The aim of this study was to optimize the water and fertilizer management of peppers, and to investigate the effects of the coupling of irrigation frequency and nutrient solution supply on the quality, yield, water use efficiency and fertilizer partial productivity of peppers grown in substrate bags.

Kailai (37-83) RZ F1 pepper was chosen as the material in the study, the irrigation amount (IA) required to maintain the water content of the substrate at 55%-60% was set as the total daily IA of single plant, and three irrigation frequencies (IF) of single plant were to supply IA according to 1 time (IF1), 2 times (IF2) and 4 times (IF3), respectively, and two nutrient solution supply amounts (NS) were the standard Yamazaki pepper nutrient solution (NS1, i.e. 250 mL/plant per day and 500 mL/plant per day during the flowering to triple layer harvesting period and after the triple layer harvesting, respectively) and the increasing nutrient solution (NS2, i.e. the initial nutrient solution supply was 250 mL/plant per day, after each layer of pepper was harvested, the nutrient supply of single plant was increased by 50 mL, and did not increase until it increased to 500 mL/plant), for a total of six coupled treatments. The principal component analysis-technique for order preference by similarity to an ideal solution (PCA-TOPSIS), membership function analysis and grey relational degree analysis were used to comprehensively evaluate fruit quality, yield, water use efficiency and fertilizer partial productivity.

The IF had a significant effect on all quality indicators except shoulder length (P<0.01); the NS had a significant effect on vitamin C, soluble protein, capsaicin and dihydrocapsaicin (P<0.01), but had no significant effect on other quality indicators; the coupling of IF and NS showed highly significant effect on the quality indicators, except the thickness of the peel (P<0.01). At the same time, IF, NS and their coupling showed extremely significant effects on pepper yield and water use efficiency (P<0.01). The evaluation results were consistent by PCA-TOPSIS, fuzzy membership function and grey relational degree, and the top two were IF1NS1 and IF2NS2. IF1NS1 treatment had the best fruit quality of pepper, for yield, water use efficiency, and N, P, K fertilizer partial productivity were the highest, with the value of 74 482.24 kg∙hm^-2, 34.21 kg∙m^-3, 625.95 kg∙kg^-1, 679.54 kg∙kg^-1, and 367.23 kg∙kg^-1, respectively. Therefore, IF1NS1 was the optimal water-fertilizer coupling treatment.

The optimal IF and NS management of peppers grown in substrate bags were as follows: IF of single plant was to maintain the water content of the substrate at 55%-60% required IA was supplied according to 1 time, the standard Yamazaki formula nutrient solution of 250 mL/plant per day and 500 mL/plant per day was supplied from the flowering to triple layer harvesting period and after the triple layer harvesting, respectively.