Effects of foliar sprays of urea, boric acid and zinc sulfate on quality and some mineral nutrients concentration in golden kiwifruit (Actinidia chinensis L.)

Kiwifruit (Actinidia chinensis) is an important subtropical fruit appreciated by consumers due to its excellent qualitative attributes and its nutritional properties, especially high ascorbic acid content, which provide health benefits. A balanced fertilization of macro- and micro-nutrients for plant nutrition is essential for enhancing fruit yield and quality (Ashouri Vajari et al., 2018). Foliar application is a complementary method for plant nutrition and is mostly used in cases where the plant urgently needs a certain element and also in early spring when the absorption of elements from the soil is low due to low root activity. Also, in cases where the soil conditions are not suitable for the absorption of elements, it is a relatively easy method and it yields quickly. Nitrogen, boron and zinc have the greatest effect on pollination, fertilization and fruit formation. In early spring, due to the cold soil temperature and the lack of sufficient leaves on the plant, the absorption of these elements from the soil is less and therefore it is necessary to provide these elements to the plant by foliar application. However, there is little information concerning the effect of nitrogen, boron and zinc application on golden kiwifruit. Thus, this research was carried out to determine the effects of preharvest urea, boric acid and zinc sulfate application on quality and some mineral nutrients concentration in golden kiwifruit.

The experiment was conducted in a randomized complete block design with three replications in Astara. Experimental treatments include: control (spraying with water), urea with a concentration of 1.5%, zinc sulfate (2000 mg L-1), boric acid (1500 mg L-1), urea 1.5% + zinc sulfate (2000 mg L-1) 1.5% urea + boric acid (1500 mg L-1 the composition of urea was 1.5% + zinc sulfate (2000 mg L-1) which in three stages of plant development including the beginning of bud swelling (early March), the beginning of sepal splitting (Late April) and the rapid fruit growth stage (early June) were sprayed. Fruit were harvested at commercial maturity in the morning and transported in an air-conditioned vehicle to to the fruit analysis laboratory at Zanjan University, and subjected to physicochemical analysis.

The results showed that foliar application of the mentioned compounds had a significant effect on the amount of total soluble solids, fruit dry weight, fruit firmness, ratio of soluble solids to total acid, ascorbic acid, total phenols, flavonoids and antioxidant capacity of fruit. All studied traits except soluble solids and the ratio of solids to total acid increased due to foliar application of nutrients. Foliar application of urea, boric acid and zinc sulfate alone or in combination increased these elements in leaves and fruits. The best treatment was for combined foliar application of 1.5% urea + sulfate on 2000 mg L-1 + boric acid 1500 mg L-1 that all the traits measured in this treatment had the highest. According to the obtained results, it can be stated that the simultaneous use of urea, boric acid and zinc sulfate increases the quality and antioxidant capacity of golden kiwifruit.

Conclusively, combined foliar application of urea (1.5%) plus zinc sulfate (2000 mg L-1) and boric acid (1500 mg L-1) at three stage can be used effectively to improve leaf nutrient status and nutritional quality of golden kiwifruit.