Roles of Exogenous α-Lipoic Acid and Cysteine in Mitigation of Drought Stress and Restoration of Grain Quality in Wheat

Abstract

Cysteine (Cys) and α-lipoic acid (ALA) are naturally occurring antioxidants (sulfur-containing compounds) that can protect plants against a wide spectrum of environmental stresses. However, up to now, there are no conclusive data on their integrative roles in mitigation of drought stress in wheat plants. Here, we studied the influence of ALA at 0.02 mM (grain dipping pre-cultivation treatment) and Cys (25 and 50 ppm as a foliar application) under well watered and deficit irrigation (100% and 70% of recommended dose). The results showed that deficit irrigation markedly caused obvious cellular oxidative damage as indicated by elevating the malondialdehyde (MDA) and hydrogen peroxide content (H₂O₂). Moreover, water stressed plants exhibited multiple changes in physiological metabolism, which affected the quantitative and qualitative variables of grain yield. The enzymatic antioxidants, including superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and peroxidase (POX) were improved by Cys application. SOD and APX had the same response when treated with ALA, but CAT and POX did not. Moreover, both studied molecules stimulated chlorophyll (Chl) and osmolytes' biosynthesis. In contrast, the Chl a/b ratio was decreased, while flavonoids were not affected by either of the examined molecules. Interestingly, all above-mentioned changes were associated with an improvement in the scavenging capacity of reactive oxygen species (ROS), leaf relative water content (RWC), grain number, total grain yield, weight of 1000 kernels, gluten index, falling number, and alveographic parameters (P, W, and P/L values). Furthermore, heatmap plot analysis revealed several significant correlations between different studied parameters, which may explore the importance of applied Cys and ALA as effective compounds in wheat cultivation under water deficit conditions.

Keywords: alpha-lipoic acid; alveographic parameters; antioxidant capacity; cysteine; grain quality; water stress; wheat.

Figure 1

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on the chlorophyll concentration of wheat plants. (A) chlorophyll a, (B) chlorophyll b, (C) total chlorophyll, (D) chlorophyll a/b ratio. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 2

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on the oxidative stress and scavenging capacity of wheat plants. (A) total antioxidant activity, (B) Lipid peroxidation (MDA), (C) Hydrogen peroxide content. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 3

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on the concentration of non-enzymatic antioxidants in wheat plants. (A) Carotenoids, (B) Flavonoids, (C) Total soluble phenols, (D) Ascorbic acid. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 4

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on the activities of antioxidant enzymes in wheat plants. (A) SOD, (B) CAT, (C) POX, (D) APX. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 5

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on the concentration of osmolytes and leaf relative water content (RWC) of wheat plants. (A) Proline, (B) Total soluble sugars, (C) RWC. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 6

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on grain yield and its components of wheat plants. (A) number of spikes, (B) number of grains, (C) Grain yield. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 7

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on the physical/chemical properties of wheat grains. (A) Moisture content, (B) Wet gluten, (C) Gluten index, (D) Falling number, (E) Weight per 1000 kernels, (F) Hectoliter weight. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 8

Effect of irrigation level, seed soaking by α-lipoic acid (ALA; 0 and 0.02 mM) and foliar application by cysteine (Cys; 0, 25 and 50 ppm) on the alveographic parameters of wheat grains. (A) P, (B) L, (C) W, (D) P/L. For each parameter, the mean values ± SD followed by a different letter are significantly different according to Tukey’s range test (p ≤ 0.05).

Figure 9

Plot of silhouette analysis values for clustering of all parameters based on “seed soaking and foliar application” treatments variables. On the y-axis each cluster is ordered by decreasing silhouette value. The silhouette value can range between −1 and 1.

Figure 10

Two-dimensional heatmap visualization shows the interaction between the irrigation treatments and both the 31 measured parameters included in the study and the six “seed soaking and foliar application” types.

Figure 11

Mosaic plot representing a contingency table of the water deficit and well watered treatments variants versus the 31 parameters included in the study. The vertical size of the cells is proportional to the number of variants found in the respective parameter; the horizontal size of the cells is proportional to the effect level of the six “seed soaking and foliar application” types for each parameter under each parameter-class. The colors of the variants (seed soaking and foliar application) are indicated at the bottom of the Mosaic plot in colored squares similar to the respective bars in the Mosaic plot. Variants were not found at all possible locations of each parameter-class, which causes the reduction of several bars to dashed lines drawn as place holders and indicating that at the particular location no variant has been found in the parameter-class.