- Email: [email protected]
Genetic differences in relative growth rate and partitioning growth components in Chrysanthemum morifolium
Scientia Horticulturae, 49 ( 1992 ) 267-275
267
Elsevier Science Publishers B.V., Amsterdam
Genetic differences in relative growth rate and partitioning growth components in
Chrysanthemum morifolium J. De Jong and J. Jansen Centre for Plant Breeding and Reproduction Research (CPRO), P.O. Box 16, 6700 AA Wageningen, Netherlands (Accepted 2 September 1991 )
ABSTRACT De Jong, J. and Jansen, J., 1992. Genetic differences in relative growth rate and partitioning growth components in Chrysanthemum morifolium. Scientia Hortic., 49: 207-275. The relative growth rate (RGR) measures the increase in weight of a plant per unit of time relative to the weight already present. Beyond the exponential phase of growth, RGR decreases with increasing plant weight. A sensible comparison of the RGR of two plants can then only be made if the corresponding weights are equal, in a growth analysis, the RGR of 15 cultivars of Chrysanthemum mor~folium ( Ramat. ) (Dendranthema grandiflora Tzvel. ) was measured at a dry weight of I g, RGR differed between eul!ivars and varied from 0.045 to 0.064 g g- t day- t. RGR was found to be correlated with leaf area ratio (LAR ~, but not with net assimilation rate (NAR), which suggests that the formation of leaf area rather than biomass production per unit leaf area explains the observed differences in RGR between cultivars. The occurrence of such a large genetic variation in RGR within a species is rare and it is argued that this is due to the particular selection pressures applied to ornamental plants. Keywords: Chrysanthemum mor(folium; cultivars; genetic variation; growth analysis; relative growth rate. Abbreviations: LAR =leaf area ratio; LWR =leaf weight ratio; NAR = net assimilation rate; RGR = relative growth rate; SLA = specific leaf area.
INTRODUCTION
For a number of crop species, the comparison of modern high-yielding cultivars with obsolete low-yielding types showed that the higher yield of modern cultivars can be attributed to an increased harvest index, i.e. a higher proportion of the dry matter produced is diverted to those pai~s of the pl,,ats that are harvested for economic purposes (Lambers, 1987 ). Hox~ever, when whole shoots are harvested, as in many vegetables and cut flowers such as Chrysanthemum morifolium, an increase in production cannot be ~'ealized by chang© 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4238/92/$05.00
268
J. DE J O N G AND J. JANSEN
ing the harvest index. Hence, higher production can only be obtained by increasing the crop growth rate. The concept of relative growth rate (RGR) was developed for the phase of exponential plant growth where RGR c~oes not depend on plant weight. After the exponential phase, RGR decreases with increasing plant weight. Therefore, comparisons with respect to RGR are only fair if plants of equal weights are used. However, it is still standard procedure to calculate RGR at a given time after the start of the experiment ,rod not at a given weight. This may lead to a wrong interpretation of estimates cf RGRo When analyzing the growth of cuttings of vegetatively propagated plants, like chrysanthemum, both unequal initial plant weights and non-exponential growth must be taken into account. "IO avoid initial differences in weight, cuttings may be weighed individually and trimmed to the required weight before rooting. Whether leaf or stem pieces are trimmed will affect future r~lowth, l-iowever, new variation in weight is created ~fter rooting has started and those plants rooting earliest are the first to resume growth. To avoid inaccuracies and biases due to differences in initial weight and non-exponential gro:vth, an extension of the standard growth analysis is introduced in which RGR is calculated as a function of weight instead of as a function of time. This approach has been used earlier by Smit (1983) in sugar beets. This paper aims to determine the RGR, in the manner described above, within the species C. morifolium at two irradiance levels, to focus on the principal components of growth and to identify those components that are accessible for genetic improvement. The approach chosen was to divide RGR into a morphological component (leaf area ratio; LAR ) and a physiological component (net assimilation rate; NAR). LAR describes the leaf area per gram dry weight and NAR the daily growth per square meter of leaf area. The LAR may be further partitioned into specific leaf area (SLA) which describes the leaf area per gram dry weight and leaf weight ratio (LWR) which is the proportion of dry matter that is located in the leaves: LAR=SLA×LWR MATERIALS AND METHODS
Experiments~ - The first experiment was carried out in 1985. Unrooted cut-
tings of 15 cultivars (Table 1 ), obtained from a commercial propagator, were graded according to weight. For each cultivar, the fraction with the highest number of cuttings was chosen. Cuttings were rooted in pots in a greenhouse at 22 ° C. After rooting, the plants were transferred to two growth rooms of the phytotron of the Institute for Horticultural Plantbreeding (IVT) (Smeets, 1978) on 24 September and grown further at 18°C. Both rooms were irradiated for 8 h day-~ with Philips 400 W HPI-T lamps
