THERMOSTABILITIES OF CELL-MEMBRANE AND PHOTOSYNTHESIS IN CABBAGE CULTIVARS DIFFERING IN HEAT TOLERANCE

Responses of two Japanese cabbage (Brassica oleracea L. var capitata) cultivars, heat tolerant Sousyu and heat-susceptible YR Kinshun, were compared in terms of cell-membrane thermostability, chlorophyll fluore scence, chlorophyll A and chlorophyll B contents, and photosynthetic r ates. High (similar to 50 degrees C) temperature caused greater relati ve injury and reduction in the values of fluorescence in YR Kinshun th an in Sousyu, especially when non-hardened for 24h prior to the measur ement. Hardening at 30-35 degrees C improved the thermostabilities of cell-membrane and chlorophyll fluorescence and decreased the chlorophy ll content, and consequently photosynthetic rates in YR Kinshun. Chlor ophyll content and chlorophyll fluorescence of Sousyu were relatively less adversely affected by the hardening and stress temperatures. Sous yu maintained higher chlorophyll content and rates of photosynthesis t han the YR Kinshun in hardened condition whereas differences between t he two genotypes were small in non-hardened condition. Heating leaves at 50 degrees C reduced photosynthetic rates of both the cultivars. So usyu, however, maintained higher rates than YR Kinshun even after the heat treatment. The results suggest that higher thermostability of cel l membranes, chlorophyll fluorescence and photosynthetic rates of Sous yu, especially under non-hardened condition, may give it an overall ad vantage in high temperature environments over YR Kinshun.