Water Deficit Effects on the Cotton Leaf Cuticle and the Efficiency of Defoliants

Inconsistent results are often experienced with chemical defoliants in cotton ( Gossypium hirsutum L.). Leaves of field‐and growth chamber‐grown plants were used in a 3‐yr study to evaluate the hypothesis that drought‐induced alterations in the leaf cuticle may be associated with the decreased efficiency of defoliants. Plants which experienced previous water stress had reduced defoliation and increased regrowth when treated with the commercial defoliant Harvade [dimethipim (2,3 dihydro‐5,6‐dimethyl‐1,4‐dithiin‐1,1,4,4‐tetraoxide) Uniroyal Chemical Co., Middlebury, CT]. Water deficit increased leaf cuticle thickness by 33%, altered the epicuticular wax composition, and increased total wax content. Chloroform extracts of the leaf cuticle indicated that water‐stressed leaves had increased concentrations of higher molecular weight waxes which increased the hydrophobicity of the leaf surface. Although the 14 C‐labelled defoliant Harvade penetrated the leaf under both water regimes, 34% less defoliant entered the previously stressed leaves. The thicker, more hydrophobic cuticles of water‐stressed leaves decreased the penetration of the 14 C‐labelled defoliant and this was reflected in the decreased defoliation efficiency. These results suggest that seasonal inconsistencies in defoliant efficiency may be related to the irrigation history of the crop. In cotton growing areas receiving inadequate rainfall or irrigation, defoliant rates may need to be increased or the choice of spray adjuvant changed.