Starch accumulation rates increased up to five-fold in the leaves of five out of six species examined when plants were shifted from a long (12 or 14-h) to a short (7-h) photosynthetic period. The five species that responded were corn (Zea mays L.), pangola (Digitaria decumbens Stent.), soybean (Glycine max L.) Merr.), spinach (Spinacia oleracea L.), and sugarbeet (Beet vulgaris L.) However, the rate of starch accumulation in alfalfa (Medicago sativa L.) was unaltered following a shift from a long to a short photosynthetic period. We conclude that photosynthate partitioning into chloroplast starch, at least in a wide variety of species, is a programmable process that can be manipulated by altering the length of the daily photosynthetic period.