Impacts of crop load on fruit growth and quality for top grafted Ruixianghong apple

Abstract: 【Objective】Crop load is one of the important indicators of orchard productivity. It is of great significance to study the suitable crop load for new varieties after top grafting. In this experiment, the effects of different crop load levels on fruit growth, fruit quality and economic benefits of Ruixianghong apple trees were studied to determine the appropriate crop load in production.【Methods】The materials were apple variety Ruixianghong trees top grafted 5 years before onto M26 dwarfing interstock Qinfu 1 trees planted in 2007 (11 years old when top grafting). Crop loads were adjusted to 1.3 (T3), 2.0 (T2), 3.0 (T1), and 4.3 fruit· cm^-2 of trunk cross-sectional area (TCSA), with the highest crop load as the control check (Control: 4.3 fruit·cm^-2 TCSA). From late May to fruit harvest, the maximum longitudinal diameter and the maximum transverse diameter of the fruit were measured regularly. Fruit weight, hardness, soluble solids, titratable acid and color parameters L, a, b, total aroma volatile compounds, and the relative content of each component were measured at harvest. The impact of crop load on economic return was analyzed.【Results】With the decrease in crop load, the growth rate of fruit longitudinal diameter and transverse diameter showed a decreasing trend, and the growth rate was in a pattern of T3＞T2＞ T1＞control. At 192 days after flowering, the fruit transverse diameter of T1, T2 and T3 increased by 8.2%, 12.7% and 17.2% respectively, and the fruit longitudinal diameter increased by 6.9%, 11.3% and 15.8% respectively compared with control. Crop load had no significant effect on fruit brightness, back-ground color, growth pattern of longitudinal and transverse diameters, fruit shape index and its change pattern. The higher the crop load, the lower the average single fruit weight. Compared with control, the single fruit weight of T1, T2 and T3 increased significantly by 28.0%, 37.2% and 58.5%, respectively. When the crop load was reduced to T2 and T3 levels, the color value showed a significant difference. At T3 level, the fruit hardness decreased significantly and the soluble solids content increased significantly. The contents of fructose and glucose in T2 and T3 increased significantly compared with control, and were not significantly different from T1. The sucrose content decreased with the decrease in crop load. Compared with control, the sucrose content in fruit of T1, T2 and T3 decreased significantly, and the titratable acid content had no significant difference. Under the condition of low crop load, the total content of aroma substances in Ruixianghong fruit could be increased, and the total aroma substance content was in an order of T3＞T2＞T1＞control. The fruit aroma substances under T2 and T3 showed significant improvement. The content of esters in fruit under all treatments exceeded 60%. The contents of esters and aldehydes under T1, T2 and T3 increased to various degrees compared with control, but the difference between T1, T2 and control was not significant. When the crop load decreased to T3 level, esters and aldehydes increased significantly. Under different crop loads, was not significantly different, which indicated that alcohol was not easily affected by crop load. Some other kinds of aroma substances were also affected by the crop load. Under T2 and T3, these aroma substances showed a significant increase compared with control, although the difference between T1 and control was not significant. The proportion of fruit with a transverse diameter smaller than 70 mm in control was as high as 53%, and that between 65 and 70 mm and between 70 and 75 mm was 34% and 40%, respectively. Under T1, T2 and T3, fruit above 70 mm accounted for 86%, 96% and 95%, respectively, and fruit larger than 85 mm could only been found in T2 and T3. Under T1 treatment, fruit between 70 and 75 mm and between 75 and 80 mm accounted for 36% and 44%, respectively, while in T2, fruit diameter fell mainly in a range of 75-80 mm, accounting for 55%. Under T3, fruit of 75-80 mm and 80-85 mm accounted for 31% and 37%, respectively. The percentage of big fruit increased significantly with the decrease of crop load. Compared with control, there was no significant difference in yield under T1; the proportion of large fruit was higher; and the estimated economic benefit was the highest. Although the yield under T2 decreased significantly, the proportion of large fruit was high, and there was no significant difference in estimated economic benefits. The proportion of large fruit under T3 was very high. However, due to the low yield, its economic benefits were not high enough.【Conclusion】Although the fruit growth rate can be higher and the fruit quality better under lower crop load, yield and economic benefit may be lower. Increasing the crop load can increase the yield, but it may reduce the fruit quality. Based our results, the optimal crop load for 5-year-old top grafted Ruixianghong apple tree is 3.0 fruit·cm-2 TCSA.