Abstract
5-Aminolevulinic acid (ALA) plays an important role in plant growth and development. It can also be used to enhance crop resistance to environmental stresses and improve the color and internal quality of fruits. However, there are limited reports regarding the effects of ALA on tomato fruit color and its regulatory mechanisms. Therefore, in this study, the effects of exogenous ALA on the quality and coloration of tomato fruits were examined. Tomato (Solanum lycopersicum “Yuanwei No. 1”) fruit surfaces were treated with different concentrations of ALA (0, 100, and 200 mg⋅L–1) on the 24th day after fruit setting (mature green fruit stage), and the content of soluble sugar, titratable acid, soluble protein, vitamin C, and total free amino acids, as well as amino acid components, intermediates of lycopene synthetic and metabolic pathways, and ALA metabolic pathway derivatives were determined during fruit ripening. The relative expression levels of genes involved in lycopene synthesis and metabolism and those involved in ALA metabolism were also analyzed. The results indicated that exogenous ALA (200 mg⋅L–1) increased the contents of soluble sugars, soluble proteins, total free amino acids, and vitamin C as well as 11 kinds of amino acid components in tomato fruits and reduced the content of titratable acids, thus improving the quality of tomato fruits harvested 4 days earlier than those of the control plants. In addition, exogenous ALA markedly improved carotenoid biosynthesis by upregulating the gene expression levels of geranylgeranyl diphosphate synthase, phytoene synthase 1, phytoene desaturase, and lycopene β-cyclase. Furthermore, exogenous ALA inhibited chlorophyll synthesis by downregulating the genes expression levels of Mg-chelatase and protochlorophyllide oxidoreductase. These findings suggest that supplementation with 200 mg⋅L–1 ALA not only enhances the nutritional quality and color of the fruit but also promotes early fruit maturation in tomato.
Highlights
Fruit quality is determined by visual attributes, including size and color, as well as non-visual attributes, such as fruit taste and nutritional value (Nour et al, 2010)
Based on the effects of the various Aminolevulinic acid (ALA) concentrations on soluble sugar, soluble solids content and firmness of tomato fruit, we considered the 100 and 200 mg·L−1 ALA treatments were more effective to increase the content of soluble sugar and soluble solid and reduce fruit firmness
Chlorophyll synthesis was inhibited in the late stage of fruit development, which resulted in the inhibition of ALA metabolism in the fruit. These results suggested that the metabolism of ALA and biosynthesis of carotenoid in tomato fruit might have some interaction regulative mechanism, which should be deeply explored in further studies
Summary
Fruit quality is determined by visual attributes, including size and color, as well as non-visual attributes, such as fruit taste and nutritional value (Nour et al, 2010). Consumer pursuit of tomato fruit qualities, such as appearance, flavor, and internal nutrition, has increased. Consumers equate the visual appearance of fresh fruits with their internal qualities (Saltveit, 1999), which jointly determines their preferences and purchase desires (Hadi et al, 2013). The most important external quality is color, which is one of the critical factors determining consumer acceptance (Yu et al, 2016). Improvement of the external qualities of fruits has become an important research area (Ye et al, 2017)
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