Alteration in chloroplast structure and thylakoid membrane composition due to in vivo heat treatment of rice seedlings: correlation with the functional changes

Exposure of 25 °C-grown, seven-day-old rice seedlings to mild heat stress of 40 °C for 24 h in dark did not cause any change in protein or pigment content of the thylakoids, but produced major disorganization of chloroplast ultrastructure. This heat induced disorganization of thylakoid structure/organization caused significant (65 percnt;) loss in PSII activity, slight loss in PSI activity, and brought about a decrease in relative quantum efficiency of PSII. The herbicide ¹⁴C atrazine binding assay revealed a decreased number of binding sites of the herbicide and altered the herbicide dissociation constant, suggesting that the heat induced disorganization of the thylakoids affects the acceptor side of PSII. Cation induced Chla fluorescence analyses at room temperature and low temperature indicated thatin vivo heat exposure of rice seedlings altered the extent of energy transfer in favor of PSI. Immunoblotting analysis of several PSII polypeptides such as D1/D2 reaction dimer and Cyt b₅₅₉ showed no major changes due to mild heat exposure except for the PSII core antenna polypeptide (CP43), which could reflect the reduction in PSII activity observed in light saturation studies. Similarly, haeme staining did not indicate any change in other cytochrome related polypeptides. Our results therefore clearly suggest thatin vivo exposure of rice seedlings to elevated (40 °C) temperature caused thylakoid structural disorganization, and this disorganization of some of the thylakoid complexes resulted in a loss in thylakoid photochemical function.