Photosynthesis Research 23: 67-72, 1990. © 1990 Kluwer Academic Publishers. Printed in the Netherlands.
Regular paper
The effect of leaf size on mutual shading and cultivar differences in soybean leaf photosynthetic capacity James A. Bunce Plant Photobiology Laboratory, USDA-Agricultural Research Service, Beltsville Agricultural Research Center, Beltsville, MD 20705, USA Received 15 November 1988; accepted in revised form 7 February 1989
Key words: leaf size, light, photosynthesis, shading, soybeans, specific leaf weight. Abstract
This study investigated the basis of the negative relationship between leaf size and photosynthetic rate per unit of area among five cultivars of soybeans. Exposure of developing mainstem leaves to light, and sizes and light saturated photosynthesis rates of those leaves at maturity were compared in cultivars grown in field plots for two years at Beltsville, Maryland, USA. Plants were grown both in stands at 2.5 cm by 1 m spacing and as isolated plants. While cultivar differences in leaf size were large and consistent in both planting arrangements, significant cultivar differences in light saturated photosynthetic rates were found only in plants grown in stands. Similarly, leaf size was significantly correlated with specific leaf weight only for plants grown in stands. The mainstem apex and developing mainstem leaves experienced more severe shading in large-leaved cultivars than in small-leaved cultivars when plants were grown in stands. Thus, cultivar differences in photosynthetic capacity were probably a consequence of differences in the exposure o f developing leaves to light.
Introduction
Among higher plants there is no obvious relationship between leaf size and maximum rate of photosynthesis per unit of area. However, negative relationships between leaf size and maximum photosynthetic rate have contributed to the frequent failure to find positive relationships between growth rates and leaf photosynthetic rates in genetic comparisons within species of annual crops (Bunce 1986). These relationships potentially limit the improvement of seasonal canopy photosynthesis by selecting for larger leaf size or higher rates of photosynthesis. Thus it is important to understand the basis of these negative correlations. Although negative relationships between leaf size and photosynthetic rate occur in many species (Bhagsari and Brown 1986, Bunce 1986, Evans and Dunstone 1970, Hesketh et al. 1981), the data in such cases do not always strongly support
the hypothesis (Hesketh et al. 1981) that large leaves have low rates of photosynthesis because of "dilution" of their photosynthetic apparatus over a larger surface area. For example, although ribulose bisphosphate carboxylase content and photosynthetic rate were positively correlated (r = 0.79) among soybean cultivars (Hesketh et al. 1981), ribulose bisphosphate carboxylase content was only weakly related to leaf area (r = -0.46). Another possible explanation of the negative relationship between leaf size and photosynthetic capacity is that genotypes with large leaves may have more severe mutual shading, which could reduce their photosynthetic capacity (Bunce 1988). I have studied five soybean cultivars grown under field conditions in stands and as isolated plants to determine to what extent differences in maximum photosynthetic rates may be related to differences in leaf size, and to differences in exposure of developing leaves to light.
68 Materials and methods Five cultivars of Glycine m a x (L) Merr., Amsoy, Clark, Hodgson, Lincoln, and Mandarin were grown in 1987 and 1988 in the field at Beltsville, Maryland, USA. These cultivars were chosen because they have a range of leaf sizes and photosynthetic rates. To insure that the desired plant spacing was achieved, seedlings were transplatned into the field plot. Seeds were planted in mid-May in peat pots filled with soil, and germinated in a glasshouse at 25°C. Two weeks after planting, seedlings were placed in pure stands every 2.5cm in rows 1 m apart, and also as isolated plants 75 cm apart in a field plot with a Codorus silt loam soil. The 2.5 cm by 1 m spacing was chosen so that all above ground leaf overlap was between plants in the same row during these experiments. Six weeks after planting, unfolding mainstem leaves were tagged on twenty individual plants per cultivar in the stands, and ten plants per cultivar for isolated plants. The leaves tagged were the sixth or seventh mainstem trifoliolate leaves in the isolated plants, and fifth or sixth leaves for the plants in stands. All plants were flowering at this time. Exposure of tagged leaves to sunlight was determined periodically until area expansion of the tagged leaves was complete. In 1987, the number of individuals in which tagged leaves were shaded by other leaves and the number of individuals with unshaded leaves was determined by direct observation near solar noon. In 1988, horizontal photosynthetic photon flux density (PPFD) above the crop and at the position of the tagged leaves was measured at various times of day. All tagged leaves were within 20 ° of horizontal. Light saturated photosynthetic rates, leaf areas, and specific leaf weights were measured eight days after tagging leaves, at which time they were found to be fully expanded. There were ten replicates per cultivar per planting arrangement. Photosynthetic rates and conductances to water vapor were determined using an open gas exchange system with a differential infrared carbon dioxide analyser and a dew point hygrometer (Bunce 1984a). A clamp-on cuvette enclosed 2.5 cm 2 of the terminal leaflet, with the enclosed area illuminated at a P P F D o f 1 . 8 m m o l m - 2 s -~ with a fibre-optic light guide from a quartz halogen lamp. Tests showed that photosynthesis was light saturated at this PPFD.
