Entry.
Boron is a typical nonmetal, with a low atomic mass. The most well-known boron compounds are boric acid and its salts – borates; boron occurs in them as a trivalent element. The complex-forming properties of the borate ion are well-known. Research on boron as a trace element dates back to the first years of this century. Research on boron as a trace element demonstrated its beneficial effect on the growth of higher plants, resulting in its inclusion on the list of essential nutrients. Despite rigorous research on the importance of boron for higher plants, symptoms of this element deficiency began to be discovered in field-grown plants. Brandenburg (1931) concluded that heart rot, then common in sugar beet plantations in Germany, was caused by a boron deficiency and could be successfully treated by adding this trace element to the soil. New, often contradictory, views on the physiological and biochemical functions of boron are constantly being put forward.
- Nutritional requirements of plants for boron.
The question of the essentiality of boron as a basic nutrient for higher plants has been definitively resolved for several years. Furthermore, it appears that boron is essential for certain algae, while fungi likely do not require this element. Plants require a constant supply of external boron throughout their growth period, because so-called boron reutilization in plants, i.e., its transfer from older, lower leaves to young, growing leaves, is either very low or nonexistent. Various researchers speculate that the reason for the low boron mobility is its binding within the plant cell, likely to structural components of cell walls. Oertli and Richardson reported on the movement of boron within plants at smaller distances, observing the circulation of this micronutrient between sieve tubes and vascular tubes, including within the leaf blade. However, their experiments were conducted under conditions of a luxurious boron supply (at a dose of 100 mg B/liter of nutrient solution). Plant nutritional requirements for boron vary significantly depending on the plant species, cultivar, and climatic conditions, such as day length and temperature, which influence growth rate. It is assumed that fast-growing plants require larger amounts of boron than slow-growing plants and show symptoms of boron deficiency earlier than the latter. The generally much lower boron requirements of monocotyledonous plants compared to dicotyledonous plants are noteworthy. In an attempt to explain these differences, the following aspects were highlighted: Monocotyledonous plants contain a higher percentage of boron in a soluble, and therefore active, form than dicotyledonous plants. Plants with a high meristematic tissue content, such as dicotyledonous plants, are characterized by a high boron requirement. Finally, differences in the quantitative boron requirements between dicotyledons and monocotyledons may result from different pathways of lignin synthesis, in which boron likely participates.
- Boron uptake by plants.
Boron is absorbed by plants in the form of the borate ion (BO33- or B4O72- ) . Its natural source in the soil is borosilicates or boroaluminosilicates, which gradually weather. The total and available boron content in the soil depends on several factors :
- type of soil – clay soils usually contain more boron than sandy soils;
- organic matter content – the richer the soil is in humus, the more boron it contains;
- on soil reaction – in alkaline soils and over-limed acidic soils, the absorption of soil boron is significantly reduced, which contributes to the occurrence of a deficiency of this nutrient in plants;
- from the presence of certain cations in the soil – in addition to calcium ions, potassium ions in higher concentrations also limit the uptake of boron by plants;
- on soil moisture – boron deficiency occurs particularly often in dry years.
- Boron content in plants
Under mature conditions, boron content in plants ranges from traces to approximately 100 ppm B based on the dry weight of the above-ground parts. However, the significant variability of these values, which are the result of a number of factors, has been repeatedly noted. Specifically, in addition to the species (and even cultivar) differences mentioned above, plant growth conditions also play a role, influencing the unequal rate of plant mass growth relative to the rate of boron uptake by plant roots. Factors influencing these relationships include the plant's growth stage, as well as diverse soil, fertilizer, and climatic conditions. Despite these many variables, researchers are attempting to define certain ranges of boron content in plants, which could characterize conditions of deficiency or abundance of this micronutrient. Boron content in different parts of the same plant also varies. Relatively high boron content occurs in leaves and generative parts, although seeds are not particularly rich in boron. In the leaf blade, most boron is found on its edges.
- Distribution and forms of boron in the plant cell.
