In rice, under low light condition, the height of the plant increases in comparison to the plants in the normal light environment. Under low light stress, the rate of photosynthesis(P) decreases. As the height of the plant is more so the food the plant produces is utilized for sustenance, instead of grain filling. So if the flag leaf is isolated at the flowering stage for the estimation of total soluble sugar(TSS) then (P) will be always positively (significantly)correlated with TSS. But as the TSS not promptly converted to starch under low light condition, there will be a non-significant correlation between the (P) and starch in the grain. However, under the normal light, the (P) will be positively correlated with TSS in the leaves at flowering stage and starch in the grain.
IS THIS ANALYSIS CORRECT?
This question is related to carbohydrate partitioning and sink-source relationship in plant, so you may want to search on those topics. Generally speaking, it depends on whether the plant is annual or perennial. Rice is an annual plant, therefore it has the tendency to give everything it gets to the seeds. Under low light condition, you will notice that seed weight is reduced much less than vegetative part as compared to normal light condition. As such, the correlation between seed weight (mostly starch of course) and PPF in low light condition in rice will be weak. Perennial plants, on the other hand, will sacrifice the fruits or seeds under low light condition to ensure the survival of the plants to wait for another, more favorable year. Therefore in these plants, there is strong correlation between PPF and fruit/seed yield. Under too low light condition, you may not even have any fruit/seed at all.
The correlation between leaf sugar and PPF is another story. This usually depends on the nature of each plant species to buffer its leaf sugar content. In many plants such as tomato, the leaf sugar content is buffered by leaf transitory starch, and as PPF varies, the transitory starch content will vary accordingly, but leaf sugar content will change at a much lesser extent. Other plants such as lettuce, and I believe rice also, do not emphasize on this buffer mechanism, and as such you will see a much clearer correlation between PPF and leaf sugar.
So, in short for your question, for rice, PPF to leaf sugar: yes, PPF to grain starch: not so.
Please refer to publications co-authored by Mark Stitt, you will find plenty of information!
Check this link
Article Effect of Leaf Sugar and Starch Concentrations on Apparent P...
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