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Trace Elements in Abiotic and Biotic Environments
within the range of 3–320 ng/m 3 , at the mean value of 67 ng/m 3 (Injuk et al. vide
Kabata-Pendias and Mukherjee 2007).
Zinc enters the atmosphere in vapor forms, as well as particles, from industrial
processes, incinerations of waste, cement plants, and fuel-fired power plants. The
coal combustion for heat and electricity production is considered as the highest Zn
source in the atmosphere.
Apparently, the wet deposition dominates and can be up to about 90% of the total
Zn fallout. The global Zn deposition in the 1980s varied between 683 and 1954/yr
(Nriagu and Pacyna 1988). In the Netherlands Zn fallout, at that period, was estimated to be 575 t/yr.
Speciation of Zn in street dust is quite variable and its amounts are in forms
(in mg/kg): exchangeable, 1.2; absorbed, 21.7; carbonate-bound, 138.7; OM-bound,
52.4; and sulfide-bound, 145.7 (Mirosławski et al. 2006).
Zinc contents of moss (Hylocomium splendens) from the mountains in Poland,
32.5–56.1 mg/kg (mean 41.8 mg/kg), are slightly lower than those of the same moss
species from Alaska, 12.0–39.6  mg/kg (mean 20.2  mg/kg). This suggests higher
atmospheric deposits of Zn in Poland than in Alaska (Migaszewski et al. 2009).
55.5 PLANTS
Mobile species of Zn are readily available to plants, and its uptake is reported to be linear with metal concentration in nutrient solutions and in soils. Organic Zn complexes
increase its bioavailablity (Degrys et al. 2006). The composition of soil solution, especially contents of Ca, has a significant impact on its uptake. However, various plants
reveal different capability to Zn uptake, which is illustrated by its variable contents
among plants from the contaminated region; the highest Zn content, mean 12.38 mg/kg,
was in aboveground Spanish foxglove (Digitalis thapsi), and the lowest in oat grass
(Arrhenatherum album), mean 0.87 mg/kg (Garcia-Salgado et al. 2012).
There is still a disagreement whether Zn uptake is an active or a passive process.
It is most probably that both processes may occur, depending on both soils and plants.
However, there are many suggestions that Zn uptake is mostly metabolically controlled.
In Zn-deficient soils, exudates of roots of cereal plants may mobilize this metal from
various stable compounds (Fussuo Zhang vide Kabata-Pendias 2011). Plants uptake
mainly two species: hydrated Zn and Zn 2+ . However, several complex Zn ions and
their organic complexes may also be readily absorbed. In general, Zn is the most easily
adsorbed and transported metal within plants, compared with other trace metals.
Zinc is mainly bound to mobile low-molecular proteins; however, Zn phytates and
other insoluble Zn complexes are also present in plants. A high part of Zn bound to
light organic compounds influenced its high mobility in plants. It is likely to be concentrated in old leaves, and accumulated in chloroplasts. Under Zn-deficiency conditions,
some plants may mobilize Zn from old leaves and transport to generative organs.
Zinc plays essential metabolic functions in plants, of which the most important is
its activity in several enzymes, such as dehydrogenases, proteinases, peptidases, and
phosphohydrolases. Therefore, Zn functions in plants are related to metabolisms of
several important compounds: proteins, carbohydrates, phosphates, as well as ribosomes and RNA. Presumably, Zn stimulates the resistance of plants to dry and hot