Gamma-ray radiation and sodium azide ( NaN 3 ) mutagenic efficiency in rice

Rice (Oriza sativa L.) cultivar IAC-1246 seeds were treated with 10, 15, 20 and 30 Krads of gamma-rays and sodium azide (SA) concentrations of 0.5, 1.0 and 5.0 mM to study their efficiency in inducing chlorophyll mutations. Combined treatments incremented the damages when compared to single treatments, especially for seedling height and M1 panicles fertility. Treatments with SA showed a higher frequency of chlorophyll mutations than the gamma-ray treatments, both in single and combined treatments. The gamma-ray spectrum was different for the various types of mutations, whether in individual or combined treatments. The SA efficiency was higher than that of gamma rays at 1.0 and 5.0 mM concentrations and lower at 0.5 mM. On average, the additive effect of the mutagenic combinations was more evident than the frequencies of the mutations.

Artificial induction of mutations is done through the use of physical and/or chemical mutagens which enlarge the mutation frequency, when compared to the spontaneous occurence.However, for extensive use of these mutants in plant breeding, high production efficiency is essential.This means that the utility of any mutagen depends not only on its effectiveness (mutation factor / dose) but also on its efficiency.The effectiveness of a mutagen has no practical implications since radiations and chemical mutagens are relatively inexpensive.On the other hand, lower levels of mutagens efficiency can limit their uses.Mutagenic efficiency is the production of desirable changes which are free from associations with undesirable genetic alterations.This is generally measured by the proportion of the mutation frequency in relation to damages associated to mutagenic treatments such as: height reduction, chromosomes breakages, sterility, lethality, etc. (Konzak et al., 1965;Gaul et al., 1972).
The possibility of inducing new variability by mutagenic agents is, therefore, of great interest to genetic improvement.This interest becomes even greater when associated with the possibility of assessing qualitatively and visibly the increases in mutation frequency through chlorophyll mutations.
Mutagenic treatments with two different combined mutagens have ben proposed by many researchers as Ando (1970), to increase the effectiveness and efficiency of mutation induction.The question whether or not combined mutagens have additive effects on the mutation frequency, and not on the physiological damages that they originate, remains unwered.Recently, some combinations of mutagens have been tested in rice: gamma-rays combined with EMS (Rao, 1977), with HA, NMU and MMU (Rao and Rao, 1983); with SA (Reddi and Rao, 1988) and with EMS and DES (Kaul and Bhan, 1977).
This study aimed at assesing the effect of gammarays and SA in both single and combined treatments on the mutagenic efficiency and spectra of chlorophyll mutations in rice.
For the sodium azide treatment, seeds were treated with 250 ml of freshly prepared 0.5, 1.0 and 5.0 mM of SA (buffered at pH 3) solution for 8 hours with continuous shaking at room temperature (25 o C + 2).Immediately after the treatment, the seeds were washed thoroughly in running water to reduce the residual effect of the mutagen on the seed coat.Next , 10 Kr and 0.5 or 1.0 mM of SA; 15 Kr and 1.0 or 5.0 mM of SA; 20 Kr and 0.5 or 1.0 mM of SA; 30 Kr and 1.0 or 5.0 mM of SA were used.The procedure adopted was similar to that used for the individual treatments.
Treated seeds were planted in pots in a glasshouse, and 30-day-old seedlings were transplanted in the field in rows.Plants spacing was 15 cm within rows and 80 cm between rows.
Data on seedling growth injury and M 1 spike sterility was recorded on 40 and 50 plants selected at random from each treatment, respectively.M 2 generation was raised from M 1 spikes, in a glass-house, and frequency of chlorophyll deficiency mutation was scored seven days after germination, based on a M 1 panicle progenie and a M 2 population, for both treated and control populations.The number of mutants by 100 M 2 seedling was also calculated, and, according to Gaul (1960), this proportion has some advantages over other indexes.An average of 31,701 plants per treatment were evaluated.
The χ 2 test was used to test the homogeneity of the mutation spectra and the additive effect of the combined treatment.In the last case, the expected number was calculated using the sum of the proportional effect of a single treatment.
The frequency of chlorophyll mutations was calculated in the following proportions: By putting together the higher efficiency levels achieved by the considered mutation factor and the different damages/deficiencies, the SA was more efficient than the gamma-rays, on average, when Msp and Msd were used, and it did not differ greatly from Mtsd.As for the Msp and Msd, higher efficiency levels were reached for the L damages and lower for the I .As for the Mtsd, the highest efficiency level was reached by the L damage and the lowest by the S. The higher efficiency observed in lethality is important; however, its practical utility is limited since there is a not very expressive efficiency in sterility, as it happened in this study.
The spectrum and frequency of the chlorophyll mutations in all treatments are presented in Table 3, where the albino type of mutations is predominant in physical and chemical mutagen-treated populations, as well as in the combined treatments with rare exceptions.In the gamma-ray treatments, the frequency of viridis was the next largest frequency after the albino, whereas in chemical treatments and in the combined treatments, some exceptions were recorded.Inequalities in other types of mutations were not very pronounced.
The predominance of the albino type over viridis, in the spectrum that resulted from the treatment with physical and chemical mutagens, is not consistently shown in the literature.Ando (1970), found a larger albino rate using ethylene oxide (EO), Ethyleneimine (EI) and ethyl methane sulfonate (EMS), as well as gamma-rays.Similar results were reached by Miah and Awan (1971) with neutrons and gamma-rays and by Rao (1977) with EMS and gamma-rays.On the other hand, Kaul and Bhan (1977) found a higher albino rate in one of the two varieties studied; nevertheless, in a third variety, a larger proportion of viridis was found.Finally, Rao and Rao (1983) produced higher proportions of albinos by applying methyl methane sulfonate (MMS), N-nitrous-N-methyl urethane (NMU) and hidroxyl amine (HA).
Larger proportions of viridis in rice using AS, were reported by Bhan and Kaul (1976) and Awan et al. (1980);by Reddi and Reddi (1984) using MMS, dms and dES;by Reddi and Rao (1988) using SA in two of the three treated varieties, and by Reddi and Suneetha (1992) using EMS.

