Effect of soil mechanical impedance on root growth of two rice varieties under field drought stress

Two upland rice varieties, Azucena and Bala, were screened for root growth under droughted and irrigated treatments in two field sites at the West Africa Rice Development Association (WARDA) experimental farm, Côte d’Ivoire, during the dry season of 1999/2000. The sites were chosen to represent contrasting soil profile penetration resistance (PR) characteristics on upland sites, although both were relatively impeding. The number of nodal root axes per unit area passing through horizontal transects (root density) was counted at 35, 56, 77 and 98 days after sowing (DAS) at 10 cm depth intervals. Azucena consistently maintained a greater root density than Bala and a greater proportion of Azucena roots grew to 30 cm depth (22.7% vs. 8.4% at 77 DAS). There was little detectable effect of water regime on root distribution but evidence of lower root numbers at depths below 20 cm in the higher PR site was revealed. A site by variety by soil depth interaction suggests that Azucena roots are more strongly affected by very high PR than those of Bala. PR between 0–30 cm depth increased greatly with decreasing soil water content during the drought as the soil dried. This increase is likely to have prevented or greatly impaired further nodal root growth within this layer. At 40 cm depth, PR was high (3–4 MPa) but did not increase during the drought. At this depth root growth rate was likely to be greatly reduced despite the availability of water. These results demonstrate that varietal differences in root morphology characterised in the laboratory can be also detected in impeding field soils as differences in the density of roots at depth. Relatively poor root growth in these fields in the absence of drought was probably due to the high mechanical impedance and/or the physiological stress of the plants in the dry season. Our results indicate that high mechanical impedance was a more fundamental constraint on root growth than soil water availability during the drought. Thus, varietal differences in root penetration ability might be very important for drought avoidance in soils of this type.