Factors determining the performance of synthetics in Vicia faba L. 1. Heterogeneity,heterozygosity, and degree of cross-fertilization

Summary The relative performance of synthetic varieties in faba beans (Vicia faba L.) as compared with line varieties is the result of the joint effect of several factors. Factors investigated in the present study were (1) effects of heterogeneity and (2) heterozygosity on agronomic performance, (3) genetic variance for the degree of cross-fertilization among inbred lines and (4) genetic variance among homozygous and heterozygous genotypes for yield. The evaluation was based on multilocational trials in Germany with 36 inbred lines and derived entries with contrasting population structure (levels of heterozygosity and heterogeneity). When inbred lines and F₁-hybrids are included in the same mixture of genotypes, the resulting heterogeneity had a positive effect on yield. Relative mid-parent heterosis for yield was between 33% and 51%. The average degree of cross-fertilization was 53.5%, and its genetic standard deviation was 8.3%. The estimates for genetic standard deviation for yield among inbred lines were between 3.4 q/ha and 5.3 q/ha. The highest yielding synthetic variety is expected to outyield markedly the highest yielding line.     

Evaluation of three sources of pollen fertility restoration genes and natural outcrossing of cytoplasmic male-sterile wheat

Summary Fertility restoration genes in Triticum aestivum L. in Texas Restorer Composite (TRC), D6301, and four CIMMYT restorer lines were studied, and selection was made for higher fertility in TRC. Mean-while, outcrossing percentages of seed set for 27 spring habit cytoplasmic male sterile (cms) varieties were evaluated for 3 to 5 years at Davis. The winter-habit TRC material did not restore reasonably good fertility, and the response to selection for higher fertility seemed to be slow. This poor fertility could be partly due to its late winter growth habit causing flowering at a period of high temperature and low humidity at Davis. The highest F₁ fertility was 46.6% in the cross cms Ramona x TRC-6, and its F₂ segregated into the ratio of 15 fertile to 1 sterile, with fertility ranging from 3.2 to 100%. Suggested for its improvement was intensive selection in the original TRC material and in the segre-gating F₂ population, followed by intercrossing. D6301 has 2 fertility restoration genes with different strengths which restore fertility up to 45.2% when both genes are heterozygous. D6301 is quite likely heterogeneous for these genes. Four CIMMYT restorer lines, D7464, D7465, D7466, and D7467, had satisfactory F₁ fertility restoration after crossing with cms Ramona 50. In 1975, the fertilities of the F₁'s ranged from 71 to 85% and were over 90% in 1976. The F₂ population of the cross cms Ramona 50 × D7464 segregated into a ratio of 3 fertile to 1 sterile, indicating that D7464 has a single dominant gene for fertility restoration. The F₂'s of crosses cms Ramona 50 × D7465, cms Ramona 50 × D7466, and cms Ramona 50 × D7467 gave a ratio of 15 fertile to 1 sterile, indicating that two gene pairs in these three lines were responsible for the fertility restoration. The best of this group was D7467 which restored fertility fully after being crossed with cms Ramona 50 (T. timopheevi cytoplasm).The early-flowering cms male-sterile varieties had higher outcrossing rates (16 to 38%) than late varieties (6 to 30%) over a 5-year period. This was due to hot and dry weather during the late growing season as well as to the rarity of windborne pollen. In 1970, 1971, 1972, and 1976, the variation among varieties was rather great. Some of them such as Roque 66 and Bajio 67, had consistently high outcrossing rates. This outcrossing ability seemed to be inherited and probably associated with the open-flowering characteristics of each variety.     

Cross-fertilization behaviour of some white flowering varieties of Vicia faba

Summary Applying a special design for maximalization of cross-fertilization and using flower colour as a marker, considerable differences in the percentage of cross-fertilization between white flowering Vicia faba varieties were observed. Part of these differences were due to differences in beginning of flowering in white and coloured varieties, part of them proves to be independent of this and, therefore appear to be characteristic for the variety. Interaction between mother and father varieties was absent.     

