Citation: Hammami, S.B.M.; Chaari, S.; Baazaoui, N.; Drira, R.; Drira, N.; Aounallah, K.; Maazoun, A.; Antar, Z.; Jorrín Novo, J.V.; Bettaieb, T.; et al. The Regulation of Ion Homeostasis, Growth, and Biomass Allocation in Date Palm Ex Vitro Plants Depends on the Level of Water Salinity. Sustainability 2022, 14, 12676. https://doi.org/10.3390/su141912676 Academic Editors: Erika Sabella and Teodor Rusu Received: 15 August 2022 Accepted: 29 September 2022 Published: 5 October 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article The Regulation of Ion Homeostasis, Growth, and Biomass Allocation in Date Palm Ex Vitro Plants Depends on the Level of Water Salinity Sofiene B. M. Hammami 1, * ,† , Sami Chaari 1,2,† , Narjes Baazaoui 3 , Riadh Drira 4 , Noureddine Drira 4 , Karim Aounallah 1 , Asma Maazoun 1 , Zied Antar 5,6 , Jesús V. Jorrín Novo 7 , Taoufik Bettaieb 1 , Hava F. Rapoport 8 and Besma Sghaier-Hammami 2,7,9, * 1 Laboratoire LR13AGR01, Institut National Agronomique de Tunisie, Université de Carthage, 43 Avenue Charles Nicolle, Tunis 1082, Tunisia 2 Laboratoire des Plantes Extrêmophiles, Centre de Biotechnologie de Borj-Cédria, B.P. 901, Hammam-Lif 2050, Tunisia 3 Biology Department, College of Sciences and Arts Muhayil Assir, King Khalid University, Abha 61421, Saudi Arabia 4 Green Biotechnology Company, P.O. Box 125, Sfax 3071, Tunisia 5 Ecole Nationale des Sciences et Technologies Avancées de Borj Cedria (ENSTAB), Université de Carthage, Hammam-Chott 1164, Tunisia 6 Laboratoire Génie des Matériaux et Environnement (LGME), Ecole Nationale des Ingénieurs de Sfax (ENIS), Université de Sfax, Sfax 3038, Tunisia 7 Agroforestry and Plant Biochemistry, Proteomics and Systems Biology, Department of Biochemistry and Molecular Biology, University of Cordoba, UCO-CeiA3, 14014 Cordoba, Spain 8 Instituto de Agricultura Sostenible-C.S.I.C., Avenida Menéndez Pidal s/n, 14004 Cordoba, Spain 9 Département Santé Végétale et Environnement, Institut National Agronomique de Tunisie, Université de Carthage, 43 avenue Charles Nicolle, Tunis 1082, Tunisia * Correspondence: sofiene.hammami@inat.u-carthage.tn (S.B.M.H.); besma.sghaier@inat.ucar.tn (B.S.-H.) † These authors contributed equally to this work. Abstract: The date palm, a central plant in the fragile oasis ecosystem, is considered one of the fruit species most tolerant to salt stress. However, the tolerance mechanisms involved are yet to be addressed and their evaluation until now was mainly based on heterogenous plant material such as seedlings or limited to in vitro experiment conditions. For these reasons, we propose to deepen our knowledge of the morphological and physiological responses to salt stress using acclimated ex vitro plants resulting from the propagation of a single genotype. The plants were irrigated with 0, 150, 300, or 450 mM NaCl solutions for four months. Our results showed that the influence of water salinity on growth and ion-homeostasis regulation was very dependent on stress levels. The 150 mM NaCl concentration was found to improve dry biomass by about 35%, but at higher salt concentrations (300 and 450 mM) it decreased by 40–65%. The shoot:root dry mass ratio decreased significantly at the 150 mM NaCl water concentration and then increased with increasing water salt concentration. The leaf:root ratio for Na + and Cl − decreased significantly with increasing water salinity up to a concentration of 300 mM NaCl, and then stabilized with similar values for 300 mM and 450 mM NaCl. In contrast to Na + and Cl − , leaf K + content was significantly higher in the leaf than in the root for all salt treatments. Unlike Na + and K + , Cl − was expelled to the surface of leaves in response to increased water salinity. Overall, date palm plants appear to be more capable of excluding Cl − than Na + and of changing biomass allocation according to salt-stress level, and their leaves and roots both appear to play an important role in this tolerance strategy. Keywords: ex vitro plants; ions exclusion and uptakes; Phoenix dactylifera; salt stress adaptation; salt stress avoidance Sustainability 2022, 14, 12676. https://doi.org/10.3390/su141912676 https://www.mdpi.com/journal/sustainability