Effect of water stress on growth,photosynthesis, and photoassimilate translocation in soybean and tropical pasture legume siratro

Siratro (Macroptilium atropurpureum) and soybean (Glycine max) were grown in pots with or without irrigation for 3 weeks at the vegetative stage to examine the effect of water stress on plant biomass production, biological N₂ fixation, CO₂ assimilation rate, stomatal conductance, leaf water potential, and the partition of ¹⁴CO₂ to plant parts. Biomass production decreased by water stress and the decrease was less conspicuous in siratro, mainly due to the maintenance of a relatively higher growth rate in stem plus petioles and roots. Siratro maintained a higher stomatal conductance and CO₂ assimilation rate at a lower leaf water potential compared to soybean. Water stress decreased the biological N₂ fixation in both species, and the decrease was more appreciable in siratro than in soybean under stress conditions. Water stress affected the export of photoassimilates from leaves in both species. The translocation of ¹⁴CO₂ photoassimilates to nodules and roots was less substantial in siratro than in soybean under water stress conditions. Translocation of photoassimilates from leaves to stem plus petioles in siratro enables the maintenance of growth of stem and petioles under water stress conditions.