Development of photoperiod-sensitive cytoplasmic male sterile (PCMS) wheat lines showing high male sterility under long-day conditions and high seed fertility under short-day conditions

A “two-line system” using photoperiod-sensitive cytoplasmic male sterility (PCMS) caused by Aegilops crassa cytoplasm under long-day photoperiods (≧15 h) has been proposed as a means of producing hybrid varieties in common wheat (Triticum aestivum). The PCMS line is maintained by self-pollination under short-day conditions, and hybrid seeds can be produced through outcrossing of the PCMS line with a pollinator line under long-day conditions. Our previous studies revealed that PCMS lines showing complete male sterility under long-day conditions are necessary for practical hybrid wheat breeding, especially to obtain high hybrid purity in F₁ seeds. Furthermore, practical PCMS lines should have high seed fertility under short-day conditions, which is associated with female fertility. Wheat cv. Norin 26 with Ae. crassa cytoplasm exhibits high seed fertility under short-day conditions, and cv. Fujimikomugi with Ae. crassa cytoplasm shows high male sterility under long-day conditions. Here we developed practical PCMS lines derived from the F₁ generation of Norin 26 and Fujimikomugi (with Ae. crassa cytoplasm) that were then backcrossed to elite wheat lines.     

Chasmogamous mutant,a novel character enabling commercial hybrid seed production in mungbean

Mungbean (Vigna radiata (L.) Wilczek) is a self-pollinating crop that displays significant hybrid vigor in seed yield of F₁ hybrids. Thus there is the possibility to use hybrid varieties as a breakthrough to raise the yield plateau of mungbean. However, hybrid mungbean seeds can only be accomplished by hand-pollination and thus commercial production is not possible. To encourage hybrid seed set, the plant breeder needs to develop characters that promote higher outcrossing rate such as open flower (chasmogamy). In this experiment, new chasmogamous mutants were induced by gamma irradiation at the rate of 100 and 200 Gy. The mutants were identified at a low rate of 0.4–0.7% in the M₂ generation of accession V1197, and observed for their purity by growing in plant-to-row in the M₃ and M₄ generations. A uniform chasmogamous line was hybridized to normal flower lines to study the inheritance of this character. All F₁ plants had normal flowers, while the F₂ plants segregated well with 3 normal : 1 chasmogamous ratio. When the F₁ was backcrossed to the chasmogamous parent, the progeny gave a ratio of 1 normal to 1 chasmogamous. Thus, chasmogamy was controlled by a single recessive gene, cha.     

Extent, variation and breeding impact of natural cross-fertilization in German winter faba beans using hilum colour as marker

Faba bean is a partially allogamous grain legume, mostly sown as spring crop in Central Europe, and in few mild coastal areas of UK and France as a winter crop. A population with a genetically wide base, conceived and selected for superior winter hardiness showing promise as winter crop in Germany was evaluated. The reproductive behaviour of this population was evaluated for its degree of cross-fertilization at five German environments (1999–2001), and inbred lines bred from it, plus one F1 bulk and further checks, were assessed for variation in cross-fertilization at three German environments (2001). Seed hilum colour, a dominant-recessive, biallelic locus, was used as a morphological marker. Cross-fertilization (population mean value of 60%) was shown to be higher than in spring faba beans and was environmentally influenced. Genetic variation was demonstrated, heritability was high (h²= 0.75), and heterosis was negative. Inbred lines with a cross-fertilization of 70% can be easily bred. Breeding synthetic cultivars is recommended, their optimum number of components is expected to be higher than the optimum recommended for spring faba beans. This revised version was published online in July 2006 with corrections to the Cover Date.     

Breeding for wide adaptation in faba bean

Summary Broadening agronomic adaptation will improve yield stability in the grain legume Vicia faba L. We gathered information on the adaptation of European and Mediterranean material to European and Mediterranean environments. The material comprised 20 inbred lines (12 European and 8 Mediterranean lines) and 99 intra- and interpool-crosses in generation F₁. These were evaluated in 9 environments: two spring-sown Southern German environments (SGermE), and seven autumn-sown Mediterranean environments (MedE) in Sicily, Puglia, Andalucia and South Africa. Standard ANOVA, stability analyses and AMMI analysis were performed. Mean yield in F₁ was 257 g/row, the overall parental mean was 144 g/row. The range of environmental means was from 94 g/row to 411 g/row. The average regression coefficient in F₁ was b _i =1.07, being significantly greater than for the parents (b _i =0.68). The opposite was true for the relative magnitude of the deviations from the regressions, which were highly correlated to the AMMI-PC1-results. The AMMI analysis clearly separated the SGermE from the MedE, as well as the germplasm pools. Though the superiority of the F₁-hybrids over their parents was striking, their pattern of interactions with the environments strictly reflected that of their parents. A number of promising crosses was identified as a nucleus of a widely adapted faba bean genepool.     

