Iron is one of the essential trace elements of plants, which plays an important role in photosynthesis, biological nitrogen fixation and electron transport of respiratory chains, and is called "green leaf element".
Distribution of iron in plants
Most plants have an iron content between 100-300 mg-kg-1 (dry weight) and often vary with plant species and plant site. Some vegetable crops have a high iron content, such as spinach, lettuce, green leaf kale, etc., generally above 100 mg·kg-1 (dry weight), up to 800 mg·kg-1 (dry weight); while rice, corn iron content is relatively low, about 60 ~180 mgkg-1 (dry weight).
In general, legumes have a higher iron content than grasses. The iron content of different plant sites is also different, such as the iron content of grass crop scale is higher than that of the seed grain, and the iron content in the grain and tubers is relatively low. In the same plant, the distribution of iron is also uneven, for example, there is often a large amount of iron precipitation in the stem nodes of corn, but the iron content in the leaves is very low, and even iron deficiency symptoms occur.
It is generally believed that Fe2+ is the main form of plant absorption, chelated iron can also be absorbed, Fe3+ has a low solubility under high pH conditions, and most plants are difficult to use. With the exception of grasses that can absorb Fe3+, Fe3+ can only be absorbed after the root surface is reduced to Fe2+. Plant root tips absorb iron at a higher rate than at the base. A variety of ions can affect the absorption of iron by plant roots, such as Mn2+, Cu2+, Mg2+, K+, Zn2+ and other metal ions, which have obvious competitive effects with Fe2+. For example, Cu2+ and Zn2+ can be displaced from the chelate Fe2+ to form the corresponding cu2+ and Zn2+ integrates, and the displaced Fe2+ is easily fixed in the soil, reducing its effectiveness, thereby limiting the absorption and utilization of this part of the iron by plants.
When Fe2+ is absorbed by the root system, it can be oxidized to Fe3+ in most root cells, and is liatized with citric acid and transported to the aboveground through xylem. The transport of iron is carried out in the form of citric acid chelatate. Citric acid and iron ions have a strong affinity. Iron citrate is found in the wound fluids of sunflowers and soybeans. It has also been reported that iron can form complexes with organic acids such as citric acid or malic acid and move to other sites in the catheter, due to the low mobility of iron in the phloem. Therefore, the new tissues of plants are prone to iron deficiency symptoms. In order to regularly ensure the iron needs of the new tissues, it is necessary to often supplement iron nutrients in moderation (such as masotide) for plants growing on iron-deficient soils.
The role of iron in plant bodies
1. Necessary for chlorophyll synthesis
In a variety of plants, most of the iron is found in chloroplasts. For example, 75% of the iron in spinach leaves is concentrated in chloroplasts. Although iron is not a component of chlorophyll, the synthesis of chlorophyll requires the presence of iron. In chlorophyll synthesis, iron is an activator of one or more enzymes, when iron deficiency chloroplast structure is destroyed, resulting in chlorophyll can not be formed, severe iron deficiency, chloroplasts become smaller, or even disintegration or vacuole. Iron is closely related to photosynthesis. It not only affects the redox system in photosynthesis, but also participates in photosynthetic phosphorylation and directly participates in the reduction process of CO2. Iron affects chlorophyll synthesis at the same time, but also affects all organs that can capture light energy, including chloroplasts, chlorophyll protein complexes, carotenoids, etc.
Because iron deficiency affects the synthesis of chlorophyll, and the mobility of iron in the phloem is very low, it is difficult to transfer the iron in the old leaves after iron deficiency to the newborn young leaves, so that the new young leaves have iron deficiency and green loss. This is completely different from the symptoms of deficiency such as nitrogen, phosphorus, and potassium.
2. Participate in in vivo redox reaction and electron transport
Another major function of iron is to participate in redox reactions and electron transport within plant cells. Its essence is the change in valence and electron transfer between trivalent iron ions (Fe3+) and divalent ferrous ions (Fe2+). This occurs frequently during biochemical metabolism in plants.
More importantly, the redox capacity of inorganic iron salts is relatively weak. If iron binds to certain organics to form heme or further synthesize heme protein, their redox capacity can be increased by a thousand or ten thousand times. For example, various cytochromes, soy hemoglobin, ferrite, etc. are all iron-containing organic matter, and their reduction ability is very strong. These different kinds of iron-containing proteins act as important electron transmitters or catalysts and are involved in a variety of metabolic activities in plants. Cytochromes are a class of ferroporphyrin-protein conjugates that are mainly found in mitochondria and play an important role in metabolic processes such as plant respiration.
Nitrogenase is necessary for nitrogen fixation in legume crops, it is composed of two oxygen-sensitive non-hemoglobin, one is a protein containing iron and molybdenum, also known as molybdenum ferritin; the other is ferrite, ferroside protein is a small molecular weight, do not contain porphyrin structure of ferritin, it is a protein that combines iron and cysteine or inorganic sulfur, so it is also called ferritin. Molybdenum ferritin is the active center of nitrogenase. When these two proteins are present alone, the nitrogenase is not active, and legumes cannot fix nitrogen, and nitrogen fixation can only be carried out when the two are combined.
In the process of oxidative phosphorylation, the transmission of electrons is done with the participation of a variety of special substances. Among them, ferrotropin and cytochrome are important organic compounds containing iron, and it is in the process of the change of valence of iron redox that the electron transport is completed. In the photosynthesis of higher plants, ferrite is an important substance on the photosynthetic electron transport chain, and it is also an electron transporter in many basic metabolic processes of the plant body.
3. Participate in plant respiration
Iron is also involved in the respiration of plant cells because it is a component of some enzymes associated with respiration. Such as cytochrome oxidase, catalase, peroxidase, etc. all contain iron. Iron is often located at the active site of the enzyme structure. When plants are iron deficient, the activity of these enzymes is affected, and further weakens a series of redox effects in the plant, electrons cannot be transmitted normally, respiration is blocked, and ATP synthesis is reduced.
Iron is also the best activator of sucrose synthase phosphate. Iron deficiency in plants can lead to a decrease in sucrose synthesis in the body.
Symptoms of iron deficiency in plants
Plant iron deficiency always begins with young leaves, and the typical symptom is the loss of green between the leaf veins and in the cell network tissue of the leaves, and the leaf veins are often obviously visible on the leaves. In severe iron deficiency, necrotic spots appear on the leaves, and the leaves gradually wither away. In addition, the accumulation of organic acids may occur in the root system during iron deficiency, mainly malic acid and citric acid. For iron supplementation, you can choose to apply bottom or multiple small amounts of leaf spray, while supplementing with functional foliar fertilizers (such as masin), supplementation is the basis, and improving or activating nutrient utilization is the key.
Ferrous poisoning often occurs on poorly drained soils and paddy soils with long-term water accumulation, and one of the important reasons for the low yield of rice is the toxic effect of iron. When the ferrous content in rice leaves > 300 mg·kg-1, iron toxicity may occur. The cause of ferrous poisoning may be that plants absorb too much ferrous iron and easily lead to the production of oxygen free radicals. Iron poisoning is often associated with zinc deficiency, which causes zinc and copper-containing superoxide dismutase (Zn-Cu-SOD) activity to decrease and biofilms to be damaged. Symptoms of iron poisoning are brown spots on old leaves and gray-black roots that are perishable. The method of prevention and control is: appropriate application of lime, reasonable irrigation or timely drainage of the field. This can also be addressed through the selection of resistant varieties.