DOI: 10.1007/s11099-011-0025-z PHOTOSYNTHETICA 49 (2): 185-194, 2011 185 Evaluation of cold stress of young industrial chicory (Cichorium intybus L.) by chlorophyll a fluorescence imaging. II. Dark relaxation kinetics P. LOOTENS *,+ , S. DEVACHT *, ** , J. BAERT * , J. VAN WAES * , E. VAN BOCKSTAELE *,** , and I. ROLDÁN-RUIZ * Institute for Agricultural and Fisheries Research (ILVO), Plant Sciences Unit, Caritasstraat 21, 9090 Melle, Belgium * Ghent University, Faculty Bioscience Engineering, Department of Plant Production, Coupure Links 653, 9000 Gent, Belgium ** Abstract Industrial chicory, Cichorium intybus L., has rather poor early vigour under the typical early spring morning conditions of low temperatures and high light intensity. Screening tools are being developed to assess the cold tolerance/sensitivity of young industrial chicory plants under these conditions. Refinement of such tools requires better understanding of the plants’ physiological responses. In this paper we discuss the effects of growth temperature (GT), measurement temperature (MT), and measuring light intensity (ML) on the relaxation of the Kautsky curve. We chose the chicory variety ‘Hera’, as it is known to possess a good average early vigour. Young plants of the variety ‘Hera’ were grown at three temperatures (GT): 16°C (reference), 8°C (intermediate), and 4°C (cold stress). The dark relaxation kinetics were analyzed at different light intensities (ML) in combination with different measurement temperatures (MT). The three components of the nonphotochemical quenching process (NPQ E , NPQ T , and NPQ I ) were determined. NPQ E was not affected by GT but was significantly affected by MT and ML. NPQ T and NPQ I were affected by all factors and their interactions. An acclimation effect for plants grown at low GT was detected. Acclimation resulted in lower NPQ T and NPQ I values. The halftime of the inhibition depending on NPQ (NPQ I ) was not affected by any of the factors investigated. Based on the data generated, we conclude that NPQ I is a valuable parameter for screening the cold sensitivity of young industrial chicory plants. Additional key words: chilling, energy-dependent quenching, nonphotochemical quenching, photoinhibition, state-transition- dependent quenching. Introduction Industrial chicory, Cichorium intybus L., is cultivated for the production of inulin, a linear fructose polymer with a terminal glucose molecule. Early sowing allows for a longer growth season and can contribute to improved yield (Baert 1997). However, the yield potential of the currently available varieties has not yet been completely exploited due to the slow youth growth that most chicory varieties display at low temperatures (Baert 1997). ——— Received 16 July 2010, accepted 26 March 2011. + Corresponding author; tel. +32 9 272 28 55, fax. +32 9 272 29 01, e-mail: peter.lootens@ilvo.vlaanderen.be Abbreviations: ANOVA − analysis of variance; Ax − antheraxanthin; Chl − chlorophyll; EC − electrical conductivity; F o − the minimum chlorophyll fluorescence in dark-adapted state; F m − the maximum chlorophyll fluorescence in dark-adapted state; F m ' − maximum fluorescence after light induction; htE − halftime of the energy-dependent quenching; htI − halftime of the photoinhibition-dependent quenching; htT − halftime of the state-transition-dependent quenching; GT − growth temperature; k P − rate constant for PSII photochemistry; LHC − light-harvesting complex; ML − measurement light intensity; MT − measurement temperature; NPQ − nonphotochemical quenching of the chlorophyll fluorescence signal; NPQ E − energy-dependent quenching; NPQ f − fast nonphotochemical quenching; NPQ I − photoinhibition-dependent quenching; NPQ T − state-transition-dependent quenching; PAM − pulse amplitude modulated; PAR − photosynthetically active radiation; PSI − photosystem I; PSII − photosystem II; q E − energy-dependent quenching; q I − photoinhibition-dependent quenching; q N − nonphotochemical quenching coefficient of the Chl fluorescence signal; q T − state-transition-dependent quenching; SE − standard error; Vx − violaxanthin; Zx − zeaxanthin. Acknowledgements: The authors thank Laurent Gevaert, Luc Van Gijseghem and Christian Hendrickx for their help with the measurements and the cultivation and maintenance of the plants. The authors also like to thank Assistant Professor E. Rosenqvist and Professor N. D’Ambrosio for useful discussions concerning the Chl a fluorescence parameters. Miriam Levenson is acknowledged for her English language review.