Limitations by mesophyll conductance do not influence estimates of Rubisco capacity obtained from CO2 response curves of several tropical and temperate rainforest trees

NHA Bahar, OK Atkin and JR Evans

Division of Plant Sciences, RSB, ANU, Canberra ACT 0200

Globally, plants growing in warm regions typically exhibit lower photosynthetic rates compared to those from cold environments. One factor that has been suggested to contribute towards lower photosynthetic rates is a greater mesophyll resistance. Using a combination of leaf gas exchange and carbon isotope discrimination measurements, we estimated mesophyll conductance of several Australian tropical and temperate rainforest trees, grown in common environment. Maximum Rubisco carboxylation capacity, Vcmax was obtained from CO2 response curves. Consistent with broader findings, tropical trees exhibited significantly lower photosynthetic rates and Vcmax than their temperate counterparts. Photosynthetic rate correlated strongly with diffusion conductance for both tropical and temperate trees. All species fell on a common photosynthesis-mesophyll conductance relationship and the drawdown of CO2 imposed by stomatal and mesophyll resistances were independent of mesophyll conductance. Limitations to CO2 diffusion imposed by mesophyll resistance were relatively constant across tropical and temperate species (~13%), irrespective of photosynthetic capacity. This indicates that tropical trees are not penalised by low mesophyll conductance, hence the lower photosynthetic rates of tropical species are unlikely to be caused by lower mesophyll conductance. Rather, lower photosynthetic rate of tropical species could be ascribed to lower nitrogen investment in Rubisco. More importantly, we found that Vcmax estimated on the basis of intercellular CO2 partial pressure, Ci, was equivalent to that on the basis of chloroplastic CO2 partial pressure, Cc, when using appropriate Michaelis-Menten constants for CO2 and O2. Our study provides evidence that a valid Vcmax estimate can be derived using either Ci or Cc, provided the appropriate Kc and Ko values are applied.