Characteristics of Photoassimilates Transportation and Distribution to the Fruits from Leaves at Different Branch Positions in Apple

Abstract

Clarifying the characteristics of photoassimilates transport and distribution to the fruits from leaves located on different branches can guide the branch composition and pruning of apple trees. A field experiment was carried out in 6-year-old ‘Fuji’3/M26/Malus hupehensis Rehd. apple trees, and the 13C-labeled tracer method was used to study the growth characteristics of the leaves on different branches (vegetative branches, long branches, mid branches, short branches, bourse branches, and cluster branches) and the translocation of 13C-photoassimilates to the fruit from these leaves at the fruit enlargement stage in 2018 and 2019. Additionally, the effect of the distance between the leaf and fruit (leaves from both short and long branches that were 20, 40, and 60 cm away from the fruit) was assessed. We found that with increased branch length, the intensity of photosynthesis and the level of photoassimilates synthesis in the leaves decreased, but increased in the leaves near the fruits. Indole-3-acetic acid, gibberellic acid, and zeatin accumulated in the leaves of the longer branches, while abscisic acid accumulated in the leaves of the shorter branches. The δ values of the 13C-labeled leaves from the short branches, the short branches 20 cm away from the fruits (S20), and the bourse branches at the fruit stage were low, and the corresponding δ values of the fruits were high. The 13C absorption of the fruit was highest in the leaves of the short branches, followed by those of the bourse branches, S20, and mid branches, and was lower in the leaves of the vegetative branches, cluster branches, and 60 cm short and long branches. The leaf 13C-photoassimilates in the short branches were more greatly transported to the fruits than those from the long branches when the distance between the leaves and fruits was the same. The 13C absorption of the fruit increased by 10.91, 5.28, and 1.14 mg at the leaf–fruit spacing of 20, 40 and 60 cm, respectively, and the leaf–fruit spacing was determined to be more important than branch type in determining the 13C absorption of the fruit.