| Abstrakt: |
The sub-tropical cropping region of north eastern Australia is characterised by summer dominant rainfall. Thus, wheat sown in autumn to avoid the hot wet summers must grow through the dry winter relying heavily on stored moisture in the deep clay soils that predominate [1]. Unfortunately over large areas, surface soil sodicity as well as subsoil salinity and phytotoxic concentrations of chloride (Cl) impose major soil constraints on wheat production [2-5]. These constraints inhibit soil moisture extraction, particularly from deep in the soil late in the season when marginal water use efficiency is high [6-8]. We aimed to determine whether genotypes tolerant to these constraints can be identified. In 2007, 2008, 2009, 2015 and 2016, we evaluated the performance of 11, 17, 17, 36 and 38 wheat genotypes, respectively. From 2007 to 2015, lines were grown on two sites <0.5 km apart one with sodic soil predicted to reduce wheat production. Compared to the nonsodic site, the sodic site had significantly higher Cl concentration (>800 mg.Cl.kg-1) in the subsoil (0.9-1.3 m soil depth) and higher exchangeable sodium percentage (ESP) (>6%) in the surface and subsoil. In 2016, a similar non-sodic site was used but a new sodic site was chosen having greater surface soil ESP (12.5%) and greater Cl (1500-2000 mg/kg) concentration at depth. Wheat yields were significantly lower at the sodic site than the non-sodic site in 2007 and 2008. They also varied significantly between genotypes. For example, in 2008 grain yield was reduced by between 8% and 33%. Soil moisture extraction, as measured by plant available water capacity (PAWC), was also between 3% and 37% lower [9]. The yield ranking for genotypes at the non-sodic site was not well correlated with that at the sodic site. Thus, selection for yield potential at non-sodic sites is not a good predictor of performance at sodic sites. The difference in site mean yields between sodic and non-sodic sites varied greatly between seasons. The contrast between sites was reduced in 2015 and 2016 due to higher than normal in-crop rainfall which reduced crop reliance on stored soil moisture. In contrast in 2009, very low availability of rainfall and stored moisture severely reduced yields of all lines at both of the sites. This demonstrates the significant influence of seasonal water availability on the impact caused by soil constraints. However, yield rankings of genotypes continued to differ between sodic and non-sodic sites. Overall, certain genotypes were relatively susceptible to sodicity while others were relatively tolerant. Most wheat genotypes grown at the sodic site in 2008 showed Ca deficiency symptoms on younger leaves having Ca concentration <0.2% in the youngest mature leaf (YML). Grain yields of wheat genotypes in 2008 increased significantly with increasing Ca and K in YML and decreased significantly with increasing Na and Cl. In step-wise regression, the PAWC of wheat genotypes was the principal determinant of variation in grain yield in 2008. Including the Ca concentration in the YML of wheat genotypes in the regression significantly improved the prediction of grain yield. This suggests that Ca concentration in YML could be a useful indicator of tolerance to soil constraints. Thus, we successfully identified and quantified useful genetic variation in tolerance to soil sodicity between wheat genotypes suggesting that selection of more tolerant cultivars could improve productivity on sodic sites. As current cultivars have largely been selected for nonconstrained soil conditions, these findings also suggest potential to breed new cultivars with superior tolerance to soil constraints. [ABSTRACT FROM AUTHOR] |
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