Foliar application of phenylalanine on nutritional value in Vitis vinifera var. Hosseini

Grape berries contain rather high amounts of polyphenolic compounds such as anthocyanins, which contribute significantly to their quality, appearance, and taste. Grape berries produce significant amounts of phenolic compounds. Phenylalanine ammonia-lyase (PAL), is first enzyme in the phenylpropanoid pathway which catalyses the conversion of phenylalanine to flavonoids, phenolics and anthocyanins. Phenylalanine ammonia-lyase (PAL) (EC 4.3.1.24) is an essential enzyme in the phenolic biosynthesis pathway that catalyzes the conversion of L-phenylalanine to trans-cinnamic acid and ammonia. Thus, PAL is the first and decisive step in the phenylpropanoid pathway and is therefore involved in the biosynthesis of phenolic compounds in the plants.The phenylpropanoid pathway involves in the synthesis of secondary compounds such as phenylalanin and phenolics. Considering the positive impacts of phenolic compounds on human health due to their antioxidant properties, their enhancement in fruits and vegetables especially by natural products such as phenylalanin might be considered as promising strategy to fight against cancerous cells and important diseases. Moreover, the increased concentration of phenolics in berries is a critical property benefitting health. In recent years, widespread attention has been focused on the potential beneficial properties on human health of anthocyanins in grapes and their products. Free radical scavenging, antioxidant, antimicrobial, and antiviral activities, nutraceutical and pharmaceutical activities like prevention of cardiovascular disease, protective effects against hepatic damage and disease, anticancer, antitumor, and antimutagenic activities, suppression of inflammatory responses, protection against age-related decline in cognitive behavior, and neuronal dysfunction are some top health beneficial properties of phenolic compounds.

This experiment was conducted in a completely randomized block design using four replications. All reagents and solvents were purchased from Sigma-Aldrich, St. Louis, MO, USA, and used without further purification. Each experimental unit consisted of two plants that each had at least three clusters of approximately the same size, maturity, and development. Vines were sprayed with four concentrations of Phenylalanin (0, 50, 100, and 200 µM) at pre-véraison (in which berries are green and hard) stage. The final volume was reached using distilled water and the pH was set with NaOH (1.0 N) to 7.0. The solutions of Phenylalanin with surfactant TWEEN® 20 were sprayed fifth at 5 days intervals (to ensure that all clusters received sufficient amounts of Phenylalanin solution at pre-véraison stage in case of any rain, deficiency or late development of some berries) on the whole cluster in the early morning. The clusters had little or no evidence of asynchrony and the berries were mostly green and hard. An equal amount of distilled water plus TWEEN® 20 was sprayed to the untreated plants (concentration 0.0 mM). The sprays were carried out with a hand sprayer. At harvest, although not all berries in a cluster were at the same developmental stage, most of the berries should have a particular coloration, softness, and level of development to be harvested.

The results showed that total phenolics and flavonoids content were significantly enhanced in Phenylalanin -treated (100.0 and 200.0 µM) berries compared to untreated ones. Phenylalanin treatment at all concentrations considerably improved the total antioxidant capacity (DPPH) in the berries and, compared with untreated berries, the activity of phenylalanine amino-lyase enzyme was higher in Phenylalanin -treated fruits. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging capacity of the fruits treated with Phenylalanin were significantly higher than those of the untreated berries. The activity of phenylalanine ammonia-lyase (PAL) in phenylalanin treated fruits significantly increased as compared with the untreated clusters. The remarkable effects of the PAL enzyme in increasing production of phenolic compounds in various plant tissues have been established for a long time, and it seems that the increases in concentrations of phenolic compounds in the present study are significantly related to this enzyme. The PAL enzyme causes the production and accumulation of secondary phenolic metabolites in plant tissues by causing a shift from primary to secondary metabolic pathways, and this accumulation usually increases the nutritional value of plant products. The PAL enzyme causes the production and accumulation of secondary phenolic metabolites in plant tissues by causing a shift from primary to secondary metabolic pathways, and this accumulation usually increases the nutritional value of plant products. Therefore, PAL plays a crucial part in improving the quality of crops by stimulating the production of secondary metabolites. The results of current research indicated that the increased activity of the PAL enzyme was entirely related to concentrations of phenylalanin, and its maximum activity was recorded in the treatment with the highest level (200.0 µM) of the phenyalanin. Moreover, a considerable increase was also observed in the evaluation of the total content of flavonoids, one of the major subgroups of phenolic compounds in plants, in grape berries. At the highest concentration of phenyalanin application, an increase of about 100% was recorded in the production of flavonoids, which can significantly help in the marketability of grape berries in addition to their increased nutritional value.

A general evaluation of the results of the present experiment leads us to the conclusion that phenylalanin might be a suitable and recommendable treatment to improve the quality and nutritional value of grape berries. The phenylalanin is an essential amino acid and is safe for human consumption as it is present in almost all plant tissues that have always been consumed by humans without causing any problems. The improvement in the quality of berries, achieved in this research, is partly due to their improved visual quality and partly due to their increased healthful quality, which results from the increase in their antioxidant property and from the accumulation of compounds in them that enjoy high nutritional value. In summary, therefore, it seems that spraying grape berries at pre-véraison can be a suitable and convenient strategy for increasing the quality and nutritional properties of grape berries.