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Effect of Paclobutrazol on Shoot Growth, Flowering Fruit Set, and Yield of Rambutan (Nephelium lappaceum L.) var. Rongrien
Dissertation Abstract:
Two experiments were conducted to evaluate the effect of four paclobutrazol concentrations (0, 1.5, 3, and 4.5 mM per tree), applied either as canopy or soil surface spray, at two different times, on six-year-old rambutan trees f rom July 1994 to July 1995.
For both methods of application. paclobutrazol treatment reduction of shoot length, leaf size. and panicle size was directly proportional to the chemical concentration. Leaf curling resulting from the treatment was noted.
Canopy application at 4.5 mM proved more effective than soil surface application. The latter. however. had a long-term effect. lasting up to eight months after application. Shoot length was significantly shorter than the control and the canopy-sprayed trees with 3.0 and 4 .5 mM concentrations.
Paclobutrazol at 4.5 mM, applied as soil surface spray three months after flushing. was the best treatment for inducing earlier flower bud initiation (14 days earlier than the untreated trees). It also gave the highest yield and percent fruit set as well as the lowest percentage of parthenocarpic fruit.
Canopy application at 1.5 mM. regardless of time of application, was the optimum level in terms of yield and percent fruit set. However, it did not induce earlier flower bud initiation.
Except for the soil surface sprayed trees (paclobutrazol at 4.5 mM) where fruit size was reduced slightly. there were no differences in fruit size and quality among the treated trees relative to the control. Two and a half months after application. all treated trees had higher carbohydrate/nitrogen (C/N) ratio in their shoots and leaves than the control. The highest values were obtained from trees treated with 4.5 mM paclobutrazol applied as soil surface spray. The increase in C/N ratio was associated with flower bud initiation.
Cost and return analysis showed that the greatest economic benefit could be derived from trees treated with 4.5 mM paclobutrazol applied as soil surface spray three months after flushing. This treatment not only gave high yield but also advanced the flowering date. Consequently. it increased the possibility of taking advantage of the higher off-season prices for this fruit.