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 yie lds are frequently reduced by heat and water stress. Eight groundnut genoty pes varying in heat tolerance were grown in controlled environments and exp osed to either high (40/28 degreesC) or near-optimum (30/24 degreesC) tempe rature from 32 days after sowing (DAS) to maturity. There was significant v ariation 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 degr eesC than at 30/24 degreesC. Days from sowing to first flowering varied amo ng 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. F ruit 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 onse t of high temperature (r(2) = 0.93; n = 7; P < 0.001), indicating that 'esc ape' was an important component of heat tolerance in this experiment. Furth er analysis showed that fruit number in all genotypes at 40/28 <degrees>C w as 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 ther efore due both to the timing of flowering and the initial rate of flower pr oduction. These data also suggest that the most sensitive stage of developm ent to high temperature in groundnut occurred around 3 days before flowers opened. Therefore, it was the timing of flowering, rather than heat toleran ce or susceptibility, that was the dominant attribute determining fruit num ber.