Drought Resistance, Water Potential and Water Content in some New Zealand Plants

Flora (1986) 178: 23-40 VEB Gustav Fischer Verlag Jena

Drought Resistance, Water Potential and Water Content New Zealand Plants

III

some

PETER BANNISTER Department of Botany, University of Otago, Dunedin, New Zealand

Summary The drought resistance of cut shoots and leaves of some 23 (mostly native) woody plants and ferns, showed a gradation from species that were resistant to water loss but damaged by relatively small water deficits (drought avoiders) to those that lost water readily but were damaged only by large water deficits (drought tolerators). Drought avoidance tended to be associated with low symplastic water content at the turgor loss point, large amounts of "bound" water and small changes in water potential with water content beyond the turgor loss point; whereas drought tolerance was associated with opposite values of the same parameters. Bulk moduli of elasticity were not significantly associated with drought resistance but were positively correlated with the water content at zero turgor. Species with low ratios of saturated water content to dry weight tend to have low solute water potentials, low relative water contents at zero turgor and show a steep decline of water potential with decreasing water content. Neither drought avoidance nor tolerance were particularly associated with any morphological or taxonomic grouping or with field observations of drought resistance, although there was some tendency for conifers to be drought avoiders and for species that are widespread in dry sites to be tolerators rather than avoiders.

Introduction The drought resistance of native New Zealand plants has not been systematically investigated, despite suggestions that drought may have had a significant role in the development of some peculiar adaptations shown by members of the New Zealand flora (e.g. the divaricating habit of many shrubs - MCGLONE & WEBB 1981). Species have been ranked with regard to drought resistance after natural droughts (e.g. ATKINSON & GREENWOOD 1972; OGDEN 1976), but few physiological investigations of water relations had been made until quite recently (e.g. JANE & GREEN 1983; GREEN & JANE 1983a, b). The drought resistance of cut shoots and leaves may be determined by allowing them to desiccate in air (e.g. JARVIS & JARVIS 1963; BANNISTER 1970, 1971) and expressed as the water content or water potential that is associated with a critical amount of damage or recovery (BANNISTER 1985) and may be divided into drought avoidance or tolerance (LEVITT 1980). Avoidance is the ability of a shoot or leaf to resist the formation of water deficits whereas tolerance is its ability to survive a given deficit. Other criteria have been suggested for differentiating between drought resistant (or xerophytic) and less drought resistant (or mesophytic) plants; in particular parameters derived from the relationship between water potential and water content. More resistant plants are expected to show smaller changes in water content for a given change in water potential than less resistant plants (WEATHERLEY & SLATYER 1957; JARVIS & JARVIS 1963). Values derived from the measurement of water potential in the pressure chamber (SCHOLANDER et al. 1964; TYREE & HAMMEL 1972) have also

24

P.

BA~NISTER

been interpreted in terms of drought resistance. Thus a high bulk modulus of elasticity (i.e. non-elastic walls) have been considered to confer drought resistam:e (CHEUNG et al. 1975) as has a low bulk osmotic potential (TYREE & KARAMANOS 1981) and high amounts of bound water (CUTLER et al. 1977). The current study investigates the drought resistance, water potential and water content of cut shoots and leaves of 23 species found in New Zealand. The fact that little is known about the water relations of any native New Zealand plants makes them eminently suitable for testing hypotheses about the relationships between various physiological parameters and drought resistance.

Materials and Methods Shoots or leaves of various species, all native except for the introduced fern Dryopteris filix. mas (Table 1) were collected from various localities in North Dunedin in summer, many from spe· cimens in the gardens of the Botany Department. Further details of sites of collection are given by BANNISTER & SMITH (1983). Leaves (or pinnae) were used for the ferns, and for Brachyglottis repanda and Pseudopanax colensoi among the woody plants; otherwise shoots were sampled. The principal determinations of drought resistance were in the summer of 1980/81 and those of P-V parameters in the subsequent summer. Some measurements on Dacrycarpus dacrydioides and Dacrydium cupressinum were made during the 1982/83 summer. Drought resistance Shoots or leaves were placed with their cut ends in water, covered with polythene bags and allowed to take up water overnight. Samples were exp03ed during the following day: after being blotted dry they were weighed to find a saturated weight and then allowed to lose water at laboratory temperature (approx. 20 DC) for 0, 0.5, 2, 4, 8, 24 and, ifneeded, 36 h. Five replicate samples were usually used at each time interval. Samples were then wrapped in damp paper towelling, placed within polythene bags and left for visible signs of damage to develop (about two weeks). Where samples were partially damaged, the damaged proportion was separated and weighed and expressed as a percentage of the total fresh weight. Various measures of drought avoidance and drought tolerance were made (Table 2) and expressed both in terms of turgid and symplastic water content using data from P-V curves (see below). The water potential at 50 % damage was estimated from the P-V curves, usually by interpolation, although for some of the species extrapolation had to be used. In the latter cases the values might differ from those obtained by interpolation because of a changed relationship between applied pressure (P) and water content (V) in damaged tissue. Relationships between water potential (P) and water content These were derived from measurements made in a pressure chamber (SCHOLANDER et al. 1964): the particular chamber used in this study is more fully describes by POLLOCK (1979). Shoots and leaves were collected and pretreated as in the determinations of drought resistance, but the exposure of the samples was staggered so as to produce a wider range of water contents and to ensure continuous use of the pressure chamber. Only one determination of water content and balancing pressure (P = -P) was made per shoot and curves are consequently made up of a number (15-30) of independent samples. This method was employed because the labour involved in making multiple determinations of a series of single shoots for all 23 species proved too great. The use of independent samples allows a statistical evaluation of differences between slopes (Table 4) and was considered preferable to making evaluations based on only one or two shoots (cf. TYREE & HAMMEL 1972; CHEUNG et al. 1975). Some multiple determinations on series of single shoots were made in the winters of 1981 and 1983 and have been used for comparative purposes (see Table 6). The relationship between applied pressure (P) and water content (V) has been analysed by TYREE & HAMMEL (1972); CHEUNG et al. (1975, 1976) and recently reviewed by RICHTERet aJ. (1981), generally in the form of graphs of liP against V (the Type II transformation of TYREE &

