Long coleoptile wheats – for deep seeding and optimising sowing window options

Take home message

• Ensure high seed quality (seed weight, gemination % and vigour) to maximise emergence in adverse conditions
• Sowing deep early in the optimum sowing window will out yield sowing shallow or deep late in the window in almost all scenarios
• Ensure press wheel pressure does not further exacerbate stress on emerging seedlings
• Soil temperature above 19°C can have a negative effect on coleoptile length and needs to be considered when timing sowing early in the planting window.

Background

Since the green revolution when dwarfing genes were introduced into wheat varieties to increase grain yield, there has been unintended selection for shorter coleoptile length, reducing the plants’ ability to emerge from deep sowing or in unfavourable conditions. Long coleoptile genes have long been investigated as a possible solution to increase coleoptile length. However, in recent times this trait is being investigated to assist with changing autumn rainfall patterns to optimising more timely sowing opportunities.

Extensive laboratory and field testing conducted in southern and western grain regions of Australia showed that the substitution of the Rht-B1b and Rht-D1b dwarfing genes for alternative Rht13 and Rht18 genes can significantly increase the length and diameter of the coleoptile without increasing the overall height of the plant (Rebetzke et al., 1999). However, it was also discovered that coleoptile length can be almost halved in warm soils (>19 degrees) for both short and long coleoptile wheats alike (Rebetzke et al., 2004).

The work reported here aimed to test the ability of LCW (which have coleoptiles nearly double the length of traditional dwarf wheats) and shorter coleoptile wheat (SCW) to emerge from deep sowing in the heavy vertosol soils of central and southwest Queensland.

All trials were planted on 50 cm row spacings, at two depths. Shallow sowing was aimed at representing standard sowing practice (SSP), with no more than 3 - 5 cm of soil over the seed (including subsidence) post plant. The deep sowing (DS) treatments were targeted to have 9 - 11 cm of soil over the seed (including subsidence). Despite many positive anecdotal observations across the five trials, the two-year project was not able to statistically conclude that the LCW lines improved emergence in Southern and Central Queensland compared with SCW. Reasons for this are explained in this paper.

Significant benefits were identified around optimising deep sowing results regardless of coleoptile type.

Possible emergence limitations

1. Soil temperature

Soil temperature has been proven to limit/reduce coleoptile length and diameter, particularly when soil temperatures exceed 19^oC. (Rebetzke et al., 2004). Trials conducted since 2021, actively measuring soil temperatures at SSP (3 - 5 cm) and DS (9 - 11 cm) and demonstrated that temperatures >20⁰C are often experienced (Figure 1).

The data in Figures 1 (Emerald) & 2 (southern Qld sites) represent the range, mean and median of air and soil temperature at both standard and deep sowing depths, observed during the first 20 days post sowing for six trials over three years. The temperatures experienced at the Emerald Central Queensland Smart Cropping Centre (CQSCC) location were significantly warmer (Figure 1) than the southern Queensland (SQ) sites (Figure 2). But it is important to note that the late April sown SQ site in 2022 did experience soil temperatures above the 19^oC threshold found to significantly limit coleoptile length and diameter (Figure 2).

Figure 1. Air and soil temperature for two sowing depths at the Central Queensland Smart Cropping Centre, Emerald based on 15 minute observations (TOS 1 only). The box shaded area represents 50% of all observations, the line across the box represents the median temperature observed and the X marker represents the mean temperature for the 20-day period. The graph shows soil temperatures, while variable over the 24 hour, 20-day period, averaged greater than 20°C, with the 2022 TOS 1 treatment averaging soil temperatures above 25°C, both shallow and deep.

Figure 2. Air and soil temperature for two sowing depths at the southern Queensland trial sites (TOS 1 only). The shaded box area represents 50% of all observations, the line across the box represents the median temperature observed and the X marker represents the mean temperature for the 20-day period. The graph shows soil temperatures, while variable over the 24 hour, 20-day period, were significantly cooler than the CQ site, though the late April sowing at both Lundavra and Ranges Bridge (Macalister) show these sites did experience average soil temperatures at or above the 19^oC threshold discussed.

2. Seed quality

The quality of seed wheat can be assessed in a multitude of ways, and often influence emergence. Factors that impact seed quality include:

• Climatic conditions during grain fill
• Indications of disease on seed
• Colouring/appearance/protein content
• Time since harvest
• Storage temperature and moisture post-harvest
• Insect damage in storage
• Seed treatments applied (and when).

But when trying to compare one seed lot with another, typically it will come down to three key attributes:

• Germination (%) - the percentage (%) of seeds that will germinate, given suitable moisture conditions, within a defined period, typically seven to ten days.
• Seed size - Presented as the weight of a representative number of seeds. Often presented as grams per 300 or 1000 seeds.
• Seed vigour - Generally understood in concept, but poorly/inconsistently assessed. At its most basic level, it is an assessment of the seedling’s ability to germinate and then emerge from defined a seedbed environment in a timely manner.

