1. Introduction
Phosphorus (P) is a critical macronutrient required for plant growth; it may also play a role as an alleviator of salt stress as its supply affects Na transport within plants and the ultimate salt tolerance of crops [1]. Inorganic phosphate (Pi) is the main available form for plant uptake among many complex chemical P forms present in soil [2,3]. However, Pi availability is affected by several factors, including soil pH. Pi can be precipitated with calcium at high pH and aluminum at low pH. To address low P availability—a worldwide issue restricting crop growth and final yield—large amounts of P fertilizers are applied to sustain crop production [4], which can cause P accumulation in the topsoil of intensive farming systems and P leaching [5]. Sustainable agriculture requires strategies such as breeding and P management to improve the P economy, as rock phosphate is a non-renewable resource [6,7,8,9]. Optimizing P placement under salt stress is of significance for improving the crop P economy grown on salt-affected soils.
Plants have self-adjustment mechanisms to cope with low Pi availability, including morphological/architectural adaptations in roots and shoots, biomass allocation, seed development [10], regulation of Pi concentrations, exudation of organic anions and phosphatases, and symbiotic associations with soil microorganisms [11,12]. Root system architecture (RSA) plays a crucial role in resource acquisition [13,14]; for instance, shallower root systems can distribute more lateral roots in the topsoil to improve Pi acquisition [15]. Nakhforoosh et al. (2021) reported that a durum wheat landrace’s root system explored deep soil, while monococcum-type wild einkorn roots explored more of the topsoil [16]. Such genotypic differences in root systems offer selective breeding opportunities for improved nutrient utilization. Moreover, the dwarf genes which caused genotypic variation between tall and dwarf wheat genotypes may have an impact on root growth and nutrient utilization.
RSA is highly plastic in response to changes in environmental conditions. Uneven P distribution due to localized P supply can stimulate root development within a nutrient-enriched zone [17]. Increasing P supply can enhance plant growth through improved spatial root growth [18,19], likely enhancing P acquisition opportunities in the Pi-enriched zone. Fertilizer application in strips/bands is a common method for improving P utilization in crop production. Crops grown with banded P can adjust their RSA in response to the more heterogeneous Pi distribution than conventional broadcast P [20,21].
In addition to changes in RSA, changes in P status could impact nutrient uptake (e.g., Ca, Mg, Na, and K) and thus change the plant’s nutrient composition; increasing P nutrition, for instance, enhanced Mg and Ca uptake [22,23,24]. Furthermore, salt stress can affect plant uptake of essential elements [25]. Salt-stressed plants usually have increased Na/K ratios in roots and shoots [26] and altered associations between plant growth parameters and nutrient uptake [27].
Generally, applying the proper amount of P fertilizer to saline soils will increase salt tolerance, as reported in sorghum [28], durum wheat [29] and barley [30]. In contrast, a study suggested that P limitation can increase salt tolerance in maize [31]. These inconsistent results might be due to the interactive effects of P supply and salt stress on parameters such as root growth and nutrient uptake.
We conducted a column culture experiment with five P placements, two salt stress levels, and two wheat genotypes to better understand the effects of P placement and salt stress on spatial root distribution, nutrient utilization, and plant agronomic performance in wheat. We tested the following hypotheses: (1) plant belowground and aboveground biomass accumulation is affected by P placements in terms of P input amount and placement depth; (2) a modified spatial root distribution and nutrient allocation is required for a growth compromise in response to P status and salt stress; (3) differences in P utilization exist between tall and dwarf wheat genotypes.
2. Materials and Methods
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Plant material
One dwarf Chinese wheat cultivar, ‘LX99’, widely grown in North China, and one tall American genotype, ‘Alice’, were used in this study.
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Experimental set up
The experiment was conducted under greenhouse conditions in Binzhou University, China, from October 2021 to June 2022. A 100 cm column (diameter: 15.8 cm) was divided into three segments (0–20 cm, 20–40 cm, and 40–100 cm), connected with plastic tape to avoid soil leakage. Each column was filled with 12.5 kg pre-washed and dry Greens Grade® soil amendment (PROFILE Products LLC, Buffalo Grove, IL, USA). This soil amendment had a porosity of 74%, pH 5.5, and cation exchange capacity (CEC) of 33.6 m Eq/100 g; its main chemical composition was SiO2 (74%), Al2O3 (11%), and Fe2O3 (5%). The experiment had a completely randomized block design with three factors and three replications. The two wheat genotypes, ‘LX99’ and ‘Alice’, were two levels for the factor ‘genotype’. Factor ‘P placement’ had five levels (‘Top Dressed High P’ (TopHP, 2.4 g P placed in 0–3 cm soil layer), ‘Top Dressed Reduced P’ (TopRP, 1.2 g P placed in 0–3 cm soil layer), ‘Deep Banded High P’ (DeepHP, 2.4 g P placed in 20–23 cm soil layer), ‘Deep Banded Reduced P’ (DeepRP, 1.2 g P placed in 20–23 cm soil layer), and ‘No P added’ (−P)), using corresponding amounts of monocalcium phosphate (Ca(H2PO4)2, also known as calcium superphosphate, as the P source. Factor ‘salt stress’ had two levels: salt stress and non-salt stress.
Seeds were germinated on moist filter paper in a Petri dish placed in the dark for 2 days at 25 °C. Three uniform seedlings (~0.4–0.6 cm shoot length and ~0.6–1.0 cm root length) were transferred to a pot pre-filled with 1.0 kg Greens Grade® soil amendment and vernalized for two weeks in a 4 °C growth room before transferring them to the 100 cm column.
Each column was irrigated with either 10 L of 50 mM NaCl for the salt stress (+S) treatment or 10 L deionized H2O for the non-salt stress (−S) treatment before transferring the seedlings, allowing the soil amendment to be totally saturated for each. After transplanting, each column received 5 L of 50 mM NaCl in the +S treatment or 5 L deionized H2O for the non-salt stress (−S) treatment weekly until Zadok’s stage Z14, then a higher salinity level, i.e., 100 mM NaCl, was employed in the +S treatment until anthesis (Zadok’s stage Z65). The two salt stress levels and five P placements resulted in ten treatments: ‘Top Dressed High P combined with non-salt stress’, TopHP−S; ‘Top Dressed Reduced P combined with non-salt stress’, TopRP−S; ‘Deep Banded High P combined with non-salt stress’, DeepHP−S; ‘Deep Banded Reduced P combined with non-salt stress’, DeepRP−S; ‘No P added combined with non-salt stress’, −P−S; ‘Top Dressed High P combined with salt stress’, TopHP+S; ‘Top Dressed Reduced P combined with salt stress’, TopRP+S; ‘Deep Banded High P combined with salt stress’, DeepHP+S; ‘Deep Banded Reduced P combined with salt stress’, DeepRP+S; and ‘No P added combined with salt stress’, −P+S.
