There is a strong correlation between leaf thickness and the light-saturated rate of photosynthesis per unit leaf area (Pmax). However, when leaves are exposed to higher light intensities after maturation, Pmax often increases without increasing leaf thickness. To elucidate the mechanism with which mature leaves increase Pmax, the change in anatomical and physiological characteristics of mature leaves of Chenopodium album, which was transferred from low to high light condition, were examined. When compared with leaves subjected to low light continuously (LL leaves), the leaves transferred from low to high light (LH leaves) significantly increased Pmax. The transfer also increased the area of chloroplasts facing the intercellular space (Sc) and maintained a strong correlation between Pmax and Sc. The mesophyll cells of LL leaves had open spaces along cell walls where chloroplasts were absent, which enabled the leaves to increase Pmax when they were exposed to high light (LH). However, the LH leaves were not thick enough to allow further increase in Pmax to the level in HH leaves. Thus leaf thickness determines an upper limit of Pmax of leaves subjected to a change from low to high light conditions. Shade leaves would only increase Pmax when they have open space to accommodate chloroplasts which elongate after light conditions improve.