Research advance in fruit quality of sweet cherry

Fruit quality is an important basis for breeding and selection of high-quality fruits. However, there are problems such as insufficient fruit quality in the production of sweet cherries. In order to scientifically improve the quality and yield of sweet cherry fruits, this article focuses on the appearance quality parameters such as fruit shape, size, and skin color, as well as a series of physiological and biochemical changes such as texture, sweetness and sourness, flavor, hormones, and other internal quality formation. The influence of internal and external factors are also analyzed, such as variety and rootstock, temperature, light, water, and nutrients, on the formation of fruit quality; Secondly, the research progress in different measures to improve the quality of sweet cherry fruits is summarized; Finally, prospects are proposed for the research content and direction of improving cherry fruit quality. Sweet cherry fruit is a stone fruit, and its texture characteristics include hardness, brittleness, and juiciness, which directly determine consumers' choices, and the storage and transportation properties of the fruit. The sweetness is soluble solids content (SSC), the acidity is titratable acidity (TA), and the optimal SSC/TA ratio is between 1.5 and 2.0. Cherry fruits belong to the sugar accumulation type, with glucose and fructose as the main sugar components. The sucrose content is low and the fruit does not accumulate starch and sorbitol. The flavor substances in cherries mainly include aldehydes, alcohols, ketones, acids, esters, and terpene compounds. Sweet cherries are rich in dietary phenolic compounds. It is mainly composed of phenolic acid (hydroxy cinnamic acid) and flavonoids (xanthone alcohol, flavonoids, flavanone, flavan-3- ol, lavones and anthocyanins). The total phenolic content in the flesh and peel of sweet cherries accounts for 3.99% to 39.01% and 61.99% to 76.02% of the total phenolic contents in the entire fruit, respectively. They are rich in minerals such as calcium (14 mg · 100 g- 1 ), magnesium (10 mg · 100 g- 1 ), phosphorus (20 mg · 100 g-1 ), and potassium (200 mg · 100 g-1 ). Cherry fruits are nonclimacteric and the ripening process is mainly controlled by the plant hormone abscisic acid (ABA). Their content rapidly increases during the color transition period and reaches the highest level 4 days before harvest. The fac-tors that affect the quality of sweet cherry fruit vary greatly among cherry varieties in terms of fruit hardness. The interaction between scions and rootstocks affects the water relationship, leaf air exchange, mineral element absorption, plant size, flowering, fruit setting time, fruit quality, and yield efficiency of the grafted tree. Low temperature is beneficial for the accumulation of sugars, starch, and other substances in plants. In areas with large temperature differences between day and night, fruits accumulate more dry matter, and have larger sizes with high sugar content. Adequate light can promote the rapid accumulation of secondary metabolites during fruit development. Drought stress leads to severe physiological fruit drop and yellowing of sweet cherries, which affects fruit yield. The impact of precipitation on fruit quality is mainly manifested in the inhibition of root growth, absorption, and transportation activities before fruit maturity, resulting in inhibition of above ground growth. Mechanical damage to sweet cherry skin is isotropic, and its mechanics mainly depend on the epidermis and subcutaneous tissue, rather than the stratum corneum. The measures to improve the quality of sweet cherry fruit include cultivation management mode, surface coverage, reasonable fertilization, exogenous hormones, and storage methods. Drip irrigation and summer pruning will increase fructose levels and total phenolic content. The position of trees in the orchard and the position of fruits on the trees affect fruit weight and quality, and fruit size and sweetness decrease with increasing plant density. Grass covering cultivation can significantly increase the yield of sweet cherries and improve the quality of their fruits. Long term film covering can lead to certain enhancement of photosynthetic characteristics in the leaves and improve the coloring of cherry fruits, but there is no difference in the size of cherry fruits produced with or without film covering. Increasing the dosage of N has a positive impact on the required fruit quality attributes (such as fruit color and flesh hardness), while excessive potassium application has no positive impact on fruit quality, and may even have a negative impact. The most suitable potassium application amount is 400 g K2O per plant, and a high dosage of 600 g leads to a decrease in other nutrients, as well as FF, SS, and TA. Boron fertilizer has no effect on the vitality and yield of sweet cherry trees, and B fertilizer has no effect on the average fruit weight, titratable acidity and fruit cracking sensitivity. In addition, foliar calcium spraying before fruit ripening can significantly increase the content of Ca, Mg, K, Na, and Fe mineral elements during fruit storage. The application of growth regulators can improve the storage, increase the red color and fruit size, and reduce cracks. The application of cytokinin at the early stage of fruit development, or the application of synthetic auxin at the time of stone hardening stage can stimulate the growth of sweet cherries, thus stimulating their final fruit size. In addition, auxin has also been proved to play an important role in calcium transport and absorption of fruits during development in plants. Several types of polysaccharide coatings have been used for sweet cherry preservation, such as sodium alginate, chitosan, whey protein isolate, almond gum, acacia senegal and aloe gel, guar gum (GG) and ginseng extract.