Temperatures of the enclosed leaf were measured with a fine thermocouple pressed against the underside of the leaflets, and were within 2°C of ambient air temperature. Leaf temperatures were 27 + I°C in 1987, and 25 _ I°C in 1988. Water vapor pressure deficits in air in the cuvette were 1.1 + 0 . 2 k P a i n 1987, and 1.6 _ 0 . 1 k P a i n 1988. Air from 2 m above the crop was pumped at constant flow rate through the cuvette. The carbon dioxide pressure of this air was 34 to 35 Pa. Cuvette boundary layer conductance to water vapor was 0 . 5 m o l t o 2s ~, as estimated from wet filter paper placed in the cuvette. Substomatal pressures of carbon dioxide were calculated from the carbon dioxide pressure of air around the leaf in the cuvette, which was 33.0 ___ 0.5Pa, the photosynthetic rate, and leaf and boundary layer conductances to water vapor (Jones 1983). Sampling of photosynthetic rates was begun about 3 h before solar noon on sunny days, and completed before solar noon. Sampling was done systematically so that average time of day of measurement was the same for all cultivars within a planting arrangement. The timing was chosen so that leaves were at high light at least I h before measurements, while the higher air saturation deficits for water vapor of up to 2.8 kPa which occurred later in the day were avoided. After gas exchange measurement, terminal leaflets were removed, enclosed in a plastic bag with wet filter paper, and kept on ice until measurement of area with a photoelectric area meter. Dry mass was then determined after oven drying at 70°C for 24 h. Petiole and internode lengths were also measured in 1988 on ten plants per cultivar for plants in the stands. One-way analysis of variance was used to analyse cultivar effects separately for each year and for each planting arrangement. An LSD was calculated in cases where the F-test was significant at the 5% probability level. Correlations between variables were tested using mean values for the cultivars.
Results The two years gave very similar patterns of results, although absolute values of leaflet areas and photosynthetic rates were lower in 1988, probably because the 1988 season was abnormally warm and dry. In both years cultivars differed significantly in
69 Table 1. Light saturated photosynthetic rates (P), leaflet areas (A), and specific leaf weight (SLW) of five soybean cultivars grown in stands at 2.5cm by I m spacing, and as isolated plants for two years at Beltsville, Maryland, USA. LDS's are given when the F-test was significant among cultivars, and ns indicates no significant difference at the 5% probability level.
Cultivar
1987
1988
P
A
SLW
P
A
(pmol m - 2 s- i )
(cm2)
(g m - 2)
(~mol m - 2 s- t )
(cm 2)
SLW (g m - 2)
19.0 13.3 19.1 13.7 18.1 2.9
68.7 84.8 69.4 83.1 72.7 8.2
43.2 35.6 49.0 36.5 47.2 6.6
13.7 11.3 14.0 11.2 12.4 2.1
24.1 34.7 26.4 34.6 31.2 4.6
46.8 34.5 45.0 34.6 42.1 5.5
23.7 21.8 24.2 22.4 24.8 ns
73.7 96.7 68.5 95.0 83.8 11.7
59.5 59.4 62.6 58.3 61.4 ns
16.6 16.5 16.9 15.5 15.4 ns
23.8 30.0 23.7 35.5 32.0 4.2
49.7 48.3 53.7 47.0 54.6 5.2
Stands
Amsoy Clark Hodgson Lincoln Mandarin LSD Isolated
Amsoy Clark Hodgson Lincoln Mandarin LSD
photosynthetic rate for plants grown in stands, but no differences occurred for isolated plants in either year (Table 1). For plants in stands, Amsoy and Hodgson had high rates in both years, and Clark and Lincoln had low rates both years, with Mandarin somewhat intermediate. Substomatal carbon dioxide pressures were 21.5 +__ 1.5Pa in all cultivars in both years and both planting arrangements. Specific leaf weight also differed among
cultivars when grown in stands in both years, but not when grown as isolated plants in 1987 (Table 1). Area per leaflet differed among cultivars in both planting arrangements in both years. Clark and Lincoln generally had large leaves, and Amsoy and Hodgson had small leaves, regardless of planting arrangement (Table 1). The range of leaf sizes was at least as great for isolated plants as for plants in stands.
