Boron (B) is an essential micronutrient in higher plants, although it is toxic in excess. In soils, the concentration range between B deficiency and toxicity is generally narrow and differs among crops. Both these stress conditions (B deficiency and toxicity) severely reduce crop yield and quality worldwide. In particular, B toxicity is more difficult to manage agronomically and can be dealt with by using B-tolerant crop varieties. A typical symptom of B toxicity is the appearance of chlorotic and/or necrotic spots at the margins and tips of older leaves. Although much evidence indicates that several key cellular processes are sensitive to B toxicity, the molecular mechanisms are not fully understood. The B tolerance is most commonly associated with the ability to maintain low B concentration in shoots. Indeed, B-tolerant varieties differ from sensitive ones by their ability to exclude B more effectively from roots and to translocate less to shoots. In this chapter, we try to explain B chemistry in soil, root uptake, translocation, and the metabolic processes in which B is involved. In addition, a molecular mechanism regarding tolerance of B toxicity and their potential utility in plant-breeding programs is also discussed.
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