The influence of nitrogen and phosphorus supply and genotype on mesophyll conductance limitations to photosynthesis in Pinus radiata

Mesophyll conductance, g(m), may pose significant limitations to photosynthesis and may be differentially affected by nutrition and genotype in Pinus radiata D. Don. Simultaneous measurements of gas exchange and chlorophyll fluorescence were made to determine g(m), using the constant J method (Harley, P. C., F. Loreto, G. Di Marco and T. D. Sharkey. 1992. Theoretical considerations when estimating the mesophyll conductance to CO2 flux by analysis of the response of photosynthesis to CO2. Plant Physiol. 98: 1429-1436), in a fast-and a slow-growing clone of P. radiata grown in a greenhouse with a factorial combination of nitrogen (N) and phosphorus (P) supply. Values of g(m) increased linearly with the rate of photosynthesis at saturating irradiance and ambient CO2 concentration, A(sat) (g(m) = 0.020A(sat), r(2) = 0.25, P < 0.001) and with stomatal conductance to CO2 transfer, g(s) (g(m) = 1.16g(s), r(2) = 0.14, P < 0.001). Values of g(m) were greater than those of stomatal conductance, g(s), and the ratio (g(m)/g(s)) was not influenced by single or combined N and P additions or clone with a mean (+/- SE) value of 1.22 +/- 0.06. Relative limitations to mesophyll conductance, L-m (16%) to photosynthesis, were generally greater than those imposed by stomata, L-s (13%). The mean (+/- SE) CO2 concentration in the intercellular air spaces (Ci) was 53 +/- 3 lmol mol(-1) lower than that in the atmosphere (C-a). Mean (+/- SE) CO2 concentration in the chloroplasts (C-c) was 48 +/- 2 lmol mol(-1) lower than C-i. Values of L-s, L-m and CO2 diffusion gradients posed by g(s) (C-a - C-i) and g(m) (C-i - C-c) did not significantly differ with nutrient supply or clone. Mean values of V-cmax and J(max) calculated on a Cc basis were 15.4% and 3.1% greater than those calculated on a C-i basis, which translated into different slopes of the J(max)/V-cmax relationship (C-c basis: J(max) = 2.11V(cmax), r(2) = 0.88, P < 0.001; C-i basis: J(max) = 2.43V(cmax), r2 = 0.86, P < 0.001). These results will be useful for correcting estimates of V-cmax and J(max) used to characterize the biochemical properties of photosynthesis for P. radiata.