25

Some New Zealand Plants 6

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Fig. 1. Graph of reciprocal of water potential against relative water deficit (100 VEjVT) for Weinmannia 1'acemosa. TLP = turgor loss point, for further explanation see text. RICHTER 1981, 1982 - Fig. 1). A Type II procedure has been used here; attempts to fit complex functions to the whole curve (SINCLAIR & VENABLES 1983; JANE & GREEN 1983) proved unsatisfactory because of the greater variation which resulted from the sampling method used. The abbreviations used to describe parameters derived from P-V curves are given in Table 2. V is essentially a measure of the volume of the symplast and is equivalent to water content if the density of water is taken as unity. In this paper, all values of V are expressed relative to the water content at full turgor (V T) which is set either to 100 (as in measures of V p, V 0, B in Fig. 1) or to unity (as in slopes of relationships between P and V - Tables 2 and 4, Fig. 2). Although the graph of P against V may be approximated by a straight line (Table 4), it translates into a hyperbola when IjP is plotted against V (Fig. 1) and is divided into two portions. The first, at high water contents, represents the relationship for turgid cells: the second, less steep and linear, portion represents non-turgid cells where symplast volume is inversely related to applied pressure in accord with the Boylejvan t-Hoff equations. This second phase describes the relationship between bulk osmotic pressure (II = -'P s) and water content. Its intersect with the vertical axis gives the osmotic pressure at full turgor (II 0) and the intersect with the horizontal axis (ljP = 0, P = (0) gives a limit for the volume of of "free" or symplastic water taking part in osmotic exchanges (V o ). Consequently the difference between Vo and the water content at full turgor (VT) is often termed "bound" water (B = V T - V 0)' The intersect of the two portions of the graph gives a point of zero turgor, the turgor loss point (TLP), which may be expressed both as a water content (V p , or V p B) or as an osmotic pressure (lIp). Water content (V) may be expressed relative to V T as relative water content (the ratio of the volume of extruded water (V E = V T - V) to V T is the relative water deficit). V may also be ex-

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26

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Relative water deficit (V =VE/VT) Fig_ 2. Graph of turgor pressure ('P"p) against relative water content (100 V /V T) for Asplenium bulbiferum. E = bulk modulus, TLP = turgor loss point, for further explanation see text .

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Fig. 3. Principal component ordinations based on drought resistance parameters. (a) Correlation of the drought resistance parameters with the first two components of the ordination (see Table 2 for further explanation). (b) Position of the species on the first two components (see Table 1 for abbreviations).

pressed relative to Yo, as relative osmotic water content (CHEUNG et al. 1975) or as free water content (F, HELLKVIST et al. 1974); whereas CHEUNG et al. (1976) used VE/V p in their calculation of a bulk modulus. The initial phase of the relationship is generally used to calculate a bulk modulus from some function of turgor pressure ('P"p) which is derived from the difference between the applied and os· motic pressures (as 'P" = -P, 'P"p = 'P" - 'P"s = II - Pl. The bulk modulus (E) usually defined as the slope of relationship between turgor pressure and water content. Water content may be defined in terms of Vo (HELLKVIST et al. 1974), V p (CHEUNG et al. 1976) or VT (MELKONIAN et al. 1982). The use of V p requires the predetermination of a point of zero turgor; in this study the point

27

Some New Zealand Plants

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Fig. 5. Principal components ordination based on P-V and related parameters. (a) Correlation of the parameters with the first two components of the ordination (see Table 2 for further explanation). (b) Position of the species on the first two components (see Table 1 for abbreviations).

of zero turgor has been determined from the point on a graph of turgor pressure against water content where 'Jfp = 0, and the bulk modulus from the slope of the graph (Fig. 2). This method is not dissimilar to that of MELKONIAN et aJ. (1982) who determined a bulk modulus from the initial slope of a graph of P against V. The bulk moduli obtained in this paper can be converted to those used by other authors as V p and V 0 are known. The overall slope of the P-V curve (approximated as a straight line) and slopes both before (i.e. bulk moduli from 'Jfp against V) and after (e.g. liP or P against V/VT or VE/VT) the turgor loss point have been used to compare species (Tables 2, 4, 5).

28

P. BA="NISTER

Statistical analyses Linear regressions and correlations have been computed and compared by analyses of co· variance (SNEDECOR 1(56) using the statistical package TEDDYBEAR (WILSON 1975). Differences between slopes were determined by a version of Duncan's multiple range test which is similar to the Q test of SNEDECOR (1956). Principal component analyses of correlation matrices (Figs. 3, 5) have been used to produce a "parsimonious summarization of a mass of observations" SEAL (1964), and were executed by an unpublished general program for multivariate analyses (GaLLI. WaG by J. B. WILSON). All analyses were run on the Burroughs B6700 or B5930 computers at the University of Otago.

Results Drought resistance A principal components analysis of drought resistance parameters (based on VT ) accounts for 84 % of the total variation and shows an inverse relationship between drought avoidance and tolerance (Fig. 3a). Species with high avoidance and low tolerance are displaced to the right of Fig. 3b (e.g. Pseudopanax colensoi, Phymatosorus diversifolius and most of the conifers) in contrast to those with high tolerance and low a voidance (e.g. Blechnum penna-marina, Cyathodes froseri). Other species show intermediate values of avoidance and tolerance (e.g. Dacrycarpus dacrydioides, Pteridium esculentum, Brachyglottis repanda). Some species deviate from this inverse relationship as a high position on the second axis is associated with increased tolerance. Thus Plagianthus betulinus appears to be the least drought resistant as it combines moderate avoidance with poor tolerance. Other species appear more drought resistant. N othofagus fusca, Podocarpus hallii and Weinmannia racemosa combining moderate to high avoidance with increased tolerance, whereas Drypoteris filix-mas, Asplenium bulbiferum and Pernettya macrostigma combine high tolerance with increased avoidance. A very similar ordination of species i'l obtained for values expressed in terms of symplastic water content (F). The first axis of such an ordination is highly correlated (r = 0.8) with that in Fig. 3b (Table 5). Otherwise the position on the first axis of the ordination (which can be taken as a measure of relative drought avoidance or tolerance) is not strongly associated with any other measured parameters. There are significant correlations with V p, B, VIP and SF (Tables 3 and 5) so that high avoidance is associated with low symplastic water contents at zero turgor, high bound water contents, steep slopes of liP against relative water content, and shallow slopes of P against F beyond the turgor loss point. High tolerance is associated with opposite values of these parameters. The best multiple regression (accounting for 51 % of the variation) relates position to V p and SF. Slopes of relationships between P and V (Ta ble 4) The overall relationships of P against V or F (i.e. PI V, PI F) can be approximated by a straight line, but as they are highly correlated with slopes after the turgor loss point (SR, SF), only the latter are considered in detail. The difference between slopes of P against V and those of P against F is a function of the amount of bound water, thus species with low amounts of bound water show similar slopes for both relationships (e.g. Cyathodes fraseri, Asplenium bulbiferum) , whereas species with high amounts of bound water (e.g. Dracophyllum longifolium, Phymatosorus diversifolius) show large reductions in slope. Although the differences in slope are highly significant, the principal differences are between the extremes and are reduced or even eliminated when P is plotted against F (Table 4).

