EFFECT OF COLD HARDENING ON SENSITIVITY OF WINTER AND SPRING WHEAT LEAVES TO SHORT-TERM PHOTOINHIBITION AND RECOVERY OF PHOTOSYNTHESIS

Photoinhibition of photosynthesis and its recovery were studied in wheat (Triticum aestivum L.) leaves grown at nonhardening (20-degrees-C) and cold-hardening (5-degrees-C) temperatures. Cold-hardened wheat leaves were less susceptible to photoinhibition at 5-degrees-C than nonhardened leaves, and the winter cultivars, Kharkov and Monopol, were less susceptible than the spring cultivar, Glenlea. The presence of chloramphenicol, a chloroplastic protein synthesis inhibitor, increased the susceptibility to photoinhibition, but cold-hardened leaves still remained less susceptible to photoinhibition than nonhardened leaves. Recovery at 50 mumol m-2 s-1 photosynthetic photon flux density and 20-degrees-C was at least biphasic, with a fast and a slow phase in all cultivars. Cold-hardened leaves recovered maximum fluorescence and maximum variable fluorescence in the dark-adapted state during the fast phase at a rate of 42% h-1 compared with 22% h-1 for nonhardened leaves. The slow phase occurred at similar rates (2% h-1) in cold-hardened and nonhardened leaves. Full recovery required up to 30 h. Fast-recovery phase was not reduced by either lowering the recovery temperature to 5-degrees-C or by the presence of chloramphenicol. Slow-recovery phase was inhibited by both treatments. Hence, the fast phase of recovery does not require de novo chloroplast protein synthesis. In addition, only approximately 60% of the photochemical efficiency lost through photoinhibition at 5-degrees-C was associated with lost [C-14]atrazine binding and, hence, with damage to the secondary quinone electron acceptor for photosystem II-binding site. We conclude that the decrease in susceptibility to photoinhibition exhibited following cold hardening of winter and spring cultivars is not due to an increased capacity for repair of photoinhibitory damage at 5-degrees-C but reflects intrinsic properties of the cold-hardened photosynthetic apparatus. A model to account for the fast component of recovery is discussed.