Weekly irrigations occurred with 5 L of nutrient solution containing 1 mM KNO3, 0.5 mM MgSO4, 0.6 mM KCl, 1.5 mM CaCl2, 1 μM HBO3, 0.1 μM (NH4)6Mo7O24, 0.5 μM CuSO4, 1 μM ZnSO4, 1 μM MnSO4, and 0.01 mM FeNa-EDTA (pH adjusted to 6.8).
Light and temperature during the growing period were set to 10 h light at 25 ± 3 °C and 14 h dark at 15 ± 3 °C for four weeks from transplanting and then adjusted to 14 h light at 25 ± 3 °C and 10 h dark at 15 ± 3 °C. Humidity within the greenhouse was maintained at 45–65%, while the soil relative water content was maintained at 55–70% under current growth condition.
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Root length measurements
Roots at 0–20, 20–40, and 40–100 cm soil depth were harvested separately, dried at room temperature, and shaken to remove attached soil particles on the root surface. Root length was measured using the WinRHIZO Root Analysis System (Regent Instruments, Montreal, QC, Canada).
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Elemental measurements
At harvest, plants were separated into grain, straw, and roots at 0–20, 20–40, and 40–100 cm soil depth. The samples were washed twice using deionized water, oven-dried for 72 h at 80 °C, and ground into a fine powder. A 30 mg sample was digested with 13 mL nitric acid and 2 mL H2O2 using a microwave digestion instrument. Na, K, Ca, Mg, and P concentrations were measured using an Inductively Coupled Plasma Optical Emission spectroscopy (ICP-OES, Thermo Fisher, Seattle, WA, USA).
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Calculation of nutrient accumulation parameters
Grain weight, total aboveground biomass (AGB; including grain and straw), and root dry weight were determined using a digital balance (accuracy 0.1 mg) after oven-drying at 80 °C.
The total Na, K, Ca, Mg, and P accumulation in grain, straw, roots at different soil depths, and whole plant (mg) were calculated as follows:
Total accumulation of an individual element in grain (mg/pot) = grain weight × Cgrain;
Total accumulation of an individual element in straw (mg/pot) = straw weight × Cstraw;
Total accumulation of an individual element in roots at 0–20 cm soil depth (mg/pot) = root weight at profile 0–20 cm × Croot 0–20cm;
Total accumulation of an individual element in roots at 20–40 cm soil depth (mg/pot) = root weight at profile 20–40 cm × Croot 20–40cm;
Total accumulation of an individual element in roots at 40–100 cm soil depth (mg/pot) = root weight at profile 40–100 cm × Croot 40–100cm;
Total accumulation of an individual element in whole plant (mg/pot) = grain weight × Cgrain concentration + straw weight × Cstraw + root weight at 0–20 cm soil depth × Croot 0–20cm + root weight at 20–40 cm soil depth × Croot 20–40cm + root weight at 40–100 cm soil depth × Croot 40–100cm,
where Cgrain, Cstraw, Croot 0–20cm, Croot 20–40cm, and Croot 40–100cm are the concentrations (mg/kg) of each measured element in grain, straw, roots at 0–20, 20–40, and 40–100 cm soil depth, respectively.-
Calculation of phosphorus efficiency
Three indices were used to evaluate P efficiency: P utilization efficiency (PUE), P agronomic efficiency (PAE), and P physiology efficiency (PPE), calculated according to [32]:
PUE = (P uptake in treatment − P uptake in control)/(P input amount) × 100%;
PAE (kg/kg) = (Grain yield in treatment − Grain yield in control)/(P input amount);
PPE (kg/kg) = (Grain yield in treatment − Grain yield in control)/(P uptake in treatment − P uptake in control).
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Statistical analysis
Statistical analysis was conducted using SPSS software (version 16.0, Chicago, IL, USA, SPSS Inc.). Full factorial ANOVA was used to test the effect of factors and their interactions on plant biomass, root growth, elemental concentrations, and total elemental accumulation. α = 0.05 and α = 0.01 were used as two significant levels to examine significance. Multiple comparisons (Duncan’s) were further used to observe the significance of values among different P placement treatments.
3. Results
3.1. Root Weight and Distribution at Different Depth Profiles
Root weight at 0–20 cm soil depth at maturity was significantly affected by salt stress (p < 0.01), P placement (p < 0.01), genotype (p < 0.01), and their interactions (Table S1). Root weight at 0–20 cm soil depth for Alice ranged from 1.23–1.80 g/pot under non-salt stress and 0.86–1.74 g/pot under salt stress, while LX99 ranged from 0.37–0.52 g/pot under non-salt stress and 0.34–0.48 g/pot under salt stress (Figure 1). Generally, top P placements had more root weight at 0–20 cm soil depth than deep P placements for both genotypes. The lowest root weights at 0–20 cm soil depth occurred with DeepRP under non-salt stress (1.23 g/pot) and −P with salt stress (0.86 g/pot) for Alice and −P irrespective of salt stress for LX99 (0.37 g/pot under non-salt stress, 0.34 g/pot under salt stress).
Similarly, root weights at 20–40 cm and 40–100 cm soil depth were significantly affected by salt stress (p < 0.01), P placement (p < 0.01), genotype (p < 0.01), and their interactions (Tables S2 and S3). Deep P placement increased root weight at 20–40 cm soil depth by 6.4–31.2% for Alice and 7.2–17.1% for LX99, relative to top P placement under non-salt stress (Figure 2). Under salt stress, DeepHP increased root weight while DeepRP decreased root weight at 20–40 cm soil depth for Alice. Deep P placement increased root weight at 20–40 cm soil depth for LX99. The No-P treatment had the most root weight at 40–100 cm soil depth for both genotypes under non-salt stress but the least root weight under salt stress relative to the P-added treatments (Figure 3).
