Chapter 8 - 2 Nutrient Relations : Co 2

Figure 1. Peat moss, Sphagnum fimbriatum, covering the largest area of carbon sink in the world. Photo by Michael Lüth. Normally we don't think of carbon as a limiting resource, although experiments on higher plants have shown that increased carbon dioxide usually increases productivity. Mosses are typically C 3 plants with high CO 2 compensation points (Raven et al. 1998). In other words, they require high levels of CO 2 to balance the CO 2 lost to respiration. C 3 plants are those plants that have no special mechanism for storing carbon from CO 2 temporarily in a compound such as malate or oxalate. Instead, they put all their CO 2 directly into the photosynthetic pathway in a 3-carbon compound, hence the term C 3. This pathway is less efficient because the enzyme Rubisco (Ribulose bisphosphate carboxylase/oxidase) is much less effective at binding the atmospheric CO 2 into a 3-C compound within the cell than is PEP carboxylase, the enzyme used in the C 4 and CAM pathways to put the carbon in temporary storage compounds (C-4) for later use in photosynthesis. However, mosses are not limited by guard cell closure in obtaining CO 2 and thus should be able to obtain CO 2 any time of the day. Bryophytes must live in a delicate balance between sufficient moisture and sufficient CO 2. When the leaves are wet on the outside, that water offers significant resistance to CO 2 diffusion. Surprisingly, a thin cuticle permits greater diffusion than even a thin film of water, so mosses living in very wet habitats often are protected from waterlogging by well-developed waxes or other cuticular material (Proctor 1984). Polytrichum commune and P. strictum are good examples of this, but less obvious examples are Saelania glaucescens (Figure 2), Schistostega pennata, Pohlia cruda, P. wahlenbergii, Bartramia pomiformis, and Philonotis, all of which have a whitish appearance to the naked eye (Proctor 1984). Sphagnum partially solves this balance by having water-holding cells (hyaline cells) that bathe the photosynthetic cells (Figure 3), while exposing at least one surface (in most) of the photosynthetic cell to the atmosphere. Furthermore, air bubbles become trapped among the leaves and between the leaves and the stem, thus providing an additional source of CO 2. Robinson (1985) considered that no CO 2 was obtained from the hyaline (water-holding) cells because all the chloroplasts of the cells were positioned along the wall most …