Some New Zealand Plants

29

Beyond the turgor loss point the slope of the curve is related to the bulk osmotie potential of the tissue and should give a linear relationship between liP and V. This is the case, although a regression of P against V gives an equally good fit. Species with a steep slope of liP against F generally show shallow slopes of P against F and vice versa, but the same is not true of slopes of liP against V and those of P against V because steepness of the former is associated not only with low values of II 0 but also with high values for bound water (e.g. Phymatosorus diversifolius shows a steep slope of both liP against V and P against V because of its high value for bound water). Steep slopes of P (or shallow slopes of liP) against V or F are associated with high values for IIo and IIp and low water contents (Rp or Fp) at the turgor loss point. Steep slopes of both P and liP against V are associated with low symplastic water contents at the turgor loss point and high amounts of bound water. Steep overall slopes of P against F (and shallow slopes of liP against F) are associated with low saturated water contents per unit dry weight (TI, T2) which indicate more sclerophyllous shoots or leaves (Table 5). Slopes of turgor pressure against water content (Table 4) The relationship between turgor pressure and water content determines the bulk modulus (Fig. 2). Species with high bulk moduli are likely to have a combination of high turgor pressure at saturation (which will be reflected in a low osmotic potential or high IIo) and a high relative water content at the turgor loss point (e.g. Brachyglottis repanda) but moderately high bulk moduli are shown in species which have high initial turgor and turgor loss at lower water contents (e.g. Pernettya macrostigma, Cyathodes fraseri, Nothofagus spp.) or lower initial turgor and turgor loss at high water contents (e.g. Asplenium bulbiferum, Pseudopanax colensoi, Leptospermum scoparium) or intermediate values for both (e.g. Weinmannia racemosa, Dacrydium spp., Plagianthus betulinus). Low bulk moduli are associated with combinations of low initial turgor andlor turgor loss at relatively low water contents (e.g. Agathis australis, Blechnum penna-marina, Pte ..idium esculentum). In some of these cases it is possible that the low bulk moduli represent failure to achieve full saturation. Differences between slopes are highly significant (P < 0.001) but tend to be associated with the extremes (Table 4). Bulk moduli are associated with few other measured parameters (Table 5), although bulk moduli calculated on the basis of relative water content (MR) are highly correlated with those based on symplastic water content (MF), despite differences in bound water content. Otherwise bulk moduli are positively correlated with water contents at the turgor loss point (Vp, R p, Fp) as shown in Fig. 4. Relationships between other parameters (Ta bles 3 and 5) Osmotic pressures, both at full turgor (IIo) and turgor loss point (IIp), are, apart from the expected correlation with one another, inversely correlated with water contents both at zero turgor (Rp, Fp) and per unit dry weight (TI, T2); whereas Rp and Fp are positively correlated with TI and T2. The symplastic water content at zero turgor (Vp) is inversely correlated with bound water content (B) and also shows some positive correlation with the relative symplastic water content (Fp = Vp/V o ). The broad relationships between the many parameters (other than those relating to drought resistance) is evident in a principal components analysis (Fig. 5a). Here, steep slopes of P or V against F (i.e. PI V, P/F, SR, SF) and high osmotic pressures (IIo, IIp) are positively associated with the first axis, whereas high water contents (Tl, T2, Fp) and steep slopes of liP against F (F/P) are negatively associated. The second axis shows negative association with steep slopes of liP against V (VIP) and

P I

30

P.

BANNISTER

high bound water (B) and is positively associated with high values for bulk modulus (MF) and the symplastic water content at the turgor loss point (V pl. Bulk moduli (MR) and the relative water contents at turgor loss point (Rp) that are calculated in terms of VT are best assoeiated with a third axis (not shown). The resulting speeies ordination (Fig. 5 b) is quite dissimilar to the ordination based on drought resistance (Fig. 3b). Ferns are displaced to the left on account of their high water eontents (T1, T2, R p , Fp) and their low osmotic pressures (Ilo, IIp). However the two ferns characteristic of more exposed habitats, Blechnum and Pteridium, are most displaced to the right whereas the species from the most sheltered habitat, Aspleniu111, is furthest to the left. Similarly, Nothofagus solandri and N. menziesii which are charactelistic of drier or more exposed habitats than N. fusca are further displaced to the right. The rest of the species, however, show no marked separation; e.g. there is little separation of Dacrycarpus and the Dacrydium spp., which are often found in wet habitats, from Podocarpus hallii which is often found in dry and exposed sites. Water relations in summer and winter (Table 6) Some parameters derived from P-V curves were determined for a limited number of species in winter (1981, 1983) and provide an opportunity for comparison with summer values (Table 6). Those species examined in both summer in winter show a higher osmotic pressures, particularly at the turgor loss point, in winter. This is a consequence of a higher osmotic pressure at full saturation (in all species except Leptospermum scoparium) and lower relative water contents at the turgor loss point. Other measured parameters show no consistent change between summer and winter.

