Evaluation of different concentrations of Fe and Zn in nutrient solution on yield, biochemical, and nutrient content of spinach in soilless conditions

  Spinach is a leafy vegetable rich in nutrients and can be consumed either raw or prepared in various ways. Incorporating micro-elements like Zn and Fe can increase nutrient concentration in agricultural products and boost plant yield. This, in turn, plays a crucial role in improving food quality and promoting better health for society. High concentrations of Zn can impede plant growth by affecting photosynthesis and oxygen-free radical production. Conversely, Fe is a vital micronutrient contributing to several plant metabolic processes. This study aims to identify the optimal levels of Fe and Zn elements to enhance spinach yield, element concentration, and vegetative growth.

The plants were cultivated in washed sand media (free from any nutrients). Plants were treated with Zn (0.22, 5, and 10 mg L-1) and Fe (3, 6, and 12 mg L-1). After the first two leaves appeared in each pot, 500 ml of nutrient solution was manually added.

The ANOVA results indicate that varying levels of Zn and Fe have a significant impact on various traits in spinach, including leaf size, fresh and dry shoot, and root weight, photosynthetic pigments, total antioxidant activity, total soluble protein and carbohydrate content, as well as leaf and root elements. It is important to note that the nutritional components of both spinach leaves and roots were impacted by the combinations of treatments applied. Specifically, the levels of Mg, Zn, and Cu increased after using varying concentrations of Fe and Zn compared to those plants watered with the lowest levels of these minerals. The simultaneous application of Zn and Fe increased the Fe content of leaves and roots. Still, with the increase in the concentration of these treatments, it was observed a reduction in the Fe content so that the lowest amount of Fe in the leaves and roots was observed in the highest concentration of Zn in the food solution (12 mg L-1 Fe + 10 mg L-1 Zn). Spinach plants under irrigation by 5 mg L-1 Zn + 6 mg L-1 Fe had the highest leaf K content, which improved by 31.79% compared to the control treatment. When the concentration of Zn in the nutrient solution was increased to 10 mg L-1, it resulted in a decrease in K content in both leaves and roots. The treatment of Fe 6 mg L-1 + Zn 5 mg L-1 had the highest fresh weight (33.977 g) and dry weight (3.487 g) content and also resulted in an increase in leaf size (length 8.933 mm and width 4.477 mm). However, with the increment of Fe concentration to 12 mg L-1 + 10 mg L-1, these traits showed a significant decrease, as well as the highest content of chlorophyll a and b and carotenoids were obtained at a concentration of 6 mg L-1 of Fe + 5 mg L-1 of Zn, which showed an increase of 289, 687 and 406% compared to the plants treated with the lowermost Zn and Fe concentration, respectively, and the lowest values of chlorophyll a and b, and carotenoids were found in the control plants, the protein content varied from 1.038 mg g-1 FW in the lowest treatment to 4.256 mg g-1 FW in the 12 mg L-1 Fe + 5 mg L-1 Zn treatment, the highest total soluble content was observed in 6 mg L-1 Fe + 5 mg L-1 Zn, which was led to an increase of 72%. The lowest total soluble carbohydrate was also observed in the control plants. The highest and lowest total antioxidant activity was recorded in 12 mg L-1 Fe + 5 mg L-1 Zn (10.75%) and the control spinach plants (6.705%), respectively.

Increasing the Zn concentration up to 10 mg L-1 caused a reduction in growth parameters and the content of photosynthetic pigments and elements of K and Fe in leaves and roots. However, the measured trait concentration for the studied treatments was higher than in the control plant. The present results showed that the spinach plant is somewhat resistant to the high concentration of Zn micronutrient. Based on the obtained results, Fe and Zn treatments combined were more effective for promoting healing than separate treatments. 6 mg L-1 of Fe and 5 mg L-1 of Zn was the most effective treatment.