Effects Of Low Content Chlorophyll-b On Photosynthetic Apparatus And Thermostability In Rice

Photosynthesis is the basis of growth and development of plant, it provides the substratesand energy for growth and development. High temperature inhibits seriously photosynthesisand increase of crop productivity. Therefore, it is important to study the mechanisms thathow high temperature stress effects photosynthesis.Low content chlorophyll b rice mutant and the wild type rice (Zhenhui 249) were used tostudy the pigmentation content, chlorophyll fluorescence, microstructure and ultrastructure.The. results showed that the low content chlorophyll b rice mutant, as compared withwild-type rice, had lower chlorophyll content, but higher the ratio of Chl a to Chl b andF_v/F_m. In microstructure, the mutant was mainly characterized by the thinness leaves.Ultrastructural observation revealed that the chloroplast structure in the mutant degenerated.As compared with wild-type rice, the mutant has slight decrease in the number of granumthylakoids, disordered grana arrangement in the chloroplast. These results are consistentwith the physiology character of lower photosynthetic rate but higher photosyntheticefficiency in the mutant.Photosynthesis at different temperature especially stability to high temperature werestudied in low content chlorophyll b rice mutant and the wild type rice(Zhenhui 249) bymeasuring pigmentation content, fluorescence parameters, polyphasic chlorophyllfluorescence transients, H₂O₂ accumulation and electrolyte leakage under differenttemperatures. The temperature of F_o rising threshold was shifted 3℃lower in the mutant(43℃) than in the wild type (46℃). With increasing temperature the rate of decline influorescence parameters were larger in mutant than the wild type. Pigmentation analysisand electrolyte leakage were also accorded with the above results in two species. Withincreasing temperature the content of H₂O₂ increased in leaves of mutant and wild type rice. At temperature up to above 44℃, significant H₂O₂ accumulation in leaves were found inthe mutant, whereas in the wild type plant, less H₂O₂ was accumulated. All results indicatedthat the main part of heat damage to Photosystemâ…¡was oxygen-evolving activities(OEC). The mutant showed a tendency for higher thermosensitivity, and the loss ofchlorophyll b in LHCII could lead to less thermostability of PSII structure and function.Heat damage to photosynthetic apparatus, might be partially attributed to the internaloxidative stress produced at severely high temperature.