Iodine in Plants: Essential or Beneficial?

Iodine is essential in human nutrition, but its place in plant nutrition is less settled. Plants can absorb iodine, incorporate it into proteins and, under some experimental conditions, respond to small additions with changes in growth, flowering or crop yield. Those findings support a biological role without demonstrating that every plant requires an iodine supplement.

The useful question for growers is therefore not simply whether iodine is present in a plant. It is whether supplying it improves a relevant outcome in a particular crop, growing system and environment, at a concentration that does not cause injury.

A nutritional role is not the same as universal essentiality

Under the classical definition, an essential element is required for a plant to complete its life cycle, has a function that cannot be replaced by another element and is directly involved in plant metabolism. A beneficial element can improve performance under particular conditions without meeting all of those criteria. The distinction is explained more broadly in essential and beneficial elements in plant nutrition.

Iodine is difficult to study through complete exclusion because trace amounts can enter a growing system through water, salts, substrate and the atmosphere. Nevertheless, difficulty removing an element is not itself proof that the plant requires it. Evidence of improved growth following supplementation also needs to be distinguished from evidence that a plant cannot complete its life cycle without it.

Kiferle and colleagues proposed a nutritional role for iodine on the basis of growth experiments, gene-expression responses and protein analysis. A subsequent commentary by Nascimento and colleagues welcomed the findings while emphasizing the need to test their wider relevance across crop species. Iodine is consequently best discussed as an element with demonstrated benefits in some systems and an incompletely resolved nutritional role, rather than as a universally established essential micronutrient (Kiferle et al., 2021; Nascimento et al., 2022).

What the protein research actually found

In their 2021 study, Kiferle and colleagues found evidence of protein iodination and reanalysed existing mass-spectrometry datasets from Arabidopsis thaliana. They identified iodinated peptides corresponding to 42 protein accessions in leaf datasets and 40 in root datasets. These numbers describe the proteins identified through that analysis, not a complete inventory of every protein that can contain iodine.

Interaction analysis of the leaf proteins produced a network containing 40 iodinated proteins, of which 31 were associated with photosynthesis. These included components of photosystems, light-harvesting complexes, electron transport and carbon fixation. The denominator matters: 31 out of 40 is approximately 78% of the iodinated proteins in that particular network. It does not mean that 78% of all photosynthetic proteins contain iodine.

The root analysis showed enrichment of functions associated with stress responses, oxidative stress and peroxidase activity. This places iodination in biologically relevant parts of plant metabolism, but detecting iodine in a protein does not establish whether that modification improves, reduces or leaves its activity unchanged. The function of individual iodination events remains a separate experimental question (Kiferle et al., 2021).

Growth responses in Arabidopsis

The same study grew Arabidopsis in a hydroponic solution prepared to minimize background iodine. Adding potassium iodate at 0.20 or 10 micromoles per litre influenced flowering, biomass and seed production. Treated plants flowered earlier, and several growth and reproductive measurements were higher than in the low-iodine control. The response was not identical for every organ or measurement; for example, the reported increase in whole-plant fresh weight was mainly associated with the inflorescence rather than a significant increase in rosette fresh weight.

Related experiments compared potassium iodide, sodium iodide and potassium bromide. The bromide treatment did not reproduce the iodine-associated flowering response, helping the researchers distinguish that response from a general effect of adding potassium or a related halogen. Gene-expression analysis also identified iodine-associated responses, including defence-related processes (Kiferle et al., 2021).

The study also illustrates why an apparently favourable response should not be judged in isolation. At 30 micromoles per litre, the iodide treatments reduced growth even though flowering was still promoted. Earlier flowering is therefore not, by itself, proof of better overall plant performance. None of these Arabidopsis concentrations should be treated as a general application rate for other crops.

Tomato research: yield, salinity and dry matter

A 2022 study examined iodine in tomato using two distinct growing systems: growth-chamber experiments with the small-fruited cultivar ‘Micro-Tom’, and a greenhouse trial with ‘Cartesio F1’ grown on rockwool under hydroponic drip irrigation. The doses and treatment schedules differed between those systems, so their results should not be combined into a single universal recommendation (Kiferle et al., 2022).

In the greenhouse trial, nutrient solutions received no added potassium iodate, 0.2 micromoles per litre or 10 micromoles per litre, alongside treatments with no added sodium chloride or with 30 or 60 millimoles per litre sodium chloride. The irrigation water already contained a small background amount of iodine, so the control was not an absolute iodine-free environment.

Without added sodium chloride, the two iodate treatments increased fresh fruit yield by approximately 22% and 42%, respectively, compared with the control. Fruit dry-matter percentage decreased, however. A larger harvest measured as fresh weight should therefore not be described as an equivalent increase in dry-matter production.

At 30 millimoles per litre sodium chloride, the higher iodate treatment significantly increased final fresh fruit yield by approximately 13.5% relative to the corresponding treatment without added iodine. At 60 millimoles per litre sodium chloride, the observed tendencies towards more or larger fruits did not produce a statistically significant increase in fruit yield. These results support a benefit under some of the tested conditions, not protection against any degree of salinity.

Two authors of the tomato study declared employment with SQM International. That commercial involvement does not invalidate the experiment, but it is relevant context when weighing the evidence and the value of independent replication.

