DIURNAL REGULATION OF LEAF BLADE ELONGATION IN RICE BY CO2 - IS IT RELATED TO SUCROSE-PHOSPHATE SYNTHASE ACTIVITY

The relationship between leaf blade elongation rates (LER) and sucrose-phosphate synthase (SPS) activity was investigated at different times during ontogeny of rice (Oryza safiva L. cv Jarrah) grown in flooded soil at either 350 or 700 mu L CO2 L(-1). High CO2 concentrations increased LER of expanding blades and in vivo activity (V-limiting) SPS activity of expanded blades during the early vegetative stage (21 d after planting [DAP]), when tiller number was small and growing blades were strong carbohydrate sinks. Despite a constant light environment, there was a distinct diurnal pattern in LER, V-limiting SPS activity, and concentration of soluble sugars, with an increase in the early part of the light period and a decrease later in the light period. The strong correlation (r = 0.65) between LER and V-limiting SPS activity over the diurnal cycle indicated that SPS activity played an important role in controlling blade growth. The higher V-limiting SPS activity at elevated CO, at 21 DAP was caused by an increase in the activation state of the enzyme rather than an increase in V-max. Fructose and glucose accumulated to a greater extent than sucrose at high CO2 and may have been utilized for synthesis of cell-wall components, contributing to higher specific leaf weight. By the mid-tillering stage (42 DAP), CO2 enrichment enhanced V-limiting and V-max activities of source blades. Nevertheless, LER was depressed by high CO2, probably because tillers were stronger carbohydrate sinks than growing blades.