EFFECTS OF PANICLE REMOVAL ON THE PHOTOSYNTHETIC CHARACTERISTICS OF THE FLAG LEAF OF RICE PLANTS DURING THE RIPENING STAGE

All panicles were removed from rice plants (Oryza saliva L.) at anthesis and the effects of this treatment on photosynthetic gas-exchange rates and the underlying biochemical properties of the blade of the flag leaf were examined during senescence. Panicle removal retarded the decrease in photosynthesis in the flag leaf during senescence. Similarly, the levels of ribulose-1,5-bisphosphate carboxylase (Rubisco), chlorophyll and Cyt f, and the activities of sucrose-phosphate synthase (SPS) and ADP-glucose pyrophosphorylase in panicle-free plants remained relatively high during leaf senescence and decreased more slowly than those in the control plants. Regression analysis showed no difference between the treated and untreated plants in the relationship between CO2-limited photosynthesis (measured at an intercellular CO2 pressure of 15 Pa) and Rubisco content, or in the relationships between CO2-saturated photosynthesis (measured at an intercellular CO2 pressure above 60 Pa), Cyt f content and SPS activity. These results indicated that the change in photosynthesis caused by panicle removal at anthesis could be explained by the changes in the amounts and/or activities of the major biochemical participants in photosynthesis. The temporary accumulation of starch and sucrose in the flag leaf was observed in the panicle-free plants, but the accumulation was not significant. However, in the panicle-free plants, the weight of shoots, excluding panicles, increased by 200% and that of roots increased by 150% at the harvesting stage. Thus, it appears that, since the effective translocation of photosynthate to other organs, such as late tillers and roots, can occur, the removal of panicles has no effect on the relationship between the rate of photosynthesis and the levels of the major biochemical participants in photosynthesis.