In general, Alice had a greater total root length (TRL; 69.92–145.30 m) than LX99 (23.75–41.88 m) across all treatments (Table 1). Both genotypes allocated the most root length to the 0–20 cm soil layer (Alice: 62.32–76.18%; LX99: 49.68–72.55%). Deep P placement increased the root length at 20–40 cm soil depth more than top P placement in both genotypes under non-salt stress (Table 1).
3.2. Aboveground Biomass and Agronomic Traits as Affected by Salt STRESS and P Placement
Salt stress (p < 0.01), P placement (p < 0.01), and genotype (p < 0.01) significantly affected aboveground biomass and grain weight/pot (Tables S4 and S5). TopHP produced the most aboveground biomass, while −P produced the least for both genotypes (Table 2). The aboveground biomass of Alice ranged from 5.06–6.00 g/pot under non-salt stress and 4.01–5.42 g/pot under salt stress, while LX99 ranged from 2.26–3.02 g/pot under non-salt stress and 1.86–2.56 g/pot under salt stress. Salt stress produced slightly lower aboveground biomass than non-salt stress. TopHP also produced the highest grain weight for both genotypes, irrespective of salt stress (Table 2). P placement and salt stress significantly affected the agronomic traits of both genotypes in terms of plant height, grain number/pot, and grain weight/pot (Table 2).
P placement, salt stress, genotype, and some of their interactions significantly affected the TRL/AGB ratio (Table 2 and Table S6). For example, −P−S treatment produced the highest TRL/AGB ratios for Alice (30.86 m/g) and LX99 (17.60 m/g). In contrast, −P+S treatment produced the lowest TRL/AGB ratio for Alice (17.46 m/g) and LX99 (12.84 m/g).
3.3. Mineral Concentration and Accumulation as Affected by Salt Stress and P Placement
Na concentration varied between genotypes, salt stress levels, P placement, and plant parts (grain, straw, and roots) (Tables S7 and S8). Salt stress produced significantly higher shoot and root Na concentrations at maturity than non-salt stress. Na concentrations also differed between plant parts. For example, Alice had straw Na concentrations 3.5–6.8 times higher than grain under non-salt stress and 20.8–34.6 times higher than grain under salt stress, while LX99 had straw Na concentrations 4.3–8.9 times higher than grain under non-salt stress and 29.2–69.2 times higher than grain under salt stress. Straw Na concentrations of both two genotypes were significantly affected by P placement. DeepHP under non-salt stress and TopRP under salt stress resulted in the highest straw Na concentrations for Alice; DeepHP under non-salt stress and −P under salt stress resulted in the highest straw Na concentration for LX99. In addition, salt stress significantly increased straw and root Na/K ratios relative to non-salt stress. For example, straw Na/K ratios ranged from 0.03–0.09 for Alice and 0.04–0.15 for LX99 under salt stress, compared to 6.1 × 10−3–10.8 × 10−3 for Alice and 6.2 × 10−3–16.6 × 10−3 for LX99 under non-salt stress. Similarly, Alice and LX99 had 2.3–8.1 and 2.1–5.2 times higher root Na/K ratios under salt stress than non-salt stress.
Genotype, P placement, salt stress, and plant parts significantly affected Ca and Mg concentrations (Tables S7 and S8). With regard to the change of Ca and Mg concentrations modified by P placements, genotype Alice and LX99 responded differently. For example, TopHP and DeepRP led to the highest and lowest grain Ca concentration of Alice under non-salt stress, respectively; DeepRP and −P led to highest and lowest grain Ca concentration of LX99 under non-salt stress, respectively. Overall, more Mg translocated from roots to aboveground plant parts and from straw to grain than Ca. The grain Ca and Mg concentrations were 6.0–12.0% and 67.4–109.6% of that in straw across all treatments, respectively. Aboveground plant parts contained 45.8–77.1% of the total Ca for Alice and 47.3–62.1% for LX99 and 63.2–82.9% of the total Mg for Alice and 72.1–80.0% for LX99 across all treatments (Table 3).
Genotype, salt stress, P placement, organ type, and their interactions significantly affected P concentrations (Tables S7 and S8). No significant difference of both straw and grain P concentrations were examined among different P placements under non-salt stress (except LX99 grown under DeepRP). In contrast, different P placements significantly affected straw P concentration of both two genotypes Alice and LX99. Straw and grain had significantly higher P concentrations than roots. Different P placements altered total mineral accumulation, including P (Table 3). The changes in measured parameters of plant growth and mineral accumulation under different treatments were compared to the −P−S control treatment for two wheat genotypes (Alice and LX99), as shown in Figure 4 and Figure 5.
3.4. P Utilization Efficiency among Different Treatments
PUE significantly differed among treatments (Table 4). Alice had a higher PAE under non-salt and salt stresses than LX99. For Alice, TopRP produced the highest PUE and PAE under non-salt and salt stresses, while DeepRP produced the highest PPE. For LX99, DeepRP and TopRP produced the highest PUE and PPE, respectively, under non-salt and salt stresses, while TopHP produced the highest PAE under non-salt stress, and TopRP produced the highest PAE under salt stress.
4. Discussion
This study investigated the effect of different P placements on root traits and nutrient accumulation in wheat plants under salt and non-salt stresses. Root traits have a relatively lower broad-sense heritability than aboveground plant traits [33], suggesting that they are plastic traits easily affected by environmental factors. A meta-analysis revealed that P deficiency decreases root length and biomass by 14% and 25%, respectively [34]. In contrast, a localized P supply stimulated root development in a hydroponic system [17], glasshouse [35], and field trials [18,19]. In the current study, P placement depth played an important role in regulating root distribution, with TopHP producing the greatest root distribution at 0–20 cm soil depth for both genotypes (Alice and LX99) and combined DeepHP and non-salt stress producing the greatest root distribution at 20–40 cm soil depth. Combined DeepHP and salt stress, however, did not result in greater root distribution at 20–40 cm soil depth, suggesting the promoting effect of localized P on root growth was affected by soil salinity.
