Root penetration ability of wheat through thin wax-layers under drought and well-watered conditions

Abstract

Sand over clay duplex soils and those compacted by heavy farm machinery restrict water infiltration and root growth because roots cannot penetrate hard soil. Under drought, restriction of roots to soil above the hard layer results in the early onset of plant water-deficit, unless roots can penetrate the hard layer to reach soil water and nutrients at depth. There is little to no information on the ability of roots of bread wheat (Triticum aestivum L.) to penetrate hardpans. Here we report on 3 experiments undertaken in a controlled environment in pots that validate and explore the use of thin Paraffin wax–Vaseline (WV) layers of different strengths to simulate a hardpan under contrasting water regimes. Seeds produced an average of 5 seminal roots, which all penetrated the low-impedance wax-layer (0.03WV), in such a way that seminal root dry matter (DM) was evenly distributed throughout the soil profile. The number and depth of penetrating seminal root axes declined as wax-layer strength increased, and a significant proportion of total length and DM of main seminal root axes was instead restricted to the 0–0.12-m soil layer above the wax layer. No roots penetrated the 0.60WV, which was equivalent to ~1.50 MPa penetrometer resistance. The distribution of seminal roots was less affected by water regime than nodal roots, which were severely reduced in number when drought was imposed at 14 days after sowing (DAS), compared with well-watered conditions. Growth of the seminal roots into soil beneath the wax-layer determined the pattern of stomatal conductance and volumetric soil water content (Jv) over the period of drought stress, as few nodal roots reached and penetrated the wax layer. Stomatal conductance declined suddenly at 19 days after the last irrigation, and partially recovered as water extraction increased in the 0.40–0.60-m soil depth. Reasons for this are discussed. The wax-layer technique requires validation for wheat in the field, but the technique offers promise for screening breeding lines for the ability to penetrate a hardpan.