Varietal variation in northern grain responses to phosphorus

Take home message

After accounting for other varietal choice factors (climate, disease, yield, rotation etc.) some wheat and barley varieties are better suited to certain soil P scenarios. We would not advise varietal selection purely on the basis of P responsiveness, however, if all other things are equal, these traits may assist decision making. There remains potential to target varieties to make better use of the prevailing nutritional environment.

Baxter, Kennedy and Hindmarsh are good varieties under low P nutrition. Kennedy, Sunco, Suntop and Shepherd grow well in fertile situations. Kennedy, Spitfire and Clearfield® Scope barley may acquire more P from deep P bands. Gregory, Suntop and most barley varieties may grow well where surface soil is P enriched, but subsoil P reserves are low.

Minimal early season root plasticity was observed in the 6 chickpea varieties tested.  Yorker was the most responsive to a P band.

Introduction

GRDC recently invested in quantifying the variation in root response of 10 northern wheat and 5 barley lines to P, K, S and N nutrition. A modification to the project also included responses of 6 chickpea lines to P and K nutrition. Interestingly, and for another paper perhaps, it was not possible to measure significant responses to N, K or S nutrition when encountered in low N, K or S scenarios. It seems P plasticity is the main root adaptation for these lines and this paper focusses on P nutrition.

The data acquired was used to generate a table identifying which varieties may respond well in low soil P, high soil P, stratified P (high surface and low subsoil P) and where growers have placed subsoil banded P. These are conditions growers may face under a range of fertiliser application strategies. For example, low soil P may arise with a history of cropping with minimal P replacement. High soil P may occur where land is recently cleared, or fertiliser application has been maintained with both surface and subsoil placement. Stratified P scenarios arise with regular application of starter P rates or controlled traffic scenarios, and deep placement is a recent development following recognition of subsoil P resource depletion.

Materials and methods

Ten common Australian wheat varieties (cv. Baxter, Crusader, Gregory, Kennedy, Spitfire, Sunco, Sunguard, Suntop and Sunvale) and five barley varieties (cv. Clearfield® Scope, Commander, Gairdner, Hindmarsh and Shepherd) were investigated. These varieties represent the most grown, and most likely to be grown, in the major central-eastern Australian cereal areas over the next 3-5 years.

Nutrient acquisition trials were undertaken to assess root proliferation responses to applied fertiliser, critical internal nutrient requirements and annual uptake patterns with a view to recommending varieties best suited for a range of soil nutrition scenarios. 

Root plasticity experiments

Five hundred grams of sieved (<5 mm) P and N responsive Red Ferrosol subsoil from Kingaroy was packed in the closed cell polyethylene tubes (aka ‘pool noodles’). The soil moisture was maintained at 80% of field capacity throughout the trial. A 4 cm P band (150 mg P kg-1) was placed 3 cm below the surface of the soil. The P was supplied as MAP and the soil above and below the P band was adjusted to the N concentration in the band (68 mg N kg-1) using urea. Basal K, S and Ca were supplied. A negative control containing only N and basal nutrients (No-P), and a positive control containing P at the banded rate throughout the entire soil volume were also established (Uniform P).  One germinated seed of wheat was planted approximately 3 mm below the soil surface of each tube. The treatments were replicated 4 times and grown in a glasshouse with day and night time temperatures of 25°C and 14°C, respectively.  All plants were grown to the 4th leaf stage.  The plants were then harvested, dried at 40°C in oven and weighed.  Shoots were digested and analysed for P. Roots were extracted, washed and scanned on a flatbed at 600 dpi (i.e. 42 µm) and analysed with WinRhizo® v. 2009c software to determine root length and average root diameter of each section.  Six chickpea varieties (cv Yorker, Pistol, Boundary, Slasher, HatTrick, Kyabra) were also examined under the above conditions, with a growth period of 25 days.

Nutrient recovery experiments

40 kg of fertile Vertosol soil was packed in large bins and planted to the same 15 wheat and barley varieties.  Subsamples were taken for nutrient analysis at 5 dates from 30 days after emergence until maturity; biomass and nutrient concentrations measured at each time point.  Basal, N, P, K and S were applied so that growth was nutritionally unconstrained. Data was plotted as nutrient uptake relative to plant dry matter production.

