The Vital Role of Micronutrients in Plant Growth and Health
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Micronutrients are mineral elements that plants need in tiny amounts yet cannot grow normally without — they regulate the physico-chemical state of protoplasm colloids, drive carbohydrate and protein metabolism, support chlorophyll synthesis, and form part of many plant enzymes that they also activate. Alongside macronutrients, these trace elements are essential for healthy plant development.
The role of micronutrients in plant life is far larger than their concentrations suggest. Even in minute quantities they influence:
- the physico-chemical state of protoplasm colloids,
- the metabolism of carbohydrates and proteins (more detail: composition of plant cells),
- the synthesis of chlorophyll,
- the structure and activity of certain plant enzymes.
How do micronutrients act on plant development?
Micronutrients act on plant development by forming organo-mineral complexes that carry out specialised functions in metabolism. These complexes are central to many processes, which is why a deficiency of any single trace element produces distinct, often diagnosable symptoms. The sections below describe the most important micronutrients one by one — iron, boron, manganese, zinc, copper and molybdenum — and the disorders that appear when each is lacking.
Why is iron essential for chlorophyll?
Iron is essential for chlorophyll formation even though it is not part of the chlorophyll molecule itself. The German agrochemist Wilhelm Knop (1817–1891) observed that, in the absence of iron, plants turn chlorotic and lose their green colour. It was first assumed that iron was a building block of chlorophyll, but the German organic chemist Richard Willstätter (1872–1942) — a Nobel laureate for his work on plant pigments — established that chlorophyll contains magnesium, not iron.
Iron remains indispensable to chlorophyll synthesis because that synthesis is catalysed by iron-containing enzymes. Its role is not limited to chlorophyll, however: iron is also required by non-chlorophyll organisms, which shows the element serves functions well beyond pigment production.
Later research showed that iron is a component of oxidation-reduction enzymes and plays a major part in plant respiration and in photosynthesis in leaves. Without iron the growing point of the stem dies, buds drop, internodes shorten, chloroplasts break down and living cells die. Iron is usually not applied to soil because most soils already contain enough of it in an available form.
Strongly calcareous, alkaline soils are the main exception, since iron there can become unavailable to the plant. In this situation plants develop chlorosis: the youngest leaves pale first and then lose their colour entirely, while the disorder spreads gradually to lower leaves and the lowest leaves keep their green. Loss of green begins at the base of the leaf — in the growing zone — and moves towards the tip.
Treating chlorosis early reverses it in the same pattern. If a plant is given iron in an available form during the early stage of chlorosis, the green colour returns starting from the base of the leaf and, across the plant, spreads from the young leaves back to the old ones.
Advanced chlorosis is harder to undo. As it progresses, spots appear on the leaves followed by browned patches that mark complete cell death, and iron does not move from the lower green leaves up to the affected upper ones. Chlorosis of this kind can be seen in grapevine, citrus, hops and other plants.
This disorder reduces yield, so correcting it on alkaline soils calls for iron chelates — complex compounds of organic anions with metals. Ordinary iron salts applied to alkaline soil react with other elements and become unavailable to the plant, whereas chelates do not.
Iron chelates are highly stable, enter the plant readily through both roots and leaves, and fully meet its iron requirement: the organic part of the chelate molecule breaks down and the iron is taken up by the plant. This is why chelated forms are the standard remedy for lime-induced chlorosis.
Why can't many plants grow without boron?
Boron is the most thoroughly studied of all the micronutrients, and many crops — flax, buckwheat, tobacco, beet and others — cannot grow at all without it. Boron is needed by every other plant too; its absence causes a range of growth and development disorders and a loss of resistance to pests and diseases. Dicotyledonous plants remove up to 350 g of boron per hectare from the soil, while monocots take only 8–20 g.
