Effect of terminal locations of pods on biomass production and ¹³C partitioning in a fasciated stem soybean Shakujo

Effect of terminal locations of pods on biomass production and 13 C partitioning in a fasciated stem soybean Shakujo 1 first-author 1 2 3 corresp ‡Corresponding Author ‡Corresponding Author †E-mail:helshemy@hotmail.comhuzita.kounosuke@cameo.plala.or.jp †E-mail:helshemy@hotmail.comhuzita.kounosuke@cameo.plala.or.jp helshemy@hotmail.com 4 1 5 1 6 1 corresp ‡Corresponding Author ‡Corresponding Author †E-mail:helshemy@hotmail.comhuzita.kounosuke@cameo.plala.or.jp †E-mail:helshemy@hotmail.comhuzita.kounosuke@cameo.plala.or.jp helshemy@hotmail.com Department of Environmental Dynamics and Management, Graduate School of Biosphere Sciences, Hiroshima University, Higashi-Hiroshima 739-0046, Japan Department of Environmental Dynamics and Management, Graduate School of Biosphere Sciences, Hiroshima University, Higashi-Hiroshima 739-0046, Japan Nagano Chushin Agricultural Experimental Station, Shiojiri 399-0700, Japan Nagano Chushin Agricultural Experimental Station, Shiojiri 399-0700, Japan Faculty of Agriculture Research Park (FARP) and Biochemistry Department, Faculty of Agriculture, Cairo University, Giza 12613, Egypt Faculty of Agriculture Research Park (FARP) and Biochemistry Department, Faculty of Agriculture, Cairo University, Giza 12613, Egypt School of Life Sciences, Sambalpur University, Jyoti Vihar, Sambalpur Orissa, India School of Life Sciences, Sambalpur University, Jyoti Vihar, Sambalpur Orissa, India Hokkaido Prefectural Kitami Agricultural Experiment Station, Tokoro, Hokkaido 099-1400, Japan Hokkaido Prefectural Kitami Agricultural Experiment Station, Tokoro, Hokkaido 099-1400, Japan Hiroshima Prefectural Agriculture Research Center, Hara, Hachihonmatsu, Higashi-Hiroshima 739-0151, Japan Hiroshima Prefectural Agriculture Research Center, Hara, Hachihonmatsu, Higashi-Hiroshima 739-0151, Japan This study addresses the hypothesis that stagnation of soybean yield on the farm can be improved by selection of a physiological trait favoring carbon assimilate partitioning to terminally placed pods versus genotypes having axillary pods at close plant spacing. 13 C was fed to source-sink units comprising a leaf, axillary/terminal pods, and petioles at upper and lower positions of the stem axis in two soybean cultivars, namely Shakujo and Enrei, at different densities of populations. The cultivars differ significantly in architecture, Shakujo bearing a few hundreds of pods in close succession to one another in a terminally placed raceme, in contrast to Enrei having axillary racemes. Pod yield per plant was higher in Enrei than in Shakujo at low density, but Shakujo out-yielded Enrei at close spacing. Population density decreased yield per plant and altered the pattern of assimilate partitioning significantly within the plants for both varieties. At high density more assimilates moved to the upper parts at the cost of the lower parts. The terminally placed pods of Shakujo were advantaged to receive assimilates under density stress. No benefit was accrued to pod filling of Enrei, however, under this condition. Fasciated stem Grain yield Photosynthesis Photosynthate partitioning Population density Soybean Source-sink unit Introduction 1 Soybean seed is a rich source of edible proteins and oil for human beings. It ranks fifth among the grain crops cultivated worldwide (Egli and Crafts-Brandner, 1996 1996 ). The seed yield of this important crop has improved through cultivar development (Specht et al., 1999 1999 ). The rise in annual production has ranged between 0.5% and 0.7% in North America, the world’s greatest soybean-producer (Kumudini, 2002 2002 ). However, to sustain growth of soybean production, a physiological understanding of yield improvement is necessary, in particular, by comparison of the improved genotypes with the traditional genotypes under various cultural practices. Similar to other grain crops, yield is a product of interaction between genotypes and the environment (Liu and Herbert, 2002 2002 ); the most suitable combination is desired for maximizing grain yield. In this context, the response of soybean genotypes to planting density has attracted considerable attention (Egli, 1988 1988 ; Bullock et al., 1998 1998 ; Gan et al., 2002 2002 ). The spatial distribution of soybean plants has been reported to influence growth and yield responses (Duncan, 1986 1986 ; Robinson and Wilcox, 1998 1998 ) by affecting canopy photosynthesis and light interception capacities (Wells, 1991 1991 ; Board, 2000 2000 ; Singer, 2001 2001 ). Close spacing enhances light interception. The canopy photosynthesis increases and supports greater seed yield (Christy and Porter, 1982 1982 ; Board et al., 1992 1992 ; Singer, 2001 2001 ). A positive correlation of seed yield with population density is not always evident in soybean. There have been instances in which seed yield did not increase commensurate to the increase of population density (Weaver et al., 1991 1991 ; Kane and Grabau, 1992 1992 ; Board, 2000 2000 ). Among the genotypes which