Rhizospheric application with 5-aminolevulinic acid improves coloration and quality in 'Fuji' apples

5-Aminolevulinic acid (5-ALA), an eco-friendly plant growth regulator, has been increasingly applied to crop and fruit production. In the actual production, many fruits are often covered by plastic bags which will not be removed until the fruits are harvested. Therefore, although 5-ALA shows promotive effect on fruit quality, fruit spray cannot be implemented in the orchards using plastic bags. In this study, rhizospheric application of 5-ALA increased the endogenous 5-ALA content in the leaves and fruits, suggesting that 5-ALA is transported from roots to aboveground. Using a multiple plant efficiency analyzer (M-PEA), we also found that rhizospheric application of 5-ALA significantly improved PS II and PS I photosynthetic activities of apple leaves. It means that 5-ALA treated apple trees might accumulate more assimilates for fruits to improve their quality. Meanwhile, rhizospheric application of 5-ALA obviously increased fruit weight, the content of flavonoids, total soluble solids and ascorbic acid, as well as activities of antioxidant enzymes. Most importantly, rhizospheric irrigation of 5-ALA also significantly improved fruit coloration. Using calli induced from the flesh of 'Fuji' fruits, we found that 5-ALA up-regulated the expressions of many genes involved in flavonoid anabolism. These results suggest that rhizospheric application of 5-ALA solution is effective to improve external and interior qualities of apple fruits although 5-ALA was not directly applied to fruit surface. Taken together, we conclude that rhizospheric application of 5-ALA can be transported to the leaves and fruits of apple trees, where it enhances leaf photosynthetic capacity and improves fruit coloration and interior qualities simultaneously. This finding provides an effective and handy method to improve apple quality in modern orchards.