Discussion The results presented in this paper are from an extensive rather than intensive study. Each species is represented by a single set of results and cannot represent the range of variation shown by that species with regard to both site and season. The object of this paper is, however, not to show differences between species but rather to use value.;; obtained for different species to make generalisations about the relationships between drought resistance and water relations parameters. Such generalisations are more likely to be valid if made between species than is a more intensive investigation of variation within a particular species. Some indication of seasonal variation is given in Table 6. As far as possible the material collected in summer was of current mature growth, but material of Leptospermum scoparium, Pteridium esculentum and Metrosideros robusta was of recent new growth. Young material in summer is often characterised by low osmotic pressures (TYREE et al. 1978; KARLIC & RICHTER 1983; GREEN & JANE 1983 b) and low bulk moduli (d. TYREE et al. 1978). The fronds of Pteridium were only recently expanded and show a very low bulk modulus whereas Metrosideros and other species sampled in winter show increased Ilo (Table 6). An increase in bulk modulus is apparent only in Phymatosorus. In all other cases lower bulk moduli are associated with much lower water contents at the turgor loss point (d. Fig. 4) and this would have the effect of producing a lower bulk modulus if the maximum turgor pressure wab similar in both cases. Comparisons with the results of other workers The osmotic components (Il 0, Il p) are generally within the range shown by other workers for woody plant" (e.g. Populus spp. show values for Ilo from 1.09-2.16 MPa and 1.37-2.57 MPa for IIp, TYREE et al. 1978; SINCLAIR & VENABLES 1983), although

Some New Zealand Plants

:31

somewhat lower values are recorded for Acer saccharum (TYREE ct al. 1978). The annual range shown by conifers (Picea, Tsuga) is also similar (TYREE et al. 1978). Values for other New Zealand plants are in accord with those in this paper and specific values for Weinmannia racemosa and N othofagus menziesii are in the same range (JANE & GREEN 1983). Investigations of drought-hardy shrubs (HINCKLEY et al. 1980 1983) in summer give a wider range (lIo from 0.79-2.98 MPa and IIp from 1.80 to 3.94 MPa) with a strong bias to higher values (> 2 MPa). Values for the water content at turgor loss (expressed as Vp/V o ) for the 23 species in this paper range from 70-95 %. CHEUNG et al. (1975) give a range of 60-85 % for 10 species and JANE & GREEN (1983) 80-88 % for five New Zealand species. The higher values (> 90 %) for some species in this paper are associated with drought avoidance (e.g. Phymatosorus, Pseudopanax) or with high bulk moduli (e.g. Brachyglottis). Values for symplastic (Vo) or bound water (B = VT-V o ) vary with season (TYREE et al. 1978) and with site (JANE & GREEN 1983) so that the potential range is wide (e.g. Vo from 48-96 % in JANE & GREEN 1983) which compares with 47-100 % in this paper and 5 -96 % in CHEUNG et al. (1975). Specific comparisons show that V 0 was slightly greater in N othofagus menziesii and less in Weinmannia than recorded by JANE & GREEN (198:3). Bulk moduli are calculated in various ways, often as some sort of exponential relationship (e.g. TYREE & HAMMEL 1972; HELLKVIST et al. 1974; SINCLAIR & VENABLES 1983; JANE & GREEN 1983) which is extrapolated to full saturation. Such results cannot be validly compared with those in this paper which are based on a linear approximation. The E MAX of CHEUNG et al. (1976) is more comparable and their range of 6-26 MPa is very similar to the 2-27 MPa that is obtained when the bulk moduli from this paper are rEo-expressed in terms of Vp. They record a value of 8 MPa for Nothofagus betuloides which is very similar to the 7-10 MPa for three New Zealand species of Nothofagus, although their value of 15 MPa for Podocarpus nubigenus is considerably higher than the 5 MPa calculated for the morphologically similar P. hallii. Effects of sampling shoots or leaves It is possible that some of the differences between species and associations between characters might be associated with the fact that some species (e.g. ferns, Brachyglottis, Pseudopanax) were sampled as leaves while the rest were not. Species sampled as leaves had a significantly higher water content per unit dry weight (P < 0.001) than those sampled as shoots. Hence, some of the parameters correlated with water content (Tl, T2) are also associated with whether shoots or leaves were sampled (e.g. lIo, R p, Fp), whereas others wcre not (e.g. lIo, P/V, F/P, P/F). Overall, parameters were less associated with whether shoots or leaves were sampled than they were with water content. Furthermore, bound water (B) would be expected to be associated with the higher amounts of xylem in woody twigs (d. JANE & GREEN 1983). It is not, and the range of bound water is as great in species sampled as leaves as in those sampled as twigs despite the greater water content per unit dry weight of the former. There is, therefore, no strong evidence to suggest that sampling species either as leaves or as shoots has had a great effect on the results obtained; but verification of this observation can be obtained only by further experiments that compare results obtained using both shoots and leaves of the species involved. Relationships between P - V and associated parameters Some of the relationships shown in Table 5 are only to be expected; e.g. the relationships between similar values such as osmotic pressures (lIo, IIp), slopes (SR,

32

P.

BANNISTElt

PIV; SF, P/F, F/P) and water contents (Tl, T2; R p , Fp). Others are related by their

derivation: e.g. both IIo and B must be related to the slope of liP against V (and all correlated slopes, e.g. FjP), whereas Vo (and hence Vl') is inversely related to B (Fig. 1). Even the relationship of water content (R p , Fp) to bulk modulus (MR, MF) is predictable if maximum turgor pressures are similar for all species (Fig. 2) as the slope of a linear regression of lJI p against water content is a function of its intercepts with both axes. JANE & GREEN (1983) show that their bulk modulus is dependent upon II 0 as a consequence of the relationship that they use for its calculation. In their case there is no clear relationship with Fl'. The association of water content at the turgor loss point with osmotic pressure (IIo, IIp) might be expected if the osmotic contents of all species were the same (which they are not) and then the associations with slope (e.g. SF) follow (as SF and IIo or IIl' are positively correlated). Inspection of the data of JANE & GREEN (1983) shows a similar relationship of water content at turgor loss with osmotic potentials. Other correlations cannot be so readily accounted for; e.g. those of osmotic pressure (IIo, IIp), water content (Rl', Fl') and slope P/V, F/P, P/F) with water content per unit dry weight, and the correlations with drought resistance (Table 5); although some of the conclmiions of CUTLER et al. (1977) on the effect of cell size may help to explain these relationships. Thus the correlation of parameters has both revealed relationships that are partially a consequence of the analysis but still are of ecological interest (e.g. the associations of water content at turgor loss with bulk moduli and osmotic pressures) and those which are novel and unexpected (e.g. the association of water content at turgor loss with the slope beyond it, and the various cOITelations with water content per unit dry weight). Relationships with field

