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Boron Functions in Plants, Sources, Mobility, Deficiency, and Toxicity

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Boron is one of the essential micronutrients that plants need, whose essentiality for higher plants was recognized by 1923. Recent studies have also found it critical in ferns and algae.

Today, we will talk about boron functions and tell you some of the plants where it is mobile and those where it is immobile. We will also give you some of the sensitive plants, moderately tolerant and very tolerant to boron.

Afterward, we will give you boron sources, including organic ones, deficiency and toxicity symptoms, and fertilizers.

Boron in plants toxicity signs
Boron deficiency signs in plants: Photo credit: Malcolm Manners from Lakeland, FL, USA, Wikimedia, CC BY 2.0.


Boron (B), atomic number 5, is a chemical element and one of the essential plant micronutrients. In nature, it does not exist in elemental form. Instead, it exists in combined forms like boric acid, kernite, ulexite, borax, borates, or colemanite.

Note that soluble boron is mobile in the soil, i.e., it moves up as the water evaporates or leaches down when you irrigate soil with fresh water.


Plants absorb boron as boric acid H3BO3 (uncharged leaches easily) or borate, i.e., H2BO3 (hydrogen borate) or HBO32- (dihydrogen borate) via roots with some foliar formula available.

Is boron mobile in plants or not?

Yes. Boron is mobile in the xylem transport systems of all plants, i.e., as plants transpire, this nutrient can move from roots through the xylem (transpiration stream) to various plant tissues that need it. However, in most plants, it is immobile, while in a few, it is mobile. Sounds confusing? It shouldn’t.

a). Plants where boron is immobile

Boron is immobile in most plants, i.e., such plants cannot move it via the phloem transport system. Therefore, once it reaches and is incorporated into tissues, it cannot move or be remobilized to another plant tissue that needs it.

Plants where it is immobile produce fewer amounts of polyols. Also, the plants will have the highest concentration of older leaves at the base since they have transpired more.

Examples of plants where boron is immobile include pecans, strawberries, tomatoes, alfalfa, beans, lettuce, potato, pistachio, corn, cotton, walnut, sorghum, sugar beets, peanut, wheat, tobacco, soybean, fig, etc.

b). Plants where boron is mobile

Boron is highly mobile in plants that produce enough polyols (simple sugars) in primary photosynthetic processes. Such plants can mobilize it from one tissue to another where it requires it. These sugars bind with this nutrient forming a complex that can move in the phloem.

Examples include a relatively higher number of nut and fruit plant species, including apples, grapes, apricot, olive, pear, peach, pomegranates, loquats, plums, cherries, nectarines, and almonds. Other plants in this category include canola (limited mobility) and coffee.

Also, it is mobile in vegetables like rutabaga, pea, celery, cauliflower, broccoli, carrot, radish, onion, and asparagus.

Lastly, since it is mobile, a deficiency in the soil will not show in growing apical. Instead, these plants will continue utilizing soluble portions in mature leaves until depletion. Then signs will start showing.

What does boron do for plants – functions or effects  

It is one of the essential micronutrients plants require for optimum growth and development. Also, it is necessary for the best crop quality and yield (fruits, tubers, or seeds) and better resistance to diseases or pests.  

The main functions include:

1. Part cell membrane structure and helps improve its integrity 

Most of the boron in leaves (95-98%) is in cell walls, forming part of the cell wall structure that controls porosity and tensile strength. Also, it participates in protein and enzyme functioning in the cell membrane and ensuring its integrity.

2. Aids in calcium movement in plants, K and P absorption

Besides being involved with calcium in the cell wall structure, it helps in calcium movement into and within plants. Also, boron helps in the absorption of phosphorus and potassium, and the transport of potassium to stomata

3. Needed in cell division or growth sites

Actively growing plant regions where cell division is high, like root tips or where new leaves, branches, or buds develop, require boron. It will ensure conductive (for moving water, organic compounds, or nutrients) and storage tissues remain healthy.

Also, it is essential for nodule growth and nitrogen fixation in legumes. Also, it plays a role in mycorrhizal fungi root colonization which will improve phosphorus uptake.

A deficiency will affect roots in species where this nutrient is not mobile as they can only get it from the soil by adsorption, not the xylem transport system.

4. Increases sugar transport rates

Boron increases the sugar transport rate from where they form during photosynthesis to storage, development sites, or sites for making other compounds. For instance, in bolstering sugar transport, it helps ensures it is available in roots that need it for growth, and legumes can develop normal nodules.

Without it, sugar transport from photosynthetic sites will slow, leading to an accumulation that will affect the process.

5. Promotes flowering and fruiting

Boron helps plants to produce and retain flowers. Also, it increases the pollen tube’s elongation, seed germination, and development of seeds/fruits. For these reasons, plants need more for their reproductive growth than any vegetative part.

6. Helps regulate plant hormones

Boron has a role in regulating plant hormone levels. These hormones regulate growth and reproduction functions, including flowering, fruit development, tissue formation, root elongation, formation of tissue, etc.

7: Protein and RNA formation  

It helps in protein synthesis and the formation of genetic material (RNA and DNA). It helps in uracil synthesis (nitrogenous base) necessary for ribosome formation, which without protein synthesis will be deficient.

