Effect of atmospheric CO2 enrichment on Rubisco content in herbaceous species from high and low altitude

Atmospheric CO2 enrichment reduces Rubisco content in many species grown in controlled environments; however, relatively few studies have examined CO2 effects on Rubisco content of plants grown in their natural habitat. We examined the response of Rubisco content to atmospheric CO2 enrichment (600-680 mu mol mol-l in place of ppm) in 5 herbaceous species growing in a low altitude grassland (550 m) near Basel, Switzerland, and 3 herbaceous species from Swiss alpine grassland at 2 470 m. At low elevation, the dominant grass Bromus erectus and the subdominant dicot Sanquisorba miller exhibited 20% to 25% reduction of Rubisco content following high CO2 exposure; no CO2 effect was observed in the subdominants Carex flacca, Lotus corniculatus and Trifolium repens. At the Alpine site, the subdominant grass Pea alpina maintained 27% less Rubisco content when grown at high CO2 while the co-dominant forb Leontodon helveticus had 19% less Rubisco in high CO2: Rubisco content was unaffected in the tundra dominant Carer curvula. Because the degree of Rubisco modulation was similar between high and low elevation sites, it does not appear that differences in local partial pressure of CO2 (altitude) or differences in stress in general induce different patterns of modulation of photosynthetic capacity in response to high CO2. In addition, the degree of Rubisco reduction (<30%) was less than might be indicated by the low biomass response to CO2 enrichment previously observed al these sites. Thus, plants in Swiss lowland and alpine grassland appear to main tain greater Rubisco concentration and photosynthetic capacity than whale plants can effectively exploit in terms of harvestable biomass.