Evaluation of Mutant Generations of Irradiated Soybean (Glycine max (L.) Merrill) in the Guinea Savannah Agroecology of Ghana

MATERIALS AND METHODS

Experimental site: The researches were conducted at the experimental field of the University for Development Studies (UDS) in the Faculty of Agriculture, Nyankpala campus in the Northern Region of Ghana. Nyankpala is located about 16 km west of Tamale Metropolis and lies on the latitude 09°24’15.9’’N and longitudes 01°00’12.1’W and an altitude of 183 M (msl) of the interior Guinea Savannah agro-ecological zone of Ghana. The studies were carried out during the cropping seasons of 2015, 2016 and 2017, respectively. The experimental site is characterized by tree and weed species such as Neem tree (Azadirachta indica), Shea tree (Vitellaria paradoxa), Dawadawa tree (Parkia biglobosa), Teak (Tectona grandis), Broom weed (Sida acuta), Speargrass (Imperata cylindrica L. ), Pigweed (Boerhavia diffusa) Andropogon gayanus⁷. The soil in the area is Nyankpala series and is made up of mainly sand and loam. Rainfall in the experimental area starts in late April and reaches a peak in July-September, there is a sharp decline and little or no rain in November and December⁸. The area experiences total precipitation of about 1,100 mm per annum, with a range from about 800-1500 mm (Table 1). The experimental area has an average ambient temperature usually high all year round (about 28°C) but the harmattan months of December and January, are characterized by the minimum temperature that may fall to 13°C at night, while March and April, may experience 40°C in the early afternoon (Table 2). The experimental site also has an average Relative Humidity (RH) of 64 during May-December, (Table 3).

Land preparation, experimental design and crop husbandry: The field was ploughed and harrowed by a tractor and later manually levelled, ridged and demarcated according to the planting distance of 50×20 cm for soybean in the various cropping seasons. This was done with the aid of tape measure, lines and pegs. A plot size measuring 2×2 m, with 1.5 m space between blocks and 1 m space between plots and replication were considered. The trials were laid out using Randomized Complete Block Design (RCBD) with three replications. Soybean variety (Jenguma) were irradiated to generate M₁ seeds in 2015 using 150, 200, 250 and 300 Gy doses of gamma irradiation from Cobalt-60 source and planted. Some un-irradiated seeds (0 Gy) were also planted to serve as the control. The harvested M₁ seeds were planted to obtain M₂ seeds in 2016. Selected M₂ seeds were planted and evaluated for improved agro-morphological traits in 2017. Weeding was carried out manually on 2 weeks’ interval basis to control the growth of weeds.

Data collection and analysis: Data generated from 2015-2017 from the various generations (plant height, chlorophyll content, number of branches, percentage pod shattering, number of days to 50% flowering, number of days to maturity, number of seeds per pod, number of pods per plant, pod length, 100 seed weight and total grain yield) were subjected to combined analysis for variation in factorial experiments (factor 1 being mutant genotypes and factor 2 being mutant generations) in RCBD. Genstat (18 edition) statistical package was used in the analysis. Means were separated using the Least Significant Difference test (LSD) at a 5% probability level.

RESULTS

Number of secondary branches: Irradiation dose and generation effect as well as their interactions significantly (p<0.05), influenced the number of branches. The 150 Gy mutant lines recorded the highest number of branches, while the 250 Gy mutant lines recorded the lowest number of branches (Fig. 2a). The M₃ generation in general recorded the highest number of branches whilst plants from the M₁ generation recorded the lowest number of branches (Fig. 2b). However, plants irradiated with 150 Gy recorded the highest number of branches at M₂ whilst those treated with 200 Gy dose recorded their highest number of branches at M₃ (Fig. 2c).

Fig. 1(a-b):

Influence of (a) Irradiation dose and (b) Generations on plant height of soybean (Glycine max (L.) Merrill) mutants during field experimentation Error bars represent Mean±SEM

Fig. 2(a-c):

Influence of (a) Irradiation, (b) Generations and (c) Genotype×generation on the number of branches of soybean (Glycine max (L.) Merrill) mutants Error bars represent Mean±SEM

Days to 50% flowering: Treatments with gamma irradiation doses and generation effect as well as the interaction between the 2 factors significantly affected (p<0.05) number of days to 50% flowering. Plants from 150, 200 and 250 Gy recorded earliness in terms of flowering (Fig. 3a), whilst plants from the M₂ and M₃ showed the same number of days to 50% flowering (Fig. 3b). Plants from 150, 200 and 250 Gy had a similar number of days to 50% flowering at the M₂ and M₃ generations (Fig. 3c).

