The responses of carbon exchange rate (CER), stomatal conductance (gs), 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 (Chl) 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, gs, activities of the photosynthetic enzymes and contents of Chl and Tsp were observed from leaf water potentials (Ψw) of -0.37 MPa. During water stress, CER and gs, decreased in a non-linear way, activities of the five enzymes and contents of Chl and Tsp decreased linearly with decreasing leaf Ψw. The changes of gs 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 non- stomatal limitations. Above leaf �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-stornatal 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 Ψw suggesting that PPDK is very likely to be the limiting enzyme to photosynthesis under water stress.

The aim of this study was to investigate the mechanism of photosynthetic changes in sugarcane leaves in response to chilling temperature by using three species (Saccharum sinense R. cv. Yomitanzan, Saccharum sp. cv. NiF4 and Saccharum officinarum L. cv. Badira) differing in origin and cold sensitivity. Yomitanzan is native to subtropical areas, Badira is native to tropical areas and NiF4 is a hybrid species containing genes of both tropical and subtropical species. At exposure to chilling temperature (10°C), the photosynthetic rate in the leaves at either 10°C or 30°C showed a greater decrease in Badira than in NiF4 and Yomitanzan. After 28 h exposure of plants to the chilling temperature, the extractable activities of pyruvate, orthophosphate dikinase (PPDK) and NADPmalate dehydrogenase (NADP-MDH) increased or were relatively stable in the leaves of NiF4 and Yomitanzan, but decreased substantially in Badira. Correspondingly, there was a substantial accumulation of aspartate, and the level of alanine increased in Badira leaves during the chilling treatment. It is suggested that NADP-MDH and PPDK are key enzymes which may determine the cold sensitivity in photosynthesis of sugarcane.

During a slowly induced water stress, almost all measured activities of enzymes, including the important enzymes associated with the Calvin cycle, the C4 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 (Ψw)), 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 C4 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 Ψw), 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.