Root system characteristics in wheat for effective absorption of water under drought

Vikender Kaur^1*, Rashmi Yadav¹, Anita Kumari² and Sukham Madaan²

¹:Germplasm Evaluation Division, ICAR-National Bureau of Plant Genetic Resources, New Delhi-110012, India.

²: Department of Botany and Plant Physiology, CCS Haryana Agriculture University, Hisar-125004, India.

*Corresponding author: Vikender Kaur

E-mail: vikender@nbpgr.ernet.in, drvikender@yahoo.in

Root architecture traits in wheat are important in deep soil moisture acquisition and may be used to improve adaptation to water limited environments. Developing wheat cultivars with enhanced drought adaptation and higher yield has been the focus of many wheat improving programs. The limited research effort in improvement of roots may be because of the difficulty in observing, measuring, and manipulating them. However, a better understanding of root functional traits and how traits are related to whole plant strategies to increase crop productivity under different drought conditions is needed. Increased root biomass, root length density (RLD) and rooting depth are often considered to be primary drivers of drought avoidance. Today most rainfed wheat in India is in the Central and Peninsular regions, and accounts for 30% of India’s total production with little access to irrigation. Furthermore, wheat production in many regions still relies on use of excessive water and chemical fertilizers which is not sustainable either ecologically or economically. The trend in Indian agriculture is towards less water for irrigation, and storing and use of deep water, either summer rains or irrigation, will become increasingly important. Thus, an area of recent interest is improvements of root traits that increase efficient deployment of tissues for foraging of soil water and, especially, the maintenance of productivity under water deficit. Here we explore which root traits are most likely to be valuable for improving water uptake to increase yield, and discuss efficient ways to select for these traits in breeding.

Contribution of deep soil water extraction to grain yield in wheat

Many wheat production systems could benefit from improvement of the storage of soil moisture (through management) (Hunt and Kirkegaard, 2012) and soil moisture exploitation (through root genetics). In India, rain falls almost entirely in the summer, during the monsoon season when crops such as rice or sorghum are grown or the land is fallowed. All wheat is grown in the winter when it is dependent on water stored in soil after the monsoonal rains plus any supplemental irrigation. ‘Rainfed’ Indian wheat typically is irrigated once before sowing to allow the crop to germinate and emerge, and then relies entirely on water stored in the soil. The value of targeting the capture of deeper soil moisture with selected root traits in a breeding programme is 2-fold. First, the moisture stored beyond the evaporation zone becomes a known source of crop water, while the in-season rainfall is unpredictable at the time of sowing. The second value of deeper soil moisture is that its uptake generally coincides with grain development when crops are vulnerable to terminal drought. Water use at this time has a very high conversion efficiency into grain (water-use efficiency) as vegetative growth has finished and all photosynthate is used for grain growth. Most of the increase in yield from late season subsoil water use is due to increases in the harvest index (ratio of grain to shoot weight) of the wheat crop (Passioura and Angus, 2010). Using rainout shelters in the field and drip irrigation, it was shown that 10 mm of subsoil water absorbed between depths of 1.35 m and 1.85 m after anthesis would increase grain yield by 0.62 t ha^-1, equating to 59 kg ha^-1 for every additional millimetre (Kirkegaard et al., 2007).

Two types of roots occur in cereals, the seminal roots coming direct from the embryo and the later, nodal roots emerging at lower tiller nodes. The seminal roots penetrate the soil earlier and deeper than nodal roots, so are considered important for deep soil moisture extraction. Manschadi et al. (2006) have reported that the drought-tolerant CIMMYT wheat line ‘SeriM82’

exhibited better yielding ability than the Australian wheat line ‘Hartog’ under water shortage conditions. They showed that the narrow root system architecture of ‘SeriM82’ is beneficial for the efficient extraction of soil moisture, and that the architecture of the entire root system is influenced by the traits of the seminal root. In addition, they found that the drought-tolerant line allocated less root growth laterally and produced more root length at deep layers. Oyanagi (1994) have found that wheat varieties bred for western Japan tend to have shallower root systems than varieties bred for eastern Japan, and explained this regional difference as an adaptation to the more abundant soil moisture in western Japan. Thorup-Kristensen et al. (2009) have suggested that the deeper rooting of winter-type wheat enables more effective usage of N in the lower levels of the soil due to N leaching. Manschadi et al. (2008) have found that drought-tolerant varieties tend to have deeper root systems compared to susceptible varieties. These results illustrate the close relationship between root system architecture and soil environmental conditions. Winter-type varieties exhibited deeper root architecture than spring-type varieties. These results support the idea that deep rooting is beneficial to obtaining water from deep soil layers. Traits such as rooting depth and RLD in wheat have shown high heritability across different environments and have also been related to improvements in grain yield under certain conditions (Sayar et al., 2007).

