Challenging the dogma of a narrow supply range between deficiency and toxicity of boron

Abstract

It is generally accepted that there is a narrow range of boron (B) supply between B deficiency and B toxicity in plants. In this study, five temperate crop species and cultivars were grown in flowing solution culture for 28 to 30 days at carefully maintained solution B concentrations from 0.15 to 640 µM. A no added boron (NBA) treatment was also included. Most of the species grew poorly in the NBA treatment, with relative whole plant dry matter yields ranging from 1.8% for barrel medic (Medicago truncatula)to 89% for wheat (Triticum aestivum). However, a solution B concentration between 0.15 and 2.5 p.M was sufficient to produce maximum yield in all species and cultivars. Yield reduction due to B toxicity became apparent at 160 µM, and at 640 µM B relative yield ranged from 35% for barrel medic to 83% for field pea (Pisum sativum). Hence, there was a wide range of solution B concentrations over which maximum or near-maximum yield was produced. A lack of sensitivity in internal B concentration to solution B concentration between 0.15 and 40 µM was consistent with the operation of a B absorption regulation mechanism. There was no evidence to support the notion that there is a narrow range of B supply between B deficiency and B toxicity.