Key Points:
• Crop root traits associated with improved drought tolerance include deeper rooting, greater root dry weight and root hair density, more aerenchyma (lower metabolic cost) and early root vigour
• Rooting depths can vary substantially between environments, crop varieties and crop species
• Increased rooting depth from 0.9 to 1.3 m reduced drought stress occurrence by up to 35% in spring barley crops in the UK
• Drought stress occurrence was dependent on the timing of rainfall in the growing season, and not necessarily the total quantity
• Limitations to crop rooting and yield potential can be better understood by estimating the depth of water extraction
Introduction
With the UK having recently experienced its driest spring in over a century, in addition to below average rainfall in June, the need for more drought resilient crops is greater than ever. The Horizon Europe Roots to Resilience (Roots2Res) project is very timely as it is investigating how to increase the resilience of cropping systems to climate change impacts through the characterization of below ground ideotypes (ideal root and rhizosphere traits) and identification of tools for rapidly assessing genotypes. There is a current lack of knowledge of the genetic variability of root traits within crop species and how much these traits are influenced by environment, these knowledge gaps are being addressed in Roots2Res to support the harnessing of these ideal traits.
The project is running from 2022 – 2027 with 22 partners from across Europe and Africa. Roots2Res focuses on tolerance against abiotic stresses (namely water and phosphorus stress) primarily in barley, potato and bean crops; and aims to deliver simple to use high-throughput screening tools to facilitate breeders to consider root traits for resilience and hasten the development of stress tolerant varieties. For more information about Roots2Res, visit the webpage.
Rooting traits associated with drought resilience
To identify belowground crop traits associated with water stress tolerance a literature review was undertaken to understand what specific root and rhizosphere traits are important for resilience to water stress, across the main crops being studied in Roots2Res. The rhizosphere is defined as the soil zone around the roots in which the soil chemistry and microbial biomass is impacted by the presence of the roots.
This review identified a range of traits important for drought stress tolerance, evidenced through past field and controlled environment experiments (Table 1). Deeper rooting was identified as important to reduce drought stress due to the capture of water stored deeper in the soil profile, which may otherwise be lost to drainage. This water is less vulnerable to evaporation and therefore a more stable water supply later in the growing season. Other root traits frequently associated with improved drought stress tolerance included early root vigour, and greater root hair density.
Table 1. Root and rhizosphere traits associated with superior crop performance in water limited environments, sourced from a literature review. Bolded traits were evidenced in more literature sources. Root traits Early water deficit (pre flowering) Late water deficit (post flowering) Deeper roots with greater root length density at depth ✓ ✓
Early root vigour
a Aerenchyma are tissue containing a high proportion of gas-filled spaces and provides a low-resistance pathway for long-distance gas transport. In water stressed conditions, large root cortical aerenchyma reduces respiration, nutrient content of root tissues, and the metabolic cost of soil exploration.
Modelling the impact of deeper rooting to reduce drought stress
To quantify the impact of increasing the maximum root depth on drought stress, a modelling exercise was undertaken using IRRIGUIDE, a field-scale model of evaporation, transpiration and drainage, to assess soil-crop water relations. IRRIGUIDE was originally designed to help farmers schedule irrigation by calculating Soil Moisture Deficits (SMD) and crop available water capacity (AWC) using a Penman-Monteith evapotranspiration approach (Bailey and Spackman, 1996). The model calculates SMD using the following formula:
SMD(d) = SMD(d-1) + AET(d) - R(d) - I(d) + D(d) + r(d)
Whereby d = day in question, AET = actual evapotranspiration (derived from potential evapotranspiration, SMD, and rooting depth), R = rainfall, I = irrigation, D = drainage from the root zone and r = runoff.
The IRRIGUIDE model can calculate SMDs for a range of different crop types. As part of the Roots2Res project, IRRIGUIDE has been developed to allow maximum rooting depth to be specified as either low, medium or high (Table 2). These ranges have been sourced from the literature, combined with expert judgement, to quantify the impact of rooting depth on drought stress occurrence for a range of crop species. It is also advantageous to be aware of inter species differences in rooting ability to understand their tolerance to drought and legacy effects within a rotation. Given most available literature reports the maximum rooting depth achieved by common cultivars, it can be assumed these ranges represent commercially relevant scenarios; maximum rooting depths at which sufficient water can be extracted when soil depth or condition is not a major limitation.
Table 2. The range of rooting depths for different crop species at full development, reached at crop maturity, sourced from the literature.
*Assumed wheat references are applicable to barley.
To quantify the impact of increased rooting depth on tolerance to drought stress, IRRIGUIDE was run using weather and cropping data from spring barley field trials carried out as part of the Roots2Res project. These trials took place in Nottinghamshire in the two growing seasons of 2023 and 2024 and the three rooting depth scenarios of low (0.9 m), mid (1 m) and high (1.3 m) were modelled. The trial site had a sandy loam soil texture and SMD was assumed to be 0 mm (i.e. soils were at their full water holding capacity) from the end of winter, prior to the crop being sown. Crops were considered drought stressed when 75% of the accessible water was depleted, assuming the crops could only access 75% of the available soil water to root depth (because rooting densities decline exponentially with soil depth, limiting water uptake). It was assumed drought stress cannot occur after 0 Green Area Index (~GS89), due to a lack of transpiration.
