The effect of paclobutrazol injected into the soil on vegetative growth and yield of Vitis vinifera L., cv. Thompson Seedless

Summary

Grapevines, cv. Thompson Seedless were treated with the plant growth regulator paclobutrazol in the spring of 1984. The treatments consisted of four soil application rates (including a zero control) of paclobutrazol on two phenological dates (budbreak and bloom). Vegetative growth, as measured by pruning weight, vine yield and fruit maturity characteristics, were determined in 1984 and in the next three growing seasons. There was no significant difference in vine yield between treatments at harvest in 1984; however, the fruit of vines receiving paclobutrazol at budbreak had accumulated less sugar at that time. Paclobutrazol application significantly (P < 0.01 ) reduced pruning weights in 1984 for all vines receiving paclobutrazol. Only the highest rate of paclobutrazol significantly reduced pruning weights in 1985. The number of clusters per vine and yield were significantly less for the treated vines than for the control ones in 1985 and 1986. Pruning weights in 1986 and yield in 1987 showed no significant differences between treatments. The results indicate that a single soil application of paclobutrazol was effective in reducing vegetative growth of Thompson Seedless grapevines for a period of two years if the rate of application was greater than 1.1 kg a. i. ha^-1. However, the adverse effect of this growth regulator on vine yield indicate that it would be of little practical use when applied via the soil to this seedless cultivar.