1967 season.
s•
116
P.
BANNISTER
inverted so that peaks represent maximum resistance (i.e. a maximum amount of undamaged tissue). In all species except Vaccinium the maximum amount of damage was sustained at the beginning of the growing season. In Calluna there is a steady increase in resistance during the growing season to reach a maximum level by October whilst in Erica tetralix and E. cinerea there is a marked similarity in reaction (r = 0.7954). The maximum levels are attained much earlier (June in E. cinerea; July - August in E. tetralix ) and appear to decline during the second winter. All species appear to be more resistant to damage in the second winter than in the previous winter. The shoots of Vaccinium appear to be most susceptible to damage in July which is well into the growing season; buds, stems and leaves all show a greater resistance earlier in the growing season. The leaves are easily damaged in the autumn, and there is not much difference in the winter resistance of stems in the two winters as the leafless stems are highly resistant to the induction of water deficits (Fig. 6). The consideration of all the preceding factors seems to indicate that in Calluna and V accinium there is maximum drought resistance in winter with a minimum during the growing season, whilst in the two species of Erica there is a rapid increase in drought resistance during the early part of the growing season so that the shoots may be more resistant to drought during the summer than in the winter. undamaged !issue f%1 mo.-~--------~
a
b
r
.
• I ~/ ~\,/
20
Fig. 10. The average amounts of undamaged tissue after a standard desiccation treatment. 1967-1968. a. Erica cinerea; b. E. tetra!ix; c. Ca!!una vulgaris; d. Vaccinium myrti!!us. Da:,hed lines indicate shoots current in the 1966-1967 season.
The annual course of drought etc.
117
3. Specific differences in resistance to drought and heat
It is apparent that the main similarities are between the two spcies of Erica or between Calluna and V accinium. It is possible to represent these similarities in a graphical manner (Fig. 11) in a system where the proximity of points representing the individual species is related to the degree of similarity of their reactions. The method of representation is that of BRAY & CuRTIS (1957) [See also BANNISTER (1968)] and the measure of distance is the complement (1-r) of the correlation coefficient (r). In the drought resistance diagram the coefficient of similarity is derived from the average interspecific correlations coefficient derived from all five parameters considered under 'drought resistance', whilst the coefficients for heat resistance are derived from interspecific correlations. It can readily be seen that Erica tetralix and E. cinerea show a similar pattern of drought resistance which is markedly different from that of V accinium, whilst Calluna shows some similarity to both extremes. There is little difference between the patterns of heat resistance, but the sequence of Erica tetralix, E. cinerea, Calluna and V accinium is similar to the separation in drought resistance. 4. Physiological correlations Heat resistance (Fig. 4) appears to be correlated with the annual course of growth of the plant. One parameter that also parallels the growth of the plant is saturated wafer content/dry weigh/
a
VfJCcinium (/1/ •
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• Callur7fJ (CJ 2
• Erica cinerefJ ( EcJ
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12 Fig.ll. Diagrammatic representation of interspecific relationships in drought (a) and heat (b) resistance. Fig. 12. Annual fluctuations in the ratio of saturated water content to dry weight of shoots. 1967-1968. Open circles with solid lines - Call una; open circles and dashes -Erica cinerea; filled circles and solid lines - Vaccinium; filled circles and dashes - E. tetralix.
118
P.
BANNISTE!t
the ratio of the water content at saturation to dry weight. This value is highest in very young tissues and lowest in fully dormant tissues. There is a significant relationship (always accounting for more than 75% of the total variation in heat resistance) between the annual course of heat resistance and the reciprocal of this parameter (i. e. the ratio of dry weight to saturated water content) and the age of the shoot; thus the change in heat resistance is more influenced by differences in the water content in winter than in summer. In Erica cinerea and E. tetralix the age of the shoot also influences the resistance; this influence is more marked in Erica tetralix (the independent effect of age accounts for 46% of the total variation) where the current growth is always less resistant to heat than than the previous season's growth, but is confined to the first few months' growth of the growth of thenew season in E. cinerea (hence the best regression is obtained by using a logarithmic scale of age (23% of the total variation). The average damage sustained in the drought experiments (Fig. 6) was strongly correlated with the average water deficit attained during that experiment. This relationship always accounted for more than half the total variation in damage. However if both the average deficit and desiccation resistance were related to average damage, a much higher proportion (always more than 75 %) of the variation was accounted for. These two parameters have been further examined for correlations with environmental factors. 5. Environmental correlations Local meteorological data (kindly supplied by Glasgow Weather Centre) were used in attempts to determine what environmental conditions were responsible for determining the patterns of heat and drought resistance (monthly averages given in Fig. 1). Data for maximum and minimum temperatures, daily rainfall and sunshine were available and various combinations of these factors (averaged over periods of one week to one month before the investigation) were used. Few significant correlations were found. The most significant correlations were with accumulated temperatures above 6 °C (i. e. those temperatures likely to be associated with growth). A significant proportion of the variation in deficits (always greater than 60 %) induced in the laboratory could be attributed to the accumulated temperature of the previous month and the accumulated temperature over the year and the desiccation resistance of Calluna could also be well explained by these two parameters. Only in Erica tetralix did any other environmental factor have any obvious effect; here the combination of monthly accumulated temperature and monthly precipitation accounted for the highest proportion of the variation. There is obviously an inverse correlation of the annual course of temperature and that of heat resistance; although upon closer examination the decrease in resi-
T
119
The annual course of drought etc.
Table 2 a. The relationship of heat resistance (y) to the date (days from the winter solstice) (x1) and age of the shoot (days from 1st April) (x2) percentage of total variation accounted for by:x2 -xi xl + x2 xl + x2
Erica cinerea y = 46.4940- 0.0263x1 + 1.6379~log10 X2 Erica tetralix y = 49.3528 - 0.0314x1 + 0.0078x2 Calluna vulgaris y = 51.2411 - 0.398x1 - 0.0009x2 l'accinium myrtillus (stems) y = 51.4755- 0.0309x1 + 0.0021x2
92
50
18
82
52
28
84
80
1
90
67
1
b. The relationship of desiccation resistance (y) to the date (days from the nearest equinox) (x1) and the age of the shoots (x2) xl
Erica cinerea y = 52.7877 + 0.1600x1 Erica tetralix y = 39.1131 + 0.1257x1
+ x2
xl-x2
x2-xl
+ 151.17fx2
80
36
24
+ 0.0440x2
76
21
62
(The column under x1 + x2 gives the total variation accounted for by the regression; the values under x1 - x2 give the amount of variation that is accounted for by x1 independently of x2 or of x2 independently of x1 .) The logarithm or reciprocal of age was in the equations for Erica cinerea as the age effect fell off markedly with the age of the shoot.
stance in late winter and spring and the increase in resistance in late summer are not well correlated with the annual course of temperature. However there is a consistenty high correlation of heat resistance with what at first appears to be a rather improbable factor, the date, measured as days from the winter solstice (Table 2). Again in Erica cinerea and E. tetralix the age of the shoots has significance in the determination of resistance. The correlation with date appears less surprising when day lengths can be shown to be almost linearily related to this expression of date. Day length has an effect on the growth of the plant through the control of the induction and cessation of growth as well as through photosynthetic effects. The use of day length instead of date in the regression analyses makes no difference to the significance of the results. The annual trends in desiccation resistance showed no consistent correlation with any environmental parameters (except in Calluna where there was the strong association with monthly accumulated temperatures). Data for V accinium were
120 Table 3
P.
