Physiological persistence of paclobutrazol on the ‘Tommy Atkins’ mango (Mangifera indicaL.) under rainfed conditions

Abstract

SummaryTo investigate the feasibility of using paclobutrazol (PBZ) to control chemically mango tree size under rainfed conditions, the physiological 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 harvest in June, 1990. Each dosage was dissolved in 2 1 of water and applied in a 15 cm depth trench at 1.5 m around the tree. Shoot length was significantly reduced only with 10 g PBZ per tree and greater concentrations. PBZ at 20 and 40 g per tree drastically reduced vegetative growth for a year after treatment. The effect of 10 g PBZ per tree on shoot growth was modest and lasted one year. With the exception of 40 g PBZ per tree, the effect of any level of PBZ on shoot length disappeared by year three. Increases in PBZ levels increased the earliness of flowering from 16 d (2.5 g PBZ per tree) to 40 d (40 g PBZ per tree) in the 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 three after treatment. PBZ levels from 2.5 to 20 g per tree did not affect the number of mature fruit per m2 of canopy during the three experimental years. PBZ at 40 g per tree resulted in the lowest production of mature fruits for the first two years after application. However, for the third year this treatment produced most mature fruit per m2 of canopy. All PBZ rates decreased average fruit weight, but, the smallest fruits were produced with 15, 20 and 40 g PBZ per tree. Total soluble 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 tree and greater concentrations. PBZ at 10 g per tree and above drastically 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 were detected in the leaf content of N and K. The use of 10 g PBZ per tree may have a dual effect, as it may reduce tree size and induce early flowering. To keep this effect, we speculate that under rainfed conditions, 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.