Effects of different fruit loads on fruit quality and yield of cylinder-shaped pear trees in high-density orchards

Abstract:【Objective】Fruit load impacts quality and yield of fruit in apple, pear, and peach. According to the structural characteristics of cylindrical pear trees in high density systems, we used‘Zaosu’(Pyrus bretschneideri Rehd.) and‘Huangguan’(Pyrus bretschneideri Rehd.) for the study. The pear trees were grown at the experimental center of Wuwei Forestry Research Institute. Understanding the effect of fruit load on quality and yield is important for fruit thinning management of cylindrical pear trees in high density systems.【Methods】Two pear varieties which were relatively consistent in height, diameter and number of fruiting branches were selected. According to the number of fruiting branches, 4 fruit load treatments were set. Each fruiting branch group was allowed to bear an average of 3, 6, 9, and 12 fruit (T1, T2, T3 and T4, respectively), equivalent to 78, 150, 216, and 295 fruit in‘Zaosu’, and 62, 127, 194 and 264 fruit in‘Huangguan’, respectively. The experiments used single plant plot with 3 bio- logical replicates. Fruit thinning operation was carried out on May 30, 2017 and June 10, 2019. And the 3rd to 4th order fruit on inflorescence were selected. After thinning, 6 fruit with similar size were select-ed from each treatment and their vertical and horizontal diameters were measured every 7 days. Ripe fruits were harvested to determine yield per plant, and unit area production was calculated according to a density of 167 plants per mu. Fruit weight was measured with an electronic balance with an accuracy of 0.1 g. The diameters of the main stem and fruiting branch were determined with a digital display cali- per. The fruit longitudinal diameter was the length from the top to the bottom, and the fruit transverse di- ameter was the largest width along the fruit equator. The hardness of fruit was measured using a GY-4 digital hardness tester. An Atago PAL-1 sugar analyzer was used to measure the soluble solid content. Titratable acid was determined with titration method and soluble sugar content with anthrone sulfuric acid method. The content of stone cells was determined by freezing method.【Results】Changes in fruit vertical and horizontal diameters displayed a slow-fast-slow pattern. With the increase in fruit load, the growth rate of fruit vertical and horizontal diameters decreased. Similarly, the single fruit weight‘Zao- su’pear reduced with increase in fruit load. It was the highest in T1 and T2, which were 208.1 g and 199.5 g in 2017, 200.0 g and 179.8 g in 2019, respectively. Compared with the other treatments, single fruit weight in the two treatments was significantly higher. In 2017, the single fruit weight of‘Huang- guan’was 278.6 g and 270.6 g in T1 and T2, 318.5 g and 313.7 g in 2019, respectively, which were sig- nificantly greater than in the other treatments. Hence, the two pear varieties had a similar change pattern. Different fruit load had no significant influence on fruit shape index. Compared with other treatments, soluble solid content in‘Zaosu’and‘Huangguan’pears was higher in T1 and T2. With the increase in fruit load, soluble sugar content in both cultivars decreased. In‘Zaosu’pear, the titratable ac- id content increased with fruit load. In‘Huangguan’, it was the lowest in T1 and T2, and the highest in T3. The ratio of sugar to acid also decreased with the increase in fruit load. There was no significant difference in the hardness of fruit among different fruit loads. In‘Zaosu’, the stone cell content in T1 was16.7% and 28.5% lower than in T3 and T4, respectively, and that in T2 was 33.3% and 42.9% lower than T3 and T4, respectively. In 2019 and 2017, the trend of the stone cell content was similar. However, there was no significant difference between the two seasons. The proportion of fruit cores increased with fruit load and was the smallest in T1 and T2. The yield of two pear varieties increased with fruit load and was the lowest in T1 and the highest in T4. The yield of fruit in‘Zaosu’was 4 830.4 kg per 666.7 m2 in 2017 and 4 837.9 kg per 666.7 m2 in 2019. In 2017 and 2019, fruit yield was 5 441.3 kg and 5 693.1 kg per 666.7 m2, respectively, in‘Huangguan’.【Conclusion】The results showed that contents of soluble solids and soluble sugars were highest in T2, in which the ratio of sugar to acid was suitable and the proportion of fruit core was low. The yield was high with quality assurance in the treatment and it had no influence on the return flower in next season.