The effect of perturbations in temperature and photon flux density on the growth and photosynthetic responses of five pasture species to elevated CO2

Abstract

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.