Seasonal Investigations of Soluble Carbohydrates and Pigment Levels in Antarctic Bryophytes and Lichens

Abstract

Levels of soluble carbohydrates, moisture, and pigments in the major bryophytes and lichens from the Windmill Islands, continental Antarctica, were monitored over a 14-month period. Gas-liquid chromatography revealed little seasonal change in soluble carbohydrate levels. Water content of vegetation varied throughout the season with maximum hydration occurring over the summer months. Pigment levels also varied seasonally with a decrease in total chlorophyll and chlorophyll a/b ratios during winter. Levels of total carotenoids increased over summer in response to high ambient irradiation. In contrast with the seasonality of soluble carbohydrates reported in some cryptogams from subpolar regions, the lack ofany significant seasonal changes in the Antarctic bryophytes and lichens may be due to the extreme climate and rapid temperature fluctuations in the continental Antarctic environment. Bryophytes and lichens form a dominant component of vegetation at high latitudes due to their ability to withstand extreme polar environments (Longton 1988; Tenhunen et al. 1992). An important factor in the resistance of these poikilohydric plants is considered to be the accumulation and maintenance of relatively high levels of cell solutes including lipids, amino acids, and soluble carbohydrates (Farrar 1976, 1978; Karunen & Salin 1982; Levitt 1980; Longton 1988). Although it has long been recognized that soluble carbohydrates confer frost resistance to higher plant cells by increasing their tolerance to osmotic and thermal stress (Carpenter & Crowe 1988; Levitt 1980), the significance of soluble carbohydrates in the frost resistance of polar bryophytes and lichens is unclear. Studies from subpolar regions have indicated seasonal trends in carbohydrate levels of mosses and lichens with the accumulation of photosynthates and inorganic nutrients during the periods preceding winter (Hicklenton & Oechel 1977; Smith 1978; Tearle 1987). These differences were high enough to suggest a role in frost resistance. In the latter study, maritimeantarctic lichens were found to have much higher soluble carbohydrate levels than those reported in lichens from temperate regions. Given the hostile nature of the antarctic environment, vegetation on continental Antarctica may similarly be expected to contain high levels of cell solutes in response to cold and desiccation stress. However, Rastorfer (1972) reported that many moss species are frost resistant despite low soluble sugar contents. Furthermore an initial survey offrigid-antarctic (sensu Longton 1988) cryptogams (Roser et al. 1992) showed soluble carbohydrate levels to be comparable to those found in temperate cryptogams and much lower than those reported by Tearle (1987) for maritime-antarctic cryptogams. However, as the former survey involved a single sampling over summer the possibility remained that seasonal increases in soluble carbohydrate levels may occur. In this paper the results of a long-term monitoring of the seasonal cycle of soluble carbohydrates in several of the dominant continental antarctic moss and lichen species are presented for the first time. To give some parallel measures of other growth paameters results are also given for seasonal variations in the moisture content and pigment levels of the plants. This information is important not only to clarify the adaptive physiology of plants in the extreme antarctic habitat but also in terms of the nutrients made available to the antarctic terrestrial ecosystem. The flush of soluble carbohydrates during the spring thaw is thought to contribute to the increase in soil microbial activity over this period in the maritime antarctic (Smith 1985; Tearle 1987; Wynn-Williams 1982). In continental Antarctica where the terrestrial biomass is much less, the nutrient input from cryptogamic vegetation is likely to be the major source of nutrients in areas away from bird colonies.