Boron toxicity

Whilst of lesser prevalence than B deficient soils, B-rich soils are important, causing B toxicity in the field and decreased crop yields in different regions of the world. The highest naturally occurring concentrations of soil B are in soils derived from marine evaporites and marine argillaceous sediment. In addition, various anthropogenic sources of excess B may increase soil B to levels toxic for plants. The most important source is irrigation water, but others include wastes from surface mining, fly ash, and industrial chemicals. Ameliorating high-B soils is extremely difficult. A commonly used method of reclaiming high B soils is to extensively leach with low B water. Though used successfully, leaching may not be a permanent solution and causes difficulties with the disposal of the leachates. Other amelioration methods include the use of soil amendments (e.g. lime, gypsum) and the planting of plant genotypes that are tolerant of high external B concentrations. Although there are various methods available to determine the levels of B in soils, soil analysis can provide little more than a general risk assessment for B toxicity. Similarly, diagnosing B toxicity in plants, either by visible symptoms or tissue analysis has limited applicability. Thus at present, neither soil nor plant analysis can be recommended to precisely predict the growth of plants on high soil B. Recent physiological and genetic studies have provided some understanding of genetic variation in the response of plants to high concentrations of B. Moreover, these studies have facilitated the breeding of tolerant genotypes for cultivation on high B soils. Considerable genetic variation in response to high B has been identified in a wide range of plant species, most of which share a similar tolerance mechanism - reduced uptake of B in both shoots and roots. The tolerance mechanism appears to be under the control of several major additive genes, and specific chromosomal locations have been identified for the genes in some species. Considerable success has been achieved in breeding for tolerance to B toxicity, a process that is greatly aided by the ease with which genotypic variation for this characteristic can be assessed and the range of methods available to screen breeding populations.