Dynamic Study On PSⅡ And Photosynthetic Key Enzyme In Rice Under High-temperature Stress

To explore how high-temperature impact the photosynthesis, the measurement for gas exchange, chlorophyll (Chl) fluorescence, A-Ci curve, activity and content of Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) and Rubisco activase (RCA) were conducted with the fully expanded top leaves of rice plant, grown under the moderate high-temperature stress (40℃) and normal temperature (30℃), during a chase of 0d, 1d, 2d, 4d, 7d, 11d, 16d and thereafter, respectively, 10 day-recover in normal temperature condition after the stress. We also investigated the activity and content of Rubisco and RCA of the plant leaf grown under either 42℃or 45℃for Oh, 1h, 2h, 4h, 8h, 16h in order to understand short-time higher-temperature injury to photosynthesis of rice and the possible role of RCA in heat-resistance of plant.The results showed that the net photosynthetic rate (Pn) decreased significantly under high-temperature stress (40℃), but had a potential to return to normal under 30℃as long as the stress time was less than 7 days. The change of Pn had little correlation with intercellular CO₂ concentration (Ci), which indicated that the reduction of Pn under 40℃were mainly due to non-stomatal factor. Among this factor, Rubisco and RCA first responded to heat. At the beginning of heat stress, we found no damage to photosystem (PS) II. However, the moderate heat stress had reduced the initial activity of Rubisco and photosynthetic rate remarkably. The measurements of A-Ci curve indicated that carboxylation efficiency (CE) and the rate of RuBP regeneration decreased significantly under the stress. The direct measurement for Rubisco and RCA also confirmed that the trends of photosynthetic rate were in accordance with the initial activity and content of Rubisco. However, RCA content level in heat-stress rice remained higher than that in the unstressed one when the treatment lasted over 10-days.Although Chl fluorescence showed little change in moderately high-temperature stressed plant compared to that in control plant in the early stage of heat stress, Fv/Fm as an index of injury to PSâ…¡decreased remarkably upon the 16-day stress. These results indicated that the reduction of photosynthetic capacity on the anaphase of high temperature was attributed to both the injury to PSâ…¡and Rubisco.The measurement under 45℃showed that both the initial and total activities of Rubsico decreased dramatically. But RCA content under 45℃was higher than either the control plant or that treated with 42℃high-temperature, which implied that the level of RCA expression was promoted by heat stress, but this RCA could activate Rubisco no longer.