0

bservations of drought resistance

ATKINSON & GREENWOOD (1972) and OGDEN (1976) have shown both Podocarpus totara (which is morphologically and ecologically similar to P. hallii) and Metrosideros robusta to be drought resistant. These two species are drought avoiders (Fig. 3b). On the other hand, Leptospermum spp., which are tolerators, are also drought resistant in the field. Dacrycarpus dacrydioides and Dacrydium cupressinum were found to be drought sensitive, but cannot be differentiated to any great extent, either in terms of laboratory measures of drought resistance or any other water relations parameters, from Podocarpus hallii. Likewise, Brachyglottis repanda was found to be moderately drought sensitive but is not readily differentiated from the Leptospermum species in terms of drought tolerance and other measured parameters (Figs. 3b and 5b). The drought resistance of cut shoots is not necessarily correlated with any field observations of drought resistance, principally because the water relations of the shoots of intact plants are influenced by their roots. Species from wet areas (e.g. Dacrycarpus and Dacrydium spp.) may be susceptible to drought because of poor rooting in wet substrates (cf. GREEN & JANE 1983a) irrespective of the drought resistance of their shoots, while other species may have extensive root systems tapping deeper sources of water and thus escape damage even though their tissues may not appear so drought resistant (e.g. HINCKLEY et al. 1980, 1983). As each species is likely to be adapted to the environment in which it is found, different strategies (e.g. tolerance and avoidance) are to be expected (d. HARPER 1982). It is possible that the survival of extreme drought is not as suitable a criterion as the ability to grow and develop under moderate water stress. This latter is likely to be a function of stomatal aperture, which is related to turgor, and stomatal closure has been shown to coincide with the turgor loss point in some instances (RICHTER

Some New Zealand Plants

33

et al. 1980; HINCKLEY et al. 1980, 1983). Plants that are able to delay stomatal closure when under water stress ",ill be able to photosynthesise for longer periods (e.g. the dwarf shrub. Calluna vulgari8, shows stomatal closure at lower relative water contents in plants from drier or more open habitats than in plants from wet or sheltered habitats. BANNISTER 1964, 1971). Such a strategy is one of tolerance rather than avoidance and in the investigated New Zealand species (e.g. the dwarf shrubs Cyathode8 fra8eri and Pernettya macro8tigma) it is often associated with low relative water contents at the turgor loss point. Plants that show turgor loss at high relative water contents are able to delay turgor loss (and hence stomatal closure) if they possess high bulk moduli (e.g. Brachyglotti8 repanda); thus a large change in water potential can be accommodated by only a small loss of water content. The significant correlation that exists between bulk moduli and the water contents at stomatal closure suggests that these two strategies are complementary (Fig. 4). The association of high IIo and high I/P with low water contents at the turgor loss point (Table 5) implies that turgor loss will occur at lower water potentials in drought tolerators. Beyond the turgor loss point, a steep curve of P against V (or water potential against water content) will allow species to adjust their water potentials at the expense of only small changes in water content. Such steep slopes are again characteristic of tolerators; however steep slopes, low osmotic potentials and low water contents at the point of turgor loss (all of which may be interpretated as "drought tolerance") are more highly associated with species with low ratios of saturated water content to dry weight (i.e. sclerophylly) than with drought resistance. These correlations support the predictions of CUTLER et al. (1977), although no observations of cell size were made. When the bulk water potential of a plant's tissues is lowered (e.g. by decreased availability of soil moisture), the drought avoider with a high bulk modulus will resist the initial loss of water but will lose turgor (and show stomatal closure) at a relatively high water potential and content. Subsequently it must either show a large loss of water content (so that water potential is lowered sufficiently for water to be taken up from the environment) or rely on supplies of stored water. The latter implies an interchange between "bound" and symplastic water which some have suggested might not exist (e.g. TYREE & KARAMANOS 1981). In contrast the tolerator will maintain turgor (and open stomata) over a wider range of water contents and water potentials and be able to reduce water potential by losing lesser amounts of water after turgor is lost. The conclusion of this analysis is that drought tolerators are better adapted to grow and develop in habitats subjected to moisture stress than avoiders, whose strategy merely allows survival. Similar arguments to the above have been advanced by RICHTER & WAGNER (1983), who discuss the tolerance and avoidance of periods with restricted photosynthesis, and by HINCKLEY et al. (1980, 1983). If these criteria are used then species with low water potentials at turgor loss aml which show large changes in water potential for small losses of symplastic water after turgor loss should be the most characteristic of dry habitats. If species are ranked on these two criteria then the first seven species are, in rank order, Cyathode8 fra8eri, Pernettya macro8tigma Nothofagus 8olandri, N. menziesii, Blechnum penna-marina, Plagianthu8 betulinu8, Leptospermum ericoides. All of these, with the possible exception of Plagianthus, are found in dry or exposed habitats and the two Nothofagus spp. are found at tree-line. Plagianthus was noted as not particularly drought resistant in laboratory studies; it is, however, the only deciduous species examined and some of the habitats on which in grows (e.g. alluvial soils) could be subject to periodic drought. The seven lowest rankings are, from the lowest, Asplenium bulbiferum, Metrosideros robusta, Phymatosorus diversifolius, Podocarpus hallii, Pteridium esculentum, Dacry3

Flora, Bd. 178

P. BANNISTER

34

Table 1. Names, families, and abbreviations for the species used Ferns Asplenium bulbiferum FORST. f. Blechnum penna-marina (POIR.) KUHN Dryopteris filix-mas (L.) SCHOTT Phymatosorus diversifolius (WILLD.) PIC. SER. Pteridium esculentum (FORST. f.) KUHN

( Aspleniaceae ) (Blechnaceae) ( Aspidiaceae ) (Polypodiaceae) (Pteridiaceae)

As Bl Dry Phy Pte

(Epacridaceae) (Epacridaceae) (Ericaceae)

Cy Drl Pny

( Araucariaceae) (Podocarpaceae) (Podocarpaceae) (Podocarpaceae) (Podocarpaceae)

Ag Dcy Dbd Dcu Pdh

( Asteraceae) (M yrtaceae ) ( M yrtaceae ) ( M yrtaceae) (Fagaceae) (Fagaceae)

Br Le Ls

(Fagaceae) ( M alvaceae ) ( Araliaceae) (Cunoniaceae)

Ns Pig Px Wn

Ericaceous and epacridaceous shrubs/dwarf shrubs

Cyathodes fraseri (A. CUNN.) ALLAN Dracophyllum longifolium (J. R. et G. FORST.) R. BR. Pernettya macrostigma COL. Coniferous trees and shrubs

Agathis australis SALISB. Dacrycarpus dacrydioides (A. RICH.) DE LAUBENFELS Dacrydium bidwillii HOOK. f. ex KIRK Dacrydium cupressinum LAMB. Podocarpus hallii KIRK Other trees and shrubs (all evergreen except Plagianthus)

