Effects of foliar application of some biofertilizers on quantitative and qualitative traits and antioxidant capacity of Red Delicious apple fruit

Background and objectives: Apple is one of the most important fruit trees in temperate regions and is a native of central Asia. Apple fruit is rich in potassium, sodium, calcium, boron, phosphorus, and significant amounts of vitamins C, B and A. Optimal nutrition for fruit trees is one of the effective strategies to achieve maximum fruit yield and increase product quality. But, long-term use of chemical fertilizers leads to harmful impacts on the soil, environment and humane health. Recently, one of the basic solutions to overcome these problems is the use of mineral, organic and biological fertilizers as an alternative to chemical fertilizers. Commercial bio-stimulants include important compounds such as seaweed extracts, humic substances, amino acids, etc. Therefore, in this study, we evaluated the effect of foliar application of some of biofertilizers (seaweed extract, humic acid, amino acid) on quantitative and qualitative traits and antioxidant capacity of Red Delicious apple fruit, under the climatic conditions of Zanjan region, Iran.

Materials and methods: The experiment was conducted using a randomized complete block design with three replications in apple orchard of university of Zanjan during 2023. The experimental treatments included foliar spraying of seaweed extract (Alg; 0.075 and 0.15%), amino acid (GF Amino; 0.1 and 0.2%), humic acid (HA; 0.03%), commercial fertilizer Homarang, (Homa fert; 0.5%), combined chemical fertilizer of nitrogen, zinc and boron (Combinate fert; 1% urea, 0.3% zinc chelate and 0.1% boric acid) and foliar spraying with distilled water (control). Foliar application of treatments started 40 days after full bloom, and was repeated three times with an interval of 30 days until the physiological ripening of fruits. The fruits were harvested based on the fruit growth curve at the physiological maturity stage, which occurs 130 days after full bloom. Ten fruits were randomly harvested from each experimental unit, and then the samples were transferred to the laboratory for quantitative and qualitative trait measurements. Quantitative and qualitative traits of the fruit, such as fruit length, diameter, weight, volume, and dry weight, phenols, flavonoids, antioxidant capacity, ascorbic acid, protein, and anthocyanin content of the fruit were examined.

Results: Results showed that the highest value of titratable acidity and vitamin C (22.25 mg 100g-1FW) were obtained in apple trees treated with seaweed 0.075%. The maximum amount of total phenol (9.62 mg 100g-1FW), flavonoid (1.59 mg 100g-1FW) and antioxidant capacity (70.17 %) were observed in fruits treated with amino acid fertilizer 0.1 and 0.2%. The highest fruit length (70.56 mm), diameter (71.16 mm), weight (186.51 grams) and fruit firmness (43.54 N) were obtained with foliar application of HA. Also, the highest anthocyanin content of fruit skin (16.9 µmol g-1FW) was achieved with the use of commercial Homa fertilizer, as this fertilizer had the greatest effect on fruit color quality. The application of combined chemical fertilizer had a negative impact on fruit quality indices, reducing the quality of skin color and fruit firmness.

Conclusion: The results of this study showed that foliar application of biofertilizers and chemical fertilizer had a significant effect on the quantitative and qualitative characteristics and antioxidant capacity of Red Delicious apples. According to the results, biofertilizers, especially seaweed extract (0.075%), amino acid (0.1%), and humic acid (0.03%), had a positive effect on the quantitative and qualitative characteristics and nutritional value of the Red Delicious apple fruit. So that, the combined or separate application of these biofertilizers is recommended to improve the quantitative, qualitative, and nutritional properties of the Red Delicious apple fruit under climatic conditions of Zanjan.