Proposed Physiologic Functions of Boron in Plants Pertinent to Animal and Human Metabolism

Abstract

Boron has been recognized since 1923 as an essential micronutrient element 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 metabolism, RNA metabolism, respiration, indole acetic acid metabolism, phenol metabolism and membrane transport. However, the mechanism of boron involvement in each case remains unclear. Recent work has focused on two major plant-cell components: cell walls and membranes. In both, boron could play a structural role by bridging hydroxyl groups. In membranes, it could also be involved in ion transport and redox reactions by stimulating enzymes like nicotinamide adenine dinucleotide and reduced (NADH) oxidase. There is a very narrow window between the levels of boron required by and toxic to plants. The mechanisms of boron toxicity are also unknown. In nitrogen-fixing leguminous plants, foliarly applied boron causes up to a 1000% increase in the concentration of allantoic acid in leaves. In vitro studies show that boron inhibits the manganese-dependent allantoate amidohydrolase, and foliar application of manganese prior to application of boron eliminates allantoic acid accumulation in leaves. Interaction between borate and divalent cations like manganese may alter metabolic pathways, which could explain why higher concentrations of boron can be toxic to plants. - (1). Since that time, boron nutrition has been studied extensively; and although several roles of boron have been proposed, its exact metabolic function has not been determined. The quantity of boron required by plants and found in their tissues shows clear-cut variation within the plant kingdom. Species from the Gramineae require and contain less boron than the other monocots and all dicots. The Gramineae are also unique in several other ways. Gramineous plants contain large quantities of silicon and much less pectin in their cell walls than other monocots and the dicots (2). Loomis and Durst (3) point out that gramineous plants are uniquely resistant to 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophen-