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Export of ¹⁴C-Assimilates in Cotton Leaves¹

A study was made of the export of ¹⁴C-assimilate in mature leaves of ‘Stoneville 213’ cotton (Gossypium hirsutum L.). Fruiting branches and subtending leaves of field grown plants were pulse-labeled with ¹⁴CO² for 5 min at about mid-day. The amount of ¹⁴C-assimilate transported to the cotton boll from the subtending leaves, bracts or boll wall, as well as the amount of ¹⁴C-assimilate remaining in the leaves and bracts, were determined over a 22-hour period. During this translocation period, leaves and bracts export ¹⁴C-assimilate to the boll wall, ovules and lint; whereas the boll wall does not export ¹⁴C. assimilate. The leaves are the dominant source of assimilate for cotton bolls throughout the season.

There is a differential rate of export and retention of H₂O soluble and insoluble (starch) assimilate from the leaves and bracts. There is an initial rapid phase of transport of H₂O soluble compounds from both leaf and bract sources. About 80% of these compounds disappear 6 hours after exposin the sources to ¹⁴CO₂. After 22 hours, over 90% of this pool is depleted. In contrast, there is a retention of ¹⁴C in the starch fraction up to 6 hours after pulse-labeling the sources with ¹⁴CO₂. After 6 hours, there is a slow export of ¹⁴C-assimilate from the starch fraction stored in the sources. Cotton leaves export assimilate stored as starch slowly. At the end of 22 hours, 90 to 95% of the ¹⁴C-assimilate disappears from mature leaves and bracts.

Key Words: Translocation • ¹⁴C-photosynthate • Source-sink

² Professor of plant physiology, Dep. of Plant Sci., Tex. A&M U., College Station, TX 77843, and research geneticist, ARS, USDA, College Station, TX 77843.

Received for publication October 19, 1974.

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