COLD-ACCLIMATION AND FREEZING TOLERANCE - A COMPLEX INTERACTION OF LIGHT AND TEMPERATURE

By comparing growth under five different temperature and irradiance re gimes (20 degrees C and 800, 250, and 50 mu mol m(-2) s(-1) and 5 degr ees C and 250 and 50 mu mol m(-2) s(-1)), we have examined the effects of light, temperature, and the relative reduction state of photosyste m II on plant morphology, freezing tolerance (lethal temperature at wh ich freezing injury occurs [LT50]), transcript levels of Lhcb and two cold-stimulated genes (Wcs19 and Wcs120), and photosynthetic adjustmen t in winter rye (Secale cereale L. cv Musketeer). We show, for the fir st time to our knowledge, that in addition to adjustments in photosynt hetic capacity, nonphotochemical quenching capacity and tolerance to p hotoinhibition, the accumulation of the cold-induced transcript Wcs19, and the compact plant morphology usually associated with cold-hardeni ng are correlated with the relative reduction state of photosystem II rather than with growth temperature or growth irradiance per se. In co ntrast, the acquisition of maximal LT50, as well as Lhcb and Wcs120 mR NA accumulation, appears to be dependent on both growth temperature an d growth irradiance but in an independent, additive manner. The result s are discussed with respect to the possible role of the modulation of chloroplastic redox poise in photosynthetic acclimation to fold-harde ning temperatures and the attainment of maximal LT50.