Branch age and angle as crucial drivers of leaf photosynthetic performance and fruiting in high -density planting: A study case in spur-type apple "Vallee Spur" (Malus domestica)

Right branch angle could result in higher light interception as planting density increases. The influence of branch age and angle on light capture and nutrition accumulation has been noticed in previous studies, however, the optimum branch age and angle remained to be determined in high-density planting. The conducted study focused on the impact of branch age (from 2- to 8-year-old) and angle (horizontal, vertical and downward) on leaf photosynthetic performance and fruiting (leaf area, leaf SPAD value, mineral elements (N, P and K) content, photosynthetic and chlorophyll fluorescent parameters, endogenous hormone content, enzyme activities and fruit quality) in the cultivar "Vallee Spur" in high-density planting. Here we showed that leaf growth from 3- to 6-year-old branches was obviously superior to 2-, 7- and 8-year-old branches. Superior fruiting capacity was observed from 3- to 6-year-old horizontal branches, 3- and 4-year-old downward branches. Principal component analysis revealed that the scores from 7- and 8-year-old were the lowest in the three branch angles, thus, 7- and 8-year-old branches could be removed in high-density planting. Therefore, leaf photosynthetic performance and fruiting capacity of 3- to 6-year-old horizontal branches are optimum in high-density planting. Linear relationships were discovered between fruit quality and leaf growth, single fruit weight-IAA (R-2 = 0.936, p < 0.05) in horizontal branches, and titratable acid content-leaf nitrate reductase (NR) activity in vertical branches (R-2 = 0.954, p < 0.05) and in downward branches (R-2 = 0.916, p < 0.05). These results provide evidence for produce superior quality apples in spur-type and new clues for fruiting capacity assessment based on leaf growth by modulating branch ages and angles.