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Effects of a Phosphate Defoliant on Cotton Leaves: Water Relations and Free Amino Acids¹

The phosphate defoliant DEF (S,S,S-Tributylphosphorothrithioate) was sprayed on presquaring greenhousegrown cotton (Gossypium hirsutum L.) plants. Plant water status and leaf area data were taken at 3-hour intervals during the 1st day and daily thereafter. Stomatal diffusion resistance was measured during the 1st day only. Leaves were analyzed for total N, soluble N, and distribution of free amino acids. Little change in any of the parameters measured was noted the 1st day. On subsequent days the DEF-treated leaves had lower leaf water potential recovery values (predawn measurement) and lower specific weights (fresh weight basis). DEFtreated leaves, however, had a higher specific leaf weight on a dry weight basis. Leaf N percent did not change as a result of the DEF treatment. Soluble N increased up to twice that of the controls by the 3rd day. The distribution of free amino acids in DEF treated leaves was altered. The concentrations of all amino acids except aspartic acid, glutamic acid + glutamine, proline and valine were significantly increased in the DEF treated leaves. The exceptions remained stable or tended to decrease. The results indicated that DEF-treated leaves may be mobilizing the leaf protein prior to abscission, but that transport out of the leaf is impeded. Further experiments are needed to substantiate this notion.

Key Words: Gossypium hirsutum L. • DEF • Asparagine • Soluble N

² Plant physiologists, SEA, USDA, Stoneville, MS 38776, Mississippi Agric. and For. Exp. Stn. Cooperating.

Received for publication September 17, 1977.