Table 2. Exposure to sunlight o f recently unfolded mainstem leaves on the day of unfolding and the following two days for five soybean cultivars grown at 2.5 cm by 1 m spacing. In 1987 the percent of plants in which leaves were not shaded at solar noon was determined. In 1988 the horizontal photosynthetic photon flux density at the position of the leaf was determined at various times of day. In both years measurements were made on twenty plants per cultivar.
1987: Percentage of plants with sunlit leaves Cultivar Day 1
Day 2
Day 3
Mean
Amsoy Clark Hodgson Lincoln Mandarin
50 0 33 33 67
77 43 83 40 77
55 14 48 24 48
1988: Percentage of maximum P P F D Cultivar Day 1, I pm
Day 2, 9 am
Day 2, 1 pm
Day 3, 4 pm
Mean
Amsoy Clark Hodgson Lincoln Mandarin
91 45 78 42 71
86 67 85 72 80
82 49 80 50 73
89 55 82 53 71
38 0 27 0 0
97 57 86 46 59
70 F o r p l a n t s g r o w n in isolation, d e v e l o p i n g m a i n s t e m leaves were fully e x p o s e d to sunlight f r o m u n f o l d i n g to c o m p l e t i o n o f e x p a n s i o n in all cultivars (not shown). F o r p l a n t s in stands, differences a m o n g cultivars existed in the a m o u n t o f s h a d i n g o f d e v e l o p i n g m a i n s t e m leaves for at least three d a y s after u n f o l d i n g (Table 2). S h a d i n g was least in A m s o y a n d H o d g s o n , a n d greatest in C l a r k a n d L i n c o l n in b o t h years. Leaves o f all cultivars b e c a m e fully e x p o s e d to light a b o u t five d a y s after u n f o l d i n g (not shown). M e a n i n t e r n o d e lengths a n d lengths o f m a t u r e petioles did n o t differ between cultivars when g r o w n in stands (Table 3). H o w e v e r , petiole lengths o f u n f o l d i n g leaves a n d o f leaves one n o d e lower were l o n g e r in C l a r k a n d Lincoln t h a n in A m s o y a n d H o d g s o n , with M a n d a r i n i n t e r m e d i a t e . T h e n u m b e r o f leaves c a p a b l e o f s h a d i n g the apex was also greatest in C l a r k a n d Lincoln a n d least in A m s o y a n d H o d g s o n ( T a b l e 3). F o r p l a n t s g r o w n in stands, c o r r e l a t i o n s between p h o t o s y n t h e t i c rate a n d the variables area, specific l e a f weight, a n d m e a n light e x p o s u r e were all significant a m o n g cultivars in b o t h years (Table 4).
L e a f size a n d specific leaf weight were negatively related for p l a n t s g r o w n in stands, b u t n o t for p l a n t s g r o w n in i s o l a t i o n ( T a b l e 4).
Discussion The r a n k i n g o f the cultivars in m a x i m u m p h o t o s y n t h e t i c rates a n d leaf sizes when g r o w n in stands was in a g r e e m e n t with d a t a o f o t h e r workers. H e s k e t h et al. (1981) f o u n d H o d g s o n a n d A m s o y to have higher rates t h a n C l a r k a n d Lincoln. Sinclair (1980) r e p o r t e d that A m s o y h a d higher rates o f p h o t o s y n t h e s i s t h a n Lincoln. D o r n h o f f a n d Shibles (1971) f o u n d A m s o y to have higher rates t h a n Lincoln, with M a n d a r i n intermediate. H e s k e t h et al. (1981) r e p o r t e d H o d g s o n to have smaller leaves t h a n C l a r k a n d Lincoln. L e a f size a n d p h o t o s y n t h e t i c rate were inversely related for p l a n t s in s t a n d s in b o t h years, as also f o u n d by H e s k e t h et al. (1981). H o w e v e r , the lack o f significant differences in p h o t o s y n t h e t i c rates for p l a n t s g r o w n in isolation, in spite o f v a r i a t i o n in leaf size at least as large as for p l a n t s in stands, calls
Table 3. Lengths of mature petioles and internodes, lengths of petioles of the unfolding and the next older mainstem leaf, and the number of mainstem leaves capable of shading the apex for five soybean cultivars grown at 2.5 cm by I m spacing in 1988. Mature petiole lengths are means for mainstem trifoliolates 3 through 5. Internode lengths are means for mainstem nodes 3 through 6 above the cotyledons. Values are means for ten plants per cultivar. LSD's are given when the F-test was significant among cultivars, and ns indicates no significant difference at the 5% level of probability.
