Ross O. Nable

Resistance to boron toxicity amongst several barley and wheat cultivars: A preliminary examination of the resistance mechanism

The mechanism of resistance toB toxicity in barley and wheat was studied in a solution culture experiment using several cultivars displaying a large range of sensitivity to excessB supply. Plants were cultured for 35 d atB concentrations ranging from normal to excessive (15 to 5000 μM, respectively) then examined for dry matter production and theB distribution between roots and shoots.In both species, increasedB supply was accompanied by increased tissueB concentrations, development ofB toxicity symptoms and depressed growth. At each level ofB supply, however, resistant cultivars accumulated considerably lessB than did sensitive cultivars, in both roots and shoots. Even at the lowestB supply, at which noB toxicity symptoms developed and growth was not affected, resistant cultivars maintained relatively low tissueB concentrations. No cultivar displayed an ability to tolerate high tissueB concentrations.These results indicate that sensitivity toB toxicity in barley and wheat is governed by the ability of cultivars to excludeB. If theB concentrations of tissues is used to indicate resistance toB toxicity, then cultivars have the same ranking whether cultured at a normal or excessB supply.

Physiological and genetic control of the tolerance of wheat to high concentrations of boron and implications for plant breeding

Physiological and genetic studies have been undertaken to further the understanding of genetic variation in response to high concentrations of B in the soil and so facilitate the breeding of tolerant varieties for cultivation in high B regions. Genetic variation in response to high concentrations of B has been identified for a number of crop and pasture species of southern Australia, including wheat, barley, oats, field peas and annual pasture medics. The wheat variety Halberd, which was the most widely grown variety in Australia during the 1970s and early 1980s, is the most tolerant of the current Australian wheat varieties. The mechanism of tolerance for all species studied is reduced accumulation of B by tolerant genotypes in both roots and shoots. Results from experiments of uptake kinetics indicate that control of B uptake is a non-metabolic process. The response of wheat to high B supply is under the control of several major additive genes, one of which has been located to chromosome 4A.

Genotypic differences in boron accumulation in barley: Relative susceptibilities to boron deficiency and toxicity

Boron accumulation in genotypes of barley displaying a large range of susceptibility to B toxicity was studied in solution culture experiments. The relationship between susceptibility to B toxicity and to B deficiency was also examined.

Further evidence that zinc is required throughout the root zone for optimal plant growth and development

The effect of variable Zn supply with depth in a soil profile was examined in two wheat genotypes differing in their Zn efficiency. ‘Gatcher’ (Zn-inefficient) and ‘Excalibur’ (Zn-efficient) were grown in a low Zn soil in pots with two treatment zones. The upper zone (10 cm) was supplied with Zn while Zn was either supplied or withheld from the lower zone (25 cm). In both genotypes, withholding Zn from the lower zone had no effect on root growth in either the upper or lower zones; neither did it affect plant appearance prior to booting. However, withholding Zn from the lower zone delayed head emergence in Gatcher by some 10 d and depressed grain yield by 20%. In Excalibur, Zn treatment had no effect on head emergence or grain yield. In Gatcher, withholding Zn from the lower zone depressed water usage by 12% during a 60 d period preceding maturity. No effect of Zn treatment on water usage was seen in Excalibur. Tissue Zn concentrations closely reflected the lower zone Zn treatments in both genotypes. Irrespective of the Zn treatment, Excalibur had higher Zn concentrations in flag leaves but lower concentrations in grain than Gatcher. In whole shoots, genotypic differences in Zn concentration only occurred when Zn was added to the lower zone; Excalibur having almost twice that of Gatcher. Clearly, if Zn is not supplied to the entire root system there is the potential for impaired root function and plant development, and for reduced grain yield. Approaches to managing this problem are discussed.

Problems associated with the use of foliar analysis for diagnosing boron toxicity in barley.

