Effects of regulating deficit at young fruit stage on yield, quality, and water use efficiency of citrus with different plastic film materials

Regulated deficit irrigation has been one of the most typical characteristics of saving water and fertilizer, in order to improve the quality and efficiency. In this study, an attempt was made to introduce surface mulching technology with regulated deficit irrigation. The water retention and water control effect of mulching were then utilized to treat the soil moisture shortage and later yield reduction caused by water deficit. The research object was selected as the citrus trees aged 10 years old at the Cangwubang citrus experimental base (30°75′N, 110°41′E, altitude: 343 m) in the western Hubei Province of China. A series of field experiments were conducted to investigate the effects of water deficit on the citrus quality, yield, and water use efficiency (WUE) under film mulching. Three levels of water deficit were set at the young fruit stage: Light (L: 80%-90% of field capacity), moderate (M: 70%-80% of field capacity), and severe (S: 60%-70% of field capacity). Four mulching levels were set: A (Japanese permeable film), B (Dupont Tyvek film), C (Chinese silver-black double-color film), and no mulching. The field experiment was conducted with full irrigation as the control, and 13 treatments were set up from 2019 to 2021. The results showed that the water deficit and film mulching significantly improved the quality of citrus fruits. The L treatment significantly increased the single fruit weight and excellent fruit rate, whereas, there was a decrease in the S treatment. The degree of water deficit was positively correlated with the soluble solids and reduced sugar content, while negatively correlated with the juice yield. Plastic film mulching significantly increased the single fruit weight, fruit color, excellent fruit rate, VC content, soluble solids content, and reduced sugar content (P<0.05), whereas, there was a significant decrease in the peel thickness, and titratable acid content (P<0.05), among which the A and B films were the best. A comprehensive evaluation of fruit quality showed that the M-B treatments were ranked first in 2019 and 2020, and the M-A treatment ranked first in 2021. There were the best fruit shape index and single fruit weight of M-B treatment in 2019. The best was found in the excellent fruit percentage and Vc content of M-B treatment in 2020. By contrast, the best was observed in the single fruit quality and excellent fruit percentage of M-A treatment in 2021. The M-A and M-B treatment were the best to improve the citrus quality. It infers that the moderate-deficiency treatment was the best treatment for better citrus quality, whereas, the A and B materials were the most suitable film mulching materials in such treatment. A significant level was also reached in the effects of water deficit and film mulching on the yield. Both L and M treatments significantly increased the citrus yield, whereas the S treatment significantly reduced the citrus yield. The interaction between the water deficit and film mulching posed a significant effect on the yield (P<0.05). Film mulching effectively reduced the effect of severe water deficit on the citrus yield reduction. Three kinds of film materials significantly reduced the ET of citrus trees (P<0.05) under the same water deficit condition. The water-saving effect of C material was better than that of A and B materials. Both water deficit and plastic film mulching effectively improved the WUE (P<0.05). The interaction between them posed a significant effect on WUE (P<0.05). The yield and WUE of M-A and M-B treatments reached the highest level. From 2019 to 2021, the output of M-A and M-B treatments were 44 793.6, 45 325.1, 43 126.8 kg/hm2, and 44 870.5, 44 727.7, 41 783.5 kg/hm2, respectively, and the WUE was 7.31, 7.68, 7.08 kg/m3 and 7.47, 7.47, 6.89 kg/m3, respectively. Therefore, the optimal treatment can be the M-A and M-B treatments with higher levels of citrus quality, yield, and WUE. This finding can also provide a theoretical basis for the irrigation management and quality improvement of the citrus industry in western Hubei of China.