Critical tissue P concentration

One kg of Red Ferrosol subsoil was treated with increasing rates of P (or N) and each of 15 cereal varieties was grown in the glasshouse for 6 weeks.  Moisture was maintained at 80% of FC and all other nutrients were applied such that only the nutrient in question was limiting.  Plants were harvested, dried, weighed and analysed for P uptake to determine the internal P requirement of wheat and barley varieties.

Results

Root plasticity

All varieties responded to P application in the band, and grew faster as a consequence. All varieties showed plasticity of roots (i.e. proportional RLD increase in band compared to the surface) and produced between 3 and 18 cm cm-3 of root in the P enriched patch (3-7 cm) in P band treatment (Fig. 1).

Figure 1. Example of the increase in root length density (RLD) of wheat varieties over 13 cm following either no P application (No-P), a band containing 150 mg P kg-1 (P band) or a uniform profile with 150 mg P kg-1 throughout (Uniform P).

Figure 1. Example of the increase in root length density (RLD) of wheat varieties over 13 cm following either no P application (No-P), a band containing 150 mg P kg-1 (P band) or a uniform profile with 150 mg P kg-1 throughout (Uniform P).

The greatest increase in RLD between the surface and the band was observed in Kennedy (Fig. 2).  However, statistically speaking there was no difference between the varieties in their ability to proliferate roots in response to encountering banded P. This lack of significant difference is a function of the large variation in root system expression in cereals. The extra root length produced in the P band was up to 4 times higher, and in some varieties 8 times higher.  Barley and wheat were not different in their plasticity responses although barley varieties were less plastic at the high end, then wheat varieties.

Recovery of P over time

Most wheat varieties took up P from the soil as they needed it.  The relative accumulation of P was in advance of plant growth in most of the barley varieties, but Gregory and Suntop also took up P before they needed it for biomass accumulation (Figure 3).

Figure 2.  Plasticity of 10 wheat and 5 barley varieties over 13 cm following either no-P application (No-P), a band containing 150 mg P kg-1 (P band) or a uniform profile with 150 mg P kg-1 throughout (High P), and comparison of the P use efficiency of varieties when encountering the band of P.

Figure 2.  Plasticity of 10 wheat and 5 barley varieties over 13 cm following either no-P application (No-P), a band containing 150 mg P kg-1 (P band) or a uniform profile with 150 mg P kg-1 throughout (High P), and comparison of the P use efficiency of varieties when encountering the band of P.

Acquisition efficiency

The ability of varieties to acquire P under low soil P status was measured through comparing the recovery of P from a low P soil and comparing it with recovery under P sufficient conditions in the same soil.  Considerable variation in varietal ability to acquire P from a scarce environment was observed (Figure 4). Baxter and Kennedy, Gairdner and Hindmarsh were particularly efficient at acquiring P when in low soil supply, recovering up to twice as much P from the same soil as less efficient varieties.

Figure 3.  Phosphorus utilisation efficiency (Dry matter/P uptake) of 10 wheat and 5 barley varieties grown to maturity in 40 kg of Vertosol soil.  Relative accumulation of P for varieties that acquired P in advance of growth requirements indicated on the right.  All other varieties had overlapping P uptake and dry matter curves.

Figure 3.  Phosphorus utilisation efficiency (Dry matter/P uptake) of 10 wheat and 5 barley varieties grown to maturity in 40 kg of Vertosol soil.  Relative accumulation of P for varieties that acquired P in advance of growth requirements indicated on the right.  All other varieties had overlapping P uptake and dry matter curves.

Figure 4.  Phosphorus acquisition efficiency (P uptake low P/P uptake sufficient P x 100) of 10 wheat and 5 barley varieties grown in a P responsive Red Ferrosol subsoil

Figure 4.  Phosphorus acquisition efficiency (P uptake low P/P uptake sufficient P x 100) of 10 wheat and 5 barley varieties grown in a P responsive Red Ferrosol subsoil

Discussion

Root plasticity of wheat and barley varieties

Root architectural variation in northern wheat, barley and chickpea lines in response to banded P application had not been measured prior to this experiment. All cereal varieties showed root plasticity in the P band by producing more branch roots. These seminal root branches in the P band produced thin roots with higher specific surface area. The diameter of roots in the P band was significantly lower than that of surface layers, suggesting that lateral roots specifically increased specific surface area only where P supply was highest.  This study revealed that variation in this trait is small for varieties selected for the northern cereal zone.  Two of the wheat varieties tended towards increased plastic responses. Comparison of these with modern southern varieties, and, more importantly, with Federation, landrace or wild wheat lines may indicate how much scope there is for selecting traits that exploit bands of P fertiliser more efficiently.  Interestingly, there was limited capacity for early stage lateral root expression in response to banded P in the 6 chickpea varieties tested.  Chickpeas were grown for longer time periods than cereals, yet still failed to respond with increased root allocation to P enriched soil.  This may account for some of the variable field responses to banded fertiliser applications by chickpea.