respond positively to population density, the effect of plant density on seed yield varies independent of genotype (Gan et al., 2002 2002 ). This evidence indicates differences in physiological interactions among the genotypes in response to population density. The chemical nature of these responses is thus far elusive. Seed yield of grain crops is a direct outcome of biomass partitioned from the source to the sink, but there have been fewer studies in soybean to unravel the influence of population density on this aspect and to understand how genotypes differ in response to plant density. In the present study, biomass partitioning has been examined in a fasciated stem soybean, Shakujo, in which the architecture is modified to shift the spatial locations of the sources and sinks, although this genotype has not been intensively studied in terms of biomass and grain production (Albertsen et al., 1983 1983 ). It seems that the peculiar architecture of the genotype precludes mutual shading under constraints of population density stress and is congenial for easy flow of assimilates to sink from the source, in comparison with common cultivars of Japan. In the process here, a concept will be developed for the plant ideotype for better partitioning of assimilates for high grain yield. Materials and methods 1 Two soybean ( Lysine max L.) genotypes, namely a fasciated stem (f/f) soybean Shakujo and a normal stem (F/F) soybean Enrei were used in the experiment. Shakujo originated in Japan in the course of natural mutation. Expression of a recessive gene is responsible for the swelling of the apical internodes. This genotype bears a few hundred of pods in close succession to one another on the flowering raceme located at the top of the stem. The leaves are found in whorls, according to the verticulate type of phyllotaxy. In contrast, Enrei is a common genotype widely cultivated in Japan. On the axillary position of many leaves, a flowering axis containing a raceme type of inflorescence is borne. Each raceme consists of a maximum number of three pods. The leaves are arranged in alternate sequence on the stem and the order of development of pods is acropetal. Experiment 1: 13 C export and partitioning from leaves at different positions on the main stem 2 Shakujo and Enrei cultivars were cultivated in the experimental field of Hiroshima University from May to October in 2002 and 2003. The soil was a granite aerosol. Commercial fertilizers consisting of N, P, and K were added to the soil at the ratio of 50:98:139 kg/ha. Soil pH was adjusted to 6.0 with dolomitic calcium carbonate prior to a basal dressing. Three seeds were planted in a hill spaced at 40 cm apart in a row and the rows were spaced 70 cm apart, equivalent to 3.7 plants/m 2 . There were 40 rows in an individual plot. The plants in a hill were thinned to one plant 21 d after cultivation. The statistical design was a randomized block with three replicates. 13 CO 2 feeding 3 A trifoliated leaf at different positions of the stem was exposed to 13 CO 2 14 d after flowering according to the method described by Nobuyasu et al. ( 2003 2003 ). The leaf was placed inside a transparent plastic bag containing 1 g of Ba 13 CO 3 (atomic fraction 99% 13 C). A total of 4 ml lactic acid was injected into the bag to release the 13 CO 2 . The leaf was allowed to assimilate 13 CO 2 for 2 h under average photosynthetically active radiation (PAR) of 1 000 µmol/(m 2 ∙s). The plants were harvested immediately after termination of 13 CO 2 assimilation. Harvested plants were immediately separated into the 13 C fed leaf, 13 C fed leaf petiole, pods of the same unit with 13 C fed leaf (in Shakujo, it is referred to pods at top unit), other pods, and the rest of the leaves and stem above or below the 13 C fed leaf, which are referred to as upper or lower leaves and stem. The plant parts were dried in an open air oven at 70 °C for 3 d, weighed and ground to powder with a vibrating sample mill (Model TI-100 Heiko Co., Ltd., Fukushima, Japan). 13 C analysis 3 The 13 C abundance in the sample was determined with a mass spectrometer (Finnigan MAT Delta Plus) using 2 μg of the powdered materials. The amount of labeled C ( A ) was calculated using the equation as follows: A =( 13 C abundance in the sample/ 13 C abundance of Ba 13 CO 3 )×amount of C in the sample. Experiment 2: grain weights and source activities of upper and lower units 2 In this experiment both of the genotypes were grown in the field conditions at hill and row distances of 20 and 50 cm, respectively. The method of cultivation was similar to Experiment 1. During the pod filling stage, the uppermost source-sink unit (referred to as upper unit) consisting of the leaf, petiole, axillary pod, and sub-tending stem was separated from the rest by girdling treatment. The fifth unit (lower unit), consisting of similar segments, was also excluded from the rest by a similar technique. Girdling was done by heating a part of the stem in a circular fashion just above and below the unit with the help of an electric burner. On the following day, 13 C was fed to the leaves