In carob tree (Ceratonia siliqua) radiant energy saturated net photosynthetic rate (PN) during summer was about 10 % of the spring values. This was accompanied by a reduction in stomatal conductance (gs), which only partially explains the strong reduction in PN. Photosynthetic capacity (Pmax) and quantum yield (Φ), both measured under saturating CO2, had the maximum in spring (about 34 μmol m-2 s-1 and 0.08 mol mol-1, respectively) and both decreased in late summer to about 55 % of their spring values. Despite strong decreases in Φ, photoinhibition of photosystem 2 (PS2) was negligible or easily reversible in carob leaves subjected to summer drought, since Fv/Fm, measured in the morning, did not show appreciable changes. The recovery of affected parameters was very rapid after the first rains in late October. The chlorophyll (Chl) alb ratio in the end of the summer was 2.6, a value significantly lower than 3.6 obtained in the spring, suggesting that Chl a was preferentially reduced. and J. C. Ramalho, J. A. Lauriano, M. A. Nunes.
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 (gs) and photosynthetic activity (PN) 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 (ψs) 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 (Pmax) 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 PN decrease observed in this genotype is mainly a consequence of stomatal closure. Such Pmax maintenance suggests the existence of a high mesophyllic ability to cope with increasing tissue dehydration in Vg. and P. Scotti Campos ... [et al.].