Differences in levels of nutrients of mepiquatchloride treated cotton have been reported for controlled environment and greenhouse studies, but information from field studies Is lacking. The objectives of this research were 1) to determine the effect of foliar applications of mepiquat chloride (N,N-dimethyl piperidinium chloride), B, and urea on petiole NO₃-N, leaf N content, and selected nutrient levels (Ca, Mg, K, and PI and 2) to examine specific leaf weight and yield relationships to these treatments of 'Stoneville 825' cotton grown under field conditions. Treatments, arranged in a randomized complete block design, consisted of a nontreated control, MC (49,4 g ha^-1) applied at early bloom, four applications of B (0.224 g ha^-1) initiated at the 6-leaf growth stage, urea applied as determined by petiole NO₃ monitoring, and all combinations of MC, B, and urea. The fourth leaf from the terminal (fully expanded) was sampled throughout the study to maintain consistent physiological age of the tissue sampled.

Significant increases were obtained in specific leaf weight of MC-treated tissue over the non-treated control within 2 weeks of treatment that remained until late in the season. A significant treatment by date interaction for leaf nutrient levels was not observed. However, MC treatment increased levels of Ca (6.2%) and Mg (7.9%). decreased, levels of P (5.6%), and had no effect on K or N levels over levels in nontreated cotton leaf tissue, Patterns for leaf N and P were strikingly similar. Over years, petiole NO₃-O levels of MC-treated cotton were numerically higher than the controls. As the season progressed, the levels of petiole NO₃ -N in MC treated cotton tended to become more pronounced. Lint yield of treated cotton was increased by 9.5% (116 kg ha^-1) over nontreated cotton (1983-1986).