The biological and genetic mechanisms of fruit drop in apple tree (Malus x domestica Borkh.)

The apple tree (Malus × domestica Borkh.) is one of the most popular fruit trees for growing us a food tree worldwide. Global apple fruit production has grown rapidly over the past two decades. It has increased by 54%. The most popular is fresh fruits of high quality, which depend on different internal and external fruit parameters, like size, colour, acidity, sweetness, and maturity of the fruits. Yield self-regulation in apple trees helps to have high quality of crop. Only 5–10% of blooms can develop into marketable-quality fruits. Self-regulation is a complex biological process that is influenced by environmental and genetic factors. Too much blooming and overcropping are big problems for apple growers. Over cropping determines many small and low-quality fruits. Apples and other fruit trees have a natural system of fruitlet self-elimination. The natural optimal selfelimination of apple fruitlets is rare, and only a few apple tree varieties naturally thin fruitlets up to one fruit per inflorescence. In most cases, apple trees do not eliminate enough fruitlets to produce a qualitative fruit. The genetic programming within the tree determines how many of these buds will develop into fruit-bearing structures and how many immature fruitlets will be dropped. Regulation of the fruitlets’ number on the tree is an essential technological aspect in horticulture, as flower or fruitlet thinning is used to optimise the yield. This can be conducted by hand, mechanically, or by using chemical substances. The biological and genetic mechanisms of yield self-regulation in apple trees are important. The internal factors like plant physiology, genetics, and hormones, as well as external influences like environmental cues and management practices. The internal factors are the focus of this review.