Evaluation of Seed Yield of Nigerian Bambara Groundnut [ Vigna subterranea ( L . ) Verdc . ] Landraces under Varying Water Conditions

Seed yield of selected Nigerian Bambara Groundnut landraces under two varying water conditions was evaluated. Twenty eight (28) Bambara groundnut accessions were planted in a randomized complete block design with two treatments (Nonwater stressed and Water stressed) and replicated five times. The experiment was conducted at the Experimental Garden of the Department of Biological Sciences, Federal University of Technology, Minna, Niger State. The accessions were planted in planting bags during the cropping season of 2016 between July and October. The two treatments were rain fed until maturity except during the period of exposure to water stress where the water stressed plants were transferred to a screen house made with polyvinyl ceiling to prevent water during the flowering stage for two weeks. It was observed that water stress reduced the yield of the Bambara groundnut landraces by 47%. Accessions NGB-01646-B, NGR-NI-20-H and NGB-01491 which had relatively high yield in the stressed and nonwater stressed condition were considered the most drought tolerant accessions and accession NGR-KG-02-C with low yield both in stressed and non-stressed conditions was identified as the most susceptible accession. Accessions which had high yield only in water stressed conditions could serve as elite line for Bambara groundnut for areas which have problem of drought. These drought tolerant accessions identified in the research can serve as potential parent lines in breeding programmes aimed at the development of drought tolerant varieties of Bambara groundnut.


Introduction
Bambara groundnut is a leguminous crop that is well cultivated by farmers in Nigeria.It is believed to have originated from Nigeria and Cameroon as the distribution of wild Bambara groundnut was found to extend from Jos Plateau and Yola in Nigeria, to Garoua in Cameroon (Goli, 1997).It is in West Africa that most of the world's Bambara groundnut is grown and where the crop is most prominent in the traditions of rural communities.Bambara groundnut is now widely distributed in the semi-arid zone of sub-Saharan Africa (SSA) and most authors seem to support the view that it is the third most important food legume after cowpea (Vigna unguiculata) and groundnut (Mkandawire, 2007).In Nigeria, the Igbo people call it Okpa, Hausa call it Kwaruru or Gurjiya, Ebira call it Ezi, Igala call it Pangudu, Nupe call it Edzu and Gbaji call it Abwui.
It is an herbaceous annual crop that grows for about 0.30-0.35m in height; it has both erect and prostrate forms (Bamshaiye et al., 2011).It forms seed and pods in subsurface of the soil.The pods are round wrinkled and over half inch long each containing one or two seeds that are round smooth and very hard when dried.The seeds vary in terms of colour from white, cream, dark brown, red or black which may be patterned with combination of these colours.
The seed contain sufficient amount of protein (19%), carbohydrate (63%) and fat (6.5%).The red seeds are useful in areas where there is deficiency in iron because they contain almost twice iron as that of the cream seeds (Mayes et al., 2011).Bambara groundnut seeds have been processed

Source of seeds
The seeds used for the study were collected from farmers in seven States in Northern Nigeria such as Niger, Kogi, Plateau, Kaduna, Nassarawa, Adamawa and Jigawa States.Some seeds were also collected from National Agency for Genetic Resources and Biotechnology (NACGRAB) Ibadan, Nigeria.

Description of study site
The seeds were sown at the Department of Biological Sciences Experimental garden, Federal University of Technology Minna, Niger State.Minna is located between latitude 90 º 31 ' and 90 º 45 ' North and longitude 6 º 31 ' and 6 º 45 ' East of the equator.The area falls within the Southern Guinea savannah vegetation zone of Nigeria with an annual precipitation varying from 1,100-1,600 mm, mean temperature between 21 º C and 36.5 º C and relative humidity between 50 to 61% (The Nigerian Congress, 2007).

