Mungbean (Vigna radiata (L.) Wilczek) or green gram as a dryland grain legume is exposed to varied timings and severity of drought stress, resulting in variability in photoassimilate production. In order to evaluate some physiological responses in one mungbean cultivar to timing and severity of drought stress, a 3-replicate RCBD (randomized complete block design) field experiment was carried out during year 2004 year at the experimental farm of University College of Agriculture and Natural Resources, University of Tehran. Plants were exposed to either moderate or severe water deficit at either vegetative (VS) or reproductive growth stages (RS). Parameters were recorded at the middle stages of vegetative growth. In general, water stress reduced net photosynthesis rate (Pn), stomatal conductance (gs), transpiration rate (Tr) and leaf relative water content (RWC) while increasing leaf temperature (T). The greatest effects of drought on these parameters occurred at reproductive stage. Photosynthetic water use efficiency increased and decreased during VS and RS respectively. The results showed a clearly close correlation between RWC and gs in VS (r=0.912**) and RS (r=0.976**) treatments. Although water stress at vegetative growth stage significantly decreased Pn, gs, Tr, and RWC, these parameters were recovered upon rewatering at reproductive stage. It is concluded that to maximize mungbean Pn in dry zones, irrigation should be extended across all growth stages, especially during the reproductive stage.
Keywords: Mungbean; Drought stress; Net photosynthesis rate; Stomatal conductance