269
t 1~,t (;RI I ~ " l ! ! R.-%TEOF ('.liRi'S..INITtEMUM
I ;~itLE i
Height. number of leaves and growth parameters of Chrysanthemum cultivars grown at two irradiancc levels. The data presented were calculated at a shoot dry weight of 1 g and averaged over 15 cultivars lrradiance level (pE m - 2 s-~ )
Height (cm) No. of leaves RGR (g g- t day- ~) DRGR ( (g g-I day- i ) g - i ) LAR (m 2 g- t ) SLA (m: g-~ ) LWR (g g-~ ) NAR (g m--" day-~ )
80
160
33.0 18.6 0.0426 0.0077 0.0278 0.0442 0.6278 1.5547
25.5 16.0 0.0676 0.0148 0.0241 0.0365 0.6598 2.8551
SED
2.0 0.7 0.0009 0.0058 0.0007 0.0019 0.0155 0.2630
giving at the top of the plants 80 uE m-2 s-~ photosyntbe ucally active irradiation in Room l (low-irradiance treatment) and 160 pE m - 2 s- ~in Room 2 (high-irradiance treatment). Incandescent lamps gave 2.7 pE m -2 s-~ during the remaining 16 h in order to secure vegetative development. The distance from the lamps to the tops of the plants was kept constant. The daily light integrals chosen arc appropriate to winter-raised glasshouse crops in the Netherlands. In each growth room, four replicates were laid down. Each replicate consisted of six plants of each cultivar put in a row. The initial spacing was 64 plants m-2. On six successive occasions, one plant was taken at random from each row and harvested. The position of the remaining plants was adjusted over the total area in order to restore uniform spacing and to avoid inter-plant competition. For the low-irradiance treatment, harvests were carried out 0, 13, 23, 31, 41 and 50 days after the start of the experiment, while for the highirradiance treatment harvests were carried out 0, 8, 15, 22, 30 and 36 days after the start of the experiment (Day 0 is 24 September). The experiment was repeated in 1986. The grading in this second experiment differed from that in the first experiment: for each cultivar, the fraction of cuttings closest to 1 g fresh weight was selected. For the low-irradiance treatment, harvests were carried out 0, 12, 25, 3 l, 44 and 48 days after the start of the experiment, while tbr the high-irradiance treatment harvests were carried out 0, 10, 17, 24, 30 and 37 days after the start of the experiment (Day 0 is 3 October). R e l v t i v e gro,,vth rate. -
RGR(t)-
1 dW --W dt
The RGR is defined by the equation
din(W) dt
270
J. DE J O N G AND J. JANSEN
(Hunt, 1982), where W(t) is the dry weight of the plant at time t. For the present situation, it was found that the logarithm of dry weight at time t can be well approximated by a quadratic equation
ln{W(t)}~a+bt+ct 2
(l)
Since the data do not provide information about an end-point to growth, asymptotic growth functions (Gompertz, Logistic or Richard's) cannot be used in the current situation. From eqn. ( l ), it follows by differentiation that RGR, as a function of time t, can be written as RGR(t) = b + 2ct
(2)
where a is the natural logarithm of the dry weight at t=0, b is the relative growth rate at t = 0 and c is half the average change in the RGR per unit time. The time at which the dry weight In{ W(t) } attains a value I4Iois obtained by solving the equation
a+bt+ct2=ln( Wo) The solutions to this equation are given by
t~,2 = { - b + ( b 2 - 4 [ a - I n ( W o ) ]c)t/2}/2c
(3)
of which the solution corresponding to a positive RGR is of interest to us. By putting this solution into eqn. (2), a relationship between RGR and the dry weight W is found RGR(W)
=
{ b a - 4 [ a - l n ( W ) ]c} ~/2
The graph of the function RGR (W) can be drawn once estimates of the parameters a, b and c have been obtained. The decrease in RGR is obtained by differentiating - R G R (W) with respect to W, and is equal to DRGR ( W ' ) m --
2 c W - t R G R (W) -i
Second-order polynomials were also fitted to leaf area and leaf weight. The results of eqn. (3) for plant dry weight were then used to calculate the leaf area and the leaf dry weight for plants of I g. These values and the values of plant dry weight and RGR were used to calculate NAR, LAR, SLA and LWR for plants of I g.
Statistical analysis. - in both experiments, it was found that the standard error of dry. weight increased approximately proportional with the average dry weight. A logarithmic transformation was used to obtain data with constant variance (Snedecor and Cochran, 1967; Hunt and Parsons, 1974). Transformation to logarithms also provides a way to estimate the parameters a, b and c by simple least squares. For every plot in the experiment, each consisting of
RELATIVE
GROWTH
271
CilRYSAN771EMUM
RATE OF
six plants, separate estimates of a, b and c were obtained. These estimates were used as three new variables (Rowell and Waiters, 1976; Keen et ~ , 1986). The same approach could be followed for leaf area and leaf dry weight. RESULTS
Analysis of variance of RGR showed significant differences between cultivars as well as between irradiances (P < 0.05 ), but cultivar × irradiance interactions were absent, indicating that no differences could be observed between cultivars in the response to changing irradiance. Data could, therefore, be averaged over irradiances and cultivars.
