Zinc Sprays Improve Apple Quality, Nutrition

SHANDONG PROVINCE, ChinaSpraying apple trees with zinc sulfate or sugar alcohol zinc sprays throughout fruit development yields improved fruit, with increased zinc concentration, size, fruit firmness, soluble sugar and vitamin C levels, according to a new study published in the journal HortTechnology.

Researchers at Shandong Agricultural University sprayed Gala and Fuji apple trees with zinc sulfate (ZnSO4) and sugar alcohol zinc separately during four different developmental stages2 weeks before budbreak; 3 weeks after bloom; the termination of spring shoot growth; and 4 weeks before harvest, to determine if the sprays improved fruit quality and zinc concentration.

Results concluded zinc sprays during the four different developmental stages increased zinc concentration of peeled and washed fruit at harvest without phytotoxicity. The treatments 3 weeks after bloom and 4 weeks before harvest increased average fruit weight of both Gala and Fuji apples. Other combinations of treatments were determined to increase fruit firmness, soluble sugar and vitamin C levels in the cultivars. The scientists said the effects of sugar alcohol zinc applications were equal to and "more pronounced" than those of ZnSO4.

"Although the apple trees showed no zinc deficiency symptoms and the leaf zinc nutrition was at a low level, continuing zinc sprays on these trees was required to increase fruit quality," the researchers said. Apples tend to be highly susceptible to zinc deficiency, which can affect the fruit's carbohydrate metabolism, photosynthesis and sugar and starch synthesis.

These experimental results offer new strategies for apple growers. Based on a cost analysis, the authors recommend that growers spray ZnSO4 or sugar alcohol zinc, with 0.1% zinc and 0.04% nitrogen in the spraying solution, on the abaxial/adaxial surfaces of apple leaves to runoff at 4 weeks before harvest.