Effects of gamma irradiation on submergence tolerance of two selected varieties of lowland rice (Oryza sativa L.)

The study was carried out to assess the effect of different doses of gamma irradiation on agro-morphological traits under simulated flooding of two rice varieties (FARO 44 and FARO 60) in order to identify the most effective radiation dose in creation of genetic variability for submergence tolerance. Seeds of FARO 44 and FARO 60 cultivars were collected from National Cereal Research Institute (NCRI) Badeggi, Nigeria and exposed to different doses of gamma irradiation (0, 50,100, 150 and 200 Gy) at Center for Energy and Research Training, Zaria, Nigeria. The treated and the controls seeds were evaluated for submergence in a Randomized Complete Block Design with four replicate each. Each experimental bucket was flooded for a period of 10 days and was allowed to grow till harvest after submergence period. From the results, gamma irradiation had significant (P≤0.05) positive effect on survival percentage of FARO 44 after submergence with 150 Gy having the highest (93.75%) compared to the control (56.25%). The irradiation doses (150 and 200 Gy) had significant positive effects (p≤0.05) on the plant height and yield of FARO 44. In Faro 60, low doses of gamma irradiation (100 Gy) had significant (p≤0.05) effects on the days to 50% flowering and weight of 100 grains (g). It was observed that gamma radiation doses of 150 Gy and 200 Gy had positive effects on submergence tolerance indices of FARO 44 used in this research. Further research therefore should be carried out on submergence tolerance of the promising mutants.


Introduction
Rice is one of the most important food crop in the world, consumed by nearly 3 billion people daily.Rainfed lowland and deep water rice cultivars are cultivated on approximately 33% of global rice farmlands [1].This accounts for about 50 million hectares of the estimated 150 million hectares of rice fields under cultivation worldwide [2].More than 16 million hectare of rice lands of the world in lowland and deep-water rice areas are unfavorably affected by flooding.Submergence caused by flash flood is a key factor limiting the yield of lowland rice.In Nigeria, approximately 70% rainfed lowland rice farms are prone to seasonal flooding, this results in total loss of the crop.These losses affect rice farmers in rained and flood-affected areas where alternative livelihoods are limited.Therefore, the incidences and severity of poverty in these areas are high [1].During any given year, yield losses in Nigeria resulting from flooding may range from 10 percent to total destruction [3].
Submergence tolerance is defined as the ability of a rice plant to survive and continue growing after being completely submerged in water for several days [1].Recently, the extent of submergence stress has increased due to extreme weather events such as unpredicted heavy rains that have flooded wider areas across many states of the country.Among the most frequently and severely affected states in Nigeria are Kebbi, Niger, Kogi and Taraba states which together account for over 80% of lowland rice ecology in Nigeria.The experts say the situation may become worst as climate change progresses.Flash floods are highly unpredictable and can occur at any growth stage of the rice crop, resulting in yield loss of 10% to 100%, depending on water depth, duration of submergence, temperature, turbidity of water, light intensity, and age of the crop [4].Flooding is a serious constraint to rice plant growth and survival in rainfed lowland and deepwater areas.This is because it results in partial or complete submergence of the plant [3].Gamma (γ) rays are physical mutagens; gamma irradiation has proven to be a useful method for introducing new trait variations that may result in crop improvement and can be used as a complementary tool in plant breeding [5].Induced rice mutants have been useful research tools for genetic and physiological assessments of yield-limiting factors in rice.Mutants have made it possible to identify critical elements for developing high yield potential varieties exhibiting desirable traits such as semi-dwarfism, early maturity, a greater number of panicles/plant and increased fertility.By 2003, 440 mutant rice varieties had been developed.Of these, 264 were produced by the direct application of mutagens and 176 were created by cross-breeding with induced mutants [6].
Submergence tolerance has long been regarded as an important breeding objective for rain-fed lowland and deep water rice areas [7].Despite this recognition, there has been limited success in developing improved submergence tolerance in Africa particularly Nigeria.Keeping in view of this problem, the importance of improving submergence tolerance of some Nigerian cultivars cannot be over emphasized in the present rice improvement in Nigeria.The most relevant intervention to reduce vulnerability of lowland rice farms to submergence is to develop varieties that are submergence tolerance.Thus this research is design to evaluate for submergence tolerance of Faro 44 and Faro 60 under the effect of gamma irradiation.