Irradiance. Figure 1 (a) shows RGR as a function of dry weight for the two experiments separately. The data are averaged over the 15 cultivars. The results of the two experiments appear to be fairly similar and they have been averaged to generate the data presented in Table 1 in which-~he height, num-
o.og ~
(a)
......................
(b) 3
]60
~
I 'D
t3
, 0.07.
'o
r 0.07
ol w
f_
14
5 g
]1 Q
~ 0.06_ "
0.05
L
I I I I
I I o.o3
__1 --T
-I logarithm
t
;
0
I
of dry w e i g h t
(log e g)
0.05
i
t
I
-1
0
logarithm
of dry w e i g h t
t !
(lo%
g~
Fig. !. (a) Relationship between RGR and the logarithm of dry weight for ~he low (80 pE m -2 s -I ) and high ( 160/~E m -2 s -t ) irradiance levels in two separate experiments. The solid line represents the data of Experiment 1, the dashed line those of Experiment 2. The data are averages over 15 cultivars. (b) Relationship between RGR and logarithm of dry weight for five cultivars of C. morifolium averaged over irradiance levels and experiments (0 indicates the value of RGR at Day 20). 2, 'Byron'; 3, 'Byoux'; 13, 'Daymark'; 14, 'Regoltime'; 15, 'Album'.
272
J. DE JON(3 A N D J. JANSEN
ber of leaves and the growth parameters for a plant dry weight of I g are presented. Doubling the irre~diance level increased RGR by 59%. This increase is realized with a 13% reduction in LAR and an 84% increase in NAR. Partitioning LAR into SLA and LWR showed that at the higher irradiance the SLA is 17% lower, which indicates that the leaves are thicker or more dense than at the lower irradiance. Cultivate. - In Fig. 1 (b), the relationship between RGR and dry weight is presented for five cultivars. RGR decreases with increasing weight of the plants, thus DRG R values will be negative. The dots indicate the weight of these cultivars 20 days after the start of the experiment. It can be observed that RGR calculated at a fixed time may differ considerably from RGR calculated at a fixed weight (compare cultivar 13 with 15 ). Table 2 contains RGR and the decrease of RGR and other growth components, all determined at a dry weight of I g. It is clear that significant differences exist between cultivars in RGR, the decrease in RGR and the partitioning growth components. The cultivar 'Daymark' is the fastest growing clone ~ith a RGR that exceeds that of cultivar 'Album' by 42%. The variations in the growth components are equally large: for LAR, the score of the cultivar with the highest value exceeded the score of that with the lowest value by 50%. For SLA this was 36%, for LWR 30% and for NAR 36%.
TABLE 2 The RGR and partitioning growth components of 15 cultivars of Chrysanthemum determined at a shoot dry weight of l g. For the units, see Table I Cultivar 'Daymark' 'Pink Pompon' 'Byoux'
'Penny Lane' 'Rivalry' 'Caroussel' "FrankfoW 'Byron' 'White Spider' 'Cream Pearr 'ReCur' 'Statesman' "Delta' "Regoltime' "Mum" SED DF
RGR
")Rt~R
LAR
SLA
LWR
NAR
0.0642 0.0610 0.0599 0.0598 0.0574 0.0572 0.0548 0.054~ 0.0537 0.0536 0.0530 0.0513 0.0508 0.0504 0q452
0.0176 0.0192 0.0141 0.0129 0.0096 0.0046 0.012~ 0.0073 0.0089 0.0094 0.0105 0.0107 0.0139 0.0060 0.0115
0.0270 0.0310 0.0262 0.0306 0.0290 0.0260 0.0218 0.0236 0.0284 0.0239 0.0235 0.0258 0.0248 0.0263 0.0208
0.0410 0.0468 0.0406 0.0401 0.0442 0.0400 0.0371 0.0403 0.0401 0.0405 0.0393 0.0410 0.0387 0.0409 0.0344
0.661 0.669 0.648 0.765 0.657 0.654 0.589 0.590 0.708 0.591 0.600 0.633 0.642 0.645 0.605
2.411 2.008 2.346 2.002 2.039 2.429 2.009 2.376 1.920 2.289 2.295 2.015 2.105 1.938 2.203
0.0055 14
0.0058 14
0.0022 14
0.00033 14
0.032 14
0.281 14
RELATIVE GROWTH RATE OF ('llRYSANI'II~'L~I{'M
273
TABLE 3 Correlation coefficients beiween RGR and DRGR on the one hand and the partitioning growth on the other hand
LAR SLA LWR NAR
RGR
DRC/R
0.65** 0.6 i * 0.46 0.20
0.28 0.25 0.20 0.02
*and ** indicate statistical significance at the $% and 1% level, respectively.