of these two units and partitioning of the isotope was traced in different plant parts as described in Experiment 1. In addition, leaf area, photosynthetic rates of the leaves, and grain yields of the two units were also measured. There were three replicates for each observation. Photosynthetic rate was measured by a portable infra-red gas analyzer (Model LI-6400, LI-COR Co., Ltd., Lincoln, USA) under natural sunlight. The PAR was more than 1 700 µmol/(m 2 ∙s) during the measurements. The leaf area was determined by an auto leaf area meter (Model AAM-5, Hayashi Denko Co., Ltd., Osaka, Japan). The dry weights of the plant parts were measured at maturity. Experiment 3: effects of population density on biomass, grain production, and 13 C partitioning 2 Both genotypes were grown in the field conditions similar to Experiment 1 at four levels of population densities: 6.5, 12.5, 18.0, and 22.9 plants/m 2 . The experiment was conducted in a randomized block design with three replicates. The production structure at 106 d after treatment (DAT) was investigated. The light transmission ratio (LTR), leaf area, and pod weight at the different strata were measured. LTR was investigated by measurement of solar radiation intensity at the various heights of the canopy with a portable infra-red gas analyzer (Model LI-6400, LI-COR Co., Ltd., Lincoln, USA). The photosynthetic rates of the upper and lower strata at various radiation levels were measured. The 13 CO 2 feeding, as described in Experiment 1, was carried out using leaves at the upper (the first) and lower (the fifth) units counted from the uppermost leaf in different densities of population during the pod filling stages. Statistical analysis 2 All the experiments were analyzed for the effect of treatments according to the expected mean squares given by McIntosh ( 1983 1983 ). For the separation, treatment sum of squares was partitioned by the method of orthogonal contrasts. The coefficient of variation for all response variables was below 10%. Results 1 Experiment 1: 13 C export and partitioning from leaves at different positions on the main stem 2 Biomass production 3 Biomass production similarly increased with time in both soybean cultivars, Shakujo and Enrei (data not shown). This increase in pods initiated earlier in the former than in the latter. 13 C export and partitioning 3 In Shakujo, 13 C partitioning to the apical pods was maximum from the uppermost leaf. The contributions from leaf Nos. 3 and 5 were also significant (Fig. 1 1 ). However, leaf No. 7 did not contribute any of the 13 C fed to it for development of the apical pods. Instead, some of 13 C from this leaf moved into the axillary pods. Leaf Nos. 3 and 5 contributed more 13 C to axillary pods in comparison with the apical pods. Leaf No. 1 retained nearly 60% of the 13 C fed, but the rate of transport was higher in leaf Nos. 3 and 5 within the same period. In contrast, transport of 13 C was the poorest in leaf No. 7; it retained about 80% of assimilates. 13 C was fed to leaf Nos. 1, 3, 5, and 7 in Shakujo, and leaf Nos. 1 and 5 in Enrei. The leaves were counted from the uppermost leaf. * Apical pods for Shakujo and unit pods for Enrei; ** Axillary pods for Shakujo and upper pods for Enrei. The experiment was conducted in 2002 Similar to Shakujo, export of 13 C from an upper leaf was more than that of the lower leaf in the Enrei cultivar; the lower leaf retained nearly 80% of the fed isotope (Fig. 1 1 ). A major part of the assimilate export from the upper leaf went to the pods of the 13 C fed unit and the pods above it. The pods below the fed leaf received a minor share. In contrast, export of 13 C from the lower leaf was identical to the pods on its own unit and below. Experiment 2: grain weights and source activities of upper and lower units 2 In Shakujo, the upper source-sink units possessed considerably more numbers of pods and grains compared with the lower units (Table 1 1 ). However, the average grain weight did not differ as much between the units. Unlike Shakujo, the pod and grain numbers did not differ between the upper and lower units in Enrei cultivar, but the grain weight of the upper unit was significantly higher than that of the lower unit. m grain : grain weight per unit; n pod : number of pods per unit; n grain : number of grains per unit; m 100-grain : 100-grain weight Figures in the parentheses are percentages relative to whole plant grain weight Grain yield and yield components were measured at 149 and 135 DAP in Shakujo and Enrei, respectively. LSD: least significant difference. The experiment was conducted in 2003 Cultivar Position m grain (g) * * n pod n grain m 100-grain (g) 2  Shakujo Upper units 29.70 (86.8%) 126.3 219.8 13.5 Lower units 0.75 (2.1%) 4.1 6.9 11.6 2  Enrei Upper units 1.68 (4.2%) 3.0 8.2 21.9 Lower units 1.02 (2.6%) 2.8 7.7 13.0 6 2  LSD (0.05) 14.3 61.5 106.1 4.7 The source capacity of the upper unit was considerably higher than that of the lower unit in both Shakujo and Enrei cultivars during the period of pod filling (Table 2 2 ). With passage of time, the source activity of the upper