Element partitioning, element localisation and transcriptome responses of date palms exposed to NaCl

Date palm (Phoenix dactylifera L.) is an economically important fruit crop in many (semi)arid regions. Large intra-species variation in susceptibility to NaCl has been reported. Controlled experiments were conducted to evaluate sodium (Na) and chlorine (Cl) partitioning in two-year old plants of six date palm varieties and to determine tissue location of Na and Cl and transcriptome responses to NaCl of two varieties. The largest Na concentrations were found in secondary roots, especially under NaCl treatment. By contrast, only small differences in within-plant partitioning of Cl were observed with less differences between varieties than for Na. Variety Sultana had the largest Na translocation factor. The variety Khalas exhibited better restriction of Na mobility, which may be a consequence of Na hotspots in roots, presumably immobilising Na in roots, which were absent in Sultana. In the root, shoot base and lower part of leaves, Cl accumulated in fibre bands and sclerenchyma sheaths of vascular bundles. RNAseq analysis revealed that Sultana and Khalas respond very differently to NaCl: Khalas modified expression of NaCl-specific genes in both roots and leaves, while Sultana responded manly in leaves. Gene ontology analysis showed that the expression changes for genes linked with photosynthesis and carbohydrate metabolism predominated as a response to NaCl. In Khalas, differentially expressed genes encoding for metal transporters and those reported to be involved in heat, salt and osmotic stress response were found. Large variability in Na and Cl accumulation in different date palm varieties may have practical agronomical consequences.