Effects of leaf age and tree size on stomatal and mesophyll limitations to photosynthesis in mountain beech (Nothofagus solandrii var. cliffortiodes)

Mesophyll conductance, g m, was estimated from measurements of stomatal conductance to carbon dioxide transfer, g s, photosynthesis, A, and chlorophyll fluorescence for Year 0 (current-year) and Year 1 (1-year-old) fully sunlit leaves from short (2m tall, 10-year-old) and tall (15m tall, 120-year-old) Nothofagus solandrii var. cliffortiodes trees growing in adjacent stands. Rates of photosynthesis at saturating irradiance and ambient CO 2 partial pressure, A satQ, were 25 lower and maximum rates of carboxylation, V cmax, were 44 lower in Year 1 leaves compared with Year 0 leaves across both tree sizes. A lthough g s and g mwere not significantly different between Year 0 and Year 1 leaves and g s was not significantly different between tree heights, g mwas significantly (19) lower for leaves on tall trees compared with leaves on short trees. Overall, V cmax was 60 higher when expressed on the basis of CO 2 partial pressure at the chloroplasts, C c, compared with V cmax on the basis of intercellular CO 2 partial pressure, C i, but this varied with leaf age and tree size. To interpret the relative stomatal and mesophyll limitations to photosynthesis, we used a model of carbon isotopic composition for whole leaves incorporating g meffects to generate a surface of 'operating values' of A over the growing season for all leaf classes. Our analysis showed that A was slightly higher for leaves on short compared with tall trees, but lower g mapparently reduced actual A substantially compared with A satQ. Our finding s showed that lower rates of photosynthesis in Year 1 leaves compared with Year 0 leaves were attributable more to increased biochemical limitation to photosynthesis in Year 1 leaves than differences in gm. However, lower A in leaves on tall trees compared with those on short trees could be attributed in part to lower g mand higher stomatal, L s, and mesophyll, L m, limitations to photosynthesis, consistent with steeper hydraulic gradients in tall trees.