Relationship between RGR, DRG R and growth components. - To answer the question which growth component is best related to the observed variation in RGR or the rate of decrease of RGR, the linear correlation coefficients were calculated (Table 3)~ The observed variation in LAR was significantly correlated with the observed variation in RGR. This was not so for NAR. From the partitioning components of LAR, the SLA showed the highest correlation with RGR. No significant correlation was found between RGR and the rate of decrease of the relative growth rate, DRG R ( r = 0 . 4 8 ) . DISCUSSION
Differences in RGR between species have frequently been reported (Poorter, 1990), but variation in RGR between genotypes within a species, where reported, is small. Smeets and Garretsen (1986) found differences in the RGR of 16 cultivars of tomato that ranged from 93 to 103% as compared to a standard. Wilson (1982) successfully selected for a low rate of respiration in Lolium perenne, but the RGR of slow- and fast-respiring selections did not differ significantly at the 5% level. Where differences were found, they coincided with subspecies of Piantago major (Dijkstra and Lambers, 1989) or distinct plant architectures in genotypes of Phlox paniculata (Garbutt, 1986) where prostrate and erect types were compared. Our observations on Chrysanthemum cultivars indicate the existence of large variation in RGR within the species C. morifolium. The cultivars used belong to the same species, are of the same erect type and they were all developed for the production of cut flowers in greenhouses. Large differences in RGR within a species have also been reported for another cut ornamental crop, the carnation (L.D. Sparnaa~, personal communication, 1990). The occurrence of variation in RGR within the species may depend on the selection pressures asserted. In crop plants, rapid growth and high yield are absolute requirements for the introduction of a cultivar and only those cultivars with maximum crop growth rate are retained. In cut flow-
274
J. DE J O N G AND J. JANSEN
ers, slow growth may be acceptable in novelties fetching high prices per flower. Thus, genetic variation in growth rate has been retained in ornamental plants which makes these suitable for the analysis of genetically controlled factors that regulate growth. Poorter (1990), who summarized 60 publications on RGR, found that differences between species in RGR are explained by LAR for 80-90% and by NAR for 10-20%. The differences we observed between genotypes within a species are similarly dominated by accompanying differences in LAR, while NAR has little to do with the variation in RGR. Thus light-intercepting capacity, rather than the efficiency of its use, seems to vary within Chrysanthemum. When LAR is divided into LWR and SLA, both LWR (which describes the allocation to the leaves) and the SLA (which describes the leaf area formed per gram dry weight) play a role, although the latter seems to be the more determining factor. In conclusion, large differences between cultivars in RGR have been observed. Rapid growth was associated with the rapid development of a large leafarea. The biomass production per unit leaf, although highly variable, does not relate significantly to RGR. When selecting for a high RGR, fast deployment of leaf area should be the main criterion.
REFERENCES Dijkstra, P. and Lambers, H., 1989. A physiological analysis of genetic variation in relative growth rate within Plantago major L. Funct. Ecol., 3: 577-587. Garbutt, K., 1986. Genetic differentiation in leaf and whole plant photosynthesic capacity and unit leaf rate among clones of Phh~xpaniculata. Am. J. Bot., 73:1364- i 371. Hunt, R., 1982. Plant Growth Curves. The Functional Approach to Plant Growth Analysis. Edward Arnold, London, 248 pp. Hunt, R. and Parsons, I.T., 1974. A computer program for deriving growth-functions in plant growth-analyses. J. Appl. Ecol., I I: 297-307. Keen, A., Thissen, J.T.N.M., Hoekstra, J.A. and Jansen, J., 1986. Successive measurement experiments. Stat. Neerl., 40: 205-223. Lambers, H., 1987. Does variation in photosynthetic rate explain variation in growth rate and yield? Neth. J. Agric. Sci., 35:505-5 ~9. Poorter, H., 1990. lnterspecific variation in relative growth rate: on ecological causes and physiological consequences. In: H. Lambers, M.L. Cambridge, H. Konings and T.L. Pons (Editors), Causes and Consequences of Variation in Growth Rate and Productivity of Higher Plants. SPB Academic Publishing, The Hague, pp. 45-68. Roweli, J.G. and Waiters, J.E., 1976. Analysing data with repeated observations on each experimental unit. J. Agric. Sci., 87: 423-432. Smeets, L., 197S. The phytotron of the Institute for Horticultural Plant Breeding (IVT), Wageningen, The Netherlands. A revision of previous descriptions. Neth. J. Agric. Sci., 26: 8-12. Smeets, L. and Garretsen, F., 1986. Growth analysis of tomato genotypes grown under low night temperatures and low light intensity. Euphytica, 35:701-715. Smit, A,L., 1983. Influence of external factors on growth and development of sugar-beet (Beta
RELATIVE GROWTH RATE OF CtlRYSANI'HEMUM
275
vulgaris L. ). Agric. Res. Rep. 914, Centre for Agricultural Publishing and Publication, Pudoc, Wageningen, 109 pp. Snedecor, G.W. and Cocl~ran, W.G., 1967. Statistical Methods.6th Edn. Iowa State University Press, Ames, IA, 593 pp. Wilson, D., 1982. Response to selection for dark respiration rate of mature leaves in Lolium perenne and its effects on growth of ,,,~, . . . . v .~""'~ J ~-..~, . . . . . . ". ." "u simulated swards. Ann. Bot., 49: 303312.
