BORON TOXICITY

Whilst of lesser prevalence than B deficient soils, B-rich soils are i mportant, causing B toxicity in the field and decreased crop yields in different regions of the world. The highest naturally occurring conce ntrations of soil B are in soils derived from marine evaporites and ma rine 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 s urface mining, fly ash, and industrial chemicals. Ameliorating high-B soils is extremely difficult. A commonly used method of reclaiming hig h B soils is to extensively leach with low B water. Though used succes sfully, leaching may not be a permanent solution and causes difficulti es with the disposal of the leachates. Other amelioration methods incl ude 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 o f B in soils, soil analysis can provide little more than a general ris k assessment for B toxicity. Similarly, diagnosing B toxicity in plant s, either by visible symptoms or tissue analysis has limited applicabi lity. Thus at present, neither soil nor plant analysis can be recommen ded to precisely predict the growth of plants on high soil B. Recent p hysiological and genetic studies have provided some understanding of g enetic variation in the response of plants to high concentrations of B . Moreover, these studies have facilitated the breeding of tolerant ge notypes for cultivation on high B soils. Considerable genetic variatio n in response to high B has been identified in a wide range of plant s pecies, most of which share a similar tolerance mechanism - reduced up take of B in both shoots and roots. The tolerance mechanism appears to be under the control of several major additive genes, and specific ch romosomal 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 gen otypic variation for this characteristic can be assessed and the range of methods available to screen breeding populations.