Effect of water stress on pyruvate, P-i dikinase and phosphoenol pyruvate carboxylase activities in the leaves of two cultivars of sorghum (Sorghum bicolor L)

The effect of withholding water on the specific activities of pyruvate, orthophosphate dikinase (PPDK, EC 2.7.9.1.) and phosphoenol pyruvate carboxylase (PEPC, EC 4.1.1.31) was studied in two cultivars of Sorghum bicolor L. different in their sensitivity to water deficit. The drought sensitive cultivar ICSV 1063 and the more resistant cultivar MIGSOR were subjected to water deficit in controlled conditions. The results showed a decrease in both enzyme specific activities, higher for drought sensitive cultivar ICSV 1063, when leaf water potential (psi) was lowered to -2.3 MPa. Following rewatering, enzyme specific activities increased in both cultivars, with the increase being more significant in MIGSOR. Relative water content (RWC) decreased significantly for water-stressed ICSV 1063 cultivar, lower for MIGSOR and returned quickly to that of control plants upon rehydration. Net photosynthesis showed a decrease for water-stressed plants, higher for ICSV 1063. At minimal psi, net photosynthesis was completely inhibited, with the stomata being closed. After rehydration, MIGSOR showed a better recovery in photosynthesis but never reached the initial values of day 0. Water stress had a striking effect, both on net photosynthesis by regulation of stomatal aperture and on PPDK and PEPC activities, although the enzymes were still active when photosynthesis ceased. Therefore, the level of PPDK and PEPC activities may contribute to the limitation of photosynthetic carbon dioxide fixation.