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a USDA-ARS, P.O. Box 1090, Stuttgart, AR 72160
b Sugar Research Station, P.O. Box 604, St. Gabriel, LA, 70776
c University of Arkansas, Rice Research and Extension Center, P.O. Box 351, Stuttgart, AR 72160
d USDA-ARS, 1509 Aggie Drive, Beaumont, TX 77713
* Corresponding author (jnrutger{at}spa.ars.usda.gov)
The ARS-USDA and the Arkansas Agricultural Experiment Station released six induced semidwarf mutants of rice (Oryza sativa L.), KBNT 4 (Reg. no. GP-80, PI 632276), KBNT 5 (Reg. no. GP-81, PI 632277), LGRU 12 (Reg. no. GP-82, PI 632278), LGRU 13 (Reg. no. GP-83, PI 632279), ADAR 10 (Reg. no. GP-84, PI 632280), and ORIN 172 (Reg. no. GP-85, PI 632281), in March 1999. The mutants were induced at Stuttgart, AR, in four tall Arkansas rice cultivars, Kaybonnet (KBNT) (Gravois et al., 1995), LaGrue (LGRU) (Moldenhauer et al., 1994), Adair (ADAR) (Gravois et al., 1994), and Orion (ORIN) (Moldenhauer et al., 1992), to obtain semidwarfism in adapted germplasm quickly. These mutants, which have height reductions from 10 to 26% of their tall parents, provide breeding sources of semidwarfism nonallelic to the worldwide semidwarfing gene sd1, in tropical japonica germplasm adapted to the southern USA and similar climatic areas. Such mutants provide alternative semidwarfing sources should genetic vulnerability problems arise from widespread use of sd1.
Approximately 4000 dry seeds of each parent cultivar were treated with 200 or 250 Gy of 
rays in 1993 or 1994. The M1 of the medium grain cultivar ORIN was grown in the 1993-1994 Puerto Rico winter nursery, while the M1 generations of the other three cultivars were grown at Stuttgart in 1994. Because of extremely poor growth in the 1993-1994 winter nursery, only 121 panicles were harvested from the 200 Gy and 60 panicles from the 250 Gy treatment of ORIN. In the three long grain cultivars, ADAR, LGRU, and KBNT, stand reductions in the 250 Gy-treated M1 generations were so severe that the treatment was abandoned. However, about 1000 random panicles were harvested from each 200 Gy treatment of these cultivars. Each M1 panicle was sown in an M2 row in the succeeding summer or winter nursery as appropriate. In the M2 generation semidwarf selections were made in rows segregating for more than one semidwarf plant per row. Usually about one-fourth of the plants in M2 rows that were selected had semidwarf plant height, which virtually assured that recessive mutants were being selected. The general criteria in selecting semidwarfs were to look for plants that were 10 to 25% shorter than their parent, but otherwise were normal in appearance.
Allelism tests to sd1, the worldwide semidwarf source, were conducted by crossing the M3 or M4 semidwarf generation mutants as females to ‘Calmochi-101’ (C101) (Carnahan et al., 1986), ‘S-101’ (S101) (Johnson et al., 1989), or ‘ED7’ (Rutger et al., 1982), three lines known to carry sd1 from ‘Calrose 76’ (CA76) (Rutger et al., 1977). Calrose 76 is itself an induced semidwarf mutant. Height of the three known sd1 sources was 80 to 90 cm, which was similar to or slightly taller than the new mutants. All three sd1 sources from Calrose 76 have pubescent leaves and hulls, while the new mutants and their parents carry the recessive gene for glabrous leaves and hulls. The F1 generations were grown in the greenhouse and checked for pubescence to assure true crosses, but height data were inconclusive under these conditions. The F2 and F3 generations were grown in the field. Allelism tests among the six mutants were not conducted.
