Iron naturally forms a number of compounds, such as oxides, hydroxides, carbonates, sulfides, and others. In all of these compounds, iron occurs in the second or third oxidation state. It also tends to form complex compounds. Due to its exceptional economic importance, it is considered the most important metallic element. The first mentions of iron's essentiality for plants date back to the 19th century. Specifically, in 1844, Eusebe Gris found that a lack of iron in the nutrient solution led to plant chlorosis, and painting leaves with a solution of an inorganic iron salt caused them to turn green within 2 to 3 days. Classical studies by Sachs and Knop on the mineral nutrition of plants, conducted in the second half of the 19th century, confirmed these conclusions. Iron was included in the list of 10 elements essential for plants (along with C, O, H, N, S, P, K, Mg, and Ca), later called "macroelements." The inclusion of iron in this group of elements was rather accidental, however, as the iron content in plant tissues is much lower than, for example, potassium, phosphorus, or even sulfur or magnesium. Therefore, iron is now increasingly being classified as a trace element, especially since, due to its physiological functions, it shares many characteristics with other heavy metals such as copper, manganese, and molybdenum—typical trace elements.
Iron Uptake:
Iron is one of the most common elements in soils (after oxygen, silicon, and clay). It occurs in the form of silicates, phosphates, oxides, hydroxides, and other mineral compounds with varying degrees of solubility, as well as in combination with organic matter. It is also strongly bound in ion form by the soil sorption complex. Plants can absorb iron in the form of ferrous ions (Fe 2+ ) or ferric ions (Fe 3+ ), as well as in the form of chelate compounds (as ions or even whole particles). Iron absorption is significantly influenced by the pH and air conditions of the soil. The lower the soil pH, the greater the amount of available iron available to plants, while a pH higher than 6 inhibits iron uptake. This is associated with the precipitation of iron hydroxides in the soil, which are poorly absorbed by plants. It has been found that iron in the trivalent state (Fe 3+ ) precipitates at a much lower pH than iron in the secondary oxidation state (Fe 2+ ). The presence of iron in the secondary state depends largely on oxygen availability. The lower the oxygen availability, the more the Fe 2+ Fe 3+ balance shifts in favor of iron in the divalent state. Under conditions of severe anaerobicity, excess reduced iron (Fe 2+ ) can accumulate in the substrate, which has a detrimental effect on plant root systems. Phosphorus also has a significant impact on iron uptake. High phosphate concentrations significantly inhibit iron uptake by plants. Chlorosis occurs, which is the result of the precipitation of insoluble iron phosphate on the root surface or in the vascular-sieve bundles. Iron uptake also depends on the individual properties of the plant itself. It has been observed that different plant species growing in the same substrate uptake different amounts of iron. It has been shown that plants less sensitive to iron deficiency are able to acidify the substrate more strongly, which likely promotes the availability of sparingly soluble forms of soil iron. Iron Complexes. Much attention has been devoted to research on iron uptake from chelates. Jacobson (1951) was the first to publish on the usefulness of a complex iron compound with EDTA (ethylenediaminetetraacetic acid), known as Fe-EDTA, for plants. Numerous researchers have pointed out that while iron absorption from inorganic salts is strongly influenced by pH and redox potential, Fe uptake from complex compounds is largely independent of these influences. On the other hand, studies have shown that iron uptake from complex compounds and its transport within plants were inhibited by various heavy metals, particularly those that readily form chelates, primarily copper, nickel, cobalt, and zinc, and least significantly by manganese. This phenomenon is more pronounced the more stable the chelate, e.g., DTPA or HBED. The question of whether iron from chelates is taken up in the form of ions or whole molecules is also debated. Experiments have shown that under iron-deficient conditions, plants took up more of the metallic component than the chelated component. However, when the iron content in the nutrient solution was sufficient, iron and chelate were sorbed in equivalent amounts. It is also hypothesized that iron chelates undergo photoreduction in the leaves, and the active iron thus released remains available to the plant. However, in some cases, chelates can negatively impact iron uptake. If a plant actively takes up iron from the chelate, the chelating agent is released on the root surface, which can then react with other cations in the substrate, forming secondary chelates. However, if such cations are not present, the chelating agent accumulates excessively near the root and, as a result of competitive action, contributes to limiting iron uptake by the plant.
Iron transport in plants.
