Plant productivity and yield depend on the capacity of leaves to fix CO₂ during photosynthesis and on the capacity of sink tissues such as developing seeds or fruits to convert the fixed carbon into dry matter. The capacity to convert triose-phosphate to sucrose or starch in the leaf can limit the maximal rate of photo synthesis, and we have demonstrated that photosynthetic capacity and growth correlate with the capacity to make leaf starch. The findings presented here reveal that in Arabidopsis thaliana with increased activity of the major starch regulatory enzyme, ADPglucose pyrophosphorylase (AGPase), there is a greater capacity for starch biosynthesis compared to wildtype and a starch-deficient mutant. This resulted in increased CO₂ as similation due to suppression of photosynthetic feedback and in higher starch turnover rates in these plants. Collectively these effects translated to increased growth and seed biomass. Previous studies from our laboratory showed that rice is limited by the capacity to convert triose-P to carbohydrates under ordinary environ mental conditions. Therefore, productivity may benefit by increasing the conversion of triose-P into leaf starch through increased AGPase activity.