267
Elsevier Science Publishers B.V., Amsterdam
Genetic differences in relative growth rate and partitioning growth components in
Chrysanthemum morifolium J. De Jong and J. Jansen Centre for Plant Breeding and Reproduction Research (CPRO), P.O. Box 16, 6700 AA Wageningen, Netherlands (Accepted 2 September 1991 )
ABSTRACT De Jong, J. and Jansen, J., 1992. Genetic differences in relative growth rate and partitioning growth components in Chrysanthemum morifolium. Scientia Hortic., 49: 207-275. The relative growth rate (RGR) measures the increase in weight of a plant per unit of time relative to the weight already present. Beyond the exponential phase of growth, RGR decreases with increasing plant weight. A sensible comparison of the RGR of two plants can then only be made if the corresponding weights are equal, in a growth analysis, the RGR of 15 cultivars of Chrysanthemum mor~folium ( Ramat. ) (Dendranthema grandiflora Tzvel. ) was measured at a dry weight of I g, RGR differed between eul!ivars and varied from 0.045 to 0.064 g g- t day- t. RGR was found to be correlated with leaf area ratio (LAR ~, but not with net assimilation rate (NAR), which suggests that the formation of leaf area rather than biomass production per unit leaf area explains the observed differences in RGR between cultivars. The occurrence of such a large genetic variation in RGR within a species is rare and it is argued that this is due to the particular selection pressures applied to ornamental plants. Keywords: Chrysanthemum mor(folium; cultivars; genetic variation; growth analysis; relative growth rate. Abbreviations: LAR =leaf area ratio; LWR =leaf weight ratio; NAR = net assimilation rate; RGR = relative growth rate; SLA = specific leaf area.
INTRODUCTION
For a number of crop species, the comparison of modern high-yielding cultivars with obsolete low-yielding types showed that the higher yield of modern cultivars can be attributed to an increased harvest index, i.e. a higher proportion of the dry matter produced is diverted to those pai~s of the pl,,ats that are harvested for economic purposes (Lambers, 1987 ). Hox~ever, when whole shoots are harvested, as in many vegetables and cut flowers such as Chrysanthemum morifolium, an increase in production cannot be ~'ealized by chang© 1992 Elsevier Science Publishers B.V. All rights reserved 0304-4238/92/$05.00
268
J. DE J O N G AND J. JANSEN
ing the harvest index. Hence, higher production can only be obtained by increasing the crop growth rate. The concept of relative growth rate (RGR) was developed for the phase of exponential plant growth where RGR c~oes not depend on plant weight. After the exponential phase, RGR decreases with increasing plant weight. Therefore, comparisons with respect to RGR are only fair if plants of equal weights are used. However, it is still standard procedure to calculate RGR at a given time after the start of the experiment ,rod not at a given weight. This may lead to a wrong interpretation of estimates cf RGRo When analyzing the growth of cuttings of vegetatively propagated plants, like chrysanthemum, both unequal initial plant weights and non-exponential growth must be taken into account. "IO avoid initial differences in weight, cuttings may be weighed individually and trimmed to the required weight before rooting. Whether leaf or stem pieces are trimmed will affect future r~lowth, l-iowever, new variation in weight is created ~fter rooting has started and those plants rooting earliest are the first to resume growth. To avoid inaccuracies and biases due to differences in initial weight and non-exponential gro:vth, an extension of the standard growth analysis is introduced in which RGR is calculated as a function of weight instead of as a function of time. This approach has been used earlier by Smit (1983) in sugar beets. This paper aims to determine the RGR, in the manner described above, within the species C. morifolium at two irradiance levels, to focus on the principal components of growth and to identify those components that are accessible for genetic improvement. The approach chosen was to divide RGR into a morphological component (leaf area ratio; LAR ) and a physiological component (net assimilation rate; NAR). LAR describes the leaf area per gram dry weight and NAR the daily growth per square meter of leaf area. The LAR may be further partitioned into specific leaf area (SLA) which describes the leaf area per gram dry weight and leaf weight ratio (LWR) which is the proportion of dry matter that is located in the leaves: LAR=SLA×LWR MATERIALS AND METHODS
Experiments~ - The first experiment was carried out in 1985. Unrooted cut-
tings of 15 cultivars (Table 1 ), obtained from a commercial propagator, were graded according to weight. For each cultivar, the fraction with the highest number of cuttings was chosen. Cuttings were rooted in pots in a greenhouse at 22 ° C. After rooting, the plants were transferred to two growth rooms of the phytotron of the Institute for Horticultural Plantbreeding (IVT) (Smeets, 1978) on 24 September and grown further at 18°C. Both rooms were irradiated for 8 h day-~ with Philips 400 W HPI-T lamps
