Seasonal changes in the physiology of shade leaves of Acer saccharum

Changes in the physiology of shade leaves of sugar maple trees (Acer saccharum Marsh.) in eastern Massachusetts were monitored throughout several growing seasons. In the late spring, following leaf expansion, the sugar maple leaves showed high photosynthetic rates and a high carboxylation efficiency which increased only slightly into the early part of summer. In contrast, pigment and protein concentrations increased sharply from late spring to mid summer. The average photosynthetic rates remained fairly high during the summer and started to decrease in early September. This decrease was accompanied by decreases in leaf pigment, protein and nitrogen concentrations. In the last few weeks before leaf abscission photosynthesis was not limited by stomatal conductance as indicated by high internal CO2 concentrations. However, photosynthesis may have been CO2 limited at times during the summer. Low quantum yields of carbon fixation and low photosynthetic rates on some days during the summer indicated temporary stress. Experiments revealed that maple leaves responded to short-term stress by temporarily downregulating PSII. In general, leaves exhibited high Fv/Fm values throughout most of the year. Sustained photoinhibition as indicated by reduced Fv/Fm was only observed during the last few weeks before leaf abscission. Photochemical quenching remained high throughout the fall, indicating that the efficiency of excitation energy capture by open PSII reaction centers was the major reason for the decrease in photochemical efficiency of PSII during the last few weeks before the leaves were shed. At the same time the non-photochemical quenching increased throughout the fall and was only low in extremely yellow leaves.