Abstract
While evaluating impact of Au nanoparticles on seed germination and early seedling growth of cowpea, HAuCl4 was used as control. Seedlings of cowpea raised in HAuCl4, even at concentration as high as 1 mM, did not show any suppression in growth. Accordingly, Au3+, despite being a heavy metal, did not alter levels of stress markers (viz. proline and malondialdehyde) in cowpea. Interestingly, cowpea turned clear pale yellow HAuCl4 solutions colloidal purple during the course of seed germination and seedling growth. These purple colloidal suspensions showed Au-nanoparticle specific surface plasmon resonance band in absorption spectra. Transmission electron microscopic and powder X-ray diffraction investigations confirmed presence of crystalline Au-nanoparticles in these purple suspensions. Each germinating seed of cowpea released ∼35 nmoles of GAE of phenolics and since phenolics promote generation of Au-nanoparticles, which are less/non toxic compared to Au3+, it was contemplated that potential of cowpea to withstand Au3+ is linked to phenolics. Of the different components of germinating seed of cowpea tested, seed coat possessed immense power to generate Au-nanoparticles, as it was the key source of phenolics. To establish role of phenolics in generation of Au-nanoparticles (i) seed coat and (ii) the incubation medium in which phenolics were released by germinating seeds, were tested for their efficacy to generate Au-nanoparticles. Interestingly, incubation of either of these components with Au3+ triggered increase in generation of Au-nanoparticles with concomitant decrease in phenolics. Accordingly, with increase in concentration of Au3+, a proportionate increase in generation of Au-nanoparticles and decrease in phenolics was recorded. In summary, our findings clearly established that cowpea possessed potential to withstand Au3+-stress as the phenolics released by seed coat of germinating seeds possess potential to reduce toxic Au3+ to form non/less toxic Au-nanoparticles. Our investigations also pave a novel, simple, green and economically viable protocol for generation of Au-nanoparticles.
Highlights
With the rapid expansion of electronic industry, the demand and cost of gold has increased markedly over past four decades
Our results showed for the first time that (i) cowpea posesses excellent potential to withstand Au3+-stress due to the presence of phenolics; and (ii) phenolics released during the course of seed germination and early seedling growth of cowpea play a vital role in detoxification of Au3+ by forming Au-nanoparticles
Impact of Au3+ on Seedling Growth While evaluating impact of Au-nanoparticles on seed germination and early seedling growth of cowpea, HAuCl4 was used as control
Summary
With the rapid expansion of electronic industry, the demand and cost of gold has increased markedly over past four decades. Gold comes into the environment from primary (i.e. ores) and secondary sources such as electronic scrap and waste electroplating solutions [1,2,3]. Au, whose density is 19.32 g cm, like other heavy metals, has a negative impact on physiology and biochemistry of microorganisms and animal systems including humans [4,5]. Researchers working with animal systems could trace Au in various organs including ovaries, hypothalamus, liver, adrenals, kidneys, testes, lymph nodes and pituitary glands [6]. To the best of our knowledge, no significant studies have been carried to investigate the impact of Au on plant growth and development. There are reports on accumulation of Au, synthesis and accumulation of Au-nanoparticles in cells of plants exposed to Au salts [7,8,9,10]
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