Yasuor, H., RH Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, Rehovot, 76100, Israel
Riov, J., RH Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, Rehovot, 76100, Israel
Rubin, B., RH Smith Institute of Plant Sciences and Genetics in Agriculture, Faculty of Agricultural, Food and Environmental Quality Sciences, Rehovot, 76100, Israel

Glyphosate-resistant cotton (GRC) was introduced experimentally to Israel in 1998. Field and laboratory studies were conducted in 1998-2000 to evaluate the effect of the timing and rates of glyphosate application on cotton development and fruit set. No damage to the vegetative parts of cotton plants was observed when glyphosate was applied over-the-top (OTT) at all growth stages examined. However, glyphosate applied OTT, at the 8-10 leaf stage (late OTT), caused severe damage to the reproductive parts. Cotton grown in different climatic regions responded differently to late OTT application of glyphosate, indicating a strong interaction with environmental conditions. Late OTT application resulted in a development of abnormal flowers, non-dehiscent anthers containing irregularly shaped and less viable pollen grains. In contrast, the stigma and other female organs of treated plants were functional and did not show any injury symptoms. Male-sterile flowers usually produced deformed bolls with one or more degenerated compartments leading to a "moon" or "beak" shape bolls. Bolls developed on the early fruiting branches (5th-10th) of late OTT-treated plants were smaller as compared to those developed at the same position on untreated plants. In general, the cotton plants succeeded to compensate for the early damage caused by glyphosate by producing additional bolls on the upper internodes. These additional bolls did not reach maturity until harvest time. In spite of the visible injury to flowers and bolls, late OTT application did not cause significant yield reduction. Combined late and very late (12-13 leaf stage, before cotton canopy closes) glyphosate application resulted in a significant yield reduction. Further studies are in progress to elucidate the mechanism involved in the glyphosate-induced male sterility in transgenic cotton. © 2006 Elsevier Ltd. All rights reserved.