During the ‘Green Revolution,’ the incorporation of dwarf genes in wheat resulted in an increased harvest index, which was suggested to be associated with reduced investment of dry matter in the root systems and thus a lower root–shoot ratio [36]. This increased harvest index, therefore, may be partially due to altered root growth habits. The tall genotype Alice had a larger root system compared to the dwarf genotype LX99 and thus had different responses to P status; for example, the RP and −P treatments increased the root distribution at 20–40 cm soil depth for Alice but decreased that of LX99 compared to TopHP under non-salt stress. Root distributions at 40–100 cm soil depth for both genotypes in the −P treatments increased under non-salt stress but decreased under salt stress relative to that in the P-added treatments. These findings suggest that the effect of P application on root distribution depends on genotype and soil environment factors such as salinity.
Deep-banded -P fertilizer placed at an optimal depth increases PUE [37,38,39,40] by altering RSA and root anatomy. Liu et al. (2022b) reported that 16 cm was a particularly effective P placement depth [21]. In the current study, the 20 cm placement depth increased the root distribution at 20–40 cm soil depth except for Alice under combined DeepRP and salt stress, suggesting that the beneficial effect of deep-placed P depends on placement depth and the soil environment. Therefore, P placement depth plays a crucial role in regulating root distribution and should be adjusted according to the growth process of aboveground plant parts. For instance, applying P at a key growth stage, such as the tiller-forming stage when the roots reach the Pi-enriched zone, may benefit plants the most.
The current study revealed that salt stress and P treatment significantly affected the spatial distribution of roots (root weight and length) in segmented soil profiles and wheat agronomic traits, indicating the need for coordinated growth between aboveground and belowground plant parts in response to environmental changes. As roots grow downwards to forage for nutrients, aboveground plant parts grow upwards, requiring a balance between aboveground and belowground plant growth. The difference in P availability in deep soil profiles between top P and deep P applications may alter root growth and subsequently affect aboveground plant development and biomass accumulation. One study showed that P deficiency increased the root weight/aboveground biomass ratio [41], while a meta-analysis revealed no consistent root/shoot ratio response to P deficiency [34]. In the current study, the root weight/aboveground biomass ratio of both genotypes increased under −P−S but decreased under −P+S relative to the P-added treatments (Table 2), suggesting that the root/shoot ratio response to P deficiency depends on soil salinity.
The aboveground biomass and grain yield of both genotypes under P-added treatments increased relative to −P treatment regardless of the salt stress level in the current study, suggesting that the application of a proper amount of P fertilizer under salt stress could improve agronomic performance. Both P deficiency and salt stress adversely affect plant growth [42], leading to changes in nutrient uptake and biomass accumulation. Reduction of root growth by salt stress can be alleviated by supplemental Ca [43]. One study showed that applying superphosphate (with 26 kg P ha−1) under coastal saline soil conditions increased soil Ca concentrations by 20–42% and decreased soil Na concentrations by 14–18% across all soil layers [9]. In the current column culture system, the different P placements resulted in a localized increase in P concentration within the column, as evidenced by the altered root distribution. For example, DeepHP produced the highest root P concentrations at 20–40 cm soil depth in Alice under non-salt and salt stresses, while TopHP produced the highest in LX99 (Table S8). Root weight and total Ca accumulation in the roots of both genotypes, Alice and LX99, under all P-added treatments combined with salt stress was obviously increased relative to −P+S treatment (Figure 1, Figure 2 and Figure 3; Table 3); this promoting effect can be partially attributed to Ca from the applied monocalcium phosphate.
The effects of P application and salt stress on PUE raise the question of whether the optimal P placement method observed under non-saline conditions could also be effective under saline conditions. Loudari et al. (2022) found that adequate P fertilization positively affected durum wheat plants under salt stress [29]. There were large differences in terms of biomass accumulation and root length between genotypes Alice and LX99 in the current study; these differences could be contributors affecting the accumulation of P (and other nutrients) in plants. Sustainable agriculture requires a higher PUE with less P input, and the highest PUE occurred at TopRP for Alice and DeepRP for LX99 under non-salt and salt stresses in the current study. However, reduced P input did not necessarily result in a higher P efficiency in terms of all three P utilization parameters (PUE, PAE, and PPE) than high P input. Genotype, P placement depth, and soil salinity significantly affected PUE, and these factors should be considered when improving wheat PUE under saline conditions. A proper P amount and suitable P placement imposed on a salt stress-tolerant wheat genotype may result in a higher PUE.
5. Conclusions
This study investigated the effect of P placement and salt stress on spatial root distribution, plant agronomic performance, and nutrient utilization. Deep P placement under non-salt stress increased root distribution at 20–40 cm soil depth for both genotypes, Alice and LX99, compared to top P placement. P application under salt stress increased root weight at all three soil depths in both genotypes and P accumulation in aboveground plant parts (except for LX99 under TopRP). Changes in the P utilization efficiency of genotypes Alice and LX99 responded differently to P placement, with the highest P utilization efficiency occurring in Alice under TopRP and in LX99 under DeepRP regardless of salinity level. The results demonstrate the importance of coordinated adaptation in biomass allocation between belowground and aboveground plant parts and of altered nutrient utilization in reaching a growth compromise in response to P supply changes and salinity.
D.-Y.Z.: conceptualization; D.-Y.Z., X.-L.Z. and Z.-W.Z.: methodology; S.-P.Z., G.-L.L., X.-L.Z., X.-P.L. and W.-F.Z.: investigation; D.-Y.Z. writing; D.-Y.Z., S.A.K. and K.H.M.S.: writing—review and editing. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in the study are included in the article/
The authors declare no competing interest.
Footnotes
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Figure 1. Comparison of root weight at 0–20 cm soil depth for wheat genotypes Alice (A) and LX99 (B) grown under combined P placement and salt stress treatments. Note: different letters within a salt stress treatment indicate significant differences at the 0.05 level.
Figure 2. Comparison of root weight at 20–40 cm soil depth for wheat genotypes Alice (A) and LX99 (B) grown under combined P placement and salt stress treatments. Note: different letters within a salt stress treatment indicate significant differences at the 0.05 level.
Figure 3. Comparison of root weight at 40–100 cm soil depth for wheat genotypes Alice (A) and LX99 (B) grown under combined P placement and salt stress treatments. Note: different letters within a salt stress treatment indicate significant differences at the 0.05 level.
