Fruit Development and Primary Metabolism in Apple

Abstract

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.

Physicochemical Properties, Image Textures, and Relationships between Parameters of Red-Fleshed Apples Collected on Different Harvest Dates

“…In our research, the delay in the harvest date caused the increase in fruit weight. This confirms the theory that fruits increase in diameter and weight until harvest [34][35][36]. During the first harvest date, the IEC was approximately 0.1 ppm, which is often taken as the beginning of the climacteric stage [37].…”

Rootstock and Crop Load Effects on ‘Honeycrisp’ Photosynthetic Performance and Carbohydrate Accumulation

“…Lastly, significant differences in SSC were observed in 'Royal Gala' trees between all crop load treatments (3, 4, and 5 fruit cm −2 limb cross-sectional area, LCSA), with the highest crop load presenting an average value of 1.1 • Brix lower than the lowest crop load [61]. These results suggest that the effect of crop load on fruit SSC could also depend on other factors, such as apple cultivar, growing conditions, training system, timing of thinning, and additional agronomic practices [62,63].…”

Fruit Photosynthesis: More to Know about Where, How and Why

“…However, the same authors verified that tomato photosynthesis had a considerable role in seed development, impacting on seed set, composition, and morphology during the early developmental stage. In the same way, a recent review about the primary metabolism of apples, suggested that apple photosynthesis may be mainly used to support seed development and accumulate malate via the malate-CO 2 shuttle [127]. This hypothesis was based on the low rates of CO 2 assimilation in immature apples, as compared to those of the leaves, meaning that the CO 2 fixed in the peel also has little effect on growth and development in this crop [128,129].…”

“…Indeed, the CBB cycle can supply precursors for pathways of primary and secondary metabolism (as reviewed for tomato by Cocaliadis et al [113]; and for apple by Tijero et al [127], who highlighted the importance of apple photosynthesis for its primary metabolism). Primary metabolites, such as sugars, organic acids, and amino acids, produced at the early stages of development, determine most of the final fruit quality, as they are the main substrate for many reactions of secondary metabolism.…”