BANNISTEH
Correlations between heat and drought resistance
Erica cinerea E. tetralix Calluna vulgaris Vaccinium myrtillus (stems)
heat resistance and desiccation resistance
heat resistance and average damage
0.0724ns 0.5442* 0.5305+ 0.8549*
0.001ns 0.0137ns 0.7479** 0.8695***
(The significance of the correlation coefficient is indicated as follows: -
(P
ns (not significant,
> 0.1), +(P < 0.1), *(P < 0.05), **(P < 0.01), ***(P < 0.001).
sparse and no general conclusions can be based on the small amount of data, however there is a strong correlation between heat resistance, desiccation resistance, andjor average damage (Table 3) in both V accinium and Calluna which suggests that the two resistances are closely related. No such correlation exists in either species of Erica (the similarity between heat and desiccation resistance in E. tetralix is due chiefly to the increase of either resistance with age). Again there is a significant relationship when the date of observation is taken into account. In this case the date is expressed as the time in days to the nearest equinox. The age of the shoot of Erica tetralix is the major determinant of its desiccation resistance, in E. cinerea the resistance is decreased only in very young shoots so that there is an excellent fit with the reciprocal of the age of the shoot and the date (Table 2b). Thus the pattern of resistance in these two species can be visualised as increasing in response to the lengthening days after the vernal equinox and shortening days after the autumn equinox whilst resistance decreases as the days shorten after the summer solstice and lengthen after the winter solstice.
C. Discussion
It is evident that the greater proportion of variation in both drought and heat resistance in the plants that have been examined in this paper can be explained by the state of growth of the plant. Younger and growing tissues are generally less resistant than older or dormant tissues; thus there is a basic pattern of resistance which is scarcely influenced by fluctuations in climate other than the normal progression of seasons. This is more marked in the consideration of heat resistance where there is scant evidence of any adaptive increase in heat resistance during the warmer part of the year. In the expression of drought resistance, the interaction between the ability to resist the formation of water deficits and the actual desiccation resistance (the level of water deficit at which a critical level of damage is sustained) allows the formation of a greater variety of patterns of resistance. In Calluna and Vaccinium
The annual course of drought etc.
121
the desiccation resistance shows a minimum in summer, but whilst in Calluna this low summer resistance is offset by increased resistance to the formation of deficits, in Vaccinium the leafy summer shoots lose water much more readily than the leafless winter stems and thus the susceptibility to desiccation is increased. In both species of Erica there is an increase in desiccation resistance in the summer which is reinforced by an increased resistance to the formation of water deficits, thus shoots of these species are highly resistant to drought for most of the growing season, but not as resistant to winter drought as are Calluna and V accinium. These observations are capable of some ecological interpretation. Erica cinerea has an oceanic but somewhat southern distribution and typically occurs in dry habitats. In such a situation it is exposed to summer drought, and the course of drought resistance appears to be adapted to such a situation. However the desiccation resistance. is low in winter, and whilst a dry cold spell appeared to increase the drought resistance of Calluna and V accinium, there was no similar increase in Erica cinerea. The potential susceptibility to winter drought might be a factor in limiting this species to the oceanic areas of Europe where the winters are mild. A not dissimilar pattern is shown by Erica tetralix, however this species shows a much heightened resistance in the older tissues and therefore winter resistances may he quite high. E. tetralix has a more continental and northern distribution than E. cinerea and occurs in Scandinavia and the Baltic region. The increase in summer resistance is not as marked as in E. cinerea but this plant is characteristic of wetter habitats where the dangers of summer drought are less. The ability to resist the formation of deficits is enhanced when the previous month is wet. This is in accord with the observation of BANNISTER (1964a) that Calluna plants from wet habitats showed a more immediate control of water loss than those from drier habitats. Calluna and V accinium are most resistant in winter and showed a rapid increase in drought resistance when more 'continental' conditions prevailed. This maximum resistance in winter suits them to their more continental and northern distribution patterns. V accinium myrtillus is very characteristic of woodland and in such a situation may escape summer drought as it may in the more northern and sub-alpine parts of its range. Calluna too is a woodland plant in its most easterly continental extent (GrMINGHAM [1960]) but does penetrate to southern regions with dry summers. The resistance to drought in summer is greater than that of Vaccinium and it may be that an adaptive drought resistance can be invoked in more southerly climates.
D. Summary The resistance to drought and heat of heath plants with a continental distribution ( Calluna rulgaris, Vaccinium myrtilltls) has been contrasted with that of two more oceanic species (Erica
122
P.
BANNISTER
tetralix, E. cinerea). All material was collected from the same site near Glasgow (Scotland) in a region which has a markedly oceanic climate. 1. Resistance was estimated as the degree of temperature or water deficit necessary to induce a critical level of damage (usually 20 %). Various rapid methods of assessing damage were tried. The degree of resaturation of shoots gave some indication of drought damage but was useless in the determination of heat resistance where completely damaged shoots showed a successful resaturation. The conductivity of water extracts of the shoots was also related to damage but there were marked differences in the relationships between conductivity and damage for the drought and the heat resistance determinations. The relevance of these findings to techniques depending on the measurement of conductance is briefly discussed. The resistance determinations are based on visual assessment of damage and the estimate of the critical points by eye from drawn graphs was found to be just as effective, and less time consuming, than the more accurate method of pro bit analysis. •
2. Heat resistance is highest in the winter and lowest in the summer. There is scarcely any eYidence for adaptive changes in heat resistance during the summer. The development of a small water deficit in the plant increases the heat resistance, larger increases in deficit show a less marked effect. 3. Drought resistance is shown to be compounded of a variety of factors. In Calluna and Vaccinium the overall pattern of drought resistance is not dissimilar to that of heat resistance but in the two species of Erica there is a heightened resistance in summer and a lower resistance in winter. 4. The annual course of heat resistance is strongly correlated with the water content of the tissues whilst the physiological parameters that best explain the average level of damage attained in drought experiments are the ability to withstand the induction of water deficits and the ability to survive large water deficits without damage. 5. There was little evidence of any adaptive changes in either heat or drought resistance. However both heat and drought resistance showed correlations with the time of year which was presumably related to the prevailing day length. The ease of induction of deficits was related to the temperatures available for growth; (in all species except Vaccinium there was an increase in resistance as accumulated temperatures above 6 °0 increased). In Vaccinium the increase in accumulated temperature increased the susceptibility to the formation of water deficits (presumably because of increased leaf area). In all cases the resistance to the formation of deficits was increased by the accumulated temperatures over the whole year. Thus both resistances appear to be connected with the state of growth of the plant, rather than adaptive response to the prevailing temperature or potential for desiccation. 6. The patterns of drought resistance may offer a partial explanation of the phytogeographical distribution of the species. Those species with an oceanic distribution are less well adapted to severe winters than those with a more continental distribution.
