EFFECTS OF SOURCE-TO-SINK RATIO ON CARBOHYDRATE PRODUCTION AND SENESCENCE OF RICE FLAG LEAVES DURING THE RIPENING PERIOD

Partial removal of spikelets and partial defoliation were conducted in rice at anthesis to evaluate the effects of the source-to-sink ratio (ratio of leaf area to grain number). The ratio was 0.75 in the control plants and ranged from 0.24 to 1.53 for the various treatments. The apparent photosynthetic rate was not influenced significantly by the treatments 12 days after anthesis. Contents of chlorophyll and Rubisco decreased substantially 3 weeks after anthesis, but remained relatively high when the sink size was reduced. Three weeks after anthesis, both the chlorophyll and Rubisco contents were positively correlated with the source-to-sink ratio (P<0.01). Therefore, it appears that a large sink size (i.e., smaller source-to-sink ratio) promotes flag leaf senescence. However, no significant differences in the enzyme activities related to sucrose synthesis were observed among the treatments. In spite of the large differences in sucrose and starch contents in leaf sheath and culm among tile treatments, the trends of change in sucrose and starch contents in the flag leaf blades during the ripening period were similar. Thus, the ability of the flag leaves to synthesize sucrose was not greatly reduced during senescence. The nitrogen content of flag leaf blade was negatively correlated with sink size. These results. suggest that the difference in leaf senescence under various source-to-sink ratios may be associated with the different rates of nitrogen withdrawal from leaf blades to grains.