Boron

Boron in the soil

The boron content of soils in humid climates ranges between 5-80 mg kg^-1. Soils rich in sand typically contain a lower boron content (5-20 mg kg^-1) than soils rich in clay and organic matter (typically 30-80 mg kg^-1). In saline soils, boron concentration may be so high that it can reach levels that are toxic to plants. Boron is present in the soil solution in the form of boric acid (H₃BO₃) which is produced during weathering of mica and tourmaline. Boric acid dissociates above pH 6.3 and the negative charge of the anion produced,is attracted to the positive surfaces of iron and aluminium oxide, clay minerals and organic substances thus limiting availability to the plant. Since boron is taken up with the soil water, boron deficiency mainly occurs during dry periods.

Boron in the plant

Boron belongs to the group of essential micro-nutrients and affects many processes in the plant metabolism. The requirement of the various crops for boron is very different. For example, monocotyledonous plants such as cereals generally have a lower requirement for boron than dicotyledonous crops. This is thought to be due to key differences in the cell wall structures of the two groups. Boron is taken up by plants mainly in the form of boric acid.

Functions of boron in the plant

Promotes synthesis of structural carbohydrates in the cell wall.
Improves stability and function of cell membranes.
Enhances sucrose synthesis and transport of assimilates to storage organs.
Regulates RNA synthesis, which in turn affects synthesis of nucleic acids and therefore proteins.
Supports plant growth by stimulating cell division.

Boron deficiency symptoms

Deficiency symptoms are visible first on the youngest growing points.
Subsequently, roots and shoot tips die and young leaves wilt probably as a result of an inadequate supply of assimilates as well as the interrupted water supply.
Next the growth of side shoots is increased because of the lack of apical dominance.
Flower formation and fertilization are affected.
Transpiration is increased and the water balance is negatively affected.
Rhizobia development in the roots of legumes is inhibited.
Typical deficiency symptoms in beet and chard include brown heart and dry rot. In alfalfa, tip yellowing is commonly seen.

Boron surplus in the plant

Yellowing of older leaves which results in necrotic and perforated tissue.
Cucumbers and legumes are very susceptible as the range between adequate supply and toxicity is very narrow.