Experimental design
Bambara groundnut accessions were grown in two different treatments: T1-Non water stressed condition (Plants were rain fed throughout the experiment) and T2-Stressed condition (Plants were deprived of water for 14 days during the flowering period by transferring plants to a shade house made of transparent polyvinyl ceiling).The accessions were grown in planting bags in a completely randomized block design with five replicates.Two seeds were sown per bag and the bags were given a spacing of 30 x 30 cm (inter and intra row spacing) and later thinned to one 2 weeks after planting.

Determination of seed yield
The seed yield was determined by counting the number of seeds (NS) produced per plant at the time of harvest by sun drying and opening the pods and counting the number of seeds which was determined by their large size and firmness.

Determination of drought tolerant accessions
In order to determine the most drought tolerant accessions, drought tolerance indices were determined according to the method of Wonderwosen et al. (2012).The following drought tolerance indices were determined for each accession.
and used in several ways.It is used to make a paste out of the dried seeds, which is then used in the preparation of various fried or steamed product such as "moin-moin" and "akara" (Okpuzor et al., 2010).In Nigeria this crop has been used as a favourable dish, "Okpa" which is a doughy paste that is wrapped in banana leaves and then boiled.The demand for Bambara groundnut is increasing due to its many uses, high nutritional value and medicinal value as it serves as a major diet for diabetic patients.Despite its high nutritive value and many uses, the crop is under-utilized; landraces are unimproved due to neglect of the crop by scientists (Berchie et al., 2010).In Nigeria the cultivation of Bambara groundnut in the Sahel and Sudan Savannah zone has declined over the past two decades.Farmers in this region estimated that the cultivation of Bambara groundnut presently is about 5 to 20% of that of 20 years ago and attributed this reduction to drought (Vuraiyai et al., 2011a, Mohammed, et al., 2013).But this crop has been increasingly cultivated in the Guinea and Forest zone of Nigeria during this same period.The increase in the amount of crops produced in these areas is thought to be due to the fact that this crop brings higher prices now than it was before which makes the crop a potential crop to boost nutrition and alleviation of poverty for farmers.
The shift in cultivation of Bambara groundnut from the dried part to the wet part of the country has declined the overall production of the crop because there are fewer farmers producing the crop in the wetter areas, the cost of land clearing is much, soil in the wet area are more difficult to cultivate as they are heavier and more prone to weeds and disease problems are encountered in wetter area because of the prevalent high humidity (Mohammed et al., 2013).The problem of drought and lack of improved varieties in Bambara groundnut requires that effort should be made towards the development of drought tolerant varieties hence the need to evaluate Bambara groundnut accessions for their seed yield in varying water conditions, so as to determine the best drought tolerant accessions that can be used for breeding purpose and to recommend accessions for different areas in Nigeria with different water availability.This will help to increase farmer's productivity of the crop.Being that farmers are interested in increasing their yield, so also plant breeders are interested in reducing the yield loss by farmers, there is therefore need to evaluate Bambara groundnut seed yield under varying water conditions so as to be able to recommend accessions for different parts of the country with varying water availability and to improve farmers yield.
The understanding to be gained would serve as a basis for selection of baseline parents with potentials for drought tolerance and high yielding to be adopted for overall productivity of the crop.This will help to increase farmer's productivity of the crop.The present effort will further stimulate farmer's interest in the cultivation of the crop to meet market demands thereby improving their livelihoods, alleviate poverty and facilitate food security in the face of many problems posed by climate change.
The accessions were ranked based on these drought tolerance indices, the rank sum was calculated and the mean rank sum was used to determine the most drought tolerant accession.

Statistical analysis
The yield parameters for all accessions were subjected to analysis of variance (ANOVA) using the Statistical Analysis System (SAS).Treatment means were compared using the Least Significance Difference (LSD) at probability level of 0.05.

