Résumé
Two upland rice varieties, Azucena and Bala, were screened for root response to drought in two field sites with slightly different penetration resistance characteristics at the West Africa Rice Development Association (WARDA, Côte d'Ivoire) during the dry season of 1999/2000. The soil was a freely drained sandy clay loam Paleudult containing frequent gravelly iron concretions. The number of nodal root axes per unit area passing through horizontal transects (root density) was counted at 35, 56, 77 and 98 DAS at 10 cm depth intervals. Azucena consistently maintained a greater root density and rooting depth than Bala. Azucena was also able to access water at greater depth although it did not extract significantly more soil water from the profile. There were no consistent site differences but root density and depth were significantly greater on irrigated plots. There was no variety by treatment interaction suggesting that the response of Azucena and Bala were similar. Penetration resistance (PR) between 0-30 cm depth increased greatly during the drought, in line with a reduction in soil water content. This was likely to prevent or greatly impair further nodal root growth within this layer. At 40 cm depth, PR was high (3-4 MPa) and did not increase under drought, at this depth root growth rate was likely to be greatly reduced despite the availability of water. Our results suggest that varietal differences in root morphology characterised in the laboratory can be detected in impeding field soils as difference 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 dry season screens. Further reductions in root growth under drought treatments could be due either to the higher mechanical impedance in the upper portions of the soil or the further increase in plant water stress as the soil dries. We suggest that root response to mechanical impedance was a more fundamental constraint than root response to water stress during the drought. Thus, varietal differences to impedance could be importance for drought avoidance in soils of this type. (Résumé d'auteur)