Brachyglottis repanda J. R. et G. FORST. Leptospermum ericoides J. R. et G. FORST. Leptospermum scoparium J. R. et G. FORST. M etrosideros robusta A. CUNN. Nothofagus fusca (HOOK. f.) OERST. N. menziesii (HOOK f.) OERST. N. s,olandri (HOOK f.) OERST. var. cliffortioides (HOOK. f.) POOLE Plagianthus betulinus A. CUNN. Pseudopanax colensoi (HOOK f.) PHILIPSON Weinmannia racemosa LINN. f. Table 2. Parameters measured during the investigation

Parameters from P-V curves P Balancing pressure (MPa, = -P) lIo Osmotic pressure (-Ps) at full turgor (MPa) IIp Osmotic pressure (-Ps) at turgor loss point (MPa) VT Water content at full turgor VE Extruded water content (% ofVT) Vo Maximum symplastic water content (% ofVT) B "Bound" water content (B = V T - V 0) (% of V T) V Symplastic water content (% OfVT) R % Relative water content (V/VT) F % Relative water content (V /V 0) VP Symplastic water content (Vp) at turgor loss point (% of VT) Rp % Relative water content (V/VT) at turgor loss point Fp % Relative water content (V/Vo) at turgor loss point T1 VT/dry weight (1980/1981) (g/g) T2 VT/dry weight (1981/1982) (g/g) Slopes beyond turgor loss point (V T, V 0 = 1) SR Slope of P against V E/V T (MPa) SF Slope of P against V E/V 0 (MPa) V/P. Slope of l/P against V/VT (l/MPa) F/P Slope of l/P against VIVo (l/MPa)

Mr Nf Nm

1

35

Some New Zealand Plants

Table 2 (Continued)

I

Overall slopes; Bulk moduli (V T, V 0 PlY P/F MR MF

=

1)

Overall slope of P against VE/V T (MPa) Overall slope of P against VE/V O (MPa) Bulk modulus (slope of lJ'P against V/V T) (MPa) Bulk modulus (slope of lJ'P against VIVo) (MPa)

Drought avoidance LR Time taken (loglO h) to desiccate to 50 % relative water content (1980/81) LG Time taken (loglO h) to dessiccate to a balancing pressure of 3.45 MPa (500 pounds per square inch) (1981/82) DA % Relative water content (V/VT ) after 8 h desiccation FA % Relative water content (V/Vo ) after 8 h desiccation Drought tolerance DT % Relative water deficit (VE/V T ) at 50% damage FT % Relative water deficit (VE/V 0) at 50 % damage PT Water potential (-MPa) at 50% damage Species' positions on first axis of drought ordinations AR AF

First axis of drought ordination using V /VT First axis of drought ordination using V /V 0

Table 3. Drought resistance and other water relations parameters for the investigated species arranged by life·form and decreasing order of drought avoidance within each group (see Tables 1 and 2 for a fuller explanation of abbreviations) LR

LG

DA

DT

PT

JIo

JIp

Rp

B

Tl

T2

1.6 1.1 0.6 0.9 0.3

1.7 0.7 1.1 0.5 0.5

88 58 49 52 21

37 74 77 47 80

3.6 7.2 3.3 4.7 7.7

1.27 1.36 1.10 1.14 1.68

1.39 1.60 1.16 1.29 1.87

96 91 95 94 93

51 35 8 50 28

3.7 2.3 5.3 2.0 2.6

3.7 3.4 4.6 2.0 3.4

Ericaceous and epacridaceous shrubs/dwarf shrubs 1.49 70 57 6.1 0.6 1.3 Drl 6.2 1.69 0.9 0.5 49 69 Pny 1.98 70 7.6 0.2 0.3 24 Cy

1.67 2.37 2.78

95 79 73

50 26 7

0.8 1.4 1.1

1.2 1.5 1.2

Ferns Phy Dry Asp Pte Bl

Coniferous trees and shrubs 1.6 1.5 1.1 1.6 1.1

84 76 71 62 67

35 65 43 36 58

4.0 4.7 3.5 2.7 3.9

1.10 1.22 1.52 1.60 1.49

1.56 1.51 1.88 1.88 1.77

86 88 85 87 90

53 37 21 8 19

1.8 1.5 1.7 1.3 1.3

1.8 1.8 1.4 1.5 1.7

Other trees and shrubs 1.5 1.6 Psx 1.5 1.5 Mr 1.5 1.0 Wn 1.3 Nf 1.4 0.6 1.4 Nm 0.6 1.2 Ns

84 76 73 60 74 62

36 >52 61 66 45 49

3.5 3.1 5.2 7.3 5.0 4.2

1.45 1.25 1.52 1.62 1.90 1.82

1.62 1.51 1.81 1.95 2.56 2.37

95 86 91 92 83 85

34 18 44 52 36 34

2.5 1.7 1.4 2.0 1.1 1.2

2.7 2.6 1.6 1.6 1.2 1.2

Ag Pdh Dcu Dbd Dcy

3·

1.6 1.4 1.1 1.0 1.1

t 36

P. BANNISTER

Table 3 (Continuod) LR

LG

Other trees and shrubs 0.8 0.5 0.5 0.8 0.8 0.4 0.7 0.5

Ls Br PIg Le

DA

DT

PT

lIo

IIp

Rp

B

T1

T2

31 29 44 26

63 60 34 65

3.7 4.7 3.4 4.7

1.45 1.70 1.66 1.40

1.55 1.83 2.07 1.78

93 94 85 89

1 16 26 36

1.7 2.8 2.7 1.8

1.5 2.6 2.3 1.2

NB. FA, FT, Fp may be oalculated from DA, DT, R p ; either as 100 (Rr-B){(100-B) (from DA, Rp) or as 100Dj(100-B) (from DT) where Rand D are relative water content (VjV T ) and water deficit (VEjVT). The symplastic water content at turgor loss point is Rp-B.

