Photosynthesis, pigments, and chloroplast ultrastructure of an Antarctic liverwort from sun-exposed and shaded sites

Abstract

POST, A., and VESK, M. 1992. Photosynthesis, pigments, and chloroplast ultrastructure of an Antarctic liverwort from sun-exposed and shaded sites. Can. J. Bot. 70: 2259-2264. Adaptations to severe environmental conditions were studied in the only recorded continental Antarctic hepatic, Cephaloziella exiliflora (Tayl.) Steph. During the summer growing season Antarctic plants melt out and many are exposed fully to the sun. C. exiliflora plants collected from sun-exposed and shaded sites were compared. Most plants were sun exposed, with dark purple leaves containing an anthocyanin-like pigment bound within the thick cell walls of the leaves. The purple plants grew in a dense turf, were larger with more closely spaced leaves, and had a higher carotenoid to chlorophyll ratio than the green plants. Shaded green plants contained more chlorophyll per unit weight. The chlorophyll a to chlorophyll b ratio did not vary. At low irradiances the green plants had higher photosynthetic oxygen evolution rates. Comparing leaves in similar positions along the stems showed that the chloroplasts in green leaves contained more appressed thylakoids than the chloroplasts in purple leaves. These differences are consistent with responses to varying light exposure. An understanding of these effects of changing irradiance on bryophytes is needed for monitoring the impact of increased ultraviolet light owing to the seasonal depletion of stratospheric ozone.

Sequential reduction of UV-B radiation in the field alters the pigmentation of an Antarctic leafy liverwort

“…During this period we also recorded a rise in chlorophyll a / b ratio from 1.45 to 2.46, the same value as that recorded by Post and Vesk (1992). This is analogous to the response recorded by Martin and Churchill (1982), who found that chlorophyll a / b ratio declined in a range of mosses after canopy closure in a temperate forest.…”

“…Similar effects of UV-B radiation on the accumulation of anthocyanins in plant tissues are well documented (Chalker-Scott, 1999). Furthermore, our study indicates that the reduced chlorophyll concentrations in sun-exposed C. varians tissues reported by Post and Vesk (1992) are also most probably owing to UV-B radiation exposure. After 35 d, chlorophyll concentrations in tissues of plants exposed to reduced UV-B were almost twice those of tissues exposed to high doses of UV-B.…”

“…The samples were returned to a laboratory c. 500 m from the gully in which the screens were deployed. The uppermost 2-3 mm of foliage was cut from each sample and, to facilitate grinding, it was blotted on absorbent paper until no free water could be drawn from the tissues (Post and Vesk, 1992). Preliminary analyses, using the spectrophotometric methods described below, indicated that this process removed c. 2% of the pigments analysed for in this study from tissues.…”

“…The extracts were then centrifuged at 6000 rpm for 10 min at 0 °C and 700 Fl of these extracts were passed through 0.45 Fm filters (Whatman International Ltd., Maidstone, UK). To extract UV-B screening pigments and the anthocyanin-like pigment, 3 ml of a 70:20:1 mixture of methanol, water and HCl was added to the second subsample and the tissues were ground for 1 min (Post and Vesk, 1992). The extracts were heated at 65 °C for 10 min and were then centrifuged at 6000 rpm for 10 min, the supernatants removed and placed in a refrigerator, and a further 1 ml of the methanol, water and HCl mixture was added to the pellet, which was resuspended before heating again at 65 °C for 10 min.…”

“…To estimate concentrations of UV-B screening pigments, the absorbance of each acidified methanol extract was measured between 280 and 315 nm (step 1 nm). The concentration of the anthocyanin-like pigment was estimated by measuring the absorbance of acidified methanol extracts at 495 nm (Post and Vesk, 1992). Concentrations of UV-B screening pigments were expressed in arbitrary units as the area under the absorbance curve (AUC per g dry weight of tissue.…”