Yield and Biochemical Attributes of Polyhouse Grown Spinach as Affected by Proportions of NO₃^-: NH₄⁺ in the Growing Media

Spinach (Spinacia oleracea L.) was cultivated under polyhouse conditions using a soilless growing medium, while a parallel crop was established in open field conditions following conventional agronomic practices. In both environments, sowing was carried out three times annually: October (main season), February (off-season), and July (off-season). In the polyhouse system, plants were supplied with nitrogen through nutrient solutions containing varying nitrate (NO₃^–) to ammonium (NH₄^–) ratios: 100:0 (T₁), 75:25 (T₂), 50:50 (T₃), 25:75 (T₄), and 0:100 (T₅). Foliage was harvested 30 days after sowing, with subsequent harvests conducted at 30-day intervals. At each harvest, various growth parameters, including plant height, leaf area, fresh and dry shoot biomass, and total yield per plant were recorded. Additionally, physiological attributes such as total chlorophyll content, carotenoids, soluble sugars, starch, soluble proteins, total free amino acids, and proline content were measured. The activities of antioxidant enzymes including superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were also assessed. Across all sowing dates, spinach grown under polyhouse conditions and supplied with a 75:25 NO₃^–:NH₄^– ratio (T₂) exhibited significantly higher yields compared to the open field-grown crop. The T₂ nutrient treatment also resulted in enhanced enzymatic activity and improved physiological quality parameters. These findings suggest that a 75:25 NO₃^–: NH₄^– ratio is the most effective for optimizing spinach production under polyhouse conditions.