Effect of Auxin and Gibberellin Foliar Application and Fruit Immersing Treatments on Vegetative and Reproductive Growth of Strawberry. CV. ‘Gaviota‘

Strawberry is known as one of the most important temperate small fruits which is cultivated in the field, greenhouse and high tunnels in the most regions of the world. The role of gibberellin and auxin in increasing fruit size of strawberries have been reported. One of the major problems of strawberry production in greenhouse is the fruits malformation which has reduced the market value of the fruit. Gibberellin and auxin have been different effects in vegetative and reproductive of growth stages. The purpose of this research was to improve the vegetative and reproductive indices of the fruit and to control the malformation problem by stimulating the development of seeds on the fruit and stimulating the growth of the fruit receptacle by gibberellin GA3.

Experiments were conducted in a commercial greenhouse in Karaj, Hashtgerd New City, Phase 7 during 2017-2019. Two separate experiments (spraying hormones on whole plant or immersing individual fruitlets on hormone solution) were designed and implemented in a commercial greenhouse unit. In the first experiment: GA3 was at concentrations of 50, 100 and 150 ppm and IBA at concentrations of 20, 40 and 60 ppm in growth stages of 60 and 65 according to BBCH worldwide model. In the second experiment, fruits were immersed in NAA with concentrations of 20, 40 and 60 ppm and GA3 with concentrations of 25, 50 and 100 ppm at fruit growth stages 70 and 73 according to BBCH procedure. After reaching the red stage, the fruits were manually harvested and transferred to the laboratories for additional tests. Vegetative and reproductive traits were evaluated including leaf area, number of leaves, crown circumference, plant height, number of flowers and fruit dimensions. Fruit biochemical characteristics include: total soluble solids, titratable acid content of fruit, anthocyanin and antioxidant capacity, total phenolics, catalase and superoxide dismutase enzymes activities and content of vitamin C were evaluated. The experiments were designed and implemented in the frame of a randomized complete block design. The data were analyzed by SAS ver.9.4 and SPSS ver. 22 software, the mean data were compared based on Duncan's multi-range test, and the graphs were drawn by Excel 2013 software.

Foliar spraying with gibberellin had a significant effect on several characteristics, including leaf area, crown circumference, number of flowers, and plant height at the 1% level of significance. However, it did not yield a significant impact on the attribute of leaf number. The increase in vegetative growth could be due to the synthesis of more amino acids in the stimulation of gibberellin treatment. Gibberellin treatments increased the vegetative growth of the plant and increased the number of flowers, but their growth was stopped after fruit formation and no fruits were formed in gibberellin spraying. This can be due to the existence of an antagonistic relationship between vegetative parts and fruit developments. Auxin hormone treatments had a significant effect on increasing fruit size and reducing the percentage of malformed fruits. GA3 at concentration of 50 ppm improved plant vegetative characteristics such as: plant leaf area, plant crown circumference, plant height and number of flowers, while IBA reduced fruit deformity by 40% and also increased the amount of total soluble solids, the titratable acid content of the fruit and the antioxidant content of the fruit. In general, gibberellin at a concentration of 50 ppm in order to increase plant vegetative indices and auxin at a concentration of 60 ppm were statistically effective. In second experiment, it seems that three levels of gibberellin treatment have increased all traits related to fruit compared to auxin and control. GA3 at a concentration of 100 ppm, causing an increase in fruit size, fruit weight and the biochemical properties of the fruit and the deformity decreased by 37/5% while NAA at a concentration of 60 ppm increased the amount of fruit anthocyanin as well as total fruit phenolics and vitamin C content.

In the first experiment, gibberellin foliar application at a concentration of 50 ppm had the greatest effect on vegetative growth indicators. Also, in the same experiment, it was found that the use of GA3 in high concentrations has the negative effects on flowering and fruit growth., while it increases the amount of runner production. Also, IBA at a concentration of 60 ppm was the most effective treatment in fruit indices. In the second experiment, gibberellin at a concentration of 100 ppm was the most effective treatment in fruit and its biochemical traits, while NAA auxin in the second experiment showed a lower response to fruit than IBA auxin in the first experiment.