We have relatively little information about the distribution of boron within plant cells. Probably about 50% of both boron and 70% of calcium are bound in the cell wall or the intercellular matrix. However, the boron content in chloroplasts is low. Comparing the boron distribution in leaf cells with boron deficiency, normal boron levels, and increased boron content led to the conclusion that boron plays an important role in the cytoplasm and cell wall, but is insignificant in chloroplasts. It has been hypothesized that the metabolism of boron, calcium, and an unexplored cell wall component—perhaps pectin or protopectin—is closely related. Currently, it is believed that only a negligible portion of the absorbed boron is present within the cell in the form of ions or mineral particles. However, attention has been drawn to the possibility of complex combinations of the borate ion with various organic compounds containing hydroxyl groups, such as carbohydrates, alkaloids, hydroxy acids, and others, occurring in plants. In these complexes, trivalent boron is often linked to organic molecules via additional coordination bonds. The ability to form such complexes in vitro is no longer in doubt. It has also been found that the borate ion, although very weakly dissociating, dissociates more strongly when complexed with organic compounds, which could be of significant importance for boron's physiological functions. For many years, the highly suggestive assumption has been made that boron exists in plants primarily in the form of the complexes discussed; however, this has not been experimentally confirmed.
- Symptoms of boron deficiency and excess in plants.
The most typical symptom, occurring in all plants suffering from boron deficiency, is growth inhibition and eventual death of growth cones, both in the above-ground parts and the roots. This symptom appears within a few days, or even hours, after boron is removed from the nutrient solution. Boron deficiency also often causes uneven growth of individual tissues, resulting in various types of curling, cracking, and growth retardation, often characteristic of a given plant species. These morphological symptoms are accompanied by characteristic anatomical changes: abnormal development and death of the meristematic tissues of the growth cones and cambium, as well as poor development and degeneration of xylem, phloem, and parenchyma. Certain symptoms of boron deficiency can sometimes be observed very early, long before external signs appear. Namely, as early as the second or third day after removing boron from the nutrient solution, plants lost their ability to respond geotropically, and their response to X-rays changed. Boron deficiency also leads to other profound changes in plant metabolism, including excessive accumulation of carbohydrates in leaves due to impaired carbohydrate metabolism. These changes are being studied in detail in the hope that they may help decipher the mechanism of boron's action in plants. Cases of boron toxicity to plants are also known. These symptoms manifest themselves externally as the "drying" of edges, leaves, and even entire plants. They are the result of inappropriate use of boron-containing fertilizers, especially in acidic environments. It has been found that under such conditions, the boundary between optimal and toxic doses is very narrow.
- Physiological and biochemical functions of boron in plants.
Boron influences certain physiological and biochemical processes, which in turn intertwine with other processes. A whole chain of causes and effects emerges, making it difficult to distinguish between what is a manifestation of boron's primary effect and what is a secondary reaction. A significant difficulty in studying the role of boron is the fact that boron compounds in plants have not yet been identified, nor has the mechanism of boron's action on enzymatic systems been discovered. Participation in enzymatic reactions is a characteristic feature of several micronutrients. However, boron—a typical nonmetal—occupies a distinct position among them. Despite new discoveries in the field of enzymatics, it has not yet been proven that boron is essential for the activity of any enzyme or that it is a component thereof. On the other hand, there is a wealth of data, based on both in vitro and in vivo studies, indicating that either the addition or absence of boron alters many enzymatic reactions. Therefore, the current view is that boron may affect enzymatic reactions indirectly by forming complex bonds with a number of enzymatic substrates, which specifically prepares the substrate for reaction with the enzyme. Due to these difficulties, the primary method for studying boron function has so far been studying changes in plant metabolism under conditions of boron deficiency, particularly in the initial stages of boron deficiency before secondary deficiencies occur. When formulating new hypotheses, information that may shed light on the issue is also considered, such as the distribution of boron within the plant cell, the fact that it is poorly reutilized in the plant, and differences in requirements depending on the species.
-Anna Nowotna-Mieczyńska "Physiology of mineral nutrition of plants. PWRiL 1965,
-Lityński T., Jurkowska H "Soil fertility and plant nutrition" PWN 1982,
– Franck B. Salibury, Cleon Ross "Plant Physiology" PWRiL 1975,
-Zurzycki J. Michniewicz M. "Plant Physiology" PWRiL 1979,
-Otis F. Daniel G. Curtis Clark "Introduction to Plant Physiology" PWRiL 1958.