Figure 4. Changes in aboveground and belowground biomass of wheat genotype Alice grown under combined P placement and salt stress treatments. Abbreviations: AGB, Aboveground biomass; DeepHP−S, Deep Banded High P combined with non-salt stress; DeepRP−S, Deep Banded Reduced P combined with non-salt stress; DeepHP+S, Deep Banded High P combined with salt stress; DeepRP+S, Deep Banded Reduced P combined with salt stress; GW, Grain weight; −P−S, No P added combined with non-salt stress; P+S, No P added combined with salt stress; RW, Root weight; TopHP−S, Top Dressed High P combined with non-salt stress; TopRP−S, Top Dressed Reduced P combined with non-salt stress; TopHP+S, Top Dressed High P combined with salt stress; TopRP+S, Top Dressed Reduced P combined with salt stress.
Figure 5. Changes in aboveground and belowground biomass of wheat genotype LX99 grown under combined P placement and salt stress treatments. Abbreviations: AGB, Aboveground biomass; DeepHP−S, Deep Banded High P combined with non-salt stress; DeepRP−S, Deep Banded Reduced P combined with non-salt stress; DeepHP+S, Deep Banded High P combined with salt stress; DeepRP+S, Deep Banded Reduced P combined with salt stress; GW, Grain weight; −P−S, No P added combined with non-salt stress; P+S, No P added combined with salt stress; RW, Root weight; TopHP−S, Top Dressed High P combined with non-salt stress; TopRP−S, Top Dressed Reduced P combined with non-salt stress; TopHP+S, Top Dressed High P combined with salt stress; TopRP+S, Top Dressed Reduced P combined with salt stress.
Root length (RL) distribution within the soil profile and total root length (TRL) of wheat genotypes Alice and LX99 under combined P placement and salt stress treatments.
Genotype | Salt Stress Level | P Placement | RL0–20cm (m) | RL20–40cm (m) | RL40–100cm (m) | TRL (m) |
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Alice | Non-salt stress | TopHP | 99.95 ± 8.26 a | 24.30 ± 1.42 c | 21.05 ± 0.95 c | 145.30 ± 10.30 a |
DeepHP | 94.35 ± 4.31 ab | 30.31 ± 2.17 a | 18.95 ± 1.24 c | 143.62 ± 6.79 a | ||
TopRP | 86.46 ± 4.03 b | 26.60 ± 1.37 bc | 24.31 ± 1.61 ab | 137.37 ± 6.91 a | ||
DeepRP | 85.10 ± 9.47 b | 29.74 ± 0.89 a | 21.71 ± 0.82 bc | 136.54 ± 7.85 a | ||
−P | 92.32 ± 5.50 ab | 28.04 ± 1.12 ab | 26.10 ± 2.12 a | 146.45 ± 6.68 a | ||
Salt stress | TopHP | 99.25 ± 4.06 a | 21.84 ± 1.68 b | 17.09 ± 1.58 b | 138.18 ± 6.66 a | |
DeepHP | 91.17 ± 7.54 ab | 24.23 ± 1.01 a | 22.41 ± 1.92 a | 137.81 ± 10.29 a | ||
TopRP | 85.77 ± 5.93 b | 19.94 ± 0.66 b | 17.83 ± 1.07 b | 123.54 ± 7.54 b | ||
DeepRP | 63.72 ± 4.79 c | 17.06 ± 1.38 c | 9.14 ± 0.95 c | 89.92 ± 6.69 c | ||
−P | 53.27 ± 5.59 d | 13.33 ± 0.45 d | 3.32 ± 0.58 d | 69.92 ± 6.57 d | ||
LX99 | Non-salt stress | TopHP | 22.43 ± 1.14 a | 8.87 ± 0.77 a | 4.90 ± 0.59 d | 36.20 ± 2.37 bc |
DeepHP | 22.03 ± 0.82 a | 10.07 ± 1.02 a | 9.78 ± 1.18 b | 41.88 ± 1.54 a | ||
TopRP | 20.23 ± 1.44 ab | 5.48 ± 0.63 c | 7.20 ± 0.66 c | 32.91 ± 2.73 c | ||
DeepRP | 19.42 ± 0.84 b | 6.38 ± 0.70 bc | 8.24 ± 0.36 bc | 34.04 ± 1.74 c | ||
−P | 19.69 ± 1.65 b | 7.27 ± 0.60 b | 12.68 ± 1.23 a | 39.64 ± 3.48 ab | ||
Salt stress | TopHP | 25.47 ± 1.29 a | 7.88 ± 0.71 a | 7.86 ± 1.06 ab | 41.21 ± 3.02 a | |
DeepHP | 21.04 ± 2.13 b | 8.55 ± 0.78 a | 8.99 ± 0.80 a | 38.59 ± 3.66 a | ||
TopRP | 20.49 ± 1.12 b | 6.13 ± 0.79 b | 6.88 ± 0.47 b | 33.50 ± 2.36 b | ||
DeepRP | 19.93 ± 1.67 bc | 8.36 ± 0.90 a | 2.24 ± 0.29 c | 30.53 ± 2.73 b | ||
−P | 17.23 ± 1.53 c | 4.77 ± 0.14 c | 1.75 ± 0.41 c | 23.75 ± 1.85 c |
Note: Within a column for a particular cultivar and salt stress treatment, letters indicate significant differences for P placement treatments at the 0.05 level. Abbreviations: DeepHP, Deep Banded High P; DeepRP, Deep Banded Reduced P; −P, No P added; TopHP, Top Dressed High P; TopRP, Top Dressed Reduced P.
Agronomic traits of wheat genotypes Alice and LX99 under combined P placement and salt stress treatments.
