Effects of drought on photosynthetic performance and water relations of four Vigna genotypes

The effect of drought on plant water relations and photosynthesis of Vigna glabrescens (Vg) and Vigna unguiculata (cvs. 1183, EPACE-1 and Lagoa), which differ in their drought resistance, was compared. With the increase of drought severity, Vg showed a more gradual stomatal closure and maintained significantly higher levels of stomatal conductance (g(s)) and photosynthetic activity (P(N)) than the other genotypes even when minimum relative water content (RWC) values were observed. Furthermore, Vg was the only genotype able to accumulate significant amounts of proline already under moderate water deficit, what could explain the lower osmotic potential (ψ(s)) values observed in these plants. The three V. unguiculata cultivars presented a similar stomatal control under increasing water deficit. A mesophyllic impairment of photosynthetic capacity (P(max)) was detected for cv. 1183 from the beginning of drought onset (85-75% RWC) while in the Vg plants the values remained unaffected along the whole drought period, indicating that P(N) decrease observed in this genotype is mainly a consequence of stomatal closure. Such P(max) maintenance suggests the existence of a high mesophyllic ability to cope with increasing tissue dehydration in Vg.