Pathways to deeper roots: anatomical phenes of maize under impedance

When roots grow through soil they experience mechanical impedance to varying extents. When levels of mechanical impedance become greater, for instance when soils become compacted or soil moisture decreases, roots can become obstructed. As a consequence the uptake of nutrients and water from the soil reduces, which can reduce plant growth and ultimately negatively impact yield. In this work field trials were conducted at two different field sites (one at the Apache Root Biology Centre in Willcox, Arizona and one at the Russell E. Larson Agricultural Research Center in Rock Springs, Pennsylvania) to study the differential distribution of maize roots following the interaction with a compacted soil profile. In soils with compacted plots the rooting depth of coarse roots was not correlated with coarse root length, which indicated that nodal roots of some genotypes were able to grow under impeding conditions while other genotypes were not capable of growing through impeding conditions. Furthermore genotypes were identified which had similar rooting depths but contrasting coarse root lengths, these genotypes were equally able to reach to deeper depths. The amount of roots formed by the root system therefore does not determine the ability of roots to grow deeper under impeded conditions. Root thickening, a response of roots often seen when submitted to mechanical impedance, varied among genotypes. The same field trial were also used to investigate the role of root anatomy and adaptation to soil mechanical impedance. Root anatomy varied according to genotype and nodal position. Deeper rooting was facilitated by root anatomical phenes such as reduced cortical cell file number in combination with greater middle cortical cell area for node 3 and increased aerenchyma for node 4. In a separate pot trial the hypothesis that radial expansion was related to the ability of roots to cross a compacted layer in four different genotypes was tested. Radial expansion of roots was mainly attributed to the cortex. Cortical expansion of a single root axis was caused by cellular expansion and not an increase in cell file number. The ability of roots to reach the compacted layer was dependent on the root growth angle. Genotypic variability was present for the ability to cross the compacted layer, and genotypes that did not radially expand in response to impedance had more roots crossing the layer and reached deeper past the layer. The same genotypes were tested in a hydroponics experiment, which showed that genotypes that did not thicken in response to ethylene were the same as those that were able to overcome impedance. It can be concluded that radial thickening should be seen as a response to mechanical impedance rather than a positive adaptation. Genotypic variation was related to rooting depth, and anatomical adaptation was more important for thinner than thicker root classes. Understanding the functional utility of root anatomical phenotypes under abiotic stress, such as impedance, is important for the breeding of new crop cultivars with superior adaptation to edaphic stress. Furthermore this work illustrates that root systems are highly adaptive across genotypes but also within an individual plant. The understanding of such adaptability is important, as edaphic stresses such as impedance influence global agriculture

Root anatomical traits contribute to deeper rooting of maize under compacted field conditions

© The Author(s) 2020. To better understand the role of root anatomy in regulating plant adaptation to soil mechanical impedance, 12 maize lines were evaluated in two soils with and without compaction treatments under field conditions. Penetrometer resistance was 1–2 MPa greater in the surface 30 cm of the compacted plots at a water content of 17–20% (v/v). Root thickening in response to compaction varied among genotypes and was negatively associated with rooting depth at one field site under non-compacted plots. Thickening was not associated with rooting depth on compacted plots. Genotypic variation in root anatomy was related to rooting depth. Deeper-rooting plants were associated with reduced cortical cell file number in combination with greater mid cortical cell area for node 3 roots. For node 4, roots with increased aerenchyma were deeper roots. A greater influence of anatomy on rooting depth was observed for the thinner root classes. We found no evidence that root thickening is related to deeper rooting in compacted soil; however, anatomical traits are important, especially for thinner root classes

Soil penetration by maize roots is negatively related to ethylene-induced thickening

Radial expansion is a classic response of roots to a mechanical impedance that has generally been assumed to aid penetration. We analysed the response of maize nodal roots to impedance to test the hypothesis that radial expansion is not related to the ability of roots to cross a compacted soil layer. Genotypes varied in their ability to cross the compacted layer, and those with a steeper approach to the compacted layer or less radial expansion in the compacted layer were more likely to cross the layer and achieve greater depth. Root radial expansion was due to cortical cell size expansion, while cortical cell file number remained constant. Genotypes and nodal root classes that exhibited radial expansion in the compacted soil layer generally also thickened in response to exogenous ethylene in hydroponic culture, that is, radial expansion in response to ethylene was correlated with the thickening response to impedance in soil. We propose that ethylene insensitive roots, that is, those that do not thicken and can overcome impedance, have a competitive advantage under mechanically impeded conditions as they can maintain their elongation rates. We suggest that prolonged exposure to ethylene could function as a stop signal for axial root growth

