Photosynthesis of Lolium perenne L. at low temperatures under low irradiances

To develop mechanistic models for winter survival of grasses under climate change, more knowledge is needed of photosynthetic activity at low irradiance and temperature. Photosynthetic activity of small stands of Lolium perenne was continuously studied for up to two weeks under low photosynthetic photon flux density (PPFD) in the air temperature range from -3 degrees C to +9 degrees C. The photosynthetic rate of plants growing at 2, 6 and 9 degrees C was similar at 120 mu mol m(-2) s(-1) PPFD but the rate increased with increasing PPFD at all temperatures, particularly 9 degrees C. Light saturation of photosynthesis was reached at approx. 300 mu mol m(-2) s(-1) at 2 and 6 degrees C, and at approx. 600 mu mol m(-2) s(-1) at 9 degrees C. At 300 mu mol m(-2) s(-1), the CO2 exchange rate (CER) at 2 degrees C was approx. 60% of that at 9 degrees C. When the temperature increased gradually from 0 degrees C at start to +5 degrees C at the end of the 8-h photoperiod, CER decreased by about 20% compared with that at constant 5 degrees C. Changing the temperature from constant +5 degrees C to a diurnal variation between -2 and +2 degrees C gradually decreased daytime CER to approx. 10% after five days, partly due to leaf area losses (similar to 50% loss) when the poorly acclimatised plants were exposed to frost. At start of the photoperiod at -2 degrees C CER was negative, but became slightly positive when temperature exceeded -1 degrees C after two hours. Total daily photosynthesis was negative due to night-time respiration in this treatment. Soil heating to avoid freezing when the diurnal air temperature fluctuated between -3 and +3 degrees C had no effect on CER at this low PPFD level (150 mu mol m(-2) s(-1)). In contrast to the -2/+2 degrees C treatment, total daily photosynthesis was slightly positive in the -3/+3 degrees C treatment, where the plants were better acclimatised to frost. Increasing the CO2 concentration from 350 to 600 mu mol mol(-1) had no effect on CER at 2 and 6 degrees C, but increased it by 20% at 9 degrees C. The data indicate that the minimum temperature for photosynthesis in the cultivar studied is about -4 degrees C. The results can be applied in different photosynthesis models. (C) 2010 Elsevier B.V. All rights reserved.