Photosynthetic Responses of Four Legume Crops to Fluctuations of Evaporative Demand Following the Rainy Season

In the typical climate of East Asia, summer crops develop large shoots with relatively poor root systems during the rainy season, leading to restricted photosynthetic activity in the following summer months. The objectives of the present study were 1) to determinate if there are specific differences in the photosynthetic responses of four legume crops to daily fluctuations of evaporative demand and 2) to identify physiological attributes responsible for the differences. Soybean, azuki bean, cowpea and peanut were grown in a field, and apparent leaf photosynthetic rate (AP) and gas exchange parameters as well as water potential and transpiration rate were measured at midday on three consecutive summer days. When leaf-to-air vapor pressure deficit (VPD_) varied from 1.72 to 3.44 kPa during the experimental period, the four species responded differently. The AP of soybean and that of azuki bean decreased with increasing VPD_ over a range of 1.72 to 3.44 kPa, whereas the activities of cowpea and peanut were greatest at VPD_ around 2.53 kPa. The leaf water potentials of soybean, azuki bean and cowpea reached minima at VPD_ around 2.53 kPa, while that of peanut was fairly constant over the VPD_ range of 1.72 to 3.44 kPa. The transpiration rates of soybean, azuki bean and cowpea were greatest at VPD_ around 2.53 kPa and decreased beyond that range of VPD_, while peanut transpired actively with increasing VPD_. AP of soybean was correlated with leaf water potential, whereas that of cowpea and peanut was correlated with transpiration rate. With respect to water relation, peanut was most tolerant to increasing VPD_ among the four species tested, presumably because it maintained higher water potential and transpiration rate than the other species under the condition of high VPD_

Salinity Tolerance of Super-Nodulating Soybean Genotype En-b0-1

Salinity stress causes various physiological dysfunctions in soybean (Glycine max (L.) Merr.). For example, reduced nitrogen (N) uptake due to salt-induced depression of nodule formation severely limits soybean growth and yield. Super-nodulating soybean genotypes were previously identified by their superior N2 fixation and photosynthesis. Here, we have tested our hypothesis that the super-nodulating En-b0-1 genotype is more salinity tolerant than a normal-nodulating genotype. The super-nodulating genotype and its parental normal-nodulating cultivar Enrei were grown in pots and subjected to saline conditions during the pre-flowering and reproductive growth stages. Under saline conditions imposed during pre-flowering, En-b0-1 formed heavier nodules, resulting in greater N uptake, higher photosynthetic activity, and greater biomass production compared with Enrei. Saline treatment increased the concentrations of sodium (Na) and chlorine (Cl) in all plant parts regardless of genotype; but in En-b0-1, the concentrations of these elements in shoots were significantly lower, while those in roots and nodules were higher than in Enrei. When the salinity treatment was imposed during the reproductive growth stages, En-b0-1 maintained higher N uptake, leading to better alleviation of salinity-induced yield reduction than in Enrei. The super-nodulating genotype En-b0-1 was more tolerant to salinity than its parental normal-nodulating cultivar, due to its superior nodulation and prevention of excessive accumulation of Na and Cl in shoots, which were retained in roots and nodules, thus supporting our hypothesis

Intra-Raceme Variation in Pod-Set Probability Is Associated with Cytokinin Content in Soybeans

A large proportion of flowers abscise during development in soybean (Glycine max L. Merr.). A reduction in this abscission might increase pod and seed number, and thereby can lead to an increased yield. Previous studies showed that pod-set probability was greater at proximal positions of individual racemes, and that the probability was enhanced by the exogenously applied cytokinins. However, whether intra-raceme variation in the pod-set probability relates to endogenous cytokinin levels remains unknown. To address this question, intra-raceme variation in cytokinin content and pod-set probability was investigated. A soybean genotype 1X93-100, which has long racemes, was grown in an environmentally controlled chamber (30/20°C day/night temperature, 15 h day length, 600 μmol m–2s–1 photosynthetic photon flux density). Flowers, which were divided into three floral positions (proximal, middle, distal) on individual racemes, were sampled at intervals after anthesis. The cytokinins in the samples were identified by gas chromatography-mass spectrometry (GC-MS) and further quantified by enzyme immunoassay (EIA). The GG-MS analysis revealed that cis-zeatin riboside (c-ZR) and isopentenyl-adenosine (iPA) were predominant forms of cytokinin in soybean racemes. The total amount of these cytokinins in racemes, which was monitored by EIA, peaked one to two weeks after the first flowering on a raceme, when pod development was initiated. Within individual racemes, the total cytokinin concentrations were greater at more proximal floral positions, as was the probability of pod set. Removal of proximal flowers at anthesis enhanced both cytokinin concentrations and pod set at middle positions on the raceme. Thus, pod-set probability was significantly associated with the cytokinin concentration at different floral positions within individual soybean racemes

Correlation of Leaf Nitrogen, Chlorophyll and Rubisco Contents with Photosynthesis in a Supernodulating Soybean Genotype Sakukei 4

Soybean requires more nitrogen (N) than gramineous crops because it accumulates a large amount of N in seeds, and its photosynthetic rate per leaf N is low. The supernodulating genotype Sakukei 4 has a superior symbiotic N2 fixation capability, and thereby is potentially high-yielding. In our previous study, Sakukei 4 was characterized by having a superior ability to maintain high leaf N content and high photosynthetic rate. The objectives of this study were to know photosynthetic characteristics of Sakukei 4 in detail, especially, the responses to CO2 concentration and light intensity, and to elucidate how the photosynthetic characteristics of Sakukei 4 are associated with the amounts of photosynthesis-related N compounds (chlorophyll and Rubisco). The three genotypes (Sakukei 4 - supernodulating cultivar derived from Enrei, Enrei - normally nodulating cultivar, En1282-non-nodulating line derived from Enrei) were grown at various N levels in this study. The CO2 exchange rate (CER) in Sakukei 4 was higher than, or equal to that in Enrei at wide ranges of CO2 concentrations (150-700 μmol mol-1) and light intensities (200-1,500 μmol m-2 s-1 PPFD). Sakukei 4 had higher leaf N (Nl), chlorophyll (ChlL) and Rubisco (RubL) contents per leaf area, but lower chlorophyll and Rubisco contents per leaf N content (ChlL/Nl, RubL/Nl) than Enrei. The specific leaf weight (SLW) and leaf area trended to be lower in Sakukei 4 than in Enrei. These results indicate that the superior photosynthetic rate in Sakukei 4 is attributed to higher total N, chlorophyll and Rubisco contents per leaf area, but not to high rate of allocation of total N to these N compounds