Boron mobility in plants.

Boron (B) is a micronutrient essential for the normal growth of monocots, dicots, conifers, ferns and several diatom species. Boron deficiency causes many anatomical, physiological and biochemical changes, making it difficult to identify a primary role for it; however, evidence does indicate that B is involved at the membrane level. Whatever the role(s), it likely involves the complexation of B with compounds containing cis-hydroxyl groups. Boron deficiency in crops is more widespread than deficiency of any other micronutrient. Nutritional disorders in vegetables include brown heart in rutabaga, turnip and radish roots, and hollow stem in cauliflower and broccoli. The occurrence of these disorders even when B is in ample supply suggests that they are physiological in nature and related to the mobility of Bin the plant. The distribution of B is related to the loss of water from shoot organs, suggesting that it is primarily xylem-mobile with limited retranslocation in phloem. However, research has shown that B is present in the phloem, albeit at low concentration, and that it is generally retranslocated in the phloem to satisfy the demands of sink organs that do not readily transpire. Further progress into the mechanism(s) of B retranslocation will be facilitated by insights into the role and metabolism of B in plants. [ABSTRACT FROM AUTHOR]