Temporal variation in root penetration ability of wheat genotypes through thin wax layers in contrasting water regimes and in the field

A significant proportion of arable land in south-western Australia is highly susceptible to subsoil compaction, which limits access of roots of wheat to water and nutrients at depth. Australian wheat cultivars were shown to differ in penetration ability. The objective of this paper was to examine the dynamics of penetration ability and temporal variation in root growth using contrasting cultivars, with and without a thin wax layer, in contrasting water regimes, and in the field on two contrasting soil types at Merredin, Western Australia. The presence of a wax layer had a significant impact on root dry matter (DM) distribution and root number, which were more abundant above the wax layer. Root axes below the wax layer were around 50% shorter compared with columns without wax layers. Around 15% more nodal roots were produced in columns containing wax layers than those without and this difference was apparent from an early stage of plant growth, as was the impact of water deficit that decreased root DM and nodal root number above the wax layer. Root axes of genotypes Bonnie Rock, Castle Rock, Halberd and Spear all penetrated the wax layer, while CM18 and Cranbrook had limited or no success, regardless of water regime. The maximum root depths in field soils at 75 and 90 DAS were relatively shallow at 35–48 cm, but with a relatively small variance. On a sandy duplex containing a hardpan, CM18 and Cranbrook had 28% shorter roots than Bonnie Rock and Halberd at 75 DAS, which was consistent with the poor ability of roots of CM18 and Cranbrook to penetrate wax layers. Adaptation of roots to soil physical constraint thus provides benefit for root access to deeper soil layers.

► Compacted soil was simulated by a thin wax-petroleum layer in soil columns.
► The wax layer had an impact on root axis distribution, number and length.
► Wheat genotypes varied in ability to penetrate the wax layer.
► Lines with greater penetration ability attained a greater root depth in the field.
► Penetration ability should benefit root access to deeper soil layers in compacted soils.