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 s oybean (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 st ress on the leaf water potential, stomatal conductance, biomass production, biological nitrogen fixation, and nitrogen accumulation. Whole plant weigh t decreased under water stress conditions and the decrease was less pronoun ced in siratro than in desmodium and soybean. Decrease in total leaf area w as the largest and dry matter partition to stem and petioles was the highes t in siratro. Decrease in leaf water potential was lower in desmodium and s oybean than in siratro. Although water stress decreased biological nitrogen fixation in all the species, the decrease was relatively less pronounced i n 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 pot ential and higher biological nitrogen fixation by siratro under water stres s conditions.