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II. Breeding Applications of Semidwarf Mutants

II. Breeding Applications of Semidwarf Mutants

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Early Watar ibunr






Smooth No.4


C 6 Smoorh







FIG.1. Ancestry of publicly developed short- and medium-grain rice cultivars in California. Semidwarf cultivars are in double boxes, tall

cultivars in single boxes. Introductions, breeding lines, and proprietary cultivars are underlined. Female parent is on the left, except for three

cases in which it is designated by 0 . The cross that produced S-201 was actually between a sister line of S6 and the induced mutant D51, a

mutant which has the same semidwarfing gene as Calrose 76.

Table I

Semidwarf Public Cultivars and Released Germplasm Lines Derived from Induced Mutation

or Cross-Breeding with Induced Mutants in California

Cultivar or

germplasm line





Calrose 76








Germplasm lines

CI 11033 (D66)

CI 11034 (D24)

CI 11035 (D38)

CI 11036 (DDI)

CI 11045 (S-6190-96)

CI 11046 (S-6190-110)

CI 11047 (S-8158-39)

CI 11048 (WC 1403-12)

CI 11049 (72/16439)

CI 11050 (S-6193-3)

'sd? # sd,




of release










y-Ray induced mutant of Calrose

Calrose 76/CS-M3

CS-M3/Calrose 76//D31

Calrose 76/CS-M3//M5

Calrose 76/CS-M3//S6

Calrose 76/CS-M3//M5


y-Ray induced mutant of Terso

Rutger et al. (1977)

Camahan et al. (1978a)

Rutger et al. (1979b)

Johnson et al. (1980)

Camahan et al. (1980)

Johnson et al. (1981)

Camahan et al. (1981a)

Camahan et a[. (1981b)











y-Ray induced mutant of Calrose

y-Ray induced mutant of Calrose

y-Ray induced mutant of Colusa

Calrose 76/CI 11033

y-Ray induced mutant of M5

y-Ray induced mutant of M5

y-Ray induced mutant of Maxwell

y-Ray induced mutant of WC 1403

y-Ray induced mutant of Calrose

y-Ray induced mutant of Tsuru Mai

Rutger et al.

Rutger ef al.

Rutger ef al.

Rutger et al.

Rutger er al.

Rutger et al.

Rutger ef al.

Rutger er al.

Rutger et al.

Rutger et al.












means that this genotype has a semidwarfing gene nonallelic to sd, but that its relationship to other semidwarfing loci is unknown.



Davis group investigated a breeding method, induced mutation, and took the lead

in developing two semidwarf cultivars, Calrose 76 and M-101, and several

germplasm lines (Rutger et al., 1979a, 1982a). The Biggs group has developed

numerous additional cultivars, both tall and semidwarf, as well as germplasm

lines (Carnahan ef al., 1982).

Height of the California semidwarfs is generally about 90 cm, compared to

120-130 cm for the previous tall cultivars. The semidwarfs yield more because

of their increased responsiveness to nitrogen fertilizer (Fig. 2). In addition to

showing greater lodging resistance at the higher nitrogen levels, the semidwarfs

show increased numbers of panicles per square meter (Dat et al., 1978).

Because rice yields in California were at a relatively high base level, semidwarf cultivars did not produce the dramatic doubling of yields that frequently

occurred in the Green Revolution cultivars in Asia. Nevertheless, the cumulative

evidence indicates that the semidwarfing gene increases rice yields 15%, and

when the semidwarf cultivars are used with intensified cultural practices, farm

yields increase about 25%. In 1981, approximately 95% of California’s 245,000

ha of rice were sown to semidwarfs [54% to cultivars carrying the Calrose 76induced mutant source and about 35% to the semidwarf cultivar M9, which

derives its semidwarfing gene from IR8 (Carnahan et al., 1978b)I. The combination of favorable growing conditions, intensive cultural practices, and high-




- 70001






Nitrogen (kg/ha)

FIG. 2. Response of the semidwarf cultivars Calrose 76 and M7 and the tall cultivar CS-M3to

nitrogen fertilization. Adapted from Brandon er al. (1981).



yielding semidwarf cultivars resulted in record California average yield of 8.07

metric tons/ha in 1981.

