Screening of African Yam Bean Accessions for Imbibition and Seed Physiological Quality

African yam bean is a nutritionally-important but neglected food crop with several health benefits. But its large scale cultivation and consumption are still limited by lack of systematic genetic improvement and breeding programme for the crop. Preliminary studies have established that the inherent field establishment potential of African yam bean is low. Hence the need to screen available accessions to identify those with outstanding seed physiological qualities for selection as parental materials for further improvement. Thirty-four African yam bean accessions were collected from different farmers in five states of Southeastern Nigeria. Seed physiological quality (viability and vigour) was assessed using hundred seed weight, water imbibition, standard germination, accelerated ageing and conductivity tests. The mean squares due to varietal effect were significant (p < 0.05) for germination index and highly significant (p < 0.01) for all other traits. Accessions ENAGag, ENAGmg 1 and, ENAGmg 2 had high standard (> 80%) and accelerated ageing (> 70%) germination percentages. Only 11 of the accessions had an accelerated ageing germination percentage of above 50%. All but one of the 34 accessions had conductivity values less than 5.00 μScm g. On the average, African yam bean seeds require up to 76.9% moisture of their initial weight over a period of approximately 53 hours prior for the commencement of germination process. Accessions ENAGag, ENAGmg 1, ENAGmg 2, ENNKob, ENNSog, IMISis, IMOKeo 1 and IMOKeo 3 were outstanding and are therefore potential candidates for genetic improvement of seed quality in African yam bean.


Introduction
African yam bean [Sphenostylis stenocarpa (ex. A. Rich.) Harms] is an underutilized indigenous legume consumed for its nutritious grains and tubers (Potter & Doyle, 1992). It is cultivated throughout West Africa countries particularly, Cameroon, Cote d'Ivore, Ghana, Nigeria and Togo (Potter, 1992). It is a perennial climbing species whose morphotypes may be prostrate or erect and about 1-3 m in height. Its leaves are trifoliate, 2.7 to 13 cm long and 0.2 to 5.5 cm broad (Nnamani et al., 2017). The edible seeds have high amino acid content (lysine and methionine) which has been reported to be higher than those of pigeon pea, cowpea, and Bambara groundnut (Uguru & Madukaife, 2001). It is commonly cultivated by traditional farmers along the South West and South East regions of Nigeria (Nnamani et al., 2017). The propagation of African yam bean (AYB) is predominantly through seeds and despite its great nutritional potentials, the consumption and cultivation of the legume is still limited by lack of systematic genetic improvement studies on the crop. While 55% of farmers cultivating AYB in South East Nigeria reported poor availability of seeds, 60% reported poor seed quality as constraints to the extensive cultivation of African yam bean (Nnamani et al., 2017).
Seed viability and vigour constitute the physiological component of seed quality. Seed viability is the ability of the seed to germinate under optimal conditions, while seed vigor denotes inherent ability of seeds to emerge uniformly even under adverse environmental conditions in the field (Kandasamy et al., 2020). Low-vigor seeds emerge less uniformly than high-vigor seeds (Egli & Rucker, 2012), and delay in emergence affects the

Hundred Seed Weight
One hundred seeds in three (3) replicates of each accession were counted and weighed using a sensitive measuring scale.

