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

Abstract

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.

Sugarcane Response to Water-Deficit Stress during Early Growth on Organic and Sand Soils

“…Significant increase in Ci for the WS plants on sand soil (Fig. 2c) between 20 and 25 days in 2009 or from 10-17 days in 2010 after initiating the WS treatment suggested that under severe water stress, decreased leaf Pn was mainly caused by non-stomatal limitation, such as photoassimilate enzyme activities (Du et al, 1998).…”

Diurnal Changes in Photosynthesis in Sugarcane Leaves: II. Enzyme activities and metabolite levels relating to sucrose and starch metabolism

“…Determinations of metabolites and carbohydrates Triose-P, glucose-6-phosphate, glucose-1-phosphate, fructose-6-phosphate and FBP were extracted and determined as described earlier (Du et al, 1998). Sucrose, glucose, fructose and starch were extracted as before (Du et al, 1998), and determined photometrically using a modified enzymatic method Qones et al, 1977). The assay mixture ( 1.0 mL) contained 100 mM imidazo1-HC1 of pH 6.9, 0.5 mM NADP, 1.0 mM ATP, 5.0 mM MgC1 2 , 2 units of glucose-6-phosphate dehydrogenase and 20 J1.…”

“…The extraction and assays of SPS, cytosolic FBPase, UDPG-PPase and ADPG-PPase were conducted as described earlier (Du et al, 1998), with SPS activity measured in an assay mixture without inhibitor orthophosphate.…”

Diurnal Changes in Photosynthesis in Sugarcane Leaves: I. Carbon dioxide exchange rate, photosynthetic enzyme activities and metabolite levels relating to the C4pathway and the Calvin cycle

“…The extraction and determinations of the metabolites were carried out as described previously (Du et al, 1998). Figure IA shows the diurnal patterns of PAR of sunlight and CER in sugarcane leaves.…”