Genetic studies have shown that the induced mutant semidwarfing gene sd, is

allelic to the major semidwarfing gene in Deo-Geo-Woo-Gen (DGWG) and the

widely grown Green Revolution cultivars derived from DGWG (Foster and

Rutger, 1978; Mackill and Rutger, 1979). Because the three widely used indica

semidwarf cultivars DGWG, I-geo-tze, and TNl all carry the same recessive

semidwarfing gene (IRRI,1966), the simplified designation DGWG will be used

for the indica semidwarf gene source in this article. In F, generations of crosses

between sd, and the DGWG types, no truly tall recombinants have been recovered, although considerable variation exists in height of the F, semidwarfs

(Foster and Rutger, 1978; Mackill and Rutger, 1979). In crosses with tall

cultivars, the recessive sd, mutant gene shows rather discrete semidwarf versus

tall segregation. On the other hand, the DGWG semidwarfing source shows

more continuous variability, which is usually attributed to the presence of a

single major gene plus minor gene modifiers (Aquino and Jennings, 1966).

Thus, although it is possible to obtain a broad range of semidwarf heights in

crosses of tall lines with DGWG, a narrower range of semidwarf heights is

recovered in crosses of tall lines with the induced sd, source. The major gene

plus modifier nature of the DGWG source also would explain the variation in

semidwarf height that is seen in crosses between the induced mutant sd, and

DGWG sources. The sd, allele appears to be a recumng mutation, as it has been

induced independently in three California cultivars: Calrose, to produce Calrose

76 (Rutger, 1982a); Colusa, to produce a germplasm line (Rutger, 1982a); and

Terso, to produce M-401 (Carnahan et al., 1981b).

In addition to the 8 semidwarf cultivars which have been developed from use

of induced mutants, 10 semidwarf germplasm lines have been released from the

California programs (Table 1). Allelism tests have shown that at least three

independent, recessively inherited semidwarf genes were induced in the tall

cultivar Calrose: the sd, locus present in Calrose 76 and its six crossbred derivative cultivars, the sd, locus in CI 11033, and the sd, locus in CI 11034 (Foster

and Rutger, 1978; Mackill and Rutger, 1979). Typically, F,s among the nonallelic semidwarfs are tall, and 9 tall : 6 semidwarf : 1 doubledwarf ratios are

observed in the F,. However, neither the sd, nor the sd4 source has been as

agronomically useful as the sd, source. The sd, source reduces height only 15

cm and thus is still somewhat lodging susceptible at high fertility levels (sd,

reduces height about 30 cm, a more desirable reduction). The sd, source also

reduces height only 15 cm and has an additional pleiotropic effect for a 20%

reduction in seed size. The inheritance of height in three other induced semidwarfs of Calrose was investigated, but these three (D32, D23, and D25) were

not released because genetic studies showed that D32 was allelic to the Calrose

76 sd, source and to DGWG and that D23 and D25 were allelic to the CI 11034






FIG. 3. Allelic relationships of induced semidwarf mutants from Calrose and Deo-geo-woo-gen

(DGWG). Genotypes at the same comer of the triangle are allelic; those at different corners are

nonallelic. Adapted from Mackill and Rutger (1979).

sd, source (Fig. 3). Another induced semidwarf, CI 11035, from the cultivar

Colusa, also was found to be allelic to the sd, source. The released doubledwarf

carrying sd, sd, (CI 11036) and an unreleased doubledwarf carrying sd, + sd,

have been too short (about 65-75 cm) and not as productive agronomically as the

sd, semidwarf (Rutger, 1982b). The third doubledwarf, sd, + sd4, was created,

but as its height was about the same as the sd, semidwarf(85-95 cm), it was not


After the three independent sernidwarfiig alleles, sd,, sd,, and sd, were

identified, subsequent genetic studies concentrated only on determining if new

mutants were allelic to sd,. Two semidwarf mutants induced by Carnahan and

co-workers from the tall cultivar M5 (CI 11045 and CI 11046) were thus found to

be nonallelic to sd, (Table I). Both are about 30 cm shorter than their parent and,

except for a tendency to show discolored hulls at harvest, both are phenotypically

identical to the sd, source (Rutger et al., 1982a). Although neither of the M5

“raw” semidwarfs has been more productive than its tall parent (and thus by

inference each is less productive than the sd, source), neither has been evaluated

for yield potential after crossing to other genotypes.

