Responses to water stress of enzyme activities and metabolite levels in relation to sucrose and starch synthesis, the Calvin cycle and the C4 pathway in sugarcane (Saccharum sp.) leaves

During a slowly induced water stress, almost all measured activities of enzymes, including the important enzymes associated with the Calvin cycle, the C-4 pathway, and sucrose and starch synthesis, and the pool sizes of metabolites, including hexose phosphates, 3-phosphoglycerate, triose phosphates, malate, pyruvate and PEP, in leaves of sugarcane (Saccharum sp. cv. NiF4) were not or only moderately reduced by mild water stress (above -0.9 MPa leaf water potential (Psi(W))), and the magnitudes of reductions in those parameters were less than the reductions in photosynthetic rates. We conclude that the biochemical processes of sucrose and starch synthesis, the Calvin cycle and the C-4 pathway in sugarcane leaves were not seriously affected by mild water stress, and the changes in those processes were not the cause for the decline in photosynthesis; mild water stress induced decline in photosynthesis is caused by stomatal closure. Under severe water stress (-1.2 MPa leaf Psi(W)), most metabolite levels and enzyme activities decreased significantly compared with those under mild water stress. But the enzyme activities and metabolite levels relating to sucrose and starch synthesis, and the Calvin cycle still remained at high levels compared with the corresponding photosynthetic rate. PPDK activity and pyruvate content decreased to very low levels. It is suggested that PPDK is a possible limiting enzyme for photosynthesis in leaves of sugarcane under severe water stress.