Multiseriate cortical sclerenchyma enhance root penetration in compacted soils

Mechanical impedance limits soil exploration and resource capture by plant roots. We examine the role of root anatomy in regulating plant adaptation to mechanical impedance and identify a root anatomical phene in maize (Zea mays) and wheat (Triticum aestivum) associated with penetration of hard soil: multiseriate cortical sclerenchyma (MCS). We characterize this trait and evaluate the utility of MCS for root penetration in compacted soils. Roots with MCS had a greater cell wall to lumen ratio and a distinct UV emission spectrum in outer cortical cells. Genome-wide association mapping revealed that MCS is heritable and genetically controlled. We identified a candidate gene associated with MCS. Across all root classes and nodal positions, maize genotypes with MCS had 13% greater root lignin concentration compared to genotypes without MCS. Genotypes without MCS formed MCS upon exogenous ethylene exposure. Genotypes with MCS had greater lignin concentration and bending strength at the root tip. In controlled environments, MCS in maize and wheat was associated improved root tensile strength and increased penetration ability in compacted soils. Maize genotypes with MCS had root systems with 22% greater depth and 39% greater shoot biomass in compacted soils in the field compared to lines without MCS. Of the lines we assessed, MCS was present in 30-50% of modern maize, wheat, and barley cultivars but was absent in teosinte and wild and landrace accessions of wheat and barley. MCS merits investigation as a trait for improving plant performance in maize, wheat, and other grasses under edaphic stress

Pathways to deeper roots: anatomical phenes of maize under impedance

When roots grow through soil they experience mechanical impedance to varying extents. When levels of mechanical impedance become greater, for instance when soils become compacted or soil moisture decreases, roots can become obstructed. As a consequence the uptake of nutrients and water from the soil reduces, which can reduce plant growth and ultimately negatively impact yield. In this work field trials were conducted at two different field sites (one at the Apache Root Biology Centre in Willcox, Arizona and one at the Russell E. Larson Agricultural Research Center in Rock Springs, Pennsylvania) to study the differential distribution of maize roots following the interaction with a compacted soil profile. In soils with compacted plots the rooting depth of coarse roots was not correlated with coarse root length, which indicated that nodal roots of some genotypes were able to grow under impeding conditions while other genotypes were not capable of growing through impeding conditions. Furthermore genotypes were identified which had similar rooting depths but contrasting coarse root lengths, these genotypes were equally able to reach to deeper depths. The amount of roots formed by the root system therefore does not determine the ability of roots to grow deeper under impeded conditions. Root thickening, a response of roots often seen when submitted to mechanical impedance, varied among genotypes. The same field trial were also used to investigate the role of root anatomy and adaptation to soil mechanical impedance. Root anatomy varied according to genotype and nodal position. Deeper rooting was facilitated by root anatomical phenes such as reduced cortical cell file number in combination with greater middle cortical cell area for node 3 and increased aerenchyma for node 4. In a separate pot trial the hypothesis that radial expansion was related to the ability of roots to cross a compacted layer in four different genotypes was tested. Radial expansion of roots was mainly attributed to the cortex. Cortical expansion of a single root axis was caused by cellular expansion and not an increase in cell file number. The ability of roots to reach the compacted layer was dependent on the root growth angle. Genotypic variability was present for the ability to cross the compacted layer, and genotypes that did not radially expand in response to impedance had more roots crossing the layer and reached deeper past the layer. The same genotypes were tested in a hydroponics experiment, which showed that genotypes that did not thicken in response to ethylene were the same as those that were able to overcome impedance. It can be concluded that radial thickening should be seen as a response to mechanical impedance rather than a positive adaptation. Genotypic variation was related to rooting depth, and anatomical adaptation was more important for thinner than thicker root classes. Understanding the functional utility of root anatomical phenotypes under abiotic stress, such as impedance, is important for the breeding of new crop cultivars with superior adaptation to edaphic stress. Furthermore this work illustrates that root systems are highly adaptive across genotypes but also within an individual plant. The understanding of such adaptability is important, as edaphic stresses such as impedance influence global agriculture

The ability of maize roots to grow through compacted soil is not dependent on the amount of roots formed

© 2020 Elsevier B.V. Mechanical impedance is a primary constraint to root growth and hence the capture of soil resources. To investigate whether rooting depth and root length under mechanical impedance caused by compaction are correlated we evaluated 12 maize lines at two field sites. To distinguish between lateral and nodal roots, roots were sorted into different diameter classes. Coarse roots had diameters >1 mm and represent nodal root axes. Greater proportions of coarse roots on compacted plots were found at both field sites however results were driven by genotypic variation. Soil compaction reduced total rooting depth (in all diameter classes) and coarse rooting depth at both sites compared to non-compacted plots. Root distribution was influenced by compaction with greater root length densities closer to the soil surface. Root length and root depth were not related to each other under impeded conditions. Coarse roots of some genotypes became obstructed on the compacted plots, while other genotypes were capable of growing through the impeding soil and reached deeper soil strata resulting in differential distribution of roots through the soil profile. On compacted plots we observed genotypes with similar root depths but with contrasting coarse root lengths. The ability of roots to grow through compacted soils is therefore not dependent solely on the coarse root length formed by the root system