Genotype | Salt Stress Level | P Placement | PH (cm) | GN/pot | 1000-GW (g) | AGB/pot (g) | GW/pot (g) | RW/AGB | TRL/AGB (m/g) |
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Alice | Non-salt stress | TopHP | 77.33 ± 7.51 a | 92.67 ± 5.03 a | 27.82 ± 0.77 b | 6.00 ± 0.15 a | 2.57 ± 0.11 a | 0.46 ± 0.04 b | 23.89 ± 2.18 b |
DeepHP | 78.50 ± 6.06 a | 80.00 ± 2.00 b | 28.10 ± 1.87 b | 5.57 ± 0.22 b | 2.24 ± 010 b | 0.47 ± 0.02 b | 25.77 ± 0.30 b | ||
TopRP | 74.37 ± 8.45 a | 71.67 ± 4.16 c | 30.26 ± 0.48 a | 5.40 ± 0.16 b | 2.17 ± 0.09 b | 0.48 ± 0.03 b | 25.48 ± 1.55 b | ||
DeepRP | 72.07 ± 4.46 a | 71.00 ± 2.65 c | 30.27 ± 1.09 a | 5.34 ± 0.26 b | 2.15 ± 0.14 b | 0.45 ± 0.03 b | 25.83 ± 2.64 b | ||
−P | 69.32 ± 3.53 a | 63.00 ± 3.46 d | 27.28 ± 0.40 b | 5.06 ± 0.14 c | 1.72 ± 0.07 c | 0.56 ± 0.02 a | 30.86 ± 0.96 a | ||
Salt stress | TopHP | 68.16 ± 7.00 a | 81.00 ± 3.61 a | 28.93 ± 1.04 bc | 5.42 ± 0.22 a | 2.34 ± 0.10 a | 0.48 ± 0.01 a | 25.51 ± 0.53 a | |
DeepHP | 67.68 ± 8.69 a | 72.33 ± 2.08 b | 30.34 ± 0.61 ab | 5.20 ± 0.21 ab | 2.20 ± 0.13 a | 0.48 ± 0.01 a | 26.45 ± 0.91 a | ||
TopRP | 67.19 ± 6.90 a | 65.33 ± 3.51 c | 30.62 ± 0.77 ab | 4.92 ± 0.25 bc | 2.00 ± 0.09 b | 0.44 ± 0.02 a | 25.08 ± 0.23 a | ||
DeepRP | 65.24 ± 6.70 a | 58.67 ± 4.51 d | 30.95 ± 0.63 a | 4.75 ± 0.20 c | 1.81 ± 0.08 c | 0.35 ± 0.03 b | 18.97 ± 2.15 b | ||
−P | 60.56 ± 4.07 a | 46.33 ± 3.51 e | 28.01 ± 1.40 c | 4.01 ± 0.27 d | 1.30 ± 0.06 d | 0.30 ± 0.02 b | 17.46 ± 1.55 b | ||
LX99 | Non-salt stress | TopHP | 63.71 ± 3.42 a | 48.00 ± 2.00 a | 33.26 ± 0.36 a | 3.02 ± 0.27 a | 1.60 ± 0.07 a | 0.29 ± 0.03 b | 12.08 ± 1.86 c |
DeepHP | 62.81 ± 3.02 a | 46.33 ± 2.52 ab | 32.01 ± 0.91 ab | 2.80 ± 0.16 b | 1.48 ± 0.11 b | 0.36 ± 0.02 ab | 14.99 ± 0.58 b | ||
TopRP | 57.45 ± 5.90 ab | 45.67 ± 3.21 ab | 30.99 ± 0.45 b | 2.52 ± 0.17 c | 1.41 ± 0.08 b | 0.31 ± 0.03 b | 13.09 ± 1.17 bc | ||
DeepRP | 58.59 ± 5.50 ab | 44.00 ± 2.00 ab | 32.15 ± 0.47 ab | 2.65 ± 0.15 bc | 1.42 ± 0.10 b | 0.31 ± 0.02 b | 12.85 ± 0.60 c | ||
−P | 52.11 ± 3.01 b | 42.67 ± 2.52 b | 29.31 ± 1.04 c | 2.26 ± 0.24 d | 1.25 ± 0.06 c | 0.40 ± 0.03 a | 17.60 ± 1.21 a | ||
Salt stress | TopHP | 60.34 ± 7.50 a | 46.33 ± 1.15 a | 30.25 ± 1.18 a | 2.56 ± 0.25 a | 1.40 ± 0.10 a | 0.35 ± 0.04 a | 16.27 ± 2.77 a | |
DeepHP | 59.37 ± 4.47 a | 46.00 ± 2.00 a | 29.70 ± 1.35 a | 2.50 ± 0.16 ab | 1.37 ± 0.05 a | 0.36 ± 0.01 a | 15.41 ± 0.66 a | ||
TopRP | 57.03 ± 5.27 ab | 43.67 ± 4.73 ab | 30.11 ± 1.20 a | 2.34 ± 0.15 ab | 1.30 ± 0.09 ab | 0.33 ± 0.03 a | 14.35 ± 0.98 ab | ||
DeepRP | 52.67 ± 3.79 ab | 39.33 ± 2.08 bc | 31.48 ± 0.69 a | 2.31 ± 0.19 b | 1.25 ± 0.11 b | 0.30 ± 0.02 ab | 13.20 ± 0.86 b | ||
−P | 49.00 ± 3.00 b | 34.33 ± 3.06 c | 28.45 ± 2.96 a | 1.86 ± 0.11 c | 0.97 ± 0.08 c | 0.27 ± 0.02 b | 12.84 ± 1.44 b |
Note: Within a column for a particular cultivar and salt stress treatment, letters indicate significant differences for P placement treatments at the 0.05 level. Abbreviations: DeepHP, Deep Banded High P; DeepRP, Deep Banded Reduced P; −P, No P added; RW/AGB, Root weight/Aboveground Biomass; TopHP, Top Dressed High P; TopRP, Top Dressed Reduced P.
Elemental accumulation in belowground and aboveground parts of wheat genotypes Alice and LX99 under combined P placement and salt stress treatments.
