Effects of Gibberellic Acid And Putrescine on 'Thompson Seedless' Grapes

Gibberellic acid (GA3 ) or putrescine (Put) were sprayed on ‘Thompson seedless’ grapevine clusters, at three different fruit developmental stages, i.e. before (E-L 18), during (E-L 23) and after bloom (E-L 27, 29, 31). Both substances stimulated cluster and berries growth. Berry weight of vines treated with GA3 or Put was approximately 2 and 2.7 fold greater than control, respectively. Treatments significantly increased berry length and width. The smallest than usual number of GA3 applications reduced berry drop and maintained it at control levels while Putrescine improved further more berry attachment. GA3 increased ornithine decarboxylase (ODC) and arginine decarboxylase (ADC) activity compared to the control. ODC activity in clusters and rachis was much higher than the corresponding ADC activity after the first and last GA3-treatment. Finally, the activity for both enzymes was higher in rachis than in berries.