Morphological and Physiological Effects of Foliar Applied Paclobutrazol on Apple Trees
Deyton, D. E., C. G. Milne, and C. E. Sams.  2013.  Proceedings abstracts, 2013 Program/Abstracts, 40th Annual Meeting of the Plant Growth Regular Society of America & ISHS XII International Symposium of Plant Bioregulators in Fruit Production, July 28-August 1, Orlando, Florida, p. 25.

Paclobutrazol is a plant growth regulator (PGR) that acts by reducing gibberellin production. It has been used commercially to reduce vegetative growth of fruit trees, other woody plants and floriculture crops. A review in 2004 found that nearly 80% of apple [Malus domestica (Borkh.)] orchards in Great Britain were treated with PGRs and paclobutrazol accounted for 54% of PGR usage. Payback® (250 g•L-1 paclobutrazol, a.i.) was labeled for use apple, stone fruit and mango trees in Australia in 2004. Syngenta (UK) issued a 2013 label for Cultivar® for use on apples and pears. The Essence Group of China advertises paclobutrazol with described uses on fruit trees; pot-grown ornamentals and flower crops; rice (Oryza spp.); turf; and grass seed crops. This report is of a series of trials conducted to evaluate the morphological and physiological effects of foliar applied paclobutrazol of on apple and peach [Prunus persica (L.)]. Soil/media surfaces were covered with plastic to reduce soil contact during spray application. One-year-old ‘Golden Delicious’/MM.106 apple and ‘Redhaven’/Lovell peach trees were established in 19 liter plastic pots. Treatments of 0, 300, 600, 1200 or 2400 mg•L-1Paclobutrazol were sprayed of when new terminal growth was approximately 10 cm length. Treatments were arranged in a randomized complete block design with eight replications with two trees/pots per experimental unit. Whole plant photosynthesis reading were taken the first year by placing trees in a temperature controlled, closed static system assimilation chamber (0.91 m by 0.91 m by 1.1 m tall). A Shimadzu gas chromatograph measured net photosynthetic (Pn) rates. In the second year after treatment, Pn and stomata conductance rates were measured four times at approximately two week intervals using an ADC portable CO2 analyzer and a Licor steady state porometer, respectively. Measurements were made on a tagged leaf at the eighth node and on a newly expanded leaf. Tree height, trunk diameter, and total shoot growth were measured each year. Half of the trees were partitioned each year and dry weights determined. The experiment was repeated, but with measurements of leaf Pn rates and not whole tree Pn rates. In first trial, no differences in whole plant Pn rates were found among treated apple or peaches trees when measured 37 and 42 days after treatment. In the second trial, tagged older leaves of paclobutrazol treated apple trees tended to have higher Pn rates than the controls starting about 50 days after treatment. The leaves at the eight node of apples sprayed with 2400 m•gL-1 had higher Pn rates (P≤0.01) than control leaves at 64 days after treatment. . Early (Julian date=162) in the second growing season, Pn rates of older leaves had a positive linear relationship (P≤0.01) with paclobutrazol concentration, but data was variable after that. The Pn rates of newly expanded leaves were unaffected by treatment in the year of treatment or the next.