Influence of Moderate to High Salinity on the Phytochemical Profiles of Two Salinity-Tolerant Spinach Genotypes

Abstract

Freshwater is a major concern in terms of meeting the growing food demand for the growing population. Recycled waters are currently seen as an alternative, but their higher salinity may impact the yield of most crops and their phytochemical content. In this study, we investigated the effects of five salinity treatments combined with two levels of potassium (K) (deficient K, T1–T5, and enough K, T6–T10) on the phenolic profiles and soluble sugars of two spinach genotypes, Raccoon and Gazelle. We also compared colorimetric assays with chromatographic assays for phenolic quantification. Principal component analysis showed significant variations in near-infrared fingerprints with salinity treatments T1–T5 or T6–T10 for each genotype. On average, the total phenolic content (TPC) [measured by the Folin–Ciocalteu (FC) assay], antioxidant capacity [determined by the ferric reducing antioxidant power (FRAP) assay], the levels of polyphenols, phenolic acids, and sugars gradually decreased with an increase in salinity. Analysis of different extracts by UHPLC-UV-HRESI-MSn resulted in the identification of 13 polyphenols, four phenolic acids, and three soluble sugars. While the correlation between the sum of polyphenols identified by HPLC and the FC assay was comparatively strong (r2 = 0.648), it was weak between HPLC and the FRAP assay (r2 = 0.357). Also, a moderate correlation (r2 = 0.534) was observed between the colorimetric FRAP assays and TPC. As the salinity influenced the leaf sugar concentrations and sugars interfere with FC and FRAP assays, caution is advised when interpreting the colorimetric assay results for phenolics quantification. These results are relevant to researchers interested in growing crops with recycled and saline waters as sustainable alternatives to freshwater for the increased demand for global food production.