Root traits to increase deeper water uptake

Root traits contributing to plant productivity under water limited environments in cereals have been enlisted in Table 1. These traits mainly include: (a) Deep roots to increase the amount of subsoil moisture which the root system could access (b) Root angle and deep penetration in soil (c) Greater root length density to enable more complete uptake of the soil moisture (d) Reduced root length in surface soil (e) Reduced resistance to water movement from soil to shoot by longer and denser root hairs (f) reducing xylem size to increase axial resistance to water transport.

(a) Deeper root systems: The duration of root descent in wheat is approximately related to the duration from sowing to flowering as downward growth ceases around the time of flowering and onset of grain development (Gregory et al., 1978). One effective approach to increase root depth has been to increase the time to reach flowering. Time to flowering has been manipulated in Australian breeding programmes by sowing earlier in the season with varieties containing vernalization and photoperiod genes that extend the pre-flowering period (Richards, 2006). Earlier sowing improves water-use efficiency and root depth in Australian studies (Gomez-Macpherson and Richards, 1995; Kirkegaard et al., 2007). Precise knowledge of wheat vernalization and photoperiod genes allows for marker-assisted breeding to adjust flowering times and duration of vegetative growth to maximize root depth and capture of soil water in a specific region and at a specific sowing time (Eagles et al., 2011).

(b) Root Angle: The angle at which roots penetrate the soil may also relate to root depth. The angle at which roots emerge from the seed could be used as a proxy for deep rooting characteristics, particularly if it reflects an underlying gravitropic tendency in the root system. Modelling suggests that selection for a narrow angle in wheat root systems results in deeper root growth and higher yields (Manschadi et al., 2008). The root growth angle of Japanese winter wheat varieties, assessed in controlled environments, was shown to correlate negatively with the vertical root distribution of those varieties in the field (Oyanagi and Nakamoto, 1993). In summary, evidence suggests that the angle of early root formation may determine rooting depth. Identification of the genetic factors controlling growth angles of roots at the seedling stage is an important key to predicting root system architecture in cereals. By comparing the results obtained using a gel-filled chamber to measure root growth angle at the seedling stage and a soil- filled chamber at the 4-5 leaf stage (40 days after planting), Manschadi et al. (2008) have also suggested that selection for seminal root traits might be useful in breeding to increase the drought tolerance of wheat varieties with deep root architecture.

(c) Increased root length density in medium and deep soil layers: Increased depth increases the volume of soil moisture available for capture; more length from branches is needed to improve the capture of that water. Breeding for plants with less root length density (RLD, root length per soil volume) in shallow soil layers and increased RLD in medium and deep layers has been proposed as an efficient growth strategy in environments where deep water could be available to crops later in the growing season (Wasson et al., 2012; Lynch, 2013). Also the importance of deep water extraction would be more if its timing coincided with the time of most critical water demand e.g. recent evidence indeed shows that higher grain yield in wheat (Kirkegaard et al., 2007; Manschadi et al., 2006), were related to water availability during the grain filling period.

(d) Reduced root length density in the topsoil: Roots in the topsoil of older wheat plants are primarily nodal axile roots and their branches. It may be possible to redirect them to deeper layers, perhaps by selecting for narrower angles of growth. Deep or profuse rooting would have no effect in shallow soil, in soil where there is no water at depth, or under conditions of mild water stress.

