Carbon isotope discrimination in alpine succulent plants supposed to be capable of crassulacean acid metabolism (CAM)

Abstract

1. The 4 alpine species of Sempervivum (S. arachnoideum L., S. montanum L., S. soboliferum Sims, S. wulfenii Hoppe) analyzed in this study showed acidification during the dark period in their natural habitat. The δ¹³C value of these species varied according to the water supply at their natural habitat, being less negative at dry sites. These data are consistent with a larger contribution of dark CO₂ fixation via Phosphoenolpyruvate-Carboxylase in relation to light CO₂ fixation via Ribulose-1, 5-diphosphate Carboxylase. These alpine Sempervivum spp. behave, therefore, like typical CAM-plants. 2. In contrast, the 3 alpine species of Sedum analyzed in this respect (S. acre. L., S. alpestre Vill., S. rosea (L.) Scop.) showed no pronounced dark acidification. The δ¹³C value of the 8 alpine Sedum species examined in this study (the 3 mentioned above and S. album L., S. atratum L., S. dasyphyllum L., S. reflexum L., S. sexangulare L.) was much more negative than in the Sempervivum spp., indicating that the dark CO₂ fixation does not play a great role in the carbon metabolism of the alpine Sedum spp. in the natural habitats. Water supply in the natural habitat has no clear-cut influence on the δ¹³C value. 3. All alpine species of Saxifraga analysed (S. seloides L., S. oppositifolia L., S. bryoides L., S. aizoides L., S. squarrosa Sieber, S. paniculata Mill., S. caesia L.) show δ¹³C values typical of C₃ plants and they were not influended by the water conditions in the natural sites. Saxifraga paniculata showed no dark acidification in its natural habitat and we suspect that alpine Saxifrages are not CAM plants. 4. Based on δ¹³C values the alpine plants Pinguicula alpina L., Thesium alpinum L., and Linaria alpina (L.) Mill. are typical C₃ plants.