In an eight-replication characterization test at Stuttgart in 1997, KBNT 4 was 76 cm tall, or 27 cm shorter and 3 d later than its parent. In the cross KBNT 4/KBNT, F2 plant heights in 1996 ranged from 50 to 125 cm, with a break in the bimodal distribution between 90 and 95 cm, resulting in a segregation of 147 tall:54 semidwarf, a satisfactory fit (0.50 < P < 0.75) to a 3:1 ratio indicating a recessive single gene in the mutant. In this same year, the cross KBNT 4/C101 was observed to have a continuous distribution for F2 plant height ranging from 60 to 130 cm, indicating inconclusive segregation data. In F3 progeny tests of this cross in 1997, using subjective scoring for height, it was deduced that the F2 phenotypes had been 115 tall:69 semidwarf:1 double dwarf, where the two semidwarf parental phenotypes could not be distinguished from one another, and where the double dwarfs were 10 to 20 cm shorter than either semidwarf parent. This was not a satisfactory fit (0.001 < P < 0.005) to a 9:6:1 ratio for nonallelism, but pooling the semidwarf plus double dwarf groups resulted in a 115 tall:70 pooled group, which was a satisfactory fit (0.10 < P < 0.25) to a 9 tall:7 pooled ratio. In the cross KBNT 4/S101, F2 distributions for height in 1996 again were continuous and thus inconclusive, but in F3 progeny tests conducted the following year it was deduced that the F2 plant phenotypes had been 106 tall:45 semidwarf:9 double dwarf. This was a marginal fit (0.025 < P < 0.05) to a 9:6:1 ratio for nonallelism. However, in a new F3 population of the cross KBNT 4/S101, tested in the summer 2002, it was deduced that F2 plant phenotypes had been 171 tall:114 semidwarf:19 double dwarf, a satisfactory fit (0.25 < P < 0.50) to a 9:6:1 ratio for nonallelism. The recessiveness of the new semidwarf in the cross to its parent, plus the segregations in the crosses to known sd1 sources, indicates that KBNT 4 carries a recessive semidwarfism gene nonallelic to sd1.
In a genotype x nitrogen (N) fertility test at Stuttgart in 1996, with N levels of 112, 168, 224, and 280 kg ha–1, KBNT 4 had similar yield potential as its parent at the two lower levels, but suffered a yield decline at the two higher levels while the parent remained stable. Neither the mutant nor the parent showed lodging, even at the highest N level. The lack of a positive response to N by the semidwarf was unexpected, as similar work in California with semidwarfs showed that semidwarfs were more yield-responsive to increased N than were tall cultivars (Brandon et al., 1981). Averaged over three trials conducted in Arkansas in 1996 and one trial at Beaumont, TX, in 1997, KBNT 4 yielded 6120 compared with 7020 kg ha–1 for its parent. Lodging, observed only in one trial, was 45% for both the mutant and its parent. In an inoculated versus noninoculated sheath blight test at Stuttgart and Pine Tree, AR, in 1997, inoculated plots of KBNT 4 showed 10% yield reduction because of disease as compared with 2% yield loss in its parent. The greater loss in yield appeared to be due to a greater proportion of the culm in the semidwarf being infected than in the parent.
In the 1997 characterization test, KBNT 5 was 79 cm tall, or 24 cm shorter, and 3 d later than its parent. In the cross KBNT 5/C101, subjective scoring of F2 plant segregation was 72 tall:47 semidwarf:10 double dwarf, a satisfactory fit (0.75 < P < 0.90) to a 9:6:1 ratio for nonallelism. Averaged over three tests conducted in Arkansas and Texas during 1996 and 1997, KBNT 5 yielded 6250 compared with 6790 kg ha–1 for its parent. Lodging, observed only in one test, was 45% for both the mutant and its parent. In the 1997 sheath blight inoculation test, KBNT 5, like KBNT 4, showed greater yield reduction (20%) than its parent (2%) because of disease.
In the 1997 characterization test, LGRU 12 was 82 cm tall, or 23 cm shorter, and 4 d later than its parent. In the cross LGRU 12/C101, subjective scoring of F2 plant segregation was 87 tall:56 semidwarf:3 double dwarf, a satisfactory fit (0.10 < P < 0.25) to a 9:6:1 ratio for nonallelism. Averaged over four tests in Arkansas, LGRU 12 yielded 8230 compared with 8090 kg ha–1 for its parent. Lodging, observed only in one test, was zero compared with 45% for its parent.
