Elevated temperature effects on functional yield drivers in cotton

Gossypium hirsutum L. (cotton) yields are limited by heat stress. Breeding for heat tolerance is an important goal, yet the most important functional contributors to heat-induced yield loss must first be defined. Although Monteith defined crop yield decades ago as the product of intercepted photosynthetically active radiation, radiation use efficiency (RUE), and harvest index (HI), a review of heat stress effects on these functional yield drivers or their underlying processes is still needed for cotton. Heat stress is known to affect canopy development, which could impact radiation interception. Specifically, leaf area declines when plants are exposed to maximum air temperatures greater than 35°C. However, direct evidence for a yield-limiting decline in intercepted radiation under high temperature is lacking. Though information is limited in peer reviewed literature, RUE (photosynthetic efficiency of the canopy) may be an important contributor to heat-induced yield loss, and this review explores the heat sensitivity of RUE and underlying photosynthetic processes. HI is the ratio of economic yield to biological yield, and current evidence indicates that reproductive processes are more heat sensitive than vegetative growth processes. Therefore, lower HI is discussed as a potentially significant contributor to heat-induced yield loss, and the relative heat sensitivity of reproductive processes and specific yield components is explored in detail. In the future, there is a need for (1) field studies addressing the temperature responses of canopy-scale yield drivers, (2) further exploration of nighttime temperature effects, and (3) targeted selection for increased heat tolerance of physiological yield drivers.