[Effects of low temperature in the light on antioxidant contents in rice (Oryza sativa L.) indica and japonica subspecies seedlings].

To study the nature and mechanisms of resistance of rice plants to chilling stress, the effects of low temperature treatment (8 degrees C) on the photosynthetic rate and some important compounds forming redox cycles were measured. The rice varieties used are two japonica rice varieties, i.e., Taipei 309 and Wuyujing; three indica rice varieties, i.e., IR64, Pusa and CA212; and one intermediate type, i.e., Shanyou 63. Three types of varieties were studied by comparing. The light intensity-photosynthesis curves, CO2-photosynthesis curves, primary photochemical efficiency (Fv/Fm), active oxygen species (AOS) (O2*- and H2O2), glutathione (both oxidized and reduced forms) and ascorbate contents in their six-week old seedlings were measured before and after chilling treatment. The results showed that relative to the rice varieties chilling tolerance such as Taipei 309 and Wuyujing, the sensitive ones indica IR64, Pusa and CA212 exhibited a stronger inhibition of maximum photosynthetic rate (Pmax) (Figs.1 and 2) and a decrease in Fv/Fm (Fig.3), which led to the accumulation of AOS (Fig.6). It was found that the glutathione disulphide (GSSG) content in glutathione pool and that of dehydroascorbate (DHA) in ascorbate pool of the leaves of these sensitive ones under chilling were induced to increase obviously (Table 3). The correlation coefficient between the increases in GSSG, DHA and the decrease of Chl content were -0.701**, -0.656** respectively (Table 4). This indicated that the regeneration of reduced glutathione (GSH) and ascorbate was inhibited, resulting in accumulations of AOS and the reduction of Chl content (Fig.4) and the inhibition of photosynthetic activity (Fig.1 and Fig.2). The changes in japonica Taibei 309 and Wuyujing were small. And the changes in indica hybrid were lying between the above-mentioned types. Particularly, the ratio of AsA/DHA and GSH/GSSG (Fig.7) showed similar changes as those in Chl content (Fig.4). The correlation coefficient among Chl content and AsA/DHA, GSH/GSSG were 0.811**, 0.728** respectively (Table 4), significant at 0.01 probability levels. The levels of AsA/DHA and GSH/GSSG ratio in rice leaves may be the physiological indexes associated with the sensitivity to chilling in rice varieties.