Effects of different types of foliar fertilizer on the flavor quality of overwintering tomato in solar greenhouse

Tomato quality degradation can happen under solar greenhouse production in winter, due to the low temperature and weak light. Taking the "184" tomato as the research object, an experiment was conducted to clarify the effects of the five types of foliar fertilizers on the composition and content of soluble sugar, organic acid components, and volatile substances. High Performance Liquid Chromatography (HPLC), Headspace Solid-Phase Micro Extraction (HS-SPME), and Gas Chromatography-Mass Spectrometry (GC-MS) techniques were also used to characterize the tomato samples. The results showed that three soluble sugars and seven organic acids were detected in the tomato fruit by HPLC. Three soluble sugars included sucrose, glucose, and fructose, the latter two of which were the two main soluble sugar of tomato fruit. Spraying selenium liquid fertilizer significantly increased the sucrose and glucose contents of tomato fruits at the white ripening, mature, and full-ripe stage (P < 0.05). The silicon and amino acid liquid fertilizer significantly increased the sucrose and glucose contents of tomato fruits at the white ripening, color turning, and mature stage, respectively (P < 0.05). Seven organic acids contained the oxalic acid, α-ketoglutaric acid, citric acid, quinic acid, malic acid, succinic acid, and fumaric acid. Among them, citric acid, malic acid, and succinic acid were the three main organic acids of tomato fruit. The contents of α-ketoglutaric acid, citric acid and malic acid at the mature stage significantly decreased by the selenium and fish protein aqueous fertilizer (P < 0.05). The selenium liquid fertilizer significantly increased the sugar-acid ratio of tomato at the white ripening stage and turning color stage by 15.32% and 12.15% compared with CK, respectively (P < 0.05). The GC-MS showed that 71 volatile substances were detected in the tomato fruits during the mature period, including 20 aldehydes, 9 ketones, 17 alcohols, 5 esters, 10 hydrocarbons, and 10 other substances. The aldehydes content was the highest in all treatments, and the esters content was the least. The five volatile substances with a high content were 2-hexenal, methyl heptenone, n-hexal, 2-isobutylthiazole, and guaiacol. Sixteen kinds of aldehydes were detected under the treatment of selenium liquid fertilizer, fish protein aqueous and fertilizer amino acid liquid fertilizer, which were two more kinds than those in the CK. All five foliar fertilizers increased the aldehyde content, indicating an increase of 21.89% under the treatment of silicon liquid fertilizer than that under the CK (P < 0.05). The 2-hexenal was the highest in the silicon liquid fertilizer treatment, which increased by 33.54%, compared with the CK. All the detected volatile substances contained nine characteristic aroma components, which were mainly divided into three types: Floral, fruity, and fragrant. Furthermore, the selenium, silicon, and seaweed essence water-soluble fertilizer treatments significantly increased the total content of characteristic aroma components of tomato by 7.93%, 17.68%, and 8.33%, respectively (P < 0.05), compared with the CK. In conclusion, the spraying of selenium and silicon liquid fertilizer can be popularized and applied in the production of tomatoes cultivated in the overwintering facility substrate, in order to improve the sugar component, the volatile aldehydes, and characteristic aroma components of tomato fruits, particularly for the flavor quality of tomato. The finding can provide the data support and practical reference for the high-quality cultivation of facility tomatoes.