Diurnal Changes in Photosynthesis in Sugarcane Leaves : I. Carbon dioxide exchange rate, photosynthetic enzyme activities and metabolite levels relating to the C_4 pathway and the Calvin cycle

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Diurnal changes in carbon dioxide exchange rate (CER), stomatal conductance (g_s), photosynthetic enzyme activities and metabolite levels relating to the C_4 pathway and the Calvin cycle in sugarcane (Saccharum sp. cv. NiF4) leaves were characterized during a natural 24 h day-night cycle. The activities of phosphoenolpyruvate carboxylase (PEPcase), NADF-malic enzyme (NADP-ME), pyruvate, P_1 dikinase (PPKD), ribulose-1, 5-bisphosphate carboxylase / oxygenase (Rubisco) and chloroplast fructose-1, 6-bisphosphatase (FBPase) all exhibited distinct diurnal changes. The levels of C_4 metabolites, malate, phosphoenolpyruvate (PEP) and pyruvate, showed large diurnal fluctuations, but the oxaloacetate levels were extremely low and exhibited no significant changes in the day-night cycle. Diurnal changes in CER and g_s paralleled the chages in radiation of sunlight. The results in this study suggest that at midday when CER is in steady-state and saturated by full sunlight, CER is limited by PPDK activity; under lower radiation of sunlight, I.e., before CER is saturated in the morning and during the period when CER is decreasing in the afternoon, CER is possibly limited by PEPcase activity or g_s.
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