Sodium–major cation interplay and morphophysiological adjustments enabling castor bean (Ricinus communis L.) to withstand extreme salinity in dryland phytomanagement

• Salt tolerance was assessed in castor under progressively increasing NaCl levels. • In hypersaline drylands, reused water is a valuable resource for crop production. • Castor morphometric and physiological traits were monitored under salt stress. • Combined morphometric and physio‑chemical traits explained castor salt response. • Salt tolerance in castor relates to tissue nutrient distribution and Na balance. High and worldwide increasing salinity is impacting crop growth and yield, especially in drylands. In arid environments, the water scarcity has induced to irrigate with reused sanitized wastewaters. In drylands, large and increasing volumes (>50 billion m 3 year -1 ) of water potentially suitable for irrigation might originate from underground formations alongside oil and gas reservoir. The extremely high salinity (mainly due to sodium chloride, NaCl) of such water is the major limiting factor for its reuse as irrigation water. In these areas, non-food industrial crops adapted to drought and marginal lands, if tolerant to excess salt, represent a sustainable solution. Young plants of castor ( Ricinus communis L.) were grown in hydroponics under a wide range of high and increasing levels of NaCl (from 2.5 to 25 g L -1 , or 43 to 428 mM) for 24 days; the effects on growth, morphophysiological traits, photosynthesis, nutrients and sodium (Na) distribution and balance were studied. Plants began to show functional impairments in presence of 15 g L -1 (257 mM) NaCl, recovered at 20 g L -1 NaCl and went on growing and absorbing Na up to 25 g L -1 NaCl. The uptake of Na and its distribution in the plant tissues differently altered the concentration of major cations, while maintaining their relative content. The relationships among morphometric, physiological and chemical traits explained the castor tolerance to extreme saline concentrations. The high values of Na translocation and bioconcentration factors give good prospects for its bioaccumulation and removal by castor in applications of phytomanagement and/or phytodesalinization.