Fertilizer International 509 Jul-Aug 2022
31 July 2022
Iodine: the newly recognised plant nutrient
NEW PRODUCTS
Iodine: the newly recognised plant nutrient
New research findings strongly suggest that iodine behaves as a plant nutrient. SQM International has been quick to follow up on this discovery by launching a new speciality iodine fertilizer for fertigated crops.
Breakthrough research
“Results are strongly suggestive of the role of iodine as a plant nutrient.” That was the main conclusion of a landmark paper published by Italian scientists last year1 .”
It has long been known that iodine is essential for human and animal health. But these researchers have now demonstrated that plants need micro doses of iodine as well. For the first time, they identified and described the presence of 82 naturally-occurring iodine-containing proteins in higher plants.
Their research has shown – based on phenotyping, genomics and proteomics studies – that plants need iodine for
- Leaf and root growth
- Efficient photosynthesis
- Timely flowering
- Increased seed production
- The activation of an early warning system that defends the plant against damage from abiotic and biotic stress.
Iodine deficiency may also cause lower crop yields and poorer fruit quality, particularly in growing regions where the soil and water are naturally low in iodine.
To make it easier and safer for growers to provide the right source of iodine, at the right dose and at the right time, commercial products are now available that combine iodine with potassium nitrate in a single speciality fertilizer. This is a useful combination as the plant’s iodine demand is well-synchronised with the uptake of nitrate and potassium from nutrient solutions.
Breakthrough product
SQM, the leading global speciality fertilizer producer, has been quick to follow up on these latest discoveries. In response to new information highlighting the importance of iodine as an essential plant nutrient, the Chilean-based company has developed a speciality fertilizer with iodine for fertigated crops. This allows growers to apply iodine as a plant micronutrient in a form that is guaranteed to be safe and at an effective science-based dose.
The newly-launched product, known as Ultrasol® ine K Plus, combines two essential plant macronutrients – potassium and nitrate nitrogen – with iodine. The product ensures that they are applied at well-defined application rates. This makes it easy for the grower to maintain an effective and safe concentration of iodine in the root zone. As a result, Ultrasol® ine K Plus can prevent iodine deficiency in crops without the risk of excessive iodine application.
The product has already been extensively tested globally and is backed by more than 100 well-documented trials with growers. The experience of these growers has confirmed that iodine can deliver distinct benefits – including improvements to:
- Root growth
- Above ground plant growth
- Photosynthesis
- Nitrogen metabolism
- Tolerance to abiotic stress
- Flowering and fruit quality with less fruit rot and better shelf life.
Typically, the application of iodine to crops delivers 10 percent more marketable yield. This is the average yield improvement from trials on 34 farms located in nine countries with coverage of 10 different crops. These trials compared Ultrasol® ine K Plus to potassium nitrate without iodine for the same crop, on the same planting date with the same fertilizer programme. Crops included: tomato, lettuce, sweet pepper, cucumber, musk melon, sugarcane, pomegranate, papaya, banana and coffee.
Overall, the trials demonstrated that Ultrasol® ine K Plus enables iodine to be easily applied and improve crop performance – with this leading to higher yields, improved quality and therefore better revenues. The product will be available on the European market from mid-July 2022.
Iodine as a new plant nutrient
Previously, elements classed as plant nutrients include:
- Primary nutrients (C, H, O, N, P, K)
- Secondary nutrients (Ca, Mg, S)
- Micronutrients (Fe, Zn, Cu, Mn, B, Cl, Mo, Co and Ni).
Research suggests that this list of plant nutrients can now be expanded to include iodine. This would be the first new micronutrient to be added since nickel in 1987.
The case for classifying iodine as a crop nutrient was made in a recently published paper by a mainly Italian team of plant scientists1 . This was led by Professor Pierdomenico Perata and Dr Claudia Kiferle of the Plant Lab at the Institute of Life Sciences, Scuola Superiore Sant’Anna, Pisa, Italy. Other scientists from Italy’s National Research Council were also involved. Katja Hora and Harmen Tjalling Holwerda from SQM International also participated. The research was jointly funded by the Scuola Superiore Sant’Anna and SQM.
The scientists concluded that1 : “Iodine has a nutritional role in plants. Considering that plant nutrients are chemical elements that are components of biological molecules and/or influence essential metabolic functions. Further studies… will help to complete the picture on the functional role of iodine as a plant nutrient.”
