Cadmium-induced impairment in growth, photosynthetic apparatus and redox regulation in green amaranth (Amaranthus viridis L.) plant attenuated by salicylic acid and methyl jasmonate

Abstract

The hazardous Cadmium (Cd) contamination in vegetables from anthropogenic Cd-abundant agroecosystems is a decisive threat to plants and human health. This study examined the prospective roles of salicylic acid (SA) and methyl jasmonate (MeJA) in alleviating Cd-induced toxic effects in green amaranth plants. The seeds of green amaranth (cv. Ghretokanchan) plant were primed with SA (100 and 250 µM) and MeJA (2.5 and 5 µM) and 21-d-old seedlings were set in six conditions under a hydroponic system: (i) Control, (ii) Cd (10 µM CdCl2·H2O), (iii) Cd with 100 µM SA, (iv) Cd with 250 µM SA, (v) Cd with 2.5 µM MeJA, and (vi) Cd with 5.0 µM MeJA. The experiment was set in a completely randomized design having three replications. Cd exposure for three weeks markedly impaired plant growth, pigment contents, leaf gas exchange, and photosystem-II efficiency; increased malondialdehyde (MDA) and hydrogen peroxide (H2O2) levels indicating induced oxidative stress; and enhanced the activities of superoxide dismutase, guaiacol peroxidase and catalase enzymes. However, seed priming and supplementation of SA and MeJA significantly remediated these Cd-mediated adverse effects. Compared to Cd-alone treatment, 100 and 5 µM of SA and MeJA considerably improved total dry weight by 45% and 94%, respectively whereas leaf MDA and H2O2 levels were substantially reduced by 100 µM SA (37% and 14%) and 5 µM MeJA (35% and 21%). An extensive activity of antioxidants and considerable reduction in Cd uptake and translocation from medium-root-shoot was apparent due to the supplementation of SA and MeJA. The study effectively optimized the levels of SA and MeJA for the improvement of Cd tolerance based on metal uptake, morpho-physiology, and redox regulation, and concludes that either 100 µM SA or 5 µM MeJA could be used for the alleviation of Cd-toxicity in green amaranth plants in Cd-contaminated soils through further field trials.

Fig 1. Growth of 42-day-old green amaranth plants under Cd stress with or without salicylic acid (SA) and methyl jasmonate (MeJA) supplementation in hydroponic conditions. (Treatment description- 1: Control; 2: Cd = 10µ M CdCl₂.

H₂O; 3: CdSA₁₀₀= Cd with 100 µM SA; 4: CdSA₂₅₀= Cd with 250 µM SA; 5: CdMJ_2.5= Cd with 2.5 µM MeJA; 6: CdMJ_5.0= Cd with 5 µM MeJA).

Fig 2. Leaf Malondialdehyde (MDA; A) and Hydrogen Peroxide (H₂O₂; B) contents in 42-day-old green amaranth plants grown in control and Cd with or without SA and MeJA supplementation in hydroponic conditions.

The upright bars relate to SEM (n = 3). Treatment means with unlike lowercase letters within the treatments denote significant variations corresponding to Tukey’s HSD test at p < 0.05. Treatment explanation: Cd = 10 µM CdCl₂.H₂O; CdSA₁₀₀= Cd with 100 µM SA; CdSA₂₅₀= Cd with 250 µM SA; CdMJ_2.5= Cd with 2.5 µM MeJA; CdMJ_5.0= Cd with 5 µM MeJA.

Fig 3. Leaf proline contents (A) and enzymatic antioxidants activities: Superoxide Dismutase (SOD; B), Catalase (CAT; C), and Guaiacol Peroxidase (GPX; D) of 42-day-old green amaranth plants grown in control and Cd with or without SA and MeJA supplementation in hydroponic conditions.

The upright bars relate to SEM (n = 3). Treatment means with unlike lowercase letters within the treatments denote significant variations corresponding to Tukey’s HSD test at p < 0.05. Treatments are the same as Fig 2.

Fig 4. Cd contents in plant organs (µg g^-1 DW), bioconcentration factor (BCF) and translocation factor (TF) in 42-day-old green amaranth plant grown in control and Cd with or without SA and MeJA supplementation in hydroponic conditions.

The upright bars relate to SEM (n = 3). Treatment means with unlike lowercase letters within the treatments denote significant variations corresponding to Tukey’s HSD test at p < 0.05. Treatments are the same as in Fig 2.

Fig 5. Hierarchical clustering of treatments (row-wise) and measured traits (column-wise) of green amaranth plants displayed as a heatmap.

The normalized Stress Tolerance Index (STI) values were used to construct the heatmap. The STI scores are illustrated on a normalized scale of −2 (darker red) to 2 (darker green). Traits explanation: H₂O₂- Hydrogen Peroxide; MDA- Malondialdehyde; Chl b- Chlorophyll b; Total Car- Total Carotenoids; Chl a- Chlorophyll a; Total Chl- Total Chlorophyll; SDW- Stem Dry Weight; RDW- Root Dry Weight; SL- Shoot Length; LDW- Leaf Dry Weight; ΦPSII- Quantum yield of PSII; ETR- Electron Transport Rate; A- Photosynthesis rate; Pro- Proline; gs- stomatal conductance; Fv/Fm- Maximum photochemical efficiency of PSII; E- Transpiration rate; LRL- Longest Root length; CAT- Catalase; LA- Leaf Area; TDW- Total Dry Weight; RFW- Root Fresh Weight; SOD- Superoxide Dismutase; SFW- Stem Fresh Weight; TFW- Total Fresh Weight; LFW- Leaf Fresh Weight; GPX- Guaiacol Peroxidase.

Fig 6. PCA-biplot exhibiting the variability among six growth conditions considering 30 measured traits in green amaranth plant.

The primary two principal components (PCs) described about 84.6% (69.1% by PC1 + 15.5% by PC2) of total flexibility. The arrow length of each trait denotes the magnitude of contribution along with a color ramp from red to blue (prolonged with a darker blue arrow refers to a higher contributing trait). Traits are the same as Fig 5.

Fig 7. Percent contribution (a) and Eigenvalues (b) of top 10 principal components (PCs) extracted from the PCA-biplot.

The first 20 contributed traits to PC1 (c) and PC2 (d). Red-dashed lines represent the threshold limit of contribution to the respective PC.