Resistance in Cucumis sativus L. to Tetranychus urticae Koch. 5. raising the resistance level by the exploitation of transgression

summary After crossing partially resistant varieties some lines with a markedly higher resistance level were selected. This transgression for resistance indicates a polygenic inheritance of the resistance. On the most resistant F₅ lines selected, oviposition was reduced by 50 to 60% and the economic damage threshold was reached 5 to 8 weeks later than in the susceptible control. The level of acceptance was not influenced by the selection.The selection of individual F₂ plants was hampered by low heritabilities, whereas the heritabilities of F₃- and subsequent line means were generally high enough. Selection for higher resistance levels was attended by an unintended increase in cucurbitacin content. This is explained by linkage of genes for resistance and bitterness rather than by identity of these genes.     

Comparison of two fertility restoration systems against photoperiod-sensitive cytoplasmic male sterility in wheat

A ‘two‐line system’ using photoperiod‐sensitive cytoplasmic male sterility (PCMS) caused by Aegilops crassa cytoplasm under a long‐day photoperiod ( 15 h) has been proposed as a new means of producing hybrid varieties in common wheat. The PCMS line is maintained by self‐pollination under short‐day conditions, and hybrid seeds can be produced through outcrossing of the PCMS line with a pollinator under long‐day conditions. Two kinds of fertility restoration systems against the PCMS are known. One is involved with a set of multiple fertility‐restoring (Rf) genes in the wheat cultivar ‘Norin 61’ located on (at least) chromosomes 4A, 1D, 3D and 5D. The other is controlled by a single dominant major Rf gene, Rfd1, located on the long arm of chromosome 7B in the wheat cultivar ‘Chinese Spring’. To examine the degree of fertility restoration by these two systems, nine PCMS lines were crossed with ‘Norin 61’ and ‘Chinese Spring’ as the restorer lines, and the F₁ hybrids were investigated. The degree of fertility restoration was estimated by comparing the seed set rates in the F₁ hybrids having the Ae. crassa cytoplasm and those with normal cytoplasm. The results revealed that the fertility restoration ability of a set of multiple Rf genes in ‘Norin 61’ was higher than that of the Rfd1 gene in ‘Chinese Spring’.     

Resistance to the cabbage root fly, Delia radicum, within Brassica fruticulosa

Two transgenic Bt rice lines, KMD1 and KMD2, both containing a synthetic cry1Ab gene from Bt, were crossed with conventional rice varieties. The inheritance of resistance to SSB of KMD1 and KMD2was investigated through LSB and field examination of their progenies, e.g. F₁, BC₁ and F₂ populations. In LSBs, 100.0% of newly hatched SSB larvae died on the second day after feeding on leaf tissues of F₁ and GUS positive BC₁ plants, of which the area of leaf tissues consumed by SSB is also similar to that of transgenic parents. These results imply that the resistance of Bt rice to SSB is dominantly controlled and could be easily exploited in hybrid rice production. Field evaluation showed that segregation ratios for SSB resistance to susceptibility in BC₁ populations fit the ratio of 1:1, which was also confirmed by LSBs. However, in F₂ populations, the ratio was significantly smaller than 3:1 for resistant to susceptible plants in all 6 indica × japonica (KMD1 and KMD2) crosses, though it fitted 3:1 in all 4 japonica × japonica crosses. The results implied that the resistance of Bt rice to SSB was controlled by a dominant gene which was present in a homozygous condition in both KMD1 and KMD2, but the inheritance could be affected by other factors. Assays for Cry1Ab protein showed that, in most crosses, the content of Cry1Ab is significantly higher in leaves of GUS positive F₁, BC₁ and F₂ plants than that in transgenic Bt parent plants, which accounts for the high resistance observed in these plants to SSB. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hybrid performance and AFLP- based genetic similarity in faba bean