Cultivar Amsoy Clark Hodgson Lincoln Mandarin LSD
Lengths (cm) of petioles of: Mature Unfolding
Next older
Internode length (cm)
Number of leaves shading the apex
9.9 10.0 I 1.1 10.8 10.5 ns
4.8 8. I 5.2 9.3 6.8 1.2
3.0 2.9 3.4 3.0 3.2 ns
1.7 3.7 1.8 3.8 2.7 0.8
1.9 3.7 2.1 3.7 2.9 0.6
Correlations among variables of mean values for five cultivars of soybeans. Plants were grown either in stands at 2.5 cm by 1m spacing, or as isolated plants, ns indicates no significant correlation at the 5% level of probability. Table 4.
Variables Photosynthesis-area Photosynthesis-specific leaf weight Photosynthesis-percentage light Area-specific leaf weight Area-specific leaf weight
plant arrangement Stand Stand Stand Stand Isolated
Correlation coefficient 1987
1988
- 0.997 + 0.924 + 0.976 - 0.895 - 0.707 ns
- 0.966 + 0.962 + 0.968 -- 0.964 - 0.368 ns
71 into question any direct relationship between leaf size and photosynthetic rate among these cultivars. The lack of significant correlation between leaf size and specific leaf weight for isolated plants in which developing leaves were not shaded is also evidence against a "dilution" effect. A possible explanation for the abscence of differences in photosynthetic rates and specific leaf weight in spite of differences in leaf size for isolated plants is that developing leaves even of the large-leaved cultivars have more than sufficient supplies of carbon and nitrogen to support leaf development when grown in isolation. However, this is not consistent with the observation that even for isolated plants grown outdoors, maximum photosynthetic rates are closely related to variation in light during development (Bunce 1985). Differences among cultivars in the exposure of developing leaves to light, which occurred for plants in stands but not for plants grown in isolation, is the probable explanation of the differences in maximum photosynthetic rates. Several studies have shown that shading of developing leaves can result in lower maximum photosynthetic rates at maturity (Bowes et al. 1972, Bunce 1988, Lugg and Sinclair 1980). It was not coincidental that in stands, cultivars with large leaves had more shading of their own developing leaves. Large leaves did not have longer petioles at maturity, but had longer laminas, and even with equal petiole and internode lengths, were more capable of shading developing leaves at higher nodes. Among these cultivars, large leaf size at maturity was also associated with long petioles on immature leaves, i.e. earlier petiole extension. For these two reasons, unfolding leaves and the mainstem apex were more shaded in cultivars with large leaves. It is thought that shading early in leaf development is more effective in reducing photosynthetic capacity than later shading (Jurik et al. 1979, Bunce et al. 1977), but we can not rule out possible differences in timing of emergence from shade as important in photosynthetic differences among these cultivars (Bunce 1988). The very similar cultivar rankings in leaf size for plants in stands and in isolation indicates that the leaf size differences were not the result of differences in shading during development. These results indicate that cultivar differences in leaf size resulted in differences in shading of developing leaves. These differences in shading were
probably responsible for the differences in photosynthesis capacity among these soybean cultivars when grown in stands. It is not known whether negative relationships between leaf size and photosynthetic capacity in other species have the same basis. These results do not mean, of course, that cultivars can not also differ in photosynthetic rate for other reasons, especially when measured at other stages of plant development (Enos et al. 1982, Gordon et al. 1982) or under other environmental conditions (Bunce 1984b, Caulfield and Bunce 1988). However, it may be that considering leaf display in soybean improvement may allow the negative relationship between leaf size and photosynthetic rate to be broken.
