BIOLOGIA PLANTARUM 52 (2): 385-390, 2008 385 BRIEF COMMUNICATION Leaf gas exchange, water relations, nutrient content and growth in citrus and olive seedlings under salinity J.C. MELGAR*, J.P. SYVERTSEN**, V. MARTÍNEZ*** and F. GARCÍA-SÁNCHEZ*** 1 Departmento de Agronomía, Universidad de Córdoba, Edificio Celestino Mutis, Ctra. Madrid-Cádiz km. 396, E-14071, Córdoba, Spain* UF/IFAS, Citrus Research and Education Center, 700 Experiment Station Road, Lake Alfred, FL 33850, USA** Centro de Edafología y Biología Aplicada del Segura, CSIC, Campus Universitario de Espinardo, Espinardo, E-30100, Murcia, Spain*** Abstract The effects of salinity on growth, leaf nutrient content, water relations, gas exchange parameters and chlorophyll fluorescence were studied in six-month-old seedlings of citrus (Citrus limonia Osbeck) and rooted cuttings of olive (Olea europaea L. cv. Arbequina). Citrus and olive were grown in a greenhouse and watered with half strength Hoagland’s solution plus 0 or 50 mM NaCl for citrus, or plus 0 or 100 mM NaCl for olive. Salinity increased Cl - and Na + content in leaves and roots in both species and reduced total plant dry mass, net photosynthetic rate and stomatal conductance. Decreased growth and gas exchange was apparently due to a toxic effect of Cl - and/or Na + and not due to osmotic stress since both species were able to osmotically adjust to maintain pressure potential higher than in non- salinized leaves. Internal CO 2 concentration in the mesophyll was not reduced in either species. Salinity decreased leaf chlorophyll a content only in citrus. Additional key words: chlorophyll fluorescence, leaf chlorophyll content, NaCl, photosynthesis, rootstock. ⎯⎯⎯⎯ The salinity tolerance of crops varies widely but citrus trees are considered salt sensitive whereas olive trees are moderately tolerant to salinity (Maas and Hoffman 1977). Since all commercial citrus trees are grafted onto rootstocks, the salt tolerance of citrus trees can be associated with the ability of the root system to restrict the uptake and/or transport to saline ions to shoots (Levy and Syvertsen 2004). High concentrations of Cl - and/or Na + in the leaves of citrus trees have been frequently related to nutrient imbalances and reductions in gas exchange and water relations (Zekri and Parsons 1992, Walker et al. 1993). Rangpur is one of the most salt- tolerant citrus rootstocks since trees grafted onto it accumulate Cl - at a relatively slower rate than trees on other rootstocks (Zekri and Parsons 1992). Olive trees are grown from propagated cuttings and their salinity tolerance is a cultivar-dependent characte- ristic (Marín et al. 1995, Tattini 1994) that has been related to a mechanism of salt ion exclusion by roots, preventing salt translocation rather than salt absorption (Benlloch et al. 1991, Tattini 1994). Since considerable variations in salt tolerance exist among citrus and olive trees, we used two different NaCl concentrations in the irrigation water to study their respective salt tolerance. In citrus, we used 50 mM NaCl in the irrigation water since this concentration can cause a ⎯⎯⎯⎯ Received 20 September 2006, accepted 17 February 2007. Abbreviations: Chl - chlorophyll; c i - internal CO 2 concentration; E - leaf transpiration rate; E wp - whole plant transpiration rate; F m - maximum fluorescence of dark-acclimated leaves; F 0 - minimum fluorescence of dark-acclimated leaves; F v /F m - maximum quantum efficiency of photosystem 2; g s - stomatal conductance; LDM/area - leaf dry mass to area ratio; P N - net photosynthetic rate; S/R - shoot to root ratio; TPDM - total plant dry mass; WUE - water use efficiency; Φ - effective quantum yield; Ψ P - pressure potential; Ψs - osmotic potential; Ψ W - water potential. Acknowledgements: J.C. Melgar was a visiting PhD student in J. Syvertsen’s lab, supported by UF/IFAS and the Comisión Interministerial de Ciencia y Tecnología, Spain, project no. AGL2001- 2447. Dr. F. García-Sánchez was funded by a post-doc fellowship from the Ministerio de Educación, Cultura y Deportes of Spain (AGL2003-O8502-CO4-02/AGR). Grateful acknowledgement is made to Eva Ros and Jill Dunlop for their skilled technical assistance. 1 Corresponding author; fax: (+34) 968 396213, e-mail: fgs@cebas.csic.es