Abstract:

The current study aims to investigate the effects of trunk heights on leaf photosynthetic characteristics and fruit qualities at different canopy locations in Open-Central Canopy（OCC）of apple. By using the 36-year-old apple（Malus × domestica‘Nagafu 2’，grafted on the rootstock Malus robusta Rehd.）of OCC with trunk heights of 0.8-1.0 m（as the control，CK），1.2-1.5 m（low trunk treatment，LT），1.5-1.8 m（middle trunk treatment，MT），and 1.8-2.1 m（high trunk treatment，HT）as materials，changes of photosynthetically active radiation（PAR），diurnal changes and responses to PAR and intercellular CO₂ concentration（C_i）of leaf gas exchange，kinetics of chlorophyll fluorescence quenching and fruit quality indices at different canopy locations were measured and compared carefully. Results showed that PAR increased most significantly in different locations（at 1.5 m above ground in the east and west of outer-crown and 1.5 m and 2.5 m above ground in inner-crown）of HT with increment of 132.47%，178.29%，113.7% and 32.37% compared to those of CK，respectively. The maximum values of net photosynthetic rate（P_nmax,P）and characteristic parameters[light saturation point（LSP），light utilization range（LUR）and apparent quantum efficiency（AQE）]derived from the fitted-curves of P_n-PAR responses of HT were significantly higher than those of MT，LT and CK. In comparison with CK，maximum electron transport rate under saturated light intensity（J_max）and triose phosphate utilization rate（V_tpu）of HT & MT increased significantly. The average values of light energy capture efficiency（F_v'/F_m'），actual photochemical efficiency of PSⅡ（Φ_PSⅡ）and photochemical quenching coefficient（q_P）were different significantly among all treatments and CK with the most significant increment in HT. Comparing to CK，the nonphotochemical quenching（NPQ）in the outer-crown of HT，MT，and LT was lower significantly. There was no significant difference among the inner-crown of HT，MT and LT. At the height of 1.5 m and 2.5 m above ground，single fruit weight and chromatic value a of HT were significantly higher than those of CK. At east and west sides of 2.5 m above ground，the soluble solids content and the ratio of solid to acid of HT were significantly higher than those of CK，while fruit acidity was lower than that of CK. The chlorophyll content in peel of HT was significantly lower than that of CK among different canopy locations. The anthocyanin content，total phenol content，total flavonoid content and capacity of scavenging free radical（FRAP，ABTS⁺ and DPPH）of peel in east side of HT were significantly higher than those of CK with the ranking of east side > west side > inner-crown. In summary，the leaf photosynthetic characteristics and fruit qualities of apple showed significant spatial heterogeneity in OCC with different trunk heights. The rate，capacity and efficiency of leaf photosynthesis as well as light energy utilization efficiency were significantly enhanced，thus the fruit qualities（especially the east side）were significantly boosted in open-central canopy with high trunk.

Key words: apple, open-central canopy, trunk height, canopy location, photosynthetic characteristics, fruit quality