Dg. Blevins et Km. Lukaszewski, PROPOSED PHYSIOLOGICAL FUNCTIONS OF BORON IN PLANTS PERTINENT TO ANIMAL AND HUMAN METABOLISM, Environmental health perspectives, 102, 1994, pp. 31-33
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.