Results
The Bambara groundnut accessions collected from farmers in the seven Northern States of Nigeria and from National Agency for Genetic Resources and Biotechnology (NACGRAB) were made up of a variety of seed coat and eye colours.The summary of the phenotypic characteristics of the seeds are represented in Table 1.
The result showed significant differences (P ≤ 0.05) in the number of seeds and weight of seeds produced by the Bambara groundnut accessions both in the non-stressed and stressed conditions (Table 2).It was also observed that water stress reduced the number of seeds produced by the Bambara groundnut accessions in the water stressed plants.
In the non -water stressed plants, the seed yield of the Bambara groundnut accessions ranged from 13 to 51 seeds per plant with an average seed yield of 32 seeds per plant while in the stressed plants the seed yield of the Bambara groundnut accessions ranged from 6 to 28 seeds per plant with an average seed yield of 17 seeds per plant.(Table 3) In the non-stressed plants, accession NGB-01646-B had the highest number of seeds per plant (51) which was significantly different from all other accessions except accessions NGR-NI-23-C and NGB-01646-C.This was followed by accession NGB-01646-C with 40 seeds per plant and the least number of seeds were produced by accession NGR-NI-18 with 13 seeds per plant.It was also significantly different from all other accessions.In the stressed plants, the highest number of seeds per plant was produced by accession NGR-NI-20-H (28 seeds per plant) which was significantly different from all other accessions.This was followed by accession NGB-01646-B with 26 seeds per plant.2).Accession NGR-AD-27-B had the highest weight (27g) and it was significantly different from all other accessions except for accessions, NGR-JG-17-B, NGR-NS-15, NGB-01486-A and NGB-01493 while accession NGR-KG-02-C had the lowest weight (3.03g) in the non-stressed environment.It was significantly different from all other accessions except accession NGR-NI-18 (5.06g) but in the stressed environment, accession NGR-NI-20-H (15.5g) had the highest weight of seeds followed by accession NGR-JG-17-A (13.41g).They were not significantly different from each other but significantly different from all other accessions.The smallest weight of seeds per plant in the stressed environment was observed in Accession NGR-KG-02-C and it was not significantly different from accession NGR-NI-23-C but statistically different from all other accessions.Some drought tolerance indices which provide a measure of drought based on loss of yield under stressed conditions in comparison to non-stressed conditions have been used for screening drought tolerant genotypes.Such drought tolerance indices derived from mathematical calculations such as stress tolerance index (STI), Mean Productivity (MP), Geometric mean productivity (GMP), Yield index and harmonic mean (HM) were used to determine drought tolerance in Bambara groundnut (Table 3).
Stress Tolerance index is usually used to identify genotypes that produce high yield under both stress and non-stress conditions.The highest STI was observed in Accession NGB-01646-B (1.80) followed by accession NGR-NI-20-H (1.41) and then accession NGB-01491 (0.96).The least STI was observed in accession NGR-KG-O2C (0.08) which was followed by accession NGR-NI-22 (0.15) and then accession NGR-NI-25-A (0.17  Mean productivity is the average yield of plants in stress condition (Ys) and non-stress condition (Yp).The highest mean productivity was observed in accession NGB-01646-B (38.63), followed by Accession NGR-NI-20-H (32.63) and then accession NGB-01491 (27.88).The lowest mean productivity was observed in accession NGR-KG-02-C (7.75) followed by accession NGR-  The Geometric Mean productivity is often used by breeders interested in relative performance since drought stress can vary in severity in field environment.Accession NGB-01646-B also had the highest geometric mean productivity (36.59) followed by accession NGR-NI-20-H (32.63) and then accession NGB-01491 with Geometric mean productivity of 26.81.The least geometric mean productivity was observed in accession NGR-KG-02-C( 7.65) followed by accession NGR- .For the Harmonic mean, the highest harmonic mean was observed in accession NGB-01646-B followed by accession NGR-NI-20-H.The least was observed in accession NGR-KG-02-C.
All the accessions used in the study were grouped under four categories based on their performance in stressed and non-stressed conditions.Accessions which had high yield under both non-stressed and stressed conditions were put in group A. The estimate indicators of drought tolerance (Table 3) indicated that the identification of drought tolerant accessions based on a single criterion may be contradictory.To determine the most desirable drought tolerant accession according to all indices determined, rank sum and mean rank of the drought tolerance indices were calculated and based on these criteria, the most desirable drought tolerant cultivars was identified as accession NGB-01646-B with a mean rank 1.29.This was followed by accession NGR-NI-20-H with a mean rank of 2.00 and then accession NGB-01491 with a mean rank of 3.57.The most susceptible accession was identified as accession NGR-KG-02-C with a mean rank of 25.29, this was followed by accession NGR-NI-22 with a mean rank of 24 and then accession NGR-NI-25-A with a mean rank of 23.43 (Table 4).