“Crop trials have demonstrated that Ultrasol®ine K Plus leads to higher yields, improved quality and therefore better revenues.”
This compelling evidence for classifying iodine as a plant nutrient (Figure 1) is backed by a new scientific definition recently proposed by leading scientists2 .
The paper by Dr Kiferle and colleagues has made public the new discovery that plants bind iodine in 82 different proteins. These proteins have an important role in biological processes and compounds such as:
- The protein rubisco for efficient photosynthesis in leaves
- Peroxidase enzymes which defend the plant from abiotic and biotic stress
- The enzyme ATPase which supplies energy for plant growth and the transport of nutrients.
Consequently, iodine deficiency in crops is expected to cause yield losses, like those experienced by plants suffering from other micronutrient deficiencies. This new research also suggests that plants will need to be supplied with the right dosage of iodine to optimise crop production.
Plants can accumulate iodine
The presence of iodine in the biosphere is widespread, albeit usually in tiny quantities. The highest amount of iodine is found in seawater, with an average concentration of 0.5 micromole per litre. Rainwater, soil solutions and irrigation water, in contrast, contain lower concentrations, typically less than 0.2 micromole per litre. Furthermore, less than 10 percent of the total iodine present in soils is usually available for plant uptake.
It has been known for a long time that plants do take up iodine through their roots and store this in their leaves and fruits. Indeed, its benefits, in terms of plant growth and in providing resilience to stress, have been observed in many previous studies. Having reviewed the published evidence, the Italian scientists agreed that plants accumulate iodine because of its benefits to1 :
- Plant growth
- Nitrogen metabolism
- Resistance to salinity stress in the root solution
- And, in particular, the production of antioxidants by the plant.
Providing both the right dose (not too little, not too much) and the right form of iodine is also important. For example, the type of iodine present in disinfectants (free iodine, I2 , and iodide, I-) may have harmful effects at a lower dose compared to other forms.
Why do plants need iodine?
Until now, iodine’s value as a plant nutrient – when applied at the right dose – has not been fully recognised, despite the published evidence. Perceptions are, however, beginning to change following the publication of the landmark paper by Dr Claudia Kiferle and colleagues1 .
They described the results of experiments carried out on Arabidopsis thaliana. This plant species is quick to grow in the laboratory – its growing cycle taking only six weeks from seed-toseed. Arabidopsis thaliana has also been extensively studied. This means that knowledge about its genes and metabolism is freely available and widely shared by plant scientists globally.
The Italian researchers found that plant growth and blooming were much slower in the absence of iodine compared to plants that were given a dosage of 0.2-10 micromole of iodine per litre. Experiments suggest that the application of iodine at micromolar concentrations increases root and shoot growth, seed production and advanced flowering.
The scientists also investigated the genetic response of the plant to the presence or absence of iodine in the root solution. They discovered that iodine specifically regulates the expression of several genes involved in:
- Photosynthesis
- The salicylic acid (SA) stress response pathway
- Plant hormone response
- Ca2+ signalling
- Plant defence to pathogen attack.
These results confirm and further explain the previously published observation that iodine helps prevent plant damage from biotic or abiotic stress. Repeat experiments with bromine, a similar type of halogen element, also demonstrated that the observed plant growth and gene responses were unique to iodine1 .
Finally, a series of experiments using radioisotope labelling showed how iodine is incorporated in plant proteins. These include the enzymes or structural building blocks that are needed for all cell functions, as well as for collaboration and communication between cells and between plant organs.
A total of 82 iodinated proteins were identified in Arabidopsis thaliana. In plant shoots, these are mainly associated with the chloroplast and are involved in photosynthesis. Those in the roots, meanwhile, are essential for root growth and include peroxidase enzymes (important in stress-signalling) or enzymes involved in peroxidase activity. Other iodinated proteins were discovered with a crucial role in nitrogen metabolism, phytohormone regulation and energy production in both root and leaf cells1 .
These iodine-containing proteins occur widely in the plant kingdom. They were discovered not only in Arabidopsis thaliana, but also in tomato, maize, wheat and lettuce, for example1 .
These latest research findings open a new window on the pivotal role of iodinated enzymes. This is undoubtedly a discovery that will trigger renewed interest in the importance of iodine in crop production.
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