DOI: 10.1007/s10535-012-0234-8 BIOLOGIA PLANTARUM 57 (1): 33-40, 2013 33 Photosynthetic parameters of Ulmus minor plantlets affected by irradiance during acclimatization M.C. DIAS 1 *, G. PINTO 1 , C.M. CORREIA 2 , J. MOUTINHO-PEREIRA 2 , S. SILVA 1 , and C. SANTOS 1 Department of Biology and Centre for Environmental and Marine Studies (CESAM), University of Aveiro, 3810-193, Aveiro, Portugal 1 Centre for Research and Technology in Agro-Environmental and Biological Sciences (CITAB) and Department of Biology and Environment, University of Trás-os-Montes e Alto Douro, 5001-801, Vila Real, Portugal 2 Abstract In order to set up large-scale acclimatization protocols of micropropagated plants, an in-depth knowledge of their physiological responses during in vitro to ex vitro transfer is required. This work describes the photosynthetic performance of Ulmus minor micropropagated plants during acclimatization at high irradiance (HI; 200 ± 20 µmol m -2 s -1 or low irradiance (LI; 100 ± 20 µmol m -2 s -1 ). During this experiment, leaf pigment content, chlorophyll a fluorescence, gas exchange, stomata morphology, the activity of the Calvin cycle enzymes, and content of saccharides were measured in persistent and new leaves. The results indicated that HI induces a higher photosynthetic performance compared to LI. Therefore, plants acclimatized under HI are likely to survive better after field transfer. Additional key words: Calvin cycle enzymes, chlorophyll, elm, fluorescence, micropropagation, net photosynthetic rate, stomata, transpiration rate. Introduction The advantages of tissue culture for the rapid production of high-quality plants are widely recognized (Park 2002). Large-scale micropropagation is particularly useful in commercial plantation forestry (Park 2002, Merkel and Nairn 2005). Nowadays, improvement programs can be found for Eucalyptus species (e.g. Pinto et al. 2008, 2011), Pinus and Picea species (Park 2002), and Olea maderensis (Brito et al. 2009). However, for the large- scale acclimatization of micropropagated species, processes that are particularly sensitive during this stage (e.g. plant-water relations and photosynthesis) must be well understood in order to maximize survival rates. In vitro plants are grown in a culture medium with a large concentration of sugars, under low irradiance, high air humidity and low CO 2 concentrations. Once transferred to ex vitro conditions, the plants are very susceptible to several stresses since they have not developed adequate anatomical, morphological, and physiological features required to survive in the new environment (Pospíšilová et al. 1999, Pinto et al. 2011). During in vitro culture, plantlets usually show low or undetectable net photosynthetic rate (e.g. Fuentes et al. 2005, Osório et al. 2005, Rybczyński et al. 2007). The low amount and activity of RuBisCO seems to be one of the major constraints in in vitro culture (Hdider and Desjardins 1995). Fila et al. (2006) also point to the low CO 2 mesophyll conductance of in vitro plants as an important limitation for their photosynthesis. In addition, the deficient functioning of the overall water house- keeping system (e.g. poor stomatal control and cuticular abnormalities) also contributes to the problems occurring after ex vitro transfer (Pospíšilová et al. 1999, Hazarika 2006). Ulmus spesies belong to Europe´s noble hardwoods. ⎯⎯⎯⎯ Received 11 August 2011, accepted 8 February 2012. Abbreviations: Car - carotenoids; Chl - chlorophyll; DM - dry mass; c i /c a - ratio of intercellular to atmospheric CO 2 concentration; E - transpiration rate; F v /F m - variable to maximum chlorophyll fluorescence (maximal efficiency of PSII); g s - stomatal conductance; HI - high irradiance; LI - low irradiance; P N - net photosynthetic rate; PSII - photosystem 2; qN - non-photochemical quenching; qP - photochemical quenching; RH - relative humidity; RuBisCO - ribulose-1,5-bisphosphate carboxylase/oxygenase; sFBPase - stromal fructose-1,6-bisphosphatase; Φ PSII - effective quantum efficiency of PSII. Acknowledgement: This work was supported by the Portuguese Foundation for Science and Technology through a post-doctoral fellowship of M.C. Dias (SFRH/BPD/41700/2007) and S. Silva (SFRH/BPD/74299/2010). The authors acknowledge Dr. K. Bennett for her assistance in proofreading the manuscript. * Corresponding author; fax: (+351) 234 370 985, e-mail: celeste.dias@ua.pt