High sodicity in surface and subsoil, high salinity and phytotoxic concentration of chloride (Cl) in subsoil, and alkaline surface soils with acidic subsoil are common soil constraints in many dryland Vertosols of the semi-arid, sub-tropical regions of north-eastern Australia [1
]. Sodic soils tend to have severe soil structural problems, poor aeration, and restricted water transmission, resulting in reduced root growth [3
]. Subsoil constraints, on the other hand, reduce the ability of the crop roots to extract water and nutrients from the deeper layers in the soil, especially from soils high in salt content and Cl concentration [4
The yield of grain crops grown on Vertosols, which occupy 72% of the cropping soils of the region, is potentially limited by many factors; however, water supply is the dominant factor. Successful dryland crop production depends on utilising soil moisture accumulated in the period preceding sowing [5
]. Due to the high clay contents, these soils can potentially store 200–250 mm of water in the soil profile [6
]. However, soil constraints, especially in the subsoil, reduce the effective rooting depth and the amount of water and nutrients that plants can obtain from the soil, resulting in reduced crop yield [4
]. Several soil physiochemical constraints in the surface and subsoil interact with each other to determine the local environment for root growth at a given time. Rarely do the various soil constraints occur independently [3
]. Moreover, variable distribution of soil constraints, both spatially within a field, across the landscape, and with depth in the soil profile, combined with the complex interactions that exist among the various physio-chemical constraints [8
], limit the agronomic and management options. Variability in the impact on the crop growth and yield is compounded by the complex interactions between the physiochemical constraints and environmental factors, particularly the timing and amount of rainfall relative to the crop development cycle.
Several management options, including application of chemical ameliorants, compost, and/or physical amendments such as deep ripping, have been assessed to overcome soil sodicity in the region with variable success [2
]. Application of amendments generally affects only the surface soil in the short term, and they need to be leached through to the subsoil to have a significant effect in deeper layers. The use of genetic solutions to develop better adapted crops and/or genotypes of crops may provide a long-lasting tangible solution to overcome both surface soil sodicity and underlying subsoil constraints if genotypes tolerant to the multiple and variable subsoil constraints can be identified [12
]. Improving salt tolerance of barley (Hordeum vulgare
L.) and wheat (Triticum aestivum
L.) has been of long term interest, particularly in Australia [13
]. However, information about the extent of genetic variability for the performance of barley and wheat genotypes in sodic soil with varying subsoil salinity in the soils of north-eastern Australia is limited.
Since sodic soil with high subsoil Cl concentrations induces both physical and chemical stresses, growth depression is related to both soil structural degradation as well as the concentrations of soluble salts in the soil solution. The common soluble cations in the soil solution are sodium (Na+
), calcium (Ca2+
), magnesium (Mg2+
), and potassium (K+
). The common anions are chloride (Cl−
), sulphate (SO42−
), and carbonate (CO32−
dominate the majority of the sodic soils with varying subsoil constraints in north-eastern Australia. Most studies have used Na+
as an indicator of salt stress with little attention to Cl−
]. However, both Na+
occur in high concentration and are metabolically toxic to plants, therefore both should be given equal consideration [4
]. Variation in salt tolerance in crop cultivars has been found to be associated with low rates of Na+
uptake and transport, and a high selectivity for K+
] and/or restricted Cl−
]. In sodic and/or saline-sodic soils, high Na+
concentrations accumulate in plants. This reduces the uptake of essential nutrients [18
] such as K+
in particular [19
], which affect the integrity and functioning of the cell membrane [16
]. High uptake of Na+
also leads to deficiencies of other elements such as zinc (Zn2+
), copper (Cu2+
), and manganese (Mn2+
]. Therefore, it is necessary to assess tissue ion concentrations against salt tolerance of crop species in order to identify whether exclusion of Na+
, or selectivity for K+
, and/or ion imbalances are appropriate selection criteria to identify plants tolerant to sodicity and/or salinity.
