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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 (2 m tall, 10-year-old) and tall (15 m 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. Although  g  s  and  g  m  were not significantly different between Year 0 and Year 1 leaves and  g  s  was not significantly different between tree heights,  g  m  was 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  m  effects 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  m  apparently reduced actual  A  substantially compared with  A  satQ . Our findings 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  g  m . However, lower  A  in leaves on tall trees compared with those on short trees could be attributed in part to lower  g  m  and higher stomatal,  L  s , and mesophyll,  L  m , limitations to photosynthesis, consistent with steeper hydraulic gradients in tall trees.

tree physiology

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