CASE STUDY 16.1  Boron toxicity and Australian agriculture

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Jeff Paull

Boron (B) is an essential micronutrient, but is potentially toxic to plants over much of southern Australia where high concen-trations of soil B reduce crop yields, especially in sensitive varieties.

Toxic levels of soil B can derive from over-application of B when correcting a deficiency, contamination in industrial and sewerage sludges, high concentrations in groundwater used for irrigation and high levels in parent material, including marine sediments. Seawater normally contains appreciable levels of B and as a consequence deposits that remain after ancient seas recede contain high levels of B and other soluble salts. As these deposits weather, B is released and slowly redistributed by leaching.

Boric acid, B(OH)3, is the main form of free B in soil, and of much lower solubility than the accompanying sodium chloride in soils derived from these marine sediments. As a consequence, B has not been leached from root zones in low- rainfall environments of southern Australia. In addition, hydroxyl groups of boric acid react with clay particles and remain bound under alkaline conditions, so that leaching is further decreased. Geographic regions with high soil B usually correspond to areas of marine sediments, rich in clay and subject to low rainfall.

Soil B in southern Australia has been determined from analysis of soil cores, plant shoots, harvested grain and from observation of B toxicity. Visible symptoms on leaf tips of barley include distinctive black spots within dead leaf tissue. High-B soils occur on Eyre Peninsula, mallee regions of South Australia and Victoria and low-rainfall regions of inland southeastern Western Australia. B toxicity is very widespread, and levels are generally highest in subsoils rather than root zones. Soil amelioration is thus an impractical solution, so that most research efforts have been directed towards selection of B-tolerant crop varieties.

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Figure 1 Dry matter yield of shoots of five varieties of wheat grown in a pot experiment with soil at seven levels of B (Based on Paull et al. 1991)

Genetic variation in crop response to B has been identified (Figure 1). Similar variation has also been observed for several other major crops of southern Australia, including barley, oats and peas. While there is some variation in B tolerance among Australian varieties of these crops, more tolerant lines have been identified in collections from the Middle East, India, Japan and parts of South America. In all cases, these B-tolerant crops have adapted to soils subject to either marine or volcanic influence.

Tolerance to high concentrations of B is under the control of a number of major genes, and segregation ratios observed in F2 and F3 generations conform to the Mendelian ratio of 1/4 homozygous sensitive to 1/2 heterozygous to 1/4 homozygous tolerant. Two examples of segregation ratios in wheat shown here compare observed ratios (Obs) with expected ratios (Exp); Chi-square analysis (χ2) confirms that observed frequencies agree with Mendelian prediction:

   

 Sensitive   

 Heterozygous   

Tolerant   

 χ2

Warigal × (Wl*MMC)     Obs    27  63 32  
  Exp   30.5  61 30.5 0.34
Halberd × Warigal Obs    27  63 20  
  Exp    27.5  55 27.5 2.80

These genes act in an additive manner and tolerant varieties have dominant alleles at more loci than do sensitive varieties.

Plant response to B is thus subject to control by major genes, so that genes for tolerance can be transferred to sensitive varieties by backcrossing. This involves hybridisation between a sensitive, but otherwise well-adapted, variety and a tolerant variety. Progeny are tested for response to B and tolerant selections are then crossed to the sensitive variety. Progeny of this cross are again tested for response to B and tolerant plants selected. It is possible to reconstitute the agronomic traits of the sensitive variety, but now with tolerance to B, by repeating this cycle a number of times. In one such outcome (Figure 1) three cycles of backcrossing transferred B tolerance from Halberd to the sensitive variety Schomburgk. Halberd was the most widely grown variety in Australia during the 1970s and 1980s, but more recent varieties, such as Schomburgk, have higher yield potential and better flour quality. Both tolerant and sensitive plants occurred among the progeny and a tolerant selection resulted in the new variety BT-Schomburgk.

Mechanisms responsible for root acquisition of B are not known, but B uptake is known to be closely correlated with water use and is higher under conditions conducive to rapid transpiration. Once absorbed, B moves with the transpiration stream and concentrates at xylem endings. Concentration gradients thus develop within leaves, and overall leaf concen-tration will vary with transpirational history. B concentrations are generally highest at leaf tips or along margins, depending upon venation, and are higher in old leaves compared with young leaves. For example, B concentration in the terminal
4 cm of a barley leaf, when grown under high B conditions, was 2493 mg kg–1 (dry mass), while the corresponding concen-tration for a segment 14–20 cm from the tip was only 82 mg kg–1. Average concentration for the whole leaf was 208 mg kg–1 (J.G. Paull, unpublished data).

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Figure 2 Concentration of B in shoots of four varieties of wheat when grown in a solution culture experiment with six B treatments. (Based on Nable 1988)

Visible symptoms of B toxicity coincide with regions of highest B concentration and consist initially of chlorosis followed by tissue death. Varieties that tolerate high concen-trations of B (Figure 1) are able to exclude B from their roots. They therefore have lower shoot concentrations (Figure 2) and exhibit less severe symptoms of B toxicity compared with more sensitive varieties.

In retrospect, B tolerance was probably a major factor influencing progeny selection from breeding programs for wheat varieties in Australia. Several varieties have dominated Australian wheat production in southern Australia during the twentieth century and the majority of these are tolerant to B. On the other hand, varieties grown in New South Wales and Queensland, where high concentrations of B do not occur, are invariably sensitive to soil B. For example, the variety Halberd, which is one of the most tolerant Australian varieties, was the most widely grown variety in southern Australia during the 1970s and 1980s. Cultivation of Halberd was most concentrated in areas of high soil B. At some silos on Eyre Peninsula, Victorian and South Australian mallee regions as well as inland Western Australia, more than 70% of the total grain receivals were Halberd. By contrast, in low-B regions, Halberd accounted for less than 20% of production. By implication, any successful field crops in southern Australia will have some measure of B tolerance.

References

Nable, R.O. (1988). ‘Resistance to boron toxicity amongst several barley and wheat cultivars: a preliminary examination of the resistance mechanism’, Plant and Soil, 112, 45–52.

Paull, J.G., Rathjen, A.J. and Cartwright, B. (1991). ‘Major gene control of tolerance of bread wheat (Triticum aestivum L.) to high concentrations of soil boron’, Euphytica, 55, 217–228.

 

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