Subsoil acidity in soils with N2
-fixing brigalow (Acacia harpophylla
) as the dominant natural vegetation [1
] can result in increased uptake of aluminium (Al3+
, and iron (Fe2+
), which affects root growth, promotes Al3+
toxicity in the roots, and/or induces Ca2+
deficiency in the leaves. Accumulation of excess Al3+
can also be due to the presence of anionic aluminate ions [Al(OH)4−
] in alkaline soils pHw > 9.0 [20
There were two major aims of this study. Firstly, to investigate any potential genetic variability between barley and wheat genotypes in their tolerance to sodic soils with variable subsoil constraints. Secondly, to identify the potential to use tissue ion concentrations as selection criteria for barley and wheat genotypes better adapted to sodic soils. The study also examined the extent of Na+ and Cl− ion toxicity and/or Na+ and Cl− induced ion deficiencies in barley and wheat genotypes with variation in adaptation to these soils.
The aim of the current study was to investigate any potential genetic variation for performance on sodic soils with variable subsoil constraints in barley and wheat genotypes either available to growers in the region or with known adaptation to water-limitation, and to identify any potential selection criteria to select genotypes better adapted to these conditions.
Considerable variation between genotypes was found, and some potential selection criteria were identified using measurements of leaf element concentrations.
4.1. Genetic Variation Indicates that the Crop Performance under Non-Sodic Conditions Is Not a Good Indicator of Crop Performance in the Presence of Sodic Soils
Experiments in 2008 indicated substantial differences between genotypes of both barley and wheat for yield performance under sodic soil conditions with subsoil constraints. Results indicated that performance under non-sodic conditions were not a good indicator of performance in the presence of sodic soils with variable sub-soil constraints. Comparing the grain yields of both barley and wheat genotypes grown at the sodic site to the grain yields at the non-sodic site, significantly higher ESP to a depth of 0.6 m in the sodic soil resulted in reduced grain yields of both barley and wheat genotypes. It has been shown that ESP > 6% in the surface soil [25
] and >19% in the subsoil [9
] reduces the grain yield of most crops. Grain yield on sodic soils is often less than 50% of the potential yield [29
]. Dalal et al. [30
] found that wheat yield in north-eastern Australia decreased from 3.5 t/ha to <2 t/ha as a result of sodicity (expressed as ESP) increasing from 4% to 16% in the topsoil (0–0.1 m soil depth). In southern Australia, Rengasamy [3
] reported a nearly linear decline in grain yield with increasing ESP in the surface soil for 30 different crop and pasture types.
The non-sodic site in the present study had high ECse between 0.50 and 1.1 m soil depth due to the presence of gypsum, suggesting that the calculated high ECse due to gypsum in the subsoil had little effect on water uptake and yields of wheat and barley genotypes. Generally, evidence in the literature points to gypsum either having a slightly negative or an ameliorative effect on the adverse impact of Cl [16
It is important to note that although all genotypes of wheat and barley had reduced grain yield when grown at the sodic site compared to the non-sodic site, the ranking of different genotypes of both wheat and barley varied at the different sites. In general, wheat experimental line HSF1-112 ranked low and EGA-Hume ranked high at both the sites. Similar to wheat, barley yield rankings differed markedly between sites. For example, Mackay and Kaputar ranked high at the non-sodic site but low at the sodic site. These results suggest that selection and recommendation of wheat and barley genotypes for sodic sites needs to be based on testing done in the presence of soil constraints. The decrease in grain yield of wheat and barley genotypes in the present study, besides other factors, is likely due to a combination of factors that include decreases in PAWC, and Ca and K concentrations and/or increases in the concentrations of Na and Cl in the plant tissues in the sodic soil.
