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 fluorescence, chlorophyll A and chlorophyll B contents, and photosynthetic rates. High (similar to 50 degrees C) temperature caused greater relative injury and reduction in the values of fluorescence in YR Kinshun than in Sousyu, especially when non-hardened for 24h prior to the measurement. Hardening at 30-35 degrees C improved the thermostabilities of cell-membrane and chlorophyll fluorescence and decreased the chlorophyll content, and consequently photosynthetic rates in YR Kinshun. Chlorophyll content and chlorophyll fluorescence of Sousyu were relatively less adversely affected by the hardening and stress temperatures. Sousyu maintained higher chlorophyll content and rates of photosynthesis than the YR Kinshun in hardened condition whereas differences between the two genotypes were small in non-hardened condition. Heating leaves at 50 degrees C reduced photosynthetic rates of both the cultivars. Sousyu, however, maintained higher rates than YR Kinshun even after the heat treatment. The results suggest that higher thermostability of cell membranes, chlorophyll fluorescence and photosynthetic rates of Sousyu, especially under non-hardened condition, may give it an overall advantage in high temperature environments over YR Kinshun.