Boron deficiency (plant disorder)

This article is about boron deficiency in plants. For the same deficiency in animals, see boron deficiency (medicine).

Boron (B) deficiency is an uncommon disorder affecting plants growing in deficient soils and is often associated with areas of high rainfall and leached soils. Boron may be present but locked up in soils with a high pH, and the deficiency may be worse in wet seasons. Most of what is known about boron is from the observations of plants grown in boron deficient conditions. The primary role of boron is its involvement in the stabilization of the primary cell walls in plant cells. Boron is also involved in the carbohydrate metabolism in plants, protein synthesis, seed and cell wall formation, germination of pollen grains and growth of pollen tubes and sugar translocation.

Extreme case of boron/calcium imbalance induced "watercore" in apple

Symptoms

Symptoms include dying growing tips and bushy stunted growth, extreme cases may prevent fruit set. Crop-specific symptoms include;

• Apple- interacting with calcium, may display as "water core", internal areas appearing frozen
• Beetroot- rough, cankered patches on roots, internal brown rot.
• Cabbage- distorted leaves, hollow areas in stems.
• Cauliflower- poor development of curds, and brown patches. Stems, leafstalks and midribs roughened.
• Celery- leaf stalks develop cracks on the upper surface, inner tissue is reddish brown.
• Celeriac- causes brown heart rot
• Pears- new shoots die back in spring, fruits develop hard brown flecks in the skin.
• Strawberries- Stunted growth, foliage small, yellow and puckered at tips. Fruits are small and pale.
• Swede (rutabaga) and turnip- brown or gray concentric rings develop inside the roots.
• Arecaceae (Palm Tree) - brown spots on fronds & lower productivity.references/

Soil Conditions

Boron is present in the soil in many form, the most common being Boric Acid (H₃BO₃). An adequate amount of boron in the soil is 12 mg/kg. If the boron content of the soil drops below 0.14 mg/kg then boron deficiency is likely to be observed. Boron deficiency is also observed in basic soils with a high pH because in basic conditions boric acid exists in an undissociated form which the plant is unable to absorb. Soils with low organic matter content (<1.5%) are also susceptible to boron deficiency. Highly leached sandy soils are also characteristic of boron deficiency because the boron will not be retained in the soil. Boron toxicity is also possible if the boron content of the soil is high enough that the plant cannot cope with the excess boron. The levels at which boron is toxic to plants varies with different species of plants.

Boron Requirements

Boron is an essential micronutrient which means it is essential for plant growth and development, but is required in very small quantities. Although Boron requirements vary among crops, the optimum boron content of the leaves for most crops is 20-100 ppm. Excess boron can result in boron toxicity and the toxicity level varies between plants.

Treatment

Boric acid (16.5%boron), borax (11.3% boron) or SoluBor (20.5% boron) can be applied to soils to correct boron deficiency. Typical applications of actual boron are about 1.1 kg/hectare or 1.0 lb/acre but optimum levels of boron vary with plant type. Borax, Boric Acid or Solubor can be dissolved in water and sprayed or applied to soil as a dust. Excess boron is toxic to plants so care must be taken to ensure correct application rate and even coverage. Leaves of many plants are damaged by boron; therefore, when in doubt, only apply to soil. Application of boron may not correct boron deficiency in alkaline soils because even with the addition of boron, it may remain unavailable for plant absorption. Continued application of boron may be necessary in soils that are susceptible to leaching such as sandy soils. Flushing soils containing toxic levels of boron with water can remove the boron through leaching.

Functions

Once boron has been absorbed by the plant and incorporated into the various structures that require boron, it is unable to disassemble these structures and re-transport boron through the plant resulting in boron being a non-mobile nutrient. This results in the symptoms of boron deficiency appearing in the young leaves first.

Cell Wall

Boron is part of the dRG-II-B complex which is involved in the cross linking for pectin located in the primary cell wall and the middle lamella of plant cells. This cross linking is thought to stabilize the matrix of plant cell walls.