5.2.4. Zinc

After iron, Zinc (Zn) is the second most abundant transition metal in living organisms. Plant zinc uptake is predominantly in the form of a divalent cation (Zn²⁺) and at high pH as a monovalent cation (ZnOH⁺). In the xylem Zn is either transported as free divalent cation or bound to organic acids. Recent studies show that there are many proteins that contain or bind zinc. Zinc is the only metal that is present in all six enzyme classes, namely: oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. Therefore, changes in metabolism induced by Zn deficiency are rather complex. But what can be generalized is that Zn plays an important role in plant tolerance to environmental stress factors (Marschner et al. 1996). Zinc deficiency in dicotyledonous plants become most apparent in stunted growth due to shortening of internodes (‘rosetting’) and a drastic decrease in leaf size (‘little leaf’).

Although Zn deficiency clearly reduces shoot growth, root growth is not reduced and might even be enhanced at the expense of the shoot growth (Cumbus 1985; Cakmak et al. 1996). Zinc deficiency increases root exudation of low-molecular-weight solutes. Examples in dicotyledonous plants are amino acids, sugars, phenolics and potassium. In monocots the main released solutes are phytosiderophores, as is typical for Fe deficiency. However, this enhanced release of phytosiderophores that occurs both during Zn and Fe deficiency are two separately regulated processes (Suzuki et al. 2006).

In leaves, the critical deficiency concentrations are below 15–20 μg Zn g^-1 dw. Grain and seed yield are lowered to a greater extent by Zn deficiency than the total dry matter production. This is most likely due to impaired pollen fertility in these Zn deficient plants. Not all plant are equally susceptible to Zn deficiency. That is rice, maize and apples are far more sensitive than oats, rye or pea. Within Gramineae crops rye has the highest tolerance to Zn deficiency, followed by triticale, barley, bread wheat, oats and durum wheat (Cakmak et al. 1997).

Zinc deficiency is widespread, it occurs in plants grown in highly weathered acid soils and in calcareous soils. On alkaline calcareous soils Zn deficiency is often associated with Fe deficiency (‘lime chlorosis’). In general Zn and Fe deficiency occur under conditions with high pH. Opposed to Fe deficiency, however, Zn deficiency in plants on calcareous soils can be mitigated by soil fertilization with inorganic Zn salts such as ZnSO₄. The low availability on calcareous soils is mainly due to the adsorption of Zn to clay or CaCO₃.

Zinc toxicity is rare and only found in crops grown on soils contaminated by industry, e.g. mining or sewage sludge (Broadley et al. 2007). In greenhouses Zn toxicity can also occur as a result of Zn containing roofs and gutters. Rain water collected for irrigation using these roofs, gutters and rain pipes contains high Zn. Zinc toxicity can also be induced by high Zn fertilization rates, but in practice this is only a problem for non-tolerant plants that will show inhibited root elongation and chlorosis in young leaves. These symptoms are most likely caused by deficiency of, Mn, Mg or Fe, most likely because of the similar ion radius of these ions. This induced deficiency and competition causes reduced RuBP carboxylase activity is presumably caused by competition with Mg and reduction in stomatal conductance and mesophyll conductance to CO₂  (Sagardoy et al. 2010).

Zinc biofortification