Accele
Significant differences existed in the seed physiological quality of the AYB seeds collected from the 5 states in Southeast, Nigeria. Seeds collected from Enugu State had outstanding seed physiological traits in terms of hundred seed weight, highest germination percentage, accelerated ageing germination percentage and highest vigour index. While seeds from Anambra State performed poorly among the other states (Table 3). Note. HSW: Hundred Seed weight (g); GPCT: Germination percentage (%); GI: Germination index; WIA: Water imbibed during accelerated aging (%); AAT: Accelerated ageing germination percentage (%); AAI: Accelerated aging germination index; COND: Conductivity per gram of seed (µScm -1 g -1 ); VI: Vigour index; WIP: Percentage water imbibed (%); DIM: Duration of imbibition (hrs).
The accessions differed widely in HSW, ranging from 22.70 to 52.83 g. Out of the 34 accessions evaluated, 11 had HSW < 30 g, 13 were 30-35 g, 7 were 36-40 g, and 3 had HSW > 40 g ( Table 4). 33 of the 34 accessions had germination percentages ranging from 2.67 to 98%. Only 9 accessions had germination percentage that was less than 50% while for more than half of the accessions, germination percentage was above 70%. Unlike seed weight (HSW) and viability (GPCT), germination index, GI, was less variable, ranging from 3.98-6.61 days with a mean of 5.07 days. The range of the amount of water imbibed varied widely. While ENIWea absorbed 130% and EBIZig and EBIZib up to 100% of the initial weight, ENAGmg series absorbed less than 50% of their respective weights (Table 4). Accessions ENAGag, ENAGmg 1, ENAGmg 2 also had germination percentages greater than 80% combined with germination percentage after accelerated ageing test (AAT) of above 70%. Although ABBEuz, EBIKea, EBIZib, EBIZwa, ENNEub and IMOKeo 2 accessions had very high mean values for germination percentage (> 80%), the percentage of germination after being subjected to accelerated ageing test drastically reduced to beyond half of the initial germination percentage. Only 11 of the accessions had an accelerated ageing germination percentage of above 50%. The accelerated ageing germination values for the accessions varied from 0% to 91.33% (Table 4). Accessions ENAGmg 1 and ENNSog revealed a higher accelerated ageing germination percentage values than their standard germination percentage values, while other accessions had lesser values. Also, the accelerated ageing index values ranged from 3.88-6.17 days similar to the germination index values. All conductivity values for the 34 accessions were less than 5.00 µScm -1 g -1 , except for ANISnn which had a value of 7.39 µScm -1 g -1 . Conversely, accession ANISnn had the least vigour index value, while others had vigour index values ranging from 29.51 to 88.92. The water imbibed percentage varied from 64.10% (ENAGag) to 88.98% (EBIKea), while the duration in which the water was imbibed varied from 42 hrs to 66 hrs. Half of the accessions had VI above 65% (88.92-65.11) and the other half below, ranging from 25.12-64.81. The first 7 accessions with the highest VI were from Enugu and Imo States (Table 4).
From the correlation result (Table 5), a highly significant (p < 0.01) negative correlation existed between hundred seed weight and accelerated ageing water imbibed, hundred seed weight and percentage water imbibed, germination percentage and conductivity, conductivity and vigour index, and between percentage water imbibed and duration of imbibition. Whereas a highly significant (p < 0.01) positive correlation existed between germination percentage and accelerated ageing germination percentage, germination percentage and vigour jas.ccsenet.org Vol. 13, No. 5; index, accelerated ageing germination percentage and vigour index. Also, a significant (p < 0.05) positive correlation existed between hundred seed weight and the duration of imbibition, germination index and duration of imbibition, accelerated ageing index and duration of imbibition. Furthermore, relationship between accelerated ageing germination percentage and conductivity, and between accelerated ageing germination percentage and percentage water imbibed were significant (p < 0.05) but negative. Note. HSW: Hundred Seed weight (g); GPCT: Germination percentage (%); GI: Germination index; WIA: Water imbibed during accelerated aging (%); AAT: Accelerated ageing germination percentage (%); AAI: Accelerated aging germination index; COND: Conductivity per gram of seed (µScm -1 g -1 ); VI: Vigour index; WIP: Percentage water imbibed (%); DIM: Duration of imbibition (hrs).

