PROPOSED PHYSIOLOGICAL FUNCTIONS OF BORON IN PLANTS PERTINENT TO ANIMAL AND HUMAN METABOLISM

Boron has been recognized since 1923 as an essential micronutrient ele ment for higher plants. Over the years, many roles for boron in plants have been proposed, including functions in sugar transport, cell wall synthesis and lignification, cell wall structure, carbohydrate metabo lism, RNA metabolism. respiration, indole acetic acid metabolism, phen ol metabolism and membrane transport. However, the mechanism of boron involvement in each case remains unclear. Recent work has Focused on t wo major plant-cell components: cell walls and membranes. In both, bor on could play a structural role by bridging hydroxyl groups. In membra nes, it could also be involved in ion transport and redox reactions by stimulating enzymes like nicotinamide adenine dinucleotide and reduce d (NADH) oxidase. There is a very narrow window between the levels of boron required by and toxic to plants. The mechanisms of boron toxicit y are also unknown. In nitrogen-fixing leguminous plants, foliarly app lied boron causes up to a 1000% increase in the concentration of allan toic acid in leaves. in vitro studies show that boron inhibits the man ganese-dependent allantoate amidohydrolase, and foliar application of manganese prior to application of boron eliminates allantoic acid accu mulation in leaves. Interaction between berate and divalent cations li ke manganese may alter metabolic pathways, which could explain why hig her concentrations of boron can be toxic to plants.