Boron deficiency strikes at the reproductive and growing tissues. In many cereal plants its absence produces a sterile ear. Without boron the normal functioning of meristematic tissues is disrupted, the conducting system is underdeveloped, the growing points of the stem die and root growth is held back; in legumes the number of nitrogen-fixing nodules falls sharply.
Boron also governs flowering and fruit set. It affects the permeability of the protoplasm and the movement of carbohydrates and, through these, hastens the onset of flowering. When boron is short, flowering and fruit set weaken, the growth of reproductive organs is delayed, and under severe boron starvation those organs die.
Boron is not re-utilised within the plant, so boron fertilisers are best applied to the soil at several points across the growing season. A shortage triggers recognisable diseases: in sugar beet the growing points die and the tissues of leaves and root rot away (heart dry rot), and in swede and turnip the core browns and dries out.
Bacteriosis of flax is likewise caused by the absence or shortage of boron, which makes adequate boron supply a direct factor in that crop's health.
What does manganese do for plants?
Manganese activates a number of enzymes and supports photosynthesis, and its content in plants varies widely. Spring wheat grain holds about 6.0 mg per kg, sunflower seeds 18 mg, and sugar-beet leaves as much as 180 mg per kg of dry weight — a thirty-fold range across tissues and species.
A lack of manganese suppresses photosynthesis and lowers the chlorophyll content of plant cells. In cereals manganese deficiency produces grey speckling and a transverse line of weakened turgor, so the leaf blade bends and hangs down.
Other crops show their own manganese-deficiency symptoms. Peas develop marsh spot — brown or black markings on the seeds — beet shows speckled yellows that curl the leaves, and many fruit trees become chlorotic when manganese is short.
What happens when plants lack zinc?
A shortage of zinc causes a range of diseases, most strikingly in fruit, citrus and tung trees. The deficiency weakens growth and stunts the plant in a characteristic way.
Lack of zinc reduces growth, produces small leaves and shortened internodes, and so causes rosetting; chlorotic mottling and a bronze leaf colour appear alongside it.
Zinc supports the synthesis of growth substances and helps build several enzyme systems. It is part of carbonic anhydrase, the enzyme that speeds the breakdown of carbonic acid (H2CO3) into water and carbon dioxide — a reaction central to carbon handling in the plant.
Why do plants need copper?
Copper is required by all plants and works mainly within oxidation systems: it is a component of many oxidative enzymes, where it is firmly bound to protein. Copper concentrates in the chloroplasts — in the chloroplast ash of sugar beet it reaches 64% of the total copper in the leaf ash.
This concentration points to copper's large role in chloroplast enzyme activity. Copper also makes chlorophyll more resistant to breakdown and improves the water-holding capacity of tissues.
Adequate copper raises frost resistance, while a shortage on peaty soils hits cereals (oats, barley and wheat) and beet hardest: leaf tips dry and curl, grain often fails to form, and in fruit trees the crown can die back (top dryness).
Applying copper fertilisers on peat soils makes it possible to grow healthy plants where they would otherwise fail, which is why these soils are routinely dressed with copper.
Why is molybdenum important for legumes?
Molybdenum is present in plants at lower levels than any other micronutrient — fractions of a milligram per kilogram of dry weight — yet it is decisive for nitrogen nutrition. It is required by nitrogen-fixing bacteria, both free-living and symbiotic, to fix atmospheric nitrogen, which makes its presence in the soil especially important under legume crops.
Molybdenum also takes part in the reduction of nitrates, because it is a component of the enzyme nitrate reductase. Through both roles — bacterial nitrogen fixation and nitrate reduction — molybdenum sits at the heart of how plants acquire and process nitrogen.
Which other trace elements do plants need?
Beyond the main six, plants also require cobalt, arsenic, iodine, nickel, fluorine, aluminium and others.
The role of micronutrients in plant life is remarkably varied: they take part in almost every process of plant metabolism despite being needed only in the smallest amounts. Matching the right trace element to the symptoms a crop shows is therefore one of the most practical tools in plant nutrition and disorder diagnosis.
