MUNGBEAN VARIETAL IMPROVEMENT S. Shanmugasundaram

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Induced Mutations and Mutants
Many studies have been made of induced mutations in mungbean,
especially in India, Pakistan, and Bangladesh. The mutagenic agent used most
frequently has been 60 Co gamma radiation, but X-rays, neutrons, and chemical
mutagens such as EMS and DMSO have been used also. The mutagenic agents
cause varying kinds of undesirable mutations in addition to the desirable ones.
More point mutations and fewer chromosomal aberrations are obtained with EMS
or DMSO than with radiations. With gamma radiation, a dose of 30 to 40 KR 60 Co
radiation was found to be most effective in producing desirable mutants (D.P.
Singh et al., 1979), although the sensitivity dosage of the mutagenic agent may
vary with the genotype used (Murray and Newcombe, 1970; Rajput, 1973; Yahya
et al., 1975; Khan and Hashim, 1978; Rangaswamy et al., 1973; V.P. Singh and
Chaturvedi, 1981).
A list of induced mutations identified have been reviewed by Gupta (1996).
He also lists the mungbean mutants released officially as varieties in different
countries.
Mutants commonly reported have been characterized by the following
traits: shorter plants, earlier and synchronous maturity, more branches-per-plant,
more pod clusters, greater pod set, MYMV resistance, higher 100-seed weight,
and higher protein content (Santos, 1969; Bhatt et al., 1972; Dahiya, 1973, 1978;
Rajput, 1974; Tikoo and Jain, 1974; Prasad, 1976; Shakoor, 1977; Subramanian,
1980; V.P. Singh and Chaturvedi, 1981; Shaikh et al., 1982). Four mungbean
cultivars have been released from mutation breeding programs, 'Pant Mung 2',
'TAP-7', and 'Co4' from India and 'NIAB Mung 2B' from Pakistan
(Shanmugasundaram, 1988). The cultivar, 'Pant Moong 2', released in 1982 is
reported to be moderately resistant to MYMV, and 'TAP-7', also released in 1982,
is reported to be tolerant to powdery mildew and a leaf spot disease.
Cytology and Cytogenetics
Cytological and cytogenetic studies with mungbean have been relatively
few due to the small size of the mungbean chromosomes and the difficulty of
working with them. The diploid chromosome number of mungbean is 2n=-2x = 22
(Karpechenko, 1926; Kumar, 1945; Krishan and De, 1965). A pachytene
chromosome analysis disclosed that the chromosomes varied in length from 28.1
to 73.3 µ with two bivalents associated with the nucleolus at pachytene (Krishnan
and De, 1965). The nuclear chromosomes varied in length and the position of
secondary constrictions at pachytene and somatic metaphase stages also varied.
The karyotypes of three wild types had shorter somatic chromosomes than two
cultivated varieties (Shrivastava et al., 1973). By comparing the chromosome
complements of several related species, Bhatnagar et al. (1974) proposed the
following karyotype formula for mungbean: 4L S '+4M s '+3M "' where L = long (2.7
to 3.5 µ), M = medium (1.9 to 2.6 µ), sm = submedian, and m = median
centromere. An idiogram of prometaphase chromosomes was presented by Joseph
and Boukamp (1978). Since mungbean has long metacentric and submetacentric
chromosomes, it is considered to be more primitive than blackgram (De and