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Boron (B) is a unique non-metal micronutrient required for normal growth and development of plants. In 1923, it was first time reported that B is essential for cell structure of plants (Warington, 1923). The possible roles of B include sugar transport, cell wall synthesis, lignification, cell wall structure integrity, carbohydrate metabolism, ribose nucleic acid (RNA) metabolism, respiration, indole acetic acid (IAA) metabolism, phenol metabolism, and as part of the cell membranes (Parr & Loughman, 1983; Welch, 1995; Ahmad et al., 2009). In soils, concentration of total B is reported to be in the range of 20 to 200 mg B kg-1 (Mengel & Kirkby, 1987), and its available concentrations also vary greatly from soil to soil.

Boron is absorbed by roots as undissociated boric acid [B (OH) 3 or H3BO3](Mengel & Kirkby, 1982; Marschner, 1995) which has a strong ability to form complexes with diols and polyols, particularly with cis-diols inside the plant system (Loomis & Durst, 1992). Among the elements required by plants that are taken up from the soil, B is the only element that is taken up by plants not as an ion, but as an uncharged molecule (Marschner, 1995; Miwa & Fujiwara, 2010). The factors affecting B uptake include soil type (texture, alkalinity/calcareousness, pH, organic matter content), B concentration, moisture, and plant species (Welch et al., 1991). Boron absorption by plant roots is closely related to pH and B concentration in the soil solution; and is probably a non-metabolic process (Brown & Hu, 1998). The supplying mechanism of B to plant roots is primarily through mass flow, while its distribution in plants is governed by the transpiration stream through …