Research progress on the mechanism of flavor formation and regulation in apricot

Abstract：Apricot is a notable deciduous fruit tree in northern China. The flavor of apricot consists of soluble sugars, organic acids and aroma, which is pivotal to determine the quality of apricot fruit. Sugars, mainly including sucrose, glucose, fructose and sorbitol, are considered as essential primary metabolite in apricot fruit. The dominant sugar in most apricot cultivars is sucrose. The flavor of apricot is also affected by the contents and types of organic acids. The primary organic acids are malic acid and citric acid in apricot. The more malic acid or citric acid in apricot, the more the fruit tasted acidic or better. Aroma plays a vital role in the sensory quality of apricot. More than 200 volatile substances have been identified in apricot fruits, including esters, alcohols, aldehydes, ketones and terpenes. The main esters include ethyl acetate, butyl acetate and γ-decalactone. The major components of alcohols are trans-3- hexenol, cis- 3- hexenol and isobutanol. Nonal, hexal and hexenal are dominant aldehydes in apricot. Geographical factors and varietal differences are critical to the flavor of apricot. The apricots cultivated in India and Xinjiang of China have higher total sugar contents than those cultivated in Turkey and Shanxi of China, respectively. In line with sugars, the contents of organic acids in China are higher than those of Oceanian and European varieties. The sugar degree in south-central China were significantly higher than those in the rest regions. Compared with the soluble sugars and organic acids, the aroma level of apricots which cultivated in North China ecologic group were stronger than those of European eco-logic group. Furthermore, the diversity of volatile substances in northwest China was higher than those of in North China, Northeast China and Southwest China. In addition, the composition of sugars, organic acids and volatiles was diversified among genotypes. The synthesis and metabolism of flavor substances in apricot exhibit the spatiotemporal specificity. For temporal specificity, fructose and glucose are mainly synthesized during early development stage. At the later stage of fruit development, the sucrose and sorbitol become the main components accumulated. Different from sugars, most organic acids increase in the early stage and decrease rapidly during fruit maturation. The main aroma differs in different ripening stages. For spatial specificity, there are no significant differences in the contents of total sugars and sucrose between pulp and peel except the fructose. Fructose in the pulp is higher than in the peel. Similar to the condition of sugars, the total amount and composition of organic acids in the pulp and the peel are the same in most apricot cultivars, but the content of various organic acids in the fruit is different. The contents and types of aroma substances in the pericarp are significantly different from those in the pulp. The nutrient elements, endogenous hormones and environmental conditions affect the accumulation of soluble sugars, organic acids and aroma. The contents of total sugars in fruit are affected by the ratio of nitrogen in different forms in the soil. Meanwhile, rational application of fertilizer improves the flavor quality and yield of apricot and reduces environmental pollution. Plant hormones are involved in the formation of fruit flavor quality by influencing plant growth, regulating enzyme activity and controlling gene expression. Abscisic acid promotes the sugar contents by regulating transport of the photosynthetic products to fruits and the expression of sucrose synthase genes. 1-methylcyclopropene, an ethylene inhibitor, is involved in reduction of sugars and organic acids in fruits via reducing the activity of enzymes involved in sugar synthesis and metabolism. In addition, methyl salicylate and cytokinin have been proved to maintain and improve fruit acidity. Environmental factors, bioactive molecules and plant resistance inducers are involved in the formation of fruit quality. Appropriate temperatures may prolong the shelf life, maintain the contents of soluble solids, delay the decline in organic acids, and affect the formation of aroma substances. It is a complex process that the synthesis, transport and metabolism of flavor substances in apricot fruit. A large number of enzyme genes, transporters and transcription factors are involved. In the aspect of enzyme genes, ParSuSys and PaSPSs are involved in the synthesis of sucrose. PaPEPC, PaMDH, PaME and PaCS have been identified to participate in the biosynthesis of malic acid and citric acid. PaFADs, PaLOXs, PaAAT1 and PaCCDs take part in the synthesis of esters and apocarotenoid. In the aspect of transporters, the transcript levels of PaSTP3, 5.1, 5.2, PaSUC4 and PaSWEET10 are up regulated with fruit ripening. It is suggested that these genes involved in sucrose transport over the tissues and cells in apricot. The transport of organic acids in apricot is mediated by vacuole transporters, carrier proteins (proton pumps) and ion channels, such as PaALMTCs, PaSFCs and PaVPP. In the aspect of transcription factors, ERF transcription factors regulate the formation of volatile substances in fruit by regulating the genes which respond to the biosynthesis of ethylene and aroma. At present, most studies focus on the differences in composition types, contents and spatio-temporal variation of the sugar, acid and aroma substances among the apricot varieties. However, the researches on apricot fruit quality which effected by external factors and key genes are weak. Here, the synthesis and metabolism of soluble sugars, organic acids and aroma substances in apricot fruits are reviewed, and the future studies are prospected in order to provide references for the flavor quality formation and new germplasm creation of apricot fruit.