The influence of water content and leaf anatomy on carbon isotope discrimination and photosynthesis in Sphagnum

The relative effect of diffusional resistance due to water films (rwf) and leaf anatomy (rp) on rates of net photosynthesis and on-line measures of carbon isotope discrimination was investigated in Sphagnum. Sphagnum species differ in the exposure of photosynthetic cells at the leaf surface. In S. affine, photosynthetic cells are widely exposed at the surface, whereas in S. magellanicum, photosynthetic cells are enclosed within water-filled hyaline cells. This difference is expected to lead to variation in diffusive resistance within leaves (rp). Net photosynthesis and on-line carbon isotope discrimination were measured at two water contents: greenhouse water content (wet) and blotted dry (dry). Without correcting for respiration, on-line carbon isotope discrimination values differed significantly between wet (23.7 parts per thousand) and dry (30.9 parts per thousand) plants. However, there was no significant difference between species means and no species X water content interaction. Corrections for respiration lowered carbon isotope discrimination values by approximately 8.1 parts per thousand and reduced the mean difference to 3.1 parts per thousand, but did not alter the rank order of treatments. Net photosynthesis also decreased by 16% in wet plants, but there was no significant difference between the two species. In addition, five populations of S. affine and S. magellanicum grown in a common garden were analysed for their organic matter carbon isotope composition . These values varied more within each species (0.9-1.2 parts per thousand) than between the two species (0.6 parts per thousand). Therefore, we conclude that variation in surface water films leads to a greater difference in resistance to CO2 uptake and carbon isotope discrimination than that due to variation in leaf anatomical properties in Sphagnum.