In situ effects of elevated CO2 on chlorophyll fluorescence and chloroplast pigments of alpine plants

Alpine vegetation responds to elevated CO2 with downward adjustment of photosynthesis. The experiments should show if doubling of ambient CO2 reduces the maximum quantum yield and the chlorophylls thus altering the pigment composition of the thylakoid membranes in typical species of an alpine grassland (Caricetum curvulae). The studies were part of a CO2 enrichment experiment with open-top chambers in the Swiss Central Alps in 2 470 m altitude over a period of four years. The leaves of Carer curvula and Trifolium alpinum were analysed ill situ under ambient (355 mu l/l) or elevated (680 mu l/l) CO2 and at two different nutrient levels. In each vegetation period both species showed a tendency to lower ratios of variable to maximum fluorescence (F-v/F-m) in plants with elevated CO2 treatment compared to the ambient variants. These reductions in F-v/F-m were statistically different only for Carer curvula in 1993 and 1995. CO2 enrichment caused reductions of leaf pigment concentrations of 10-30% especially far Trifolium alpinum whereas Carer curvula was less affected. The lower pigment contents per leaf were probably due to reductions of thylakoid membranes. In most cases, the influences of elevated CO2 or of nutrient treatments on pigment composition and primary photochemistry were very small. This indicates that the downward regulation begins at early stages in the photosynthetic process. Some changes of the photosynthetic apparatus are species-specific and possibly reflect different strategies of protective acclimation processes of alpine vegetation.