269
t 1~,t (;RI I ~ " l ! ! R.-%TEOF ('.liRi'S..INITtEMUM
I ;~itLE i
Height. number of leaves and growth parameters of Chrysanthemum cultivars grown at two irradiancc levels. The data presented were calculated at a shoot dry weight of 1 g and averaged over 15 cultivars lrradiance level (pE m - 2 s-~ )
Height (cm) No. of leaves RGR (g g- t day- ~) DRGR ( (g g-I day- i ) g - i ) LAR (m 2 g- t ) SLA (m: g-~ ) LWR (g g-~ ) NAR (g m--" day-~ )
80
160
33.0 18.6 0.0426 0.0077 0.0278 0.0442 0.6278 1.5547
25.5 16.0 0.0676 0.0148 0.0241 0.0365 0.6598 2.8551
SED
2.0 0.7 0.0009 0.0058 0.0007 0.0019 0.0155 0.2630
giving at the top of the plants 80 uE m-2 s-~ photosyntbe ucally active irradiation in Room l (low-irradiance treatment) and 160 pE m - 2 s- ~in Room 2 (high-irradiance treatment). Incandescent lamps gave 2.7 pE m -2 s-~ during the remaining 16 h in order to secure vegetative development. The distance from the lamps to the tops of the plants was kept constant. The daily light integrals chosen arc appropriate to winter-raised glasshouse crops in the Netherlands. In each growth room, four replicates were laid down. Each replicate consisted of six plants of each cultivar put in a row. The initial spacing was 64 plants m-2. On six successive occasions, one plant was taken at random from each row and harvested. The position of the remaining plants was adjusted over the total area in order to restore uniform spacing and to avoid inter-plant competition. For the low-irradiance treatment, harvests were carried out 0, 13, 23, 31, 41 and 50 days after the start of the experiment, while for the highirradiance treatment harvests were carried out 0, 8, 15, 22, 30 and 36 days after the start of the experiment (Day 0 is 24 September). The experiment was repeated in 1986. The grading in this second experiment differed from that in the first experiment: for each cultivar, the fraction of cuttings closest to 1 g fresh weight was selected. For the low-irradiance treatment, harvests were carried out 0, 12, 25, 3 l, 44 and 48 days after the start of the experiment, while tbr the high-irradiance treatment harvests were carried out 0, 10, 17, 24, 30 and 37 days after the start of the experiment (Day 0 is 3 October). R e l v t i v e gro,,vth rate. -
RGR(t)-
1 dW --W dt
The RGR is defined by the equation
din(W) dt
270
J. DE J O N G AND J. JANSEN
(Hunt, 1982), where W(t) is the dry weight of the plant at time t. For the present situation, it was found that the logarithm of dry weight at time t can be well approximated by a quadratic equation
ln{W(t)}~a+bt+ct 2
(l)
Since the data do not provide information about an end-point to growth, asymptotic growth functions (Gompertz, Logistic or Richard's) cannot be used in the current situation. From eqn. ( l ), it follows by differentiation that RGR, as a function of time t, can be written as RGR(t) = b + 2ct
(2)
where a is the natural logarithm of the dry weight at t=0, b is the relative growth rate at t = 0 and c is half the average change in the RGR per unit time. The time at which the dry weight In{ W(t) } attains a value I4Iois obtained by solving the equation
a+bt+ct2=ln( Wo) The solutions to this equation are given by
t~,2 = { - b + ( b 2 - 4 [ a - I n ( W o ) ]c)t/2}/2c
(3)
of which the solution corresponding to a positive RGR is of interest to us. By putting this solution into eqn. (2), a relationship between RGR and the dry weight W is found RGR(W)
=
{ b a - 4 [ a - l n ( W ) ]c} ~/2
The graph of the function RGR (W) can be drawn once estimates of the parameters a, b and c have been obtained. The decrease in RGR is obtained by differentiating - R G R (W) with respect to W, and is equal to DRGR ( W ' ) m --
2 c W - t R G R (W) -i
Second-order polynomials were also fitted to leaf area and leaf weight. The results of eqn. (3) for plant dry weight were then used to calculate the leaf area and the leaf dry weight for plants of I g. These values and the values of plant dry weight and RGR were used to calculate NAR, LAR, SLA and LWR for plants of I g.
Statistical analysis. - in both experiments, it was found that the standard error of dry. weight increased approximately proportional with the average dry weight. A logarithmic transformation was used to obtain data with constant variance (Snedecor and Cochran, 1967; Hunt and Parsons, 1974). Transformation to logarithms also provides a way to estimate the parameters a, b and c by simple least squares. For every plot in the experiment, each consisting of
RELATIVE
GROWTH
271
CilRYSAN771EMUM
RATE OF
six plants, separate estimates of a, b and c were obtained. These estimates were used as three new variables (Rowell and Waiters, 1976; Keen et ~ , 1986). The same approach could be followed for leaf area and leaf dry weight. RESULTS
Analysis of variance of RGR showed significant differences between cultivars as well as between irradiances (P < 0.05 ), but cultivar × irradiance interactions were absent, indicating that no differences could be observed between cultivars in the response to changing irradiance. Data could, therefore, be averaged over irradiances and cultivars.
