Physiological responses of tolerant and susceptible rapeseed (Brassica napus L.) cultivars to heat stress

Heat stress is one of the most important abiotic stresses reducing rapeseed productivity. In order to study the physiological responses of tolerant and susceptible rapeseed cultivars to heat stress, two tolerant, SAFI5 and MAHTAB, and two susceptible cultivars, DH13 and ZAMAN, were planted in a growth chamber in a factorial experiment based on a completely randomized design with three replications. Plants were moved to a chamber with 35/22 (day/night) temperature for 72 hours at the flowering stage. Leaf temperature, relative water content, electrolyte leakage, chlorophyll fluorescence (Fv/Fm), the content of chlorophyll a and b and carotenoid content were measured. Relative water content decreased significantly and leaf temperature increased under stress while electrolyte leakage did not change remarkably. Chlorophyll fluorescence decreased due to heat stress, susceptible cultivars showing more reduction. Content of chlorophyll a and b decreased under heat stress condition. Carotenoid content, however, increased in tolerant cultivars and decreased in susceptible ones. Significant high correlations were observed among relative water content, leaf temperature, chlorophyll fluorescence, and chlorophyll content. In sum, the results indicated that relative water content and leaf temperature had a greater contribution to heat tolerance and could be applied in screening tolerant varieties.