Salt Tolerance Diversity in Citrus Rootstocks Agrees with Genotypic Diversity at the LCl-6 Quantitative Trait Locus

Background/Objective: Salinity is a growing problem affecting a large portion of global agricultural land, particularly in areas where water resources are scarce. The objective of this study was to provide physiological and molecular information on salttolerant citrus rootstocks to mitigate the detrimental effects of salinity on citriculture. Methods: Ten accessions belonging to eight Citrus species and four to Poncirus trifoliata Raf. were tested for salinity tolerance (0 and 15 mM NaCl for 1 year) in terms of vegetative and Cl− tissue distribution traits. In addition, most accessions were evaluated for leaf Na+ and other cations. Results: All salt tolerant accessions tended to restrict the leaf Cl− content, although in a lower degree than the Cleopatra mandarin. However, differences in their ability to restrict leaf [Na+] were evident, contributing to a classification of trifoliate and sour orange accessions that matched their genotypic grouping based on allele sharing at a marker targeting candidate gene coding for the NPF5.9 transporter within LCL-6 quantitative trait locus. Conclusions: Our markers targeting LCl-6 candidate genes coding for NPF5.9, PIP2.1, and CHX20 (citrus GmSALT3 ortholog) could be efficient tools for managing the detected salt tolerance diversity in terms of both Cl− and Na+ homeostasis in rootstock breeding programs derived from these species, in addition to Citrus reshni.