Other crops do not always show the same outcome

More recent sweet-pepper research reported favourable responses to foliar potassium iodate under salinity, including changes in yield, water status and fruit-quality measurements. This was a leaf-applied treatment, not the same delivery route as the tomato fertigation experiment. Its results extend the evidence to another crop and application method, but do not establish a common optimum across species, cultivars or growing systems (Akınoğlu et al., 2025).

Hora and Holwerda also described eight commercial tomato and sweet-pepper case studies involving iodine supplied with potassium nitrate under heat-stress conditions. These observations are relevant to horticultural practice, but a collection of commercial case studies should not be treated as equivalent to one standardized, independently replicated experiment (Hora & Holwerda, 2023).

There is also limiting evidence. In lettuce exposed to 50 millimoles per litre sodium chloride, Maglione and colleagues reported changes in pigments and other bioactive compounds following iodine treatment, but did not find that iodine improved salt tolerance. A change in antioxidant-related compounds is not automatically evidence of better growth or greater stress tolerance (Maglione et al., 2022).

Biofortification needs similar care in interpretation. Increasing iodine in an edible crop may be useful for human nutrition, but tissue enrichment, improved plant growth and reduced stress damage are different outcomes. One cannot stand in for another.

Dose, chemical form and delivery route matter

Iodine can be supplied in different chemical forms, including iodide and iodate. Their uptake, transformation and potential toxicity depend on the plant and its environment. The response also depends on background iodine, exposure time, substrate or solution chemistry, growth stage and whether treatment reaches the roots or leaves. A concentration used successfully in one experiment is not sufficient evidence of safety or benefit in another system.

The Arabidopsis growth reduction at the higher iodide concentration provides a direct example of this limitation. More iodine did not simply produce more of the favourable response. Nor does a low concentration measured in irrigation water, by itself, establish a crop deficiency: that conclusion would require evidence linking supply to a relevant plant response, with other iodine sources and growing conditions taken into account.

Seaweeds can contain iodine, but that does not make every seaweed extract a standardized iodine source. The final formulation and its measured composition matter, and responses to a complex extract cannot automatically be attributed to iodine alone.

What this means for growers

Iodine research gives growers a reason to examine plant nutrition beyond the familiar essential-element list, but not a reason to diagnose iodine deficiency from a photograph or add iodine to every nutrient programme. Poor growth still requires a broader assessment of watering, root-zone conditions, established nutrient supply, light, temperature, pests and disease. The practical sequence in reading the plant before adding anything remains relevant.

For cannabis, results from Arabidopsis, tomato, pepper or lettuce do not establish a crop-specific iodine requirement, a safe optimum or an effect on flower yield or cannabinoid production. Those claims require suitable experiments in cannabis rather than extrapolation from another species.

Where an iodine-containing product is being considered, check the existing nutrient programme and the product's declared composition and application guidance rather than copying a research concentration. A meaningful comparison should keep other management conditions as consistent as possible and assess growth and injury as well as the desired outcome.

The evidence supports a measured conclusion: iodine participates in plant-associated chemistry and can benefit particular crops under particular conditions. How widely those benefits apply, which iodinated proteins have a necessary function and whether iodine meets classical essentiality criteria across plants remain open questions.

For the composition and application guidance of Iodine Insight, consult the product information and label. Studies of iodine salts or other formulations do not demonstrate the same effects for this finished product, and increasing the dose is not a reliable route to a better result.

References

Kiferle, C. et al. (2021). Evidences for a Nutritional Role of Iodine in Plants. Frontiers in Plant Science, 12, 616868. https://doi.org/10.3389/fpls.2021.616868.

Nascimento, V.L., Souza, B.C.O.Q., Lopes, G. & Guilherme, L.R.G. (2022). On the Role of Iodine in Plants: A Commentary on Benefits of This Element. Frontiers in Plant Science, 13, 836835. https://doi.org/10.3389/fpls.2022.836835.

Kiferle, C., Gonzali, S., Beltrami, S., Martinelli, M., Hora, K., Holwerda, H.T. & Perata, P. (2022). Improvement in fruit yield and tolerance to salinity of tomato plants fertigated with micronutrient amounts of iodine. Scientific Reports, 12, 14655. https://doi.org/10.1038/s41598-022-18301-w.

Maglione, G., Vitale, E., Costanzo, G., Polimeno, F., Arena, C. & Vitale, L. (2022). Iodine Enhances the Nutritional Value but Not the Tolerance of Lettuce to NaCl. Horticulturae, 8(7), 662. https://doi.org/10.3390/horticulturae8070662.

Hora, K. & Holwerda, H.T. (2023). Loss of yield or lower fruit quality due to iodine deficiency in tomato and sweet pepper crops under heat stress can be prevented by application of potassium nitrate with iodine. Acta Horticulturae, 1372, 251–258. https://doi.org/10.17660/ActaHortic.2023.1372.33.

Akınoğlu, G., Kiremit, M.S. & Rakıcıoğlu, S. (2025). Exploring the Potential of Iodine in Mitigating Salinity Stress: Effects On Yield, Physio-biochemical Traits, and Fruit Quality of Sweet Pepper (Capsicum Annuum L.). Journal of Crop Health, 77. https://doi.org/10.1007/s10343-025-01152-6.