References Bhagsari AS and Brown RH (1986) Leaf photosynthesis and its correlation with leaf area. Crop Sci 26:127-132 Bowes G, Ogren WL and Hageman RH (1972) Light saturation, photosynthesis rate, RuDP carboxylase activity and specific leaf weight in soybeans grown under different light intensities. Crop Sci 12:77-79 Bunce JA (1984a) Effects of humidity on photosynthesis. J. Exp Bot 35:1245-1251 Bunce JA (1984b) Identifying soybean lines differing in gas exchange sensitivity to humidity. Ann Appl Biol 105:313-318 Bunce JA (1985) Effects of weather during leaf development on photosynthetic characteristics of soybean leaves. Photosyn Res 6:215-220 Bunce JA (1986) Measurements and modeling of photosynthesis under field conditions. Crit Rev in Plant Sci 4:47-77 Bunce JA (1988) Mutual shading and the photosynthetic capacity of exposed leaves of field grown soybeans. Photosyn Res 15:75-83 Bunce JA, Patterson DR, Peet MM and Alberte RS (1977) Light acclimation during and after leaf expansion in soybeans. Plants Physiol 60:255-258 Caulfield F and Bunce JA (1988) Comparative responses of photosynthesis to growth temperature in soybean (Glycine m a x L. Merril) cultivars. Can J Plant Sci 68:419~125 Dornhoff GM and Shibles RM (1970) Varietal differences in net photosynthesis of soybean leaves. Crop Sci 10:42--45 Enos WT, Alfich RA, Hesketh JD and Wooley JT (1982) Interactions among leaf photosynthetic rates, flowering and pod set in soybeans. Photosyn Res 3:273-278 Evans LT and Dunstone RL (1970) Some physiological aspects of evolution in wheat. Aust J Biol Sci 23:728-741 Gordon AJ, Hesketh JD and Peters DB (1982) Soybean leaf photosynthesis in relation to maturity classification and stage of growth. Photosyn Res 3:81-93 Hesketh JD, Ogren WL, Hageman ME and Peters DB (1981) Correlations among leaf CO2-exchange rates, areas and
72 enzyme activities among soybean cultivars. Photosyn Res 2: 21-30 Jones HG (1983) Plants and Microclimate, p. 144. Cambridge: Cambridge Univ Press. Jurik TW, Chabot JF and Chabot BF (1979) Ontogeny of photosynthetic performance in Fragaria virgininiana under
changing light regimes. Plant Physiol 63:542-547 Lugg DG and Sinclair TR (1980) Seasonal changes in morphology and anatomy of field-grown soybean leaves. Crop Sci 20: 191-196 Sinclair TR (I 980) Leaf CER from post-flowering to senescence of field-grown soybean cultivars. Crop Sci 20:196-200
Regular paper
The effect of leaf size on mutual shading and cultivar differences in soybean leaf photosynthetic capacity James A. Bunce Plant Photobiology Laboratory, USDA-Agricultural Research Service, Beltsville Agricultural Research Center, Beltsville, MD 20705, USA Received 15 November 1988; accepted in revised form 7 February 1989
Key words: leaf size, light, photosynthesis, shading, soybeans, specific leaf weight. Abstract
This study investigated the basis of the negative relationship between leaf size and photosynthetic rate per unit of area among five cultivars of soybeans. Exposure of developing mainstem leaves to light, and sizes and light saturated photosynthesis rates of those leaves at maturity were compared in cultivars grown in field plots for two years at Beltsville, Maryland, USA. Plants were grown both in stands at 2.5 cm by 1 m spacing and as isolated plants. While cultivar differences in leaf size were large and consistent in both planting arrangements, significant cultivar differences in light saturated photosynthetic rates were found only in plants grown in stands. Similarly, leaf size was significantly correlated with specific leaf weight only for plants grown in stands. The mainstem apex and developing mainstem leaves experienced more severe shading in large-leaved cultivars than in small-leaved cultivars when plants were grown in stands. Thus, cultivar differences in photosynthetic capacity were probably a consequence of differences in the exposure o f developing leaves to light.
Introduction
Among higher plants there is no obvious relationship between leaf size and maximum rate of photosynthesis per unit of area. However, negative relationships between leaf size and maximum photosynthetic rate have contributed to the frequent failure to find positive relationships between growth rates and leaf photosynthetic rates in genetic comparisons within species of annual crops (Bunce 1986). These relationships potentially limit the improvement of seasonal canopy photosynthesis by selecting for larger leaf size or higher rates of photosynthesis. Thus it is important to understand the basis of these negative correlations. Although negative relationships between leaf size and photosynthetic rate occur in many species (Bhagsari and Brown 1986, Bunce 1986, Evans and Dunstone 1970, Hesketh et al. 1981), the data in such cases do not always strongly support
the hypothesis (Hesketh et al. 1981) that large leaves have low rates of photosynthesis because of "dilution" of their photosynthetic apparatus over a larger surface area. For example, although ribulose bisphosphate carboxylase content and photosynthetic rate were positively correlated (r = 0.79) among soybean cultivars (Hesketh et al. 1981), ribulose bisphosphate carboxylase content was only weakly related to leaf area (r = -0.46). Another possible explanation of the negative relationship between leaf size and photosynthetic capacity is that genotypes with large leaves may have more severe mutual shading, which could reduce their photosynthetic capacity (Bunce 1988). I have studied five soybean cultivars grown under field conditions in stands and as isolated plants to determine to what extent differences in maximum photosynthetic rates may be related to differences in leaf size, and to differences in exposure of developing leaves to light.