unit declined significantly, but it was not so for the lower unit. Differences in photosynthetic rate and leaf area contributed to the variation of source activities of the upper and lower units in both the cultivars, but the contribution of the upper unit was more compared with the lower unit. At 101 and 108 DAT, the photosynthetic rate of the upper unit of Shakujo was more than three times higher than that of the lower unit. However, this difference was not evident in the Enrei cultivar. PR: photosynthetic rate; S leaf : each leaf area; SC: source capacity per leaf; LSD: least significant difference. The experiment was conducted in 2003 2 t (DAT) 2 Position 3 Shakujo 3 Enrei PR (µmol/(m 2 ∙s)) S leaf (m 2 ) SC (μm/s) PR (µmol/(m 2 ∙s)) S leaf (m 2 ) SC (μm/s) 101 Upper units 15.9 0.0277 0.440 16.9 0.0240 0.406 Lower units 4.8 0.0117 0.056 12.3 0.0101 0.124 108 Upper units 10.2 0.0105 0.107 12.6 0.0255 0.321 Lower units 2.1 0.0167 0.035 6.5 0.0196 0.127 116 Upper units 4.6 0.0187 0.086 6.1 0.0252 0.154 Lower units 3.8 0.0110 0.042 3.7 0.0062 0.023 8 2  LSD (0.05) 5.2 0.0066 0.155 5.0 0.0083 0.142 Girdling of the upper and lower units restricted 13 C partitioning within the various components of the units. In Shakujo, 13 C fed to the leaf in the upper unit moved to the pods and the translocation increased with the passage of time (Fig. 2 2 ). No 13 C moved from the leaf of the lower unit to the pods, although this leaf assimilated the isotope. The situation was different for Enrei. At 109 DAT, 13 C export to the pod was very high. It declined considerably at 117 DAT, and the most isotopes were retained in the fed leaf blade. The fifth leaf could assimilate very little amount of 13 C fed. 13 C was fed to the leaf in each unit. The plants were girdled right below the unit. LSD: least significant difference. The experiment was conducted in 2003 Experiment 3: effects of population density on biomass, grain production, and 13 C partitioning 2 Grain production 3 Biomass production decreased with increasing density and the decrease was similar in Shakujo and Enrei (Fig. 3 3 ). Increase of population density significantly improved grain yield per plant in Shakujo. In contrast, the yield of Enrei was not influenced by the density (Table 3 3 ). At low density, Enrei produced more yield than Shakujo, but the latter out-yielded the former at high density. The yield advantage of Shakujo at high density was due to the difference in pod number per plant. Population density did not alter pod number in Shakujo, but it reduced the number significantly in Enrei. Grain weight of either of the two cultivars was not affected by population density. (a) 77 DAT; (b) 98 DAT; (c) 114 DAT. LSD: least significant difference. The experiment was conducted in 2003 m 1 : grain weight per unit (m 2 ) of area; m 2 : grain weight per plant; n pod : number of pods per plant; n grain : number of grains per plant; m 100-seed : 100-seed weight. Grain yield and yield components in 2003, and 2004 were measured at maturity. In 2004, Shakujo plants lodged at 110 DAT. Values are expressed as mean (±SD) In 2004 For those in column 2 without mark ‘*’, in 2003 2 Cultivar 2 Population density (plant/m 2 ) 2 Grain weight 2 n pod 2 n grain 2 m 100-seed (g) m 1 (g/m 2 ) m 2 (g/plant) Shakujo 6.5 247 38.0±14.3 171.5±39.6 233.0±75.7 16.3±0.21 12.5 441 35.3±12.2 150.6±36.1 215.2±64.0 16.4±0.35 12.5 * * 499 39.9 201.5 19.8±0.35 18.5 673 36.4±8.0 148.9±26.3 211.6±42.8 17.2±0.57 18.5 * * 588 31.8 166.5 19.1±0.57 7 Enrei 6.5 568 87.4±12.6 173.1±27.8 284.7±42.2 30.7±0.92 12.5 615 49.2±9.6 100.0±22.8 155.7±39.4 31.6±0.64 12.5 * * 512 41.0 167.3 30.6±0.51 18.5 594 32.1±11.7 73.8±18.5 104.3±31.2 30.8±1.06 18.5 * * 669 36.1 104.3 34.6±0.33 Leaf area distribution at different strata 3 The total leaf area of the plant was higher in Shakujo than in Enrei (Fig. 4 4 ). Population density stress significantly reduced the leaf area in both cultivars, but it was more effective in Shakujo. The leaf area of Shakujo at the uppermost stratum remained unchanged at 50% of total area at different densities of population. For Enrei, however, the leaf area of the uppermost stratum was more than 50% at low population density, but it decreased to less than 50% at high density. Flowering, pod elongation, and grain filling initiated at 55, 77, and 102 DAT in Enrei, and 69, 91, and 115 DAT in Shakujo, respectively. LSD: least significant difference. The experiment was conducted in 2003 Pod mass distribution at different strata 3 The total pod weight per plant was nearly two times higher in Enrei than in Shakujo at low population density (Fig. 5 5 ). The weight decreased as population density increased in both cultivars. At higher densities, the pod weight of Shakujo was more depressed than Enrei; at densities of 18.5 and 22.9 plants/m 2 , the pod weight of the former was less than a third of pod weight of the latter (Fig. 5 5 ). In Shakujo, pod weight distribution was mostly confined to the uppermost stratum of the plant, and density stress did not alter this pattern. In contrast, the distribution pattern was seriously