# Effect of Different Ratios of Cow Manure and Chemical Fertilizers on Fruit Quality of Gala Apples
## 3 Results and Discussion
### Aroma Substances  
A total number of 26 volatile compounds were identified in all the studied apples, as shown in Table 5, with the highest proportion of ester aroma substances, followed by alcohol aroma substances, as well as a small amount of aldehyde aroma substances, and a very small amount of ether and ketone aroma substances. The three most abundant esters were butyl acetate, hexyl acetate and 2-methylbutyl acetate. Figure 1: Effect of different ratios of cow manure and chemical fertilizers on total soluble solids, titratable acid, sugar-acid fraction of fruits: (**A**) total soluble solid; (**B**) titratable acid (**C**) sorbitol; (**D**) glucose; (**E**) fructose; (**F**) sucrose; (**G**) malic acid; (**H**) quinic acid; (**I**) shikimic acid. Different lowercase letters indicate significant differences between treatments (\(p<0.05\)). The fruit of 100% CM treatment contained 252.56 ug/g of butyl acetate, which was significantly higher than that of 100% CF treatment. There was no significant difference in the content of butyl acetate in the cattle manure with chemical fertilizer treatment, but all of them were significantly higher than that of the chemical fertilizer treatment alone. The 241.23 ug/g of hexyl acetate in the 100% CM treatment was significantly higher than that of the 100% CF treatment. Among the cow manure with fertilizer treatments, the fruit hexyl acetate content of 50% CF + 50% CM treatment was 287.36 ug/g, which was significantly higher than the other treatments. Fruit 2-methylbutyl acetate content was not significantly different among the treatments. The content of most of the remaining aroma substances was also higher in the 50% CF + 50% CM treatment. Most of the fruit aroma substances were higher in the treatments of cow manure with chemical fertilizer than in the treatments of chemical fertilizer alone and cow manure alone. The substances affecting apple aroma were mainly composed of various volatile organic compounds, including terpenes, aldehydes, alcohols, esters, ketones and sulfur-containing compounds. However, not all volatile substances contribute to fruit aroma, but only when their substance content reaches a certain value will they have an impact on fruit aroma. The variety of aromatic substances in plants is extremely rich, and more than 2000 species have been identified. The type and content of aroma substances vary among different plant species. There are at least four pathways from primary metabolites to the formation of various aromatic compounds [41]. Fatty acid and amino acid pathways are the two main metabolic pathways for the synthesis of esters in fruit. The production of esters aromatic volatiles is controlled by ethylene [42]. Esters are the most abundant of the volatile substances in apples. Nitrogen supply plays a role in the formation of aroma, which is limited under nitrogen deficient conditions. Fatty acids and amino acids are important substrates for the synthesis of aroma substances [43]. Therefore, we speculate that the input of nitrogen fertilizer can promote the synthesis of precursors such as amino acids in the fruit, which in turn affects the synthesis of aroma substances. Previous studies have shown that different fertilization treatments significantly affect the content of aroma substances in fruits [24], and studies on dates found higher levels of sugars, acids, and aroma substances under organic treatments [44]. In our study, it was found that esters accounted for an extremely high percentage of apple fruit aroma substances, about 75%, followed by alcohols, about 20%, and the rest were aldehydes, ethers, and ketones. Among the esters, butyl acetate, hexyl acetate, and 2-methylbutyl acetate were higher.