Literature P., (1964a). Stomatal responses of heath plants to water deficits J. Ecol. 52,151-158. (1964 b). The water relations of certain heath plants with reference to their ecological amplitude. II Field Studies. J. Ecol. 52, 423-432. (1968). An evaluation of some procedures used in simple ordinations. J. Ecol. 56, 27-34.
BANNISTER,
-
The annual course of drought etc.
123
BRAY, J. R. & CuRTIS, J. T., (1957). An ordination of the upland forest communities of southern Wisconsin. Ecol. Monogr. 27, 325-349. CLAPHAM, A. R., TUTIN, T. G. & WARBURG, E. F., (1962). Flora of the British Isles, 2nd edn. Cambridge University Press, Cambridge. FI!'>!'>EY, D. J., (1952). Probit analysis. Cambridge University Press, Cambridge. FLINT, H. L., BoYCE, B. R. & BEATTIE, D. J., (1967). Index of injury - A useful expression of freezing injury to plant tissues as determined by the electrolytic method. Can. J. Plant. Sci. 47, 229-230. GIMJNGHAM, C. H., (1960). Biological Flora of the British Isles: Calluna vulgaris (L.) HuLL. J. Ecol. 48, 455-483. HoFLER, K., MIGSCH, H. & RoTTENBERG, W., (1941). Uber die Austrocknungsresistenz landwirtschaftlicher Kulturpflanzen. Forschungsdienst 12, 50-61. KAPPEN, L., (1964). Untersuchungen iiber den Jahreslauf der Frost-, Hitze- und Austrocknungresistenz von Sporophyten einheimischer Polypodiaceen (Filicinae). Flora 155, 123-166. - (1966). Der EinfluB des Wassergehaltes auf die Widerstandsfiihigkeit von Pflanzen gegeniiber hohen und tiefen Temperaturen, untersucht an Bliittern einiger Farne und von Ramonda myconi. Flora 156, 427-445. LANGE, 0. L., (1961). Die Hitzeresistenz einheimischer immer- und wintergriiner Pflanzen im Jahreslauf. Planta, 56, 666-683. LEVITT, J., (1956). The hardiness of plants. Academic Press, New York. - SuLLIVAN, C. Y. & KRULL, E., (1960). Some problems in drought resistance. Bull. Res. Council Israel 8D, 173-180. RYCHNOVSKA-SounKov A, M., (1963). Study of the reversibility of the water saturation deficit as one of the methods of causal phytogeography. Bioi. Plant. 5, 175-180. SToCKER, 0., (1929). Das Wasserdefizit von GefiiBpflanzen in verschiedenen Klimazonen. Planta 7, 382-387. THREN, R., (1934). Jahreszeitliche Schwankungen des osmotischen Wertes verschiedener i:ikologischer Typen in der Umgebung von Heidelberg. Z. Bot. 26, 449-526. TRoSHIN, A. S., (1967). The cell and environmental temperature. Pergamon Press, Oxford. WALTER, H., (1963). Zur Kliirung des spezifischen Wasserzustandes im Plasma. II Methodisches. Ber. dtsch. Bot. Ges. 76, 54-71. Author's address: Dr. P. BANNISTER, Department of Biology, University of Stirling, Stirling, Scotland (Great Britain).
The annual c_ourse of drought and heat resistance m heath plants from an oceanic environment By PETER BANNISTER With 12 figures (Received June 9, 1969)
Many field studies of the drought and heat resistance of plants have been made in regions where there are marked annual fluctuations in both temperature and rainfall (see LEVITT (1956) and TROSHIN (1967) for a fuller consideration of resistance phenomena). However there is very little information from the oceanic regions of Britain which are characterised by a high annual rainfall (with a minimum in spring and early summer) and no great annual fluctuations in temperature (Fig. 1). Despite this mild climate, heath plants (and in particular species such as Calluna vulgaris1) which have a fairly extensive continental distribution) have been shown to suffer large water deficits during the late winter and early spring (BANNISTER 1964). The heat resistance of British heath plants does not appear to have been examined, although (LANGE 1961) has examined the heat resistance of Erica tetralix in Germany. I should like to gratefully acknowledge the technical assistance of Mrs. J. A. THORNHILL who carried out most of the resistance determinations and was also responsible for much of the primary calculation of results. I should also like to acknowledge the receipt of a grant from the British Council (Younger Research Workers Interchange Scheme) in 1966 which allowed me to visit the University of Innsbruck and the University of Giittingen. In these two universities the helpful discussions with Professor A. PISEK, Professor W. LARCHER, Professor 0. L. LANGE and others encouraged me to embark on the studies outlined in this paper.
A. Materials and methods of resistance determination
1. Plant material Four species were selected for examination; two species, Erica tetralix and E. cinerea, with a markedly oceanic distribution and two further species, V accinium myrtil!us and Cal!una vulf garis, with a more continental and northern distribution. All were collected from an area ot moorland near Strathblane, some 10 km to the north of Glasgow. Material was collected as enshoots (about 6 em in length) and enclosed in polythene tubes for transportation to the labora1) The nomenclature of species follows CLAPHAM, TuTIN & WARBURG (1962).
106
P. BANNISTER oC lfo.rimum femperolure fdai(y mean)
20
°C lfinimum lemperolurefdailr mean) 12 8
Sunshine (lola/}
mm
Rainfall f lola/)
200
0
Fig. 1. The annual course of various meteorological parameters in 1966-1967 (8). (Monthly averages or totals). Data for Glasgow Airport. The black circles indicate long-term average values. tory. Monthly collections were made over the period from February, 1967 to February, 1968. The determinations of heat and drought resistance were usually separated by intervals of-one week.
2. Resistance determinations Shoot samples were allowed to become fully saturated with water prior to the onset of resistance determinations. The method is essentially the same as that used by BANNISTER (1964) to determine the percentage relative water content1 ) of shoots from the field. On suitable (dry) days, field measurements of relative water content were made. Shoot water contents have also been expressed as percentage water saturation deficits (water deficits) as in STOCKER (1929) the water deficit is merely the difference between the percentage relative water content and 100. Heat resistance was determined after the method of LANGE (1961) and shoots were immersed in a water-bath at predetermined temperatures for periods of 30 minutes. After exposure the samples were drained and blotted before their return to polythene tubes where they were again allowed to take up water through their cut ends. In the determination of drought resistance the shoot samples were exposed to different periods of desiccation (1 / 2 , 11/2 , 21 / 2 , 31 / 2 , 41 / 2 , 51 / 2 , 61 / 2 , 71 / 2 hours) in a glass-fronted incubator that was maintained at a temperature of between 28 °C and 30 °C. Samples were weighed before and after exposure; the exposed samples were returned to polythene tubes after weighing and allowed to take up water. These weighings allowed the calculation of shoot water deficits. 1) The use of the term 'relative turgidity' has been abandoned as it can be misleading ( cf. WALTER 1963) and the determination of shoot relative water content is much more com-
parable to the method of STOCKER (1929) than to discing techniques. Dry weight changes are not detectable, water is taken up through the cut ends of shoots and so infiltration does not occur, and changes in water content due to growth do not normally constitute a serious error in the determination.
