42
Trace Elements in Abiotic and Biotic Environments
through the roots. Thus, its uptake is proportional to its concentration in water of
growth media. All B species are available, but the most easily available to plants is
boric acid, as well as all water-soluble B compounds (Kabata-Pendias 2011). Several
soil properties have an impact on B availability to plants, but the most important
seems to be the Ca:B ratio (Azarenko 2007).
Boron is an essential element for plant growth and its biochemical role is still
under investigation. Its significant functions as a structural element of plant cell
walls are closely related to cell metabolic processes (Goldbach and Wimmer 2007).
It has a complexing ability to trigger the movement of sugars and other compounds,
especially those involved in biocomplexing membrane bindings. Thus, an adequate
B supply to sugar beets is necessary for sugar synthesis. Functions of B have not
been identified with any specific enzyme; however, its activities are related to some
basic processes as follows:
r Carbohydrate metabolism and transport of sugars through membranes
r Nucleic acids (DNA and RNA) and phytohormone syntheses
r Structural integrity of the plasma membrane (boric acid is involved in the
linking of some cell components)
r Tissue and cell-wall developments
r Formation of B complexes with constituents of cell walls and plasma
membranes
B deficiency has been reported in over 130 crops from 80 countries. Its deficiency may
occur in all countries, but most often in Asian and African countries. This deficiency
affects some commercial crops such as sugar beets, celery, sunflower, legumes, and
apples. Critical B levels for most plants range at 5–30 m/kg. Mycorrhizal plants have
a greater need for B supply than do nonmycorrhizal plants. It is related to a beneficial
effect of B on the nodule formation and nitrogenase activity.
Enhanced B levels in soils can be harmful to crop plants, especially lemon and
orange trees. Several other crops, such as cereals and cotton, when cultivated in
irrigated areas might be affected by increased B contents. B toxicity is usually more
common in arid and semiarid regions. B toxicity in some crops (mainly cereals and
sunflower) occurs on soils contaminated due to (1) irrigation with municipal wastewater or B-enriched river water, in aridic climatic zones; (2) amendment with fly
ash; and (3) foliar application of B, mainly in citrus or apple orchards. Toxic B content in most plants range from 330 to 350 mg/kg, for spinach and corn, respectively
(Kabata-Pendias 2011).
Highest B contents are in fodder plants within the following ranges (in mg/kg):
alfalfa, 26–40; clover, 14–33; and grass, 5–26. Mean B contents in cereal grains are
within the range 0.69–3.3  mg/kg, with the highest value for oat grains (Eriksson
2001a).
There is often a narrow range of toxic and deficient B levels to plants. Boron
levels in the most food plants cultivated in humid zones do not exit 18 mg/kg,
whereas in fodder plants it is higher. In cotton grown on irrigated land, B contents
may be up to 422  mg/kg. For animal nutrition, B level above 150  mg/kg is not
recommended.