Targeted Metabolic Profiling of Apple Fruit Development in Response to Reduction of Crop Load

The apple fruit is a typical pome fruit with the accessory fruit (cortex) developed from the hypanthium or floral cup, and the true fruit developed from the ovary (within the pith). The growth and development of fruit is associated with changes in a variety of metabolites such as sugars, sugar alcohols, organic acids and amino acids. However, the relationship these major metabolites and spatial patterns of fruit growth is poorly understood. Further, the effects of crop load reduction on the distribution and concentration of these metabolites are not completely understood. Here, the spatial changes in these metabolites during different stages of apple fruit development and in response to hand-thinning were studied using gas chromatography. The concentrations of major sugar, sugar alcohol, and organic acid were evaluated for the cortex and pith tissue of the thinned and un-thinned ‘Golden Smoothee’ at nine developmental stages. Fruit diameter and length of the hand-thinned apples were significantly greater than those in the controls from 19 days after thinning. However, no significant difference was observed between the thinned and un-thinned apples in terms of the concentration of major sugars, sugar alcohols and organic acids such as fructose, glucose, sucrose, sorbitol, and malic acid, indicating that the concentration of these metabolites was not affected by reduction in crop load, although higher quantities of these metabolites were present within the total fruit. However, asparagine, a major translocated form of nitrogen, was significantly lower in thinned fruit during early stages of fruit development suggesting potential effects on nitrogen remobilization in response to a reduction in crop load. Interestingly, several metabolites showed significant differences in concentration between the cortex and pith tissue at various stages of development. For example, the concentration of sorbitol was consistently higher in the pith than in the cortex tissue across all developmental stages, while the opposite trend was observed for malic acid. This study indicates that while the concentration of major metabolites may not play be affected by a reduction in crop load, metabolic crosstalk between the cortex and pith tissue regulate spatial patterns of fruit growth in apple.