107
The annual course of drought etc.
In a limited number of experiments the effect of water deficits upon the heat resistance of shoots was determined. Samples were allowed to desiccate for varying periods of time and then exposed in a glass vessel submerged in a water-bath. The temperature of the water bath or the internal temperature of the vessel was maintained at a predetermined range of temperatures during these experiments.
3. Assessment of damage In the determinations of both drought and heat resistance the samples were examined after 24 hours of resaturation. The resaturated weight was determined and expressed as a percentage of the original saturated weight (cf. RYCHNOVSKA-SouDKOVA (1963)) and the electrical conductivity of a water extract of the samples was made. The measurement of resaturation was discontinued in the heat resistance determinations as it was found that heat-treated samples, irrespective of the amount of damage sustained, always took in amounts of water that were sufficient to cause their complete or over-saturation. Similar results were obtained when samples were exposed to heat in air (Tablel). After a period of one week, samples were divided into damaged and undamaged material and damage was expressed as percentage of the fresh weight of the shoot. This method was employed on account of the difficulty of making assessment in terms of leaf areas. In Vaccinium the damage to leaves, stems and buds was determined separately. Table 1 Mean levels of resaturation for heat treated samples (% of initial saturated water content)
Erica cinerea E. tetra!ix Ca!!una vulgaris Vaccinium myrtil!us
exposed in water undamaged damaged
exposed in air undamaged
damaged
106.3 104.1 113.2 106.8
104.2 105.1
112.1 115.8
104.5 105.8 114.5 108.5
4. Expression of results Heat and desiccation resistance have been expressed as the temperature or water deficit required to induce 20% damage in the samples (KAPPEN (1964)). In Vaccinium the 20% point for stems was approximately equal to the 50% point for buds and the 90% point for leaves; accordingly these two levels have been used for these organs. This critical point has been estimated from both handdrawn curves and by the statistical method of probit analysis (FINNEY (1952)). There is good agreement, in all four species, between the results of the two methods and no evidence for a significant deviation from a regression coefficient of 1.0. There was less scatter of points in the determinations of heat resistance (a mean correlation coefficient of 0.9638 as compared with 0.9459 in the drought resistance determinations) and there is a closer agreement between the visual and probit estimates in this instance. Thus there would appear to be little value in adoption of the more sophisticated and laborious method of probit analysis in determining the temperature or water deficit at the critical level of damage. Unfortunately the utilisation of a standard desiccation treatment resulted in shoots of T' accinium and Cal! una escaping damage at certain times of year. The amount of material that escaped damage, averaged over the eight exposures of the standard desiccation treatment, gave
108
P.
BANNISTER
a more generally applicable index of resistance. The average levels of deficit and resaturation at each monthly determination have also been used in the interpretation of the drought resistance of each species. Initially it was hoped that the measurement of either resaturation or conductivity made after 24 hours would provide a rapid method of estimating damage to the tissues. Figure 2 shows the generalised relationship between damage and recovery over the annual cycle of determinations for all four species; the curves are derived from pro bit' analyses of all available data. There is a somewhat different relationship for all four species and the level of recovery at 20% damage varies from 75% in the stems of Vaccinium to an estimate of over 100% in the leaves of the same species. However if the data for bud damage are taken as representative of Vaccinium then the narrow range from 86-92% includes the point at which all four species experience 20% damage. These curves represent averages for the whole year; each monthly relationship does not necessarily conform to the annual pattern for the species and the differences in each monthly relationship do not necessarily show the same seasonal changes in the four species under consideration. A less amenable situation exists in the relationships between damage and conductivity. In the heat experiments large changes in the level of damage cause only small changes in conductivity whereas in desiccation experiments a similar change in damage occasions a larger change in conductivity (Figs. 3 & 4). This could be due to mechanical damage, caused by shrinkage during desiccation, allowing an easier escape of solutes from the drought-damaged tissues or by the immobilisation of solutes as a result of heating. Thus the conductivity at 100% damage may
damage 100
80
I I
I I
'·
20
resalurolion /%) Fig. 2. The relationship between damage and resaturation in drought experiments. Solid lines - Vaccinium (upper curve for leaves, middle for buds and lowerfor stems); dotted line -Er-ica tetralix; long dashes- Calluna; shorter dashes- Erica cinerea.
109
The annual course of drought etc.
vary with the type of treatment as well as its duration and intensity; furthermore the amount of tissue involved and, very strikingly, the season of the year all contribute to the variability. Various workers (e. g. FLINT et al (1967)) have used measures which essentially express the increase in conductance due to damage in the experimental sample as a percentage of the total increase in conductance of a completely damaged (usually heat killed) sample. The full statement of index of injury as used by FLINT et al (1967) goes a long way towards eliminating some of the errors mentioned above, but fails in two important respects. Firstly if 100% injury can occur at a conductance below that possible when a sample is killed by some other means, then indices of less than 100% damage could be obtained for material that was completely dead or values of more than 100% could be obtained for material that was not completely killed (depending upon which dead sample was used as the control). The use of the index of injury in this department (A. PoL WART) has led to such errors. Secondly the relationship between damage and conductivity (or, more exactly, the logarithm of conductance) is of a sigmoid type and any estimate of damage based on proportionalities would be accurate at only three points (0, 50, 100 %) ; below 50% there would be an overestimate of damage and above an underestimate. Luckily the most commonly used critical point is that at 50% damage in the samples, and, in the event of an accurate determination of the point of complete damage, is likely to be accurately assessed. In view of these complications and the availability of values for directly observed damage, the conductivity values have not been used in the interpretation of the results of this paper. damage(%)
,,/--
100
/;
I
l
/
80
/
/
I
I
t
:
/
/
I
,'
I/ / II /
60
;
+O
f f
I
1 / /,/
I
---:
/
h
!.8
!.7
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1
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!I j:
I
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2.2
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! 1/
2.3
I
I
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20
!.9
I I I
il
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!.9'
I I
f
il H
0~-=/~-~-~--~--~--~~~--~--~--~ !.5
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I
60
log speci/ic conductivity (mlcromhos/cm)
I
I /
I
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II/ I
I
/
I
:!
I
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//
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!!
/,········~~>;;~~--------;~-:~~-;; l
20
:;..----
---~-.......
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damage{%) 100
-<~-
I
log spec/lie
conduclivily
(mkromhos I em)
o~~~~--~--~--~--~--~~~
1.7
1.8
t9
2.0
2.1
3
4 Fig. 3. The relationship between damage and the logarithm of specific conduction in drought experiments. (Symbols as in Fig. 2). Fig. 4. The relationship between damage and the logarithm of specific conductivity in heat experiments. (Symbols as in Fig. 2).
P.
