Integrated Physiological and Metabolomic Analyses Reveal the Differences in the Fruit Quality of the Blueberry Cultivated in Three Soilless Substrates

Abstract

With improving living standards, traditional blueberry planting modes cannot meet commercial demands, and blueberry cultivation with soilless substrate has become a popular solution in the blueberry industry. In this study, different soilless substrate treatments were found to markedly influence fruit appearance and intrinsic quality. The fruit in the 50:50 peat/pine bark (v/v) (FPB) treatment group had the maximum single fruit weight, largest vertical diameter, and brightest color, as well as the highest 1,1-diphenyl-2-picrylhydrazyl (DPPH) value, solid-acid ratio and anthocyanin content. The fruit in the 50:50 pine bark/rice husk (v/v) (FBR) treatment group had the highest total phenol and flavonoid levels, largest drip loss value, and lowest total pectin content and firmness value. Metabolomic analysis showed that flavonoid, carbohydrate, and carbohydrate conjugate, and amino acid, peptide, and analog levels were significantly different between groups. Fruit in the FPB group had the highest sucrose, D-fructose 1,6-bisphosphate, salidroside, tectorigenin, naringenin chalcone, trifolirhizin, and galangin contents. The increase in the relative expression of phenylalanine (Phe) promoted the synthesis of fruit polyphenols in the FBR group. Our results provide new insights into the effects of different substrates on the quality of blueberries and a reference for the soilless substrate cultivation of blueberries.

Keywords: blueberry; fruit quality; metabolite; soilless cultivation; substrate.

Figure 1

Effects of the soilless substrate type on the antioxidant system and quality indexes of blueberry fruits. (A) The generation rate of O₂⁻; (B) H₂O₂ content; (C) MDA content; (D) SOD activity; (E) POD activity; (F) CAT activity; (G) Antioxidant activity DPPH, (H) Antioxidant activity FRAP; (I) Total phenol content; (J) Anthocyanin content; (K) Flvaonoids content; (L) Ellagic acid content; (M) Soluble solids content; (N) Total acid content; (O) Solidity-acid ratio; (P) Glucose content; and (Q) Fructose content. Values are presented as the mean ± SD (n = 3). Bars with different letters indicate significant differences between the treatments according to Tukey’s tests (p < 0.05).

Figure 2

(A) Correlation matrix of the physiological and quality indicators of blueberry fruits. The results were derived from the Pearson correlation analysis. Red and blue colors indicate positive and negative correlations, respectively. * and ** represent a significant correlation at the 0.05 and 0.01 levels, respectively. (B) PCA score plot of the physiological and quality indicators of blueberry under different substrate treatments.

Figure 3

Partial least squares discrimination analysis (PLS-DA) of metabolites in blueberry fruits under three different soilless substrate treatments. PLS-DA score plot analysis of the three comparisons in positive (FPB vs. FPR (CK) (A), FPB vs. FBR (B), and FBR vs. FPR (CK) (C)) and negative (FPB vs. FPR (CK) (D), FPB vs. FBR (E), and FBR vs. FPR (CK) (F)) ionization modes. PLS-DA valid plot analysis of the three comparisons in positive (FPB vs. FPR (CK) (G), FPB vs. FBR (H), and FBR vs. FPR (CK) (I)) and negative (FPB vs. FPR (CK) (J), FPB vs. FBR (K), and FBR vs. FPR (CK) (L)) ionization modes.

Figure 4

Differentially abundant metabolites (DAMs) in blueberry fruits identified in the three comparisons (FPB vs. FPR (CK), FPB vs. FBR, and FBR vs. FPR (CK)). (A): Histogram showing the number of DAMs, including the total number, upregulated number, and downregulated number in positive and negative ionization modes. (B,C): Venn diagrams of DAMs in positive and negative ionization modes, respectively.

Figure 5

Stem diagram of the top 20 upregulated and downregulated differentially abundant metabolites (DAMs) identified from the three comparisons in positive (FPB vs. FPR (CK) (A), FPB vs. FBR (B), and FBR vs. FPR (CK) (C)) and negative (FPB vs. FPR (CK) (D), FPB vs. FBR (E), and FBR vs. FPR (CK) (F)) ionization modes. Red dots represent upregulated DAMs, and blue dots represent downregulated DAMs. The length of the stem represents the value of log2 (fold change), and the size of the dot represents the VIP value.

Figure 6

Top 20 enriched KEGG pathways of differentially accumulated metabolites (DAMs) identified from the three comparisons in positive (FPB vs. FPR (CK) (A), FPB vs. FBR (B), and FBR vs. FPR (CK) (C)) and negative (FPB vs. FPR (CK) (D), FPB vs. FBR (E), and FBR vs. FPR (CK) (F)) ionization modes.

Figure 7

Heatmap of the differentially accumulated metabolites (DAMs) involved in flavonoid (A), carbohydrate and carbohydrate conjugate (B) and amino acid, peptide, and analog (C) metabolism in blueberry fruits overlapping among the three comparisons. The scales were generated using log2 (fold change) values.