BORON MOBILITY IN PLANTS

Boron (B) is a micronutrient essential for the normal growth of monoco ts, dicots, conifers, ferns and several diatom species. Boron deficien cy causes many anatomical, physiological and biochemical changes, maki ng it difficult to identify a primary role for it; however, evidence d oes indicate that B is involved at the membrane level. Whatever the ro le(s), it likely involves the complexation of B with compounds contain ing cis-hydroxyl groups. Boron deficiency in crops is more widespread than deficiency of any other micronutrient. Nutritional disorders in v egetables include brown heart in rutabaga, turnip and radish roots, an d hollow stem in cauliflower and broccoli. The occurrence of these dis orders even when B is in ample supply suggests that they are physiolog ical in nature and related to the mobility of B in the plant. The dist ribution of B is related to the loss of water from shoot organs, sugge sting that it is primarily xylem-mobile with limited retranslocation i n 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 readil y transpire. Further progress into the mechanism(s) of B retranslocati on will be facilitated by insights into the role and metabolism of B i n plants.