Successful prediction of heterosis and performance of F₁-hybrids from the genetic similarity of their parents based on molecular markers has been reported in several crops and can be very helpful in hybrid breeding. The relationship between genetic similarities based on amplified fragment length polymorphism (AFLP) of 18 European faba bean lines and their hybrid performance and heterosis was investigated. Parental lines, 62 F₁-hybrids and their F₂-progenies were evaluated in field trials in four environments in Germany for seed yield, 1,000-seed weight and plant height. Results clearly demonstrated a stable superiority of the hybrids over their inbred parents and elite check cultivars, and showed a marked and varying amount of heterosis. Parental seed yield and F₂-hybrid yield were promising as predictors for F₁-hybrids. AFLP analysis of the 18 inbred lines using 26 EcoRI/MseI primer combinations resulted in 1202 polymorphic fragments. Cluster analysis based on genetic similarity estimates unambiguously identified pedigree-related inbred lines. No clear separation of the 18 inbred lines into subgroups was detected. Correlation coefficients between genetic similarity estimates and either heterosis or F₁-hybrid performance were small and not useful. Also correlations between specific genetic similarity and specific combining ability were too small for all traits to be of predictive value. Results showed that AFLP-based genetic similarities are not useful to predict the performance of hybrids or heterosis within the elite European faba bean gene pool.     

Genetic Structure of Three Open-Pollinated Faba Bean Varieties (Vicia faba L.)*

To study the genetic structure of open-pollinated faba bean varieties, three sets of genotypes were generated from each of the varieties ‘Minica’, ‘Kristall’, and ‘Deiniol’: (1) inbred lines developed by single-seed descent in bee-proof isolation cages, (2) intravanetal and (3) intervarietal F₁-hybrids produced by controlled hand crossing. In 1989 and 1990, a total of 144 entries, including the three open-pollinated source varieties, were grown in single-row plots with two replications at two locations in Western Germany. Performance data of the inbred lines revealed relatively large genotypic variability within each variety, for all the characters studied, which was greater in ‘Deiniol’ and ‘Kristall’ than in ‘Minica’. An average yield heterosis of 20 %, 39 % and 57 %, respectively, was found for the intravarietal hybrids, whereas that of the intervarietal hybrids varied between 70 and 73 %. Compared to the mean yield of the inbred lines the superiority of the open-pollinated source variety was small and not significant in ‘Minica’ (6 %), but it was large and highly significant in ‘Kristall’ (22 %) and ‘Deiniol’ (37 %). The present investigation revealed in faba bean that similar performance levels of open-pollinated varieties may result from either high per se performance under inbreeding with low heterosis values or from lower per se performance but with high heterosis values.     

Selection for biomass yield in wheat

Summary Biomass (above ground plant parts) yield may be a useful selection trait for yield improvement in wheat (Triticum aestivum L.). This study was conducted to estimate realized heritability of biomass yield and to determine the response to selection for high and low biomass yield in 8 genetically diverse populations of spring wheat under two production systems. Selections were made among the F₃ lines. Progenies of the selected lines were evaluated in replicated field tests in the F₄ generation under high fertility and low fertility production systems at Rampur, Nepal, in 1991. Fertility level had a significant effect on biomass yield, grain yield, effective tiller number, number of kernels per spike, thousand kernel weight, and harvest index. Selection in the F₃ for high and low biomass yield was effective in identifying F₄ lines with high and low biomass yield, respectively. Biomass yield differences between high and low selection groups in the F₄ generation, expressed as percent of the mean of the low selection group and averaged over the eight populations, were 53.9 and 36.5% higher than the mean of the low selection group under the high and the low fertility production systems, respectively. The corresponding figures for grain yield were 48.8 and 34.9% under the high and the low production systems, respectively. Also, selection for high biomass yield resulted in higher effective tiller number, and number of kernels per spike, but lower harvest index. Realized heritability estimates for biomass yield were greater at high fertility (range 0.49 to 0.85) than at low fertility (range 0.22 to 0.44). Biomass yield showed positive genotypic correlations with grain yield, effective tiller number, and number of kernels per spike but a negative correlation with harvest index. The results indicated that selection for high biomass yield should bring about positive improvements in biomass yield, grain yield, effective tiller number, and number of kernels per spike. The correlation between F₃ and F₄ generations suggested that biomass yield in the F₃ generation was a good predictor of biomass yield and grain yield in the F₄ generation. Selection for biomass yield in wheat should be made under the standard production system to obtain a realistic response.     

