MEPIQUAT CHLORIDE AND TEMPERATURE EFFECTS ON PHOTOSYNTHESIS AND RESPIRATION OF FRUITING COTTON

The bioregulator, mepiquat chloride (1,1-dimethylpiperidinum chloride), consistently suppresses rank growth of cotton (Gossypium hirsutum L.), but yield responses are erractic. The objective of this study was to determine cotton plant response to mepiquat chloride (MC) at different temperatures, so that appropriate functions can be provided to the cotton crop simulation model. Cotton plant foliage was sprayed with MC at initial bloom, and maintained at four different day/night temperatures for 49 d. Carbon dioxide fluxes were measured continuously, and used to calculate photosynthetic and respiration rates. Plant components and leaf area were measured at the end of the treatment period. Mainstem growth rates and node addition rates were reduced by MC application at all temperatures. Gross photosynthesis increased linearly when the ambient air temperature increased to 30-degrees-C and declined at 40-degrees-C. Mepiquat chloride increased the canopy gross photosynthesis (Pg) within 48 h, suggesting a direct effect on photosynthesis. The increased Pg established on Day 2 persisted throughout the season at various temperatures, but the differences decreased at the end of the season. Respiration rates increased linearly as air temperature increased and decreased with plant age when expressed on unit weight basis. Biomass yields of cotton plants harvested 49 d after treatment were closely related to net photosynthetic assimilation accumulated over the same period. We conclude that MC and temperature impact cotton growth, development, respiration, and photosynthetic rates. This information will be useful for predicting optimum timing and amounts of MC application for improved managements of cotton canopies.