110
BANNISTER
B. Results
1. Heat Resistance The annual course of heat resistance is similar in all instances (Fig. 5) and there is a strong correlation between the levels of resistance in all four species and between the measurements for different organs in Vaccinium (values of the correlation coefficient varying from a minimum of 0.6989 to a maximum of0.9338). The general pattern is that of a winter maximum falling to a minimum value in May - June and thereafter a rise to the next winter maximum. There are some small deviations from this general pattern. In Calluna and Erica tetralix the resistance of the young growth declines during the first part of the growing season. There is some evidence of an increase in heat resistance in June (which was warm and dry) and also in August (when the preceding weather was wet). The heat resistance of the current growth of Erica tetralix is appreciably lower than that of the previous season; a comparable effect can be seen in the observations of LANGE (1961) and in the course of drought resistance in this species (Figs. 9 & 10). The annual course of heat resistance would appear to be inversely related to the annual course of temperature and to be more reasonably related to the state Critical temperature (°C J
a
~5
'/
dale
JIJ/IIY Y Jll7!Yill!I I ll1ll I II
dale IIlillY Y Yf Yf!Yill!II Ill!!I II
Fig. 5. The annual course of heat resistance. a. Erica cinerea; b. E. tetralix; c. Calluna vulgaris; d. V accinium myrtillus. Dotted lines indicate growth current in the 1966-1967 season (a-c). In (d) the solid line indicates the resistance of stems; the dashed line refers to buds and the dotted line to leaves.
The annual course of drought etc.
111
of growth of the plants. However, it should be possible to use the annual course of heat resistance as a 'baseline' against which to measure the fluctuations in other resistances. In February 1967 and January 1968, the effect of water deficits upon the level of heat resistance was determined in shoots of Calluna and V accinium. In 1967 the temperature control was based on the temperature of the water-bath whilst in 1968 the control was based on the internal temperature of the glass containers. If the absolute temperatures are ignored, then the pattern of the results can be seen to be essentially the same in both years (Fig. 6). A small water deficit ( < 20 %) is sufficient to cause an appreciable increase in heat resistance (measured in this instance as the temperature causing 50% damage), whilst further increases have only a slight effect. These results may be compared with those of KAPPEN (1966) who demonstrated that large deficits (>40 %) led to a dramatic increase in heat resistance. Deficits as large as 60% were induced in Calluna in my experiments, but there is no evidence to suggest a rise of heat resistance comparable to that observed by KAPPEN. It is possible that the induction of larger deficits would have led to such increases; similarly, if KAPPEN had examined the effects of small deficits, he might have been able to demonstrate the initial rise in heat resistance that has been shown Heal resistance ("C) Sfrr--a.---.---.---.---,---r-,
0
'
0
!968
---------------0
SJ
0
wafer defictl (%) !0
20
Fig. 6. The relationship between heat resistance and water deficits in Calluna and Vaccinium. Open circles - Calluna; filled circles - V accinium.
112
P.
BANNISTER
in the experiments with Calluna and Vaccinium. It is, however, evident that the presence of water deficits, comparable to those found in the field, is sufficient to cause an increase in the heat resistance of the shoot. The heat resistance of the shoots is related to the reciprocal of the water deficit; the annual course of heat resistance is related to the reciprocal of the prevailing saturated water content of the shoot in a similar manner. 2. Drought resistance Drought resistance may be considered as a generic term which describes the ability of a plant to survive extreme water stress (cf. LEVITT, SuLLIVAN & KRULL, 1960). Three parameters are considered here as important in determining or describing this resistance to drought. a) The ability to resist the induction of large water deficits, both in the laboratory and in the field ('drought avoidance' of LEVITT et al., 1960). b) The ability to endure and recover from a given level of internal water deficit (the 'critical water deficit' of HoFLER et al., 1941). This is termed 'desiccation resistance' (cf. KAPPEN (1964) in the text as distinct from 'drought resistance' which is meant to have a more general meaning. c) The damage suffered by a standard desiccation treatment. This is the resultant of the two previous parameters (comparable in some respects to the 'drought tolerance' of LEVITT et al., 1960). a) Deficits in the laboratory and in the field Conditions in the drought chamber were kept as uniform as possible, but the absence of a precise control could possibly account for some of the differences in the levels of water deficit induced in the shoots throughout the year. However, despite the fact that species were exposed simultaneously, there is no overall similarity between reactions of the four species; moreover, there is a correlation between the response of the plants in the field and that in the laboratory. The correlation is strongest in Erica cinerea (correlation coefficient of 0.8870) and becomes significant in Calluna and Vaccinium when values for December 1967 and January 1968 are excluded, although the correlation for V accinium remains poor. These observations suggest that the deficits induced in the laboratory were related to the physiological condition of the plant in the field rather than to fluctuations in the evaporating power of the air in the drought cabinet. In Erica tetralix and E. cinerea the largest water deficits, both from the field and the laboratory, are in the winter and at the beginning of the growing season; field observations for Calluna and, to a lesser extent, Vaccinium show a similar pattern. However, in the laboratory Calluna and Vaccinium are highly resistant to the formation of deficits in January and February of the second winter; if these last
The annual course of drought etc.
113
two values are excluded, then the pattern of deficits in Calluna is quite similar to that of Erica cinerea and E. tetralix. In V accinium, the leafless stems are highly resistant to the induction of water deficits, although in April (before the expansion of the leaves) larger deficits were induced and in October (before the complete shedding of leaves) there was an appreciable decrease in the size of the induced deficit. The increased resistance to the induction of deficits in the second winter as compared with the first is marked in Calluna and V accinium but not detectable in Erica tetralix It is possible that this increased resistance is due to the 'hardening' of the plants during the growing season, as the summer of 1967 was somewhat drier, warmer, and sunnier than the summer o£1966; however it seems more likely that the resistance to the formation of deficits was induced by conditions prevailing in the field in the second winter. The period immediately before both the December and January readings was both dry and unusually cold. Large deficits were induced in the field in Calluna and Vaccinium (Fig. 7) and this, coupled with a probable increase in frost resistance which would be associated with the mobilisation of osmotically active compounds [cf. THREN(1934)], would lead to an increase in the resistance to the formation of deficits. It is interesting to note that two oceanic species of Erica showed only a slight response to what were essentially 'continental' winter conditions. The calculation of correlation coefficients (r) underlines the similarity of reaction of Erica cinerea and E. tetralix, both in the laboratory (r = 0. 7916) and in the field (r = 0.6093). Erica cinerea shows some similarity of response to Calluna [r = wafer deficil (%) 0 r---------.
~
20
60~ 0.------------,
c~ 20
'---../"
\___
¥0 60·
~
~
dale
BO I!l!l!Y Y 'If l1/l'l!IIJI I XIIIII
dale I! l!liY Y 'lll1/l'l!IIJIII!17U
Fig. 7. The annual course of water deficits from the field (upper lines and induced in the laboratory (lower lines). a . .En:ca cinerea; b . .E. tetralix; c. Calluna vulgaris; d. Vaccinium myrtillus. 8 Flora, Bd. 159
114
P.
