Cowpea (Vigna unguiculata (L) Walp) is the second most important grain legume grown in Sierra Leone after groundnut and is used mainly as a source of protein food and also supplements the income of many small scale farmers. It contains 23% protein by weight and plays an important role in providing protein requirements of Sierra Leone (MAFFS/NARCC/NUC, 2005). It helps to maintain soil fertility by fixing nitrogen in the soil. Cowpea is a cheap  source   of  protein  in  the  diet and  an  increased production and consumption will help reduce incidence of protein malnutrition especially in children. It is assumed that the name cowpea is derived from American farmers who used it as feed for their cows (IITA, 2009). Cowpea improvement has been intensified in Sierra Leone since 1988, with the inception of the Institute of Agricultural Research (IAR). This has led to the release of several improved varieties, notably Slipeas 1, 2, and 3 in 2002 and   Slipeas   4   and   5  in  2014.  The  varieties  mostly cultivated by the farmers are Tabe and Hundweh (IITA, 2011) both local varieties are preferred by the consumers because of their good taste.

Despite the development of improved cowpea genotypes in Sierra Leone, yields are still low (less than 2 t/ha) compared to yields as high as 4 t/ha obtained in Nigeria. In the evaluation and testing of genotypes most of the traits considered are morphometric and agronomic traits only germination indices such as seed vigor, germination percent, speed of germination index which measure the sum total of those properties of the seed which determine the level of activity and performance of the seed or seed lot during germination and seedling emergence are not considered. According to Shaibu and Ibrahim (2016), one of the factors that may affect crop production is the physiological quality of the seeds, which has an indirect influence on the speed and percentage of seedling emergence and final stand, or a direct influence on plant vigour. Therefore, the use of seeds of high physiological quality is paramount to increasing productivity and improving the technological level of bean cultivation (Adebisi et al., 2013). Generally, vigor tests have proven to be more useful as predictors of field emergence than the standard germination test. When planted in fields with stressed environmental conditions, especially cool, wet conditions, a high vigor seed lot can withstand the stress during germination and early seedling development longer than a low vigor seed lot (Spears et al., 2002). Thus, emergence is generally higher and seedling growth is more rapid. Vigor tests are used extensively in the seed industry to provide a sensitive, consistent, fast, simple and economic method that can be used to predict the seed performance in the field environment (McDonald, 1980). It is well known that decline in seed vigour precede those observed in germination as seed deterioration progresses. This phenomenon underscores the importance of using seed vigour tests as a more sensitive measure of seed quality and plant emergence capability of a seed lot (Copeland and McDonald, 2001). There is lack of data on these traits in the evaluation of these varieties. Sierra Leone is the third most vulnerable country affected by climate change as the rainfall patterns are currently unpredictable with rising temperatures, flooding and periodic droughts. There is therefore, a need to develop varieties that are not only early maturing to escape drought but also tolerant to multiple abiotic stresses. Seed vigor can be used to predict the field performance of a variety under stressful environmental conditions and recommendation for period of storability. Variations in genetic make-up of crops have been reported as one of the major causes of differences observed in seed quality and yield (Ajala et al., 2006; Okelola et al., 2007). Stahr (2012) stated that seed lots with similar standard germination results may emerge differently in the field due to differences in their seedling vigor. This study is therefore conducted to evaluate the seed quality  of  released  improved  cowpea varieties to identify varieties using physiological traits that can withstand environmental stresses. This information will be useful to breeders and cowpea growers to ensure productivity of improved plant population in Sierra Leone. The aim of the study is to identify improved cowpea varieties with good seed quality traits that can withstand environmental stresses in the field.

Specific objectives

(i) To identify seed quality traits associated with high germination and vigor.

(ii) To identify suitable varieties with high vigor and germination traits that can withstand environmental stresses in the field.

 

The experiment was conducted at the Njala Agricultural Research Centre Seed Laboratory in 2018 under normal room temperature and relative humidity. Seeds of the 4 released improved cowpea varieties and 1 local variety (Tabe) were used for the experiment (Table 1). Seeds were obtained from the seed store whilst the local variety was purchased from farmers. A completely randomized design with four replications was used in the study. Sterilised sand free from weed seeds was used as the medium for germination. For every 1 L of sand, 160 ml of water was added and mixed thoroughly with a hand trowel. The four replications were then placed under optimal germination conditions and 25 seeds were used for each of the four replications. The number of normal seedlings on the first count (5^th day) and final count (8^th day) was used to determine the vigor and germination ability respectively. Chlorox soaking test was used to determine percentage seed coat damage of the different varieties. 88.7 ml of Chlorox solution was dissolved in 1 gallon of distilled water. The mixture was poured in 5 petri-dishes. 100 seeds of each of the varieties were dropped in the Petri dishes containing the solution for 15 mins and the number of swollen seeds was counted to determine percentage of seeds with damaged seed coat.

