Identifying Suitable Irrigation Thresholds at Different Growth Stages to Improve Yield, Water Productivity and Quality of Drip-Irrigated Kiwifruit in Southwest China

Global water scarcity necessitates precision irrigation management to optimize the balance between yield, water productivity (WP), and fruit quality. This study employed structural equation modeling (SEM) to explore the relationships between physiological responses, growth indicators, and yield of eight-year-old kiwifruit (Actinidia chinensis, cv. Jin Yan) under drip irrigation. A two-year field experiment (2018–2019) was conducted in a seasonally dry region of Southwest China, including 17 treatments across four growth stages: bud burst to leafing (I), flowering to fruit set (II), fruit expansion (III), and fruit maturation (IV). The treatments included a control (CK) and four irrigation lower limits at different growth stages: low (LL), mild (L), moderate (S), and severe (SS). Compared with CK, I-SS, II-S, III-LL, and IV-LL treatments significantly increased leaf instantaneous water use efficiency (WUEi) by 9.05%, 4.52%, 7.89%, and 9.55% (p < 0.05) respectively. After re-watering, II-L treatment significantly boosted Pn by 13.56% and Tr by 10.12%, leading to a 3.33% improvement in WUEi. Compared with CK, I-SS, and I-S treatments reduced the length of new shoots by 4.02% and 1.18% and the diameter of new shoots by 4.84% and 1.11%, while they increased fruit volume by 5.03% and 8.01%, respectively. I-SS and I-S treatments greatly increased the yield by 0.86–1.68% (p < 0.05), and increased water productivity (WP) by 3.90–4.31% (p < 0.05), respectively. Based on correlation analysis and SEM, kiwifruit yield was directly influenced by gs, leaf relative chlorophyll content, fruit volume, and shoot growth with standardized path coefficients of 0.377, 0.367, 0.546 and −0.04, respectively. For WP, gs, leaf relative chlorophyll content, fruit volume, and shoot growth exhibited direct path coefficients of −0.501, 0.342, 0.315, and 0.508, respectively. The IV-S, IV-SS, and I-SS treatments were consistently identified as the top three performers in both years using TOPSIS method of combining weights based on game theory. The suitable irrigation pattern was irrigation thresholds of 55%FC, 80%FC, 80%FC, and 60% FC for stages I, II, III, and IV, which enhanced the yield and WP of kiwifruit in Southwest China. This study could provide a scientific basis for precise soil moisture regulation of kiwifruit for similar production conditions.