Comparison of water stress effects on growth,leaf water status,and nitrogen fixation activity in tropical pasture legumes siratro and desmodium with soybean

Siratro (Macroptilium atropurpureum), desmodium (Desmodium intortum), and soybean (Glycine max) were grown in pots with or without irrigation for 20 d at the vegetative growth stage in order to examine the effects of water stress on the leaf water potential, stomatal conductance, biomass production, biological nitrogen fixation, and nitrogen accumulation. Whole plant weight decreased under water stress conditions and the decrease was less pronounced in siratro than in desmodium and soybean. Decrease in total leaf area was the largest and dry matter partition to stem and petioles was the highest in siratro. Decrease in leaf water potential was lower in desmodium and soybean than in siratro. Although water stress decreased biological nitrogen fixation in all the species, the decrease was relatively less pronounced in siratro than in desmodium and soybean. Whole plant nitrogen concentration was higher in siratro than in soybean and desmodium. The results indicated that siratro is more tolerant to water stress than soybean and desmodium. This could be partially attributed to the maintenance of a higher water potential and higher biological nitrogen fixation by siratro under water stress conditions.