Gale el al. (1982) have stressed the importance of influence of background

genotypes on newly induced mutants. They showed data on a wheat case in

which the plot yield of an EMS-induced semidwarf mutant was increased from

14 to 38% of its tall parent by backcrossing to its parent. This “cleaning up”

process apparently helped eliminate other mutated (deleterious) genes in the

original line. The cleaning up process probably would not have such dramatic

beneficial effects on the M5 semidwarfs, as they yielded 95% of their tall parent

(Rutger et al., 1982a), but some increases might result from putting these genes



39 1

in different backgrounds. Should the sd, source ever prove genetically vulnerable, the M5 semidwarfs will certainly receive greater attention.

An induced semidwarf mutant, CI 11047, was also selected from the tall,

early-maturing cultivar Maxwell (Table I). The height of CI 11047 is similar to

the sd, sources, but its inheritance has not been studied. Yield was similar to the

tall check cultivar Earlirose, but both the semidwarf mutant and the tall check

showed considerable lodging (Rutger et al., 1982a).

Carnahan and co-workers also induced a semidwarf mutant, CI 11048 (Table

I), in the tall, weak-strawed, rice water weevil-tolerant line, WC-1403. The

mutant is about 110 cm tall, which is about 20 cm shorter than its parent (Rutger

et al., 1982a), and retains its parental level of water-weevil tolerance (Johnson

and Carnahan, 1982). Inheritance of its short stature has not been studied.

Narrow-leaf semidwarf mutants have been induced in two different cultivars

(Table I). These two mutants, CI 11049 and CI 11050, have been of interest

because they produce more than three-fourths of normal semidwarf yields with

only half as much leaf blade area (Rutger et al., 1982a; Lafitte, 1982). Thus,

these lines apparently trap sunlight energy more efficiently. It is postulated that

their reduced yield may result from being too short, as the narrow-leaf semidwarfs are 10-15 cm shorter than sd, lines. These narrow-leaf mutants may be of

interest in physiological studies on drought tolerance, because many ecotypes of

grasses from semiarid and low-rainfall areas have narrower leaves than those

from humid and higher rainfall areas. Similarly, it would be useful to determine

whether such mutants are beneficial in reducing respiration losses in rice-growing climates where high night temperatures prevail. One narrow-leaf semidwarf

mutant, CI 11049, was shown to be nonallelic to the sd, source; inheritance of

height in CI 11050 has not been studied. The semidwarfhg gene in CI 11049 has

a pleiotropic effect for short, narrow leaves and reduced seed size. Attempts are

being made to obtain narrow-leaf recombinants with 85-95 cm height that might

raise yields to a competitive level.

Another semidwarf mutant selected in California, “Short Labelle,” represents

a semidwarf in a long-grain background. Short Labelle equalled the yield of its

tall parent in one test in Arkansas (McKenzie et al., 1982), but when averaged

over four tests it was noticeably lower yielding than Labelle. Attempts to “clean

up” Short Labelle by hybridizing it with its parent and other adapted long grain

cultivars are also being made.

Three reasons that induced mutation has been so useful in California are:

1. The objectives were clearly specified and limited to characters under simple genetic control, primarily semidwarfism, early maturity, and waxy


2. Once found, the desired characters were quickly incorporated into standard



hybridization programs, frequently in a stepwise fashion (Rutger and Peterson, 1976).

