Boron Deficiency

Boron is a very important micronutrient, playing a crucial role in many physiological and biochemical functions, as well as in plant metabolism. More specifically, B is involved in cell elongation and division, cell wall biosynthesis and structure, N, carbohydrate and IAA metabolism, photosynthesis, as well as in membrane integrity, seed production, sugar metabolism, regulation of lignin biosynthesis etc. Flowering and fruit setting are two of the mostly known functions that are negatively influenced by B deficiency.

Boron deficiency is most probably found in calcareous and alkaline soils, as well as in soils formed on parent materials inherently low in B, such as sandstones. Boron is absorbed from soil solution mainly as undissociated boric acid. Boron uptake is not yet clear as to the extent to which this process is either passive or active; however, the metabolically controlled process seems to be relatively minor. The most possible explanation is that when B supply is high, B uptake by roots is believed to occur by passive diffusion. In contrast, under low B supply, a significant portion of B may be taken up via active pathways. Under low B conditions (leaf concentrations of less than 10 mg/kg dry weight in young sampled expanding leaves), flower sterility and floral abnormalities are within the first symptoms of B starvation. In addition, the formation of incomplete or damaged embryos and malformed fruits are also within the most common and important symptoms of B starvation for plants. Other symptoms of B deficiency include rapid cessation of root elongation, inhibited growth and thickening of young leaves, loss of apical dominance in trees etc.

Under B deficiency conditions some tolerance and adaptation mechanisms, such as enhanced translocation from root system to leaves and lower shoot/root dry weight ratio in resistant genotypes, usually take place. In order to alleviate B deficiency, the most commonly used B fertilizer is borax; nevertheless, its’ basic drawback is that it leaches easily from sandy soils. Other fertilizers that can be used, either as soil, or foliar application, are those of boric acid and solubor 20-21% (leafy sprays are particularly preferred when soil is potentially capable of fixing high amounts of B).

All these topics concerning B availability in soils and the factors influencing it, the uptake and transport of B, the roles of B in plant metabolism and growth, the critical concentrations of B starvation in plant tissues, the symptoms of B deficiency and the methods of its’ detection, as well as the tolerance mechanisms adopted by plants in order to face B deprivation and the B-fertilizers used to alleviate B stress in crops are fully analyzed and discussed in this chapter.

Keywords: B availability, B deficiency, B fertilizers, B mobility, B uptake, borax, boric acid, flower sterility, fruit setting.