PHYSIOLOGICAL PERSISTENCE OF PACLOBUTRAZOL ON THE TOMMY-ATKINS MANGO (MANGIFERA-INDICA L) UNDER RAIN-FED CONDITIONS

To investigate the feasibility of using paclobutrazol (PBZ) to control chemically mango tree size under rainfed conditions, the physiologica l persistence of soil applied PBZ on the vegetative growth, flowering behaviour, nutritional status, fruit set and quality of 'Tommy Atkins' mango was assessed for three consecutive years. A single application of PBZ (0, 2.5, 5, 10, 15, 20, and 40 g per tree) was made after harve st in June, 1990. Each dosage was dissolved in 2 l of water and applie d in a 15 cm depth trench at 1.5 m around the tree. Shoot length was s ignificantly reduced only with 10 g PBZ per tree and greater concentra tions. PBZ at 20 and 40 g per tree drastically reduced vegetative grow th for a year after treatment. The effect of 10 g PBZ per tree on shoo t growth was modest and lasted one year. With the exception of 40 g PB Z per tree, the effect of any level of PBZ on shoot length disappeared by year three. Increases in PBZ levels increased the earliness of flo wering from 16 d (2.5 g PBZ per tree) to 40 d (40 g PBZ per tree) in t he first year. For the second year after treatment, the effect of PBZ on time to flowering started to decline and, with the exception of 15 and 40 g PBZ per tree, earliness of flowering disappeared by year thre e after treatment. PBZ levels from 2.5 to 20 g per tree did not affect the number of mature fruit per m(2) of canopy during the three experi mental years. PBZ at 40 g per tree resulted in the lowest production o f mature fruits for the first two years after application. However, fo r the third year this treatment produced most mature fruit per m(2) of canopy. All PBZ rates decreased average fruit weight, but, the smalle st fruits were produced with 15, 20 and 40 g PBZ per tree. Total solub le solids of fruit juice were not affected at low PBZ rates (2.5 and 5 g per tree), but they were significantly higher with 10 g PBZ per tre e and greater concentrations. PBZ at 10 g per tree and above drastical ly reduced leaf P and Ca content. A similar reduction was found for Mg with levels of 15 g PBZ per tree and above. No significant changes we re detected in the leaf content of N and K. The use of 10 g PBZ per tr ee may have a dual effect, as it may reduce tree size and induce early flowering. To keep this effect, we speculate that under rainfed condi tions, a single application of 10 g PBZ per tree should be followed by a reapplication with 10 or 5 g per tree every two years.