Strong responses of growth and photosynthesis of five C3 pasture species to elevated CO2 at low temperatures

Abstract

Five cool-season C₃ pasture species (Bromus willdenowii Kunth, Festuca arundinacea Schreb, Lolium multiflorum Lam., Lolium perenne L. and Trifolium repens L.) were grown at a constant temperature of 12°C to assess effects of elevated CO₂ on growth and photosynthesis. Plants were grown at either 350 or 700 μmol mol^–1 CO₂ for 42-56 d, after which they were destructively harvested to assess biomass. At the same time, leaf gas exchange was measured at 12°C for each species at both CO₂ concentrations. Photosynthetic response to CO₂, that is the A/C_i curve, was also measured. Results showed all species responded to elevated CO₂ at low temperature by increasing total biomass, relative to that at ambient CO₂, from 105 to 158%. T. repens and B. willdenowii had the smallest response, and the two Lolium species had the greatest response. Photosynthesis also responded strongly to elevated CO₂ at low temperature, increasing on average by 40%. Only small growth CO₂ effects occurred with the A/C_i curves, with lower CO₂-saturated photosynthesis only for L. perenne and T. repens grown at elevated CO₂. The photosynthetic response to CO₂ was not correlated with the growth response. However, the product of photosynthesis and leaf area, that is carbon supply, was highly correlated with the growth rate, and growth CO₂ had a marked effect. Results suggest T. repens responded poorly to elevated CO₂ at low temperature because of a low capacity to fix carbon, while both Lolium species responded strongly at low temperature because of a high capacity to fix CO₂. Changes to photosynthetic model parameters are now required to account for growth temperature effects on the CO₂ response.