The utilization of tryptophan and glycine amino acids as safe alternatives to chemical fertilizers in apple orchards.

Our experiment was conducted during the seasons of 2018 and 2019 on 10-year-old "Anna" apple trees (Malus domestica L. Borkh) planted at 4 × 4 m apart in a clay soil under drench irrigation. Sixty uniform trees were selected and subjected to the same cultural practices during both seasons. Apple trees were sprayed three times as follows: before flowering, during full bloom, and 1 month later with the following treatments: control (water only); tryptophan at 25, 50, and 100 ppm; glycine at 25, 50, and 100 ppm; and their combinations, 25 ppm tryptophan + 25 ppm glycine, 50 ppm tryptophan + 50 ppm glycine, and 100 ppm tryptophan + 100 ppm glycine. The results demonstrated that the foliar spraying of "Anna" apple trees with glycine and tryptophan at 25, 50, and 100 ppm and their combinations significantly increased shoot length and diameter, leaf area, total chlorophyll, percentages of fruit set and yield, fruit physical and chemical characteristics, and leaf mineral composition of N, P, K, Ca, Fe, Zn, Mn, and B, whereas it reduced the fruit drop percentage in both seasons in comparison with control. Better results were obtained from the concentrations of 50 and 100 ppm which were more effective in both seasons in comparison with the concentration of 25 ppm. Moreover, the combination of 50 ppm glycine 50 ppm tryptophan was the best treatment and provided the highest results in both experimental seasons in comparison with the other applied treatments and control.