Heavy metals include the transition-metal elements essential to plant nutrition, iron (Fe), zinc (Zn), manganese (Mn), copper (Cu), nickel (Ni) and molybdenum (Mo), cobalt (Co), which is required for nitrogen fixation in legumes, and the non-essential elements, chromium (Cr), cadmium (Cd), mercury (Hg) and lead (Pb). All these elements are toxic to crop plants at high tissue concentrations. In agriculture, deficiencies of essential heavy metal elements are more common than their toxicities. Nevertheless, Mn toxicity can reduce crop yields on acidic soils, and Mn and Fe toxicities occur on waterlogged or flooded soils. Toxicities can also arise in soils enriched in specific heavy metals by the weathering of the underlying rocks or anthropogenic activities. The molecular biology of heavy metal uptake and transport within plants is well understood, and the regulatory cascades enabling heavy metal homeostasis in plant cells and tissues are being elucidated. Cellular responses to excess heavy metals are also known. Many of these responses proceed through the generation of reactive oxygen species and involve the synthesis of antioxidant compounds and enzymes. Tolerance of high concentrations of heavy metals in the environment is brought about by restricting the entry of heavy metals to the root and their movement to the xylem, and by chelating heavy metals entering the cytoplasm and sequestering them in non-vital compartments, such as the apoplast and vacuole. The mechanisms by which certain plant species are able to hyperaccumulate heavy metals are also providing insight into the ability of plants to exclude and tolerate heavy metals in their tissues.

The fragmentary information on phosphorus (P) × zinc (Zn) interactions in plants warrants further study, particularly in plants known for their high P and Zn requirements, such as cotton (Gossypium hirsutum L.). The objective of this study was to investigate the effect of P × Zn interactions in a modern cultivar of cotton grown hydroponically. Biomass, mineral nutrition and photosynthetic parameters were monitored in plants receiving contrasting combinations of P and Zn supply. Root biomass, length and surface area were similar in plants with low P and/or low Zn supply to those in plants grown with high P and high Zn supply, reflecting an increased root/shoot biomass quotient when plants lack sufficient P or Zn for growth. Increasing P supply and reducing Zn supply increased shoot P concentrations, whilst shoot Zn concentrations were influenced largely by Zn supply. A balanced P × Zn supply (4 mM P × 4 μM Zn) enabled greatest biomass accumulation, while an imbalanced supply of these nutrients led to Zn deficiency, P toxicity or Zn toxicity. Net photosynthetic rate, stomatal conductance, transpiration rate and instantaneous carboxylation efficiency increased as P or Zn supply increased. Although increasing P supply reduced the P‐use efficiency in photosynthesis (PUEP) and increasing Zn supply reduced the Zn‐use efficiency in photosynthesis (ZnUEP), increasing Zn supply at a given P supply increased PUEP and increasing P supply at a given Zn supply increased ZnUEP. These results suggest that agricultural management strategies should seek for balanced mineral nutrition to optimize yields and resource‐use efficiencies.

Optimisation of the concentrations of essential mineral elements in staple grain diet and reduction in non-essential and potentially toxic elements would considerably alleviate mineral malnutrition and improve the health of humans. Here, wheat (Triticum aestivum L.) plants were biofortified with Zn and/or Se to determine the changes across 36 elements in the grain. The element concentrations were determined by multielemental k 0 -instrumental neutron activation analysis (k 0 -INAA). In comparison to grain from non-biofortified plants, Zn fertilisation increased the grain Zn, Ca, and Mo concentrations, whereas the foliar application of Se only increased the grain Se concentrations. Double biofortification (combined Zn fertilisation and foliar Se) was more effective for the increased Se concentrations in the grain, in comparison to the Se-only biofortified plants, with the grain Zn, Ca and Mo concentrations remained at the same levels as those for the Zn-only biofortified plants. Except for Ba, Br and Rb, the concentrations of the elements analysed were below the detection limits. Double biofortification might be a feasible strategy to efficiently coordinate the mineral quality of wheat grain, although the considerable concentrations of other essential and non-essential elements should not be neglected.

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