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

Abstract

ABSTRACTThe 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 (Δ=Δδ13C) 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, photo‐synthetic 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 Δ were measured at two water contents: greenhouse water content (wet) and blotted dry (dry). Without correcting for respiration, on‐line Δ values differed significantly between wet (23.7%o) and dry (30.9%o) plants. However, there was no significant difference between species means and no species × water content interaction. Corrections for respiration lowered Δ values by approximately 8.1%o and reduced the mean difference to 3.1%o, 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 (δ13C). These values varied more within each species (0.9–1.2%o) than between the two species (0.6%o). 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.