Abstract
Under field conditions, the photosynthetic rate of single leaves (CER) declined with time of day even in the case where light intensity at the leaf surface was kept higher than the saturation point. The characteristics of this kind of variation of CER and its physiological base were examined. (1) The average degree of depression for 46 cases including various years, growth stages, weather conditions and leaf positions was estimated to be 19%. (2) This kind of depression was closely correlated with a depression in stomatal conductance (gs). (3) Degree of depression in gs with time was large in the condition that evaporation rate evaluated above the leaf canopies was large. (4) The degree of depression in CER was larger in lower leaves than in upper leaves because the depression in gs was larger in the latter leaves.
