Escape and tolerance to high temperature at flowering in groundnut (Arachis hypogaea L.)

Groundnut is an important crop of the semi-arid tropics where potential yields are frequently reduced by heat and water stress. Eight groundnut genotypes varying in heat tolerance were grown in controlled environments and exposed to either high (40/28 degreesC) or near-optimum (30/24 degreesC) temperature from 32 days after sowing (DAS) to maturity. There was significant variation among genotypes in mainstem leaf number and total flower number at 30/24 and 40/28 degreesC and rates of appearance were faster at 40/28 degreesC than at 30/24 degreesC. Days from sowing to first flowering varied among genotypes from 28 to 41 days and therefore the time plants were exposed to high temperature relative to first flowering ranged from -4 to 9 days. Fruit number for seven out of eight genotypes at 40/28 degreesC was linearly and negatively related to the time of first flowering relative to the onset of high temperature (r(2) = 0.93; n = 7; P < 0.001), indicating that 'escape' was an important component of heat tolerance in this experiment. Further analysis showed that fruit number in all genotypes at 40/28 <degrees>C was closely associated with the cumulative number of flowers that had opened between first flowering and 3 days after the onset of the high temperature regime (r(2) = 0.95; n = 8; P < 0.001). Variation in fruit number was therefore due both to the timing of flowering and the initial rate of flower production. These data also suggest that the most sensitive stage of development to high temperature in groundnut occurred around 3 days before flowers opened. Therefore, it was the timing of flowering, rather than heat tolerance or susceptibility, that was the dominant attribute determining fruit number.