Upland rice grown in soil-®lled chambers and exposed to contrasting water-de®cit regimes I. Root distribution, water use and plant water status A.H. Price a,* , K.A. Steele b , J. Gorham b , J.M. Bridges b , B.J. Moore b , J.L. Evans b , P. Richardson b , R.G.W. Jones b a Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UU, UK b Centre for Arid Zone Studies, University of Wales, Bangor, Gwynedd LL57 2UW, UK Accepted 16 January 2002 Abstract 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 identi®cation 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-®lled chambers. In each of 2 years, two treatments were used: an early waterde®cit(WD0)inwhichseedsweresownintowetsoilbutreceivednomorewater,andalatewaterde®cit(WD49)inwhich the plants were watered for 49 days and water was withheld for a ®nal 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. IntheWD0treatment,shootgrowthslowedmarkedly,whilemaximumrootlengthwasincreased,indicatingmajorchangesin partitioning.Thischangeinpartitioningwasalsorevealedasmajordifferencesinrootmass(totalandatdepth)androottoshoot ratio between treatments. There were also large differences in partitioning between similar treatments in different years, which may in part re¯ect plant responses to soil water and nutrient status. In both treatments, water was extracted ®rst from the upper sections of the rooting zone and then from progressively deeper layers. Effectivewater extraction required a root length density ofatleast0.4cmcm 3 inbothtreatments,andintheWD49treatmentwaterextractionat100cmdepthwasalsorelatedtoshoot sizeandstomatalconductance.AzucenahadthickerrootsandmorerootsatdepthcomparedtoBala,whichslowedshootgrowth 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. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Oryza sativa; Root growth; Drought resistance; Genotype  environment interaction; Water extraction; QTL; Soil moisture Field Crops Research 76 (2002) 11±24 * Corresponding author. Tel.: 44-1224-272690; fax: 44-1224-272703. E-mail address: a.price@abdn.ac.uk (A.H. Price). 0378-4290/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0378-4290(02)00012-6