BANNISTER
0.5267 (lab.), r = 0.5926 (field)]; this is not surprising as the material of these two species was collected from a limited area where both occurred together. Calluna shows a similarity to the field behaviour of Vaccinium (r = 0.5524) (the two plants were collected from different areas of the moor) whilst there is some similarity in the laboratory response of Erica tetralix and Calluna (r = 0.5675). The annual course of change in the ratio of saturated water content to dry weight shows a very similar pattern in all species; but with a slight earlier rise to a summer maximum in Vaccinium (Fig. 12). b) Recovery from induced deficits In V accinium and Calluna the pattern of recovery from the deficits induced in the laboratory was very similar (r = 0.9181), but whilst in Vaccinium there is a strong correlation between the pattern of deficits and recovery, there is no such correlation in Calluna. Recovery is best in winter and poorest in summer. In Erica tetralix and E. cinerea the differences between winter and summer are less marked; the two species of Erica show some similarity of pattern (r = 0.6657). The recovery of Erica tetralix is much more dependent upon the the initial deficit than is that of E. cinerea (Fig. 8), and is best in winter and poorest during the growing season. In Erica cinerea the recovery is poorest in spring and autumn but average in the middle of the growing season; there is a suggestion of a similar pattern in E. tetralix. recovety f% of origifiCll saluroled waler confenl/ ,-------------,
100
a
b
d
date 70 lll!I lY Y YI Y!lFJ!!I I I!.Ilf1
!! III lY Y Ff YllF/Il/I I II Ill!
Fig. 8. Recovery from induced water deficits (mean values for a standard desiccation treatment 1967-1968. a. Erica cinerea; b. E. tetralix; c. Calluna vulgaris; d. V accinium myrtillus. The dotted line indicate the recovery expected on the basis of the relationship between deficit and reco very. Stippled areas indicate a greater recovery than expectation.
115
The annual course of drought etc.
c) 'Desiccation resistance' (Fig. 9) and Average damage (Fig.10) The deficit required to produce a given level (20% except in the leaves (50%) and stems (90 %) of V accinium) of damage corresponds to the generally accepted definition of desiccation resistance (KAPPEN 1964). Here two opposing trends are seen; in Erica tetralix and E. cinerea there is an increase in resistance from the beginning of the growing season, rising to a summer maximum and falling in the autumn before rising to a maximum in the second winter, whilst in V accinium and Calluna there is a decline in resistance in the early part of the growing season which is followed by a rise to the winter level. The results for the second winter are incomplete in these last two species as the material escaped damage in the desiccation treatment; however in all cases it seems that the level of resistance is greater in the second winter than in the first. The resistance of V accinium leaves follows that of buds except that in the autumn there is a sudden decline in desiccation resistance. At this time of year even a small deficit is sufficient to cause extensive leaf-fall. On account of the lack of information on the winter resistance of the plants given by the previous measures, the average amount of damage (averaged over the whole desiccation treatment) has been used (Fig. 10). The scale of damage has been crtlical water defti:i/ f%)
a(v--
80r-----'-----,
70
60 '\
v
50
b
/r~~
.
v
eo __c_~ .. '' 70 '. v
d buds
60
\ leaYes 30
dale ll'::'lll:'::JY:':Y:':Y£-=.lll:::'YII!'==':IJ:~I;';Il;;'Ill~I
·dale IflllIF Y Yf lllllll!II II IllI
Fig. 9. Annual course of 'desiccation resistance'. 1967-1968. a. Erica cinerea; b. E. fetralix; c. Calluna vulgaris; d. V accinium myrtillus. Dashed lines (a-c) indicate 11:rowth eurrent in the 1966-1967 season.
s•
116
P.
BANNISTER
inverted so that peaks represent maximum resistance (i.e. a maximum amount of undamaged tissue). In all species except Vaccinium the maximum amount of damage was sustained at the beginning of the growing season. In Calluna there is a steady increase in resistance during the growing season to reach a maximum level by October whilst in Erica tetralix and E. cinerea there is a marked similarity in reaction (r = 0.7954). The maximum levels are attained much earlier (June in E. cinerea; July - August in E. tetralix ) and appear to decline during the second winter. All species appear to be more resistant to damage in the second winter than in the previous winter. The shoots of Vaccinium appear to be most susceptible to damage in July which is well into the growing season; buds, stems and leaves all show a greater resistance earlier in the growing season. The leaves are easily damaged in the autumn, and there is not much difference in the winter resistance of stems in the two winters as the leafless stems are highly resistant to the induction of water deficits (Fig. 6). The consideration of all the preceding factors seems to indicate that in Calluna and V accinium there is maximum drought resistance in winter with a minimum during the growing season, whilst in the two species of Erica there is a rapid increase in drought resistance during the early part of the growing season so that the shoots may be more resistant to drought during the summer than in the winter. undamaged !issue f%1 mo.-~--------~
a
b
r
.
• I ~/ ~\,/
20
Fig. 10. The average amounts of undamaged tissue after a standard desiccation treatment. 1967-1968. a. Erica cinerea; b. E. tetra!ix; c. Ca!!una vulgaris; d. Vaccinium myrti!!us. Da:,hed lines indicate shoots current in the 1966-1967 season.
The annual course of drought etc.
117
3. Specific differences in resistance to drought and heat
It is apparent that the main similarities are between the two spcies of Erica or between Calluna and V accinium. It is possible to represent these similarities in a graphical manner (Fig. 11) in a system where the proximity of points representing the individual species is related to the degree of similarity of their reactions. The method of representation is that of BRAY & CuRTIS (1957) [See also BANNISTER (1968)] and the measure of distance is the complement (1-r) of the correlation coefficient (r). In the drought resistance diagram the coefficient of similarity is derived from the average interspecific correlations coefficient derived from all five parameters considered under 'drought resistance', whilst the coefficients for heat resistance are derived from interspecific correlations. It can readily be seen that Erica tetralix and E. cinerea show a similar pattern of drought resistance which is markedly different from that of V accinium, whilst Calluna shows some similarity to both extremes. There is little difference between the patterns of heat resistance, but the sequence of Erica tetralix, E. cinerea, Calluna and V accinium is similar to the separation in drought resistance. 4. Physiological correlations Heat resistance (Fig. 4) appears to be correlated with the annual course of growth of the plant. One parameter that also parallels the growth of the plant is saturated wafer content/dry weigh/
a
VfJCcinium (/1/ •
3
• Callur7fJ (CJ 2
• Erica cinerefJ ( EcJ
r,-----
,b •EielrolirfEIJ I El Ec C Jl I
_I
•
•
dale
• • ][
III
IY
Jl
I!!
YJ[
Jl/1[
.IX
I
Xl
Ill
I
11
12 Fig.ll. Diagrammatic representation of interspecific relationships in drought (a) and heat (b) resistance. Fig. 12. Annual fluctuations in the ratio of saturated water content to dry weight of shoots. 1967-1968. Open circles with solid lines - Call una; open circles and dashes -Erica cinerea; filled circles and solid lines - Vaccinium; filled circles and dashes - E. tetralix.
118
P.