affected by density stress in Enrei. At low density, pod weight distribution was equally divided between the strata of 20–40 and 40–60 cm. But at higher densities, distribution of pod weight in favor of the lower two strata increased at the cost of the uppermost stratum. Flowering, pod elongation, and grain filling initiated at 55, 77, and 102 DAT in Enrei, and 69, 91, and 115 DAT in Shakujo, respectively. LSD: least significant difference. The experiment was conducted in 2003 Light transmission ratio in the canopy 3 LTR was higher in the leaves of the upper unit than in those of the lower unit in both genotypes, which was maintained in spite of population density (Fig. 6 6 ). In contrast to this, that in the leaves of the lower unit decreased with lowering position of unit and the decrease was more intense in Shakujo compared with Enrei. A: above plant canopy (60 cm). The experiment was conducted in 2004 Leaf photosynthetic rate 3 Leaf photosynthetic rate tended to decline temporally in both the soybean cultivars during the period of observation at 75 to 101 DAT. The upper leaves in both cultivars had similar rates of leaf photosynthesis and population density rarely affected the process (Figs. 7 7 and 8 8 ). In both cultivars, the rate of photosynthesis was higher in the upper than in the lower leaves. The rate of photosynthesis in the lower leaves was higher in Enrei than in Shakujo at low densities below 18.5 plants/m 2 ; however, the difference was not observed at high density. The upper and lower leaves are the 1st and 5th leaves. The experiment was conducted in 2004 LSD: least significant difference. The experiment was conducted in 2002 13 C partitioning 3 In Shakujo, 13 C fed to the leaf of the upper unit mostly accumulated in the pods of the same unit and nearby units (Fig. 9 9 , Table 4 4 ). Isotopes fed to the leaf of the lower unit were not translocated as much as those of the upper unit. They were poorly translocated to the pods and retained mostly in the vegetative parts like leaf or stem at low density. At high density, export from the upper unit significantly increased to the pods of its own unit and export to other pods declined severely. At high density, export to pods increased from the lower unit as well, but the increase was limited. Export of 13 C from upper and lower units was not similar in Enrei. At low density, export of 13 C was mostly in favor of the pods from both the units. At high density, export from the upper unit increased in the pods of its own unit at the cost of the other pods. The situation was, however, the opposite for the export of 13 C from the lower unit. In Shakujo, unit pods are the pods at the top on the main culm; in Enrei, they are the pods of the unit where 13 C-fed leaf positioned. LSD: least significant difference. The experiment was conducted in 2003 LSD: least significant difference. The experiment was conducted in 2004 2 Population density and 13 C fed leaf 2 Plant parts 2 Excess atomic fraction of 13 C (%) Shakujo Enrei Low density 1st unit 13 C fed leaf 0.261 0.057 13 C fed leaf petiole 0.229 0.041 Other leaves 0.000 0.000 Stem 0.001 0.001 Other pods 0.007 0.035 Pods in an unit 0.035 0.215 Root 0.001 0.004 LSD (0.05) 0.116 0.076 4 Low density 5th unit 13 C fed leaf 0.074 0.078 13 C fed leaf petiole 0.041 0.053 Other leaves 0.001 0.000 Stem 0.002 0.000 Other pods 0.002 0.004 Pods in an unit 0.000 0.067 Root 0.000 0.001 LSD (0.05) 0.029 0.035 4 High density 1st unit 13 C fed leaf 0.249 0.094 13 C fed leaf petiole 0.156 0.074 Other leaves 0.000 0.000 Stem 0.014 0.001 Other pods 0.013 0.021 Pods in an unit 0.032 0.343 Root 0.005 0.003 LSD (0.05) 0.085 0.123 4 High density 5th unit 13 C fed leaf 0.178 0.070 13 C fed leaf petiole 0.117 0.066 Other leaves 0.000 0.000 Stem 0.000 0.000 Other pods 0.007 0.005 Pods in an unit 0.001 0.020 Root 0.002 0.002 LSD (0.05) 0.073 0.031 Discussion 1 Increased light interception available in narrow row planting was proposed to generate more biomass in soybean plant (Board et al., 1990 1990 ). High vegetative biomass was considered beneficial for seed filling (Kane and Grabau, 1992 1992 ). Responses of genotypes to light perception, however, were not identical because of differences in physiological factors associated with assimilate supply to reproductive sinks (Kumudini, 2002 2002 ). The discovery made in our study supports this proposition and reveals for the first time that assimilate partitioning of soybean genotypes to reproductive organs is not identical at different population densities. The cultivar Enrei, which is widely used in Japan, failed to increase yield at a high population density. In contrast, the low yielding cultivar Shakujo responded well to increased density and out-yielded Enrei at a high density of 18.5 plants/m 2 (Table 3 3 ). It was found that the pod weight per plant decreased significantly as density increased in both cultivars (Table 3 3 , Fig. 5 5 ). Under these constraints, the terminal location of the big raceme comprised of a few hundreds of pods in Shakujo proved more advantageous for 13 C partitioning, in comparison