Genotype | Plant Part | Salt Stress Level | P Placement | Elemental Accumulation (mg/pot) | ||||
---|---|---|---|---|---|---|---|---|
Na | K | Ca | Mg | P | ||||
Alice | Aboveground plant | Non-salt stress | TopHP | 1.06 ± 0.13 a | 119.37 ±11.70 a | 6.64 ± 0.93 a | 10.53 ± 0.25 a | 26.20 ± 0.89 a |
DeepHP | 1.25 ± 0.10 a | 116.77 ± 5.42 a | 6.83 ± 0.82 a | 9.27 ± 0.70 a | 22.55 ± 0.42 b | |||
TopRP | 0.80 ± 0.05 b | 121.60 ± 10.03 a | 6.61 ± 0.03 a | 10.98 ± 0.90 a | 23.54 ± 1.86 b | |||
DeepRP | 1.08 ± 0.11 a | 123.37 ± 18.73 a | 6.82 ± 0.92 a | 9.82 ± 1.30 a | 23.35 ± 2.38 b | |||
−P | 0.79 ± 0.08 b | 111.33 ± 31.68 a | 7.05 ± 0.39 a | 10.15 ± 1.72 a | 18.15 ± 0.62 c | |||
Salt stress | TopHP | 6.83 ± 0.70 ab | 88.41 ± 7.62 c | 6.47 ± 1.08 a | 9.20 ± 0.78 bc | 22.64 ± 1.46 ab | ||
DeepHP | 6.04 ± 0.40 bc | 105.55 ± 8.20 bc | 5.93 ± 0.81 a | 8.42 ± 0.85 c | 21.50 ± 2.10 abc | |||
TopRP | 7.53 ± 0.69 a | 124.32 ± 16.69 ab | 6.66 ± 0.82 a | 10.82 ± 0.83 a | 23.77 ± 2.53 a | |||
DeepRP | 5.38 ± 0.89 c | 140.73 ± 18.23 a | 7.53 ± 1.17 a | 10.01 ± 0.72 ab | 19.62 ±1.88 bc | |||
−P | 3.51 ± 0.37 d | 124.55 ± 13.39 ab | 6.43 ± 0.46 a | 10.21 ± 0.66 ab | 18.26 ±0.31 c | |||
Roots | Non-salt stress | TopHP | 1.93 ± 0.20 bc | 7.24 ± 1.64 a | 6.04 ± 0.43 ab | 4.84 ± 0.63 a | 2.10 ±0.29 a | |
DeepHP | 1.64 ± 0.14 c | 6.01 ± 0.54 a | 6.68 ± 0.23 ab | 5.06 ± 0.23 a | 1.88 ±0.14 a | |||
TopRP | 2.40 ± 0.19 a | 6.02 ± 0.87 a | 7.80 ± 2.18 a | 4.72 ± 0.52 a | 2.18 ±0.47 a | |||
DeepRP | 2.12 ± 0.22 ab | 6.15 ± 1.56 a | 5.84 ± 1.99 ab | 4.74 ± 0.38 a | 1.77 ±0.41 a | |||
−P | 1.73 ± 0.05 c | 6.56 ±2.00 a | 5.02 ± 0.23 b | 5.01 ± 0.20 a | 1.77 ±0.58 a | |||
Salt stress | TopHP | 8.25 ± 0.91 a | 5.28 ±0.44 ab | 5.77 ± 0.53 b | 4.05 ± 0.62 b | 2.32 ± 0.36 a | ||
DeepHP | 8.72 ± 1.02 a | 5.48 ±0.95 a | 6.71 ± 0.71 a | 4.93 ± 0.65 a | 1.94 ±0.33 a | |||
TopRP | 7.86 ± 0.49 a | 4.31 ± 0.55 bc | 4.69 ± 0.44 c | 3.51 ± 0.42 bc | 1.45 ±0.15 b | |||
DeepRP | 4.57 ± 0.22 b | 2.95 ± 0.10 d | 2.56 ± 0.10 d | 2.86 ± 0.10 cd | 1.00 ± 0.05 c | |||
−P | 5.31 ± 0.36 b | 3.48 ± 0.37 cd | 1.91 ± 0.17 d | 2.06 ± 0.26 d | 0.96 ±0.10 c | |||
LX99 | Aboveground plant | Non-salt stress | TopHP | 0.53 ± 0.07 b | 57.15 ± 12.42 a | 3.54 ± 0.24 a | 6.06 ± 0.14 a | 15.10 ±1.50 a |
DeepHP | 0.74 ± 0.05 a | 48.17 ± 2.80 a | 3.28 ± 0.51 a | 5.69 ± 0.63 ab | 13.59 ±0.52 ab | |||
TopRP | 0.44 ± 0.05 b | 57.01 ± 8.51 a | 3.07 ± 0.13 a | 5.21 ± 0.42 bc | 12.67 ±1.08 bc | |||
DeepRP | 0.49 ± 0.05 b | 57.94 ± 7.95 a | 3.49 ± 0.42 a | 5.85 ± 0.41 ab | 13.61 ±0.27 ab | |||
−P | 0.33 ± 0.04 c | 48.93 ± 5.25 a | 2.41 ± 0.18 b | 4.90 ± 0.13 c | 11.02 ±0.84 c | |||
Salt stress | TopHP | 4.12 ± 0.19 a | 37.52 ± 4.43 c | 2.96 ± 0.42 a | 5.09 ± 0.60 a | 14.75 ±1.21 a | ||
DeepHP | 3.74 ± 0.39 ab | 45.11 ± 7.50 bc | 2.47 ± 0.34 a | 4.59 ± 0.52 ab | 12.48 ±1.45 ab | |||
TopRP | 2.80 ± 0.39 c | 51.72 ± 5.32 ab | 2.54 ± 0.44 a | 4.80 ± 0.59 ab | 11.85 ±1.20 bc | |||
DeepRP | 2.18 ± 0.13 d | 60.89 ± 2.54 a | 2.92 ± 0.14 a | 4.89 ± 0.18 ab | 13.26 ±0.80 ab | |||
−P | 3.31 ± 0.37 bc | 43.95 ±6.14 bc | 1.78 ± 0.28 b | 3.97 ± 0.69 b | 9.81 ±1.53 c | |||
Root | Non-salt stress | TopHP | 0.87 ± 0.10 a | 6.75 ± 1.80 a | 3.92 ± 0.49 a | 1.98 ± 0.19 a | 1.60 ± 0.40 a | |
DeepHP | 0.91 ±0.09 a | 4.26 ± 2.31 a | 2.80 ± 0.30 b | 1.89 ± 0.23 ab | 1.01 ± 0.07 b | |||
TopRP | 0.70 ± 0.06 c | 2.54 ±0.17 b | 2.56 ± 0.66 b | 1.53 ± 0.18 b | 0.92 ± 0.14 b | |||
DeepRP | 0.74 ± 0.05 bc | 4.16 ± 0.24 a | 2.64 ± 0.23 b | 1.88 ± 0.12 ab | 0.95 ± 0.13 b | |||
−P | 0.84 ± 0.08 ab | 5.42 ± 0.41 a | 2.34 ± 0.53 b | 1.86 ± 0.23 ab | 0.95 ± 0.12 b | |||
Salt stress | TopHP | 2.03 ± 0.07 c | 2.01 ± 0.26 d | 3.34 ± 0.20 a | 1.89 ± 0.26 a | 1.21 ± 0.14 a | ||
DeepHP | 2.45 ± 0.22 ab | 1.72 ± 0.20 d | 2.18 ± 0.15 b | 1.51 ± 0.14 b | 0.88 ± 0.12 bc | |||
TopRP | 2.64 ± 0.11 a | 2.46 ± 0.17 c | 1.91 ± 0.11 c | 1.35 ± 0.11 b | 0.81 ± 0.05 c | |||
DeepRP | 2.67 ± 0.14 a | 3.68 ± 0.24 a | 2.09 ± 0.08 bc | 1.50 ± 0.05 b | 1.00 ± 0.07 b | |||
−P | 2.23 ± 0.14 bc | 3.16 ± 0.35 b | 1.09 ± 0.07 d | 1.00 ± 0.13 c | 0.56 ± 0.05 d |
Note: Within a column for a particular cultivar and salt stress treatment, letters indicate significant differences for P placement treatments at the 0.05 level. Abbreviations: DeepHP, Deep Banded High P; DeepRP, Deep Banded Reduced P; −P, No P added; TopHP, Top Dressed High P; TopRP, Top Dressed Reduced P.