Number of days to maturity: The number of days to maturity followed a similar trend as the number of days to 50% flowering (Fig. 4a-c).

Pod shattering: Pod shattering was highly significant (p<0.001) among dosages and also single effect of generation and the interaction between the 2 factors had a significant (p<0.05) influence on pod shattering. The 0 Gy application dose recorded the highest number of pod-shattering, whilst the 250 Gy dose recorded the least number of pod-shattering (Fig. 5a). The M₁ generation recorded the highest number of pod-shattering per plant, whilst the M₂ generation recorded the least (Fig. 5b). The M₁ generation at 0 and 150 Gy recorded the highest pod shattering, followed by M₃ and M₂ at 0 Gy. Plants from the 200 Gy in all generations showed resistance to pod shattering (Fig. 5c).

Total grain and components of yield: Plants that were irradiated with 200 Gy of gamma-ray recorded relatively higher grain yield and components of yield especially at the M₂ generation (Table 4).

Fig. 3(a-c):

Influence of (a) Irradiation, (b) Generations and (c) genotype×generation on the number of days to 50% flowering of soybean (Glycine max (L.) Merrill) mutants Error bars represent Mean±SEM

Fig. 4(a-c):

Influence of (a) Irradiation, (b) Generations and (c) Genotype×generation on the number of days to maturity of soybean (Glycine max (L.) Merrill) mutants Error bars represent Mean±SEM

Fig. 5(a-c):

Influence of (a) Irradiation, (b) Generations and (c) Genotype x generation on pod-shattering of soybean (Glycine max (L.) Merrill) mutants Error bars represent Mean±SEM

DISCUSSION

There was significant variation among the irradiation doses, mutant generations and their interaction for grain yield and other agronomic traits measured. Gamma irradiation and selection significantly affected plant height. This agrees with the findings of the previous studies^9,10, who reported a reduction in height of basmati rice following gamma irradiation. The un-irradiated plants initially grew very slowly but later became the tallest, flowering lately and reaching maturity lately whilst plants irradiated with 150 and 200 Gy stopped growing tall and produced more branches. The biological effect of gamma-rays is based on the interaction with atoms or molecules in the cell¹¹. Induced mutation using gamma irradiation might have resulted in retardation of growth. The observation made here is similar to those researchers^12,13, who also observed the effect of high dose treatment on plant derivatives of M₁ mungbean (Vigna radiata (L.) Wilczek), where plant height decreased due to the treatment of gamma irradiation. Gamma irradiation treatment increased the average plant height compared with the unirradiated controls¹⁴. Many studies have shown that treatment with a higher dose of gamma rays was inhibitory, whereas lower exposures were sometimes stimulatory^15,16. This could be due to mutagenic induced chromosomal damage during cell division¹⁷. Gamma irradiation disturbs the synthesis of protein, hormone balance, leaf gas exchange, water exchange and enzyme activity¹⁸. However, previously¹⁹ reported an increased plant height in soybean due to gamma rays. A similar observation was made in the M₃ plants that were initially treated with a 300 Gy dose of gamma-ray because these plants outgrew the 200 and 250 Gy treated plants. In-plant production, reduction in height may be advantageous as short plants generally resist lodging and hence avoid yield losses that may be associated with it. A reduction in plant height has also been reported, following the application of 250 and 200 Gy in soybeans^20,21. The height difference was therefore more distinct in the M₃ and M₂ populations.

