Iron is classified as a micronutrient, meaning it is needed in relatively small quantities compared to macronutrients like nitrogen and phosphorus. Despite this low requirement, its function is important for plant survival and productivity. Iron participates in many processes at the cellular level. Its chemical properties allow it to easily accept and donate electrons, which makes it a frequent partner in oxidation-reduction reactions. Although iron is the fourth most abundant element in the Earth’s crust, its availability to plants is often restricted. In many soils, especially those that are alkaline or high in calcium, iron is chemically locked up in a form the plant cannot easily absorb.
Iron’s Core Roles in Energy Production
Iron’s most recognized function is its indirect but necessary role in manufacturing the plant’s light-harvesting pigment, chlorophyll. Although iron atoms are not part of the chlorophyll molecule’s structure, they are required as cofactors for several enzymes in the synthesis pathway. Specifically, iron is needed for the formation of a key chlorophyll precursor. Without sufficient iron, the plant cannot produce enough chlorophyll, leading to a visible loss of green color.
The plant’s energy generation relies heavily on the electron transport chain (ETC), a complex series of protein reactions in both the chloroplasts and mitochondria. Iron is an indispensable component of the cytochromes, which are heme-containing proteins that facilitate the movement of electrons during both photosynthesis and cellular respiration. This electron transfer drives the production of adenosine triphosphate (ATP), the chemical energy currency of the cell. Iron’s ability to switch between the ferric (\(\text{Fe}^{3+}\)) and ferrous (\(\text{Fe}^{2+}\)) states enables it to shuttle these electrons, making energy production possible.
Iron is also a required part of the nitrogenase enzyme complex. This complex is found within the symbiotic bacteria that live in the root nodules of legumes. The nitrogenase enzyme allows these bacteria to capture atmospheric nitrogen gas and convert it into a usable form for the plant. Without adequate iron, this critical process of biological nitrogen fixation is severely impaired, limiting the plant’s ability to build proteins and nucleic acids.
How Plants Acquire and Transport Iron
The primary challenge for plants is that the highly oxidized ferric form of iron (\(\text{Fe}^{3+}\)) is insoluble in water at a neutral or high pH, which is common in calcareous soils. To overcome this, plants have evolved two distinct biological strategies to make iron soluble and available for root uptake. These mechanisms are genetically regulated and induced when the plant detects iron deficiency.
Non-grass plants, including most dicots, utilize a process known as Strategy I, which involves an “acidification-reduction-transport” triad. The plant pumps hydrogen ions (\(\text{H}^{+}\)) out of the roots, lowering the pH of the immediate surrounding soil, a process called rhizospheric acidification. This acidification helps to solubilize some of the ferric iron. The iron is then reduced to the more soluble ferrous form (\(\text{Fe}^{2+}\)) by a root-surface enzyme called Ferric Chelate Reductase (FRO). Finally, the ferrous iron is transported across the root cell membrane by the Iron-Regulated Transporter 1 (IRT1).
Grass species, such as corn, wheat, and rice, employ a different method called Strategy II, which relies on chelation. These plants release specialized organic molecules called phytosiderophores into the soil surrounding the roots. Phytosiderophores are high-affinity chelators that tightly bind to the insoluble ferric iron (\(\text{Fe}^{3+}\)), forming a soluble iron-chelate complex. This entire complex is then transported directly into the root cells without the need for a reduction step.
Once iron is successfully absorbed into the root, it must be transported to the growing parts of the plant, primarily the leaves. Inside the plant, iron is bound to organic compounds like nicotianamine and citrate, which act as internal chelators to keep the iron soluble. This soluble, chelated form is then moved through the plant’s vascular system, particularly the xylem, to reach the sites of new growth.
Identifying and Addressing Iron Deficiency
A lack of available iron manifests in a highly recognizable symptom known as interveinal chlorosis, or iron chlorosis. This condition causes the leaf tissue to turn yellow or pale green while the veins remain distinctly green. Because iron is considered an immobile nutrient within the plant, it cannot be effectively moved from older leaves to supply newer growth. Consequently, symptoms of iron deficiency always appear first and most severely on the youngest, newly emerging leaves.
The cause of iron chlorosis is rarely a total lack of iron in the soil itself. Instead, the problem is most often the soil chemistry, with high soil pH being the primary culprit. In soils with a pH above 7.0, the iron precipitate is so insoluble that the plant’s uptake mechanisms struggle to acquire the nutrient. Other contributing factors include excessive amounts of phosphorus, which interferes with iron uptake, or poor soil drainage, which inhibits root function.
Addressing iron deficiency typically involves making the existing soil iron more accessible or supplying the nutrient in a readily available form. The long-term solution is to lower the soil pH by incorporating acidifying amendments like elemental sulfur or acidic organic matter, which increases iron solubility.
For a more immediate effect, gardeners can apply chelated iron products directly to the soil. Chelated iron is chemically bonded to a protective organic molecule, preventing it from binding with other soil compounds and becoming unavailable. For a rapid, though temporary, fix, a dilute solution of chelated iron can be sprayed directly onto the foliage, allowing the leaves to absorb the nutrient immediately.