Low stomatal and internal conductance to CO2 versus Rubisco deactivation as determinants of the photosynthetic decline of ageing evergreen leaves

A novel A-C-i curve (net CO2 assimilation rate of a leaf - A(n) - as a function of its intercellular CO2 concentration - C-i) analysis method (Plant, Cell & Environment 27, 137-153, 2004) was used to estimate the CO2 transfer conductance (g(i)) and the maximal carboxylation (V-cmax) and electron transport (J(max)) potentials of ageing, non-senescing Pseudotsuga menziesii leaves in relation to their nitrogen (N) content and protein and pigment composition. Both g(i) and the stomatal conductance (g(sc)) of leaves were closely coupled to V-cmax, J(max) and A(n) with all variables decreasing with increasing leaf age. Consequently, both C-i and C-c (chloroplastic CO2 concentration) remained largely conserved through successive growing seasons. The N content of leaves, as well as the amount of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and other sodium dodecyl sulfate-soluble proteins, increased during the first three growing seasons, then stabilized or decreased only slightly afterwards. Thus, the age-related photosynthetic nitrogen use efficiency (PNUE) decline of leaves was not a consequence of decreased allocation of N towards Rubisco and other proteins involved in bioenergetics and light harvesting. Rather, loss of photosynthetic capacity was the result of the decreased activation state of Rubisco and proportional down-regulation of electron transport towards the photosynthetic carbon reduction (PCR) and photorespiratory (PCO) cycles in response to a reduction of CO2 supply to the chloroplasts' stroma. This study emphasizes the regulatory potential and homeostaticity of C-c - rather than photosynthetic metabolites or C-i - in relation to the commonly observed correlation between photosynthesis and g(sc).