It was traditionally believed that transport (as well as the uptake process itself) occurs in the form of a divalent ion (Fe 2+ ), which many researchers considered the only form of active iron. It was noted that manganese and copper play a significant role in regulating the active iron content in plants, catalyzing the redox processes associated with iron (Fe 2+ Fe 3+ ). Among other things, both of these elements cause the oxidation of excessive amounts of divalent iron to the trivalent, inactive form. In addition to manganese and copper, phosphorus also influences iron activity in plants. Specifically, high phosphorus concentrations in plants can lead to the precipitation of iron phosphate in the vascular-sieve bundles, preventing further transport. Other studies suggest the possibility of another mechanism for iron transport in plants, namely in the form of natural iron chelates, which simultaneously protect iron from becoming inactive. It is generally assumed that the transfer of iron from older leaves to younger leaves or to meristematic tissues (generative tissues) is not very significant, as demonstrated by studies with the 55Fe isotope, among others. It was found that the iron contained in lower leaves is poorly available to young, developing tissues, resulting in meristematic tissues being the first to show iron deficiency.
Iron Deficiencies and Excesses – Symptoms.
A characteristic symptom of iron deficiency in plants is chlorosis, which initially appears in the youngest leaves. Chlorotic leaves take on a yellowish or even whitish discoloration, indicating a complete lack of chlorophyll in the tissues. The growing point does not die, but its development is stunted. Older leaves typically remain green or only partially discolored: along the main vascular-sieve bundles, the tissue remains green, while the rest of the leaf blade becomes almost light green. It was also found that iron deficiency resulted in a decrease in CO2 assimilation by more than half, a decrease in chlorophyll a by over 80%, chlorophyll by over 85%, carotene by over 70%, and xanthophyll by almost 60%. After iron was added to the medium, the greening returned to normal after approximately three weeks, with the youngest leaves becoming the first to turn green. The results cited indicate that iron deficiency causes a general underdevelopment of chloroplasts and a reduction in chlorophyll content. It is speculated that iron's action is related to protein synthesis, which subsequently determines the levels of chlorophylls, carotenoids, and xanthophyll. Several researchers have reported the effect of iron on protein synthesis; it has been observed that iron deficiency reduces protein content in plants in favor of soluble organic nitrogen compounds. Furthermore, iron deficiency has been found to specifically inhibit enzymatic processes catalyzed by metalloenzymes containing Fe, such as catalase, peroxidase, cytochrome oxidase, and others. Symptoms of
iron excess in plants are also known. A characteristic feature of iron excess is intense, dark leaf coloration, combined with stunted plant growth and browning of roots. CO2 assimilation is then significantly reduced.
Physiological and Biochemical Functions of Iron in Plants.
The biochemical functions of iron in plants are closely linked to two characteristics of this metal: its tendency to form chelate compounds and its ability to change its oxidation state. Thanks to these properties, iron is involved in a very versatile way in the redox metabolic processes of plants: it participates as an electron carrier in specific, catalytic reactions related to both respiration and photosynthesis, as well as in free nitrogen fixation, nitrate reduction, and other processes. Among the iron-containing systems, the cytochrome system plays an active role in tissue oxidation, working in conjunction with cytochrome oxidase to transfer electrons from the oxidized substrate to molecular activated oxygen. Independent of cytochromes is the peroxidase system, which, under certain conditions, assumes the function of a terminal oxidase. Furthermore, peroxidase, together with catalase, plays an important role in protecting plant tissues from excess peroxide.
in respiratory processes. Ferredoxin, the most electronegative component of the photosynthetic apparatus of green plants, plays an active role as an electron carrier in photosynthesis. Cytochrome phi and cytochrome b6, which are involved with ferredoxin in the cyclic phosphorylation process, are also oxidoreductive factors in photosynthesis. Furthermore, a complex containing iron and coenzyme A has been isolated from the leaves of various plants; there are reports that this complex may be the primary CO2 acceptor. Iron also indirectly influences the photosynthetic process; specifically, it affects the synthesis of protoporphyrin, from which chlorophyll is subsequently formed (in the absence of iron, plants are completely chlorotic). However, iron's effect on chlorophyll may also be indirect. In the process of free nitrogen fixation, nitrogenase, an enzyme that activates molecular nitrogen, has been found to contain two complexes containing iron, whose chemical form is not fully identified but is directly related to nitrogenase activity. Ferredoxin also participates in N2 fixation. Leghemoglobin, discovered in the root nodules of legumes (and also in non-legumes), likely does not directly participate in N2 fixation but merely regulates oxygen access to bacteroid tissue. Ferredoxin has also been found to be involved in nitrate reduction. However, the cytochrome system is likely active in ion sorption, growth, and water uptake. Ferritin, a reserve iron compound, has been detected in plants, prevalent primarily in the animal kingdom. In plants, it occurs primarily in the plastids—the cotyledons of non-germinating seeds.
-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,
-Diana Walstad "Plants in the aquarium. Ecology of aquatic plants" Oriol 2007,
Author: Marcin Kołodziejczyk