EFFECTS OF ELEVATED CO2 CONCENTRATION AND CLIMATE-WARMING ON PHOTOSYNTHESIS DURING WINTER IN LOLIUM-PERENNE

Long-term effects of atmospheric carbon dioxide concentration (ambient or 700 mu mol mol(-1)) and air temperature (simulation of field condi tions or +4 degrees C) on leaf photosynthetic rate were examined in Lo lium perenne L. cv. Vigor, exposed to natural illumination during wint er. Photosynthetic capacity was compared over a range of air temperatu res and photon flux densities of photosynthetically active radiation w hich were representative of winter climate (5-15 degrees C and 0-500 m u mol m(-2) s(-1)), with CO2 level during measurement similar to that during the experimental period, Long-term exposure to increased air te mperature 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-stimula tion was largely absent in plants grown in ambient temperature, but pr onounced in plants grown under +4 degrees C, where it compensated for two-thirds of the 23% drop. This enhancing effect of elevated CO2 leve l on leaf CO2 uptake rate observed in the warmer treatment, was strong ly dependent on measurement temperature, increasing from 5% at 5 degre es C, to up to 32% at 15 degrees C. Measurements of chlorophyll fluore scence and dry matter corresponded with the observed changes in assimi lation capacity, which could not be attributed to a deteriorated nitro gen status of the leaves as there was a similar N content on an area b asis. Several hypotheses are considered to explain the observed CO2-te mperature interactions.