The photosynthetic limitation posed by internal conductance to CO2 movement is increased by nutrient supply

The internal conductance to CO(2) supply from substomatal cavities to sites of carboxylation may pose a large limitation to photosynthesis, but little is known of how it is affected by nutrient supply. Knowing how internal conductance responds to nutrient supply is critical for interpreting the biochemical responses from A-C(i) curves. The aim of this paper was to examine the response of g(i) and photosynthetic parameters to nutrient supply in glasshouse-grown seedlings of the evergreen perennial Eucalyptus globulus Labill. Seedlings were grown with five different nutrient treatments and g(i) was estimated from concurrent measurements of gas exchange and fluorescence. Internal conductance varied between 0.12 and 0.19 mol m(-2) s(-1) and the relative limitation of photosynthesis due to internal conductance was greater than the stomatal limitation. In most species these two limitations are rather similar, but in E. globulus stomatal limitations were abnormally low due to high stomatal conductance (0.31 to 0.39 mol m(-2) s(-1)). The large positive response of photosynthesis to nutrient supply was not matched by changes in internal conductance, and thus the relative limitation of photosynthesis due to internal conductance increased with increasing nutrient supply. Failure to account for finite internal conductance led to estimates of V(cmax) that were 60% of the true value, which, in turn, led to an underestimation of in vivo Rubisco specific activity (as V(cmax)/Rubisco content). The specific activity of Rubisco in E. globulus (21 mol mol(-1) s(-1)) was close to the maximum published estimates, and thus, despite these leaves containing a large fraction of N as Rubisco (38-44%) there was no evidence that Rubisco activity was down-regulated or that the enzyme was in excess.