Effects of Regulated Deficit Irrigation on Leaf Nutrients, Enzyme Activities, and Root Zone Soil Nutrients during Fruit Expansion and Coloration-Sugar Accumulation Stages of Citrus in Western Hubei Province

［Objective］ This study investigated the effects of regulated deficit irrigation on leaf nutrient levels (nitrogen, phosphorus, and potassium), enzyme activities (superoxide dismutase, peroxidase, and catalase), and root-zone soil nutrients (nitrate-nitrogen, available phosphorus, and available potassium) during the fruit expansion (Ⅲ) and coloration-sugar accumulation stages (Ⅳ) of citrus in western Hubei Province. ［Methods］ Six-year-old“Ehime 28” citrus trees in the western Hubei region were selected as the research subjects. Full irrigation served as the control (CK: daily irrigation volumes of 4.0 and 2.5 L during the fruit expansion stage (Stage Ⅲ) and coloration-sugar accumulation stage (Stage Ⅳ), respectively). Four deficit irrigation treatments were applied during both Stage Ⅲ and Stage Ⅳ, including mild deficit (LD: 70% of CK), moderate deficit 1 (MD1: 55% of CK), moderate deficit 2 (MD2: 40% of CK), and severe deficit (SD: 30% of CK). The aim was to explore the response patterns of soil nutrients, citrus leaf nutrients, antioxidant enzyme activities, and root-zone soil nutrients to water stress under drip irrigation across different growth stages. ［Results］ 1) The Ⅳ-MD1 treatment significantly promoted the increase in leaf nitrogen, phosphorus, and potassium contents, with their respective mass fractions increasing by 8.05%, 2.13% and 12.42% compared to the control group. Moreover, this treatment achieved the highest rates of nutrient accumulation and growth in the leaves. 2) The Ⅲ-MD1 treatment effectively enhanced the antioxidant defense capacity of citrus trees, with the activities of superoxide dismutase (SOD) and peroxidase (POD) increasing by 63.3% and 104.5%, respectively. 3) The Ⅲ-MD1 treatment markedly enhanced nitrate nitrogen and available phosphorus contents in the upper and middle soil layers, with respective increases of 74.54% and 17.61% in the upper layer, and 73.53% and 43.58% in the middle layer. Additionally, the IV-MD1 treatment significantly improved the vertical distribution of soil nutrients, effectively reducing nutrient leaching losses. This treatment also increased the available potassium content in the upper and middle soil layers by 59.23% and 51.67%, respectively. 4) Canonical correlation analysis revealed significant relationships between soil and leaf nutrient elements. Leaf phosphorus and potassium contents were positively correlated with available potassium in the soil, while they were negatively correlated with soil nitrate-nitrogen, available phosphorus, and leaf nitrogen. ［Conclusion］ The Ⅳ-MD1 treatment effectively promotes the accumulation of soil and leaf nutrients and enhances the activity of protective enzymes, making it a suitable water-saving management strategy for drip irrigation for citrus cultivation in western Hubei. The findings provide a theoretical basis for optimizing citrus irrigation practices and improving fruit quality.