Water use of Thompson Seedless grapevines as affected by the application of gibberellic acid (GA3) and trunk girdling – practices to increase berry size

Seasonal water use of Vitis vinifera L. (cv. Thompson Seedless, clone 2A) was determined with a large weighing lysimeter in the San Joaquin Valley of California from 1994 to 1996. The first year of the study, the vines growing within the lysimeter were treated as would be done to produce fruit for use as table grapes; the application of gibberellic acid (GA3) and trunk girdling at berry set (approximately 2 weeks after anthesis). Both practices will increase berry size of this seedless cultivar. In 1995, the vines in the lysimeter were only girdled at berry set, no application of GA3 at that time. Reference crop evapotranspiration (ETo) between March 15th and the end of October averaged 1124 mm across the 3 years. Water use shortly after the vines were girdled in 1994 increased as would be expected for non-girdled grapevines while in 1995 water use after girdling decreased for a period of approximately 4 weeks. Once the girdles healed (callused over) in 1995 water use increased to values similar to those of the previous year. The crop coefficient (Kc) subsequent to girdling in 1994 remained constant for a period of 4 weeks while the Kc decreased after girdling in 1995. The Kc increased after the girdles healed both years and remained at a value of approximately 0.9 until the end of October. In 1996, the vines in the lysimeter received none of the treatments used the previous 2 years. The seasonal water use and maximum daily water use in 1996 of the vines in the lysimeter were greater than in 1994 and 1995. Water use of the vines was equivalent to 838, 708 and 936 mm from March 15 until the end of October while that of ETo was 1136, 1060, and 1176 during the same period in 1994, 1995 and 1996, respectively. At full canopy in 1996 the Kc leveled off at a value of 1.1 and remained such until the end of October. The results indicate that girdling the trunks of grapevines can affect water use when compared to non-girdled grapevines. Additionally, the Kc of this perennial horticultural crop does not decrease after harvest or later in the season if the vines are fully irrigated and insect pests are controlled.