(e) Reduced resistance to water movement from soil to shoot by longer and denser root hairs: Root hairs substantially increase root surface and thus have key role in water absorption and tolerance to drought, heat adaptation, and salinity. Increases in root surface area via root hairs may compensate for reductions in root elongation occurring in extremely dry soils. Root hairs may also promote root contact with soil particles as soil dries and may thus assist roots in acquiring soil water (Wasson et al., 2012 and references within). Direct selection for longer and denser root hairs, or hair development that contacts soil in gaps and pores, using imaging would be challenging given the high variability of root hairs in the field, and the possibility that seedling root hairs in the laboratory are not the same as hairs on components of mature root systems in field conditions. More research at different stages and different drought levels is necessary to assess the potential role for root hairs in water uptake under drought.

(f) Reducing xylem size to increase axial resistance to water transport: Richards and Passioura (1989) engaged in the best known example of the manipulation of a developmental root trait in wheat; reducing the xylem diameter of seminal axile roots to increase their axial resistance to water from the root system to the shoot, so that soil water uptake earlier in the season was reduced, leaving soil moisture available during grain filling where it contributed directly to the harvest index. By combining high axial resistance at the base of the root system with lower axial resistance in deeper roots, it may be possible to delay use of soil water until flowering and grain development to increase the harvest index, and exploit more fully water from deeper soil layers. The traits discussed are likely to be of value in a rainfed wheat production system with summer-dominant rainfall and evidence of stored soil moisture, particularly at depth. The deep root trait and the dense root at depth trait are likely to be valuable where there is a shortfall of water late in the growing season, a common occurrence globally. In Mediterranean environments with frequent in-crop rainfall events, there may be little or no benefit to deep root systems and they may pose an additional burden on plant development (Palta et al., 2011). Similarly, in situations where there is deep drainage, there may be little stored soil moisture to access. In environments with particularly shallow soils, deeper root systems will not be of benefit, and focus should instead be on better capture of rainfall events (where available), traits to minimize the impact that water deficit has on plant transpiration, and traits to meter out the available water over the entire season (such as reducing xylem size). Reducing root length density in the topsoil may be particularly inappropriate in Mediterranean environments, where shallow roots may increase the capture of in-season rainfall events.

Phenotyping for direct evaluation of root systems

Identifying desirable root phenotypes directly in the field would be the shortest route to the incorporation of traits of value in a crop-breeding programme. However, that approach is blocked by the lack of high-throughput phenotyping techniques for the field. Traditional studies have focused on excavation techniques, from which root depth and root length density can be determined. Trenching is labor intensive and slow (Van Noordwijk et al., 2000). Core sample processing has also been improved with automatic washing systems and then imaging of washed roots with flatbed scanners, and software packages for analyzing the washed images (French et al., 2009; Le Bot et al., 2010; Lobet et al., 2011). Minirhizotrons are a non-destructive alternative to excavation techniques, where a transparent tube is inserted into the ground and root growth abutting the tube is imaged with a camera that is inserted down the tube (Smit et al., 2000). Because minirhizotrons are non-destructive, the operator may monitor root growth and turnover. However, the roots must first grow against the tube wall, limiting what can be analyzed. Furthermore, the interface of the tube and soil is an artificial environment for root growth, which may lead to incorrect assessments of the growth characteristics of the plant. Computed Tomography (CT) was used for the remote sensing of wheat lines with deeper roots, and CT at grain filling negatively correlated to root dry weight at depth (Lopes and Reynolds, 2010). The isotopic signature of carbon in the grain may also be a valuable indicator of differences in access to water by mature root systems (Araus et al., 2003). Although high-throughput phenotyping programs, which enable rapid screening of roots were recently developed, the main challenge for plant breeding is still the lack of simple, reliable, fast, non-destructive and cost-effective methods which allow a high throughput screening of the root system in natural screening environments.

