EFFECT OF FOLIAR APPLIED PACLOBUTRAZOL ON MORPHOLOGY AND PHYSIOLOGY OF APPLE TREES

Paclobutrazol (PBZ) is used on various plant species to control growth. Trials were conducted to evaluate the effect of foliar sprays on young apple (Malus ×domestica) tree growth, photosynthesis and drought tolerance. Foliar sprays of PBZ did not significantly reduce shoot growth of six-year-old trees the first growing season, though a single spray of 3,900 mg/L (active ingredient) reduced growth by 45% compared to shoots on untreated trees. In the second growing season, single applications of PBZ had similar effects on shoot elongation as the multiple applications. Mean shoot elongation and leaf size of bearing age trees were reduced the second growing season with increasing PBZ concentration to 3,900 mg/L. Fruit firmness was increased and length to diameter ratio decreased in the second growing season as PBZ concentration increased to 3,900 mg/L. Increasing PBZ concentrations from 0 to 2400 mg/L reduced leader and shoot elongation and number of nodes/leader of container-grown trees during the first growing season. Total shoot elongation of container grown trees was reduced the second growing season, but by less than in the first season. Newly expanded leaves of container-grown trees treated with 300 to 2,400 mg/L PBZ had 20% to 35% higher leaf net photosynthesis (Pn) rates than leaves on unsprayed trees as early as 17 days after treatment. In a moisture stress trial, leaves of trees sprayed with 1,200 mg/L PBZ maintained higher Pn rates than control leaves in the first growing season.