3. The rate of incorporation was speeded by the close relationships of the

California cultivars. Except for some Japanese and Chinese cultivars, most

introduced lines are not well adapted to the long days and cool nights of

California’s climate. Thus, it has been easier in breeding programs to use

induced mutants than introduced sources, although both have been



An outstanding example of a useful semidwarf mutant in Japan has been the

cultivar Reimei, which was induced by y irradiation of seeds of the tall cultivar

Fujiminori (Futsuhara, 1968). The 15-cm height reduction of Remei was controlled by a single nondominant gene. Reimei has been used extensively in

crossbreeding. Sat0 (1980) reported that Reimei has contributed short and stiff

culm to 5 cultivars through crossbreeding, that it was a parent in the pedigree of

40 “candidate” cultivars, and that it is in the pedigree of 19 additional candidate

strains. Another short culm mutant, Fukei 71, with a 30-cm height reduction,

also was induced from Fujiminori (Futsuhara, 1968). Fukei 71 is in the ancestry

of 2 additional cultivars in Japan (Sato, 1980).

Short-culm mutants also were induced in the popular cultivar Koshihikari

(Samoto and Kanai, 1975). Koshihikari mutants have been used to breed 2

cultivars (Sato, 1980). Kawai (1982) listed 2 more cultivars that had been bred in

Japan by using induced mutants in crossbreeding, making a total of at least 11

cultivars developed in this fashion. In addition to Reimei, two other mutants

have been released directly as improved cultivars in Japan: Miyuki-Mochi, a

waxy mutant, and Miyamanishiku, a large-grain mutant (Toda, 1979).

Ikehashi and Kikuchi (1982) studied the allelism of Reimei and the DGWG

source and suggested that the semidwarfism gene in Reimei is the same as the

one from DGWG. They also studied the allelism of the native semidwarf Japanese cultivar Jikkoku and DGWG, after making four backcrosses of the respective gene sources into the cultivar Norin 29 to minimize confusing background effects from the usual wide segregation occurring in indica-japonica

hybrids. Jikkoku has been used widely in southwest Japan as a semidwarf donor.

From crosses between the two semidwarf sources, they concluded that DGWG

and Jikkoku have an identical semidwarf gene.

Okuno and Kawai (1977) studied many short-culm mutants in the rice

cultivars Norin 8 and Norin 22. Undesirable changes in other agronomic characters were often associated with the short mutants. Induced mutants could be



classified into three types; “upper shortening”, “lower shortening”, and “normal” internodes. Genetic studies of six short-culm mutants showed that culm

length was controlled by single recessive genes at different loci, although the

possibility of simultaneously induced mutant genes at closely linked loci could

not be completely eliminated (Okuno and Kawai, 1977). They noted that the

most desirable mutants would be those that showed minimal undesirable changes

in other characters.

Yamaguchi et al. (1981) described an erect, narrow, thick-leaf dwarf rice

mutant induced in the cultivar Hatsunishiki. Narrow leaf was controlled by a

single recessive gene. A semidwarf recombinant, G31, derived by crossing the

dwarf mutant to another cultivar, also had erect, narrow, thick leaves. G31 had

net photosynthetic and transpiration rates per unit leaf area about 1.5 times that

of the best check cultivar. In dense broadcast plantings, G31 yielded more rhan

did two check cultivars. The yield increase was attributed to less mutual shading

of leaves. The leaf characteristics of the mutant described by Yamaguichi et al.

(1981) appear to be similar to the two narrow leaf mutants (C1 11049 and CI

11050) found in California (see p. 391, this article; Rutger et al., 1982a).


Numerous other induced dwarf and semidwarf mutants of rice have been

reported in the literature (Hajra et al., 1980). Notable examples among indica

cultivars include the mutants Shuang-chiang 30-21, Keh-tze 20-74, and I-kungbau 4-2 in Taiwan (Hu, 1973); a short stature mutant from H4 in Sri Lanka

(Gunawardena et al., 1971); and the semidwarf mutant Jagannath in India

(Mohanty and Das, 1979). Examples among japonica cultivars include the semidwarf mutant Milyang 10 in Korea, which was reported to be controlled by a

single recessive gene (Ree,1973); two semidwarfing mutants from the cultivars

Chianung 242 and Tainan 5 in Taiwan, which were reported to be nonallelic to

each other (Woo et al., 1974); and six short mutants from the cultivar Tainung 61

in Taiwan (Buu and Huang, 1975). In the first two reports on japonica mutants,

no mention was made of allelism tests to DGWG, but in the last report one

mutant was found to be allelic to the semidwarf gene in TN1.

