Patterns of carbohydrates and carbohydrate enzymes were investigated in developing cotton ovules and leaves to establish which of these might be related to control of carbon export in leaf tissue and sink strength in developing cotton bolls. Enzymatic analysis of extracts from tissues indicated that during the first weeks of boll development sucrose synthase (EC 2.4.1.13) in tissues surrounding the embryos of immature cotton (Gossypium hirsutum L. cv. Coker 100A glandless) seeds rapidly degraded incoming sucrose. During this period of development these tissues contained substantial amounts of hexose but little sucrose; the reverse was true for embryo tissues. Starting one week after anthesis, starch was found to accumulate in ovule integuments. Two weeks later, this starch began to disappear and starch started to appear in the embryo. Galactinol synthase (EC 2.4.1.x), an enzyme necessary for galactoside synthesis, appeared in developing cotton embryos approximately 25 days after anthesis. Starting five weeks after flowering, embryo starch was degraded and embryos began to accumulate the galactosides raffinose and stachyose.

Mature cotton leaves were found to contain both sucrose synthase and sucrose phosphate synthase (EC 2.4.1.14) at comparable activities. The activity of both of these enzymes were found to cycle diurnally in mature cotton leaves. This is quite unlike their behavior in leaves of other crop plants where sucrose synthase activity is quite low in mature leaves and its activity does not vary diurnally. Leaf sucrose phosphate synthase activity in cotton was found to be proportional to leaf carbon export during daylight hours. Cotton leaf sucrose builds up during daylight hours and gradually disappears after the onset of darkness. Glucose exhibits little diurnal variation and free fructose is present only in trace amounts in cotton leaves.

Constant specific activity radiolabeling experiments showed that cotton leaves export sucrose derived from photosynthesis during daylight hours and sucrose derived from the degradation of starch during the darkened period of the day. As in many plants, the amount of starch in cotton leaves is strongly influenced by environmental factors, such as nutrition and water stress. In order to determine seasonal patterns in cotton leaf starch, leaves were sampled from a drip-irrigated plot in which plants were irrigated daily with water equal to pan evaporation to minimize water stress effects. The results showed a strong decrease in both AM and PM leaf starch content as the number of active bolls (i.e., <40 days post anthesis) per meter of row increased. These values decreased in parallel, such that their difference (i.e., the amount of carbon exported from the leaf during the night) was relatively constant. However, after the number of active bolls per meter reached a maximum and began to decrease, diurnal cycling of starch in these leaves ceased for three weeks. This cessation of starch cycling could not be explained by leaf aging due to plant 'cutout,' since leaves of comparable age have been shown to cycle starch diurnally, but at reduced rates. This observation suggests that boll load and leaf starch cycling are related in cotton. However, It also poses several questions: if nocturnal export of carbon from leaf starch degradation represents the major source of carbon for nocturnal metabolism, what supplies carbon for this purpose during this period? Is this cessation of leaf starch cycling related to the observation that cotton plants grow very little during cutout?