EFFECT OF LONG-TERM PHOTOINHIBITION ON GROWTH AND PHOTOSYNTHESIS OF COLD-HARDENED SPRING AND WINTER-WHEAT

The effect of repeated exposure to high light (1200 mumol . m-2 . s-1 photosynthetic photon flux density, PPFD) at 5-degrees-C was examined in attached leaves of cold-grown spring (cv. Katepwa) and winter (cv. Kharkov) wheat (Triticum aestivum L.) over an eight-week period. Under these conditions, Kharkov winter wheat exhibited a daily reduction of 24% in F(V)/F(M) (the ratio of variable to maximal fluorescence in the dark-adapted state), in contrast to 41% for cold-grown Katepwa spring wheat. Both cultivars were able to recover from this daily suppression of F(V)/F(M) such that the leaves exhibited an average morning F(V)/F(M) of 0.651 +/- 0.004. Fluorescence measurements made under steady-state conditions as a function of irradiance from 60 to 2000 mumol . m-2 . s-1 indicated that the yield of photosystem II (PSII) electron transport under light-saturating conditions was the same for photoinhibited and control cold-grown plants, regardless of cultivar. Repeated daily exposure to high light at low temperature did not increase resistance to short-term photoinhibition, although zeaxanthin levels increased by three- to fourfold. In addition, both cultivars increased the rate of dry-matter accumulation, relative to control plants maintained at 5-degrees-C and 250 mumol . m-2 . s-1 PPFD (10% and 28% for Katepwa and Kharkov, respectively), despite exhibiting suppressed F(V)/F(M) and reduced photon yields for O2 evolution following daily high-light treatments. Thus, although photosynthetic efficiency is suppressed by a long-term, photoinhibitory treatment, light-saturated rates of photosynthesis are sufficiently high during the high-light treatment to offset any reduction in photochemical efficiency of PSII. We suggest that in these cold-tolerant plants, photoinhibition of PSII may represent a long-term, stable, down-regulation of photochemistry to match the overall photosynthetic demand for ATP and reducing equivalents.