The Mechanism of Boron Immobility in Plants

Abstract

Abstract A mechanism is postulated to explain the immobility of boron in plants, i.e. , the absence of any significant retranslocation of this element from one leaf to another. It is shown that boron readily enters the bark and is translocated within the bark. Since it has also been shown that boron remains water‐soluble in plants, the immobility cannot be explained through a chemical fixation, lack of entry into phloem, or absence of phloem transport. Rather, boron enters the phloem in leaf margins where concentrations are high, is transported in these conduits, is lost therefrom where the xylem concentration is low, i.e. in basal areas of the leaf and in petiole, is transported back in the xylem and accumulates in terminal place of the transpiration stream. A high local mobility of boron, together with the essentially unidirectional flow of the transpiration stream, thus cause a cyclic movement of B and prevent the efflux of this nutrient; this explains the immobility over long distances.