Water-potassium coupling at different growth stages improved kiwifruit (Actinidia spp.) quality and water/potassium productivity without yield loss in the humid areas of South China

Precise water and nutrient regulation are the key to improving fruit quality and water productivity for kiwifruit orchards in Central Sichuan's hilly region where seasonal drought is becoming increasingly severe. Here, an experiment was set up to clarify how water-potassium coupling at different growth stages affects kiwifruit yield, water/potassium productivity and quality. A conventional fertigation was set as the control treatment (CK), and two water deficit treatments (60% and 80% CK, denoted as LW and HW, respectively) with three potassium (K) deficit levels (40%, 60% and 80% CK, denoted as LK, MK and HK, respectively) were applied at each growth stage of the cultivar “Jinyan” kiwifruit. The phenolic compounds and key enzyme activities were evaluated, in addition to the yield and traditional physical and nutritional properties. Results showed that the water and K deficit would reduce the yield of kiwifruit, however the yield of HWMK treatments at stages I and II showed no significant differences compared with CK (P > 0.05), HWMK treatment at stage III even increased the fruit yield. Water productivity (WP) and K partial factor productivity (KPFP) showed a trend of HW>LW, and reached the highest in HWMK treatment at all growth stages. The physical qualities were more affected by water, especially at stages I and II. HWMK treatments at stages I and II significantly increased fruit firmness by 30.20% and 21.56%, respectively (P < 0.05). Whereas, kiwifruit nutritional qualities were more affected by K, especially at stage III. Specifically, compared with CK, III-HWHK treatment significantly elevated vitamin C (Vc), total soluble solids (TSS), sugar/acid ratio, and total phenolic compounds by 22.45%, 30.58%, 24.91% and 54.87%, respectively (P < 0.05). Water-potassium coupling had significant effects on all five key quality-related enzyme activities (P < 0.05), except for the effect of water on ascorbate oxidase (AAO) activity (P > 0.05). The correlation analysis revealed a highly significant correlation between fruit nutritional qualities and key enzyme activities (P < 0.01), indicating that water-potassium coupling influenced nutritional quality by regulating the enzyme activity involved in metabolic processes. Principal component analysis (PCA) confirmed the results of correlation analysis, and found that HWMK treatment at stages I and II and HWHK treatment at stage III were most efficient in improving kiwifruit nutritional qualities. Overall, the stimulatory effect of fruit qualities to water-potassium coupling was growth stage dependent, water-potassium coupling affected fruit quality by regulating the enzyme activity. HWMK treatment at stages I and II and HWHK treatment at stage III were most efficient in improving fruit quality and water/potassium productivity without yield loss under water and fertilizer saving conditions, which could be suggested as the optimal strategy of fertigation in the humid areas of South China.