Seed collection and treatment
The genetic material (FARO 44 & FARO 60) used for the study were collected from National Cereal Research Institute (NCRI) Badeggi, Nigeria and irradiated at Centre for Energy and Research Training Ahmadu Bello University Zaria, Nigeria.A total of five groups each of equal gram of seeds for FARO 44 and FARO 60 were irradiated with different doses (0, 50, 100, 150 and 200 Gy) of gamma ray using cesium -137 source.The irradiated seeds were then raised in 10 liters experimental pots filled with sandy-loamy sand to 4 liters mark.Each treatment was replicated four times and arranged in Randomized Complete Block Design (RCBD) in the Botanical Garden of the Department of Plant Biology, Federal University of Technology Minna, Nigeria.The seedlings in the pots were subjected to complete submergence for a period of 10 days at 21 days seedling stage following the modified method of Akinwale et al. [1].All agronomic practices were carried out when necessary and the plant monitored for agro-morphological parameters.

Data analysis
The Agro-morphological characteristic that were taken include plant number, plant height before and after the submergence, percentage survival after submergence, number of panicles at maturity, number of tillers at maturity, 100 seed weight, days to 50% flowering, days to maturity.Analysis of variance (ANOVA) was used to test for significance difference among the means and Duncan's multiple range test (DMRT) was used to separate the means where there were differences.

Effects of gamma irradiation on germination percentage of FARO 44 and FARO 60
The results effects of gamma irradiation on the percentage germination showed that increase in the dose of gamma irradiation decreases the germination percentage of both FARO 44 and FARO 60.The highest seed germination percentage was observed in the control plans for both FARO 44 (100%) and FARO 60 (99%), while the least was observed at 200 Gy of irradiation dose for FARO 44 (90%) and FARO 60 (90%) (Figure 1).The decrease in seed germination percentage with increasing doses may be attributed to disturbance at cellular as well as physiological levels.This result is in line with the work of Ahsan et al. [8] who observed decreasing effects on the germination percentage as the dose of gamma irradiation increases in two varieties of local rice, MRQ74 and MR269.Mushtaq and Rakesh [9] also reported that percentage germination were decreasing with an increasing doses of gamma irradiation of their work on wheat (Triticum aestivum L).

Plant Height
The result of the effects of gamma irradiation on the plant height before and after submergence and their survival are is shown in table 1.In FARO 44, the plant heights recorded before submergence were not significantly (P≥0.05)influenced by the irradiation doses.However after submergence, a significant (P≤0.05)decrease in plant height was observed as the doses increases.In FARO 60, the control plants had the maximum plant height before submergence (13.95 cm) and after submergence (17.75 cm) respectively.On the other hand, plants exposed to 100 Gy had the lowest plant height (8.63 cm) before submergence, while plants exposed to 100 Gy and 150 Gy gamma irradiation had the lowest plant height (10.75 cm) after submergence.
Plants exposed to 150 Gy and 200 Gy gamma rays had significantly lowest plant height after submergence with the values if 19.50 cm and 19.43 cm when compare with the control (30.63 cm).This may be due to the fact that susceptible varieties tend to have rapid elongation during submergence which required carbohydrates and energy, leaving less available energy and maintenance required for survival during submergence while the tolerant varieties tend to limit stem elongation [1].This result is in agreement with the work of Mushtaq and Rakesh [9], who observed a decrease in plant height as doses increase with the control recording the highest plant height.

