On-farm assessment of new long-coleoptile wheat genetics for improving grain yield with deep sowing

Take home messages

• The trend has been for increasing summer rain and later autumn sowing breaks throughout the WA wheatbelt. Long coleoptiles will permit deep sowing into subsoil moisture stored from summer rains allowing for earlier germination, and crop growth to occur under conditions optimal for crop development and maximising water productivity.
• On-farm, deep-sowing studies at Southern Cross (WA) showed benefits of new dwarfing genes in increasing coleoptile length and seedling emergence at sowing depths of up to 140mm. Studies are underway in WA, SA, NSW and QLD to understand systems and performance benefits across a wider range of environments.
• Australian breeders are using new dwarfing and coleoptile growth genetics to fast-track the delivery of new higher-yielding, long coleoptile wheat varieties suited for deep sowing.

Aims

1. To validate coleoptile lengths measured under controlled conditions in the field, and examine the potential for seedling emergence of selected long coleoptile wheats at sowing depths exceeding 120mm; and
2. Assess the yield potential and water productivity with deep-sowing in a grower-led, on-farm field experiment.

Introduction

Optimal and timely plant establishment is critical in rainfed cropping systems. Well established crops provide ground cover to protect ameliorated soils, reduce water loss through soil evaporation, and increase crop competitiveness with weeds. Early emergence also increases yield potential of crops through increased duration for root growth, tillering and building of crop biomass while ensuring crop development coincides with conditions optimal for growth and flowering while avoiding hot, dry conditions late into grain-filling.

The coleoptile is a tube-like shoot that grows from the seed protecting the elongating sub-crown internode and crown. It is typically 60 to 85mm in length in modern semi-dwarf wheats, and this length limits the depth from which can successfully emerge. Changing weather patterns are associated with proportionally greater summer rainfall (see Fig. 1) and increasingly later sowing breaks (Flohr et al. 2021; Scanlon and Doncon 2020). There has been increasing interest in deep sowing systems (typically at 50-200mm) to utilise summer rainfall and ensure earlier establishment (Rich et al. 2021). However, the shorter coleoptile of modern wheat varieties limits our ability to utilise these. In turn, many crops are sown dry to accommodate large sowing programs. An ability to germinate and establish wheat crops from seed placed 100mm or deeper in the soil would be beneficial in situations where the subsoil is moist but the surface soil dry (Rebetzke et al. 2007; Rich et al. 2021).

A separate but concerning issue is the influence of increasingly warmer soil temperatures on reductions in coleoptile length. Earlier sowing into warmer soils will reduce coleoptile length by up to 50% so that a conventional variety with a 75mm coleoptile at 15°C will likely produce a 40mm coleoptile at 25°C soil temperature (Rebetzke et al. 2016).

Figure 1. Monthly average rainfall (mm) for Southern Cross (WA) in all years pre- and post-2000.

New dwarfing genes

The green revolution Rht-B1b and Rht-D1b dwarfing genes are present in most wheat varieties globally. They reduce plant heights to reduce lodging, increase grain number and increase crop yields. These dwarfing genes reduce cell size in plant stems to shorten plant height, but a major drawback is that they also reduce coleoptile length and seedling leaf size by as much as 40% (Botwright et al. 2005). A range of alternative dwarfing genes have been identified with potential to reduce plant height and increase yields while maintaining longer coleoptiles and greater early vigour. Some of these genes (e.g. Rht8 and Rht18) have been used commercially overseas but have not been assessed for use in Australia. Further these genes have not been assessed for their potential with deep sowing in germination and establishment.

Methods

The Rht18 dwarfing gene was bred from an Italian durum wheat variety, Icaro, into the tall, long coleoptile Halberd background. A fertile progeny was identified, ‘HI10S’, which was then used for crossing into the Mace, Magenta, Scout and Yitpi commercial backgrounds using both conventional and DNA-based selection methods. Four cycles of crossing and three rounds of selection were undertaken to develop BC₃-derived lines where the existing, conventional Rht-D1b dwarfing gene was replaced with Rht18 to reduce plant height but maintain coleoptile length. Resulting BC₃ progeny were then assessed for coleoptile length under controlled environment conditions in Canberra ACT to identify semi-dwarf, long coleoptile lines. These selected lines were seed-increased at Condobolin in NSW in 2018 before releasing to Australian breeding companies with residual seed used in subsequent experiments.

An experiment was sown on 7 May 2020 at Southern Cross in the eastern WA wheatbelt using grower planting equipment - the seed-bin was modified on a Gessner Landmaster® planter with curved points permitting sowing of small experimental seed-lots (up to 10kg) to depths of up to 200mm. Plots of size 60 × 4.5m were sown in a two-replicate experimental design at two sowing depths: 40mm (dry-sown) and 120-130mm (sown into summer sub-soil moisture). Genotypes included long coleoptile Mace (‘Mace18’), Magenta (‘Magenta18’), Scout (‘Scout18’) and Yitpi (‘Yitpi18’) breeding lines, commercial varieties Mace and Scepter, and tall check variety Halberd. Mace, Mace18 and Scepter were sown at shallow (40mm) and deep (120-130mm) depths for comparison. There was limited seed of Magenta18, Scout18, Yitpi18 which restricted sowing to the deep treatments only.