Genetics of root traits in bread wheat under drought

Genes and QTL associated with root traits: Root traits are believed to be complex and controlled by polygenes with quantitative effect, highly prone to environmental influences, difficult to observe and quantify under field conditions. Genetic loci controlling such traits are called quantitative trait loci (QTL). To overcome these problems, large efforts have been devoted to studying the genes and QTL controlling root traits in the hope that resultant resources can facilitate the root morphology and function. A number of studies have reported QTL linked to traits associated with increasing the foraging capacity of root systems. In common wheat molecular mapping of root trait QTL have been reported by a number of studies Sanguineti et al. 2007; Landjeva et al., 2008; Sharma et al., 2011; Hamada et al., 2012; Ren et al., 2012; Bai et al., 2013; Christopher et al., 2013; Liu et al., 2013 which collectively demonstrate the presence of multiple QTLs for several major root traits, such as increased total root biomass, length and number of roots, seminal root angle and number, deep root growth and seminal root number, lateral root length and number, and root surface area. These have revealed that the genetic improvement of root system architecture is necessary for improving water-and nutrient-use efficiency of crops or for enhancing their productivity under abiotic stress or suboptimal soil conditions. Root traits in cereals were associated with drought tolerance. A study by Koszegi et al. (1996) regarding the additive allelic effects and the intra- and inter-genomic epistatic interactions contributed by rye 1RS provided important information on root genetics which can be pivotal for alien introgressions of genes involved in drought tolerance. This may also be helpful in marker assisted selection by selecting for a desired combination of alleles for root manipulation toward better adaptability and stability to drought stressed environments.

In addition to QTL, some specific genes or mechanisms have been associated with variation for root traits in major cereal crops. Reduced height and semi-dwarfing genes are common in many modern wheat (Evans 1998) and barley (Hordeum vulgare) varieties (Chloupek et al., 2006) which have been shown to contribute to greater root system size. Recently, a molecular mechanism underlying hydrotropism in plant roots was partly disclosed in Arabidopsis using the mutant genes miz1 and miz2 (Miyazawa et al. 2009). Though a gene in a monocot cannot be said to have a syntenic relationship with a gene in a dicot, functional analyses of MIZ1 and MIZ2 will be helpful to clarify the regulation system of hydrotropism in wheat. Hamada et al., 2012 also detected two QTLs related to the hydrotropic response of the primary seminal root on chromosomes 1A and 2B in wheat which is the first report of QTLs related to hydrotropism in crops.

Marker assisted selection and introgression: Root QTL show great potential for marker assisted selection (MAS) when root traits chosen contribute significantly to drought tolerance in the target environment. Many of the reported markers and QTL for root traits have proven to be confounded by inadequate root phenotyping, inconsistent contribution across populations and environments, or the minor contributions made by the QTL to the variation in the trait of interest. QTL that have been identified in greenhouse or lab conditions must be validated under field conditions and should ultimately relate to improvements in productivity before use in a MAS program. For these reasons, there have been very few reports of the use of MAS for quantitative traits such as root characters in plant breeding programs.

Resources for genetic diversity: A reduction in diversity of crop species due to domestication or subsequent selection has been described as a genetic bottleneck that may have contributed to a loss in useful alleles. Root traits are no exception as the importance of developing improved root systems has often been overlooked. With a better understanding of root traits and their genetics, improvements in root systems can be made by utilizing the diversity currently found within modern cultivated germplasm. For example, a comparable amount of unexploited genetic variation contributing to stress tolerance can be found in modern cultivars as in landraces (primitive varieties) of wheat (Trethowan and Mujeeb-Kazi, 2008). Introgression of alleles from modern varieties reduces the negative effects of linkage drag from the use of wild species and landraces (Hubner et al., 2013). Wasson et al. (2012) recently proposed that, by using appropriate germplasm and combining with suitable laboratory and field screens, it is possible to develop wheat varieties with a deeper and more active root system for achieving higher yield in water limited environments.

Conclusion

Under limited moisture conditions, roots can play an outstanding role with respect to yield stability by effective absorption of water from soil. Although much is known about root traits and functioning, there is a need for better understanding of traits in the context of plant strategies for growth under water deficits. Smaller diameter roots, greater SRL, and increased root hair density or length should improve plant acquisition of water under water scarcity and reduce plant carbon investment required for that acquisition. Additionally, crop hydraulic functioning under water scarcity may be improved through increased capacity for nocturnal refilling of embolized xylem and changes in inter-vessel pit anatomy to reduce cavitation, which may not carry negative repercussions under well-watered conditions. The ability of plants to access water from deep depths in the soil profile has been documented and found to benefit crop productivity under water scarcity. Abundant progress has been made in understanding root traits and functioning in plant water acquisition with several root QTL identified. There continue to be promising prospects for increasing communication between plant ecophysiologists, geneticists, and breeders to learn more about root traits that have the potential to improve plant productivity under drought and put this understanding into practice to improve the performance of crops under water shortages.

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