Irradiance. Figure 1 (a) shows RGR as a function of dry weight for the two experiments separately. The data are averaged over the 15 cultivars. The results of the two experiments appear to be fairly similar and they have been averaged to generate the data presented in Table 1 in which-~he height, num-
o.og ~
(a)
......................
(b) 3
]60
~
I 'D
t3
, 0.07.
'o
r 0.07
ol w
f_
14
5 g
]1 Q
~ 0.06_ "
0.05
L
I I I I
I I o.o3
__1 --T
-I logarithm
t
;
0
I
of dry w e i g h t
(log e g)
0.05
i
t
I
-1
0
logarithm
of dry w e i g h t
t !
(lo%
g~
Fig. !. (a) Relationship between RGR and the logarithm of dry weight for ~he low (80 pE m -2 s -I ) and high ( 160/~E m -2 s -t ) irradiance levels in two separate experiments. The solid line represents the data of Experiment 1, the dashed line those of Experiment 2. The data are averages over 15 cultivars. (b) Relationship between RGR and logarithm of dry weight for five cultivars of C. morifolium averaged over irradiance levels and experiments (0 indicates the value of RGR at Day 20). 2, 'Byron'; 3, 'Byoux'; 13, 'Daymark'; 14, 'Regoltime'; 15, 'Album'.
272
J. DE JON(3 A N D J. JANSEN
ber of leaves and the growth parameters for a plant dry weight of I g are presented. Doubling the irre~diance level increased RGR by 59%. This increase is realized with a 13% reduction in LAR and an 84% increase in NAR. Partitioning LAR into SLA and LWR showed that at the higher irradiance the SLA is 17% lower, which indicates that the leaves are thicker or more dense than at the lower irradiance. Cultivate. - In Fig. 1 (b), the relationship between RGR and dry weight is presented for five cultivars. RGR decreases with increasing weight of the plants, thus DRG R values will be negative. The dots indicate the weight of these cultivars 20 days after the start of the experiment. It can be observed that RGR calculated at a fixed time may differ considerably from RGR calculated at a fixed weight (compare cultivar 13 with 15 ). Table 2 contains RGR and the decrease of RGR and other growth components, all determined at a dry weight of I g. It is clear that significant differences exist between cultivars in RGR, the decrease in RGR and the partitioning growth components. The cultivar 'Daymark' is the fastest growing clone ~ith a RGR that exceeds that of cultivar 'Album' by 42%. The variations in the growth components are equally large: for LAR, the score of the cultivar with the highest value exceeded the score of that with the lowest value by 50%. For SLA this was 36%, for LWR 30% and for NAR 36%.
TABLE 2 The RGR and partitioning growth components of 15 cultivars of Chrysanthemum determined at a shoot dry weight of l g. For the units, see Table I Cultivar 'Daymark' 'Pink Pompon' 'Byoux'
'Penny Lane' 'Rivalry' 'Caroussel' "FrankfoW 'Byron' 'White Spider' 'Cream Pearr 'ReCur' 'Statesman' "Delta' "Regoltime' "Mum" SED DF
RGR
")Rt~R
LAR
SLA
LWR
NAR
0.0642 0.0610 0.0599 0.0598 0.0574 0.0572 0.0548 0.054~ 0.0537 0.0536 0.0530 0.0513 0.0508 0.0504 0q452
0.0176 0.0192 0.0141 0.0129 0.0096 0.0046 0.012~ 0.0073 0.0089 0.0094 0.0105 0.0107 0.0139 0.0060 0.0115
0.0270 0.0310 0.0262 0.0306 0.0290 0.0260 0.0218 0.0236 0.0284 0.0239 0.0235 0.0258 0.0248 0.0263 0.0208
0.0410 0.0468 0.0406 0.0401 0.0442 0.0400 0.0371 0.0403 0.0401 0.0405 0.0393 0.0410 0.0387 0.0409 0.0344
0.661 0.669 0.648 0.765 0.657 0.654 0.589 0.590 0.708 0.591 0.600 0.633 0.642 0.645 0.605
2.411 2.008 2.346 2.002 2.039 2.429 2.009 2.376 1.920 2.289 2.295 2.015 2.105 1.938 2.203
0.0055 14
0.0058 14
0.0022 14
0.00033 14
0.032 14
0.281 14
RELATIVE GROWTH RATE OF ('llRYSANI'II~'L~I{'M
273
TABLE 3 Correlation coefficients beiween RGR and DRGR on the one hand and the partitioning growth on the other hand
LAR SLA LWR NAR
RGR
DRC/R
0.65** 0.6 i * 0.46 0.20
0.28 0.25 0.20 0.02
*and ** indicate statistical significance at the $% and 1% level, respectively.
