The role of mesophyll conductance in Oak photosynthesis : among- and within-species variability

Oak species show a wide range of variation in key foliage traits determining the leaf economics spectrum, including the leaf dry mass per unit area (LMA) and photosynthetic capacity. Though it is well known that stomatal conductance plays a major role in determining maximum rates of carbon assimilation, other factors such as mesophyll conductance to CO2 (g(m)) can constrain the rate of photosynthesis and, under certain conditions, be the most significant photosynthetic limitation. First, this chapter addresses the differences in the photosynthetic limitations imposed by g(m) between deciduous and evergreen oak species, covering the role of variations in several leaf anatomical traits determining the variability in gm and photosynthetic capacity. This analysis emphasizes that cell-wall thickness of mesophyll cells and the chloroplast surface facing intercellular air spaces are the two main anatomical traits contributing to changes in g(m), and as consequence have a high relevance in the carbon fixing capacity of leaves within the genus Quercus. The second part of the chapter analyses the within-species variation of g(m) and photosynthesis rate in oaks as related to long-term variations in site climate (genetic and plastic variability) and to shorter-term variation in environmental drivers (e.g. drought stress and light availability) and during leaf ontogeny. The results of this analysis demonstrate a very high variability within-species across species range and in response to shorter-term environmental drivers, ultimately underlying the success of several Quercus species in many ecosystems worldwide.