Phosphorus uptake and acquisition efficiency

There was considerable genetic variation in the capacity of plants to access P from this P responsive soil. Baxter had the highest acquisition efficiency and Sunguard  the lowest.  In a low P environment, longer root lengths may increase surface area and therefore the opportunity to acquire P from the larger surface area. Similarly, there was variation in both how much P was required by individual varieties to generate shoot biomass, and in how far in advance of biomass production varieties acquired the P from the soil.  This suggests that some varieties will grow better, and yield higher in soils with low P status.  And that some varieties, with the ability to accumulate P in advance of requirements will perform better in scenarios where there is a P rich topsoil that can be exploited and mined for P early in the season before drawing water from P depleted subsoil layers.  These traits and varieties are described and summarised in Table 1.

Table 1. Summary of 15 cereal variety responses to soil P status

Variety

Low P soil

 Uniform high P soil

Deep P application

Stratified P

Biomass

RLD

AE

PUtE

Biomass

RLD

PUtE

Biomass

Root Plasticity

PUtE

PUE

PUtE

Update Pattern

Wheat

Baxter

H

H

H

M

M

M

L

H

M

H

M

H

SPB

Crusader

M

H

M

M

M

H

M

M

M

L

L

H

SPB

Gregory

L

L

M

H

M

H

H

M

M

H

M

H

PAB

Kennedy

H

H

H

M

H

H

H

H

H

M

M

H

SPB

Dart

H

M

M

M

H

H

M

M

M

M

L

H

SPB

Spitfire

H

H

M

M

M

H

M

H

H

M

M

H

SPB

Sunco

M

H

M

M

H

H

H

M

M

M

M

H

SPB

Sunguard

M

M

M

M

M

H

M

H

L

H

H

H

SPB

Suntop

M

M

M

M

H

H

H

M

L

M

L

H

PAB

Sunvale

M

H

M

M

M

H

M

M

M

L

L

H

SPB

Barley

Clearfield Scope

M

M

M

H

M

H

H

H

H

H

H

H

SPB

Commander

M

M

M

H

M

M

M

H

H

L

H

H

PAB

Gairdner

H

M

H

M

M

H

M

H

H

M

M

H

PAB

Hindmarsh

H

H

H

H

M

H

M

H

M

L

L

H

PAB

Shepherd

H

H

M

H

H

H

H

H

M

M

M

H

PAB

Note: Ranking above is based on the maximum value obtained in each category. High (H) = 100-70% of maximum, Medium (M) = 70-40% of maximum, Low (L) = <40% of maximum. RLD = root length density, AE = P Acquisition efficiency (uptake in low P soil/uptake in sufficient P soil), PUtE = P utilization efficiency (Shoot dry matter/P uptake), PUE = P use efficiency (P utilization efficiency × P uptake efficiency), PAB = Uptake P in advance of biomass accumulation, SPB = Uptake P as biomass accumulates

In summary, we hypothesised that there would be variation amongst 15 commonly grown wheat and barley varieties in their ability to proliferate and respond to banded P applications. We observed no significant differences in mean in that trait, and observed some evidence that a band is only a temporary, and not particularly efficient, reprieve for roots foraging in low P soil environments. All varieties possess the ability to decrease root diameters and increase root length density when encountering enriched P patches, and there is evidence of increasing variability in those varieties with the greatest mean RLD response.  The question remains what level of root plasticity is required for plants to gain most from heterogeneously distributed nutrients in soil and how we can manage the soil or fertiliser placement to optimise these gains against changing environments.

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

Contact details

Chris Guppy
UNE
Agronomy and Soil Science
Ph: 02 6773 3567
Fx: 02 6773 3238
Email: cguppy@une.edu.au

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