Table 4. Slopes of regressions of balancing pressure (P) against water content or water deficit, and of turgor pressure against water content (= bulk modulus) for various species arranged, within each group, with respect to decreasing slope (SR) after the turgor loss point (values in parentheses are based on symplastic water content) slope after turgor loss point

bulk modulus

overall slope

SR

SF

VjP

FjP

PjV

PjF

MR

MF

8.9bcde 8.2abcde 8.0bcde 7.5abcd 2.9a

(4.5)a (5.9)ab (5.8)ab (3.7)a (2.6)a

1.74a 0.82b 1.12b 1. 78a 0.99b

(0.88)a (0.59)a (0.73)a (0.88)a (0.91)a

8.4bcd 8.4bcd 6.5b 8.5bcde 3.3a

(4.2)a (6.1)cde (4.2)a (4.2)a (3.0)a

3.8e 9.6de 8.5de 22.2bc 13.6de

(1.9)d (7.0)bcd (5.6)bc (11.0)bc (12.3)bc

Ferns Pte Bl Dry Phy Asp

Ericaceous and epacridaceous shrubsjdwarf shrubs 0.55b 1.34ab 0.80b

(0.51)a (0.67)a (0.59)a

11.5ef 11.3def 9.9cde

(10.7)f (5.7)abcd (7.4)de

8.7de 25.0b 9.0de

(8.1)bc (12.5)b (6.7)bcd

(5.0)ab (3.8)a (3.9)a (3.6)a (2.6)a

1.94a 1.30b 0.83b 0.68b 0.82b

(0.91)a (0.82)a (0.67)a (0.62)a (0.65)a

10.6cdef 7.3bo 7.2b 7.0b 6.7b

(5.0)abc (4.6)ab (5.8)abcd (6.4)bcde (5.4)abcd

5.ge 8.8de 16.ld 11.9de 10.6de

(2.8)d (5.6)cd (13.0)bc (10.9)bc (8.4)bc

Other trees and shrubs (4.7)ab Nf 9.7cde (5.8)ab 9.1bcde Le (5.1)ab 7.8abcd Ns 7.7abcd (5.0)ab Nm 7.0abc (5.2)ab PIg 6.6abc (3.7)a Wn Px 6.5ab (4.3)ab (5.0)ab Br 5.9ab (5.4)ab Ls 5.4ab Mr 4.3ab (3.6)a

1.27b 1.05b 0.83b 0.82b 0.81b 1.16b 1.03b 0.70b 0.70b 0.97b

(0.62)a (0.68)a (0.55)a (0.53)a (0.60)a (0.67)a (0.68)a (0.59)a (0.69)ll, (0.80)a

3.2f 10.6cde 8.4bcd 11.8ef 8.1b 9.4bcde 8.1bcd 6.5b 4.4ab 6.2b

(6.4)bcde (6.8)cde (5.5)abcd (7.6)e (6.0)abcde (5.3)abcd (5.4)abcd (5.5)abcd (4.4)a (5.1)abc

23.6b 19.0bod 14.7d 13.8de 8.1e 16.8cd 20.8bcd 34.5a 14.6d 9.4de

(11.5)bc (12.3)bc (9.7)bc (8.9)bc (7.4)bcd (9.9)bc (13.8)b (29.4)a (14.6)b (7.8)bc

Cy Drl Pny

11.6e 9.0bcde 8.0abcde

(10.8)c (4.5)ab (6.0)b

Coniferous trees and shrubs Ag Pdh Dcy Dbd Dcu

10.6de 6.0ab 4.8a 3.9a 3.3a

NB. Values in oolumns that share the same letter are not significantly different at the 5 % level. See Tables 1 and 2 for further explanation of abbreviations.

ns

+++

ns

ns

ns

ns

ns

ns

+++ ns

ns

ns

Fp

ns

ns

Tl

ns

ns

ns

T2

ns ns + +++ ++ + ++ + +++

--------

ns

ns

Rp

ns

ns

F/P

ns ns

ns

+++ ++ ns +++ +++

++

ns

ns

+ + + - --,

+++

ns

ns

ns

ns

ns

ns

ns

ns

ns

ns

ns

ns

Z CD

++

W -1

rJ)

~

::

go

"':I

0..

::

go

po

CD

N

:
ns

rn 0

S CD

+

ns

ns

ns

ns

---ns

+++ +++

respectively:

+

+ and -

ns

ns

ns

+ ns

+++

ns

ns

ns

+ +

ns

ns

ns

ns

ns

ns

ns

MR MF

ns

+ ++

ns

ns

ns

ns

ns

ns

ns

ns ns

ns

+

ns

ns

ns

ns

P/F

++ - - - +++ +++ - - - +++

SF

+

---+ ++

ns

ns

+ ns

P/V

VIP

ns

ns

ns

SR

ns ns - - - - - - - - - - - - ns

ns

ns

+

B

NB. See Table 2 for a fuller explanation of symbols. Significant positive and negative correlations are given by P < 0.001; + + - -, P < 0.01; + -, P < 0.05; ns = not significant, P > 0.05.

MR

F/P P/F

SF

VIP P/V

SR

B

T2

Tl

Fp

Rp

Vp

IIo IIp

AF

AR

Vp

IIo

AF

IIp

Table 5. Correlations between drought resistance and P-V parameters

r P. BANNISTER

38

Table 6. Differences in osmotic pressures (lIo, IIp), relative water contents at turgor loss (Rp, F p ), bulk moduli (MR, MF) and "bound" water (B) for four species in summer and winter. Values based on symplastic water content (V 0) are in parentheses (for further explanation see Table 2)

lIo

IIp

RpFp

MR

MF

B

Phymatosorus diversifolius

1.27 1.71

1.39 2.00

96 (91) 95 (89)

22.0 35.4

(11.0) (19.7)

51 summer 1982 56 winter 1983

M etrosideros robusta

1.25 1.85

1.51 2.38

86 (83) 82 (78)

9.4 8.6

(7.8) (7.0)

18 summer 1982 19 winter 1983

Blechnum penna· marina

1.68 2.29

1.87 2.92

93 (90) 78 (66)

9.6 5.6

(7.0) (7.6)

28 summer 1982 36 winter 1983

Leptospermum ericoides

1.46 1.80

1.78 2.17

89 (83) 80 (72)

19.0 7.8

(12.3) (5.7)

36 summer 1982 28 winter 1981

Leptospermum scoparium

1.45 1.20

1.55 1.84

93 (93) 70 (63)

14.6 5.6

(14.6) (4.7)

1 summer 1982 18 winter 1981

significance

(*)

*

** **

ns

ns

ns

Significance by t·test on differences between summer and winter: (*) P < 0.1, * P < 0.05, ** P < 0.01. (In the analysis of Rp a logarithmic transformation [loglo (difference 1)] was used).

