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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 14C‐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 14C‐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.

Growth chamber experiments were conducted to elucidate the morphological and physiological responses of rice to postemergence application of fenoxaprop. Two rice cultivars, ‘Newbonnet’ and ‘Mars’, and barnyardgrass were treated with 0.17 kg ai ha−1fenoxaprop at the five-leaf stage. Within 2 days of fenoxaprop application, rice cultivars developed a white chlorotic band across leaves that were in direct contact with spray droplets. Leaf elongation rates for the two rice cultivars were inhibited by 40% after 4 days and by over 50% after 14 days. Inhibition of leaf elongation by fenoxaprop contributed to an overall decrease in leaf area and shoot dry weight. Net photosynthesis was reduced by 35% in fenoxaprop-treated plants 11 days after application, although stomatal conductance was not affected. Nitrogen fertilization prior to fenoxaprop application increased foliar injury of both rice cultivars. Visible injury symptoms showed the following order of susceptibility to foliar-applied fenoxaprop: barnyardgrass > Mars rice > Newbonnet rice.

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