Effects of elevated CO2concentration and climate-warming on photosynthesis during winter inLolium perenne

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Long-term effects of atmospheric carbon dioxide concentration (ambient or 700 μmol mol−1) and air temperature (simulation of field conditions or + 4 °C) on leaf photosynthetic rate were examined in Lolium perenne L. cv. Vigor, exposed to natural illumination during winter. Photosynthetic capacity was compared over a range of air temperatures and photon flux densities of photosynthetically active radiation which were representative of winter climate (5–15 °C and 0–500 μmol m−2s−1), with CO2 level during measurement similar to that during the experimental period. Long-term exposure to increased air temperature reduced leaf CO2 fixation capacity by 23% (averaged over all measurement conditions), resulting from a decline in lightsaturated uptake rate, but not in incident-light quantum efficiency. CO2- stimulation was largely absent in plants grown in ambient temperature, but pronounced in plants grown under +4 °C, where it compensated for two-thirds of the 23% drop. This enhancing effect of elevated CO2 level on leaf CO2 uptake rate observed in the warmer treatment, was strongly dependent on measurement temperature, increasing from 5% at 5 °C, to up to 32% at 15 °C. Measurements of chlorophyll fluorescence and dry matter corresponded with the observed changes in assimilation capacity, which could not be attributed to a deteriorated nitrogen status of the leaves as there was a similar N content on an area basis. Several hypotheses are considered to explain the observed CO2-temperature interactions.