Exploring biochemical traits for heat stress tolerance in Indian mustard germplasm

Sixty four germplasm lines of Indian mustard (Brassica juncea) were screened in the field condition for heat tolerance based on differences in time of sowing. (1) Early sowing (ES) was done in the month of September when the average soil temperature was 41 ℃ and the atmospheric temperature was around 35 ℃ providing stressful atmosphere to the crop. (2) Normal sowing (NS) was done during mid-October when the soil temperature was 34.2 ℃. Biochemical analysis of leaves was done at the flowering stage. The relationship between biochemical parameters was studied using multivariate analysis, heat-mapping and the stress tolerance index (STI) to identify heat tolerant genotypes. Principal component analysis was carried out for the eight traits genotype wise. The biplot analysis was carried out and it is seen that the four principal components (PC1 to PC4) represented 92 and 87% of the  total variation under Normal sown (control condition) and Early sown (heat stress) respectively. Contrasting nature of genotypes under ES and NS is visualized by the darker and lighter colour scales produced by heat-map. The positive side and higher values represents heat tolerance of that particular genotype while values on the negative side with lower values show the susceptibility of that particular genotype, together with stress susceptibility index (SSI) genotypes EC-414320, EC-399318 and DMRS-97 identifed to be highly temperature tolerant. Total chlorophyll and total carotenoids showed higher values under ES compared to NS in all multivariate analysis which could be a potential screening index for heat tolerance.