Effects of water deficit at different growth stages under drip irrigation on fruit quality of citrus in the humid areas of South China

Fruit quality as affected by water deficit applied at different growth stages remain largely elusive. Therefore, to evaluate the effects of deficit irrigation (DI) timing and degree under drip irrigation on fruit physical and chemical quality of citrus, the low (D15%), mild (D30%), moderate (D45%) and severe (D60%) water deficit treatments were set at the growth stage I, II, III and IV, respectively, with a control treatment (CK), during two growing seasons. The application of water deficit under drip irrigation at the early growth stage (stages I and II) had remarkable effects on physical quality of citrus. The I-D45% treatment increased the single fruit weight and firmness by 5.78−9.85% and 13.25−17.14%, respectively, and the II-D15% treatment improved them by 6.67−9.15% and 1.43−14.46%. The application of water deficit under drip irrigation at the middle growth stage (stage III) had remarkable effects on physical and chemical quality of citrus. There was no remarkable reduction in fruit physical quality under the III-D15% treatment, while the contents of fructose, glucose, sucrose, vitamin C (Vc), total soluble solids (TSS), and MI (TSS/TA (titratable acidity) ratio) were significantly increased by 12.95−17.67%, 9.26−17.84%, 7.90−19.65%, 9.10−29.20%, 7.68−15.96% and 18.07−29.19% (P < 0.05), respectively, and TA was reduced by 8.82−9.91%. The application of water deficit under drip irrigation at the late growth stage (stage IV) had remarkable effects on chemical quality of citrus. The IV-D30% treatment significantly increased the contents of fructose, glucose, sucrose, Vc, TSS and MI by 22.95−25.54%, 16.47−17.79%, 18.56−19.54%, 25.63−50.23%, 12.49−20.59%, and 18.87−24.47% (P < 0.05), respectively, while reduced TA by 3.60−5.88%. Therefore, I-D45% and II-D15% treatments significantly improved physical quality of citrus by saving about 178 and 154 m3/ha at stage I and II, respectively; III-D15% and IV-D30% treatments significantly increased chemical quality of citrus by saving about 635 and 782 m3/ha at stage III and IV, respectively, which was demonstrated as the suitable water deficit pattern. And this strategy had no significant effect on yield. Among all the treatments, the IV-D30% treatment improved the fruit quality of citrus most significantly.