Each of these seed attributes are key to identifying the expected performance of planting seed, and all three are linked. Ideally, when deep sowing, growers should source the very best quality seed available. All seed should be tested for all three of these attributes prior to planting, and the following calculations conducted to optimise crop establishment.

High germination test counts, based on testing completed in a damp chux™ cloth on the kitchen table, nor excessively large wheat seed do not guarantee that seed will be able to emerge when sown deep into heavy warm vertosol soils at commercially acceptable populations.

3. Soil strength

An increase in soil strength will impede the developing coleoptile emergence. Inherently, soils with higher clay content (vertosols) tend to have a higher density, while softer scrub soils (with lower clay content) tend to have a lower density and consequently faster emergence. By just over doubling the sowing depth there is a significantly increase the energy required for a seedling to push through to the surface. Soil density or soil strength can also be exacerbated by excessive press wheel down pressure, particularly in damp soils. In some worse case scenarios the seed trench can set like rock as it dries or following rain create anaerobic conditions causing seedling death.

Observations

Data from the 2022 TOS 1 Emerald CQSCC and Lundavra trials (Figure 3) indicate that genotype emergence from deep sowing ranged from 20 - 90% of the shallow sown treatments for the first sowing date at each site. These two sites and sowing dates experienced the highest average temperatures during the initial 20-day emergence period for our trial program to date.

The response from most genotypes was remarkably similar across the two sites, given the cooler (and wetter) conditions experienced at the Lundavra SQ site. There were some notable exceptions, the first being Sunchaserwhich performed exceptionally well at the SQ site relative to the CQSCC site, something it has repeated in other trials in SQ in other years. It had the third largest seed in the trial and tested seed germination was excellent (Table 1), yet at the CQSCC site, its emergence was middle of the pack. Gregory18 and Scepteralso emerged better at the SQ Lundavra site compared to the Emerald CQSCC.

The most likely reason for the difference in emergence (relative to shallow) between sites for these three lines is being driven by the 5^oC cooler soil temperature at the SQ site over the emergence period. But if so, why did Magenta13 and the pre-release line 16Y466-012 perform better from deep sowing in the hotter CQ conditions (Figure 3) than in SQ? Testing showed both seed lines had acceptable quality seed, Magenta13 had the largest seed of all the lines and the pre-release line 16Y466-012 had tested germination of 95% (Table 1).

Figure 3. Emergence from deep sowing, presented as a percentage of shallow sown emergence, for each genotype (TOS 1, 2022 only) at both the Emerald CQSCC and SQ Lundavra sites in 2022. The response was quite consistent across sites, but there are some outliers which could be explained by either temperature or reduced vigour.
Magenta 13, LRPB Scout 18, Gregory 13, Calibre, Sunchaser, Mace 18, Mace, LRPB Mustang, Borlaug 100, Gregory 18, LRPB Flanker & Scepter are protected under the Plant Breeders Rights Act 1994

As background, the shallow/overall emergence at the Lundavra SQ site was lower than the CQSCC site, despite using the same seed. It was later found that the press-wheel down-force of the SQ planter (coil spring type rather than a solid or semi pneumatic type) was higher than first expected. While appearing to be satisfactory at planting, as the soil in the furrow dried post plant, it set very hard.

Table 1. Seed quality attributes and sowing rate for the 2022 LCW trial program at the Emerald CQSCC and Lundavra.
Italicised text indicates LCW lines while bold text indicates conventional lines.

Time of sowing 1 (April) conditions were conducive to a good planting however after significant rain TOS 2 (May) did not occur due to wet conditions. It was only just dry enough to plant TOS 3 (June) and the deep sown treatment had to be written off due to almost zero emergence. This was caused by compaction from excessive press wheel pressure on wet soil which resulted in hard setting surface.

Despite the extra compaction described above, the data seemed to indicate that while the germination and seed size numbers for both Magenta13 and 16Y466-012 looked sufficient, it was likely that seed vigour may not have been as good as other genotypes in the trial, leading to the reduced emergence compared to shallow sowing at the SQ site relative to the CQ site.

It is important to note that these observations are all ‘relative’ to each other, not absolute. It’s also important to remember that the genotypes in this trial have a range of coleoptile lengths which would also factor into performance from depth. To that point, the final observation to make from Figure 3 is when you look at the ranking of genotypes on the X-axis (Figure 3), of the 14 lines listed, only two of the top eight lines were not equipped with the alternative Rht13 and Rht18 genes. There is also a strong suspicion that the Gregory18 seed had compromised vigour, given how much better the Gregory13 performed.

