Combining Light and Mineral Nutrient Supplementation Increases Flavor, Firmness and Nutritional Value of Strawberry Fruit without Reducing Fruit Size or Sweetness

To investigate how different combinations of supplemental lighting and mineral nutrient sprays would impact strawberry fruit quality, a greenhouse experiment was conducted during winter. “Benihoppe” strawberry plants were subjected to LED supplemental lighting (white color, 1.85 mol daily light integral) or natural sunlight, combined with either calcium (CaCl2, 0.01 mol L−1), potassium (KCl, 0.01 mol L−1) or a control (deionized water) foliar spray. The results suggested that LED lighting increased the fruit cellulose, soluble protein and organic acid concentrations, and the ratio between complex carbohydrates to simple sugars; whilst the mineral sprays mainly influenced single fruit mass, fruit glucose, ascorbate and polyphenol concentrations. Among the treatments, the supplemental lighting with potassium spray (LEDK) achieved the best comprehensive fruit performance, increasing the concentrations of citric acid (13.7%), malic acid (23.5%), polyphenols (13.2%), starch (17.0%), cellulose (24.2%) and soluble protein (230.3%) compared to the controls; this treatment also had the highest fruit soluble protein accumulation (25.36 mg). The supplemental lighting with water (LEDN) also showed competitive performance, showing the highest fruit starch (7.92 g kg−1) and cellulose (48.18 g kg−1) concentrations, as well as the highest accumulation of starch (0.20 g), total fibers (1.25 g) and L-malic acid (24.84 mg). The stimulation of fruit calcium import during ripening and leaf sucrose export under daytime LED lighting could explain the higher fruit cellulose and nutritional value. Therefore, LED supplementation and foliar potassium could help to increase strawberry flavor metabolites, fruit nutrition value and firmness in winter greenhouse production.