Effects of Defoliation, Gibberellic Acid and 4-Chlorophenoxyacetic Acid on Growth and Composition of Thompson Seedless Grape Berries

Abstract

Fifty percent defoliation at berry shatter significantly reduced berry weight of Thompson Seedless grapes. Either gibberellic acid (GA₃) or 4-chlorophenoxyacetic acid (4-CPA) applied to foliated vines produced significantly heavier berries than the control (no defoliation or hormones). GA₃ applied to 50% defoliated vines at berry shatter yielded berries similar in weight to those from control vines, whereas, 4-CPA applied to similar vines had little effect on increasing berry weight. The barley endosperm bioassay of gibberellin (GA)-like substances revealed higher activity in berries from control vines than in berries from defoliated vines sampled one week after berry shatter. GA-like activity in young berries increased in response to exogenous GA₃ application in both control and 50% defoliated vines. No measurable GA-like activity was detected in berry samples collected at véraison. Defoliation reduced the total amount of sugar (total soluble solids) per berry, but not the concentration of total soluble solids. GA₃ and 4-CPA, on the other hand, significantly increased the total sugar per berry, but markedly reduced the level of total soluble solids. The concentration of arginine in mature fruit was significantly less in defoliated vines than control vines, whereas, the levels of malate, tartrate and proline did not differ significantly between these two treatments. GA₃ and 4-CPA significantly increased the level of malate in mature fruits, but reduced the concentrations of proline compared to control fruits. The concentrations of tartrate and arginine in berry juice from vines treated with GA₃ or 4-CPA did not differ significantly from control fruits.