Temporal patterns of metal ion and nanoparticle uptake by Lolium perenne in hydroponic systems in the presence of amendments ethylene diamine disuccinic acid (EDDS) and gibberellic acid

Abstract

Contamination of the environment by metal ions poses a threat to human and ecosystem health. In the past few years, concern has shifted to metal oxide nanoparticles due to the rapid rise in nanomaterial usage, resulting in their release into the environment. Despite an abundance of literature on metal uptake by plants and the use of amendments to enhance it, information on processes by which plants take up metal ions and nanoparticles and the changes over time from the use of one or more ions and amendments and translocation from roots to shoots is limited. Metal chelate complexes are known to transport from the roots to the shoots through breaks in the casparian strip, or through damaged physiological barriers. Plant hormones can also affect metal uptake as they regulate plant development and some hormones can activate plant defence mechanisms under adverse environmental conditions, thus alleviating stress. Presence of chelate-plant hormone in combination can cause a competition between plant damage and growth, thereby either allowing or restricting metal mobility in plant tissues. This thesis aimed at studying temporal translocation trends of Cu and Zn as ions and nanoparticles in the presence and absence of amendments and investigating the processes influencing this behaviour in Lolium perenne. A two-stage investigation process was designed to build on the understanding of the plant damage and growth that would allow the translocation of metals as ions and nanoparticles into ryegrass shoots. 1) Investigation of the combined effects of a commercially available plant growth regulator, gibberellic acid (GA), and a biodegradable chelating agent, ethylene diamine-disuccinic acid (EDDS), on the translocation of free ions (Cu, Zn) alone and in combination in a hydroponic solution; 2) Explaining the behaviour and translocation trends of hydroponically grown perennial ryegrass (Lolium perenne) plants when metals were supplied as copper oxide (CuO) and zinc oxide (ZnO) nanoparticles and Cu and Zn ions with different amendment combinations. Translocation of Cu and Zn ions varied over time when added individually and in combinations. Cu applied individually caused less damage with little translocation from roots to shoots. In contrast, Zn accumulation and damage in roots increased translocation over time to shoots. Cu concentration in shoots was enhanced by the co-application of Zn, while the application of EDDS and GA-EDDS, by themselves or with Cu and Zn, lowered transpiration and increased damage and translocation, while GA increased transpiration but decreased translocation. Metal ions and nanoparticles in mixtures increased the translocation patterns of both copper and zinc through increased membrane damage. EDDS affected the two nanoparticles very differently, with increased copper translocation, but decreased zinc translocation, either by increasing damage or plant growth. GA slightly increased translocation of both copper and zinc ions influenced by an increase in plant transpiration. Competition between GA and EDDS subsequently led to ion uptake through breached membrane barriers in GA-EDDS treatments. This shows that the increase in metal uptake was typically increased by EDDS that enhanced membrane damage and stress, while GA promoted plant growth and transpiration, lowering ion uptake. Overall, the results of this study demonstrate that several plant processes can affect the temporal translocation trends of Cu and Zn as ions and nanoparticles in Lolium perenne. EDDS application typically increased metal ion uptake by causing more cell damage, while GA typically lowered the damage and decreased metal uptake, even though the transpiration increased over time and plant growth occurred. Variability in toxicity of metal ions and nanoparticles in mixtures and several plant processes acting either simultaneously or individually are responsible for change in accumulation and translocation in perennial ryegrass. Such information may well be important as release of both forms of metals, ions and nanoparticles, will continue to intensify in the future.