Steep, cheap and deep: an ideotype to optimize water and N acquisition by maize root systems

Abstract

A hypothetical ideotype is presented to optimize water and N acquisition by maize root systems. The overall premise is that soil resource acquisition is optimized by the coincidence of root foraging and resource availability in time and space. Since water and nitrate enter deeper soil strata over time and are initially depleted in surface soil strata, root systems with rapid exploitation of deep soil would optimize water and N capture in most maize production environments. • THE IDEOTYPE: Specific phenes that may contribute to rooting depth in maize include (a) a large diameter primary root with few but long laterals and tolerance of cold soil temperatures, (b) many seminal roots with shallow growth angles, small diameter, many laterals, and long root hairs, or as an alternative, an intermediate number of seminal roots with steep growth angles, large diameter, and few laterals coupled with abundant lateral branching of the initial crown roots, (c) an intermediate number of crown roots with steep growth angles, and few but long laterals, (d) one whorl of brace roots of high occupancy, having a growth angle that is slightly shallower than the growth angle for crown roots, with few but long laterals, (e) low cortical respiratory burden created by abundant cortical aerenchyma, large cortical cell size, an optimal number of cells per cortical file, and accelerated cortical senescence, (f) unresponsiveness of lateral branching to localized resource availability, and (g) low K(m) and high Vmax for nitrate uptake. Some elements of this ideotype have experimental support, others are hypothetical. Despite differences in N distribution between low-input and commercial maize production, this ideotype is applicable to low-input systems because of the importance of deep rooting for water acquisition. Many features of this ideotype are relevant to other cereal root systems and more generally to root systems of dicotyledonous crops.

Screening of Maize Genotypes for Drought Tolerance Related Trait Variability

“…The same pattern of partitioning has also been observed in other plants (Gonzales et al, 2008). In response to evapotranspiration demands, shoots drive water uptake through a root system (Comas et al, 2013) and amount of water uptake is determined by root architecture, i.e., root angles, rooting depth, root diameter, number of root branches and length of root hairs (Lynch, 2013). Changes in biomass partitioning under stress determine plants ability to respond to environmental changes that alter resource availability and plants invariably respond by increasing its efficiency of the resource that tends to limit plant growth and finally change its yielding ability.…”

“…In the root system of maize, lateral roots are of major importance for the efficient short-distance exploitation of water and nutrients (McCully 1999), and they make up about eight times the surface area of their parental axile roots and take up about eight times as much water. In response to evapotranspiration demands, shoots drive water uptake through a root system (Comas et al, 2013) and amount of water uptake is determined by root architecture, i.e., root angles, rooting depth, root diameter, number of root branches and length of root hairs (Lynch, 2013). Wasson et al, (2012) proposed selection on the traits to improve root systems and water uptake in water-limited wheat crops, which includes deep roots, greater root branching at median and deeper soil layers, reduced root length density near the surface, and longer root hairs with increased xylem diameter for decreased resistance to water movement from soil to roots.…”

Multiple cropping systems as drivers for providing multiple ecosystem services: from concepts to design

“…The combination of targeted functional traits can be based on current knowledge of the relationship between traits and functions (Lavorel and Garnier 2002;Suding et al 2008) or from a mechanistic model (Hammer et al 2010;Lynch 2013). If a trait value/state (or a combination of trait values) is not found in an available variety/breeding line, a breeding strategy is required specifically for multiple cropping systems.…”

Functional implications of root cortical senescence for soil resource capture

“…The acquisition of mobile resources, including nitrate and water, is enhanced by root phenotypes that increase the exploration of deep soil domains. Over the growing season, mobile resources are available in deep soil domains due to depletion by root uptake in the topsoil as loss to leaching in the case of nitrate and direct evaporation in the case of water (Manschadi et al 2006;Lynch 2013;Trachsel et al 2013;Lynch and Wojciechowski 2015). However, immobile resources, including potassium and phosphorus, typically have the greatest availability in shallow soil domains Zhu et al 2005;Lynch 2011;Lynch 2013).…”

“…Several root anatomical phenes influence the metabolic cost of soil exploration by changing the proportion of respiring and non-respiring root tissue and affecting the nutrient and carbon cost of tissue construction and maintenance. Plants that are able to access soil resources at a reduced metabolic cost will have superior productivity because more metabolic resources will be available for further resource acquisition, growth, and reproduction (Lynch 2013;Lynch 2015). Cortical phene states that reduce living cortical tissue reduce root respiration and nutrient content, thereby permitting greater resource allocation to other plant functions including growth and reproduction Lynch 2015).…”