Effects of elevated CO2 concentration and climate-warming on photosynthesis during winter in Lolium perenne

Abstract

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-2 s-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 light-saturated 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.