The possible relationship of partitioning of photosynthate between starch and sucrose to the distribution of dry weight between the shoot and root was examined in relation to genetic variation and plant nutritional status. Soybean (Glycine max (L.) Merr.), tobacco (Nicotiana tabacum L.), and red-beet (Beta vulgaris L.) seedlings had low relative root weights and accumulated high levels of leaf starch, whereas the reverse was true for wheat (Triticum aestivum L.) seedlings. A similar relationship between starch formation and relative root weight was observed in comparisons among six peanut (Arachis hypogea L.) cultivars and with 'Ransom' soybean and two peanut cultivars exposed to a variety of nutritional treatments. The results suggested that carbon partitioned into leaf starch was preferentially utilized for growth of shoots at night. The postulate was supported by diurnal dry-weight changes of various parts of 'Ransom' soybean plants. Stems and expanding leaves increased in dry weight at about the same rate both during the day and night, whereas roots increased in dry weight during the day but during the night lost about 50% of the dry weight gain. Hence, of the total carbohydrate received by roots over a 24-h period, relatively more material was apparently obtained during the day. Overall, the results suggested that partitioning of carbon between starch and sucrose in the leaf may be one of the factors that affects the growth of shoots relative to roots.