Variation of pigment composition and antioxidative systems along the canopy light gradient in a mixed beech/oak forest: a comparative study on deciduous tree species differing in shade tolerance
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The acclimation of leaf pigments and antioxidative systems along the vertical canopy light gradient was determined in mature European beech (Fagus sylvatica L.) and sessile oak [Quercus petraea (Matt.) Liebl.] trees in a mixed beech/oak forest in northwest Germany. The aim of the study was to investigate if the higher shade tolerance of beech, relative to oak, is correlated with characteristics of the leaf pigment composition and antioxidative systems. Both species showed a similar acclimative response to increasing light availability. Compared to shade leaves, leaves of the upper crown layer exhibited higher chlorophyll a to chlorophyll b ratios and higher contents of photoprotective carotenoids, ascorbate and glutathione per unit chlorophyll, and higher mean activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase and glutathione reductase. The comparison of beech and oak leaves sampled at crown positions with similar light environments revealed "shade-type" pigment characteristics for beech, relative to oak, whereas "sun-type" characteristics were found with respect to antioxidative systems. At a given average light availability level, leaves of F. sylvatica exhibited higher antioxidant contents and higher ascorbate peroxidase activities as compared to those of Qu. petraea. The data suggest that a higher antioxidative capacity is required in leaves of the more shade-tolerant species. The supposed higher production rate of radicals and reactive oxygen species (ROS) in beech can be explained by: (1) the lower energy utilizing capacity, as beech leaves show lower light-saturated rates of photosynthesis than oak leaves, and (2) by specific features of the photosynthetic apparatus as suggested by the pigment data. A higher ROS generation rate in beech versus oak leaves could be seen as a consequence of efficient photosynthesis under low light conditions. Therefore a high capacity of protection against oxidative stress might be a prerequisite of shade tolerance.
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