Field Crops Research 120 (2011) 205–214 Contents lists available at ScienceDirect Field Crops Research journal homepage: www.elsevier.com/locate/fcr Variation in root system architecture and drought response in rice (Oryza sativa): Phenotyping of the OryzaSNP panel in rainfed lowland fields Amelia Henry ∗ , Veeresh R.P. Gowda, Rolando O. Torres, Kenneth L. McNally, Rachid Serraj International Rice Research Institute, Los Ba˜ nos, Philippines article info Article history: Received 9 August 2010 Received in revised form 5 October 2010 Accepted 12 October 2010 Keywords: Drought Rice Rainfed lowland Root architecture abstract Root growth at soil depths below 30 cm may provide access to critical soil water reserves during drought in rainfed lowland rice. In this study, the OryzaSNP panel, a set of 20 lines representing genetic diversity in rice used for the discovery of DNA sequence polymorphisms, was evaluated for root characteristics in the field over three seasons varying in drought severity. Root length density (RLD) at a depth of 30–45 cm varied up to 74–92% among genotypes under drought stress (2008–2009 dry seasons), ranging from 0.024 to 0.23 cm cm -3 in 2008 and from 0.19 to 0.81 cm cm -3 in 2009. Real-time monitoring of soil moisture profiles revealed significant differences among genotypes, and these differences were correlated with RLD at those soil depths. Among the lines evaluated, the Aus isozyme group, particularly the genotype Dular, showed greater drought resistance associated with deep root growth and the highest drought response index (less reduction in yield by drought stress). Since the set of genotypes used in this study has been completely sequenced for SNP markers, the phenotypic information on root growth and drought avoidance responses presented here could be used in initial analysis of the genetic basis of dehydration avoidance traits and in facilitating improvement in drought resistance in rice. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Drought stress occurs in a rainfed lowland rice (Oryza sativa) field when insufficient rainfall results in its conversion from an anaerobic flooded paddy to an aerobic drained field with cracked soil and progressively deepening water table, subject to intermit- tent water inputs from rainfall. Root characteristics, particularly root depth, are likely to increase plant water uptake, dehydra- tion avoidance mechanisms, and rice resistance to drought effects (Serraj et al., 2009). Root growth of lowland rice is generally con- sidered to be shallow due to its adaptation to flooded conditions, but genetic variation for root depth in rice does exist and has long been considered as an advantage in rainfed systems (Nicou et al., 1970). Root length density (RLD) in lowland rice is affected by environmental conditions, genotype, management practices, and soil depth (Beyrouty et al., 1988; Morita et al., 1988; Kang et al., 1994; Tuong et al., 2002; Samson et al., 2002). Lafitte et al. (2001) reported the existence of large genetic differences in root growth of various isozyme groups of rice grown in containers, where indica genotypes characteristically showed thin, shallow roots; japonica genotypes had deeper, coarse roots; and Aus genotypes showed ∗ Corresponding author at: Crop and Environmental Science Division, IRRI, DAPO Box 7777, Metro Manila, Philippines. Tel.: +63 49 536 2701; fax: +63 49 5367995. E-mail address: a.henry@cgiar.org (A. Henry). roots with intermediate diameter, growing to a depth similar to japonica. Similarly, Uga et al. (2009) reported differences in root morphology according to isozyme group (japonica, indica-I, and indica-II) in upland field conditions, with deeper roots from the indica-I group. In addition to isozyme group, Lafitte et al. (2001) cited adaptation to cultivation system (upland or lowland) as an important factor for predicting root growth. To understand genetic potential for deep root growth for improved drought resistance in rainfed lowland rice, it is necessary to evaluate root growth in the field under relevant drought stress conditions in a diverse set of genotypes. Rice has been characterized as having reduced water uptake under drought due to root shallowness and reduced water extrac- tion per length of root compared with maize (Kondo et al., 2000). Water uptake is a function of root length, soil and root hydraulic conductance, and transpiration demand (Nobel, 2005). Genetic differences in water uptake may provide important insights into drought-resistant germplasm that may be overlooked with root length measurements alone. In upland rice studies, genetic differ- ences in water uptake as measured with a neutron probe have been reported (Puckridge and O’Toole, 1981; Lilley and Fukai, 1994). However, a comprehensive view of genetic effects on soil moisture under drought has thus far not been presented in rice. In this study, we characterized root growth of rainfed lowland rice in terms of distribution with soil depth and among diameter classes, as well as volumetric soil moisture profiles. Drought and 0378-4290/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2010.10.003