Effect of different fruit loads and irrigation amounts on new shoot growth, yield and irrigation water use efficiency in spur-type Fuji apples

Abstract: 【Objective】The Loess Plateau is located in a typical area of arid and semi- arid, seasonal drought, low rainfall and insufficient irrigation water. When the lack of water leads to low irrigation consumption, reasonable control of crop load is a better strategy for perennial fruit crops, which not only reduces the application of water but also stabilizes the yield, so it is of great significance to explore the irrigation amount and fruit load mode of high-quality and efficient apple production in the Loess Plateau.【Methods】In this study, 9-year-old spur-type Fuji apple trees were used as the research objects, and three fruit loading capacities were set up: low load T1 (2 fruits·cm^-2 trunk cross-sectional area), medium load T2 (4 fruits· cm^-2 trunk cross-sectional area) and high load T3 (6 fruits· cm-2 trunk cross-sectional area), and six irrigation gradients: W0 (0), W1 (40% ETc), W2 (60% ETc), W3 (80% ETc), W4 (100% ETc) and W5 (120% ETc). An orthogonal experimental design was used for a total of 18 treatments. In order to comprehensively improve the growth, photosynthesis, fruit quality, yield and irrigation water use efficiency (hereinafter referred to as IWUE) of apple trees, the response pattern of fruit load and irrigation water treatment to apple canopy, photosynthesis, appearance quality, yield and IWUE was studied.【Results】Different irrigation treatments had significant effects on the length ofnew shoots and leaf area index (LAI) in the experiment. The reduction of irrigation water significantly reduced the length of new shoots and LAI, and the highest treatment was W5 and the lowest treatment was the control W0. The effect of fruit load on leaf area index was small, and under the same irrigation water conditions, leaf area index did not change significantly with the increase of fruit load, but it had a significant relationship with the length of new shoots, which decreased significantly with the increase of fruit load. In the full treatment, the final maximum length of T1W5 shoots was 49.1 cm, and the final maximum length of T3W0 shoots was 25.2 cm. The largest exponential growth rate of leaf area was as high as 65.9% with T1W5 treatment, and the lowest was with T3W0 treatment, which decreased by 28.1% compared with T1W5. The irrigation amount had a significant effect on the daily mean values of net photosynthetic rate Pn, transpiration rate Tr, intercellular CO2 concentration Ci and stomatal conductance Gs in apple leaves. Under the same fruit load conditions, the daily average values of photosynthetic rate Pn, transpiration rate Tr and stomatal conductance Gs all increased first and then decreased with the increase of irrigation volume, reaching the maximum with W3 treatment, and the intercellular CO2 concentration was generally the smallest with W4 treatment. The highest treatment of Pn was T1W3, and the lowest treatment was T1W5 treatment, which significantly reduced by 46.1%; The highest treatment of Tr was T1W3, and the lowest treatment was T3W0 treatment, which significantly reduced by 33.1%; The highest treatment of Ci was T3W1, and the lowest treatment was T1W4 treatment, which significantly reduced by 11.8%; The highest treatment for Gs was T2W3 and the lowest treatment was T1W5 treatment, with a significant reduction of 46.6%. The fruit load and irrigation amount can adjust the fruit quality, and under the same load conditions, the single fruit weight and large fruit rate showed a trend of first increasing and then decreasing with the increase of irrigation volume, and reached the maximum with W3 and W4 treatments, respectively. The fruit hardness increased with the decrease in the amount of irrigation water. W4 treatment can increase the single fruit weight and large fruit rate, and the quality performance with W0 treatment being the worst, and the quality performance of T3 apples was the worst. Among them, the highest weight per fruit was T1W4, which was 281.06 g, which was 39.51% higher than the lowest T3W0 which was 201.46 g. The hardest T2W1 treatment was 7.73 kg·cm^-2 , which was 73.32% higher than the lowest T1W4 treatment of 4.46 kg · cm^-2 . The T1W4 treatment with the highest fruit rate was 83.93%, which was 54.45% higher than the lowest T3W0 treatment of 54.34%. Load and irrigation volume had a significant impact on apple yield and irrigation water use efficiency (IWUE). T3W4 had the highest yield of 87 379 kg · hm^-2 , and T1W1 was the lowest. Compared with T1W1 treatment, the yields of remaining treatments increased by 7.3%-178.5%, and the IWUE treated by T3W1 was the largest (31.11 kg·m^-3 ), which was 6.7 times more than that of the smallest T1W5 treatment (4.63 kg ·m^-3 ).【Conclusion】Regression analysis showed that when yield and IWUE obtained the optimal solution at the same time, the combination of irrigation water and fruit load was closest to T2W4 treatment. Therefore, T2W4 can be used as a recommended method to save water and increase apple production in the Loess Plateau region.