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Field experiments were carried out at IARI research farm, New Delhi representing semi-arid climatic condition, to evaluate the growth and yield of oilseed Brassica using thermal indices. Two cultivars of Brassica juncea viz., Pusa Jaikisan and Varuna, widely grown in north and north-western parts of the country, were sown on ten different dates from 1st October to 3rd December at weekly interval. Three widely used thermal indices viz, growing degree days (GDD), heliothermal units (HTU) and photothermal units (PTU) were computed up to maximum biomass production in both the seasons. It was observed that GDD was able to explain variation in biomass, seed yield and oil content to the tune of 75, 66 and 78 per cent while PTU could explain 73, 66 and 77 per cent variations respectively. The yields and oil content of these two cultivars were highly influenced by the differential thermal environment and delay of sowing; decreased seed yield and oil content significantly.

Evaluation of the thermal heat requirement of Brassica spp. across agro-ecological regions is required in order to understand the further effects of climate change. Spatio-temporal changes in hydrothermal regimes are likely to affect the physiological growth pattern of the crop, which in turn will affect economic yields and crop quality. Such information is helpful in developing crop simulation models to describe the differential thermal regimes that prevail at different phenophases of the crop. Thus, the current lack of quantitative information on the thermal heat requirement of Brassica crops under debranched microenvironments prompted the present study, which set out to examine the response of biophysical parameters [leaf area index (LAI), dry biomass production, seed yield and oil content] to modified microenvironments. Following 2 years of field experiments on Typic Ustocrepts soils under semi-arid climatic conditions, it was concluded that the Brassica crop is significantly responsive to microenvironment modification. A highly significant and curvilinear relationship was observed between LAI and dry biomass production with accumulated heat units, with thermal accumulation explaining >or=80% of the variation in LAI and dry biomass production. It was further observed that the economic seed yield and oil content, which are a function of the prevailing weather conditions, were significantly responsive to the heat units accumulated from sowing to 50% physiological maturity. Linear regression analysis showed that growing degree days (GDD) could indicate 60-70% variation in seed yield and oil content, probably because of the significant response to differential thermal microenvironments. The present study illustrates the statistically strong and significant response of biophysical parameters of Brassica spp. to microenvironment modification in semi-arid regions of northern India.