Survival Percentage (%)
Significant difference (p<0.05) were observed in respect to survival percentage of FARO 44 after 10 days submergence period.Plants exposed to 150 Gy had the highest survival percentage (93.75%)while the control had the least survival percentage (56.25%).These values were significantly different from all the values of other doses (Table 1).In FARO 60, the control plants had the highest (62.50%) survival percentage which was significantly different from the value of all other doses.On the other hand, plants exposed to 100 Gy of gamma irradiation had the lowest (37.50%) survival percentage which was also significantly different from other doses (Table 1).
Survival percentage after submergence significantly increases as dose of gamma irradiation increases in FARO 44, however in FARO 60, the percentage survival decreases as gamma rays increases, except dose 150 Gy of gamma rays which surprisingly has the highest percentage survival over the control.According to Talebi et al. [10], the increasing frequency of chromosomal harm with increasing radiation dose may be responsible for reduction in plant survival.The reduction in plant survival due to the mutagenic treatments has also been reported in Pearl Millet [11] and Sesame [12].Contrary to this, Animasaun et al. [13] reported that survival rate increased in untreated plant compare to the treated plants.

100 grain weight (g)
A significant variation (P≤0.05) in the 100 grains weight (g) was observed in both varieties.The result showed that the 100 grain weight increases as the irradiation dose increases in the exposed plants.This is contrary to the work of Mushtaq and Rakesh [9] who reported a decrease in 100 grain weight as dose increases in wheat (Triticum aestivum L.).Animasaun et al. [13], however, reported average weight of 100 grains for 80 Gy as the highest on their work on Digitaria exills.

Number of tillers and panicles at maturity
The result as presented in table 2 revealed the effect of gamma irradiation on number of tillers and panicles of submerged rice.The result showed that, plants exposed to gamma irradiation significantly (P≤0.05)produced highest number of tillers and number of panicles than the control except for FARO 60 where the values were lower than the control.This is in agreement with the work of Raj and Rao [14] who reported that morphological and yield parameters increases as dose increases in exposed rice plants.Similar results were also reported by Falusi et al. [15] in Nigerian sesame cultivars.

Number of Days to maturity
The effects of gamma irradiation on the number of days to maturity of submerged FARO 44 and FARO 60 revealed that gamma rays delay flowering and maturity in some of the irradiated plants than the control plants in both varieties.This is in conformity with the work of Arvind et al. [16] who reported that the number of days required for flowering increases with increasing doses of gamma irradiation.

Correlation between irradiation doses and agro-morphological parameters of submerged FARO 44 and FARO 60
The result in table 3 showed the correlation between irradiation doses and agro-morphological parameters of submerged FARO 44 and FARO 60.In FARO 44, the result revealed that gamma irradiation doses exhibited significant positive (P≤0.05)correlation in survival, 100 grain weight, days to 50% flowering and days to maturity, but negatively correlated with number of panicles at maturity (-0.329) and height after submergence (-0.681).In FARO 60, only days to maturity exhibited significantly positive (P≤0.05)correlation with doses, height after submergence, survival percentage, 100 grain weight, number of panicles and days to 50% flowering are negatively correlated.This result showed that lower doses of gamma irradiation have stimulatory effects on the plant height and number of panicles at maturity, whereas, higher doses had inhibitory effects.This is in line with the result of Mishra and Singh [14] who reported that increase in dose of gamma rays increases the seedling height of Isabgol (Plantago ovate, Forsk).The effect of mutagen was also found inhibitory to the plant height as reported by many researchers [18] in Solanum lycopersicum L. The reduction in plant height and other yield components as a result of increased dose of gamma irradiation could be due to physiological processes.Similar assertion was made by [19] in Solanum lycopersicum and [20] in Coriandrum sativum L.

Conclusion
In conclusion, it was observed that gamma radiation doses of 150 Gy and 200 Gy have positive effect on survival of the plant after been submerged and also on the 100 seed weight of FARO 44 compare to FARO 60 which showed negative effect on most of the parameters.Based on the result of this study, the following recommendations were made:  Further research should be carried out on submergence tolerance of the promising mutants.

Table 1
Effects of gamma irradiation on submergence tolerance indices of FARO 44 and FARO 60 abValues are Mean ± SE.Means with the same superscript down the column are not significantly different (P>0.05),separated by Duncan multiple range test.

Table 2
Effects of gamma irradiation on yield and yield components of FARO 44 and FARO 60 bValues are Mean ± SE.Means with the same superscript down the column are not significantly different (P>0.05),separated by Duncan multiple range test.

Table 3
Correlation between irradiation dose and agro-morphological parameters of submerged FARO 44 and FARO 60