In the 1997 characterization test, LGRU 13 was 83 cm tall, or 22 cm shorter, and 3 d earlier than its parent. In the cross LGRU 13/C101, subjective scoring of F2 plant segregation was 79 tall:45 semidwarf:4 double dwarf, a satisfactory fit (0.10 < P < 0.25) to a 9:6:1 ratio for nonallelism. Averaged over five tests in Arkansas and Texas in 1996 and 1997, LGRU 13 yielded 8470 compared with 8360 kg ha–1 for its parent. In the 1997 sheath blight inoculation test, LGRU 13 showed a 9% yield reduction compared with a 2% increase in yield for its parent.
In the 1997 characterization test ADAR 10 was 88 cm tall, or 16 cm shorter, and 1 d earlier than its parent. In the cross ADAR 10/C101, subjective scoring of F2 plant segregation was 65 tall:48 semidwarf:2 double dwarf, a satisfactory fit (0.10 < P < 0.25) to a 9:6:1 ratio for nonallelism. In the 1996 two-location SIT 2, severe lodging, 50% for ADAR 10 and 80% for its parent, resulted in a yield of 7540 kg ha–1 for the mutant compared with 3770 kg ha–1 for its parent. In the 1997 two-location SIT 2, ADAR 10 showed zero lodging compared with 40% for its parent, and 8500 versus 7600 kg ha–1 for the parent. Thus weak straw characterizes both the mutant and its parent and should be considered when using this germplasm for breeding purposes.
In the 1997 characterization test ORIN 172 was 83 cm tall, or 9 cm shorter, and 1 d later than its parent. In the cross ORIN 172/ORIN, there was a continuous distribution of F2 plant segregation from 60 to 105 cm, so segregation data were inconclusive. In 1997 F3 progeny tests, lines segregated in the ratio 41 tall:55 segregating (tall and semidwarf):30 semidwarf, a satisfactory fit (0.10 < P < 0.25) to a 1:2:1 ratio for a recessive single gene in the mutant. In the cross ORIN 172/ED7, F2 tests were again inconclusive, but in 1997 F3 progeny tests it was deduced that the F2 plant phenotypes had been 105 tall:78 single dwarf:4 double dwarf, a satisfactory fit (0.05 < P < 0.10) to a 9:6:1 ratio for nonallelism. Similarly, in the cross ORIN 172/S101, F2 tests were inconclusive, but from 1997 F3 progeny tests it was deduced that F2 plant phenotypes had been 75 tall:57 single dwarf:3 double dwarf, a satisfactory fit (0.10 < P < 0.25) to a 9:6:1 ratio for nonallelism. In the genotype x N test at Stuttgart in 1996, ORIN 172 showed a similar pattern of yield decline at the highest N level as its parent, but as with the KBNT 4 and KBNT entries, no lodging was observed with ORIN 172 and ORIN. Averaged over six tests in Arkansas and Texas in 1996 and 1997, ORIN 172 yielded 7710 compared with 7980 kg ha–1 for its parent. Lodging, observed only in one test, was 43% in both the mutant and its parent. In the 1997 sheath blight test, ORIN 172 showed 8% yield reduction compared with 4% reduction for its parent.
Apparent amylose contents of the KBNT, LGRU, and ADAR mutants were similar to those of the parent cultivars (210–230 g kg–1), and amylose content of the ORIN mutant was similar to that of its parent (130–150 g kg–1). Grain dimensions of the mutants were similar to their respective parents. In general, each mutant was similar to its tall parent except for being shorter.
Germplasm amounts of seed (
5 g) of the above lines may be obtained by writing to J. Neil Rutger, Dale Bumpers National Rice Research Center, USDA-ARS, P.O. Box 1090, Stuttgart, Arkansas. Seed also will be placed in the National Small Grains Collection, USDA-ARS, 1691 South 2700 West, Aberdeen, ID 83210, where it is available for research purposes, including development and commercialization of new cultivars. If this germplasm contributes to the development of new cultivars it is requested that appropriate recognition be given to the source.
Accepted for publication June 30, 2003.
REFERENCES
This article has been cited by other articles:
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  J.N. Rutger, R.J. Bryant, and K.A.K. Moldenhauer Registration of Induced Semidwarf Rice Mutant DR1 Crop Sci., September 8, 2006; 46(5): 2340 - 2340. [Full Text] [PDF]
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