Discussion
The accessions exhibited a widely divergent variability for both physical and physiological traits. The wide variability in the performance of the African yam bean accessions across the seed physical and physiological quality tests corroborates the report of Adewale and Odoh (2013) that Nigeria is among the centers of diversity of African yam bean. Olasoji, Akande, and Owolade, (2011) also reported genetic variability for seed quality among 10 accessions of AYB studied. The variation was observed in their seed sizes, amount and rate of water imbibed in the process of germination, viability status and speed of germination, ability to withstand stress or adverse conditions and the integrity of their seed coats. Accessions ENAGag, ENAGmg 1 and ENAGmg 2 had comparative larger seeds than other accessions and showed similarities in their performances across the tests indicating that they were of a similar genetic background. Seed size is an important physical indicator of seed quality that affects seed germination, emergence, seedling survival, plant growth and performance of the crop in the field (Adebisi et al., 2013;Makinde, Oyekale, & Daramola, 2020). The outstanding performances of large-seeded accessions can be attributed to their higher reserve content. Shahi, Bargahi, and Bargali, (2015) reported that distinct seed sizes have different levels of starch and other energy reserves which may be an important factor to improve the expression of germination and initial growth of seedlings. The seed size of the accessions also varied across their location of collection. Although, it is believed that seed sizes may vary widely between the crop species and the germination and growth environment, generally, large seeds have a higher seedling survival rate, larger biomass and adaptability than small seeds, under adverse conditions (Ambika, Manonmani, & Somasundaram, 2014).
The failure of accession ANISnn to germinate under stress and ideal conditions suggests that the embryo must have been dead despite that other features of the seed remain intact and dormancy is not associated with African yam bean (Olisa et al., 2010a). This loss of physiological quality could have been as a result of the inherent low vigour of African yam bean seeds upon which is superimposed a poor storage environment. Accessions EBANau and EBIKok were also losing their germination potential, revealing a gradual decline in viability and vigour owing to physiological deterioration. Physiological seed deterioration is a phenomenon which is not visually apparent. The inability of seeds to maintain vigour for a long period poses storage problems and can cause the specie/crop to go into extinction (Finch-Savage & Bassel, 2016).
Most of the accessions had very low vigour as revealed by their low germination after being subjected to accelerated ageing. This corroborates the fact that African yam bean seeds are inherently low in seed vigour (Olisa et al., 2010a). The significant positive relationship between standard germination percentage and accelerated ageing germination percentage suggests that, accessions with higher germination under stress conditions will also exhibit higher germination under normal conditions, while poor germination under normal conditions will lead to poor performance under stress conditions. However, that some accessions like ENAGag, ENAGmg 1, ENAGmg 2, ENNKob, ENNSog, IMISis, IMOKeo 1 and IMOKeo 3 had higher and comparable standard germination and accelerated ageing germination percentages suggests that there exist a pool of high seed vigour potential among the African yam bean accessions that can be improved upon. High seed vigor enhances the ability to obtain optimal plant densities and high crop yields (Milosevic, Vujakovic, & Karagic, 2010). The germination index and accelerated ageing index values indicated that African yam bean seed germination occurs between averages of 4-7 days, whether under ideal or stressed conditions. These two test procedures (standard germination and accelerated ageing tests) alongside conductivity test were the key determinants of the vigour index of the accessions. The consistently low conductivity values for all the accessions could be as a result of their hard seed coat which could in turn reduce seed exudates. Tungate, Susko, and Rufty (2002) attributed low germination rate to the prevention of water and respiratory gases from penetrating into the seed by hard seed coats during imbibition. However, the considerably low conductivity values also indicated that the seeds were suitable for sowing. According to Hampton and TeKrony (1995) conductivity values less than 30 μScm -1 g -1 indicate that the seeds are suitable for sowing, even under adverse conditions.
Even though the amount and duration of water imbibed did not follow any definite pattern nor correlate with the number of seeds that germinated, their persistent correlation with hundred seed weight under ideal conditions (WIP) and under stress (WIA) revealed that imbibition is strongly influenced by or dependent on the size or weight of the seed. Olisa et al. (2010b) stated that the duration of each phase of imbibition in African yam bean depended on seed properties such as size, content of hydratable substances, seed coat permeability and oxygen uptake. However, the speed of germination under both ideal and stress conditions in this study were influenced by the duration of water imbibed. That is, a delay in the number of days of the seed to germinate extended also the period during which water is imbibed prior to germination, without necessarily increasing the overall amount of water imbibed. On the average, African yam bean seeds require moisture of up to 76.9% of their initial weight over a period of approximately 53 hours prior for the commencement of germination.

Conclusion
Seed size is an important trait in African yam bean that could be improved upon for the selection of landraces of high seed physiological potentials. Conductivity test was not very effective in determining the variability in seed vigour. Seed germination in AYB can be easily assessed 7 days after planting. Accessions ENAGag, ENAGmg 1, ENAGmg 2, ENNKob, ENNSog, IMISis, IMOKeo 1 and IMOKeo 3 were outstanding and are therefore recommended for genetic improvement of seed quality in African yam bean.