Abstract
The photosynthetic performances of individual leaves of a wheat (Triticum aestivum cv Bezostaya) crop were assessed daily and throughout individual days during the winter when temperature and light levels were fluctuating. Measurements of chlorophyll fluorescence induction and the maximum quantum yield of O(2) evolution were made on individual leaves. Depressions in the ratio of variable to maximal fluorescence (F(v)/F(m)) were correlated with low temperatures and high light levels throughout the winter and during the course of individual days. Depressions in F(v)/F(m) observed in the field during the day were not accompanied by any significant change in the ability of photosystem II complexes to bind 3-(3,4-dichlorophenyl)-1-dimethyl urea, indicating that the depressions in F(v)/F(m) were not attributable to photodamage to the D1 protein of the photosystem II reaction center. Decreases in F(v)/F(m) were associated with increases in the rate of dissipation of excitation energy by radiationless decay processes and decreases in the quantum efficiency of CO(2) assimilation, indicative of a rapidly reversible light-induced "downregulation" of photosynthesis. No major changes were observed in the maximum quantum efficiency of O(2) evolution of leaves throughout periods of fluctuating temperature and light, because light-induced depressions in photosynthetic efficiency recovered within the time required to make these measurements.