RESPONSES TO WATER-STRESS OF ENZYME-ACTIVITIES AND METABOLITE LEVELS IN RELATION TO SUCROSE AND STARCH SYNTHESIS, THE CALVIN CYCLE AND THE C-4 PATHWAY IN SUGARCANE (SACCHARUM SP.) LEAVES

During a slowly induced water stress, almost all measured activities o f 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 w ater stress (above -0.9 MPa leaf water potential (Psi(W))), and the ma gnitudes of reductions in those parameters were less than the reductio ns 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 photosynthesi s is caused by stomatal closure. Under severe water stress (-1.2 MPa l eaf Psi(W)), most metabolite levels and enzyme activities decreased si gnificantly compared with those under mild water stress. But the enzym e activities and metabolite levels relating to sucrose and starch synt hesis, and the Calvin cycle still remained at high levels compared wit h the corresponding photosynthetic rate. PPDK activity and pyruvate co ntent decreased to very low levels. It is suggested that PPDK is a pos sible limiting enzyme for photosynthesis in leaves of sugarcane under severe water stress.