Water Holding Capacity in Sphagnum: Micro CT Scanning Reveals Differences in Pore Structure

Species in the wetland plant genus Sphagnum engineer ecosystems by altering nutrient and water cycling and storage. Sphagnum plants hold water both internally and externally, with up to 80% held within capillary spaces among leaves and branches. Water retention and movement depend on hydraulic properties, which can be modeled within capillary spaces. These properties can help explain the water holding capacity in Sphagnum and how these differ across species. I used a comparative approach to understand variance in pore structure based on morphological characteristics using computer modeling and to estimate water holding capacity across species. To better understand external water holding among Sphagnum species, we used micro CT scanning to characterize the structure of pore water spaces within leaves and branches. Three species of Sphagnum were collected in the field, S. subsecundum, S. palustre, and S. pulchrum. These differ greatly in terms of cellular characteristics and represent distinct ecological groups in Sphagnum. Three mature branches from each type of Sphagnum were stained in Pb(NO3)2, imaged using a micro CT scanner and reconstructed to generate 3D model for comparison. Scans were unable to fully resolve cellular detail, but could reconstruct leaf morphology. We used a shrinkwrap technique to simulate the external water holding spaces within branches at different radii of curvature, which correspond to water held at particular water potentials (-1.07, -0.54, -0.36, -0.27, and -0.21 MPa). The ratios of water volume to leaf volume were calculated and were compared using ANOVA. For S. subsecundum, the mean ratio was 1.44 (n=3) and this was significantly lower (p=0.02) than S. palustre (2.22). Based on the Micro CT results for intra-plant pores, S. palustre holds more water across varying water potentials. S. palustre has a more compact structure leaf structure than S. subsecundum and S. pulchrum. The 3D concave dimensionality of S. palustre aids in pore water holding to avoid dessication. Understanding the mechanism that leads to differences in water holding capacity is helpful to understand the functional role of Sphagnum in ecosystems