Exogenous GA3 inhibits the phototropism of grape leaves by affecting photosynthetic characteristics and endogenous hormones

Abstract: 【Objective】The present experiment was undertaken to explore the inhibitory mechanism for exogenous GA3 affecting grape leaf phototropism. Phototropism means that plants are regulated by light to bend to incident light in order to obtain the most suitable light for their growth. Grapes, as photophil- ic plants, have obvious phototropism. Studies have found that exogenous GA3 can significantly inhibit the phototropism of grape leaves. Therefore, exploring the inhibitory mechanism for GA3 influencing the phototropism of grape leaves may provide a certain theoretical basis for further research on the ef- fects of exogenous hormones on the phototropism of grape leaves.【Methods】The experiment was carried out in Wuwei city, Gansu province in 2020, located in the eastern end of the Hexi Corridor (N 38°02′32′′, E 102°42′19′′), where there is a temperate continental arid climate. The altitude is 1632 m, the average annual precipitation is 191 mm, the average annual evaporation is 2135 mm, the frost-free period is 150 days, the average annual temperature is 7.8 °C, the average annual sunshine duration is 2715 h, and the sunshine is sufficient. The 6-year-old grapevines (Cultivar: Italian Riesling) was used in the experiment, with a spacing of 1.0 m×3.0 m. The vigorous plants without diseases and insect pests were selected on sunny days. The secondary canes with new tendrils on the primary cane of over 80 cm in length were removed before 8:00 and then the primary can was flattened and fixed on the third wire of the vertical hedge with plastic tape. The leaf blades were made on positive side or reverse side, but the leaves remained in a horizontal state. The directions of leaves only could be changed through the new tendrils at a knob, not through the blade by rotating or fixing the petiole to ensure that the petiole and blade were not affected by any external force; The pulling and fixing started in the early morning, and finished before 8:00. Then, the fixed leaves were treated by spraying the growth regulators, and three treatments were set up, namely 75 mg·L-1 gibberellin (GA3), 200 mg·L-1 entobutrazol (S3307) and clean water (CK). Each of 10 plants was treated with one treatment that repeated three times. Water po- tential, and photosynthetic and fluorescence parameters of functional leaves at about 20 d of age were measured every 2 h from 8:00 to 18:00. At the same time, each treated leaf was removed, and the leaves and petioles were separated and cut into pieces with a weight of 2.0 g respectively. The leaves were wrapped with tin foil and then quickly put into a liquid nitrogen tank and brought back to the laboratory. The leaves were placed in a refrigerator at -80 °C for hormone assay.【Results】The results showed that as the light-receiving time increased, the grape blades of each process gradually rotated from the abaxi- al plane (leaf with reverse side) to the abaxial plane (leaf with positive side). The angle between the blades and the horizontal plane gradually increased, and the angle between the leaf stems and the stems gradually decreased. However, in the process of rotation, the increase rate of the angle between leaf and the horizontal plane and the decrease rate of the angle between petiole and stem with GA3 treatment were significantly slower than those of the control. In addition, there was also a significant difference in leaf water potential between the control and GA3 treatment, which was significantly lower than that of the control from 10:00 to 18:00. Photosynthetic parameters such as Pn, Gs and Tr were significantly low- er than those of the control from 12:00 to 18:00, while changes in Ci were opposite to those of Pn, Gsand Tr. Chlorophyll fluorescence parameters such as Fm, Fv/Fm and ETR decreased to different degrees, while NPQ was significantly higher than that of the control. After S3307 treatment, the angle between leaf and horizontal plane increased faster in comparison with the control, and the angle between petiole and stem decreased faster than that of the control. Leaf water potential was significantly higher com- pared with the control from 14:00 to 16:00. Photosynthetic parameters such as Pn, Gs and Tr were signifi- cantly lower than those of the control from 12:00 to 18:00, and chlorophyll fluorescence parameters such as Fm, Fv/Fm and ETR increased to different degrees. At the same time, the results of endogenous hormone content determination showed that the content of endogenous hormone in leaf and petiole in each treatment also changed obviously. Among them, GA3 treatment increased the contents of GA3, IAA and ZT in leaves and petioles to varying degrees, and decreased the content of endogenous ABA, while the effect of S3307 treatment was the opposite.【Conclusion】After being fixed by pulling the tips, the grape leaves gradually rotated with the increase of the light-receiving hours. The angle between the leaves and the horizontal plane gradually increased, whereas the angle between the petiole and the hori- zontal plane gradually decreased. The GA3 treatment slowed down the rotation speed of the blade, while the synthetic inhibitor S3307 treatment significantly accelerated the rotation of the blade. The GA3 treat- ment significantly reduced the photosynthesis of the leaves, increased the content of endogenous GA3and IAA, and increased the ratio of GA3/IAA, while the S3307 treatment had the opposite change, com- pared with the GA3 treatment. In summary, exogenous GA3 inhibited the phototropism of leaves by re- ducing the photosynthesis of leaves, increasing the content of GA3 in leaves and petioles, and increasing the ratio of GA3/IAA.