This research was undertaken to identify and map quantitative trait loci (QTLs) associated with five parameters of rice root morphology and to determine if these QTLs are located in the same chromosomal regions as QTLs associated with drought avoidance/tolerance. Root thickness, root:shoot ratio, root dry weight per tiller, deep root dry weight per tiller, and maximum root length were measured in three replicated experiments (runs) of 203 recombinant inbred lines grown in a greenhouse. The lines were from a cross between indica cultivar Co39 andjaponica cultivar Moroberekan. The 203 RI lines were also grown in three replicated field experiments where they were drought-stressed at the seedling, early vegetative, and late-vegetative growth stage and assigned a visual rating based on leaf rolling as to their degree of drought avoidance/tolerance. The QTL analysis of greenhouse and field data was done using single-marker analysis (ANOVA) and interval analysis (Mapmaker QTL). Most QTLs that were identified were associated with root thickness, root/shoot ratio, and root dry weight per tiller, and only a few with deep root weight. None were reliably associated with maximum root depth due to genotype-by-experiment interaction. Root thickness and root dry weight per tiller were the characters found to be the least influenced by environmental differences between greenhouse runs. Correlations of root parameters measured in greenhouse experiments with field drought avoidance/tolerance were significant but not highly predictive. Twelve of the fourteen chromosomal regions containing putative QTLs associated with field drought avoidance/tolerance also contained QTLs associated with root morphology. Thus, selecting for Moroberekan alleles at marker loci associated with the putative root QTLs identified in this study may be an effective strategy for altering the root phenotype of rice towards that commonly associated with drought-resistant cultivars.

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In the rainfed lowlands, rice ( Oryza sativa L.) develops roots under anaerobic soil conditions with ponded water, prior to exposure to water stress and aerobic soil conditions that arise later in the season. Constitutive root system development in anaerobic soil conditions has been reported to have a positive effect on subsequent expression of adaptive root traits and water extraction during progressive water stress in aerobic soil conditions. We examined quantitative trait loci (QTLs) for constitutive root morphology traits using a mapping population derived from a cross between two rice lines which were well-adapted to rainfed lowland conditions. The effects of phenotyping environment and genetic background on QTLs identification were examined by comparing the experimental data with published results from four other populations. One hundred and eighty-four recombinant inbred lines (RILs) from a lowland indica cross (IR58821/IR52561) were grown under anaerobic conditions in two experiments. Seven traits, categorized into three groups (shoot biomass, deep root morphology, root thickness) were measured during the tillering stage. Though parental lines showed consistent differences in shoot biomass and root morphology traits across the two seasons, genotype-by-environment interaction (GxE) and QTL-by-environment interaction were significant among the progeny. Two, twelve, and eight QTLs for shoot biomass, deep root morphology, and root thickness, respectively, were identified, with LOD scores ranging from 2.0 to 12.8. Phenotypic variation explained by a single QTL ranged from 6% to 30%. Only two QTLs for deep root morphology, in RG256-RG151 in chromosome 2 and in PC75M3-PC11M4 in chromosome 4, were identified in both experiments. Comparison of positions of QTLs across five mapping populations (the current population plus populations from four other studies) revealed that these two QTLs for deep root morphology were only identified in populations that were phenotyped under anaerobic conditions. Fourteen and nine chromosome regions overlapped across different populations as putative QTLs for deep root morphology and root thickness, respectively. PC41M2-PC173M5 in chromosome 2 was identified as an interval that had QTLs for deep root morphology in four mapping populations. The PC75M3-PC11M4 interval in chromosome 4 was identified as a QTL for root thickness in three mapping populations with phenotypic variation explained by a single QTL consistently as large as 20-30%. Three QTLs for deep root morphology were found only in japonica/indica populations but not in IR58821/IR52561. The results identifying chromosome regions that had putative QTLs for deep root morphology and root thickness over different mapping populations indicate potential for marker-assisted selection for these traits.

their expression, and may demonstrate variation that is subsequently modified by adaptive traits. Adaptive traits In the rainfed lowlands, rice (Oryza sativa L.) develops roots under will be defined as those, such as root penetration index anaerobic soil conditions with ponded water, prior to exposure to aerobic soil conditions and water stress. Constitutive root system or osmotic adjustment (Zhang et al., 2001), which are development in anaerobic soil conditions has been reported to have expressed in response to water deficit or soil physical/ a positive effect on subsequent expression of adaptive root traits chemical barriers. Less research attention has been and water extraction during water stress. We examined effects of given to constitutive traits than to adaptive traits. phenotyping environment on identification of quantitative trait lociA deep and thick root system has been thought advan-(QTLs) for constitutive root morphology traits using 220 doubledtageous for improved drought tolerance in the rainfed haploid lines (DHLs) from the cross of 'CT9993-5-10-1-M' (CT9993; lowland ecosystem, based on extrapolation from experijaponica, upland adapted) ϫ 'IR62266-42-6-2' (IR62266; indica, lowence with upland rice (O'Toole, 1982; and Fukai and land adapted) in four greenhouse experiments. Broad sense heritabil-Cooper, 1995). Under anaerobic well-watered condiity (h 2 ) was 75, 60, and 64% on average for shoot biomass, deep root tions, root system development had a positive effect on morphology, and root thickness traits, respectively. Quantitative trait loci analysis identified 18 genomic regions associated with deep root subsequent plant growth during progressive water stress morphology traits, but only three were identified consistently across (Azhiri-Sigari et al., 2000; Kamoshita et al., 2000; and experiments. Three out of a total of eight QTLs for root thickness Hoque and Kobata, 1998). Azhiri-Sigari et al. (2000) traits were found in more than one experiment. The maximum genetic and Kamoshita et al. (2000) demonstrated genotypic effects caused by a single QTL were increments of 0.05 g of deep variation in constitutive root traits, and subsequent reroot mass below a 30-cm soil depth, 0.9% of deep root ratio, 1.6 cm sponses of adaptive root traits, especially in deeper soil of rooting depth, and 0.09 cm of root thickness, with phenotypiclayers. Greater root elongation to depth resulted in imvariation explained by a single QTL ranging from 6.8 to 51.8%. The proved water extraction. Improved seedling vigor was results demonstrate the importance of phenotyping environment and also valuable to growth afterward (Mitchell et al., 1998).suggest prospects for selection of QTLs for deep root morphology,