Honeycomb selection for yield in early generations of spring wheat

Summary The effectiveness of the honeycomb selection method for yield in spring wheat (Triticum aestivum L.) was evaluated using progenies from two wheat crosses, Glenlea x NB131 and Glenlea x Era. Honeycomb selection was carried out in the F₂ and F₃ generations, grown at the University of Manitoba in the summers of 1980 and 1981, respectively. In both generations, divergent selection was made for both high and low yield. Plants selected in the F₃ generation were entered in an F₄ yield test in the summer of 1982. Results of the experiment showed that honeycomb selection for yield in the F₂ and F₃ generations was effective in identifying parents of high- and low-yielding lines. F₃ plants from highyielding F₂ selections gave higher yields than those from low-yielding F₂ selections by 11.5% and 13.0% for Glenlea x NB131 and Glenlea x Era crosses, respectively. The F₄ yield test showed that high yielding selections from both crosses significantly outyielded by 8.9% low yielding selections and by 14.4% the unselected composite lines. It is concluded that the honeycomb selection method can be used for early generation selection in spring wheat.     

Factors responsible for levels of male sterility in photoperiod-sensitive cytoplasmic male sterile (PCMS) wheat lines

A `two-line system' using photoperiod-sensitivecytoplasmic male sterility (PCMS) caused by Aegilops crassa cytoplasm has been proposed as a newmeans of producing hybrid wheat. The PCMS line ismaintained by self-pollination under short-dayconditions (14.5 h light period), and F<sub>1</sub> seedscan be produced by outcrossing of the PCMS line witha pollinator under long-day conditions (15 h lightperiod). As the levels of male sterility in PCMSlines under the long-day conditions is a crucialfactor in determining hybrid purity of the F<sub>1</sub>seeds, a study was conducted into the effect ofseeding rate on male sterility in PCMS lines. Threedifferent density levels were tested using analloplasmic line of Japanese wheat cultivar `Norin 26'which exhibits PCMS. Levels of male sterility of thePCMS line increased at sparse planting, because tiller(ear) number per plant increased at low seedingdensity and late-appearing ears tended to exhibithigher levels of male sterility than early-appearingears. On the other hand, male sterility levels of thePCMS lines depended on genotype, e.g., the PCMS`Fujimikomugi' was completely male sterile, whereasthe PCMS `Norin 26' showed partial male sterility. APCMS line showing complete male sterility, such as thePCMS `Fujimikomugi', should produce F<sub>1</sub> seeds withhigh purity. However, the PCMS `Fujimikomugi' showeda lower female fertility. For practical use, it isnecessary to produce PCMS lines having high malesterility with high female fertility under long-dayconditions.     

Transfer of high kernel weight and high protein from wild tetraploid wheat (Triticum turgidum dicoccoides) to bread wheat (T. aestivum) using homologous and homoeologous recombination

Summary Wheat pentaploids were produced by hybridizing a high kernel weight (1000 grain wt=56 g), high protein (25.4%) line of wild tetraploid wheat (Triticum turgidum dicoccoides) as male parent, with the three hexaploids (T. aestivum) — normal Chinese Spring and its two homoeologous pairing mutants, ph _1b and ph ₂. The pentaploids were crossed as female parents to the two commercial hexaploid cultivars Warigal and Barkaee and 42-chromosome stable plants selected from the F₁ of the pentaploid x hexaploid crosses.Mean protein content of certain F₃ lines from all six pentaploid x hexaploid crosses was significantly higher than Chinese Spring and the respective commercial hexaploid parent (p<0.005) indicating high protein had been transferred from the tetraploid to the hexaploid level.Kernel weight amongst certain F₃ lines of the three pentaploids x Barkaee was significantly (p<0.0005) higher than either Chinese Spring or Barkaee, indicating the transfer also of high kernel weight from the tetraploid to the hexaploid level. However kernel weight was not significantly increased over Warigal in any F₃ lines of its crosses with the three pentaploids.High levels of homoeologous chromosome pairing in the ph-mutant pentaploids, plus evidence for significant modification of the composition of high-molecular weight (HMW) glutenin subunits of grain protein in certain F₃ derivatives of the ph-mutant pentaploid x hexaploid, crosses indicates that the ph-mutant-derived lines may possess novel (intergenome) genetic recombination, at least for high protein, and possibly kernel weight.     

Floral morphology affects seed productivity through pollination efficiency in radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> L.)

To examine the effect of stigma position and size on seed productivity through pollination efficiency in radish, the numbers of self and cross pollen grains on the stigmas and the seed productivity under insect-pollination were compared among four bred lines. Lines with a small stigma or a high stigma relative to the anthers on long stamens tended to receive fewer self and total (self + cross) pollen grains but showed a higher cross-pollination percentage (ratio of cross pollen grains to total pollen grains on stigmas) than those with a large stigma or a lower stigma. Additionally, a higher cross-pollination percentage was associated with a higher outcrossing percentage. This result suggests that variations in the two stigma traits affect the outcrossing percentage via the cross-pollination percentage. Therefore, it should be possible to prevent loss of F₁ purity by selecting new parental lines with floral morphology that favors cross-pollination percentage.     