Results

Coleoptile lengths in controlled environments

Coleoptile lengths of the control varieties Halberd and Scepter were 132 and 65mm, respectively, and Mace and Mace18 were 102 and 151mm, respectively, at 15⁰C controlled environment conditions in Canberra. The long coleoptile Mace18 containing a new Rht18 dwarfing gene established well with deep sowing (up to 80% of 40mm shallow depth) and was consistent with the greater coleoptile length of the tall control variety Halberd (data not shown). By contrast, the shorter coleoptile of Mace and Scepter were associated with reduced establishment with deep sowing (30-40%)(data not shown).

Field experiment

Available crop water at Southern Cross represented three large summer rainfall events (totaling 115mm in January-February) and an additional 76mm rainfall in-crop. Seedling emergence in the deep-sowing treatment commenced 18 May with the shallow-sowing emerging approximately two weeks later (following a 7 and then 5mm rainfall event May 24 and 29, respectively). Despite the dry decile 1 GSR, conditions through flowering and grain-filling were cool with modest rainfall (23mm) in mid-August.

Table 1. Grain yields and yield components for different wheat varieties and breeding lines sown deep at 120-130mm and shallow-sown at 40mm (in parenthesis). A subset of lines (Magenta18, Scout18, Yitpi18) were only sown in the deep sowing treatment.

† long coleoptile Rht18 selections; ǂ LSD for comparisons between entries with deep-sowing

Grain yields were high for shallow sowings despite the 2-week delay in emergence. Cooler conditions through grain-filling may have contributed to the increase in yields. The performance of the shallow sowing was unexpected given the reduced grain yield previously reported with delayed time of sowing (e.g. Anderson and Garlinge 2000; French and Zaicou-Kunesch 2019) and with APSIM modelling (Zhao and Wang unpublished. data). Yet despite this, comparisons between Mace and Mace18 indicated grain yields, head number and water productivities were significantly (P<0.01) larger for deep-sown Mace18 compared with deep-sown Mace (Table 1). Harvest index and grain size was commonly greater with deep sowing suggesting a favourable water balance for deep-sown crops after flowering. High yields and water productivities with deep sowing were also observed for long coleoptile selections in Magenta and to a lesser extent Scout and Yitpi genetic backgrounds. Grain protein concentrations were consistently larger with deep sowing and was particularly high in the deep Mace and Scepter sowings (Table 1).

This grower-led study highlighted the opportunity for deep-sowing in reducing risk a marginal environment characterised by a low rainfall year. Other genetic opportunities are being explored that should complement long coleoptile length in wheat variables adapted to future climates. These include high biomass ‘100-day’ wheats for late sowing, weed competitive wheats to assist in managing herbicide use and resistance, and high grain-filling rates to avoid hot and dry conditions at season end. There is also opportunity to translate learnings in breeding of long coleoptile wheats to other crops including canola and barley.

Conclusions

The long coleoptile trait has been demonstrated to provide good establishment and higher yields with deep subsoil moisture retained from summer rains. Further studies are underway including use of earlier sowing dates and multiple sites in WA, SA, NSW and QLD to assess the potential for long coleoptiles as part of a broader set of environments and farming systems. Coinciding closely with this assessment is the pursuit by breeders in selection of the long coleoptile trait in delivering new wheat varieties to Australian growers.

Acknowledgements

We would like to thank DPIRD Merredin for their assistance in harvest of the experiment, and Dr Sarah Rich for helpful discussions on the value of deep sowing into subsoil moisture.

References

Andersen W, Garlinge J (2000) ‘The wheat book: principles and practice.’ (Western Australia Department of Agriculture: Perth, WA).

Botwright TL, Rebetzke GJ, Condon AG, Richards RA (2005) Influence of the gibberellin-sensitive Rht8 dwarfing gene on leaf epidermal cell dimensions and early vigour in wheat (Triticum aestivum L.). Annals of Botany 95, 631-639.

Flohr BM, Ouzman J, McBeath TM, Rebetzke GJ, Kirkegaard JA, Llewellyn RS (2021) Spatial analysis of the seasonal break and implications for crop establishment in southern Australia. Agricultural Systems (In press).

French B, Zaicou-Kunesch C (2019) May delayed sowing and APSIM yield potential.

Rebetzke GJ, Richards RA, Fettell NA, Long M, Condon AG, Botwright TL (2007) Genotypic increases in coleoptile length improves wheat establishment, early vigour and grain yield with deep sowing. Field Crops Research 100, 10-23.

Rebetzke GJ, Zheng B, Chapman SC (2016) Do wheat breeders have suitable genetic variation to overcome short coleoptiles and poor establishment in the warmer soils of future climates? Functional Plant Biology 43, 961-972.

Rich S, Oliver Y, Richetti J, Lawes R (2021) Chasing water: Deep sowing can increase sowing opportunities across the grain growing regions of Western Australia. Perth Updates .

Scanlon TT, Doncon G (2020) Rain, rain, gone away: decreased growing-season rainfall for the dryland cropping region of the south-west of Western Australia. Crop and Pasture Science 71, 128-133.

Contact details

Greg Rebetzke
Chief Scientist
CSIRO Agriculture and Food
Greg.Rebetzke@csiro.au
+61 2 6246 5153