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).
At least 2800 proteins in the human proteome require Zn for their structural or functional activities. DNA replication is one of these cursial processes that depend on Zn proteins. As a rough indication Zn concentration in cultivated soils is approximately 65 mg kg-1 (Alloway 2009). But this can differ from soil to soil and organic manure can significantly increase soil Zn content. In the quest for cultivars with high levels of Zn in the grain, it is essential to know these soil Zn concentrations. Especially soil mixtures used in greenhouse studies and hydroponic nutrient solutions tend to have higher concentrations of available nutrients than most soils. As an example, Welch et al. (2005) grew the Indian durum wheat cultivar C306 under hydroponic conditions and reported 130 mg/g of Zn and 220 mg/g of Fe in the grain. In contrast, Ortiz-Monasterio (unpublished data) analyzed from the same cultivar grown under field conditions and found 31 and 33 mg/g of Zn and Fe, respectively, in the grain. This shows great potential of increasing Zn by usage of Zn fertilizers, i.e. manure application can significantly increase zinc levels (Ortiz-Monasterio et al. 2007).
Just as with Fe, phytic acid can bind Zn, forming insoluble or unavailable Zn-phytate complexes in seeds (Lönnerdal 2002; Schlemmer et al. 2009). The strong binding of Zn to phytic acid reduces bioavailability of Zn for humans and monogastric animals. In general, the higher phytate concentration the lower the bioavailability of Zn and Fe. However, it is possible to lower phytate concentration of seeds and grains by selection and breeding, or by P deficiency. Nevertheless, phytate is a major storage molecule for phosphate and is required for optimal crop growth. Therefore, a lower phytate concentration in seeds is accompanied with various negative effects such as reduced seedling emergence and poor agronomic performance (Oltmans et al. 2005).
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