Effects of supplemental illumination in different periods on the quality and volatile compounds of tomato fruit in solar greenhouse

The objective of this paper is to find the optimal periods of supplemental illumination that improves the yield and quality of tomato in solar greenhouse. The tomato cultivar of 'Fentailan' was selected as the target material, and LED was set as the light source. The tomato plants began to be supplemented with illumination 25 d after planting, and the supplement of illumination was completed when the first spike fruits were fully ripe. Three supplemental light periods in this study: 5 h before the curtain is opened (T1), 5 h after the curtain is covered (T2), 2.5 h before the curtain is opened and 2.5 h after the curtain is covered (T3); without supplemental light served as the control (CK). The flavor quality, volatile components and contents of the tomato were measured in each treatment. The results showed that the three supplemental illumination treatments can effectively increase tomato average yield per plant, soluble sugar, soluble solids, ratio of sugar to acid, and amount and content of volatile compounds, compared to that in the CK, but it would reduce the content of organic acid, all of which have the most significant effect under the T1 treatment (P<0.05). A total of 83 volatile compounds were detected in the four treatments, including 12 ketones, 22 aldehydes, 22 alcohols, 6 esters, 6 hydrocarbons, and 15 other compounds. The total amount and content of volatile compounds in each treatment are as follows in order: T1 (68 types, 3107.98 μg/kg) > T3 (65 types, 2610.74 μg/kg) > T2 (63 types, 2438.96 μg/kg) > CK (59 types, 2086.03 μg/kg). In every treatment, the content of alcohols is the highest, whereas the content of hydrocarbons is the lowest, and the substance with the greatest concentration is cis-3-hexen-1-ol. Compared to those under the CK, the three supplemental illumination treatments can increase the concentration of ketones, aldehydes, alcohols and other compounds in tomato fruits, but significantly decrease the concentration of hydrocarbons (P<0.05). The content of esters increased only under the T1 treatment, compared with that in CK. All the detected volatile compounds contain 11 kinds of characteristic aroma substances of tomato, which can be mainly divided into three categories: floral, fruity and green. Among them, the green aroma was the most abundant. Therefore, the T1 treatment (5 h before the curtain is opened) can effectively increase tomato yield, and improve the flavor quality of fruits, indicating the optimum supplemental illumination period for tomato cultivation in the local solar greenhouse. The finding can provide a sound scientific potential on supplementary lighting technology for tomato planting environment in protected cultivation.