Relationship between RGR, DRG R and growth components. - To answer the question which growth component is best related to the observed variation in RGR or the rate of decrease of RGR, the linear correlation coefficients were calculated (Table 3)~ The observed variation in LAR was significantly correlated with the observed variation in RGR. This was not so for NAR. From the partitioning components of LAR, the SLA showed the highest correlation with RGR. No significant correlation was found between RGR and the rate of decrease of the relative growth rate, DRG R ( r = 0 . 4 8 ) . DISCUSSION
Differences in RGR between species have frequently been reported (Poorter, 1990), but variation in RGR between genotypes within a species, where reported, is small. Smeets and Garretsen (1986) found differences in the RGR of 16 cultivars of tomato that ranged from 93 to 103% as compared to a standard. Wilson (1982) successfully selected for a low rate of respiration in Lolium perenne, but the RGR of slow- and fast-respiring selections did not differ significantly at the 5% level. Where differences were found, they coincided with subspecies of Piantago major (Dijkstra and Lambers, 1989) or distinct plant architectures in genotypes of Phlox paniculata (Garbutt, 1986) where prostrate and erect types were compared. Our observations on Chrysanthemum cultivars indicate the existence of large variation in RGR within the species C. morifolium. The cultivars used belong to the same species, are of the same erect type and they were all developed for the production of cut flowers in greenhouses. Large differences in RGR within a species have also been reported for another cut ornamental crop, the carnation (L.D. Sparnaa~, personal communication, 1990). The occurrence of variation in RGR within the species may depend on the selection pressures asserted. In crop plants, rapid growth and high yield are absolute requirements for the introduction of a cultivar and only those cultivars with maximum crop growth rate are retained. In cut flow-
274
J. DE J O N G AND J. JANSEN
ers, slow growth may be acceptable in novelties fetching high prices per flower. Thus, genetic variation in growth rate has been retained in ornamental plants which makes these suitable for the analysis of genetically controlled factors that regulate growth. Poorter (1990), who summarized 60 publications on RGR, found that differences between species in RGR are explained by LAR for 80-90% and by NAR for 10-20%. The differences we observed between genotypes within a species are similarly dominated by accompanying differences in LAR, while NAR has little to do with the variation in RGR. Thus light-intercepting capacity, rather than the efficiency of its use, seems to vary within Chrysanthemum. When LAR is divided into LWR and SLA, both LWR (which describes the allocation to the leaves) and the SLA (which describes the leaf area formed per gram dry weight) play a role, although the latter seems to be the more determining factor. In conclusion, large differences between cultivars in RGR have been observed. Rapid growth was associated with the rapid development of a large leafarea. The biomass production per unit leaf, although highly variable, does not relate significantly to RGR. When selecting for a high RGR, fast deployment of leaf area should be the main criterion.
REFERENCES Dijkstra, P. and Lambers, H., 1989. A physiological analysis of genetic variation in relative growth rate within Plantago major L. Funct. Ecol., 3: 577-587. Garbutt, K., 1986. Genetic differentiation in leaf and whole plant photosynthesic capacity and unit leaf rate among clones of Phh~xpaniculata. Am. J. Bot., 73:1364- i 371. Hunt, R., 1982. Plant Growth Curves. The Functional Approach to Plant Growth Analysis. Edward Arnold, London, 248 pp. Hunt, R. and Parsons, I.T., 1974. A computer program for deriving growth-functions in plant growth-analyses. J. Appl. Ecol., I I: 297-307. Keen, A., Thissen, J.T.N.M., Hoekstra, J.A. and Jansen, J., 1986. Successive measurement experiments. Stat. Neerl., 40: 205-223. Lambers, H., 1987. Does variation in photosynthetic rate explain variation in growth rate and yield? Neth. J. Agric. Sci., 35:505-5 ~9. Poorter, H., 1990. lnterspecific variation in relative growth rate: on ecological causes and physiological consequences. In: H. Lambers, M.L. Cambridge, H. Konings and T.L. Pons (Editors), Causes and Consequences of Variation in Growth Rate and Productivity of Higher Plants. SPB Academic Publishing, The Hague, pp. 45-68. Roweli, J.G. and Waiters, J.E., 1976. Analysing data with repeated observations on each experimental unit. J. Agric. Sci., 87: 423-432. Smeets, L., 197S. The phytotron of the Institute for Horticultural Plant Breeding (IVT), Wageningen, The Netherlands. A revision of previous descriptions. Neth. J. Agric. Sci., 26: 8-12. Smeets, L. and Garretsen, F., 1986. Growth analysis of tomato genotypes grown under low night temperatures and low light intensity. Euphytica, 35:701-715. Smit, A,L., 1983. Influence of external factors on growth and development of sugar-beet (Beta
RELATIVE GROWTH RATE OF CtlRYSANI'HEMUM
275
vulgaris L. ). Agric. Res. Rep. 914, Centre for Agricultural Publishing and Publication, Pudoc, Wageningen, 109 pp. Snedecor, G.W. and Cocl~ran, W.G., 1967. Statistical Methods.6th Edn. Iowa State University Press, Ames, IA, 593 pp. Wilson, D., 1982. Response to selection for dark respiration rate of mature leaves in Lolium perenne and its effects on growth of ,,,~, . . . . v .~""'~ J ~-..~, . . . . . . ". ." "u simulated swards. Ann. Bot., 49: 303312.