Until recently, most workers have concentrated on making direct releases of

the mutants and thus failed to realize the major benefits of mutants as donor

parents in crossbreeding programs. Also, the DGWG semidwarf source was

coming into widespread use in breeding programs, and because the gene most

frequently induced was the same as in DGWG, many breeders thought that

induced mutants would have minimal benefits. However, as the role of induced

mutation in supplementing natural genetic variability became better understood



(Nilan et al., 1977), several researchers have pursued programs which show

promise either for inducing nonallelic semidwarfs or for inducing mutants in

specialized cultivars.

Thus, Reddy and Padma (1976) induced several semidwarf and dwarf mutants

in the tall cultivar Tellakattera in India. Four dwarf and one sernidwarf mutants

were nonallelic to each other and to the DGWG semidwarf source (Padma and

Reddy, 1977).

Jacquot (1978) reported that short-strawed mutants were induced by irradiation of the line 63-83 (IRAT2) from Senegal. The mutants had increased lodging

resistance and retained other desirable attributes for upland rice. Three cultivars

resulted from irradiation of 63-83: IRAT 13, IRAT 78, and IRAT 79, IRAT 13

has been widely grown in West Africa. It is of medium height, 110 cm, and

carries a recessive gene for reduced height, although it is not considered a dwarf

(Jacquot, 1978). It has also been used as a parent in breeding programs. Another

mutant from 63-83, mutant 312A, has a recessive gene for semidwarfism that is

different from the gene in TNl.

Saini et al. (1977) induced a semidwarf mutant, PAU Mutant Basmati 370, in

the tall cultivar Basmati 370 and reported that it yielded 58% more than its parent

in India. Awan et al. (1982) induced several semidwarf mutants in Basmati 370

in Pakistan and found considerable yield advantages for the mutants. The

Basmati rice case appears to be a classic situation in which semidwarf mutants

would be useful: Basmati rices are characterized by a scent, and fine grains that

elongate greatly during cooking; thus they are highly valued by rice consumers in

several Middle East countries. Unfortunately, the original Basmati 370 cultivar

grows very tall (150-180 cm), and is very susceptible to lodging, hence low

yielding. High-yielding semidwarf Basmati cultivars developed by crossing to

DGWG or its derivatives apparently have had limited acceptance. Yield has

definitely been increased, but some consumers report that the odor and cooking

characteristics do not match the original distinctive Basmati type. Thus, it would

seem that the Basmati rices would be an ideal situation for inducing semidwarf

mutants; this should provide a means of changing a single character (height)

without disrupting the distinctive complex of Basmati adaptation and quality

characters. However, it was reported in the Pakistan study that even the induced

Basmati semidwarf mutants do not have quality characters equivalent to the

original parent (Awan et al., 1982). Backcrosses to the tall parent are being made

in attempts to “clean up” the quality deficiencies in the semidwarf mutants.

Mahadevappa et al. (1981) undertook studies to induce early maturing, shortstature mutants in eight cultivars adapted to tidal swamps in Indonesia, because

the introduction of modern semidwarf cultivars had not been successful in areas

with tidal swamps and adverse soil conditions. Mahadevappa et al. (1981) used

the mutagen ethyleneimine in attempts to improve these native cultivars and the

mutants thus induced are being tested in the tidal swamps.



Similarly, researchers at CIAT in Colombia, noting that broad-spectrum resistance to blast had been found only in tall, lodging-susceptible rice lines,

decided to induce dwarf mutants in these lines. It was anticipated that the dwarf

mutants would be more suitable as recurrent donor parents in crossbreeding

(CIAT, 1980). Several mutants were induced and now are under evaluation

(CIAT, 1981).

Buddenhagen embarked on a similar induced mutation project at IITA in

Nigeria in order to obtain shorter versions of local land races that have horizontal

resistance to blast and adaptation to local problems. Several semidwarf mutants

have been induced and are being used in breeding (I. W. Buddenhagen, personal

communication, 1982).


