FUNCTIONAL AND ULTRASTRUCTURAL INJURY TO PHOTOSYNTHESIS IN WHEAT BY HIGH-TEMPERATURE DURING MATURATION

Wheat (Triticum aestivum L. cv. Len) plants were exposed to 15/10, 25/ 20, and 35/30 degrees C day/night temperature regimes after anthesis u ntil plants grown at 25/20 degrees C reached physiological maturity. I ncreasing temperature during maturation enhanced leaf senescence, acce ntuated the loss of chloroplast integrity, increased thylakoid luminal volume and decreased extent of appressed thylakoid membrane area, and accelerated the decline of PSII-mediated electron transport. Stomatal conductance declined slower than other photosynthetic processes at hi gh temperature. Lability of PSII and stability of PSI activities to pr olonged high temperature contrasted with measurements of Chi-binding p roteins, such as LHCII, type-II LHCI and the PsaA-PsaB polypeptides. W hile the activity of PSII declined rapidly, the LHCII polypeptides rem ained as prominent thylakoid membrane components. Activity of PSI, in contrast, was sustained despite a diminished 735:685 nm fluorescence e mission ratio and severe damage to the Chi-binding proteins, such as P saA-PsaB and type-II LHCI. The dissimilar responses of LHCII and type- II LHCI to high temperature injury may be related to their localizatio n in the thylakoid membrane; LHCII predominantly in the appressed memb rane regions, may be relatively shielded from proteolytic activity as compared with type-II LHCI in the exposed membrane regions.