PUE, PAE, and PPE of wheat genotypes Alice and LX99 under combined P placement and salt stress treatments.
Genotype | Salt Stress Level | P Placement | PUE (%) | PAE (g/g) | PPE (g/g) |
---|---|---|---|---|---|
Alice | Non-salt stress | TopHP | 0.42 ± 0.02 b | 0.35 ± 0.04 a | 84.04 ± 8.20 a |
DeepHP | 0.26 ± 0.01 c | 0.22 ± 0.02 b | 82.51 ± 8.64 a | ||
TopRP | 0.63 ± 0.05 a | 0.38 ± 0.04 a | 59.57 ± 6.13 b | ||
DeepRP | 0.40 ± 0.05 b | 0.36 ± 0.04 a | 89.25 ± 9.12 a | ||
−P | – | – | – | ||
Salt stress | TopHP | 0.24 ± 0.02 b | 0.43 ± 0.04 b | 181.14 ± 17.68 b | |
DeepHP | 0.18 ± 0.02 b | 0.37 ± 0.04 b | 210.98 ± 22.32 b | ||
TopRP | 0.46 ± 0.05 a | 0.58 ± 0.06 a | 116.65 ± 11.91 c | ||
DeepRP | 0.11 ± 0.01 c | 0.43 ± 0.04 b | 376.79 ± 36.53 a | ||
−P | – | – | – | ||
LX99 | Non-salt stress | TopHP | 0.20 ± 0.03 ab | 0.15 ± 0.02 a | 73.98 ± 7.42 b |
DeepHP | 0.11 ± 0.01 b | 0.10 ± 0.01 b | 87.34 ± 8.55 ab | ||
TopRP | 0.14 ± 0.01 b | 0.13 ± 0.01 ab | 98.60 ± 8.97 a | ||
DeepRP | 0.22 ± 0.02 a | 0.14 ± 0.02 a | 65.71 ± 7.06 c | ||
−P | – | – | – | ||
Salt stress | TopHP | 0.23 ± 0.02 b | 0.18 ± 0.02 b | 76.89 ± 8.13 b | |
DeepHP | 0.12 ± 0.01 c | 0.17 ± 0.02 b | 133.57 ± 13.78 a | ||
TopRP | 0.19 ± 0.02 b | 0.28 ± 0.03 a | 144.31 ± 14.42 a | ||
DeepRP | 0.32 ± 0.03 a | 0.23 ± 0.03 a | 72.10 ± 7.80 b | ||
−P | – | – | – |
Note: Within a column for a particular cultivar and salt stress treatment, letters indicate significant differences for P placement treatments at the 0.05 level. Abbreviations: PAE, P agronomic efficiency; PUE, P utilization efficiency; PPE, P physiology efficiency.
Supplementary Materials
The following supporting information can be downloaded at:
References
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Abstract
Phosphorus (P) management in wheat grown under saline soil conditions requires a better understanding of how P placement and salt stress affect spatial root distribution, plant agronomic performance, and nutrient utilization. A column culture experiment was conducted with two wheat genotypes, Alice and LX99, five P placements (‘Top Dressed High P’, TopHP; ‘Top Dressed Reduced P’, TopRP; ‘Deep Banded High P’, DeepHP; ‘Deep Banded Reduced P’, DeepRP; and ‘No P added’, −P), and two salt stress levels (‘salt stress’, +S; ‘non-salt stress’, −S) to investigate differences in biomass accumulation, nutrient utilization, and root distribution (0–20 cm, 20–40 cm, and 40–100 cm) among treatments. Deep P placement under non-salt stress increased root distribution at 20–40 cm soil depth for both genotypes compared to top P placement. P application under salt stress increased root weight at all three soil depths in both genotypes and P accumulation in aboveground plant parts (except for LX99 under TopRP). The highest P utilization efficiency occurred in Alice with TopRP and in LX99 with DeepRP under non-salt and salt stresses. Overall, a coordinated adaptation in allocating biomass between belowground and aboveground plant parts, along with altered nutrient utilization, was necessary to reach a growth compromise in response to P supply changes and salinity. Therefore, genotype, P placement depth, and soil salinity should be considered to improve wheat P utilization efficiency under saline conditions.
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1 College of Biological and Environmental Engineering, Binzhou University, Binzhou 256603, China;
2 College of Biological and Environmental Engineering, Binzhou University, Binzhou 256603, China;
3 College of Biological and Environmental Engineering, Binzhou University, Binzhou 256603, China;
4 Department of Biotechnology, COMSATS University Islamabad, Abbottabad Campus, Abbottabad 22060, Pakistan;
5 The UWA Institute of Agriculture, The University of Western Australia, Perth, WA 6001, Australia;