Substantially higher grain yields of both barley and wheat genotypes grown on sodic and non-sodic soils in 2008 were due the higher in-crop rainfall (137 mm) received in 2008 as compared to 2009 (76 mm). Experiments in 2008 showed clear differences between the yield performance of wheat and barley under non-sodic versus sodic soil conditions, which agrees with observations for a different set of wheat genotypes at these trial sites during 2007 [31
]. However, after gypsum application, crops grown in 2009 exhibited considerably less variation between performance under sodic versus non-sodic conditions. It is likely that this reduced difference was influenced by at least two factors. Firstly, the potential ameliorative effects of gypsum application on performance under soil constraints. Secondly, reduced in-crop rainfall would have reduced the yield of all genotypes and the differences in yield between genotypes. These factors most probably contributed to the finding that results in 2009 did not give the clear differentiation of genotypic variation in adaptation to soil constraints versus those under non-sodic conditions that was as observed in 2008 (Figure 3
and Figure 4
). Thus, in order to determine factors that likely indicate differential adaptation to soil constraints, the data for 2008 was examined in more detail.
4.2. Differences in Soil Moisture Extraction (PAWC) Were the Major Determinant of Performance Differences in the Face of Sodic Soils with Sub-Soil Constraints
All genotypes of wheat and barley grown at the sodic site had reduced PAWC as compared to the non-sodic site in 2008. Shaw [23
] showed a strong negative relationship between the effect of exchangeable Na in the root zone on measured PAWC over the rooting depth of a crop in clay soils. Dalal et al. [30
] reported decreased PAWC from 120 mm to 80 mm with increasing ESP from 5% to 30% in the top 0.6 m soil depth in clay soils from north-eastern Australia. In the present study, the presence of high Cl concentrations in the subsoil (below 0.90 m soil depth) at the sodic site likely further restricted water extraction, resulting in further reduction in PAWC. Dang et al. [4
] showed that subsoil Cl concentrations had a greater effect in reducing soil water extraction and grain yields of five crop species studied than did salinity or sodicity, per se. Subsoil Cl in the 0.90–1.10 m soil depth layers at >850 mg/kg for wheat and >1000 mg for barley has been shown to reduce grain yield by 10% [4
]. In the present study, the Cl concentration in the subsoil at the sodic site was well above this threshold Cl concentration. It was interesting to note that grain yields of wheat and barley genotypes grown at the sodic and non-sodic sites had a strong linear relationship with PAWC for wheat and barley genotypes, but the relationships between grain yields of wheat and barley genotypes grown on the non-sodic site was not significant. The relationship between grain yields of wheat and barley grown at the sodic site with variable subsoil constraints with PAWC was significant for wheat (R2
= 0.36) and barley (R2
= 0.36) genotypes. This further suggests the importance of surface sodicity and subsoil Cl in restricting the ability of the roots to extract water [4
4.3. Concentrations of Ca in Wheat and K:Na Ratio in Barley May Be Good Surrogate Traits to Select for Adaptation to Soil Constraints
In the present study, the concentrations of Ca, K, Na, and Cl in YML of both wheat and barley genotypes were generally different when plants were grown on the sodic versus the non-sodic site. However, the differences in the concentrations of other elements such as P, Al, Mn, Cu, Zn, Mg, and S between the sodic site and non-sodic site were not as pronounced.
The relationship of Ca concentration with grain yield of wheat genotypes grown on the sodic and non-sodic sites was clear. The Ca concentration in YML of both wheat and barley genotypes grown on sodic soil was <0.35%, and most genotypes with Ca concentration ≤0.21% in YML of both wheat and barley exhibited typical Ca deficiency symptoms. The Ca concentration in the YML of wheat and barley at the sodic site in the present study was less than the critical limit (0.25%) for plant growth [33
]. The ‘Na-induced Ca deficiency’ may possibly be explained on the basis of both the physical condition of the sodic soil and presence of high exchangeable Na in the soils [19
]. All wheat and barley genotypes grown at the non-sodic site had Ca concentration >0.4% which was well above the critical Ca concentration in the plant tissue for the normal growth of cereals [33
]. Calcium can ameliorate Na toxicity by decreasing Na influx through nonselective cations channels [35
] and blocking K loss under high salt concentration [35
]. High sodicity and high soluble salts other than Ca in soil solution can inhibit uptake and transport of Ca and may induce Ca deficiency [19
]. Results of the present study suggest that Ca concentration in the YML of wheat could be a useful indicator of genotypes with tolerance to sodicity with high subsoil chloride. Further studies to confirm this result using a wider range of genotypes on a range of such sites would be warranted.
