## 3 The Impact of Trace Element Availability on the Induction of Oxidative Stress in Cropped Plants  
The availability of metals fluctuates between anaerobic (high moisture) conditions and aerobic (low moisture) conditions. Increased metal solubility leads to higher levels of bioavailable metals in soils leading to potentially growth-inhibiting toxicity in plants. Low metal solubility results in nutrient lockout of some trace elements, inhibiting plant growth and exacerbating other stressors on the crops [1; 24; 31]. The tendency for flood irrigation to solubilize more metals than in drier soil conditions is suspected to be due to increasing overall soil pH, increasing metal precipitation with chemical compounds such as sulphides and increased concentrations of Fe oxides [6]. Increasing heavy metal availability is also of concern to crop health and this fluctuates with irrigation strategy. Under flooded irrigation conditions, AS bioavailability increases, which in turn can lead to higher uptake of As by plants resulting in reduced plant growth or even death. Likewise, the use of sprinkler irrigation has shown to reduce concentrations of As in the soil; however, it has been shown to increase concentrations of Cd [21; 23]. The mode of action for free metal mediated oxidative stress towards living organisms arises from chemical reactions that catalyze metabolic products into ROS. For example, the Haber-Weiss reaction is the catalysis of superoxide and hydrogen peroxide to harmful hydroxyl radicals by transition metals, mostly Fe and Cu [56]. It incorporates the Fenton reaction, which occurs when Fe\({}^{2+}\) or Cu\({}^{+}\) reacts with hydrogen peroxide (e.g., produced in the mitochondrion as a cellular respiration by-product) into harmful hydroxyl radicals [57]. The Fenton reaction can be exacerbated when organisms are under Cu stress. Whilst Cu is a cofactor for SOD, higher SOD activity leads to higher levels of H\({}_{2}\)O\({}_{2}\) being produced. This particular situation has been observed in some bacterial species including _Staphylococcus aureus_[58], _Mycobacterium tuberculosis_[59], and _Pseudomonas aeruginosa_[60]. For plants, the major source of Cu\({}^{+}\) is found in the roots, whilst Fe\({}^{2+}\) and Mn\({}^{2+}\) are primarily found in shoots, where they are heavily involved in photosynthesis [61]. Excessive Cu\({}^{+}\) levels in the soil can lead to localized Fenton reactions within the root walls, forming toxic hydroxyl radicals, causing chlorosis and ultimately reducing biomass [61]. Similarly, excessive soil Fe\({}^{2+}\) or Mn\({}^{2+}\) levels may lead to an imbalance in photosynthetic enzyme activity, resulting in an excessive production of hydroxyl radicals and subsequent photoinhibition [61]. Some bacterial species have been shown to combat free metal oxidative stress by substituting enzymes that require Fe\({}^{2+}\) as a cofactor for homologues that have no requirement for it. For example, _Streptococcus pyogenes_ has been demonstrated to substitute an Fe-containing ribonucleotide reductase (RNR) enzyme for an Fe-free homologue for DNA synthesis. Similarly, _E. coli_ has been shown to be able to use Mn\({}^{2+}\) as a substitute for Fe\({}^{2+}\), as a cofactor for its RNR enzyme [62]. Excessive levels of other free transition metals, including Cd, Co, Ni, and Zn are also known to cause stress challenges towards cropped plants. In _Spinacia olerca_ L. (spinach), 500 uM treatments of spinach for 10 days with each metal resulted in chlorosis and reduced the activity of CAT. Excess Cd, Co, and Ni also saw an increase in lipid peroxidation [63]. High concentrations of Zn, Co, Ni, and Cd have been shown to significantly interfere with physiological and genetic Fe homeostasis mechanisms, mimicking Fe-deficiency in plants such as _A. thaliana_[64]. The subsequent metal toxicity and Fe-deficiency reduces root elongation and growth, shoot weight, and chlorophyll content of leaves [64].