Synthetic Indices and Multivariate Analysis for Explaining Apple Fruit Quality in Relation to Foliar Fertilization and Nutritional Status

Abstract

The study evaluated the differential response of five apple varieties ('Generos', 'Florina', 'Delicios de Voinești', 'Jonathan' and 'Pionier') to eight foliar fertilization options and an unfertilized control. The experiment was placed in a semi-intensive non-irrigated plantation, located in flat terrain conditions, with chernozem soil, within the DER (Didactic and Experimental Resort) of ULS "King Mihai I" from Timișoara – Fruit Center. Foliar concentrations of N, P, K, Fe, Cu and Zn were analyzed, as well as fruit quality indicators, titratable acidity (Ac) and soluble sugar content (SUG). To integrate the information, three synthetic indices were proposed: Nutrition Status Index (NSI), Fruit Quality Index (FQI) and Fertilization Efficiency Index (FEI). The data were analyzed by additive factorial ANOVA, Pearson correlations, PCA, interaction heatmaps and regression models. The results indicated a clear differentiation of the varieties in terms of the capacity to capitalize on foliar inputs, with higher average NSI values ​​in Pionier and Florina, while the qualitative response of the fruits did not always follow the same direction with mineral nutrition. Multivariate analysis (PCA) separated the nutritional variables from the quality indicators and demonstrated that the nutrition-quality relationship is dependent on the genotype and the fertilizer combination. The proposed approach facilitated the identification of technological variants with high integrated efficiency and provides a reproducible framework for the evaluation of foliar fertilization in apple.