The following research on citrus (Citrus aurantium L.) rootstocks transpired in 2020 at the Citrus Production Nursery of the Directorate of Horticulture and Forestry, Al-Hindiyya District, Iraq. The study aimed to explore the effect of foliar application of amino acids on citrus rootstocks and their various characteristics. The experiment was factorial in a randomized complete block design with two factors and three replications. The first factor included four citrus rootstocks (Citrus aurantium, Troyer citrange, Volkameriana, and Swingle citrumelo), while the second factor included four amino acid treatments, i.e., control, arginine (250 mg L^-1), tryptophan (150 mg L^-1), and arginine (250 mg L^-1) + tryptophan (150 mg L^-1), symbolized as A1, A2, A3, and A4, respectively. Results showed the citrus rootstock Swingle citrumelo emerged superior in seedling height, stem diameter, number of leaves, and contents of carbohydrate, nitrogen, phosphorus, and potassium, with average values of 123.75 cm, 1.13 cm, 96.88 leaves seedling^-1, 13.86%, 1.55%, 0.46%, and 1.51%, respectively. Amino acids’ foliar application had a significant effect, with the treatment A4 (arginine 250 mg L^-1 + tryptophan 150 mg L^-1) excelling in the seedling height and diameter, the number of leaves, and contents of chlorophyll, carbohydrates, nitrogen, phosphorus, and potassium. Corresponding average values were 110.55 cm, 1.23 cm, 104.23 leaves seedling^-1, 136.98 mg 100 g^-1, 14.41%, 1.60%, 0.52%, and 1.58%.

Keywords: Citrus aurantium L., rootstocks, amino acids, vegetative growth, biochemical traits, qualitative and yield traits

Key findings: The presented study revealed the citrus (Citrus aurantium L.) rootstock Swingle citrumelo had the best vegetative growth and biochemical properties, while the rootstock Volkameriana had the highest chlorophyll content. The combined foliar application of the amino acids (arginine 250 mg L^-1 + tryptophan 150 mg L^-1) showed the best performance in vegetative growth and biochemical properties.