Upland rice grown in soil-filled chambers and exposed to contrasting water-deficit regimes I. Root distribution, water use and plant water status

Root morphological characteristics are known to be important in the drought resistance of some rice (Oryza sativa L.) varieties. As part of a research programme aimed at using molecular marker technology for the improvement of drought resistance in rice, it is necessary to identify quantitative trait loci (QTLs) associated with root morphology and other drought resistance-related traits. Stability across environments is critically important for the identification of root growth QTLs that are valuable in breeding. Here, a mapping population of 140 recombinant inbred lines and the parental varieties Bala and Azucena were screened for root growth in thin glass-sided soil-filled chambers. In each of 2 years, two treatments were used: an early water deficit (WD0) in which seeds were sown into wet soil but received no more water, and a late water deficit (WD49) in which the plants were watered for 49 days and water was withheld for a final 7 days. Plant height, visible root growth, soil moisture at four depths and water use were monitored weekly. After 56 days, shoot mass, relative water content, root mass in four depth sections and adventitious root thickness at two depths were measured.

In the WD0 treatment, shoot growth slowed markedly, while maximum root length was increased, indicating major changes in partitioning. This change in partitioning was also revealed as major differences in root mass (total and at depth) and root to shoot ratio between treatments. There were also large differences in partitioning between similar treatments in different years, which may in part reflect plant responses to soil water and nutrient status. In both treatments. water was extracted first from the upper sections of the rooting zone and then from progressively deeper layers. Effective water extraction required a root length density of at least 0.4 cm cm (-3) in both treatments, and in the WD49 treatment water extraction at 100 cm depth was also related to shoot size and stomatal conductance. Azucena had thicker roots and more roots at depth compared to Bala, which slowed shoot growth sooner and became less water-stressed than Azucena. Azucena has root traits that potentially contribute to drought resistance, while Bala has a number of shoot-related mechanisms that make it adapted to drought-prone environments, The data presented here have been used to identify QTLs for root growth traits in a companion paper. (C) 2002 Elsevier Science B.V. All rights reserved.