Environmental and Experimental Botany 70 (2011) 297–304 Contents lists available at ScienceDirect Environmental and Experimental Botany journal homepage: www.elsevier.com/locate/envexpbot Photosynthesis of Lolium perenne L. at low temperatures under low irradiances Mats Höglind ∗ , Hans M. Hanslin, Leiv M. Mortensen Norwegian Institute for Agricultural and Environmental Research (Bioforsk), Grassland and Landscape Division, Postvn. 213, 4353 Klepp St., Norway article info Article history: Received 8 September 2009 Received in revised form 30 September 2010 Accepted 9 October 2010 Keywords: Air temperature CO2 concentration Diurnal photosynthesis Gas exchange rate Growth chamber Light saturation Soil temperature abstract 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 ◦ C to +9 ◦ C. The photosynthetic rate of plants growing at 2, 6 and 9 ◦ C was similar at 120 mol m -2 s -1 PPFD but the rate increased with increasing PPFD at all temperatures, particularly 9 ◦ C. Light saturation of photosynthesis was reached at approx. 300 mol m -2 s -1 at 2 and 6 ◦ C, and at approx. 600 mol m -2 s -1 at 9 ◦ C. At 300 mol m -2 s -1 , the CO 2 exchange rate (CER) at 2 ◦ C was approx. 60% of that at 9 ◦ C. When the temperature increased gradually from 0 ◦ C at start to +5 ◦ C at the end of the 8-h photoperiod, CER decreased by about 20% compared with that at constant 5 ◦ C. Changing the temperature from constant +5 ◦ C to a diurnal variation between -2 and +2 ◦ C gradually decreased daytime CER to approx. 10% after five days, partly due to leaf area losses (∼50% loss) when the poorly acclimatised plants were exposed to frost. At start of the photoperiod at -2 ◦ C CER was negative, but became slightly positive when temperature exceeded -1 ◦ 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 ◦ C had no effect on CER at this low PPFD level (150 mol m -2 s -1 ). In contrast to the -2/+2 ◦ C treatment, total daily photosynthesis was slightly positive in the -3/+3 ◦ C treatment, where the plants were better acclimatised to frost. Increasing the CO 2 concentration from 350 to 600 mol mol -1 had no effect on CER at 2 and 6 ◦ C, but increased it by 20% at 9 ◦ C. The data indicate that the minimum temperature for photosynthesis in the cultivar studied is about -4 ◦ C. The results can be applied in different photosynthesis models. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Perennial ryegrass (Lolium perenne L.) is one of the most impor- tant forage grasses in temperate regions of the world, such as continental Europe, the UK and New Zealand. Predicted climate warming may significantly extend the growing regions of this species into areas where its use today is restricted by limited winter hardiness. However, perennial ryegrass responses to combinations of different winter stress factors and low-light conditions are not well researched and simulation models are needed to predict plant performance in different climate change scenarios. Growth models that include winter stress factors are rare. To develop mechanistic models for winter survival of grasses under climate change, more knowledge is needed of photosynthetic activ- ity at low irradiance and temperature. A model by Riedo et al. (1998) simulates annual biomass production and fluxes of carbon, nitrogen and heat. The process-based LINGRA model has been used to sim- ∗ Corresponding author. Tel.: +47 404 75 391; fax: +47 51 42 67 44. E-mail address: mats.hoglind@bioforsk.no (M. Höglind). ulate biomass accumulation and the dynamics of tillers, leaves and carbohydrates in perennial ryegrass (Rodriguez et al., 1999) and timothy (Höglind et al., 2001). The model was recently extended to include winter stress factors (van Oijen et al., 2008). Preliminary results show that it is difficult to accurately simulate the content of storage carbohydrates in the plant during winter, partly due to incomplete knowledge about photosynthesis and respiration under winter conditions. A high photosynthetic rate at low positive tem- peratures is important for the development of freezing tolerance in many plants, including C 3 grasses (Huner et al., 1993; Harrison et al., 1997; Hjelm and Ögren, 2003). The temperature responses of photosynthesis at intermediate to high temperatures are well documented (e.g. Bernacchi et al., 2009), but for temperatures below +10 ◦ C, less is known. Significant photosynthetic activity has previously been found at temperatures around freezing point and the minimum temperature for pho- tosynthesis for four C 3 grasses is reported to be -4 ◦ C(Skinner, 2007). Under snow cover at temperatures close to 0 ◦ C, positive photosynthetic rates of some Arctic evergreens have been found in springtime when the snow is wet and relatively permeable to light (Starr and Oberbauer, 2003). 0098-8472/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.envexpbot.2010.10.007