+

dium cupressinum, Pseudopanax colensoi. Most of these species are drought avoiders; two (Pteridium and M etrosideros) were sampled as young material but are, along with Podocarpus hallii, often found in dry habitats. The re&t are found in sheltered or wet habitats. In conclusion it seems that there are a number of complementary mechanisms in the water relations of the species examined. For example, those species which show a low rates of water loss during desiccation are usually damaged by relatively low water deficits, whereas those that show high rates of loss are damaged only by large water deficits. Furthermore, species with high bulk moduli (allowing large adjustments in water potential for small changes in water content) often show turgor loss at high water contents and less steep slopes of water potential against water content after turgor loss; whereas those with low bulk moduli often show turgor loss at higher water deficits and steeper slopes of water potential against water content after turgor loss. There would, therefore, seem to be no universal strategy of resistance to desiccation but rather a variety of processes that achieve the same end by different means.

Acknowledgements I should like to thank P. J. M. SMITH, T. J. LOUGH and R. M. KISSEL for their assistance in this work in successive summers from 1980/81 to 1982/83; the University of Otago for providing Sum· mer Vacation Research Bursaries for two of them (PJMS, RMK); and Mrs. J. CLOUGH who pre. pared the figures. Thanks are also due to Botany Honours students who provided some data for the winters of 1981 and 1983.

References ATKINSON, 1. A. E., & GREENWOOD, R. M. (1972): Effects of the 1969-1970 drought on two remnants of indigenous forest in the Manawatu District. Proc. N.Z. Ecol. Soc. 19: 34-42. BANNISTER, P. (1964): Stomatal responses of heath plants to water deficits. J. Ecol. 52: 423432. (1970): The annual course of drought and heat resistance of heath plants from an oceanic environment. Flora 159: 105-123.

Some New Zealand Plants

39

(1971): The water relations of heath plants from open and shaded habitats. J. Ecol. 59: 51+64. (1985): Water relations and stress. In: Methods in Plant Ecology (2nd eels, eel. P. D. MOORE) Blackwell Scientific Publications, Oxforel, in press. & SMITH, P. J. M. (1983): The heat resistance of some New Zealand plants. Flora 173: 399414. CHEUNG, Y. N. S., TYREE, M. T., & DAINTY, J. (1975): Water relations parameters on single leaves obtained in a pressure bomb and some ecological interpretations. Can. J. Bot. 53: 1342-1346. - - (1976): Some possible sources of error in determining bulk elastic moduli and other parameters from pressure· volume curves of shoots and leaves. Can. J. Bot. 54: 758-765. CUTLER, J. M., RAINS, E. W., & LOOMIS, R. S. (1977): The importance of cell size in the water relations of plants. Physiol. Plant. 40: 255-260. GREEN, T. G. A., & JANE, G. T. (1983a): Diurnal patterns of water potential in the evergreen cloud forests of the Kaimai Ranges. North Island, New Zealand. N.Z. J. Bot. 21: 379-389. - - (1983b): Changes in osmotic potential during bud break and leaf development of Nothofagus menziesii, Weinmannia racemosa, Quintinia acutifolia and Ixerba brexioides. N. Z. J. Bot. 21: 391-395. HELLKVIST, J., RICHARDS, G. P_, & JARVIS, P. G. (1974): Vertical gradients of water potential and tissue water relations in sitka spruGe trees with the pressure chamber. J. appl. Eco!. II: 637-677. HARPER, J. L. (1982): After description. In: The Plant community as a working mechanism. (ed. E.!. NEWMAN) Blackwell Scientific Publications, Oxford, pp. 11-25. HINCKLEY,T.M., DUHME,F., HINCKLEY,A.R., & RICHTER,H. (1983): Water relations of droughthardy shrubs: osmotic potential and stomatal reactivity. Plant, Cell and Environ. 3: 131-140. - - - - (1983): Drought relations of shrub species: assessment of the mechanisms of drought resistance. Oecologia (Berlin/West). 59: 344-350. JANE, G. T., & GREEN, T. G. A. (1983): Utilisation of pressure-volume techniques and non-linear least squares analysis to investigate site induced stresses in evergreen trees. Oecologia (Berlin/ West) 573: 80-390. JARVIS, P. G., & JARVIS, M. S. (1963): The water relations of tree seedlings. IV. Some aspects of tissue water relations and drought resistance. Physiol. Plant. 16: 501-516. KARLIC, H., & RICHTER, H. (1983): Developmental effects on leaf water relations of two evergreen shrubs (Prunus laurocemsus L. and Ilex aquifolium L.). Flora 173: 143-150. LEVITT, J. (1980): Responses of Plants to Environmental Stresses. Vo!' 1. Chilling, Freezing and High Temperature Stresses, Academic Press, New York. MCGLONE, M. S., & WEBB, C. J. (1981): Selective forces influencing the evolution of divaricating plants. N.Z_ J. Eco!. 4: 20-28. MELKONIAN, J. J.,WOLFE, J., & STEPONKUS, P. L. (1982): Determination of volumetric modulus of elasticity of wheat by pressure-volume relations and the effect of drought conditioning. Crop. Sci. 22: 116-123. OGDEN, J. (1976): Notes on the influence of drought on the bush remnants of the Manawatu Lowlands. Proc. N.Z. Eco!. Soc_ 23: 92-98. POLLOCK, K. M. (1979): Aspects of the water relations of some alpine species of Chionochloa. Unpublished Ph. D. thesis, University of Otago, N.Z. RICHTER, H., DUHME, F., GLATZEL, G., HINCKLEY, T. M., & KARLIC, H. (1981): Some limitations and applications of the pressure-volume curve technique in ecophysiological research In: Plants and their Atmospheric Environment (ed_ J. GRACE, E. D. FORD & P. G_ JARVIS), Blackwell Scientific Publications. Oxford, pp. 263-272. & WAGNER, S. B. (1983): Water stress resistance of photosynthesis: some aspects of osmotic relations. In: Effects of Stress on Photosynthesis (ed. R. MARCELLE, H. CLIJSTERS & M. VAN POUCKE), Martinus Nijhoff, Dr. W. Junk Publishers. The Hague, pp. 45-53. SCHOLANDER, P. F., HAMMEL, H. T., HEMMINGSEN, E. A., & BRADSTREET, E. D. (1964): Hydrostatic pressure and osmotic potentials in leaves of mangroves and some other plants. Proc. Nat. Acad. Sci. USA 51: 119-125.

40

P. BANNISTER, Some New Zealand Plants

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