to Enrei having axillary racemes. In grain legumes like soybean ( Glycine max L.) (Nobuyasu et al. 2003 2003 ) and field bean ( Phaseolus vulgaris L.) (Tanaka and Fujita, 1979 1979 ), a source-sink unit was proposed to comprise a leaf, truss below the leaf node, and the axillary raceme containing several pods on it. This proposition matched partially for the Enrei cultivar in our study, but not so for Shakujo. In Enrei, the upper leaf exported 13 C assimilates to the axillary pods as well as pods above it. The translocation was poor from the lower leaf and it exported to the axillary pod only (Fig. 1 1 ). In Shakujo, the upper leaves supplied carbon assimilates to the apical pods and when they were removed, assimilates accumulated mostly in the fed leaf (data not shown). In addition to the first leaf, some other leaves on lower nodes also contributed significantly to the apical pods (Fig. 1 1 ). These findings corroborate the supposition addressed for rice (Mohapatra et al., 2004 2004 ) that a larger panicle in a high yielding variety is composed of a source-sink unit with remotely located lower leaves in addition to flag leaf, mainly due to stronger sink activity. Namely, source-sink unit varies depending upon the capacity of sink activity: the greater the sink activity, the wider the source leaf. Broadening of the source area seems to be an advantage for Shakujo for grain filling. Another advantage for grain production in this cultivar is that the source-sink unit is located on the top of the plant, which may avoid shading by their own leaves. However, under these conditions, these upper leaves are mutually shaded by leaves in the surrounding plants. It has been reported that soybean racemes located on the lower part of the plant are frequently shaded in field grown conditions (Fig. 7 7 ) (Ofosu-Budu et al., 1990 1990 ). In Enrei, the fifth leaf poorly assimilated the carbon isotope supplied to it (Fig. 9 9 , Table 4 4 ). Similarly, the assimilation capacity of the leaf on the same node of Shakujo was also lower. The upper leaf of Enrei assimilated more carbon, but could not export it to the pods. In addition, the leaf area distribution at the lower strata of the canopy in Enrei increased when the plants were grown under high density of population and this effect was not evident in Shakujo (Fig. 4 4 ). Similar to leaf area, pod distribution increased in the lower strata of Enrei and decreased in the upper strata (Fig. 5 5 ). Since pod development on soybean is an indeterminate process, the older pods developed and filled properly at an early stage of plant development, when competition between plants was not severe at high planting density (Duncan, 1986 1986 ). Pod weight of the uppermost stratum was reduced significantly because of intense competition between plants at later stage of development. It reduced assimilate partitioning from the upper leaf to the pods in Enrei (Fig. 5 5 ). Terminal location of pods and source leaves in Shakujo precluded this effect of planting density. Although yield per plant declined, the apical pods filled and ensured good yield at high density. High planting density could have reduced pod filling on the lower nodes of the Enrei cultivar, but yield contribution form these nodes was minimal (Table 1 1 ). The gradient in pod weight between the upper and lower units was quite evident in Shakujo, but not in Enrei (Table 1 1 ). This gradient was equally matched by the differences in source capacities of the units individually in both the cultivars (Table 2 2 ). The lower leaves of Shakujo could not photosynthesize as much as that of the upper leaves. The increase of population density exacerbated the situation for the lower leaves, but not for the upper leaves (Figs. 7 7 and 8 8 ). This effect of population density could not be noted on the lower leaves of Enrei cultivar, but the spatial difference in rate of photosynthesis between the two types of leaves was similar to Shakujo. It has been reported that leaf carbon exchange capacity in soybean varies with time of origin, nodal position, developmental phase, genotypes, and environmental conditions (Dornhoff and Shibles, 1970 1970 ; Abu-Shakra et al., 1978 1978 ; Gordon et al., 1982 1982 ; Egli, 1997 1997 ). In our study, shading of the upper canopy layers and developmental stage might have impacted leaf photosynthesis of the older leaves on the lower units in both cultivars. Although source activity of upper leaves was not affected by population density in Enrei, pod development was impacted by lack of tolerance to stress. In soybean, provision of assimilate alone does not improve grain filling, unless partitioning to the sink increases substantially (Kumudini, 2002 2002 ). Apart from assimilates, hormones, nutrients, vascular constraints, and sink capacity influence partitioning of assimilates from the source to sink (Wardlaw, 1990 1990 ). Competition between individuals at high density in our study could have influenced any of the factors and reduced pod development in the apical parts of the Enrei cultivar. It was reported that plants grown in sparse stands