Ca²⁺ plays an important role in regulating the integrated growth of Capsicum annuum L. under coupled water-calcium treatment

Water and fertilizer are two key factors affecting the yield and quality of pepper in facilities. Calcium is an essential nutrient for the growth and development of plants, particularly vegetables in the Solanaceae family. However, there are few studies on the effect of coupled water-calcium on pepper yield and quality. In this study, pepper experiments involving three irrigation levels (W1: 40 %-50 % substrate moisture content, W2: 55 %-65 % substrate moisture content, and W3: 70 %-80 % substrate moisture content) and four calcium supply volumes (C1: 1 mM, C2: 1.5 mM, C3: 2 mM, and C4: 2.5 mM) were conducted over two consecutive growing seasons in 2021. Results showed that in pepper, irrigation and calcium application levels had significant effects on chlorophyll content, photosynthetic gas exchange parameters, yield, water use efficiency (WUE), and the partial factor productivity of fertilizer (PFP). At the same calcium level, the yield and WUE of pepper increased then decreased with the increase in irrigation level, peaking at the W2 irrigation level. At the W2 irrigation level, photosynthetic gas exchange parameter values and fruit quality indexes (except for nitrate nitrogen content) increased with the increase in the amount of applied calcium. A comprehensive evaluation using four algorithms, namely, principal component analysis (PCA), TOPSIS model analysis based on combination weighting, gray correlation analysis, and fuzzy affiliation function analysis, as well as a validation experiment in spring 2022, identified W2C4 as the best water and calcium management strategy. In addition, in the pepper cultivation experiment conducted in spring 2022, PCA analysis concluded that Ca2+ played a greater role than NO(3)(-)in the applied calcium fertilizer. Specifically, Ca2+ significantly increased the yield, WUE, and PFP-N of pepper. In conclusion, this study determined the optimal water-calcium management program through three seasons of pepper cultivation experiments: irrigation level of 55 %-65 % and Ca2+ concentration of 2.5 mM.