The spring barley crops received 319 mm of rain in 2023 and 243 mm in 2024 between sowing and harvest, despite this drought stress was only modelled to have occurred in 2023. Figure 3 shows the distribution of rainfall throughout the 2023 and 2024 season and suggests rainfall in the period 60-90 DAS (days after sowing) had a large influence on drought stress occurrence, due to this being the biggest difference in water input between the two seasons. Rainfall in this period is likely to be a large driver of the crop experiencing drought stress due to the water demand of the crop being high at later growth stages when the canopy size is at its maximum.
Figure 1. Rainfall during incremental 30-day periods after sowing in Nottinghamshire in 2023 and 2024. 2024 rainfall data is missing in the 121-150 interval as the growing period was 122 days. Source: OpenMeteo.
In 2023 drought stress began 91 days after sowing, when the barley crop was at GS52. Increasing the maximum rooting depth from the low (0.9 m) to the high (1.3 m) scenario reduced the number of days the crop experienced drought stress by 35%, from 49 to 32 days, primarily due to a later onset of drought stress in the high rooting depth scenario (see Figure 4). Drought stress occurred after heading; therefore, it will likely have impacted floret survival, grain fill and time of canopy senescence.
Figure 2. Days after sowing (DAS) when actual SMD was lower than the critical SMD for low, medium and high rooting depth scenarios. X axis is DAS up until 0 GAI. Bars indicate rainfall (mm). Red dashed line indicates when heading occurred. Barley crop is drought stressed when Y-axis values are negative.
What does this mean for farmers and advisors?
Although we are only now researching how crop varieties differ in their root traits and have no certain way of predicting which varieties are best, farmers have a range of tools to enhance crop rooting and the resilience of their crops in droughted conditions. These primarily relate to the physical states of their soils – their drainage and structure, particularly their strength (hence resistance to root penetration), porosity, and aeration / hydration (especially the longevity of waterlogging). The density of subsoils is inherently high (because, by definition, they must support the topsoil), consequently, cracks and biopores are both crucially important. Cracks and biopores are created by good soil management. Most biopores are created by worms, particularly the deep burrowing anecic earthworms; practices to encourage these include minimizing soil disturbance, incorporating organic matter, minimising topsoil dryness and subsoil waterlogging. Most subsoil cracks are created by soil shrinkage caused by roots withdrawing moisture. So good subsoil rooting with high moisture extraction in one crop begets better soil conditions for the next crop; subsoil improvement is thus an incremental, long-term process.
Whilst our analysis suggests increasing crop rooting depths can have substantial benefits to reducing the occurrence of drought stress, challenges remain for farmers to estimate the success of subsoil water capture on farm, and the potential to improve crop rooting via long-term subsoil management. Our suggestion is to estimate the apparent depth of water extraction year-on-year, to monitor which fields show trends of deeper rooting over years and which show limitations. The apparent depth of water extraction can be calculated using the assumption that 20 mm of water is required to produce a tonne of biomass (Foulkes et al., 2001). You can then follow the example below to estimate the depth of water extraction in a given field (using the information in Table 3 and 4):
1. Your subsoil’s Water-Holding Capacity, mm/m, adjusted for stone content (see table of typical values below). Example: silty clay with 7% stones = (150 x (100 - 7)) / 100
139.5
2. Summer rainfall, mm, April to July, or since drains stopped flowing in spring. Example: 138 mm (like North Yorkshire in 2022)
138
3. Final crop biomass, t/ha, dry yield (t/ha) ÷ Harvest Index – see table below. Example: Dry Yield = fresh yield corrected to 100% DM (9.35 t/ha); H.I. = 0.52 …
18.0
Apparent depth of water extraction (metres) = ((18.0 x 20) – 138) / 139.5 = 1.59
Table 3. Water holding capacities of different subsoil texture.
Table 4. Typical harvest indices for different crop types
Completing these calculations should give an indication of how well crops are rooting; the approach will be most telling in growing seasons with low rainfall. Results can then be compared to the ranges described in Table 2. Particular attention should be paid to crops on the lower end of the suggested range, as this is where the potential could lie to improve subsoil rooting. This could return substantial economic gains for water limited crops as increasing the rooting depth for winter wheat from a lower value of 1 m to an average value of 1.5 m on fine sands, silts & clay soils (with no stones) could return an extra 1.95 t/ha in grain yield. Hopefully this article has offered ideas of how to ensure sufficient root systems. In summary, these include:
- Improving drainage; waterlogging stunts root growth, improved drainage allows for healthy root growth.
- Preventing compaction; by minimising soil traffic, particularly when the soil is wet, and vehicles are heavy (at harvest), appropriate cultivations, growing large-rooted cover crops and encouraging earthworms.
- Drilling earlier; drilling earlier increases the growing period of the crop and therefore results in roots growing deeper down the soil profile, improving water capture later in the season.
- Including deep rooted crop species in the rotation; the deeper that each crop extracts water, the more cracked the subsoil will become, hence the deeper the next crop will be able to root. Crop resilience through sub-soil improvement requires a stringent, long-term farming strategy.
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