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Biennial bearing has long been a challenge for perennial tree fruit production. Some cultivars are more susceptible and can have excessive bloom and fruit set in one year followed by insufficient bloom and fruit set in the next year. Although crop load management can be used to reduce biennial bearing, optimum crop loads in one year can still sometimes be followed by high variability in return bloom the next. Moreover, the fundamental mechanisms that drive this behavior are poorly understood. The two primary hypotheses of biennial bearing consider hormonal control and resource competition for carbohydrates and nutrients. Through reviewing the current literature and gaining a better understanding of carbohydrate and hormone cycling/signaling within trees, a framework for biennial bearing control can be developed. With an understanding of the physiological responses contributing to biennial bearing patterns, manipulation of carbohydrate and nutrient cycling within trees through management practices may influence flowering and fruit bud formation. Improved management practices of biennial bearing can be evaluated and adjusted to suit different planting systems and species. • Fundamental mechanisms driving biennial bearing are poorly understood. • Biennial bearing is controlled by a complex of carbohydrate and hormone levels. • Improved biennial-bearing management practices need to be developed. • Key gaps in knowledge lie in the field of carbohydrate assimilation and translocation. • Manipulation of carbohydrate and nutrient cycling may influence flower bud formation. [ABSTRACT FROM AUTHOR]

Biennial bearing is a common problem encountered in perennial fruit crops. In order to determine a suitable crop load for sustainable development of dryland apple orchards, we defined the concept of “appropriate productivity” and examined five crop load levels in an apple orchard ecosystem on the Loess Plateau, in Changwu, Shaanxi province, China. The orchards were planted with Fuji apple ( Malus domestica Borkh.) trees on M. sieversii rootstock since 1996. Crop load was limited to 53–54 (I), 67–68 (II), 80–81 (III), 94–95 (IV), and 107–108 (V) fruit per tree, using flower and fruit thinning by hand. Volumetric soil water content, total nutrient (N, P, and K) contents of branches, and economic benefit were then analyzed under each treatment over a 5-year period. Soil water content was found to be closely related to rainfall, with three seasonal periods recognized annually. Average water content in 2009–2013 ranked treatment II (21.98%) > III (21.57%) > I (20.61%) > IV (20.31%) > V (20.11%), in which treatments II and III were significantly higher than other treatments. With increasing soil depth, the water content showed an “increasing-decreasing-increasing” trend, with the highest value tending to occur within a depth of 40–100 cm. The nutrient contents of branches under treatments I–III were good and trees showed strong resistance to apple Valsa canker. From an economic perspective, the percentage of high-quality fruit increased as did the economic benefits in treatments I–III. In conclusion, controlling fruit crop load effectively reduced fluctuations in production, increased the economic benefits, and maintained the health of the orchard ecosystem. The appropriate crop load level for mature apple orchards in the gully region of the Loess Plateau was deemed to be 67–81 fruit per tree. [ABSTRACT FROM AUTHOR]

Highlights: [•] Mechanical blossom thinning and hand blossom thinning were compared for peach. [•] Both methods removed flowers and decreased fruit set with increasing rates. [•] Reductions in labour requirements were observed in both treatment years. [•] Mechanical blossom thinning appears to be an effective thinning strategy for peach. [ABSTRACT FROM AUTHOR]