We have demonstrated that boron (B) is freely phloem mobile in a number of crop species and we predict that B will be mobile in all species that transport polyols (mannitol, sorbitol, dulcitol). This finding directly contradicts accepted dogma and profoundly influences the diagnosis and management of B in almond, apple, apricot, cherry, pear, peach, plum, prune, celery, and other species. In the majority plants, B moves in the xylem with the transpiration stream. Once B enters the leaf, it remains there with little or no redistribution. As a result, there is always a decreasing concentration gradient of B from old to young leaves and B toxicity symptoms always occurs in the old leaves first, typically exhibiting tip and margin burn. In species in which B is mobile, these symptoms do not occur. When almond, peach, and plum were exposed to high B in the growth medium, the predominant site of B accumulation was fruit, young stems and apical meristems. As a consequence, the earliest symptoms of B toxicity in species in which B is phloem mobile are observed in the young shoot meristems and fruits. Foliar application of 10B isotope demonstrates that B is readily transported to neighboring fruits and buds of almond, apple, and nectarine. In apple seedlings, plant B requirements can be fully satisfied solely by foliar application to a few mature leaves. This strongly suggest that foliar B applications can be used as an efficient means for B fertilization in Malus, Prunus, and Pyrus species. Details of the studies and their implications for B management will be discussed.
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