Discussion
The reduced seed yield observed in the Bambara groundnut accessions in the water stressed plants is an indication that water stress reduced seed yield through reduced seed number and weight.Although the accessions were shown to be still productive under limited water condition.Even though Bambara groundnut has been known to be drought tolerant, water stress still reduced the yield.These results are similar to others such as Berchie et al. (2012) and Mabhaudi et al. (2013) which all reported reduced seed yield in Bambara Groundnut landraces in response to limited water availability under field conditions.According to Berchie et al. (2012) the production of seeds despite the drought experienced during the flowering stage confirms Bambara groundnuts resilience under drought stress.The reduction in the number of seeds produced by the Bambara groundnut in the stressed accessions therefore means that water should be provided to Bambara groundnut where possible in order to have maximum seed production.
The significant differences observed in the number of seeds produced by the Bambara groundnut accessions both in the non-stressed and stressed conditions is an indication that genetic variation exists among the Bambara groundnut accessions with respect to seed yield.These variations in yield parameters provide a basis for selection of the best accessions that will perform well in places with different water availability.Mitra (2001), reported that the most suitable indices for selection of drought tolerant genotypes are indicators which show a relatively high correlation with seed yield in both stress and non-stress condition.Abejide et al. also reported that the best drought tolerant indices to be used in the determination of drought tolerant accessions in Bambara Groundnut are STI, YI, MP, GMP and HM because they had significant positive correlations with seed yield in the stressed and non-stressed condition.
Based on the performances of the Bambara groundnut accessions used in the study, accessions which fall in group A and B are recommended for areas with good water availability while accessions in group A and C which performed well in water stressed conditions are recommended for semi-arid regions of Nigeria experiencing the problem of drought.Based on the values of STI,YI, MP, GMP, HM and their ranking, accession NGR-NI-20-H having the best rank mean was considered the most drought tolerant accession and NGR-KG-02-C the most susceptible.The drought tolerant accessions and the susceptible accessions identified in the research can serve as potential parent lines in breeding programmes aimed at the development of drought tolerant varieties in Bambara groundnut.

Conclusions
The study revealed that the most drought tolerant accessions are NGB-01646-B, NGR-NI-20-H, NGB-01491, NGB-01493, NGB-01646 while the most susceptible are NGR-KG-02-C, NGR-NI-22, NGR-NI-25A, NGR-NI-20-B, NGR-JG-17C.The drought tolerant accessions and the susceptible accessions identified in this study provide excellent genetic resources for further studies underlying mechanisms regulating drought responses and gene discovery.This result provides vital information to support the effort of molecular breeding and genetic Engineering to improve drought tolerance of Bambara Groundnut.

Table 1 .
Sources of Bambara groundnut accessions used in the study

Table 2 .
Number of seeds and weight of seeds produced by Bambara groundnut accessions in Non-water stressed and stressed conditions Note: Values are means ± standard error.Different letters between accessions denote significant differences (Duncan test, p < 0.05).NS: Number of seeds, WS: Weight of seeds.

Table 3 .
Some drought tolerance indices of Bambara groundnut accessions

Table 4 .
Rank, rank sum, rank mean of some drought tolerance indices and drought tolerant ranking Yield in stressed environment, Yp:Yield potential, STI:Stress tolerance index , YI:Yield index , MP:Mean productivity,GMP:Geometric mean productivity , HM: Harmonic mean, DTR: Drought tolerance ranking