The number of days to (50%) flowering varied significantly among the generations of mutants of soybean. The difference was more distinct in 200 and 250 Gy mutants, which took about half the number of days taken by the 0 Gy. Zaka et al.²² had earlier reported a reduction in the number of days to flowering in pea (Pisum sativum) following irradiation. Early maturity was observed in the soybean lines especially mutants from 150, 200 and 250 Gy. The 0 Gy took almost the same number of days as 300 Gy to mature. Ranjitha et al.²³ also selected mutants from gamma-irradiated soybean that proved to be early maturing and dwarf stature. Pod shattering is the opening of pods along both the dorsal and ventral sutures of the soybean pod. It is essential for the propagation of wild plant species bearing seeds in pods but is a major cause of yield loss in legume and crucifer crops. According to Valkama et al.²⁴, the gene for the shattering-resistant genotype, pdh1, is defective in soybean, having a premature stop codon. In this study, tolerance to pod shattering varied significantly among genotypes and was high in plants treated with 200 and 250 Gy. Ionizing radiation affects cellular components, thus, potentially inducing physiological changes in plants. This may be the mechanism that prevents the pod from shattering. The number of pods per plant differed significantly among the treatments. The result in part contradicts the findings of Pasupuleti et al.²⁵, who reported that the number of pods per plant decreased in all the genotypes (of a black gram) as the dose of irradiation increased. But Rahman et al.²⁶ observed that the treatment with gamma rays stimulated most of the quantitative characters of Vigna unguiculata such as number of branches, number of clusters, number of pods and yield. Khan and Goyal²⁷, also reported more number of pods per plant in mung beans treated with gamma rays. Hundred seed weight did not differ significantly among the treatments. A study of M₁ of cowpea, groundnut and soybean using gamma-ray at the same dose reported no significant differences²⁸. Since hundred seeds weight did not significantly differ among the treatments, the number of pods that had a significant variation seemed to have caused the significant differences in the yields of soybean.

The number of pods is a good selection criterion for increasing yield in grain legumes. This finding corroborates with the observation made by Rahimi and Bahrani²⁹, who reported the highest grain yield increase in canola irradiated with 0.2 kGy of gamma rays. Similarly, they also reported a significant improvement of 1000 kernel weight and harvest index for 0.1 kGy gamma-ray treatments. The sizes of grains in the mutants were comparably high as compared to the parent and were selected for high yield. The M₂ generation recorded the highest yield compared to the other generations. The results of the M₃ generation showed that it is possible to effectively increase grain yields components at a dose of 200 and 250 Gy. Similar observations were also made in other plants like black gram, cowpea and sesame³⁰. Improvement of agronomic characteristics by using gamma radiation has also been reported in several studies. A study revealed a significant increase in chickpea grain yield using gamma irradiation at 0.6 kGy³¹. An increase has also been observed, in the number of pods per plant in all the varieties for gamma irradiation at 0.2 and 0.4 kGy doses³². However, Khan et al.³¹, reported a decrease in pod number at 0.4 kGy treatment and an increase at 50 kGy without a change in the number of seeds per pod. The selected seeds especially of 200 and 250 Gy mutant lines should be advanced for multi-locational trials to determine the suitability of the selected mutants in various agro-ecologies for release as varieties for production by farmers.

CONCLUSION

The study showed that plants treated with 200 and 250 Gy were found to be highly resistant to shattering than the un-irradiated (0 Gy/control), believed to be an improved variety against field shattering. Field performance (agronomic traits) of soybean plants treated with 200, 250, 150 and 300 Gy were on the average higher than the parental line (control/0 Gy). These mutant lines also had shorter stems as compared to the parent line, which has an advantage in terms of lodging during heavy rains and strong winds. Yield and yield components results for soybean were higher among the M₂ populations followed by M₃ while M₁ had the least. Individual plants with desired characteristics than the parental variety ‘Jenguma’ were selected for multi-locational trials in the next generation. The number of pods per plant, seed size and seed weight were particularly considered during selection.

SIGNIFICANCE STATEMENT

This study discovered that mutagenesis could help improve yield, growth parameters and pod-shattering resistance of soya beans. Through this study, the researcher had discovered that plants treated with 200 and 250 Gy were found to produce more grain yield, improved growth parameters and were highly resistant to pod-shattering than the un-irradiated (0 Gy/control). Therefore, farmers who use these mutant lines are likely to get a better grain yield. This will result in increased soybean productivity in the Guinea Savannah Agroecology of Ghana.

ACKNOWLEDGMENT

The authors of this study are most grateful to the Faculty of Agriculture of UDS for providing funds for land preparation before planting. Sincere thanks also go to all technicians of the Department of Agronomy, UDS for their help in data collection, weeding and other cultural practices.