The growth rate of plants is limited decisively by a sufficient supply of water and the ions dissolved in it. The phenotype of plants does therefore vary more than that of animals. The pH-value of the soil is very important. Most plant favour neutral or slightly acidic soils. The tolerance boundaries of most plant species cover seldom much more than one pH unit. Root hairs and mycorrhizal fungi that live in association with roots are especially sensitive.
Only few plant species can survive on soils with pH-values below 3. Deschampsia flexuosa , a grass species, is an example. How far the limit of tolerance can be surpassed depends essentially on a combination of other factors. The conditions of a habitat are rarely optimal with respect to all requirements of a plant. It has always to live with a number of compromises that decrease its height, its number of flowers, or its metabolic performances. A surplus of certain ions can be just as dangerous. In 1919, P. EHRENBERG discovered a rule that says:
"If the supply of lime of a plant with a formerly low supply of lime is increased considerably, it limits the amount of potassium ions taken up by the plant and may thus cause severe damage."
This rule is true for many plant species. Among the exceptions are Tussilago farfara. In its case, an increased uptake of calcium correlates with an increased uptake of potassium, though the uptake capacity of magnesium ions decreases and does thus diminish growth, too.
Gramineae, Cyperaceae, and Juncaceae are far better at regulating their ion metabolism than other plant families. This may be, because of the higher efficiency of the ion pumps in the root cells of their endodermis. Some species are even able to contradict the rule above by decreasing their uptake of calcium and at the same time increasing their potassium uptake.
In general and referring to their dry weight and their content of nitrogen, these plants take up less ions than other plants.
The regulation of the calcium uptake is species-specific. The calcium-content of the shoots of Agrostis stolonifera, for example, correlates linearly with the supply of calcium in the substrate. The calcium values of the related species Agrostis stolonifera in contrast reach a saturation already at a low supply of calcium. This species is therefore better adapted to lime-less soils, while Agrostis stolonifera is better adapted to soils rich in lime.
Most plant species are especially sensitive towards high concentrations of heavy metal ions in the soil. But some, the so-called heavy-metal plants are more or less tolerant, though they do always tolerate only one type of ion. They show different physiological reactions. Tolerance of copper is based on a changed permeability of the membranes of tolerant species for this ion. Zn- and Ni-tolerance is caused by the ability of the cells to store these ions in the vacuole.
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