Boron toxicity tolerance in crops: A viable alternative to soil amelioration

Research on the problems of excessive soil B has increased considerably in the past two decades, especially in the dry areas of the world such as the Mediterranean region and parts of Australia. The objectives of this review are to promote awareness of the widespread occurrence and importance of B toxicity (BT) in dry areas, and to review the availability of BT-tolerant germplasm and progress in breeding cultivars with BT tolerance. The importance of BT was not adequately recognized until the 1980s, when scientists discovered that BT caused significant crop yield reductions in South Australia. We offer several reasons for this belated awareness before describing the areas reported to have high-B soils in the world and reviewing the occurrence of two contrasting types of BT symptoms. In the field, BT in crops usually is more prominent after drought, indicating that both BT and drought tolerance are needed in crops for dry areas having high levels of subsoil B. The interaction of BT with salinity and the levels of other nutrients such as Zn and N are also discussed. As it is neither practical nor easy to detoxify high-B soil by agronomic means in most circumstances, selecting or breeding crop cultivars with high BT tolerance is the only practical approach to increase yields on high-B soils. Extensive surveys of germplasm in different crops have been performed, and a list of some BT-tolerant lines or cultivars is presented. Finally, we review the progress in breeding for BT tolerance, which has been achieved with varying success in several common crops. We believe that the shift from soil intervention to plant adaptation to solve an intractable crop nutrition constraint represents a new paradigm in the agronomic sciences.