Two apple (Malus domestica
Borkh. ) varieties ‘YH’ and ‘QC’ were chosen as materials and two treatments of spraying humic acid liquid film on 15 d and 30 d and non-bagging were used to investigate the effects of exogenous application of humic acid liquid film on photosynthetic indexes of apple leaves, fruit color difference and texture characteristics, soluble solids, titratable acid and expression level of genes related to sugar and acid content in fruits. The results showed that compared with the control, spraying humic acid liquid film treatment increased the photosynthetic characteristics of leaves, improved the coloring of fruits, increased the soluble solids content in fruits, decreased the titratable acid content, and increased the solid-acid ratio. The relative expression levels of sugar content-related genes MdTST1
, and MdERDL6-1
increased, and the relative expression levels of acid content-related genes MdMa1
, and MdWRKY126
decreased. Among them, the effect of spraying humic acid liquid film on 15 d was the most significant. The results showed that spraying humic acid liquid film treatment could improve the photosynthetic characteristics of apples, promote the growth and development of trees, improve fruit quality and improve fruit flavor.