The cumulative evidence indicates that the major semidwarfing gene at the

locus of the DGWG semidwarfing gene is a recurring mutant. First, Hu (1973)

noted that DGWG itself may be a spontaneous semidwarf mutant of the tall

cultivar Woo-Gen (Hu reported that Deo-Geo-Woo-Gen in Chinese means

“short leg” Woo-Gen). Second, Hu (1973) reported that the semidwarf gene of

induced mutants of indica cultivars in Taiwan were at the same locus as the

DGWG source. Third, Singh et al. (1979) studied 12 representative spontaneous

and induced dwarf lines and found that 11 were allelic to the DGWG source,

which further indicates that this locus is subject to recurring mutation. Fourth,

semidwarfing mutants at the sd, locus (and by direct or indirect tests, at the

DGWG semidwarf locus) have been induced independently in three separate

cultivars in California: Calrose, Colusa, and Terso (Rutger, 1982b). Fifth, both

the dwarfing genes of the induced mutant Reimei and the native semidwarf

Jikkoku in Japan seem to be allelic to the DGWG semidwarf gene (Ikehashi and

Kikuchi, 1982). Sixth, IRRI (1980) reported that about 70 semidwarf sources

have been tested against the sd, gene from DGWG. More than 40 sources were

nonallelic to the sd, gene; others were identical or belonged to the same compound locus. Interestingly, in the IRRI study Reimei was reported to have a

nonallelic gene for short stature.

The importance of the DGWG semidwarf locus in world rice production is

well known. Hargrove ef al. (1979) noted that all named cultivars from IRRI

(except IR5) and virtually all semidwarfs developed in national rice breeding

programs in Asia derive their semidwarfing gene from DGWG. Widespread

usage of a single semidwarfing gene increases the potential genetic vulnerability

of a crop, although no evidence has yet accumulated of associated detrimental

effects of the DGWG source. However, it would be desirable to have backup

semidwarfing genes that are nonallelic to the DGWG source, preferably on a



different chromosome or at least at a locus not closely linked to the DGWG

locus. The DGWG semidwarfing gene is in linkage group three (Suh and Heu,


In California, none of the induced semidwarf sources nonallelic to sd, (and, by

inference, to the DGWG source) has become important in breeding. Whether this

results from undesirable associated or pleiotropic effects of the nonallelic sources

or only from lack of usage in breeding programs is not clear at the present time.

In Japan, the two nonallelic (to DGWG) cultivars Hokuriku 100 and Kochihibiki

were noted for their high yield (Ikehashi and Kikuchi, 1982), thus useful nonallelic semidwarfs apparently can be found.

The general usefulness of the sd, locus, as opposed to other semidwarfing

loci, sometimes suggests that there is some basis, as yet unknown, for productivity with this particular locus. Following reports by wheat researchers that the

semidwarfness of the widely used Norin 10 genes in wheat are invariably associated with nonresponsiveness to gibberellic acid (GA,) (Gale and Gregory, 1977),

while the less used induced semidwarf mutants in wheat are responsive to GA,,

rice workers have been interested in possible analogies in their crop. The DGWG

semidwarf locus in indica backgrounds appears to be GA,-responsive (Harada

and Vergara, 1971). The induced mutant sd, in japonica backgrounds appear to

be no more GA,-responsive than are tall japonica cultivars, but this may be

compounded by the fact that the tall japonicas themselves are relatively GA,responsive (T. R. LaVelle and J. N. Rutger, unpublished). Further studies are

needed to clarify whether GA, response in rice semidwarfs has any economic




Because early maturing mutants are among the types of mutants most easily

identified (Micke, 1979), it is not surprising that many have been found in rice.

Good examples are the two widely grown rice cultivars in California which were

direct releases of spontaneous mutations for early maturity. The first, Earlirose,

was a proprietary cultivar which originated as an early maturing selection from

Calrose (Davis, 1965). Earlirose is 7-25 days earlier than Calrose but is similar

in height and other characteristics. In the early 1970s, Earlirose was grown on

15-20% of the California rice area. The second cultivar, M5,originated from

two early maturing, pure-line mutations from CS-M3 (Carnahan ef al., 1975)

(Table II). M5 is 10-12 days earlier than CS-M3 but is similar to its parent in

height, yield, and other characteristics. In the late 1970% M5 was grown on as

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II. Breeding Applications of Semidwarf Mutants

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