BANNISTE!t
the ratio of the water content at saturation to dry weight. This value is highest in very young tissues and lowest in fully dormant tissues. There is a significant relationship (always accounting for more than 75% of the total variation in heat resistance) between the annual course of heat resistance and the reciprocal of this parameter (i. e. the ratio of dry weight to saturated water content) and the age of the shoot; thus the change in heat resistance is more influenced by differences in the water content in winter than in summer. In Erica cinerea and E. tetralix the age of the shoot also influences the resistance; this influence is more marked in Erica tetralix (the independent effect of age accounts for 46% of the total variation) where the current growth is always less resistant to heat than than the previous season's growth, but is confined to the first few months' growth of the growth of thenew season in E. cinerea (hence the best regression is obtained by using a logarithmic scale of age (23% of the total variation). The average damage sustained in the drought experiments (Fig. 6) was strongly correlated with the average water deficit attained during that experiment. This relationship always accounted for more than half the total variation in damage. However if both the average deficit and desiccation resistance were related to average damage, a much higher proportion (always more than 75 %) of the variation was accounted for. These two parameters have been further examined for correlations with environmental factors. 5. Environmental correlations Local meteorological data (kindly supplied by Glasgow Weather Centre) were used in attempts to determine what environmental conditions were responsible for determining the patterns of heat and drought resistance (monthly averages given in Fig. 1). Data for maximum and minimum temperatures, daily rainfall and sunshine were available and various combinations of these factors (averaged over periods of one week to one month before the investigation) were used. Few significant correlations were found. The most significant correlations were with accumulated temperatures above 6 °C (i. e. those temperatures likely to be associated with growth). A significant proportion of the variation in deficits (always greater than 60 %) induced in the laboratory could be attributed to the accumulated temperature of the previous month and the accumulated temperature over the year and the desiccation resistance of Calluna could also be well explained by these two parameters. Only in Erica tetralix did any other environmental factor have any obvious effect; here the combination of monthly accumulated temperature and monthly precipitation accounted for the highest proportion of the variation. There is obviously an inverse correlation of the annual course of temperature and that of heat resistance; although upon closer examination the decrease in resi-
T
119
The annual course of drought etc.
Table 2 a. The relationship of heat resistance (y) to the date (days from the winter solstice) (x1) and age of the shoot (days from 1st April) (x2) percentage of total variation accounted for by:x2 -xi xl + x2 xl + x2
Erica cinerea y = 46.4940- 0.0263x1 + 1.6379~log10 X2 Erica tetralix y = 49.3528 - 0.0314x1 + 0.0078x2 Calluna vulgaris y = 51.2411 - 0.398x1 - 0.0009x2 l'accinium myrtillus (stems) y = 51.4755- 0.0309x1 + 0.0021x2
92
50
18
82
52
28
84
80
1
90
67
1
b. The relationship of desiccation resistance (y) to the date (days from the nearest equinox) (x1) and the age of the shoots (x2) xl
Erica cinerea y = 52.7877 + 0.1600x1 Erica tetralix y = 39.1131 + 0.1257x1
+ x2
xl-x2
x2-xl
+ 151.17fx2
80
36
24
+ 0.0440x2
76
21
62
(The column under x1 + x2 gives the total variation accounted for by the regression; the values under x1 - x2 give the amount of variation that is accounted for by x1 independently of x2 or of x2 independently of x1 .) The logarithm or reciprocal of age was in the equations for Erica cinerea as the age effect fell off markedly with the age of the shoot.
stance in late winter and spring and the increase in resistance in late summer are not well correlated with the annual course of temperature. However there is a consistenty high correlation of heat resistance with what at first appears to be a rather improbable factor, the date, measured as days from the winter solstice (Table 2). Again in Erica cinerea and E. tetralix the age of the shoots has significance in the determination of resistance. The correlation with date appears less surprising when day lengths can be shown to be almost linearily related to this expression of date. Day length has an effect on the growth of the plant through the control of the induction and cessation of growth as well as through photosynthetic effects. The use of day length instead of date in the regression analyses makes no difference to the significance of the results. The annual trends in desiccation resistance showed no consistent correlation with any environmental parameters (except in Calluna where there was the strong association with monthly accumulated temperatures). Data for V accinium were
120 Table 3
P.
BANNISTEH
Correlations between heat and drought resistance
Erica cinerea E. tetralix Calluna vulgaris Vaccinium myrtillus (stems)
heat resistance and desiccation resistance
heat resistance and average damage
0.0724ns 0.5442* 0.5305+ 0.8549*
0.001ns 0.0137ns 0.7479** 0.8695***
(The significance of the correlation coefficient is indicated as follows: -
(P
ns (not significant,
> 0.1), +(P < 0.1), *(P < 0.05), **(P < 0.01), ***(P < 0.001).
sparse and no general conclusions can be based on the small amount of data, however there is a strong correlation between heat resistance, desiccation resistance, andjor average damage (Table 3) in both V accinium and Calluna which suggests that the two resistances are closely related. No such correlation exists in either species of Erica (the similarity between heat and desiccation resistance in E. tetralix is due chiefly to the increase of either resistance with age). Again there is a significant relationship when the date of observation is taken into account. In this case the date is expressed as the time in days to the nearest equinox. The age of the shoot of Erica tetralix is the major determinant of its desiccation resistance, in E. cinerea the resistance is decreased only in very young shoots so that there is an excellent fit with the reciprocal of the age of the shoot and the date (Table 2b). Thus the pattern of resistance in these two species can be visualised as increasing in response to the lengthening days after the vernal equinox and shortening days after the autumn equinox whilst resistance decreases as the days shorten after the summer solstice and lengthen after the winter solstice.
C. Discussion
It is evident that the greater proportion of variation in both drought and heat resistance in the plants that have been examined in this paper can be explained by the state of growth of the plant. Younger and growing tissues are generally less resistant than older or dormant tissues; thus there is a basic pattern of resistance which is scarcely influenced by fluctuations in climate other than the normal progression of seasons. This is more marked in the consideration of heat resistance where there is scant evidence of any adaptive increase in heat resistance during the warmer part of the year. In the expression of drought resistance, the interaction between the ability to resist the formation of water deficits and the actual desiccation resistance (the level of water deficit at which a critical level of damage is sustained) allows the formation of a greater variety of patterns of resistance. In Calluna and Vaccinium
The annual course of drought etc.
