Improving Photosynthetic Characteristics and Enhancing Yield and Quality of Nanfeng Tangerine via Deep Irrigation in Red Soil Hilly Regions

Seasonal drought and poor soil water retention are key constraints to citrus production in China’s red soil hilly regions. This study investigated the effects of deep irrigation on photosynthetic characteristics, yield, and fruit quality in a typical Nanfeng Tangerine orchard in Jiangxi. A factorial experiment with three irrigation depths (D1: 25 cm, D2: 50 cm, D3: 100 cm) and three water levels (W1: severe deficit, W2: mild deficit, W3: full irrigation) was conducted. Results demonstrated that the D2W2 treatment was identified as the optimal condition. Compared with D1W1 and D3W3 treatments, the monthly mean net photosynthetic rate (Pn) and instantaneous water use efficiency (WUEi) of D2W2 increased by 51.50% and 25.93%, respectively. Furthermore, D2W2 achieved the highest yield per plant (58.13 kg) and superior fruit quality (soluble solids: 34.37 °Brix; titratable acidity: 0.46%; sugar–acid ratio: 15.93). The optimal performance of D2W2 was further confirmed by its top TOPSIS comprehensive evaluation score (0.95). In conclusion, a 50 cm irrigation depth combined with a mild water deficit is the recommended strategy for synchronizing water conservation, yield increase, and quality improvement in Nanfeng tangerine orchard.