RESULTS AND DISCUSSION
Table 1 shows the treatment efficiency data.Single treatments tended to be more efficient with the increase in dosage or concentration, but they decreased after reaching a maximum level.Msp higher efficiency levels were reached with the combined treatment of 20 Krads of gamma-rays and 1,0 mM of SA.Higher efficiencies were obtained with SA on an M 1 spike basis and mutants on an M 2 seedling basis.The azide treatment was remarkably higher in producing chlorophyll mutations than the gamma-rays, both on single and combined treatments.
In general, the azide treatment recorded the highest percentage of chlorophyll mutations, followed by the combined treatment and finally by gamma radiation alone.However, when the frequency of mutants in the M 2 plants basis was considered, the combined treatment 15 Kr of gamma-rays + 5 mM of SA was slightly more effective than the best single treatment (5mM of SA).In average, the frequency of chlorophyll mutations was much higher in SA than in gamma-rays.
Data on efficiency by treatments are presented in Table 2. Single treatments tend to increase with the dosages of mutagens, however ,only after reaching a maximum drop/fall.Msp high efficiency levels were reached with 20 Krads to gammarays and 0.5 and 1.0 mM to SA.As for the Msd, the maximum efficiency level varied from 10 to 20 Krads, being more sensible to Msd/S and Msd/L concerning the gamma-rays.On the other hand, great concentrations of SA were necessary to reach maximum efficiency varying from 1.0 up to 5.0 mM.In the efficiencies concerning Mtsd, the gamma-rays reached maximum values with 15 to 30 Krads (being Mtsd/L the less sensitive) and SA with 1.0 and 5.0 mM.
However, absolute values for SA efficiency are generally far superior than those for gamma-rays.The combined treatments had intermediary efficiency levels in relation to the single treatments, with some exceptions; i.e. , they increased the efficiency level of the single treatment by gammarays and decreased the efficiency level of the single treatment by SA.

2001, Sociedade Brasileira de Melhoramento de Plantas
Crop Breeding and Applied Biotechnology, v. 1, n. 4, p. 339-346, 2001 In other cultures, the predominance of albinos also has conflicting results.Aasveit (1967) found, in barley, a combination of results which favored the development of albinos and viridis in the two types of mutagens in three experiments with physical and chemical mutagens.
The predominance of albinos over viridis in the spectra of chlorophyll mutations was also observed in combined treatments of gamma-rays and EMS or diethylsulpahte (dES) Kaul and Bhan (1977).However, Reddi and Rao (1988) found larger proportions of viridis in the Fujiminori and Jaya varieties with the combined treatment of gamma-rays and AS.
Differences were observed in the spectra of the combined treatments in relation to the respective single treatments (Table 4), but with an exception.Sixteen treatments (50 Krads of gamma-ray and 1.0 mM of SA) did not differ from the spectra in treatment 5 (30 Krad).This suggests that higher dosages of gamma-rays could equal the spectra of the combined treatments, but this happened only in one case out of the eight observed.
1/ Msp-Mutation frequency on spike basis; Msd-mutation frequency on seedling basis; Mtsd-Mutation frequency on mutant basis; I -T he seedling injury (seedling height reduction); L -T he M 1 maturity lethality and S -The reduction in M 1 seed fertility.Crop Breeding and Applied Biotechnology, v. 1, n. 4, p. 339-346, 2001 1/ means that the combined treatment originated more frequency of the albino or viridis type of mutations; 2/ means that the combined treatment originated less frequency of the albino or viridis type of mutations.
Table 5 -Homogeneity χ 2 test of spectrum of chlorophyll mutations (albina and viridis) of the combined treatment of gamma-rays and SA with the respective single treatment.
As the main spectra differences were due to the albinos and viridis homogeneity, a test was carried out to analyze the relative proportions of these types of mutants (Table 5).The spectra of albinos and viridis were different in the gamma-ray, SA and combined treatments.However, some interesting aspects were observed in the homogeneity test of each combined treatment regarding the respective single treatment.The combined treatment tended to increase the frequency of albinos in six cases and to decrease in seven cases; but as for the proportions of viridis, the combined treatments tended to decrease the frequency in eleven cases and to increase in three cases.This suggests that the combined treatment tends to diminish viridis frequency.

Table 2 -
Efficiency levels in single and combined treatments with gamma-rays and SA in rice.

Table 3 -
Frequency and spectrum of chlorophyll mutations in the treatment by gamma-rays and SA.1/ Treatments are described in Material and Methods.

Table 4 -
Homogeneity χ 2 test of spectrum of clorophyll mutations of combined treatment of gamma-rays and SA with the respective single treatment.