CHILLING/FREEZING STRESS Photosynthetic Activity and Soluble Carbohydrate Content Induced by the Cold Acclimation Affect Frost Tolerance and Resistance to Microdochium nivale of Androgenic Festulolium Genotypes E. Pociecha 1 , A. Pla _ zek 1 , M. Rapacz 1 , E. Niemczyk 2 & Z. Zwierzykowski 3 1 Faculty of Agriculture and Economics, Department of Plant Physiology, Agricultural University in Krakow, Krako ´ w, Poland 2 Institute of Plant Physiology, Polish Academy of Sciences, Krako ´ w, Poland 3 Institute of Plant Genetics, Polish Academy of Sciences, Poznan ´ , Poland Introduction The survival of overwintering grasses is influenced by many factors including low temperature and winter pathogens. During autumn and early winter, falling temperatures induce hardening processes, which are impor- tant for the development of both snow mould and frost resistance (Tronsmo 1985, Gaudet 1994). At the same time, plants must face cold-induced photoinhibition, and therefore, photosynthetic acclimation to cold is observed during the pre-hardening and hardening of plants (Adams et al. 2002). Plants with higher tolerance towards cold-induced photoinhibition are often more frost resis- tant (Huner et al. 1993, Rapacz et al. 2004). On the other hand, this relationship is modified in field conditions by the environmental factors including deacclimation and the presence of a snow cover (Rapacz et al. 2008). Photo- synthetic apparatus is also considered to be a sensor of environmental changes occurring during autumn and, in this way, it may be indirectly involved in the development of winter hardiness (Gray et al. 1997). Temperature and light intensity changes trigger an imbalance between the energy absorbed in the light phase of photosynthesis and its consumption in the photosynthetic metabolism. Such changes affect the relative redox state of PSII (Huner et al. 1998). Changes in temperature and light intensity modify the balance between the energy absorbed in the light phase of photosynthesis and its consumption in the dark processes. Changes in the PSII redox state during pre-hardening and hardening may influence the freezing Keywords carbohydrates; chlorophyll-a fluorescence; cold acclimation; Festulolium; snow mould Correspondence E. Pociecha Faculty of Agriculture and Economics, Department of Plant Physiology, Agricultural University in Krakow, Podlu _ zna 3, 30-239 Krako ´ w, Poland Tel.: +48 012 425 3301 Fax: +48 012 425 3320 Email: rrchilmo@cyf-kr.edu.pl Accepted August 11, 2009 doi:10.1111/j.1439-037X.2009.00400.x Abstract The aim of the experiment was to verify whether changes in photosynthetic activity measured by means of chlorophyll fluorescence techniques and soluble carbohydrate level induced during pre-hardening and cold acclimation in androgenic forms of Festulolium are related to their winter hardiness and path- ogen resistance. The study was performed on six androgenic genotypes of Festulolium, which differ in their degrees of winter hardiness. The chlorophyll fluorescence parameters and sugar content were measured in the control plants, pre-hardened at 12 °C and cold-acclimated at 2 °C. Cold acclimation at 2 °C, preceded by pre-hardening at 12 °C, induced a decline in maximum PSII quantum efficiency (F v /F m ) and in non-photochemical quenching (NPQ). Exposure to 2 °C also resulted in reduced efficiency of energy conversion in photochemical processes (q p ). The soluble carbohydrate amount increased in the susceptible genotypes and was associated with a decline in NPQ and current quantum yield of PSII (/ PSII ). In resistant plants, a decrease in the sugar content was observed, which could be explained by their consumption in metabolic processes initiated during cold acclimation. The changes in sugar content were associated with a decline in NPQ and q p . J. Agronomy & Crop Science (2010) ISSN 0931-2250 48 ª 2009 Blackwell Verlag GmbH, 196 (2010) 48–54