Seasonal variation of photoinhibition of photosynthesis in bark from Populus tremula L.

In the bark of Populus tremula L. photochemical efficiency of photosystem 2 (PS2) determined as F-v/F-m decreased during winter. The strongest reduction was found after cold periods. The degree of reduction depended on irradiance since the lowest levels of F-v/F-m were found on the sun-exposed side of the stem and below thin phellem. Therefore, photoinhibition was partly responsible for the reduction in F-v/F-m. The photochemical efficiency of PS2 recovered in late April about a month before the trees got leaves. In the laboratory, F-v/F-m recovered within about a week under low irradiance at 20 degrees C. Rapid recovery of photochemical efficiency of PS2 in the bark may be important to reduce respiratory loss of CO2 from the stem before the trees get leaves.