Seasonal patterns in the antioxidant enzyme superoxide dismutase in the upper canopy of sugar maple (Acer saccharum Marsh.) trees

The variability of sun and shade leaves has been found to influence the response to ambient stress factors, notably ozone. Whether the upper canopy of sugar maple (Acer saccharum Marsh.) differs in response to seasonal ambient ozone concentrations due to sun/shade leaf trait acclimation is unknown. In the current study the intensity of photosynthetically-active radiation (PAR) and UV-B at the outer crown edge as well as the canopy interior were measured in the upper crown of eight, mature sugar maple. Not all morphological and physiological traits showed typical sun/shade acclimation, perhaps due to hydraulic properties and limitations of tall trees. Superoxide dismutase isoforms (Cu/ZnSOD and MnSOD) demonstrated higher activity levels in foliage of the outer crown compared to the inner, while glutathione reductase did not display activity at either position. The effect of time on SOD activity was not different between canopy positions. The similarity between years in the temporal pattern of SOD isoform activity suggests an endogenous seasonal trend. Although strong correlations were found between SOD activity and ozone concentrations experienced in the eight hours before sampling, direct experiments are required to rigorously distinguish the influence of ambient ozone from endogenous SOD activity.