Contents lists available at ScienceDirect Scientia Horticulturae journal homepage: www.elsevier.com/locate/scihorti Morpho-physiological and homeostatic adaptive responses triggered by omeprazole enhance lettuce tolerance to salt stress Petronia Carillo a , Giampaolo Raimondi b , Marios C. Kyriacou c , Antonio Pannico b , Christophe El-Nakhel b , Valerio Cirillo b , Giuseppe Colla d , Stefania De Pascale b , Youssef Rouphael b, ⁎ a Department of Environmental, Biological and Pharmaceutical Sciences and Technologies, University of Campania “Luigi Vanvitelli”, Caserta, Italy b Department of Agricultural Sciences, University of Naples Federico II, Portici, Italy c Department of Vegetable Crops, Agricultural Research Institute, Nicosia, Cyprus d Department of Agricultural and Forestry Sciences, University of Tuscia, Viterbo, 01100, Italy ARTICLE INFO Keywords: Benzimidazole Ion homeostasis Lactuca sativa L. Photosynthesis Small bioactive molecules Stomatal conductance ABSTRACT Natural or synthetic small molecules (< 500 Da), bioactive at very low concentrations, can potentially increase plant tolerance and resilience to abiotic stresses and improve the resources use efficiency (RUE) of a wide range of crops. Hence, they represent a promising tool in coping with the increasing global food demand imposed by climate change. In this study, the responses of butterhead lettuce (cv. Trocadero) treated with omeprazole (OMP), a benzimidazole inhibitor of animal proton pumps, were studied. OMP was applied as substrate drench at five rates (0, 10, 50, 100 or 200 μM) on lettuce plants grown under nonsaline or saline conditions of 1 or 30 mM NaCl. Increasing NaCl concentration decreased lettuce fresh and dry biomass by 37% and 25% in the 0 μM OMP treatment, respectively; whereas these reductions were mitigated by the 10 μM (12% and 19%, re- spectively) and 50 μM (15% and 14%, respectively) OMP application. Though OMP was not directly involved in ion homeostasis and K + /Na + ratio regulation, treatment with 10 μM OMP under saline conditions decreased Na + in leaves and Cl - in leaves and roots while increasing NO 3 - concentration in both organs. The synthesis of nitrogenous osmolytes may be implicated in increasing salt tolerance and the sustenance of transpiration and photosynthesis. Under nonsaline conditions, OMP increased root biomass, improving nutrient and water uptake, and therefore RUE. 1. Introduction Salinity is one of the most widespread threats to sustainable agri- culture, particularly severe in arid and semiarid regions, able to jeo- pardize crop production by causing a direct reduction of global mean yield by up to 20% (Munns and Tester, 2008; Aslam et al., 2017). Ions such as Na + , Cl - and SO 4 2- present in irrigation water can accumulate in the soil; while in alkaline soils, characterized by high concentrations of carbonate and bicarbonate, Ca 2+ and Mg 2+ ions may precipitate upon evapotranspiration, leaving the soluble Na + exceeding the concentration of beneficial nutrients (Rouphael et al., 2017a). Ex- cess Na + is able to increase the soil osmotic potential, thereby de- creasing the water influx into the plant roots, and interferes with the cation exchange complex of clay particles, causing soil compaction, low porosity and consequently hypoxia (Carillo et al., 2011). Intensive farming, seawater intrusion into freshwater aquifers, use of brackish water for irrigation and decrease in natural precipitation further con- tribute to soil salinization and adversely impact crop productivity (Annunziata et al., 2017). Mild salt concentrations in the root zone may not affect crop yield and product quality provided that the critical species-specific con- centration is not reached or the plants are not in a sensitive growth phase such as the germination and seedling stages (De Pascale et al., 2012; Ferchichi et al., 2018). However, when salt concentration in the root zone builds up, salt sensitive plants experience initially osmotic stress as an inhibition of water uptake and reduced transpiration rate, with a consequent inhibition of cell division-expansion and arrested plant growth (Maggio et al., 2001; Munns and Tester, 2008; Gorham et al., 2010). Subsequently, if salt stress severity and duration increase and plant exposure to salinity is prolonged or permanent, mature leaves experience first an ionic stress. In particular, Na + and Cl - interfere with K + and NO 3 - uptake mechanisms, respectively, causing https://doi.org/10.1016/j.scienta.2019.01.038 Received 20 December 2018; Received in revised form 16 January 2019; Accepted 18 January 2019 ⁎ Corresponding author. E-mail address: youssef.rouphael@unina.it (Y. Rouphael). Scientia Horticulturae 249 (2019) 22–30 Available online 26 January 2019 0304-4238/ © 2019 Elsevier B.V. All rights reserved. T