Tree thinning intensity modulates photosynthetic performance, photoprotection, and fruit quality in open-central canopy apple orchards of the Loess Plateau

Open-central canopy (OCC) has become the popular tree shape for arboreal apple orchards in full fruit period in the Loess Plateau of China. This study aimed to elucidate the physiological regulation mechanisms related to planting density optimization, photosynthetic performance enhancement, and fruit quality improvement in dense-canopy OCC apple orchards in the Loess Plateau of China through tree thinning practices at different intensities. This study used three planting densities of OCC apple trees as experimental materials. These include no thinning [NT, 3 m (plant) × 4 m (row)], low-intensity thinning [LT, i.e., first thinning based on NT: 4 m (plant) × 6 m (row)], and high-intensity thinning [HT, i.e., second thinning based on LT: 6 m (plant) × 8 m (row)]. In three successive experimental years, changes in spatiotemporal variations of photosynthetically active radiation (PAR) intercepted by canopy, leaf gas exchange, gross photosynthetic rate (P_g), photorespiration rate (P_r), nonphotochemical quenching (NPQ) dark relaxation curves, antioxidant enzyme activity [ascorbate peroxidase, catalase and superoxide dismutase (SOD)], and fruit quality were carefully measured and compared. Results showed that compared with NT, LT increased PAR by 36%, maximum net photosynthetic rate under light saturation (P_nmax,P) by 30%, carboxylation efficiency (CE) by 37%, fruit soluble solid content (SSC) by 4%, and peel anthocyanin content (AC) by 20%. Moreover, LT enhanced the photoprotective capacity through elevated ratio of P_r to P_g under PAR of 2000 μmol ∙ m⁻² ∙ s⁻¹ (52%), the reversible component in NPQ [r(qE), 9%], and antioxidant enzymes (SOD 10%). Compared with NT, HT enhanced PAR by 71%, P_nmax,P by 67%, CE by 65%, SSC by 9%, and AC by 37% and boosted photoprotection through increased P_r/P_g (116%), r(qE) (11%), and SOD activity (29%). The Pearson Correlation Coefficients between PAR and the fruit quality indices (including single fruit weight, SSC, and AC) were 0.89, 0.73, and 0.96, respectively. Single fruit weight and SSC were significantly and positively correlated with photosynthetic parameters (such as P_nmax,P and CE). In summary, after tree thinning, the light environment in the OCC of apple trees was remarkably improved, and the leaf photosynthetic and photoprotective capacities were enhanced, thus significantly boosting fruit qualities. In terms of the dose responses, high-intensity thinning demonstrated the most significant enhanced effects.