How do roots elongate in a structured soil?

Abstract

In this review, we examine how roots penetrate a structured soil. We first examine the relationship between soil water status and its mechanical strength, as well as the ability of the soil to supply water to the root. We identify these as critical soil factors, because it is primarily in drying soil that mechanical constraints limit root elongation. Water supply to the root is important because root water status affects growth pressures and root stiffness. To simplify the bewildering complexity of soil-root interactions, the discussion is focused around the special cases of root elongation in soil with pores much smaller than the root diameter and the penetration of roots at interfaces within the soil. While it is often assumed that the former case is well understood, many unanswered questions remain. While low soil-root friction is often viewed as a trait conferring better penetration of strong soils, it may also increase the axial pressure on the root tip and in so doing reduce the rate of cell division and/or expansion. The precise trade-off between various root traits involved in root elongation in homogeneous soil remains to be determined. There is consensus that the most important factors determining root penetration at an interface are the angle at which the root attempts to penetrate the soil, root stiffness, and the strength of the soil to be penetrated. The effect of growth angle on root penetration implicates gravitropic responses in improved root penetration ability. Although there is no work that has explored the effect of the strength of the gravitropic responses on penetration of hard layers, we attempt to outline possible interactions. Impacts of soil drying and strength on phytohormone concentrations in roots, and consequent root-to-shoot signalling, are also considered.

Root Tip Shape Governs Root Elongation Rate under Increased Soil Strength

“…The root elongation rate decreased with increasing soil bulk density (Table I), which is in agreement with previous findings (Atwell, 1990a;Bengough and Mullins, 1991;Young et al, 1997;Jin et al, 2013;Colombi and Walter, 2016). However, while the root elongation rate decreased with increasing soil strength in all genotypes assessed, the magnitude of this response differed significantly between genotypes (Table I).…”

Physical root–soil interactions

“…In structured soils, when a root grows across a macro-pore and meets a solid surface (Fig. 2B), it is unconfined laterally and may buckle, which reorients its growth direction (see the review by Jin et al [7]). …”

“…Since then, thorough and synthetic reviews and books in soil science and agronomy described the interplay between physical soil properties and root growth and architecture [6][7][8][9][10]. The aim of the present review is to combine these macroscopic approaches of the soil-root physical interaction with simple mechanistic modeling derived from model experiments at the scale of the individual root apex.…”

Wheat root growth responses to horizontal stratification of fertiliser in a water-limited environment

“…The promotion of water captured from subsoil by dryland wheat (Tritcum aestivum L.) can make a big contribution to yield (Kirkegaard et al 2007;Richards 2008;Lilley and Kirkegaard 2011). Increasing root system access to water deep in the soil profile may be a promising way to increase water capture under waterlimiting conditions (Mi et al 2010;Dodd et al 2011;Jin et al 2013;Trachsel et al 2013;White et al 2013). However, whether the site of nutrient application can regulate root growth, thereby altering crop water-use, is an important issue that may increase crop productivity under water-limited conditions (Hodge 2010;Jin et al 2013;Shen et al 2013), but this issue has been relatively unexplored.…”

“…Increasing root system access to water deep in the soil profile may be a promising way to increase water capture under waterlimiting conditions (Mi et al 2010;Dodd et al 2011;Jin et al 2013;Trachsel et al 2013;White et al 2013). However, whether the site of nutrient application can regulate root growth, thereby altering crop water-use, is an important issue that may increase crop productivity under water-limited conditions (Hodge 2010;Jin et al 2013;Shen et al 2013), but this issue has been relatively unexplored. Soil nutrient heterogeneity must be considered when designing soil management strategies aimed at improved overall resource use efficiency, particularly water-use efficiency (Farley and Fitter 1999;Fransen et al 1999;Hodge 2004;Tittonell et al 2005;Hodge 2006;De Kroon 2007).…”