The effect of plant growth regulators on growth, yield, quality and management of an apple crop growing under an ultra-high density planting system

Plant growth regulators (PGRs) were used to regulate vegetative and reproductive growth in apple trees. Paclobutrazol (PBZ) was applied to trees as a soil-drench and gibberellin (GA₃ or GA₄) sprays were applied to the buds growing in the leaf axils of the existing fruiting and nonfruiting spurs. Vegetative growth, fruit yield and its components, fruit quality and storage potential were studied after PGR application. Two apple cultivars 'Braeburn', and 'Oregon Spur Delicious' ('OSD') with different vegetative growth characteristics were grown at a density of 20,000 trees/ha. One group of 3-year-old trees grafted onto MM.106 rootstock, received PBZ in September 1989 at 0 or 250 mg (a.i)/tree. A second group of 4-year-old trees received PBZ at 0, 50 or 250 mg (a.i)/tree in August 1990. In both experiments PBZ at 250 mg increased fruit yield and fruit number in the first year and then in alternate years with considerable affect on fruit size for up to four years. PBZ increased the biennial bearing index from 0.32 to 0.42. The magnitude of bienniality varied between the cultivars and 'Braeburn' exhibited an 80% greater biennial bearing than 'OSD'. PBZ applications restricted the vegetative growth by 44% ('Braeburn) and 32% ('OSD') for three consecutive years when compared to the untreated trees. A further set of trials was initiated in 1993 as a continuation of the previous work. These trials investigated the effects of a repeat application of PBZ on the vegetative and reproductive growth responses of older apple trees. Trees which previously received PBZ (0-250 mg (a.i)/tree) in 1989 or 1990 were retreated with PBZ at 0 or 250 mg/tree in autumn 1993. This repeat application of PBZ to previously treated trees did not increase yield or fruit number. In contrast, trees which received PBZ for the first time in 1993 had an increase in yield and fruit number compared to the control trees. Vegetative growth was strongly reduced the 2nd year after PBZ treatment. Fruit picked at different stages of maturity were evaluated at harvest and during a storage period of up to 80 days at 10° C. PBZ caused a slight delay in maturity plus a significant reduction in fresh fruit weight and fruit length:diameter (L/D) ratio at harvest, a reduction of 6% in total soluble solids and a slight increase in flesh firmness. However, PBZ treatments did not affect the fruit quality parameters in storage. Experiments designed to determine whether GAs could counteract the deleterious effects induced by PBZ on fruit bienniality and fruit quality were conducted on 5-year-old trees of a strongly biennial cultivar 'Braeburn' grafted to MM.106 apple rootstock. Trees were grown at a density of 10,000 trees/ha. Trees were treated with PBZ as a soil-drench at 0 or 250 mg (a.i)/tree in August 1993. GA₃ or GA₄ was sprayed on the buds of 2-year-old and older shoots on the entire tree at 0, 10 or 100 µg/bud at three times between 22 November and 22 December 1992 (before the PBZ treatment), or at similar dates in 1993 (after the PBZ treatment). The time of GA application over this period did not affect reproductive and vegetative growth or fruit quality at harvest. PBZ still decreased fruit yield and number, two years after application, but the reduction in fruit yield and number was less when GAs were applied to those buds. Shoot number and total shoot length after PBZ application for two years was consistently reduced compared to control. GA application in combination with PBZ did not cause full recovery of the vegetative growth of the trees. Consistent with earlier findings, PBZ reduced fruit fresh weight, fruit L/D ratio and ⁰Brix values and increased fruit flesh firmness. However, the effects were non-significant. The addition of GAs to PBZ treated trees generally reversed the PBZ-induced effects. Time of GA application or level of PBZ GA treatment did not affect the quality during storage of 90 days at 10° C.