Celery flavonoid-rich extract significantly reduces cucumber powdery mildew severity and enhances plant defense responses

Abstract

This study assesses the antifungal efficacy of celery flavonoid-rich extract (CFRE) against cucumber powdery mildew, caused by Podosphaera fusca, in a controlled greenhouse setting. The application of CFRE at a concentration of 4 mg mL^{- 1} resulted in a remarkable 97% reduction in disease severity. High-performance liquid chromatographic (HPLC) analysis identified apigenin as the predominant flavonoid in CFRE. Furthermore, CFRE treatment induced a robust defense response in cucumber leaves, marked by elevated levels of flavonoids, phenolics, chlorophyll, and defense enzymes such as β-1,3-glucanase, chitinase, peroxidase, phenylalanine ammonia-lyase, and polyphenol oxidase. The study also observed upregulation in the expression of three investigated genes associated with β-1,3-glucanase, chitinase, and phenylalanine ammonia-lyase. Notably, a positive correlation was established between the activity of defense enzymes and their gene expression, as well as between defense enzymes and antioxidant compounds. These findings underscore the potential of CFRE as an environmentally benign alternative to chemical fungicides for managing P. fusca infections.

Keywords: Cucumis sativus; Podosphaera fusca; Apigenin; Enzyme activities; Gene expression.

Fig. 1

Severity of cucumber powdery mildew after treating with different concentrations of celery flavonoid-rich extract one d before (left) and after (right) inoculation of cucumber seedlings, compared to inoculated control treated with distilled water. Different lowercase letters on the column indicate a significant difference, according to two-way ANOVA and Least Significant Difference test at P ≤ 0.001.

Fig. 2

Severity of cucumber powdery mildew in (a): an inoculated plant treated with celery flavonoid-rich extract (6 mg mL^− 1 one d after inoculation) and (b): an inoculated control plant treated with distilled water, 10 days after inoculation.

Fig. 3

Effect of celery flavonoid-rich extract (4 mg mL^− 1) on (a): chlorophyll, (b): total flavonoid compounds, and (c): total phenolic compounds content of cucumber leaves inoculated with cucumber powdery mildew at 0, 1, 2, 4, and 8 d after treatment with the extract. CFRE celery flavonoid-rich extract, Untreated treated with distilled water. Different lowercase letters on the column indicate a significant difference, according to two-way ANOVA and Tukey’s test at P ≤ 0.001.

Fig. 4

Effect of celery flavonoid-rich extract (4 mg mL^− 1) on (a): β-1,3-glucanase, (b): chitinase, (c): peroxidase, (d): phenylalanine ammonia-lyase, and (e): polyphenol oxidase activities of cucumber leaves inoculated with cucumber powdery mildew at 0, 1, 2, 4, and 8 d after treatment with the extract. CFRE  celery flavonoid-rich extract, Untreated  treated with distilled water. Different lowercase letters on the column indicate a significant difference, according to two-way ANOVA and Tukey’s test at P ≤ 0.001.

Fig. 5

Effect of celery flavonoid-rich extract (4 mg mL^− 1) on gene expressions of β-1,3-glucanase (β-Glu; a1–2), chitinase (CHI; b1–2), and phenylalanine ammonia-lyase (PAL; c1–2) in both non-inoculated and cucumber leaves inoculated with cucumber powdery mildew at 0, 1, 2, 4, and 8 d after treatment with the extract. CFRE  celery flavonoid-rich extract, Untreated  treated with distilled water. Different symbols indicate significant differences between each set, and their corresponding control group at the same time interval according to Tukey’s multiple comparison test. Significant symbols: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05, ns: non-significant.

Fig. 6

Correlation between gene expressions of β-1,3-glucanase (β-Glu; a1–2), chitinase (CHI; b1–2), and phenylalanine ammonia-lyase (PAL; c1–2) and the corresponding enzyme activities in cucumber seedlings inoculated with cucumber powdery mildew (a1, b1, c1) and non-inoculated cucumber leaves (a2, b2, c2) at 0, 1, 2, 4, and 8 d after treatment with celery flavonoid-rich extract (4 mg mL^− 1).