Photosynthetic ecophysiology of evergreen leaves in the woody angiosperms - a review

Evergreen plants are an important component of many ecosystems of the world and occur in numerous evolutionary lineages. In this article we review phenotypic traits of evergreen woody angiosperms occurring in habitats that regularly experience frost. Leaf anatomical traits such as sclerenchymatic tissues or prominent cuticles ensure mechanical strength while often enhancing tolerance of water deficit. The low ratio of photosynthetic to nonphotosynthetic tissues as well as modified cell wall structure and nitrogen allocation patterns in evergreen leaves result in lower mass-based photosynthetic rate and photosynthetic nitrogen use efficiency in comparison with deciduous leaves. Their photosynthetic apparatus is adapted for the survival of frost in a down-regulated state with potential for photosynthetic activity in winter during periods of permissive temperatures. Leaf structure interacts with the mechanisms of frost survival. Stem xylem in evergreen plants tends to contain smaller diameter conduits incurring greater resistance to freeze/ thaw induced cavitation than in deciduous plants, although at the cost of reduced hydraulic efficiency. In contrast, no such differences in hydraulic conductivity have been documented at the leaf level. There is evidence for reduced structural plasticity of evergreen leaves in response to variability in irradiance, however photosynthetic downregulation occurs in mature leaves in response to self shading. Some evergreen species exhibit slow leaf development and “delayed greening”, while in many species aging is also a very protracted process. Finally, evergreen leaves may participate in carbohydrate and, less obviously, in nitrogen storage for the support of spring shoot and foliage growth, although the importance of this function is under debate. In conclusion, the evergreen leaf habit is correlated with numerous structural and functional traits at the leaf and also at the stem level. These correlations may generate trade-offs that shape the ecological strategies of evergreen plants.