Spinach (Spinacea oleracea, L.) Tolerance to Saline Irrigation: Nutritional Value, Antioxidant Capacity, and Gene Expression.

Scarcity and price of good-quality water for irrigation of vegetable crops are major impediments to meet the food production required by a growing world population. Cheaper recycled waters are often available but their salt concentration is a concern for growers. Our previous work determined that spinach is more salt-tolerant than previously reported. However, the effects of salinity on leaf mineral composition, antioxidant capacity, oxalic acid, photosynthesis, and gene expression have not been established. Spinacea oleracea cv. Racoon was grown under greenhouse conditions with average irrigation water salinity of electrical conductivities (EC_iw) of 1.4, 3.6, 5.9, and 9.4 dS m^-1. Irrigation waters contained four levels of NaCl (2, 20, 40, and 80 mmol L^-1) and three levels of potassium (3, 5, and 7 mmol L^-1). After 23 days of exposure to salinity, spinach plants had significantly higher concentrations of Na and Cl in both shoots and roots, as salinity increased, and regardless of the concentration of potassium in the irrigation water. However, there were no differences in shoot levels of mineral nutrients, antioxidants, and oxalic acid (ca. 9% w/w) compared to the control salt treatment. Photosynthetic parameters, gene expression, and shoot biomass accumulation were also unaltered suggesting that the salinity levels tested did not impose significant stress on spinach plants. These results clearly indicate that spinach can tolerate twice the salinity of irrigation previously established and is an ideal winter vegetable that can be cultivated in semi-arid areas where soils/irrigation water are moderately high in salts or where the EC of the soil extract is 4-5 dS m^-1. Spinach is a salt tolerant vegetable that maintains its nutritional and antioxidant capacity and that can be grown with most recycled or drainage waters.