Involvement of Phenolic Acids in Short-Term Adaptation to Salinity Stress is Species-Specific among Brassicaceae

Abstract

Salinity is a major abiotic stress negatively affecting plant growth and consequently crop production. The effects of short-term salt stress were evaluated on seedlings of three globally important Brassica crops-Chinese cabbage (Brassica rapa ssp. pekinensis), white cabbage (Brassica oleracea var. capitata), and kale (Brassica oleracea var. acephala)-with particular focus on phenolic acids. The physiological and biochemical stress parameters in the seedlings and the levels of three main groups of metabolites (total glucosinolates, carotenoids, and phenolics) and individual phenolic acids were determined. The salt treatments caused a dose-dependent reduction in root growth and biomass and an increase in stress parameters (Na⁺/K⁺ ratio, reactive oxygen species (ROS) and glutathione (GSH)) in all seedlings but most prominently in Chinese cabbage. Based on PCA, specific metabolites grouped close to the more tolerant species, white cabbage and kale. The highest levels of phenolic acids, particularly hydroxycinnamic acids, were determined in the more tolerant kale and white cabbage. A reduction in caffeic, salicylic, and 4-coumaric acid was found in Chinese cabbage and kale, and an increase in ferulic acid levels was found in kale upon salinity treatments. Phenolic acids are species-specific among Brassicaceae, and some may participate in stress tolerance. Salt-tolerant varieties have higher levels of some phenolic acids and suffer less from metabolic stress disorders under salinity stress.

Keywords: Brassica crops; carotenoids; glucosinolates; phenolic acids; polyphenols; salinity stress; seedlings; tolerance.

Figure 1

Root growth (A) and biomass production (B) of Brassica seedlings under short-term (24 h) salinity stress (0–200 mM NaCl). Points labelled with different letters differ significantly at p < 0.05. Data are average ± SD, n = 30.

Figure 2

Brassica seedlings’ NA⁺/K⁺ ratio (A), and Na⁺ and K⁺ content (B) under control and salt stress (200 mM NaCl) treatments. Data are averages ± SD (n = 4). Points labelled with different letters differ significantly at p < 0.05.

Figure 3

Biochemical stress markers: proline content (A) and fluorescence intensity due to the reactive oxygen species (ROS) H₂O₂ (B), and superoxide (SO) (C), as well as glutathione (GSH) (D) in the roots of Chinese cabbage, white cabbage, and kale seedlings under increasing salt concentrations (0, 50, 100, and 200 mM NaCl). Data in figures B, C, and D were normalized according to the corresponding controls (horizontal lines normalized to 1). Data are averages ± SD (n = 5). Points labeled with different letters differ significantly at p < 0.05.

Figure 4

Two-dimensional principal component analysis (2D-PCA) of specialized metabolites in Chinese cabbage (C. c), white cabbage (W. c.), and kale (Kale) after 24 h exposure to 0 (control), 50, 100, and 200 mM NaCl. The green, orange, and red symbols indicate the positions in the score plots of glucosinolates (GLU), carotenoids (CAR), and phenolic compounds (total phenolics (TP), total flavonoids (TF), total phenolic acids (TPA), total flavonols (TFL)), respectively. The blue symbols indicate the positions of the Brassica crops following each of the treatments. PCA was created based on data presented in Supplementary Table S1. The correlation matrix used, eigenvalues, factor loadings, and factor scores are given in Supplementary Tables S2–S6.

Figure 5

Total hydroxycinnamic acids (A) and total hydroxybenzoic acids (B) in Brassica seedlings upon salinity treatments (0–200 mM). Data are averages ± SD (n = 5). Points labeled with different letters differ significantly at p < 0.05.