experience greater red/far-red light ratio within the canopy compared to those in dense stands (Sanchez et al., 1993 1993 ), and high red/far-red light ratio increased partitioning of dry matter into branches (Board, 2000 2000 ). Increased exposition of plant parts to the high level of far-red light within the canopy at high density could have impaired assimilates transport and grain yield per plant in our study for both varieties. The effect was more evident, however, in Enrei than in Shakujo (Fig. 9 9 , Table 4 4 ), because assimilates could not go to branches in the former. Improved assimilate export could not sustain pod development in the upper parts of Enrei (Fig. 5 5 ). Shakujo was relatively less affected because of the terminal location of pods. Taking this evidence into account, it is concluded that Enrei is more versatile for adaptation to density stress than Shakujo. The physiological basis for this adaptation remains oblivious presently. Lack of this information, however, does not preclude Shakujo for consideration as an ideotype for soybean cultivation. References SS Abu-Shakra , DA Phillips , RC Huffaker . Nitrogen fixation and delayed leaf senescence in soybeans . Science , 1978 . 199 ( 4332 ): 973 - 975 . 17752368 DOI: 10.1126/science.199.4332.973 http://doi.org/10.1126/science.199.4332.973 . Nitrogen fixation and delayed leaf senescence in soybeans MC Albertsen , TM Curry , RG Palmer , CE Lamotte . Genetics and comparative growth morphology of fasciation in soybeans ( Glycine max [L.] Merr.) . Bot Gaz , 1983 . 144 ( 2 ): 263 - 275 . DOI: 10.1086/337372 http://doi.org/10.1086/337372 . Genetics and comparative growth morphology of fasciation in soybeans (Glycine max [L.] Merr.) J Board . Light interception efficiency and light quality affect yield compensation of soybean at low population density . Crop Sci , 2000 . 40 ( 5 ): 1285 - 1294 . DOI: 10.2135/cropsci2000.4051285x http://doi.org/10.2135/cropsci2000.4051285x . Light interception efficiency and light quality affect yield compensation of soybean at low population density JE Board , BG Harville , AM Saxton . Growth dynamics of determinate soybean in narrow and wide rows at late planting dates . Field Crops Res , 1990 . 25 ( 3-4 ): 203 - 213 . DOI: 10.1016/0378-4290(90)90003-T http://doi.org/10.1016/0378-4290(90)90003-T . Growth dynamics of determinate soybean in narrow and wide rows at late planting dates JE Board , M Kamal , BG Harville . Temporal importance of greater light interception to increased yield in narrow row soybean . Agron J , 1992 . 84 ( 4 ): 575 - 579 . DOI: 10.2134/agronj1992.00021962008400040006x http://doi.org/10.2134/agronj1992.00021962008400040006x . Temporal importance of greater light interception to increased yield in narrow row soybean D Bullock , S Khan , A Rayburn . Soybean yield response to narrow rows is largely due to enhanced early growth . Crop Sci , 1998 . 38 ( 4 ): 1011 - 1016 . DOI: 10.2135/cropsci1998.0011183X003800040021x http://doi.org/10.2135/cropsci1998.0011183X003800040021x . Soybean yield response to narrow rows is largely due to enhanced early growth AL Christy , CA Porter . Govindjee . Canopy Photosynthesis and Yield in Soybean . Photosynthesis—Application to Food and Agriculture , 1982 . : New York Academic Press , 499 - 511 . . Canopy Photosynthesis and Yield in Soybean GM Dornhoff , RM Shibles . Varietal differences in net photosynthesis of soybean leaves . Crop Sci , 1970 . 10 ( 1 ): 42 - 45 . DOI: 10.2135/cropsci1970.0011183X001000010016x http://doi.org/10.2135/cropsci1970.0011183X001000010016x . Varietal differences in net photosynthesis of soybean leaves WG Duncan . Planting patterns and soybean yields . Crop Sci , 1986 . 26 ( 3 ): 584 - 588 . DOI: 10.2135/cropsci1986.0011183X002600030033x http://doi.org/10.2135/cropsci1986.0011183X002600030033x . Planting patterns and soybean yields DB Egli . Plant density and soybean yield . Crop Sci , 1988 . 28 ( 6 ): 977 - 981 . DOI: 10.2135/cropsci1988.0011183X002800060023x http://doi.org/10.2135/cropsci1988.0011183X002800060023x . Plant density and soybean yield DB Egli . Cultivar maturity and response of soybean to shade stress during seed filling . Field Crops Res , 1997 . 52 ( 1-2 ): 1 - 8 . DOI: 10.1016/S0378-4290(97)00005-1 http://doi.org/10.1016/S0378-4290(97)00005-1 . Cultivar maturity and response of soybean to shade stress during seed filling DB Egli , SJ Crafts-Brandner . E Zamski , RA Schaffer . Soybean . Photoassimilate Distribution in Plants and Crops—Source Sink Relationships , 1996 . : New York Marcel Dekker Inc. , 595 - 623 . . Soybean Y Gan , I Stulen , H van Keulen , PJC Kuiper . Physiological response of soybean genotypes to plant density . Field Crops Res , 2002 . 74 ( 2-3 ): 231 - 241 . DOI: 10.1016/S0378-4290(01)00212-X http://doi.org/10.1016/S0378-4290(01)00212-X . Physiological response of soybean genotypes to plant density AJ Gordon , JD Hesketh , DB Peters . Soybean leaf photosynthesis in relation to maturity classification and stage of growth . Photosynth Res , 1982 . 