Analysis of environmental effects on expression of root penetration QTLs in upland rice
In the first year, two upland rice varieties (Azucena and Bala), were screened for root response to drought at the West Africa Rice Development Association (WARDA), Cote d’Ivoire, in two fields of slightly different soil penetration resistance (PR) characteristics. Changes in soil PR and soil water content were monitored during the drought period. Root density and depth were significantly greater for Azucena than Bala, and on the irrigated plots compared to the droughted plots, although no consistent site differences in root density were observed. At each site, on the droughted subplot, soil PR quickly increased near the surface (0-30 cm) in response to reduction in soil water content and soil matrix potential caused by root water extraction. It is likely that this increase in PR would have either prevented or reduced the rate of downward growth of new roots entering or growing within this layer. Under these conditions, varietal differences in root response to impedance would be important for drought avoidance. In the second year, also at WARDA, a mapping population based on a cross between Azucena and Bala were tested in two fields of contrasting soil physical properties and QTL for root density at 35 cm were identified. There was no agreement between sites. Site characterisation prior to field screening revealed the two sites to be very different in terms of soil texture and water relations. These site differences would have restricted root growth in different ways and are likely to be reasons behind the lack of agreement in root density QTL between sites. In the third year, near-isogenic lines (NILs) differing only in single or multiple root growth QTL were screened in fields at the International Rice Research Institute (IRRI), Philippines, in soils of lower mechanical impedance than sites used at WARDA. No major differences were observed for root density indicating the importance of interaction between root traits and the environment when considering contribution to drought resistance.