Light: Effects of High Intensity

Figure 1. Encalypta rhabdocarpa in the alpine region where high-intensity UV light can damage chlorophyll and DNA. Photo by Michael Lüth. Exposure to UV light has been hypothesized as a major deterrent of evolution to land. Both chlorophyll and DNA are easily damaged by high intensities of direct sunlight (Figure 1). In fact, it has been suggested that a major role of lignin, absent in bryophytes, is to protect cells against UV light. Hamerlynck and coworkers (2002) hypothesized that because of its strong desiccation tolerance characters, the moss Syntrichia ruralis (Figure 2) would be unable to acclimate to different light intensity regimes. However, they found that sun plants had lower biomass, and lower tissue N, C, and chlorophyll concentrations than shade plants of the species (Figure 3). Interestingly, while the carotenoid:chlorophyll ratios of sun plants were typical of sun plants, they found that as in most bryophytes the chlorophyll a:b ratios were typical of shade plants. When transplanted to shade, sun plants were able to adjust to the lower light level by increasing their photosystem II yields; these decreased in shade plants transplanted to the sun. Conversely, sun plants transplanted to shade continued to be out-performed there by non-transplanted shade plants. They suggest that in this species, shade plants may be able to adjust relatively quickly to disturbance that exposes them to greater light and desiccation. One danger of high light intensity in bryophytes is damage it can do to chlorophyll when the moss is dry. In experiments with a number of species, Churchill and Nelson (unpubl. report 1994) have found that the light intensity transmitted through a wet moss leaf is about twice that transmitted through a dry leaf. Takács et al. (2000) found that the non-chlorophyll blue-green fluorescence of Syntrichia ruralis and two lichens increased by an order of