Trace elements for aquatic plants

Essential elements

Boron

Boron is an essential trace element for all higher plants. Even plants with a very low boron requirement need it at least to reproduce.

Plants react to a deficiency with typical symptoms: the shoot tips die off, the root system atrophies (in beetroot, for example, the well-known heart and dry rot occurs with the death of the youngest leaves and the beet head), the tissue becomes brittle.

Measurements carried out on individual cells using the cell pressure probe show that these are secondary reactions. Primarily, however, a temporary loosening of the cell wall can be observed just a few minutes after the boron supply is interrupted.

The cross-relation to calcium metabolism is particularly important. Boron is partly present in the apoplast as a negatively charged, ester-like complex. It is therefore to be expected that under deficiency conditions the ratio of bound to free cations (especially calcium) is altered by the loss of these charges, which can subsequently influence enzyme activities. Studies on apoplast solutions show a change in free calcium a few minutes after interrupting the boron supply and indicate that the boron complexes are additionally stabilised by calcium.

Boron is particularly required by rapidly growing plant parts. Using boronate-coupled fluorescent dyes, the distribution of boron-binding ligands could be investigated in vivo. Investigations show a heterogeneous longitudinal and transverse distribution of such ligands across the root. Particularly high fluorescence intensities were found in the areas of most intensive longitudinal growth.

The uptake of boron occurs mainly as undissociated boric acid and is strongly temperature-dependent; only hydroxyl ions OH- have a competitive effect, but not the anions of the nutrient salts (nitrate, phosphate, sulphate). Due to the relatively good fat solubility of boric acid, the mobility of boron in the plant is severely restricted; the tissues that are most dependent on an optimal boron supply (root and shoot tips) are the first to suffer from a deficiency.

Chloride

Chloride is an essential micronutrient, even if the actual content in the plant is often quite high. Young, growing plant parts in particular are very rich in chloride. The chloride is mainly found in the vacuoles (on average responsible for 20 % of the osmotic pressure).

The importance of chloride lies in its function as an osmotic regulator, which significantly influences turgor pressure and stomatal mechanism.
Chloride ions are involved in the photolysis of water in photosystem II (Hill reaction) as co-factors of a manganese-containing enzyme.
Chloride is also of some importance as a co-factor of asparagine synthetase. (Asparagine plays a role in the long-distance transport of nitrogen in some plants).
Other non-specific effects on other enzymes are also known.

Chloride deficiency symptoms have hardly been observed under field conditions, whereas they can easily be provoked in nutrient solution experiments.
The deficiency symptoms are then very similar to those of manganese deficiency.

Copper

Copper is absorbed as a ^Cu2+ ion or in the form of chelates. There is a pronounced antagonism between copper and other heavy metals, primarily iron and manganese. High copper levels can therefore induce iron and/or manganese deficiency.

The most important function of copper is the involvement of copper-containing enzymes in various redox systems.

The fact that 70 % of the total copper is contained in metabolically active leaves suggests that it is directly involved in photosynthesis. The copper protein plastocyanin is formed with the participation of copper-containing phenol oxidases. Through a reversible transition from Cu⁺ Cu²⁺ + e^-, it is involved in electron transfer in photosystem I. Cu²⁺ ions can fulfil a similar function to Mn²⁺ ions in the photolytic splitting of water in some plants.

Other copper-containing enzymes are mainly found in phenol metabolism, namely in the form of various phenol oxidases (tyrosinase, diphenol oxidase, laccase etc.). They catalyse the reaction phenol quinone by changing the oxidation state.

Ascorbic acid oxidase (ASO) catalyses the reaction ascorbic acid dehydroascorbic acid with the consumption of glutathione and thus contributes to the regeneration of the tissue's own antioxidant systems.

There are also involvements in lignification (storage of lignin), alkaloid synthesis, nitrate reduction, and oxygen radical degradation (superoxide dismutase).

Nickel

Ni(II) ions are easily absorbed, competing with other bivalent cations (Ca, Mn, Fe, Zn).

Nickel is the metal component of the enzyme urease as well as some hydrogenases in bacteria and legumes. Nickel deficiency induces urea toxicity, which can lead to "burns" and inhibits protein synthesis.

An old Kosmos volume (Herrmann Römpp, Trace Elements, Kosmos, 1954, Stuttgart, Franckh'sche Verlagshandlung) mentions a so-called "nutrient solution A - Z" for complete trace element supply according to R.D. Hoagland (1884 - 1949), which contains, among other things, tin(II) chloride in small quantities.

Unproven essentiality

Elements with unproven essentiality are discussed below.

Silicon

Due to its high affinity to iron/manganese oxides and hydroxides, the silicate anion is able to mobilise sorbed phosphate anions there.

Uptake occurs mainly passively in the form of undissociated acid.
In plants, silicic acid is relatively difficult to move and is found either in amorphous form or as a galactose ester in the cell walls of the epidermis in monocotyledons, but in the cell interior and in plant hairs in dicotyledons.

Silicon is considered indispensable for certain crops (e.g. rice) and useful for others.

Specific physiological functions have been sought for a long time, but so far with little success.
It is certainly involved in the stabilisation of cell walls.

Silicon deficiency leads to an inhibition of the phosphate conversion into ATP and sugar phosphates as well as lignin biosynthesis.

A good silicate content increases manganese tolerance by inhibiting iron and manganese uptake.

Tin

Amberger does not provide any information on tin.