68 Materials and methods Five cultivars of Glycine m a x (L) Merr., Amsoy, Clark, Hodgson, Lincoln, and Mandarin were grown in 1987 and 1988 in the field at Beltsville, Maryland, USA. These cultivars were chosen because they have a range of leaf sizes and photosynthetic rates. To insure that the desired plant spacing was achieved, seedlings were transplatned into the field plot. Seeds were planted in mid-May in peat pots filled with soil, and germinated in a glasshouse at 25°C. Two weeks after planting, seedlings were placed in pure stands every 2.5cm in rows 1 m apart, and also as isolated plants 75 cm apart in a field plot with a Codorus silt loam soil. The 2.5 cm by 1 m spacing was chosen so that all above ground leaf overlap was between plants in the same row during these experiments. Six weeks after planting, unfolding mainstem leaves were tagged on twenty individual plants per cultivar in the stands, and ten plants per cultivar for isolated plants. The leaves tagged were the sixth or seventh mainstem trifoliolate leaves in the isolated plants, and fifth or sixth leaves for the plants in stands. All plants were flowering at this time. Exposure of tagged leaves to sunlight was determined periodically until area expansion of the tagged leaves was complete. In 1987, the number of individuals in which tagged leaves were shaded by other leaves and the number of individuals with unshaded leaves was determined by direct observation near solar noon. In 1988, horizontal photosynthetic photon flux density (PPFD) above the crop and at the position of the tagged leaves was measured at various times of day. All tagged leaves were within 20 ° of horizontal. Light saturated photosynthetic rates, leaf areas, and specific leaf weights were measured eight days after tagging leaves, at which time they were found to be fully expanded. There were ten replicates per cultivar per planting arrangement. Photosynthetic rates and conductances to water vapor were determined using an open gas exchange system with a differential infrared carbon dioxide analyser and a dew point hygrometer (Bunce 1984a). A clamp-on cuvette enclosed 2.5 cm 2 of the terminal leaflet, with the enclosed area illuminated at a P P F D o f 1 . 8 m m o l m - 2 s -~ with a fibre-optic light guide from a quartz halogen lamp. Tests showed that photosynthesis was light saturated at this PPFD.
Temperatures of the enclosed leaf were measured with a fine thermocouple pressed against the underside of the leaflets, and were within 2°C of ambient air temperature. Leaf temperatures were 27 + I°C in 1987, and 25 _ I°C in 1988. Water vapor pressure deficits in air in the cuvette were 1.1 + 0 . 2 k P a i n 1987, and 1.6 _ 0 . 1 k P a i n 1988. Air from 2 m above the crop was pumped at constant flow rate through the cuvette. The carbon dioxide pressure of this air was 34 to 35 Pa. Cuvette boundary layer conductance to water vapor was 0 . 5 m o l t o 2s ~, as estimated from wet filter paper placed in the cuvette. Substomatal pressures of carbon dioxide were calculated from the carbon dioxide pressure of air around the leaf in the cuvette, which was 33.0 ___ 0.5Pa, the photosynthetic rate, and leaf and boundary layer conductances to water vapor (Jones 1983). Sampling of photosynthetic rates was begun about 3 h before solar noon on sunny days, and completed before solar noon. Sampling was done systematically so that average time of day of measurement was the same for all cultivars within a planting arrangement. The timing was chosen so that leaves were at high light at least I h before measurements, while the higher air saturation deficits for water vapor of up to 2.8 kPa which occurred later in the day were avoided. After gas exchange measurement, terminal leaflets were removed, enclosed in a plastic bag with wet filter paper, and kept on ice until measurement of area with a photoelectric area meter. Dry mass was then determined after oven drying at 70°C for 24 h. Petiole and internode lengths were also measured in 1988 on ten plants per cultivar for plants in the stands. One-way analysis of variance was used to analyse cultivar effects separately for each year and for each planting arrangement. An LSD was calculated in cases where the F-test was significant at the 5% probability level. Correlations between variables were tested using mean values for the cultivars.
Results The two years gave very similar patterns of results, although absolute values of leaflet areas and photosynthetic rates were lower in 1988, probably because the 1988 season was abnormally warm and dry. In both years cultivars differed significantly in
69 Table 1. Light saturated photosynthetic rates (P), leaflet areas (A), and specific leaf weight (SLW) of five soybean cultivars grown in stands at 2.5cm by I m spacing, and as isolated plants for two years at Beltsville, Maryland, USA. LDS's are given when the F-test was significant among cultivars, and ns indicates no significant difference at the 5% probability level.
