Physiologia Plantarum 130: 532–542. 2007 Copyright ª Physiologia Plantarum 2007, ISSN 0031-9317 Responses to flooding and drought stress by two citrus rootstock seedlings with different water-use efficiency Francisco Garcı ´a-Sa ´ nchez a,c, *, James. P. Syvertsen a , Vicente Gimeno c , PabloBotı´a b and Juan G. Perez-Perez b a Citrus Research and Education Center, Institute of Food and Agricultural Sciences, University of Florida, 700 Experiment Station Road, Lake Alfred, FL 33850, USA b Departament of Citriculture, Instituto Murciano de Investigacio ´ n y Desarrollo Agrario y Alimentario, 30150 La Alberca, Murcia, Spain c Centro de Edafologia y Biologia Aplicada del Segura, CSIC, Campus Universitario de Espinardo, Espinardo, 30100 Murcia, Spain Correspondence *Corresponding author, e-mail: fgs@cebas.csic.es Received 25 January 2007; revised 27 March 2007 doi: 10.1111/j.1399-3054.2007.00925.x Leaf water relations, net gas exchange and leaf and root constituent responses to 9 days of drought stress (DS) or soil flooding were studied in 6-month-old seedlings of Carrizo citrange [Citrus sinensis (L.) Osb.  Poncirus trifoliata L.; Carr] and Cleopatra mandarin (Citrus resnhi Hort. ex Tanaka; Cleo) growing in containers of native sand in the greenhouse. At the end of the drought period, both species had similar minimum stem water potentials but Cleo had higher leaf relative water content (RWC) and higher leaf osmotic potential at full turgor (C 100 p ) than Carr. Flooding had no effect on RWC but osmotic adjustment (OA) and C 100 p were higher in Cleo than in Carr. Net CO 2 assimilation rate (A CO2 ) in leaves was decreased more by drought than by flooding in both species but especially in Carr. Leaf water-use efficiency (A CO2 /transpiration) was lower in Carr and was decreased more by DS and flooding stress than in Cleo. Higher values of intercellular CO 2 concentration (C i ) in stressed plants than in control plants indicated that non-stomatal factors including chlorophyll degradation and chlorophyll fluorescence [maximum quantum efficiency of PSII (Fv/Fm, where Fm is the maximum fluorescence and F 0 , minimum fluorescence in dark-adapted leaves)] were more important limitations on A CO2 than stomatal conductance. In both genotypes, leaf proline was increased by drought but not by flooding, whereas both stresses increased proline in roots. Soluble sugars in leaves were increased by DS, and flooding decreased leaf sugars in Cleo. In general, DS tended to increase the concentrations of Ca, K, Mg, Na and Cl in both leaves and roots, whereas flooding tended to decrease these ions with the exception of leaf Ca in Cleo. Based on water relations and net gas exchange, Cleo was more tolerant to short-term DS and flooding stress than Carr. Abbreviations – A CO2 , net CO 2 assimilation rate; A/E, leaf water-use efficiency; Carr, Carrizo; C i , intercellular CO 2 concentration; Cleo, Cleopatra; DS, drought stress; E, leaf transpiration rate; Fm, maximum fluorescence; F 0 , minimum fluorescence in dark- adapted leaves; Fv/Fm, maximum quantum efficiency of PSII; g s , stomatal conductance; OA, osmotic adjustment; C p , leaf osmotic potential; C 100 p , leaf osmotic potential at full turgor; C S , stem water potential; QAC, quaternary ammonium compounds; RWC, relative water content; WUE, water-use efficiency. 532 Physiol. Plant. 130, 2007