Mesophyll conductance limitation of photosynthesis in poplar under elevated ozone.

Finite mesophyll conductance (g) reduces the rate of CO diffusion from the leaf intercellular space to the chloroplast and constitutes a major limitation of photosynthesis in trees. While it is well established that g is decreased by stressors such as drought and high temperature, few studies have investigated if the phytotoxic air pollutant ozone (O) affects g. We quantified the relative importance of three different types of limitations of photosynthesis in poplar trees exposed to elevated O: decreases in stomatal conductance, g and biochemical photosynthetic capacity. The O-induced reductions in light-saturated net photosynthesis were linked to significant declines in g and biochemical photosynthetic capacity (in particular carboxylation). There was no significant effect of O on stomatal conductance. Of the O-induced limitations on photosynthesis, g limitation was by far the most important (-16%) while biochemical limitation (-8%) was rather small. Both limitations grew in magnitude over the study period and varied in response to leaf-specific O exposure. Our findings suggest that declines in g may play a key role in limiting photosynthesis of plants exposed to elevated O, an effect hitherto overlooked.