Out-crossing frequency and genetic analysis of hybrids between transgenic glufosinate herbicide-resistant rice and the weed, red rice

The potential of transferring herbicide resistance from transgenic rice (Oryza sativa L.) varieties to sexually compatible weeds is of paramount importance for development of effective weed control strategies. The objective of this research was to determine the genetic control and frequency of natural outcrossing between a transgenic, glufosinate-resistant rice line and a Louisiana biotype of red rice (Oryza sativa L.). Molecular and phenotypic data showed that outcrossing in field plots between a non-transgenic purple marker line and red rice did occur within one field season, but at a low rate of <1%. Similarly, molecular and phenotypic data demonstrated that glufosinate resistance was transferred from the transgenic line to the red rice biotype in the field within one year at a low frequency of 0.3%. Compared to parental lines, the transgenic-red rice hybrids were extremely late, tall, and never set seed during the normal field season. Genetic analyses in all F₂ populations showed glufosinate resistance behaved in a Mendelian fashion as a single, dominant gene. Presence of the bar gene for glufosinate resistance did not increase fitness or seed fecundity in hybrids or subsequent progeny. The genetic analyses and outcrossing results from this study suggest that an effective management program can be developed to prolong the usefulness of transgenic, glufosinate herbicide technology. This revised version was published online in July 2006 with corrections to the Cover Date.     

Protein yield of oats as determined by protein percentage and grain yield

Summary Relationships among the traits protein percentage, grain yield, and protein yield of oats were studied with F₂-derived lines in F₃ and F₄ from 27 matings obtained by crossing high-protein with high-yield oat lines. High-protein parents were (a) selections from an Avena sativa bulk, (b) selections from three-way matings in which an initial parent was A. sterilss, and (c) cultivars. High-yield parents were derived from backcross populations involving A. sterilis accessions as donor parents.Significnnt genetic variation existed among F₂-derived lines for grain and protein yield in all matings and for protein percentage in all but one mating.Protein percentage had a highly significant negative correlation with grain yield (r=–0.33^**) when pooled over all matings, but in five, these two traits were not correlated. Overall, protein percentage showed a small negative correlation with protein yield (r=–0.09^*), and protein and grain yields had a high positive association (r=0.98^**). F₂-derived lines with both high protein percentage and high grain yield were obtained.High transgressive segregates for protein percentage occurred in two matings, for grain yield in nine, and for protein yield in 14. Most high transgressive segregates for protein yield were high because of high grain yield only, but in four matings, lines were found where protein yield was increased by concurrent increases in both protein percentage and grain yield.Only a few specific parental combinations between high-protein and high-yield parents produced segregates in which increased protein percentage contributed materially to high-protein yields.Journal Paper No. J-11264 of the Iowa Agriculture and Home Economics Experiment Stn., Ames, Iowa 50011. Project 2447.     

Root pulling resistance in rice: Inheritance and association with drought tolerance

Summary Avoidance of drought stress is commonly associated with root system characteristics and root development. The inheritance of root pulling resistance in rice (Oryza sativa L.) was investigated and its relationship with visual field scores for drought tolerance was studied. Transgressive segregation for high root pulling resistance was observed in 3 crosses (high x high, low x high, and intermediate x intermediate). Both dominant and additive genes control the variation. F₁ superiority for high root pulling resistance was observed and could be exploited in an F₁ hybrid breeding program. F₂ distribution curves indicated that plants highly resistant to root pulling can be obtained not only from low x high and high x high crosses, but also from intermediate x intermediate crosses. Root pulling resistance in rice has a low heritability (39 to 47%). Thus, breeding for a high root pulling resistance may best be accomplished by selection based on line means rather than individual plant selection. Field screening showed significant differences in leaf water potential among random F₃ lines. F₃ lines with higher leaf water potential had better visual scores for drought tolerance. Visual drought tolerance scores were correlated with root pulling resistance. Plants with high root pulling resistance had the ability to maintain higher leaf water potentials under severe drought stress. The usefulness of the root pulling technique in selecting drought tolerant genotypes was confirmed.