Drought is a major cause of yield loss in rain-fed rice (Oryza sativa L.), grown on over 40 million ha in Asia. The objective of this study was to evaluate the effectiveness of direct selection for ...yield under drought stress in upland rice in populations derived from crosses between irrigated high-yielding cultivars and upland-adapted cultivars. Random F2:4 lines from five populations were screened for grain yield in fully irrigated lowland fields under nonstress conditions and in uplands under severe reproductive-stage drought stress. Stress caused mean yield reduction of 64% across populations. Broad-sense heritability for yield was not consistently lower in stress than in nonstress trials. Response to selection was evaluated in two crosses in subsequent seasons. Stress-selected lines had a yield advantage of 25 to 34% over random lines when evaluated at stress levels similar to those in which they were selected. Yield gains under very severe stress occurred only in a population derived from a highly tolerant parent. Direct selection usually gave greater response under stress than indirect selection under nonstress conditions. Direct selection under dry-season stress also gave response under naturally occurring wet-season stress. These results support the hypothesis that selection for yield under reproductive-stage drought stress is effective in rice, and that choice of donor is very important in breeding drought-tolerant rice.
Drought tolerance is an important rainfed rice breeding objective, but because the heritability (
H) of yield under drought stress is thought to be low, secondary physiological traits are considered ...better targets for selection than yield under stress
per se. This assumption has rarely been tested, and there are no reports on
H for yield under drought stress from experiments repeated over seasons in rainfed lowland rice. To assess the potential for improving yield under drought stress via direct selection, and to identify associated quantitative trait loci (QTL), doubled haploid lines with a narrow range of flowering dates, derived from the population CT9993-5-10-1-M/IR62266-42-6-2, were screened under full irrigation and severe drought stress induced by draining the paddy before flowering in 2000–2002 at Raipur, India. Drought stress reduced mean yield by 80%.
H was similar in stress and non-stress trials, as was the relative magnitude of the genotype and genotype
×
year variances. The genetic correlation between yield in stress and non-stress conditions was 0.8, indicating that about 64% of the genetic variation for yield under stress was accounted for by differences in yield potential also expressed in irrigated environments. These results indicate that direct selection for yield under drought stress can produce yield gains under stress without reducing yield potential. There was no secondary trait for which selection resulted in greater predicted response in yield under stress than direct selection for stress yield
per se. A QTL was detected on chromosome 1 near
sd1 that explained 32% of the genetic variation for yield under stress, but only 4% under non-stress. Its effect was consistent across years. This QTL accounted for much of the variation in drought yield not accounted for by variation in yield potential.
Iron (Fe) toxicity is recognized as one of the most widely spread soil constraints for rice production especially in West Africa. Oryza glaberrima the cultivated rice species that originated from ...West Africa is well-adapted to its growing ecologies. The aim of this study was to identify the promising O. glaberrima accessions tolerant to Fe toxicity from the 2106 accessions held at the AfricaRice gene bank. The screenings were conducted over a four-year period and involved evaluating the entries under Fe-toxic field conditions in West Africa, selecting good yielding accessions and repeating the testing with newly selected lines. Three accessions (TOG 7206, TOG 6218-B and TOG 7250-A) were higher yielding than O. sativa checks under stress but with similar yields under control conditions. These accessions yielded over 300g/m2 under both Fe toxicity and control conditions. In conclusion, these materials could be used as donors in breeding programs for developing high yielding rice varieties suited to Fe toxicity affected areas in West Africa.
Drought is a major abiotic stress factor limiting rice production in rainfed areas. In this study we identified a large-effect quantitative trait locus (QTL) associated with grain yield under stress ...in five different populations on chromosome 1. The effect of this QTL was further confirmed and characterized in five backcross populations in a total of sixteen stress and non-stress trials during 2006 and 2008. In all the stress trials (eight in total) qDTY1.1 showed strong association with grain yield explaining on average 58% of the genetic variation in the trait. Homozygotes for the tolerant parent allele (Apo) yielded on average 27% more than the susceptible parent allele (IR64) homozygotes. Using an Apo/3*IR64 population, the peak of this QTL (qDTY1.1) was mapped to an interval between RM486 and RM472 at 162.8 cM at a LOD score of 9.26. qDTY1.1 was strongly associated with plant height in all the environments; this was probably due to the presence of the sd1 locus in this genomic region. In a Vandana/3*IR64 population segregating for sd1, a strong relation between plant height and yield under stress was observed. The observed relation between increased height and drought tolerance is likely due to tight linkage between qDTY1.1 and sd1 and not due to pleiotrophy of sd1. Thus there is a possibility of combining reduced plant height and drought tolerance in rice. The large and consistent effect of qDTY1.1 across several genetic backgrounds and environments makes it a potential strong candidate for use in molecular breeding of rice for drought tolerance.
