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), whic h differ in their drought resistance, was compared. With the increase of dr ought severity, Vg showed a more gradual stomatal closure and maintained si gnificantly higher levels of stomatal conductance (g(s)) and photosynthetic activity (P-N) than the other genotypes even when minimum relative water c ontent (RWC) values were observed. Furthermore, Vg was the only genotype ab le to accumulate significant amounts of proline already under moderate wate r deficit, what could explain the lower osmotic potential (psi(s)) values o bserved in these plants. The three V. unguiculata cultivars presented a sim ilar stomatal control under increasing water deficit. A mesophyllic impairm ent of photosynthetic capacity (P-max) was detected for cv. 1183 from the b eginning of drought onset (85-75 % RWC) while in the Vg plants the values r emained unaffected along the whole drought period, indicating that PN decre ase 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.