Yield and Reproductive Development of Soybean As Affected By Elevated Temperature Stress During Different Phases of the Life Cycle.

With the greenhouse effect, periods of elevated temperature as well as generally higher air temperature are anticipated.  Our objective was to evaluate how continuous as well as periods of elevated temperature affected reproductive phenology, pod-set, yield, and seed traits of soybean.  Three experiments were conducted on Maverick soybean (MG 3) grown to maturity in pots in temperature-controlled greenhouses at temperatures of 30/22, 34/26, 38/30, and 42/34 °C (12 h day/night thermoperiods) at either 21 or 32% oxygen.  The CO2 level was 700 ppm to reduce photorespiration.   In a fourth experiment, treatments were exposed for 10-day periods at 38/30 or 42/34°C at four growth stages, prior to moving back to 30/22°C.  Reproductive and vegetative stages were observed, and final biomass, pod yield, seed yield, harvest index, and seed size were determined.   Oxygen (32%) had minimal effects beyond minor reduction in biomass and seed yield, presumably because photorespiration increased.  At 38/30°C, anthesis, pod-set, and maturity were delayed 7 to 8 days, while node formation and vegetative growth were prolonged.  At 42/34C, flowering was delayed and no pods formed since heat stress interfered with pollen development, resulting in extensive vegetative growth.  Biomass, pod yield, and seed yield responses will be discussed.  For the fourth experiment, pod numbers were recorded non-destructively at weekly intervals which documented that 10-day periods of elevated temperature (42/34°C) applied prior to flowering and at flowering caused approximately 10- to 14-day delays in onset of pod formation as well as the time of peak pod number.  Soybean showed remarkable compensation abilities for 10-day heat stress periods.