1371 Roots Are Necessary for the Responses of In Vitro-Cultured Citrus Plants to High Salinity but Not to Osmotic Stress Rosa M. Pérez Clemente, Almudena Montoliu, Vicente Vives-Peris, Valeria Muñoz Espinoza, Sara I. Zandalinas and Aurelio Gómez-Cadenas * Universitat Jaume I (UJI), Deptartament de Ciències Agràries i del Medi Natural, Castelló de la Plana, Spain *Corresponding author: aurelio.gomez@uji.es Abstract In the field, plants are exposed simultaneously to variable biological and environmental conditions that can make physiological studies very difficult. The in vitro tissue culture techniques can overcome some of these limitations. In the present work, this methodology was applied to the study of salt and osmotic stress conditions on ‘Carrizo’ citrange. The stress conditions were generated by adding either NaCl or polyethylene glycol, to the culture medium. Micropropagated shoots, growing under salt- or osmotic-stress, shown symptoms of leaf damage very similar to those found in intact plants, which confirmed the incidence of the imposed stress on plant physiology. In whole plants, it has been reported that physiological responses to water and salt stress are essentially identical; in the opposite, in shoots cultured in vitro, levels of stress markers such as malondialdehyde (MDA) and proline increased only under water deficiency but not under elevated salt conditions. Differences were also observed in the hormonal regulation of the shoots subjected to each abiotic stress. Abscisic acid concentration increased in shoots grown under osmotic stress conditions whereas no differences with the controls were observed in salt-stressed ones. It can be concluded that, at least when culture in vitro, citrus roots are necessary for the perception and signaling of the salt stress conditions. On the contrary, the presence of this organ is not necessary to modulate the response of shoots to osmotic stress. Keywords: Carrizo citrange, Cleopatra mandarin, polyethylene glycol, proline, malondialdehyde, abscisic acid INTRODUCTION Among various abiotic stresses that limit plant growth, drought is the most complex and devastating worldwide. As a consequence of the Climate Change, the number of extreme drought events and mean drought duration are likely to increase by factors of two and six, respectively, by the 2090s (Burke et al., 2006). A decrease in summer precipitation, accompanied by rising temperatures, which enhance evaporative demand, would inevitably lead to reduced summer soil moisture and more frequent and intense droughts (Douville et al., 2002). Citrus, being a perennial tree, are frequently exposed to drought. Water deficit periods negatively affect citrus plant productivity in many aspects, including reduction in growth and metabolism, which leads to a decrease in fruit yield and quality (Pérez-Pérez et al., 2008). It has been reported that, in intact plants, the responses to water and salt stress conditions are essentially identical. The similarity between both abiotic stress situations involves most metabolic processes apart from those relating to ion transport. Hormonal responses are similar and a decrease in photosynthesis takes place in both, water- and salt- stressed plants (Munns, 2002). However, the study of the effects of a specific stress condition on the performance of plants cultivated under field conditions is difficult due to different uncontrolled factors such as interactions among abiotic and biotic stressors or changes in temperature among different trials, etc. It has been shown that the root system plays an important role in controlling water and chloride uptake (Moya et al., 2002), which means that the roots are a key organ in the responses of plants to abiotic stress. Roots are also crucial in the Proc. XII th Intl. Citrus Congress Eds.: B. Sabater-Muñoz et al. Acta Hort. 1065, ISHS 2015