Fruit development and primary metabolism in apple

Apples (Malus domestica Borkh.) represent one of the most produced fruits worldwide, along with citrus and bananas. As high quality is an important trait for the consumer, many studies have focused on the research of new techniques to ensure and preserve the optimal organoleptic characteristics of this fruit. However, despite the huge number of studies on recent technological advances dealing with fruit final quality, less research has focused on the physiological aspects of apple development, including a variety of processes triggered after fertilization, such as photosynthesis, assimilation of carbohydrates, cell division, and cell enlargement, which determine apple final quality. In the present review, we summarize some of the most important changes and mechanisms linked to the primary metabolism of apples, as well as the effect of agronomic practices, such as fruit thinning, as key factors to improve apple quality and meet consumer demands, with the aim of amassing available information and suggesting future directions of research.