Effects of water stress on carbon exchange rate and activities of photosynthetic enzymes in leaves of sugarcane (Saccharum sp)

The responses of carbon exchange rate (CER), stomatal conductance (g(s)), activities of phosphoenolpyruvate carboxylase (PEPcase), NADP malic enzyme (NADP-ME), ribulose-1,5-bisphosphate carboxylase (Rubisco), fructose-1,6-bisphosphatase (FBPase) and pyruvate, orthophosphate dikinase (PPDK), and contents of chlorophyll (Chi) and total soluble protein (Tsp) in leaves of sugar cane (Saccharum sp, cv. NiF4) to gradually developed water stress were investigated. The initial inhibitions of CER, g(s), activities of the photosynthetic enzymes and contents of Chl and Tsp were observed from leaf water potentials (Psi(W)) of -0.37 MPa. During water stress, CER and g(s) decreased in a non-linear way, activities of the five enzymes and contents of Chi and Tsp decreased linearly with decreasing leaf Psi(W). The changes of g(s), the photosynthetic enzymes, Chl and Tsp were highly related to the changes of CER. The decline in CER during water stress was caused by both stomatal and nonstomatal limitations. Above leaf Psi(W) of -0.85 MPa, the decline in CER was caused by stomatal closure, below -0.85 MPa, the decline in CER was caused by non-stomatal limitation. Among non-stomatal components, PPDK activities decreased 9.1 times during water stress, much more than other enzymes which decreased from 2 to 4 times. Measured PPDK activities were only a little higher than the corresponding CER values at various leaf Psi(W), suggesting that PPDK is very likely to be the limiting enzyme to photosynthesis under water stress.