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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 CO 2  flux by analysis of the response of photosynthesis to CO 2 . 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 CO 2  concentration,  A  sat  ( g  m  = 0.020 A  sat ,  r  2  = 0.25,  P  &lt; 0.001) and with stomatal conductance to CO 2  transfer,  g  s  ( g  m  = 1.16 g  s ,  r  2  = 0.14,  P  &lt; 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) CO 2  concentration in the intercellular air spaces ( C  i ) was 53 ± 3 μmol mol −1  lower than that in the atmosphere ( C  a ). Mean (±SE) CO 2  concentration in the chloroplasts ( C  c ) was 48 ± 2 μmol mol −1  lower than  C  i . Values of  L  s ,  L  m  and CO 2  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  C  c  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  = 2.11 V  cmax ,  r  2  = 0.88,  P  &lt; 0.001;  C  i  basis:  J  max  = 2.43 V  cmax ,  r  2  = 0.86,  P  &lt; 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 .

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