What Kind of Iron is Present in Plants? Ferrous, Ferric, and Chelated Forms

January 3, 2026 •

2 min read

Iron is an essential micronutrient for plants, despite being the fourth most abundant element in the Earth's crust. It is important to know what kind of iron is present in plants and how it is acquired, as most soil iron is in an insoluble form, making it largely unavailable for direct uptake.

Quick Summary

Plants utilize iron in its ferrous ($ ext{Fe}^{2+}$) form for metabolism but must also access the more common, insoluble ferric ($ ext{Fe}^{3+}$) form in soil. They employ different uptake strategies, involving either reduction or chelation, and chelate the iron for transport within the plant.

Key Points

Dual Forms: Iron exists in plants as either ferrous ($ ext{Fe}^{2+}$), which is highly available, or ferric ($ ext{Fe}^{3+}$), which is less soluble and bioavailable.
Distinct Strategies: Plants employ two primary strategies for iron uptake from the soil, known as Strategy I for non-grasses and Strategy II for grasses.
Reduction vs. Chelation: Strategy I plants reduce insoluble ferric iron to absorbable ferrous iron, while Strategy II plants chelate ferric iron with secreted phytosiderophores.
Transport and Storage: Within the plant, iron is chelated by molecules like citrate and nicotianamine for safe transport and is primarily stored within ferritins and vacuoles.
Adaptation: These sophisticated uptake and transport mechanisms allow plants to thrive despite the low availability of soluble iron in many soil environments, especially at high pH.
Critical Functions: Iron is essential for numerous plant physiological processes, including photosynthesis, respiration, and chlorophyll synthesis.

The Dual Forms of Iron: Ferrous vs. Ferric

Iron plays a critical role in various plant metabolic processes, including photosynthesis, respiration, and nitrogen fixation. While abundant in soils, its availability is limited by its chemical state. Iron in plants and soil exists primarily as ferrous ($ ext{Fe}^{2+}$) and ferric ($ ext{Fe}^{3+}$) forms. Ferrous iron ($ ext{Fe}^{2+}$) is more soluble and readily available, whereas ferric iron ($ ext{Fe}^{3+}$) is largely insoluble, especially in alkaline soils. {Link: Frontiers in Plant Science https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1190768/full}

The Two Main Strategies for Iron Uptake

Plants use two distinct strategies for iron acquisition from soil: Strategy I (non-grasses) and Strategy II (grasses).

Strategy I: The Reduction-Based Approach

Used by non-grass species, this strategy involves several steps under iron deficiency. Roots release protons to lower soil pH, and Ferric Reductase Oxidase (FRO) on root membranes converts soluble $ ext{Fe}^{3+}$ to absorbable $ ext{Fe}^{2+}$. Transporters like IRT1 move $ ext{Fe}^{2+}$ into root cells. Some plants secrete coumarins to aid in ferric iron reduction.

Strategy II: The Chelation-Based Approach

Characteristic of grass species, this strategy focuses on chelation. Under iron deficiency, roots produce and secrete phytosiderophores (PS) from the mugineic acid family. PS tightly bind insoluble $ ext{Fe}^{3+}$, and transporter proteins absorb the $ ext{Fe}^{3+}$-phytosiderophore complex into root cells. Iron is released inside the cell for use.

A Combined Approach: The Case of Rice

Some plants, like rice, combine aspects of both strategies, secreting phytosiderophores while also directly absorbing ferrous iron.

Internal Transport and Storage

After uptake, iron is transported and stored safely within the plant to prevent toxicity. It is always bound to chelating molecules during transport. Iron moves from roots to shoots via the xylem, often chelated by citrate. For redistribution, it travels through the phloem, primarily chelated by nicotianamine (NA). Excess iron is stored in protein shells called ferritins, mainly in plastids. {Link: BNL Newsroom https://www.bnl.gov/newsroom/news.php?a=110558}

Comparison of Iron Uptake Strategies

{Link: Frontiers in Plant Science https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1190768/full}

The Role of Iron in Plant Physiology

Iron is vital for numerous physiological processes, including chlorophyll synthesis, photosynthesis, respiration, and nitrogen fixation.

Conclusion

Plants utilize iron in both ferrous ($ ext{Fe}^{2+}$) and ferric ($ ext{Fe}^{3+}$) forms. They employ distinct uptake strategies: Strategy I (reduction) for non-grasses and Strategy II (chelation via phytosiderophores) for grasses. Internal iron management involves chelation with organic molecules like citrate and nicotianamine for transport and storage in ferritins and vacuoles. These adaptations allow plants to acquire iron effectively, which is crucial for essential processes like photosynthesis and chlorophyll synthesis. Understanding these mechanisms supports efforts in sustainable agriculture and biofortification.

For a more in-depth look into the complex molecular mechanisms governing iron transport in plants, refer to this comprehensive review: {Link: PubMed Central https://pmc.ncbi.nlm.nih.gov/articles/PMC2764373/}.

Frequently Asked Questions

The two primary forms are ferrous iron ($ ext{Fe}^{2+}$) and ferric iron ($ ext{Fe}^{3+}$). Ferrous iron is the more soluble and readily available form for plant uptake, while ferric iron is less soluble, particularly in high-pH soils.

Non-grass plants, or Strategy I plants, use a reduction-based strategy. They acidify the soil around their roots to increase iron solubility and then use enzymes called ferric reductases to convert insoluble $ ext{Fe}^{3+}$ into absorbable $ ext{Fe}^{2+}$.

Grasses, or Strategy II plants, use a chelation-based strategy. They secrete organic molecules called phytosiderophores that bind with insoluble $ ext{Fe}^{3+}$ to form a soluble complex, which is then absorbed by specific transporters on the root surface.

Iron deficiency is common because although iron is abundant in soil, it is often in the poorly soluble $ ext{Fe}^{3+}$ form, especially in alkaline (high pH) soils. This makes it difficult for plants to access the iron they need.

After being absorbed, iron is chelated by compounds like citrate and nicotianamine (NA) for safe transport through the plant's vascular system (xylem and phloem).

Plants store excess iron in two main locations: within protein shells called ferritins, primarily found in plastids, and by sequestering it within cellular vacuoles, particularly in seeds.

Phytosiderophores are specialized organic chelating molecules secreted by the roots of grass species in response to iron deficiency to specifically bind and mobilize insoluble ferric iron for absorption.

Iron deficiency, also known as chlorosis, is characterized by the yellowing of young leaves, especially between the veins, while the veins themselves remain green. In severe cases, the entire leaf may turn pale or white.