Effects of different thinning methods on ecophysiology of dense apple orchard

In order to choose optimum thinning method of closed and matured ‘Red Fuji’ orchards in the Loess Plateau, 18-year old ‘Red Fuji’ apple trees in Qingyang County, Gansu Province, were used to study the effects of three thinning methods — interlaced thinning (T1), septum strain thinning (T2) and cutting one for every 3-tree (T3) — on orchard light distribution, leaf quality, leaf structure, fruit distribution and quality, and yield and economics of apple trees. The results showed that thinning methods significantly decreased orchard coverage rate, with 22.01%, 18.01% and 10.14% decrease under T1, T2 and T3, compared with the control (CK). The transmittance between plants in a line under T1, T2 and T3 treatments increased by 108.59%, 191.98% and 57.45%, respectively, compared with the control (CK). The transmittance between lines increased 259.20%, 220.11% and 64.86%, respectively. Transmittance under crown increased respectively by 102.80%, 155.32% and 37.43%. Canopy photosynthetically active radiation (PAR) increased respectively by 38.02%, 45.18% and 18.43%. PAR three dimension distributions of apple trees under T1 and T2 was better than under CK and T3 treatment, PAR of T2 treatment was improved more obvious. Also thinning methods effectively improved the structure of apple leaf. T1 and T2 significantly increased upper epidermis thickness of leaves inside canopy and palisade tissue thickness of all leaves, while T3 significantly increased lower epidermis thickness of leaves inside and outside canopy. Under T2 treatment, leaf palisade tissues were more tidily and closely arranged than under other treatments. Leaves quality was significantly changed under different thinning methods. Leaf chlorophyll, N and K contents were increased in 3 thinning methods treatments. T2 was the best in terms of increase in the contents of cChlorophyll a, cChlorophyll b and carotenoids. Single fruit weight increased by 25.03%, 34.83% and 9.81%, fruit color index increased by 6.00%, 6.26% and 3.30%, soluble sugar content increased by 35.98%, 39.14% and 22.98%, and peel anthocyanin content increased by 104.41%, 101.47% and 30.88%, while titratable acid content decreased by 15.38%, 23.08% and 17.95%, respectively, under T1, T2 and T3 conditions, compared with CK.. Under CK treatment, there was not fruit in positon 0.5 m away from stem, most fruits were 2 m away from stem and 3 m high, showing outside moving of fruit-setting position. Under thinning conditions, fruits in inside canopy and in lower canopy increased. Despite yield per hectare under T1, T2 and T3 treatments decreased by 7 601.89 kg, 5 219.45 kg and 6 056.80 kg, respectively, orchard productivity value per hectare increased by 14 959.54 Yuan, 34 363.70 Yuan and 9 081.13 Yuan.