Grower experience – timing of deep sowing

An important component of the DAQ2104-005RTX project was understanding what experience growers have had with deep sowing winter cereals. While most were confident deep sowing chickpea, the feedback was quite different for deep sowing cereals. There was a cohort of growers in both CQ and SQ who had the equipment and felt reasonably comfortable being able to chase moisture down to depths of 10 to 15 cm, without getting too much soil back over the seed. But even within this cohort, the experience had been at best neutral to negative relative to their chickpea experience.

Of those who indicated that the experience had been less than ideal for them, (and ignoring any seed quality issues), the majority had only tried deep planting at the later end of their traditional planting window. They typically had moisture at depth late in the fallow, however it was out of reach at the beginning of the sowing window for a traditional planting.

By late May, expected rain hadn’t come, or insufficient rain had fallen – more than 30+ mm was needed to join up the profile. Deep planting was attempted however seeders struggled to maintain a consistent sowing depth (keeping the tine in the moisture) and the moist layer was now deeper. They were using varieties which would normally be planted a month earlier as they were not prepared to purchase new seed for this type of high-risk scenario, and generally it is a less than ideal situation.

As a result, emergence was well down and generally patchy. The established plants developed well, but as flowering and grain fill were delayed due to reduced competition for resources. This resulted in heat stress during flowering and grain fill, significantly reducing yield potential.

Optimum flowering dates

There has been a large body of research which shows flowering date will have a significant effect on yield of any given genotype. For Queensland conditions, aim for flowering in the coolest, lowest stress conditions possible for the crop while being mindful of frost risk at flowering and/or heat stress during grain fill. Across the LCW trial programs, wherever there has been a multi-date sowing, we have consistently seen a reduction in yield the later the sowing date, a response which is exacerbated when deep sowing (Figure 4).

Figure 4. Yield response to TOS x sowing depth at all trials from 2021 - 2023 which included multiple sowing dates. For 2021, the interval between sowing dates was 1 month, for 2022 the interval was 2 months due to wet conditions, for 2023 the interval was 3 weeks for CQ and 5 weeks for the SQ site (Ranges Bridge). Columns within each graph not labelled by a common letter are significantly different (P = 0.05). The 2023 data is still being analysed so only average difference is shown, any significant differences have yet to be determined.

The emergence from deep sowing for TOS 1 dates has been relatively consistent across the program, when compared to shallow emergence. In addition to the 2022 trials discussed above, the emergence for the 2021 CQSCC trial was worse at depth (Figure 4a), with average emergence across all lines being 25% of shallow. Figure 4a shows that despite this low emergence, the early (TOS 1) deep sown treatment significantly out yielded the May and June, shallow and deep sown treatments. When sown early, the ability of low population, deep sown wheat to compensate is considerable. Late sown wheat simply do not have sufficient time to recover, as temperatures rapidly begin rising from GS 55 onwards.

Implications

Table 2. Pros and cons of shallow and deep sowing and sowing timing

Both soil strength/density and soil temperature can have a significant effect on emergence, no matter what depth you are planting. To try and counter issues around soil strength, be very mindful of press wheel down pressure. Soil seed contact is essential, but don’t put any more strain on the emerging seedling than necessary. Equally, if planting early, try to avoid any forecast high temperature periods during the 10 - 20 day emergence period.

Sowing to target a variety’s optimum flowering window for a given location will pay dividends. Any emerging coleoptiles need to be able to drive up and out of the ground as quickly as possible so it can begin photosynthesis before the “tank is empty”. Seed with high germination, large seed size and excellent vigour (ideally tested in conditions to replicate a deep sowing environment) is essential to maximise returns from any deep sowing opportunity.

References

Rebetzke GJ, Richards RA, Fischer VM, Mickelson BJ  (1999) Breeding long coleoptile, reduced height wheats. Euphytica 106, 159–168

Rebetzke GJ, Richards RA, Sirault XRR, Morrison AD (2004) Genetic analysis of coleoptile length and diameter in wheat. Australian Journal of Agricultural Research 55, 733-743.

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.

I would like to thank AGT, Longreach and Intergrain for their generosity in providing seed lines, both current and pre-release, for testing in these projects. Also, the feedback from key personnel within each of these businesses for their invaluable feedback.

I would like to acknowledge Dr. Greg Rebetzke and CSIRO for all the background support they gave the original project and ongoing support within the current national project. Finally, I would like to acknowledge the significant effort put in by the DAF Regional Research Agronomy Team staff who have contributed to the work. In particular, Cameron Silburn & Jack Speedy who have overseen and managed the southern QLD on farm trials over the last 3 year, and also Jane Auer as our lead technician and all the work she has put in, in the background.

Contact details

Darren Aisthorpe
Department of Agriculture and Fisheries - Queensland
99 Hospital Rd, Emerald QLD 4720
Ph: 0427 015 600
Email: Darren.aisthorpe@daf.qld.gov.au

Date published
March 2024

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