Response of fruit yield, fruit quality, and water use efficiency to water deficits for apple trees under surge-root irrigation in the Loess Plateau of China

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This study investigated the response of the fruit quality, fruit yield, and water use efficiency (WUE) to regulated deficit irrigation (RDI) during the different growth stages of apple trees (Malus pumila Mill) in the Loess Plateau of northern China. Different water deficit treatments were applied in 2016 and 2017 on a field planted with 5-year-old apple trees. The treatments included low (L), moderate (M), and severe (S) water deficit treatments during the bud burst to leafing (I), flowering to fruit set (II), and fruit growth (III) stages. Compared with full irrigation (FI), water deficit treatment during the different growth stages had significant effects on the fruit quality, fruit yield, and WUE of the apple trees. The L and M water deficit treatments during stage III significantly reduced the apple yield by 10.89% and 13.46% in 2016 and 3.66% and 10.10% in 2017, respectively. A water deficit during stage III decreased the single fruit weight, excellent-fruit percentage, and fruit water content by 2.79%–11.31%, 15.24%–20.36%, and 4.26%–10.07%, respectively, and increased fruit firmness, soluble solid content, and soluble reducing sugar content by 12.70%–21.31%, 13.83%–33.60%, and 10.13%–21.48%, respectively. The L and M water deficit treatments applied during stage I resulted in apple quality and yield that were similar to those resulting from the FI treatment, but the WUE was significantly higher in the L and M water deficit treatments than in the FI treatment. The optimal period for water deficit treatment is stage II, during which the highest yield and WUE were found. The L and M treatments during stage II increased the fruit yield by 13.93% and 13.28% in 2016 and 17.94% and 17.13% in 2017, respectively. The WUE of the apple trees was higher with the I I-L and I I-M treatments (greater than 7 kg m−3) than with other treatments. In addition, water deficit treatment during stage II caused a slight increase in fruit firmness and a slight decrease in fruit water content, which produces apples suitable for storage. Single fruit weight, excellent-fruit percentage, and soluble solid and soluble reducing sugar content were significantly improved, making the apples sweeter; thus, a water deficit during stage II had a significant positive effect on apple quality, with the I I-M treatment being optimal and the II-L treatment being second best. The optimal water deficit treatment of the II-M treatment enhances the fruit quality, yield, and WUE of apple trees in water-scarce environments.