The effect of perturbations in temperature and photon flux density on the growth and photosynthetic responses of five pasture species to elevated CO 2

Five pasture grass species with known differences in responses to elevated CO2 were grown at 18 and 28°C, 350 and 700 mol mol –1 CO2 and a photon flux density of 700 mol m –2 s –1 in controlled conditions. After 2 (28°C)–3 (18°C) weeks growth, plants were transferred to reciprocal temperature conditions and to 70% shade while other plants were maintained as controls. The objective was to impose environmental perturbations to evaluate how carbon supply and sink demand affected CO2 responses in different species. Responses were assessed by biomass accumulation and photosynthesis. Ranking of the growth response to CO2 was similar between 18 and 28°C. Lolium perenne and Festuca arundinacea showed the lowest response to elevated CO2 and Poa annua and Trifolium repens the highest response. Response to CO2was highest at 28°C for all species and significantly lower at 18°C. When transferred to different conditions, the growth response to CO2 of all species changed markedly, and their relative ranking also changed. Photosynthetic responses to CO2 were consistently higher for all species at 28 than at 18°C, but was enhanced by transfer to low temperature and suppressed by transfer to high temperature. The photosynthetic response to CO2, when multiplied by the leaf area of the plants, was linearly related to the growth response to CO2 across almost all treatments and species. This implies that there is an intrinsic relationship between the responses of the two processes of photosynthesis and growth when plants are exposed to elevated CO2.