Exogenous Application of Salicylic Acid and Hydrogen Peroxide Ameliorate Cadmium Stress in Milk Thistle by Enhancing Morpho-Physiological Attributes Grown at Two Different Altitudes.

Mereen Nizar,Noreen Zahra,Manzer H Siddiqui,Ali Raza,Hayssam M Ali,Abdul Samad,Kanval Shaukat,Qasim Ali,Muhammad Bilal Hafeez

doi:10.3389/fpls.2021.809183

Abstract

Cadmium (Cd+2) is a potential and widespread toxic environmental pollutant, mainly derived from a rapid industrial process that has inhibitory effects on growth, physiological, and biochemical attributes of various plant species, including medicinal plants such as Silybum marianum L. Gaertn commonly known as milk thistle. Plant signaling molecules, when applied exogenously, help to enhance/activate endogenous biosynthesis of potentially important signaling molecules and antioxidants that boost tolerance against various abiotic stresses, e.g., heavy metal stress. The present study documented the protective role of salicylic acid (SA;0.25 μM) and hydrogen peroxide (H2O2; 10 μM) priming, foliar spray, and combinational treatments in reducing Cd+2 toxicity (500 μM) in milk thistle grown at two diverse ecological zones of Balochistan Province of Pakistan i.e., Quetta (Qta) and Turbat (Tbt). The morpho-physiological and biochemical attributes of milk thistle were significantly affected by Cd+2 toxicity; however, priming and foliar spray of SA and H2O2 significantly improved the growth attributes (root/shoot length, leaf area, and root/shoot fresh and dry weight), photosynthetic pigments (Chl a, b, and carotenoids) and secondary metabolites (Anthocyanin, Soluble phenolics, and Tannins) at both altitudes by suppressing the negative impact of Cd+2. However, the oxidative damage parameters, i.e., MDA and H2O2, decreased astonishingly under the treatment of signaling molecules, thereby protecting membrane integrity under Cd+2 stress. The morphological variations were profound at the low altitude (Tbt) as compared to the high altitude (Qta). Interestingly, the physiological and biochemical attributes at both altitudes improved under SA and H2O2 treatments, thus hampered the toxic effect of Cd+2. These signaling compounds enhanced tolerance of plants under heavy metal stress conditions with the consideration of altitudinal, and ambient temperature variations remain to be the key concerns.

Highlights

Data for MDA content in the shoot of milk thistle revealed significant data (p < 0.05) in which the maximum MDA content was reported in control under Cd+2 stress at Tbt; the H2O P + foliar spray (FS) under Cd+2 stress at Qta was reported to have a minimum content of MDA (Figure 5C)
In the roots, the highest content of tannins was recorded in salicylic acid (SA) spray under Cd+2 stress at Tbt, while the lowest tannins content was observed in control treatment of Cd+2 at the same altitude (Tbt)
The toxicity of heavy metal (Cd+2) hampered the morphological and physiological attributes of milk thistle grown at two different altitudes

Summary

Introduction

Gaertn) is a medicinal weed belonging to the Asteraceae family. It grows well at an altitude of 1,800–2,400 m a.s.l in sandy or rocky soil. The optimum temperature for germination of milk thistle seeds ranges between 28 and 29◦C. Medicinal plants are of very high significance because they provide the community with health care and prevention from diseases. A rapid increase in the population and pharmaceutical industry needs medicinally important plants with active ingredients to enhance and promote their cultivation and production (Valková et al, 2021; Zahra et al, 2021c). Compounds of pharmaceutical properties are derived from milk thistle fruits, i.e., achenes. Seed and dry pericarp accumulate a large flavonolignans group called silymarin, being the precursor taxifolin (Bijak, 2017)

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