The effects of evapotranspiration conditions on the distribution of B in leaves and critical values for B toxicity measured in shoots were examined in solution culture experiments with barley.Increased water use resulted in increased B accumulation by plants and B was concentrated in the leaf tips. The relationship between shoot DM production and shoot B concentrations was markedly affected by evapotranspiration conditions, but the effect could be removed by not analysing leaf tips. Excluding the leaf tips also decreased the shoot B concentration at which shoot DM production was depressed.Regularly spraying plants with water removed considerable B from leaves without affecting DM production.The present results indicate a number of problems in the establishment of critical values and the use of foliar analysis for diagnosing B toxicity. These problems may account for conflicting reports of critical values and discrepencies between results from glasshouse- and field-cultured plants.

Response of shoot growth and transpiration to soil drying in sugarcane

The relationship between plant-available water (PAW) and shoot extension and transpiration is required to model crop response to water stress, and has not been previously defined for sugarcane (Saccharum spp. (L.)). We subjected sugarcane plants at the 5–6 leaf stage to a continuous drying cycle in large (42 L) pots to determine the threshold fraction of plant available water (PAWt) at which plants slowed shoot extension and transpiration relative to plants watered daily. Transpiration rate was measured as the daily mass loss from the pots and shoot extension as the height increase from ground level to the tip of the youngest actively expanding leaf. Three experiments were conducted with cultivar Q115 covering a range of soil types (and hence PAW) and rates of soil drying. To compare the response with sugarcane, sorghum (Sorghum bicolor (L.) Moench s.lat.), a species that has been well characterized for the relationship between PAW and transpiration and shoot extension, was grown in two additional experiments. For the same species, response curves and PAWt for either shoot extension or transpiration were very similar for the different experiments. This similarity occurred despite there being different soils, different environmental conditions, different PAW, different times taken for the pots to dry down, and hence different rates of stress development. In sugarcane, there was almost no threshold in PAWt (0.92) for shoot extension and a very small threshold in PAWt for transpiration (0.85), while in sorghum PAWt for sorghum shoot extension (0.54) and plant transpiration (0.47) were consistent with those published previously. The present data extend previous reports that sugarcane stalk extension is very sensitive to water stress, and we discuss several factors that could provide the physiological basis for the sensitivity.

Distribution of boron within barley genotypes with differing susceptibilities to boron toxicity

Abstract The B toxicity resistance mechanism was investigated by examining the distribution of B amongst and within plant parts of four barley genotypes transferred from normal to excessive B. After transfer, the B concentrations increased in all plant parts. In corresponding parts (roots and leaves) however, the B concentrations always reflected the relative susceptibility of each genotype to B toxicity, with susceptible genotypes having higher B concentrations than less susceptible genotypes. Despite large differences amongst genotypes in the B concentrations of corresponding leaf segments, the relative B concentrations along the leaves were similar for all genotypes. Similarly, the amount of B in all parts increased after transfer, with B accumulation by leaves closely reflecting leaf size. Roots contained up to 10 percent of the total B accumulated at each harvest. This pattern was essentially the same for each genotype at each harvest, despite the large differences in total B accumulated. These resul...

Selection parameters for assessing the tolerance of wheat to high concentrations of boron

Three wheat genotypes known to differ in response to high concentrations of boron in the field were grown under controlled glasshouse conditions in soil containing 0, 20 and 60mgkg-1 applied boron. Plant growth was assessed both destructively and non-destructively at regular intervals to identify the responses to high concentrations of boron that best discriminated between the tolerant and the sensitive genotypes, and to determine the optimal growth stage at which to rate the genotypes.

The control of boron accumulation by two genotypes of wheat

The mechanism of boron (B) uptake in wheat was studied using two genotypes with known differences in their ability to accumulate B. Influx and efflux of B was measured in the roots of intact 21 d old plants.

Effects of rainfall on the use of foliar analysis for diagnosing boron toxicity in field-grown wheat

The effect of rain on foliage elemental composition, especially B, was assessed using samples of wheat collected at three harvests from a field trial conducted in soil containing excessive levels of B. Moderate rainfall substantially decreased both the B concentration and content of whole shoots and young leaves. The change in B concentration due to rain suggests that foliar analysis is unreliable for diagnosing B toxicity. For the other elements examined (Ca, Cu, Mg, Mn, Mg, P, S, Zn), rainfall had little effect.