Cultivar
1987
1988
P
A
SLW
P
A
(pmol m - 2 s- i )
(cm2)
(g m - 2)
(~mol m - 2 s- t )
(cm 2)
SLW (g m - 2)
19.0 13.3 19.1 13.7 18.1 2.9
68.7 84.8 69.4 83.1 72.7 8.2
43.2 35.6 49.0 36.5 47.2 6.6
13.7 11.3 14.0 11.2 12.4 2.1
24.1 34.7 26.4 34.6 31.2 4.6
46.8 34.5 45.0 34.6 42.1 5.5
23.7 21.8 24.2 22.4 24.8 ns
73.7 96.7 68.5 95.0 83.8 11.7
59.5 59.4 62.6 58.3 61.4 ns
16.6 16.5 16.9 15.5 15.4 ns
23.8 30.0 23.7 35.5 32.0 4.2
49.7 48.3 53.7 47.0 54.6 5.2
Stands
Amsoy Clark Hodgson Lincoln Mandarin LSD Isolated
Amsoy Clark Hodgson Lincoln Mandarin LSD
photosynthetic rate for plants grown in stands, but no differences occurred for isolated plants in either year (Table 1). For plants in stands, Amsoy and Hodgson had high rates in both years, and Clark and Lincoln had low rates both years, with Mandarin somewhat intermediate. Substomatal carbon dioxide pressures were 21.5 +__ 1.5Pa in all cultivars in both years and both planting arrangements. Specific leaf weight also differed among
cultivars when grown in stands in both years, but not when grown as isolated plants in 1987 (Table 1). Area per leaflet differed among cultivars in both planting arrangements in both years. Clark and Lincoln generally had large leaves, and Amsoy and Hodgson had small leaves, regardless of planting arrangement (Table 1). The range of leaf sizes was at least as great for isolated plants as for plants in stands.
Table 2. Exposure to sunlight o f recently unfolded mainstem leaves on the day of unfolding and the following two days for five soybean cultivars grown at 2.5 cm by 1 m spacing. In 1987 the percent of plants in which leaves were not shaded at solar noon was determined. In 1988 the horizontal photosynthetic photon flux density at the position of the leaf was determined at various times of day. In both years measurements were made on twenty plants per cultivar.
1987: Percentage of plants with sunlit leaves Cultivar Day 1
Day 2
Day 3
Mean
Amsoy Clark Hodgson Lincoln Mandarin
50 0 33 33 67
77 43 83 40 77
55 14 48 24 48
1988: Percentage of maximum P P F D Cultivar Day 1, I pm
Day 2, 9 am
Day 2, 1 pm
Day 3, 4 pm
Mean
Amsoy Clark Hodgson Lincoln Mandarin
91 45 78 42 71
86 67 85 72 80
82 49 80 50 73
89 55 82 53 71
38 0 27 0 0
97 57 86 46 59
70 F o r p l a n t s g r o w n in isolation, d e v e l o p i n g m a i n s t e m leaves were fully e x p o s e d to sunlight f r o m u n f o l d i n g to c o m p l e t i o n o f e x p a n s i o n in all cultivars (not shown). F o r p l a n t s in stands, differences a m o n g cultivars existed in the a m o u n t o f s h a d i n g o f d e v e l o p i n g m a i n s t e m leaves for at least three d a y s after u n f o l d i n g (Table 2). S h a d i n g was least in A m s o y a n d H o d g s o n , a n d greatest in C l a r k a n d L i n c o l n in b o t h years. Leaves o f all cultivars b e c a m e fully e x p o s e d to light a b o u t five d a y s after u n f o l d i n g (not shown). M e a n i n t e r n o d e lengths a n d lengths o f m a t u r e petioles did n o t differ between cultivars when g r o w n in stands (Table 3). H o w e v e r , petiole lengths o f u n f o l d i n g leaves a n d o f leaves one n o d e lower were l o n g e r in C l a r k a n d Lincoln t h a n in A m s o y a n d H o d g s o n , with M a n d a r i n i n t e r m e d i a t e . T h e n u m b e r o f leaves c a p a b l e o f s h a d i n g the apex was also greatest in C l a r k a n d Lincoln a n d least in A m s o y a n d H o d g s o n ( T a b l e 3). F o r p l a n t s g r o w n in stands, c o r r e l a t i o n s between p h o t o s y n t h e t i c rate a n d the variables area, specific l e a f weight, a n d m e a n light e x p o s u r e were all significant a m o n g cultivars in b o t h years (Table 4).
L e a f size a n d specific leaf weight were negatively related for p l a n t s g r o w n in stands, b u t n o t for p l a n t s g r o w n in i s o l a t i o n ( T a b l e 4).
