Boron toxicity and tolerance in crop plants

Boron toxicity is a common problem for many crop plants, especially those growing on soil with high levels of boron and low rainfall. Boron is transported through the plant in the xylem and boron not absorbed by other tissues is deposited at the end of leaf veins where necrosis develops. Inhibition of growth occurs at much lowerconcentrations than those required to cause necrosis. The causes of boron toxicity are poorly understood, but most probably involve disruption of genetic processes, such as transcription and translation. A wide variation in tolerance to boron toxicity has been reported for different crop species but also for different cultivars of the same species. Most of our current understanding of mechanisms of tolerance to high boron levels has come from physiological and molecular studies on cereals. The dominant tolerance mechanism is mediated by active efflux of boron from roots, and the genes encoding the efflux transporters in wheat and barley have recently been identified. At least one other gene, unrelated to membrane transporters, has been shown to be able to alter the sensitivity of some plants to high levels of boron. Attempts to improve crop yields by exploiting these tolerance mechanisms, have largely been unsuccessful because boron-toxic soils often contain other stresses such as high salinity or low moisture content that are more limiting to growth.Robert J. Rei