Agronomic differences in growth and yield between BT and conventional cotton treated with mepiquat chloride

Abstract

Cotton (Gossypium hirsutum L.), an indeterminacy plant, may exhibit rank vegetative growth under conditions of high fertility and high moisture. Traditionally, mepiquat chloride (l,l-dimethylpiperidinium chloride) has been applied to cotton for suppressing vegetative growth. Mepiquat chloride (MC) blocks the biosynthesis of the hormone gibberellic acid. Applications of MC produce plants that are consistently shorter than control plants due to decreased stem elongation. Cotton cultivate containing a gene that encodes an insecticidal protein were released commercially during 1996. The gene, transferred from the bacterium Bacillus thuringiensis, proved especially effective in reducing cotton damage caused by lepidopteran insects. However, Bt cotton cultivate exhibit rank vegetative growth. Current crop simulation models suggest that ten to twelve mg MC per kg of biomass is required for reducing main stem elongation in conventional cotton. However, Bt cultivate may behave differently toward MC than conventional cultivars. To test this hypothesis, a field experiment was conducted at the Texas Agricultural Experiment Station--Corpus Christi during 1997 and 1998. The factorial field experiment, which consisted of two cultivate and five rates of MC, was arranged as a randomized complete block with four replications. The cultivars planted included a Bt cultivar(DPL 33[]) and the same cultivar without the Bt gene (DPL 5415). MC was applied to the cotton near the 12-node stage at 15.3, 30.6, 45.9, and 61.2 g a.i. ha⁻¹. Data collected in the experiment included plant height, node number, dry weight, yield, and plant mappings. Data were analyzed using regression, ANOVA, LSD, and ANCOVA. Significant differences in plant height were observed between the different MC rates. The control plants were the tallest, followed by plants treated with the 15.3, 30.6, 45.9, and 61.2 g a.i. ha⁻¹ rates, respectively. No significant differences in the growth rate and height were detected between the Bt and the conventional cultivars. The MC concentration-height reduction models developed in this study were similar to an existing MC concentration-height reduction model. Thus, Bt cultivate behave similarly to applications of MC with respect to vegetative growth as conventional cultivate. Therefore, current MC application strategies should suffice for suppressing vegetative growth of Bt cotton.