Discussion Paper: Boron — How can the Critical Level be Defined?

The correct diagnosis of boron deficiency is not yet completely resolved for species where boron is phloem immobile. As some deficiency reactions in the extension zone of young roots cannot be reversed after only one hour of deficiency, boron should be supplied continuously. Several early deficiency reactions are shown to be related to the concentration of ”free” boron. It is suggested that there is a ”minimum” or ”critical level” of soluble boron (i.e. not tightly bound to rhamno‐galacturonan II) which is needed in the most boron requiring tissues to avoid deficiency reactions. Determination of this ”soluble” or ”exchangeable” boron fraction might improve the diagnosis of boron deficiency.