Reduction of grain yield in rice shaded during the early grain filling period is not the result of inhibition of potential grain dry-matter increase

A shortage of carbon assimilate supply to rice grain during the first ten d ays after heading (DAY(0-10)) has been considered to reduce the potential g rain dry-matter increase(Delta GP). It was, however, uncertain whether assi milate supply after the shading period was sufficient to support Delta GP w hen the reduction of grain dry-matter increase (DeltaG) was observed. Our o bjective was to demonstrate that DeltaG after the shading treatments was re duced not through the decrease in Delta GP but through a lack of assimilate supply, based on our model analysis. In our model, DeltaG is determined by assimilate supply (current dry-matter production (DeltaW) plus reserved as similate in straw (SP)) and the DeltaG has an upper limitation defined by D elta GP. In this study Delta GP was estimated under a sufficient supply of assimilates which were provided by periodical thinning of the plants. By th e shading during DAY(0-10), DeltaG as well as DeltaW were reduced even afte r the end of the shading treatment. Observed DeltaW and SP at the heading w ere input to the model to calculate DeltaG and straw dry-matter increase (D eltaS). The calculated dry weight of the grain and straw coincided closely with the actual observed data. We herewith suggest that the grain dry-matte r increase by shading during DAY(0-10) was not suppressed by inevitable red uction of Delta GP but rather by a lack of assimilate supply during the act ive DeltaG period.