Physiological Mechanisms Affecting Seed-Set, Seed Growth, and Yield of Grain Crops to Elevated Extreme Temperatures.

General circulation models project that air temperatures will increase 2 to 4 ^oC as a consequence of greenhouse gas emissions over the next century.  While rising CO₂ is beneficial, the rising temperatures are anticipated to adversely affect crop growth and yield.  The effects of elevated temperature are more detrimental to reproductive processes than to vegetative growth.  In this paper, we review evidence of elevated temperature effects on reproductive processes and grain yield of several crops including rice, soybean, peanut, sorghum, common bean, and tomato.  Each of these crops has an optimum temperature for grain yield (or harvest index, HI) above which yield declines slowly at first because of shortened grain-filling duration, followed by a more rapid decline associated with increasing failures of grain-set as well as slower individual grain growth.  There is an upper failure point temperature at which grain yield and HI reach zero, attributed to the temperature at which grain-set fails caused by pollen non-viability.  Rice and sorghum have an optimum for yield at or below 25 ^oC mean temperature, with total failure at 35 ^oC.  For soybean and peanut, the optimum for yield is also at or below 25 ^oC, but the failure point temperature is 39-40 ^oC.  Common bean and tomato have a lower optimum for yield and also a lower failure point temperature at 32 ^oC mean.  The common denominator is pollen viability in all these crops, but evidence of cultivar variation in heat tolerance is sparse (there is some variation in rice and peanut).  Poorly formed and non-viable pollen occurs as a result of elevated temperatures during the 8-10 day pollen-formation phase prior to flower opening and the day of anthesis, as shown in stage-dependent heat stress on peanut, soybean, and sorghum.  Once fertilized, the ovules are relatively more tolerant of the elevated temperature.