EARLY-SEASON ETHEPHON APPLICATION EFFECTS ON COTTON PHOTOSYNTHESIS

Cotton (Gossypium hirsutum L.) can compensate for a limited amount of early floral bud (square) loss induced by early ethephon ethephon [(2-chloroethyl)phospbonic acid] application, but ethephon has been found to alter the photosynthetic process in many species. A field study was conducted in 1989 and 1990 to determine how early season ethephon application affected cotton photosynthesis throughout the growing season. Early season squares were removed by hand or induced to abscise with a spray application of 0.28 kg ha-1 ethephon. Dry matter harvests were collected and light-saturated leaf CO2 exchange rates (CER) measured periodically throughout both growing seasons. Concentrations of leaf chlorophyll, soluble protein, ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco), and thylakoid proteins CPI and light harvesting complex along with the specific leaf weights were periodically determined on leaves tested for CER. The net assimilation rate was 13 to 17% lower in the ethephon-treated plots directly after treatment application in both years. In 1990, leaf CER was 15% lower in the ethephon-treated plants than either the check or hand-removal treatments 2 d after treatment application, but was 5% greater than the two other treatments 13 d later. Stomatal conductance of the ethephon-treated plants was 23% lower during the period of lower CER and net assimilation rate, but the internal CO2 concentration was unchanged, suggesting depressed photosynthetic activity in the chloroplasts, in addition to stomatal closure. Other than the 1- to 2-wk period after treatment application, CER was unaffected by treatments. Perhaps superior carbohydrate partitioning or remobilization is more important than elevated photosynthesis in compensating for early square loss.