Drought is a major production constraint in rainfed rice (
Oryza sativa L.). Lack of effective selection criteria is a major limitation hampering progress in breeding for drought tolerance. In an ...earlier report, we showed in two populations that one cycle of direct selection was effective in increasing grain yield under stress. In the present study, we retested the efficiency of direct selection for grain yield under drought stress in rice using four populations derived from crossing upland-adapted, drought-tolerant varieties (Apo, Vandana) to high-yielding, lowland-adapted, drought-susceptible varieties (IR64, IR72). Each population was subjected to two cycles of divergent selection either under drought stress in upland or under nonstress conditions in lowland conditions. Following selection, approximately 40 high-yielding lines selected under each protocol from each population, along with a set of unselected lines, were evaluated in a series of selection response trials over a range of moisture levels. Significant response to direct selection under stress was realized in 9 out of 15 combinations of populations and stress environments, and in 6 of the 7 severe stress trials. Averaging over all the populations and stress environments, the stress-selected lines had a yield advantage of 25 and 37% over nonstress-selected and random lines, respectively. In contrast to this, under nonstress, the nonstress-selected lines had an average yield advantage of only 7 and 13% over stress-selected and random lines, respectively. Direct selection in managed stress trials during dry seasons gave significant response (25% on average relative to indirect selection in nonstress conditions) under naturally occurring wet season stress. In addition, direct selection under stress in upland gave an average gain of 16 and 45% over nonstress-selected and random lines, respectively, under stress in lowland. The yield advantage of the stress-selected lines appears to result mainly from maintenance of higher harvest index. These results show that direct selection for grain yield under stress is effective and does not reduce yield potential. Overall, this is the first report in rice demonstrating that (a) selection under managed drought stress in the dry season can result in yield gains under natural stress in the wet season, and (b) that selection under upland drought stress can, at least under the conditions of the present study, result in gains under lowland drought conditions.
A large-effect QTL associated with grain yield in aerobic environments was identified in three genetic backgrounds, Apo/2*Swarna, Apo/2*IR72, and Vandana/2*IR72, using bulk-segregant analysis (BSA). ...Apo and Vandana are drought-tolerant aerobic-adapted varieties, while Swarna and IR72 are important lowland rice varieties grown on millions of hectares in Asia but perform poorly in aerobic conditions. Two closely linked rice microsatellite (RM) markers, RM510 and RM19367, located on chromosome 6, were found to be associated with yield under aerobic soil conditions in all three backgrounds. The QTL linked to this marker, qDTY6.1 (DTY, grain yield under drought), was mapped to a 2.2 cM region between RM19367 and RM3805 at a peak LOD score of 32 in the Apo/2*Swarna population. The effect of qDTY6.1 was tested in a total of 20 hydrological environments over a period of five seasons and in five populations in the three genetic backgrounds. In the Apo/2*Swarna population, qDTY6.1 had a large effect on grain yield under favorable aerobic (R 2 ≤ 66%) and irrigated lowland (R 2 < 39%) conditions but not under drought stress; Apo contributed the favorable allele in all the conditions where an effect was observed. In the Apo/IR72 cross, Apo contributed the favorable allele in almost all the aerobic environments in RIL and BC1-derived populations. In the Vandana/IR72 RIL and BC1-derived populations, qDTY6.1 had a strong effect on yield in aerobic drought stress, aerobic non-stress, and irrigated lowland conditions; the Vandana allele was favorable in aerobic environments and the IR72 allele was favorable in irrigated lowland environments. We conclude that qDTY6.1 is a large-effect QTL for rice grain yield under aerobic environments and could potentially be used in molecular breeding of rice for aerobic environments.
Biofortification (delivery of micronutrients via micronutrient-dense crops) can be achieved through plant breeding and offers a cost-effective and sustainable approach to fighting micronutrient ...malnutrition. The present study was conducted to facilitate the initiation of a breeding programme to improve the concentration of iron (Fe) and zinc (Zn) in peanut (Arachis hypogaea L.) seeds. The experiment was conducted with 64 diverse peanut genotypes for 2 years in eight different environments at the International Crops Research Institute for the Semi-Arid Tropics, Patancheru, India to assess the genetic variation for Fe and Zn concentrations in peanut seeds and their heritability and correlations with other traits. Significant differences were observed among the genotypes and environments for Fe (33–68 mg/kg), Zn (44–95 mg/kg), protein (150–310 mg/g) and oil (410–610 mg/g) concentration in seeds and their heritability was high, thus indicating the possibility of improving them through breeding. As seen in other plants, a significant positive association between concentrations of Fe and Zn was observed. Trade-offs between pod yield and Fe and Zn concentrations were not observed and the same was also true for oil content. Besides being high yielding, genotypes ICGV 06099 (57 mg/kg Fe and 81 mg/kg Zn) and ICGV 06040 (56 mg/kg Fe and 80 mg/kg Zn) had stable performance for Fe and Zn concentrations across environments. These are the ideal choices for use as parents in a breeding programme and in developing mapping populations.