3 ( 2 ): 81 - 93 . DOI: 10.1007/BF00040706 http://doi.org/10.1007/BF00040706 . Soybean leaf photosynthesis in relation to maturity classification and stage of growth MV Kane , LJ Grabau . Early planted, early maturing soybean cropping system: growth, development and yield . Agron J , 1992 . 84 ( 5 ): 769 - 773 . DOI: 10.2134/agronj1992.00021962008400050002x http://doi.org/10.2134/agronj1992.00021962008400050002x . Early planted, early maturing soybean cropping system: growth, development and yield S Kumudini . Trials and tribulations: a review of the role of assimilate supply in soybean genetic yield improvement . Field Crops Res , 2002 . 75 ( 2-3 ): 211 - 222 . DOI: 10.1016/S0378-4290(02)00027-8 http://doi.org/10.1016/S0378-4290(02)00027-8 . Trials and tribulations: a review of the role of assimilate supply in soybean genetic yield improvement XB Liu , SJ Herbert . Some aspects of yield physiology research in soybean . J Northeast Agric Univ , 2002 . 7 ( 1 ): 71 - 78 . . Some aspects of yield physiology research in soybean MS McIntosh . Analysis of combined experiments . Agron J , 1983 . 75 ( 1 ): 153 - 155 . DOI: 10.2134/agronj1983.00021962007500010041x http://doi.org/10.2134/agronj1983.00021962007500010041x . Analysis of combined experiments PK Mohapatra , Y Masamoto , S Morita , J Takanashi , T Kato , T Itani , JJ Adu-Gyamfi , M Shunmugasondaram , NT Nguyen , H Saneoka , 等 . Partitioning of 13 C-labelled photosynthate varies with growth stages and panicle size in high yielding rice . Functional Plant Biol , 2004 . 31 ( 2 ): 131 - 139 . DOI: 10.1071/FP03177 http://doi.org/10.1071/FP03177 . Partitioning of 13C-labelled photosynthate varies with growth stages and panicle size in high yielding rice H Nobuyasu , S Liu , JJ Adu-Gyamfi , PK Mohapatra , K Fujita . Variation in the export of 13 C and 15 N from soybean leaf: the effects of nitrogen application and sink removal . Plant and Soil , 2003 . 253 ( 2 ): 331 - 339 . DOI: 10.1023/A:1024836600725 http://doi.org/10.1023/A:1024836600725 . Variation in the export of 13C and 15N from soybean leaf: the effects of nitrogen application and sink removal KG Ofosu-Budu , K Fujita , S Ogata . Excretion of ureide and other nitrogenous compounds by the roots system of soybean at different growth stages . Plant and Soil , 1990 . 128 ( 2 ): 135 - 142 . DOI: 10.1007/BF00011102 http://doi.org/10.1007/BF00011102 . Excretion of ureide and other nitrogenous compounds by the roots system of soybean at different growth stages SL Robinson , JR Wilcox . Comparison of determinate and indeterminate soybean near-isolines and their response to row spacing and planting date . Crop Sci , 1998 . 38 ( 6 ): 1554 - 1557 . DOI: 10.2135/cropsci1998.0011183X003800060024x http://doi.org/10.2135/cropsci1998.0011183X003800060024x . Comparison of determinate and indeterminate soybean near-isolines and their response to row spacing and planting date RA Sanchez , JJ Casal , CL Ballard , 等 . DR Buxton . Plant Response to Canopy Density Mediated by Photomorphogenic Processes . International Crop Science I , 1993 . : Madison, WI CCSA , 779 - 786 . . Plant Response to Canopy Density Mediated by Photomorphogenic Processes JW Singer . Soybean light interception and yield response to row spacing and biomass removal . Crop Sci , 2001 . 41 ( 2 ): 424 - 429 . DOI: 10.2135/cropsci2001.412424x http://doi.org/10.2135/cropsci2001.412424x . Soybean light interception and yield response to row spacing and biomass removal JE Specht , DJ Hume , SV Kumudini . Soybean yield potential—a genetic and physiological perspective . Crop Sci , 1999 . 39 ( 6 ): 1560 - 1570 . DOI: 10.2135/cropsci1999.3961560x http://doi.org/10.2135/cropsci1999.3961560x . Soybean yield potential—a genetic and physiological perspective A Tanaka , K Fujita . Growth, photosynthesis and yield components in relation to grain yield of the field bean . J Faculty Agric Hokkaido Univ , 1979 . 59 ( Pt. 2 ): 145 - 238 . . Growth, photosynthesis and yield components in relation to grain yield of the field bean IF Wardlaw . The control of carbon partitioning in plants . New Phytol , 1990 . 116 ( 3 ): 341 - 381 . DOI: 10.1111/j.1469-8137.1990.tb00524.x http://doi.org/10.1111/j.1469-8137.1990.tb00524.x . The control of carbon partitioning in plants DB Weaver , RL Akridge , CA Thomas . Growth habit, planting date, and row-spacing effects on late-planted soybean . Crop Sci , 1991 . 31 ( 3 ): 805 - 810 . DOI: 10.2135/cropsci1991.0011183X003100030052x http://doi.org/10.2135/cropsci1991.0011183X003100030052x . Growth habit, planting date, and row-spacing effects on late-planted soybean R Wells . Soybean growth response to plant density: relationship among canopy photosynthesis, leaf area and light interception . Crop Sci , 1991 . 31 ( 3 ): 755 - 761 . DOI: 10.2135/cropsci1991.0011183X003100030044x http://doi.org/10.2135/cropsci1991.0011183X003100030044x . Soybean growth response to plant density: relationship among canopy photosynthesis, leaf area and light interception 10.1631/jzus.B0900097 2009-04-08 2010-12-01 2010-09-03 2022-01-05 Copyright © Zhejiang University and Springer-Verlag Berlin Heidelberg 2011 2011 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 01 12