Most wheat genotypes grown at the sodic site had Na concentrations <0.05%. Generally, Na becomes physiologically toxic to wheat growth when Na in plant tissues is more than 0.1% (R. Munns, pers. comm.). Most of the wheat genotypes in the present study had Na concentrations <0.1%, which corroborates with an earlier report suggesting that most of the Australian wheat genotypes accumulate Na in the tissue well below the critical level [37
]. The Na concentration in the YML of both a non-tolerant genotype such as EGA-Wylie (0.154%) and a tolerant genotype such as EGA-Hume were amongst the highest (0.162%). Therefore, a direct toxic effect of Na in the leaves seems unlikely to be the main causative factor for the growth depression in wheat. However, some effect cannot be excluded since even a low Na concentration in the plant can induce considerable changes in carbohydrate metabolism through its effect on activities of enzymes of carbohydrate metabolism, particularly starch synthetase [16
]. In contrast to wheat genotypes, barley genotypes grown either on the sodic or non-sodic sites accumulated substantial higher concentration of Na in the YML (>0.15%). However, Munns et al. [14
] suggested that although barley accumulates high concentration of Na and Cl in fully expanded leaves, these high concentrations do not determine the growth of barley. Barley has been shown to tolerate high Na concentrations within the leaves, probably by maintaining low levels of Na in the cytoplasm and sequestering the Na in vacuoles, whereas bread wheat has a greater ability to restrict Na uptake [39
Both wheat and barley accumulated similar concentrations of K in the YML, but the differences between genotypes grown on the sodic site compared to the non-sodic site were small. Potassium concentrations in the YML of both wheat and barley were well above the critical concentration for the growth of both wheat and barley [33
]. The K/Na ratio in the YML of barley genotypes grown on the sodic and the non-sodic sites provided a potentially useful indicator of barley genotypes tolerant to sodicity. Barley genotypes with K/Na concentrations <5 were susceptible to high sodicity. A number of studies have shown that maintenance of high K/Na ratios was important for the salt tolerance of cereals [40
]. High concentrations of Na can cause reduction of chlorophyll and inhibit the normal functioning of a large number of enzymes and proteins, resulting from the competition by Na for K binding sites [42
]. Therefore, the plant’s ability to maintain an optimal K/Na ratio has been long cited as a key feature of salt tolerance [41
]. Both wheat and barley had higher concentrations of Cl than Na in YML. However, the differences in Na concentrations in the YML between the genotypes were greater than Cl. The grain yield reduction corresponded well with the increased Cl concentrations in the YML of wheat and barley.
In the step-wise regression analyses, PAWC for both wheat and barley genotypes grown on the sodic and non-sodic sites was the principal determinant of the variation in the grain yield of both wheat and barley genotypes, respectively, which is related to the ability of the roots to extract soil water. Root system characteristics and architecture traits are of fundamental importance to soil exploration and soil water acquisition [22
]. Water availability is a key limiting factor in crop production in north-eastern Australia. Genotypes with improved adaption and/or improved ability to explore below-ground resources could potentially improve productivity of sodic soils [46
]. Further investigation would seem warranted to understand the physiological mechanisms and possible influence of root architecture on cereal tolerance to sodicity. Element concentrations in the YML, in particular Ca in wheat and K:Na in barley, significantly improved the wheat and barley grain yield prediction, respectively.