► Near-isogenic lines (NILs) are very useful in genetic and physiological investigations in crop plants. ► We report on development and characterization of NILs contrasting for grain yield under ...drought in rice. ► These NILs show that small genetic differences can cause large difference in grain yield under drought stress in rice. ► A region on chromosome linked to SSR marker RM72 on chromosome 8 appears to an important locus governing yield under drought. ► The drought-tolerant NILs generally showed higher assimilation and transpiration rates than the susceptible NILs. ► The drought-tolerant NILs seem to possess deeper root length constitutively.
The development of near-isogenic-lines (NILs) is a very important tool for both genetic and physiological dissection of drought resistance in rice. Two pairs of NILs differing for grain yield under drought stress were isolated and characterized for yield, yield related traits, and several physiological traits in a range of contrasting environments. In replicated field trials both NIL pairs differed significantly for grain yield under drought stress but showed similar yield potential, phenology, and yield component traits under non-stress conditions. A polymorphism analysis study with 491 SSRs revealed that both NIL pairs are at least 96% genetically similar. These NILs show that small genetic differences can cause large difference in grain yield under drought stress in rice. In both pairs the drought-tolerant NILs showed a significantly higher assimilation rate at later stages both under stress and non-stress conditions. They also had a higher transpiration rate under non-stress condition. The most tolerant NIL (IR77298-14-1-2-B-10) had significantly higher transpiration rate and stomatal conductance in severe stress conditions. In one pair the tolerant NIL had constitutively deeper roots than the susceptible NIL. In the second pair, which had higher mean root length than the first pair, the tolerant NIL had more roots, greater root thickness, and greater root dry weight than the susceptible NIL. Deeper root length may allow tolerant NILs to extract more water at deeper soil layers. It is concluded that enhanced rooting depth is an important strategy for dehydration avoidance and rice adaptation to drought stress, but root architecture might not be the only mechanism causing the significant yield increase we observed in lowland drought stress environments. To further dissect the drought avoidance mechanisms in rice, analysis of root hydraulic properties may be necessary.
Molecular marker loci responding to selection under drought stress were monitored in a rice breeding population obtained by crossing a tolerant parent (Apo) to a susceptible parent (IR64). The 40 ...highest-yielding lines under stress and non-stress conditions obtained after two cycles of divergent selection under drought stress and non-stress conditions, respectively were genotyped using 72 polymorphic and widely distributed SSR markers. Ten loci (RM572, RM6703, RM71, RM3387, RM5686, RM520, RM510, RM256, RM269 and RM511) showing highly significant allele frequency differences between the two sets were identified. Favorable alleles at eight of these loci came from the tolerant parent, and at two (RM3387 and RM510) from the susceptible parent (IR64). Effects of these loci on grain yield were tested in five independent experiments covering a range in soil moisture levels. Results showed that at six loci (RM572, RM6703, RM520, RM256, RM269, and RM511), Apo alleles had highly significant effects on grain yield in at least three of the four stress trials but only two of these loci (RM572 and RM511) also affected grain yield under non-stress conditions. In all these cases, the effects of loci generally increased with stress level. Apo alleles at these loci seem to enhance yield under stress mainly by increasing harvest index and reducing flowering delay. Large-effect quantitative trait loci (QTLs) affecting grain yield under upland drought stress have already been found previously in other populations near RM6703, RM520, and RM511. Thus, these regions appear to be important in explaining genetic variation for upland drought tolerance in rice.
Rice double-haploid (DH) lines of an indica and japonica cross were grown at nine different locations across four countries in Asia. Genotype-by-environment (G x E) interaction analysis for 11 ...growth- and grain yield-related traits in nine locations was estimated by AMMI analysis. Maximum G x E interaction was exhibited for fertility percentage number of spikelets and grain yield. Plant height was least affected by environment, and the AMMI model explained a total of 76.2% of the interaction effect. Mean environment was computed by averaging the nine environments and subsequently analyzed with other environments to map quantitative trait loci (QTL). QTL controlling the 11 traits were detected by interval analysis using mapmaker/qtl. A threshold LOD of >/=3.20 was used to identify significant QTL. A total of 126 QTL were identified for the 11 traits across nine locations. Thirty-four QTL common in more than one environment were identified on ten chromosomes. A maximum of 44 QTL were detected for panicle length, and the maximum number of common QTL were detected for days to heading detected. A single locus for plant height (RZ730-RG810) had QTL common in all ten environments, confirming AMMI results that QTL for plant height were affected the least by environment, indicating the stability of the trait. Two QTL were detected for grain yield and 19 for thousand-grain weight in all DH lines. The number of QTL per trait per location ranged from zero to four. Clustering of the QTL for different traits at the same marker intervals was observed for plant height, panicle number, panicle length and spikelet number suggesting that pleiotropism and or tight linkage of different traits could be the possible reason for the congruence of several QTL. The many QTL detected by the same marker interval across environments indicate that QTL for most traits are stable and not essentially affected by environmental factors.
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