121
the desiccation resistance shows a minimum in summer, but whilst in Calluna this low summer resistance is offset by increased resistance to the formation of deficits, in Vaccinium the leafy summer shoots lose water much more readily than the leafless winter stems and thus the susceptibility to desiccation is increased. In both species of Erica there is an increase in desiccation resistance in the summer which is reinforced by an increased resistance to the formation of water deficits, thus shoots of these species are highly resistant to drought for most of the growing season, but not as resistant to winter drought as are Calluna and V accinium. These observations are capable of some ecological interpretation. Erica cinerea has an oceanic but somewhat southern distribution and typically occurs in dry habitats. In such a situation it is exposed to summer drought, and the course of drought resistance appears to be adapted to such a situation. However the desiccation resistance. is low in winter, and whilst a dry cold spell appeared to increase the drought resistance of Calluna and V accinium, there was no similar increase in Erica cinerea. The potential susceptibility to winter drought might be a factor in limiting this species to the oceanic areas of Europe where the winters are mild. A not dissimilar pattern is shown by Erica tetralix, however this species shows a much heightened resistance in the older tissues and therefore winter resistances may he quite high. E. tetralix has a more continental and northern distribution than E. cinerea and occurs in Scandinavia and the Baltic region. The increase in summer resistance is not as marked as in E. cinerea but this plant is characteristic of wetter habitats where the dangers of summer drought are less. The ability to resist the formation of deficits is enhanced when the previous month is wet. This is in accord with the observation of BANNISTER (1964a) that Calluna plants from wet habitats showed a more immediate control of water loss than those from drier habitats. Calluna and V accinium are most resistant in winter and showed a rapid increase in drought resistance when more 'continental' conditions prevailed. This maximum resistance in winter suits them to their more continental and northern distribution patterns. V accinium myrtillus is very characteristic of woodland and in such a situation may escape summer drought as it may in the more northern and sub-alpine parts of its range. Calluna too is a woodland plant in its most easterly continental extent (GrMINGHAM [1960]) but does penetrate to southern regions with dry summers. The resistance to drought in summer is greater than that of Vaccinium and it may be that an adaptive drought resistance can be invoked in more southerly climates.
D. Summary The resistance to drought and heat of heath plants with a continental distribution ( Calluna rulgaris, Vaccinium myrtilltls) has been contrasted with that of two more oceanic species (Erica
122
P.
BANNISTER
tetralix, E. cinerea). All material was collected from the same site near Glasgow (Scotland) in a region which has a markedly oceanic climate. 1. Resistance was estimated as the degree of temperature or water deficit necessary to induce a critical level of damage (usually 20 %). Various rapid methods of assessing damage were tried. The degree of resaturation of shoots gave some indication of drought damage but was useless in the determination of heat resistance where completely damaged shoots showed a successful resaturation. The conductivity of water extracts of the shoots was also related to damage but there were marked differences in the relationships between conductivity and damage for the drought and the heat resistance determinations. The relevance of these findings to techniques depending on the measurement of conductance is briefly discussed. The resistance determinations are based on visual assessment of damage and the estimate of the critical points by eye from drawn graphs was found to be just as effective, and less time consuming, than the more accurate method of pro bit analysis. •
2. Heat resistance is highest in the winter and lowest in the summer. There is scarcely any eYidence for adaptive changes in heat resistance during the summer. The development of a small water deficit in the plant increases the heat resistance, larger increases in deficit show a less marked effect. 3. Drought resistance is shown to be compounded of a variety of factors. In Calluna and Vaccinium the overall pattern of drought resistance is not dissimilar to that of heat resistance but in the two species of Erica there is a heightened resistance in summer and a lower resistance in winter. 4. The annual course of heat resistance is strongly correlated with the water content of the tissues whilst the physiological parameters that best explain the average level of damage attained in drought experiments are the ability to withstand the induction of water deficits and the ability to survive large water deficits without damage. 5. There was little evidence of any adaptive changes in either heat or drought resistance. However both heat and drought resistance showed correlations with the time of year which was presumably related to the prevailing day length. The ease of induction of deficits was related to the temperatures available for growth; (in all species except Vaccinium there was an increase in resistance as accumulated temperatures above 6 °0 increased). In Vaccinium the increase in accumulated temperature increased the susceptibility to the formation of water deficits (presumably because of increased leaf area). In all cases the resistance to the formation of deficits was increased by the accumulated temperatures over the whole year. Thus both resistances appear to be connected with the state of growth of the plant, rather than adaptive response to the prevailing temperature or potential for desiccation. 6. The patterns of drought resistance may offer a partial explanation of the phytogeographical distribution of the species. Those species with an oceanic distribution are less well adapted to severe winters than those with a more continental distribution.
Literature P., (1964a). Stomatal responses of heath plants to water deficits J. Ecol. 52,151-158. (1964 b). The water relations of certain heath plants with reference to their ecological amplitude. II Field Studies. J. Ecol. 52, 423-432. (1968). An evaluation of some procedures used in simple ordinations. J. Ecol. 56, 27-34.
BANNISTER,
-
The annual course of drought etc.
123
BRAY, J. R. & CuRTIS, J. T., (1957). An ordination of the upland forest communities of southern Wisconsin. Ecol. Monogr. 27, 325-349. CLAPHAM, A. R., TUTIN, T. G. & WARBURG, E. F., (1962). Flora of the British Isles, 2nd edn. Cambridge University Press, Cambridge. FI!'>!'>EY, D. J., (1952). Probit analysis. Cambridge University Press, Cambridge. FLINT, H. L., BoYCE, B. R. & BEATTIE, D. J., (1967). Index of injury - A useful expression of freezing injury to plant tissues as determined by the electrolytic method. Can. J. Plant. Sci. 47, 229-230. GIMJNGHAM, C. H., (1960). Biological Flora of the British Isles: Calluna vulgaris (L.) HuLL. J. Ecol. 48, 455-483. HoFLER, K., MIGSCH, H. & RoTTENBERG, W., (1941). Uber die Austrocknungsresistenz landwirtschaftlicher Kulturpflanzen. Forschungsdienst 12, 50-61. KAPPEN, L., (1964). Untersuchungen iiber den Jahreslauf der Frost-, Hitze- und Austrocknungresistenz von Sporophyten einheimischer Polypodiaceen (Filicinae). Flora 155, 123-166. - (1966). Der EinfluB des Wassergehaltes auf die Widerstandsfiihigkeit von Pflanzen gegeniiber hohen und tiefen Temperaturen, untersucht an Bliittern einiger Farne und von Ramonda myconi. Flora 156, 427-445. LANGE, 0. L., (1961). Die Hitzeresistenz einheimischer immer- und wintergriiner Pflanzen im Jahreslauf. Planta, 56, 666-683. LEVITT, J., (1956). The hardiness of plants. Academic Press, New York. - SuLLIVAN, C. Y. & KRULL, E., (1960). Some problems in drought resistance. Bull. Res. Council Israel 8D, 173-180. RYCHNOVSKA-SounKov A, M., (1963). Study of the reversibility of the water saturation deficit as one of the methods of causal phytogeography. Bioi. Plant. 5, 175-180. SToCKER, 0., (1929). Das Wasserdefizit von GefiiBpflanzen in verschiedenen Klimazonen. Planta 7, 382-387. THREN, R., (1934). Jahreszeitliche Schwankungen des osmotischen Wertes verschiedener i:ikologischer Typen in der Umgebung von Heidelberg. Z. Bot. 26, 449-526. TRoSHIN, A. S., (1967). The cell and environmental temperature. Pergamon Press, Oxford. WALTER, H., (1963). Zur Kliirung des spezifischen Wasserzustandes im Plasma. II Methodisches. Ber. dtsch. Bot. Ges. 76, 54-71. Author's address: Dr. P. BANNISTER, Department of Biology, University of Stirling, Stirling, Scotland (Great Britain).