GROUNDNUT GROWTH AND DEVELOPMENT IN CONTRASTING ENVIRONMENTS-2 - HEATUNIT ACCUMULATION AND PHOTOTHERMAL EFFECTS ON HARVEST INDEX

When the same cultivars of groundnuts (Arachis hypogaea) were grown un der a rr;ide range of environmental conditions, temperature and irradi ance played a major role in determining crop duration and partitioning of dry matter to pods, the latter assessed by harvest,index. Utilizin g published data for the Virginia groundnut cultivar Early Bunch under non-limiting conditions, we show that accumulation of thermal time us ing three cardinal temperatures (T-b = 9 degrees C, T-o = 29 degrees C and T-m = 39 degrees C) has considerable potential for predicting cro p maturity. In sixteen sowings ranging from the wet tropics in Indones ia to the elevated subtropics in Australia, harvest date for Early Bun ch corresponded to the accumulation of 1808 (+/-23) degree-days after sowing. In all sowings except one in the semi-arid tropics, this value greatly with both location and sowing date, ranging from 0.31 (Indone sia) to 0.58 (subtropical Australia). Using total short-wave solar rad iation incident during the growing season and calculated values of the rmal time, the growing season for each sowing in each location was des cribed in terms of a photo-thermal quotient (PTQ, MJ m(-2) degree-day( -1)). Values for PTQ ranged from 0.99 (Indonesia) to 2.11 (subtropical Australia). Variation in harvest index could be explained largely by a curvilinear function of PTQ (R-2 = 0.98), provided data were not con founded by the effects of photoperiod. In the semi-arid tropical envir onment, decreases in the photoperiod associated with delayed sowing we re the dominant factor controlling harvest index.