Sources of boron for plants?

The primary natural or organic source of boron for plants is decomposing organic matter, including manure and worm castings. Also, some amount in the soil is from rock and minerals (mostly insoluble).

Other common sources include irrigating water and using agricultural fertilizers that contain boron, especially those intended to manage deficiency.

Boron-sensitive and tolerant plants

Here are some of the boron-sensitive, moderately tolerant, and highly tolerant plants:

  • Boron sensitive/low tolerance: Barley, beans, cucumber, grasses, melon, peas, pecan, peppers, pineapple, pumpkin, rice, rubber, rye, soybeans, squash, squash strawberry, sugarcane, sweet potatoes, and wheat, among others
  • Intermediate or moderately boron tolerant: Asparagus, carrot, citrus, clovers, coastal Bermuda grass, corn, cucurbits, eggplant, flax, grain sorghum, herbs, leeks, lettuce, okra, onions, parsnip, peaches, pears, potatoes, radish, strawberries, sweet corn, tea, tobacco, tomatoes, turf, etc.
  • Boron-tolerant plants: They include alfalfa, apples, beet, bird’s foot trefoil, broccoli, canola (oilseed rape), carnation, carnations, cauliflower, celery, chrysanthemums, coffee, collard, cotton, eucalyptus, and forage turnip. Others are gladiolus, grapes, kale, mangold, oil palm, olive, peanut, pine, rape, rutabaga, spinach, sugar beets, sunflower, swede, swiss chard, table beets, and turnip, among others.

Boron deficiency in plants

Using fertilizers or irrigation water low in this nutrient, extreme pH, drought, waterlogged soils, sandy soils, high humidity, overfarming, low organic matter, salinity, cold root zone, and high calcium may result in boron deficiency in plants.

Deficiency symptoms will first show the vegetative terminal growth points (root tips, stem tips, buds, and new expanding leaves), flowers, and fruits.

Symptoms include stunted growth, death of terminal buds, short internodes, and stubby and distorted new growth. Also, leaves may curl, harden, thicken, wrinkle, or have twisted tips, and flowering will reduce, seeds fail to set, fruits distorted, etc.

Some boron deficiencies may resemble brown spots due to calcium deficiency or twisted leaves due to tobacco mosaic disease.

To correct the deficiency, amend soil pH to 6.0-6.5 and salinity, and use tap water if you use filtered or reverse osmosis. Also, reduce stressors (raise the humidity to at least 25%, water plants well, maintain ideal temperature), add organic matter, or use boron fertilizers.

Boron fertilizers for plants

A soil and tissue test will confirm the extent of deficiency and help you determine the right boron fertilizer dosages or application rates. Usually, the sensitive plants’ application rate is 0.5 to 1 lb. per acre; for tolerant plants, you can apply up to 3 lb. per acre.

You can go for granular or liquid fertilizers, including foliar boron fertilizers (good for soils with high pH) that you apply on leaves, flowers, and fruits. Besides the foliar application, you can opt for broadcast, fertigation, side dressing, and banding. We don’t recommend banding unless you cultivate the land before seeding.

If you are looking for the best boron fertilizer, US Borax (Based in California) has various products, including solubor (foliar), Granubor, Fertibor, Solubor Flow, Zincubor, and Anhybor.

Other excellent global brands include OMEX Foliar, OrganiBOR, Foliar-Pak, WUXAL, Active Boron, Aspire, etc.

Boron toxicity in plants

This micronutrient is indeed vital to all plants. However, excessive amounts, i.e., soils with more than 1 ppm B for sensitive and up to 2 ppm for tolerant plants or irrigation with more than 0.3 ppm/L B for sensitive and up to 2 ppm/L for tolerant, will result in toxicity.

Toxicity is common in arid and semi-arid soils with low rainfall, accumulation from boron-rich irrigation water/fertilizers, and in saline soils with boron-rich parent materials.

Symptoms of boron toxicity in plants where it is immobile will first show on the lower leaves. They will include yellowing followed by browning (necrosis) starting from leaf margins and tips moving inward.

Symptoms will show new or newly expanding leaves, buds, fruits, and flowers for plants where it is mobile. They will include stunted growth, bud or twig dieback, misshapen fruits, leaf axil gumming, etc.

Other general symptoms include leaf wrinkling, deformation, thickening, curling, and bark cracking or corking.

Once a diagnostic test confirms your plants suffer from this toxicity, remedies include switching to boron-resistant plants, leaching the soil, and pausing any boron fertilizers.

If you use underground or reclaimed irrigation water, switch to surface or municipal water. Also, consider bringing in high-quality topsoil if your landscape is high in this mineral.

Frequently asked questions

1. What is the ideal boron requirement for plants?

The optimum boron content for plant leaves ranges between 20-100 ppm and soil 0.5-2.0 ppm. However, exact amounts vary from one plant, i.e., it depends on individual plant requirements.

What plants need boron?

All plants need it, i.e., it is an essential nutrient required for optimum growth, development, best yield, and resistance to diseases or pests. However, how much each plant needs may vary.

How do plants get boron?

Usually, boron is absorbed by plant roots from the soil and then moves up to various plant tissues via the xylem transport system (transpiration stream). However, plants can also get it via leaves in foliar application.