Boron deficiency is a common deficiency of the micronutrient boron in plants. It is the most widespread micronutrient deficiency around the world and causes large losses in crop production and crop quality.[1] Boron deficiency affects vegetative and reproductive growth of plants, resulting in inhibition of cell expansion, death of meristem, and reduced fertility.[2]
Plants contain boron both in a water-soluble and insoluble form. In intact plants, the amount of water-soluble boron fluctuates with the amount of boron supplied, while insoluble boron does not. The appearance of boron deficiency coincides with the decrease of water-insoluble boron. It appears that the insoluble boron is the functional form while the soluble boron represents the surplus.[3]
Boron is essential for the growth of higher plants. The primary function of the element is to provide structural integrity to the cell wall in plants. Other functions likely include the maintenance of the plasma membrane and other metabolic pathways.[4]
Symptoms include dying growing tips and bushy, stunted growth; extreme cases may prevent fruit set. Crop-specific symptoms include:
Boron is present in the soil in many forms, 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.[5] 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.[6] 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.[7]
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.[8] Excess boron can result in boron toxicity and the toxicity level varies between plants.[7]
Boric acid (16.5% boron), borax (11.3% boron) or Solubor (20.5% boron)[9] 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.[6] Borax, Boric Acid or Solubor can be dissolved in water and sprayed or applied to soil in blended dry fertilizer. Excess boron is toxic to plants so care must be taken to ensure correct application rate and even coverage.[7] While boron may be sprayed on leaves, excess will cause plant damage. 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.[5][6] Continued application of boron may be necessary in soils that are susceptible to leaching such as sandy soils.[6] Flushing soils containing toxic levels of boron with water can remove the boron through leaching.[5]
A boron-containing granular potash fertilizer, called Aspire,[10] has been developed to evenly distribute boron through potassium chloride granules. This product contains two forms of boron - sodium borate for quick release and calcium borate for gradual release - to ensure season-long boron availability. These boron-infused potash granules prevent localized boron toxicity, while the granular size allows it to be distributed by common fertilizer equipment along with typical granular NPK fertilizer blends.
Once boron has been absorbed by the plant and incorporated into the various structures that require boron, the plant is unable to disassemble these structures and re-transport boron through the plant resulting in boron being a non-mobile nutrient. Due to translocation difficulties the youngest leaves often show deficiency symptoms first.[5]
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.[11] This cross linking is thought to stabilize the matrix of plant cell walls.[11]
The B requirement is much higher for reproductive growth than for vegetative growth in most plant species. Boron increases flower production and retention, pollen tube elongation and germination, and seed and fruit development.[12]
A deficiency of B can cause incomplete pollination of corn or prevent maximum pod-set in soybeans.[12]
Photosynthesis transforms sunlight energy into plant energy compounds such as sugars. For this process to continue in plants, the sugars must be moved away from the site of their development, and stored or used to make other compounds.[12]
Boron increases the rate of transport of sugars (which are produced by photosynthesis in mature plant leaves) to actively growing regions and also in developing fruits.[12] Boron is essential for providing sugars which are needed for root growth in all plants and also for normal development of root nodules in legumes such as alfalfa, soybeans and peanuts.[12]