Discussion The r a n k i n g o f the cultivars in m a x i m u m p h o t o s y n t h e t i c rates a n d leaf sizes when g r o w n in stands was in a g r e e m e n t with d a t a o f o t h e r workers. H e s k e t h et al. (1981) f o u n d H o d g s o n a n d A m s o y to have higher rates t h a n C l a r k a n d Lincoln. Sinclair (1980) r e p o r t e d that A m s o y h a d higher rates o f p h o t o s y n t h e s i s t h a n Lincoln. D o r n h o f f a n d Shibles (1971) f o u n d A m s o y to have higher rates t h a n Lincoln, with M a n d a r i n intermediate. H e s k e t h et al. (1981) r e p o r t e d H o d g s o n to have smaller leaves t h a n C l a r k a n d Lincoln. L e a f size a n d p h o t o s y n t h e t i c rate were inversely related for p l a n t s in s t a n d s in b o t h years, as also f o u n d by H e s k e t h et al. (1981). H o w e v e r , the lack o f significant differences in p h o t o s y n t h e t i c rates for p l a n t s g r o w n in isolation, in spite o f v a r i a t i o n in leaf size at least as large as for p l a n t s in stands, calls
Table 3. Lengths of mature petioles and internodes, lengths of petioles of the unfolding and the next older mainstem leaf, and the number of mainstem leaves capable of shading the apex for five soybean cultivars grown at 2.5 cm by I m spacing in 1988. Mature petiole lengths are means for mainstem trifoliolates 3 through 5. Internode lengths are means for mainstem nodes 3 through 6 above the cotyledons. Values are means for ten plants per cultivar. LSD's are given when the F-test was significant among cultivars, and ns indicates no significant difference at the 5% level of probability.
Cultivar Amsoy Clark Hodgson Lincoln Mandarin LSD
Lengths (cm) of petioles of: Mature Unfolding
Next older
Internode length (cm)
Number of leaves shading the apex
9.9 10.0 I 1.1 10.8 10.5 ns
4.8 8. I 5.2 9.3 6.8 1.2
3.0 2.9 3.4 3.0 3.2 ns
1.7 3.7 1.8 3.8 2.7 0.8
1.9 3.7 2.1 3.7 2.9 0.6
Correlations among variables of mean values for five cultivars of soybeans. Plants were grown either in stands at 2.5 cm by 1m spacing, or as isolated plants, ns indicates no significant correlation at the 5% level of probability. Table 4.
Variables Photosynthesis-area Photosynthesis-specific leaf weight Photosynthesis-percentage light Area-specific leaf weight Area-specific leaf weight
plant arrangement Stand Stand Stand Stand Isolated
Correlation coefficient 1987
1988
- 0.997 + 0.924 + 0.976 - 0.895 - 0.707 ns
- 0.966 + 0.962 + 0.968 -- 0.964 - 0.368 ns
71 into question any direct relationship between leaf size and photosynthetic rate among these cultivars. The lack of significant correlation between leaf size and specific leaf weight for isolated plants in which developing leaves were not shaded is also evidence against a "dilution" effect. A possible explanation for the abscence of differences in photosynthetic rates and specific leaf weight in spite of differences in leaf size for isolated plants is that developing leaves even of the large-leaved cultivars have more than sufficient supplies of carbon and nitrogen to support leaf development when grown in isolation. However, this is not consistent with the observation that even for isolated plants grown outdoors, maximum photosynthetic rates are closely related to variation in light during development (Bunce 1985). Differences among cultivars in the exposure of developing leaves to light, which occurred for plants in stands but not for plants grown in isolation, is the probable explanation of the differences in maximum photosynthetic rates. Several studies have shown that shading of developing leaves can result in lower maximum photosynthetic rates at maturity (Bowes et al. 1972, Bunce 1988, Lugg and Sinclair 1980). It was not coincidental that in stands, cultivars with large leaves had more shading of their own developing leaves. Large leaves did not have longer petioles at maturity, but had longer laminas, and even with equal petiole and internode lengths, were more capable of shading developing leaves at higher nodes. Among these cultivars, large leaf size at maturity was also associated with long petioles on immature leaves, i.e. earlier petiole extension. For these two reasons, unfolding leaves and the mainstem apex were more shaded in cultivars with large leaves. It is thought that shading early in leaf development is more effective in reducing photosynthetic capacity than later shading (Jurik et al. 1979, Bunce et al. 1977), but we can not rule out possible differences in timing of emergence from shade as important in photosynthetic differences among these cultivars (Bunce 1988). The very similar cultivar rankings in leaf size for plants in stands and in isolation indicates that the leaf size differences were not the result of differences in shading during development. These results indicate that cultivar differences in leaf size resulted in differences in shading of developing leaves. These differences in shading were
probably responsible for the differences in photosynthesis capacity among these soybean cultivars when grown in stands. It is not known whether negative relationships between leaf size and photosynthetic capacity in other species have the same basis. These results do not mean, of course, that cultivars can not also differ in photosynthetic rate for other reasons, especially when measured at other stages of plant development (Enos et al. 1982, Gordon et al. 1982) or under other environmental conditions (Bunce 1984b, Caulfield and Bunce 1988). However, it may be that considering leaf display in soybean improvement may allow the negative relationship between leaf size and photosynthetic rate to be broken.
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