Distribution Patterns of Chemical Elements in Plants

As a rule, differences in (elemental) chemical compositions which exist among different species of (e.g.) plants should be caused by some unlike behaviour/differing processes in uptake or transport. For instance, there may be either active or passive transport of metal ions or other speciation forms of elements (complexes, oxoanions, organoelement species such as kakodylic [dimethylarsinic] acid or methylmetal [M, e.g., Hg, Pb, Tl] compounds/ions), producing different rates and/or equilibria of uptake. In turn abundance correlations among these very plant species appear which are at odds with chemical intuition, that is, a very low, virtually nonexistent abundance correlation in pairs of closely elements one of which is resorbed and shuttled onward to leaves/needles and fruits/seeds in a constant manner whereas the other is transported by ways/carriers

S. Fränzle, Chemical Elements in Plants and Soil: Parameters Controlling Essentiality, Tasks for Vegetation Science, vol. 45, DOI 10.1007/978-90-481-2752-8_1, © Springer Science+Business Media B.V. 2010

which depend on the corresponding species while conversely chemically apparently unrelated elements may follow similar paths. Geochemistry, including pH and wetness of soil substrate, thus provides very different patterns of elemental abundances for metals as well as certain non-metals. There are both synergistic and antagonistic relationships between uptake or use of different elements by plants. Of course these latter interactions, which partially represent the response of the plant to local geochemical conditions, in turn change the distribution patterns by mainly antagonistic interactions among essential elements (Kaim and Schwederski 1993); also consider Fig. 1.1:

Negative abundance correlations (e.g., Ca/Mg) may indicate a direct competition for the same binding centers owing to some chemical similarity among the pair of metals (Fig. 1.3). Thus, chemical similarity can bring about both highly positive and highly negative abundance correlations depending on dynamic features: if retention to biomass dominates in the end, similar coordination properties - both concerning

-synergistische Beziehung

— antagonistische Beziehung

Fig. 1.1 Network of interactions/influences among some essential chemical elements in plants (Kaim and Schwederski 1993). The parameter used for attribution of either synergy or antagonisms to an interelement relationship is rate of plant growth

-synergistische Beziehung

— antagonistische Beziehung

Fig. 1.1 Network of interactions/influences among some essential chemical elements in plants (Kaim and Schwederski 1993). The parameter used for attribution of either synergy or antagonisms to an interelement relationship is rate of plant growth binding strength and ligand selectivities - will result in positive abundance correlation whereas control by transport mechanisms, including competition for low-concentration carriers, rather gives a negative correlation. However, it is unlikely that both effects will cancel, producing no discernible abundance relationship across various plant species whatsoever. Notably, Fig. 1.3 does not display dynamic features like a rate of plant growth but "simply" the abundance relationship among the elements and plant species. Thus, Figs. 1.1 and 1.2 cannot be directly compared even for identical pairs of elements.

Local enrichment of certain elements within some plant may be due to both complexation with polymeric components of biomass and to precipitation of solid, insoluble, sometimes even crystallinic phases. Before an element may be enriched or separated in any of these kinds, three other factors contribute to the series of events, besides the conditions of uptake, namely:

- Speciation of elements next to its rhizosphere, respectively

- Mechanisms and kinetics of uptake by roots (or fungal mycelia) or leaves (especially in aquatic plants)

- Mobility inside the plant, controlled, e.g., by phosphate in the xylem

For example, the relatively large amounts of Rb present in plants may be involved in chemical signalling much like Na or K and will obviously contribute to osmo-regulation, but the latter effect does not render Rb essential because it can be replaced by other ions (or even organic compounds such as glycerine) for this purpose, and other, (more) specific uses are not obvious from analysis alone. Although some chemical details of paleobiochemistry may be inferred from appropriate fossile samples such as chitin in amber inclusions, analytical data will never reveal what element actually was required by some extinct organism. Though differences in essentiality patterns among protist, animals, plants or fungi are well-known for now (Table 2.1), and "genetic clocking" allows for temporal reconstruction of the separations of their common ancestors (Feng et al. 1997), the corresponding changes among essentiality patterns upon evolutionary radiation are not accessible. This holds the more for results of thorough geochemical changes during evolution or for such extinct organisms which apparently do not fit into patterns and categories of recent-time

Pb V

Pb V

Fig. 1.2 Highly positive (straight connection lines) and highly negative (broken connection lines) abundance correlations among pairs of chemical elements in 13 species of plants (from Markert 1996)

Fig. 1.2 Highly positive (straight connection lines) and highly negative (broken connection lines) abundance correlations among pairs of chemical elements in 13 species of plants (from Markert 1996)

— Evolution of life from inorganic to organic compounds

salt water > to Fresh water

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