The effect of foliar application of amino acids on some nutritional properties, antioxidant capacity and some other physiologic parameters of African marigold (Tagetes erecta L.), Taishan ‘Yellow’ and ‘Orange’

African marigold (Tagetes erecta L.) is one of the most well-known ornamental, medicinal and edible flowers in the world. A factorial experiment based on completely randomized design with 20 treatments in 3 replications, 60 plots and 5 plants per plot were conducted in order to investigate the effect of amino acids on growth, nutritional parameters and antioxidant capacity in African marigold. Experimental treatments included two cultivars of African marigold (‘Yellow’ and ‘Orange’) and foliar application of three amino acids (arginine, glutamine and proline) each one at three levels (100, 500 and 1000 μM). Distilled water was used as control. The results showed that the application of amino acids increased plant height, display life, fresh weight and dry matter of flower, leaf total chlorophyll and petal carotenoid compared to the control in both African marigold cultivars. Petal protein content and malondialdehyde (MDA) were not affected by the above treatments, but the use of amino acids, especially arginine and proline, increased proline and calcium, zinc and iron in the petals of both African marigold cultivars. Treatments of 100 μM arginine and 1000 μM proline induced the highest vitamin C in both African marigold cultivars. In ‘Orange’ cultivar, the highest enzyme activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX) belonged to 1000 μM proline and the highest peroxidase (POD) activity belonged to two treatments of 100 μM arginine and 1000 μM proline. In ‘Yellow’ cultivar, all three levels of arginine and proline together with 1000 μM glutamine significantly increased SOD and POD activity compared to other treatments. Also, the highest APX enzyme activity was recorded for 100 μM arginine. The results of the present study showed that external application of amino acids, especially arginine and proline, improved the nutritional properties of both African marigold cultivars by increasing the activity of antioxidants.