 

 

Data were collected on the following seed quality parameters, Seed length, width and thickness, seed moisture Content, 100 seed weight, vigor percentage, germination percentage, speed of germination index, percentage seed coat damage, seedling root length and seedling shoot  length. Seed length, width and thickness were measured using an electronic Vernier calliper. Seed moisture content was measured by moisture meter. 100 seed weight was done by counting 100 seeds of each variety and weighing using electronic compact scale (Virgo). Vigor percentage was calculated by dividing the number of seeds that germinated at first count that is, after 5 days against number of seeds sown. Germination percentages were calculated by dividing the number of emerged seedlings against total number of seeds sown after 8 days multiplied by 100.

 Germination percentage = Number of emerged seedlings at final count / Total number of seeds sown × 100.

Speed of germination index was determined by number of normal seedlings (first count) /days to first count + Number of normal seedlings (final count) / days to final count. Percentage seed coat damage was determined using the Chlorox soaking test and seedling root length and shoot were determined by measuring the tap root  and  seedling  height  of  5 seedlings and means recorded respectively.

Data were subjected to analysis of variance (ANOVA) using Genstat Release Version 10.3DE and Least Significant Difference (LSD) test was used for mean separation at 5% level of probability. Regression and correlation analysis were explored to examine relationships among various parameters (Table 1).

 

 

Morphometric seed characteristics of cowpea varieties

Seed length, width and thickness of cowpea varieties

Highly significant differences (P < 0.001) were observed in mean seed length among the cowpea varieties studied. The newly released improved cowpea variety Slipea 5 had the longest seed length (9. 95 cm) whilst the local cultivar Tabe had the shortest seed length of 8.11 cm (Table 2). However, there was no significant (P < 0.001) difference at 5% level of probability between Slipea 5 and Slipea 4 although these two varieties were significantly different from Slipeas 1 and 2 in seed length. The results of the study showed significant (P = 0.004) differences between the four improved varieties and the local Tabe variety in mean seed width. The local cultivar Tabe had the least seed width (5.46 mm) whilst the improved varieties Slipea 4 (6.81 mm) and Slipea 5 (6.78 mm) had the highest and second highest seed width respectively (Table 2). Analysis of variance results show significant differences (P < 0.001) in mean seed thickness among the cowpea varieties. The  improved  varieties  (Slipeas 4 and 5) which had the highest seed length and width also had the highest seed thickness (5.28 and 5.64 mm) respectively. The local Tabe cultivar had the least seed thickness of 3.8 mm (Table 2).

 

 

100 seed weight

The results of the study showed that the Local cultivar (Tabe) recorded the lowest 100 seed weight (13 g) whilst the improved cowpea varieties Slipea 2 and 5 had the highest 100 seed weights (23 g) (Figure 1). The higher 100 seed weights obtained by improved varieties could be related to the higher yields of the improved varieties over the local variety. The higher yield of improved cowpea over local variety was supported by work done by Singh et al. (2002) who showed that the use of improved varieties led to the realization of 4 tonnes per hectare. Local cultivars were found to be poor in resource capture and utilization resulting in lower seed weight.

 

 

Percentage seed coat damage

The results indicate that the local variety (Tabe) had the highest percentage seed coat damage (40%) followed by Slipea 5 ( 30%)  whilst Slipea 1  had the least seed coat damage  of  10% (Figure 2). The high percentage of seed coat damage observed in the local variety through the high number of swollen seeds could be due to poor handling of seeds during processing by the farmer. The results  showed  that  the  coloured  variety  had the least seed coat compared to the other white seeded Slipeas and Local Tabe. This trend of results also conform with Peksen et al. (2004) who reported that cowpea cultivars with completely or partially cream/beige testas imbibed water more rapidly than cultivars with coloured testas probably  due  to  high  seed  coat  ratio  of  the  coloured variety. However, contrary results were reported by Ilunga (2014) who found out that Brown mix variety had more leakage (53.2%) than the White birch variety (18%), indicating that seeds of White birch variety had less cell membrane permeability compared with Brown mix variety.

 

 

Physiological seed traits of cowpea varieties

Vigor and germination percentages of the cowpea varieties

The results of the study indicated that Slipeas 2 and 4 had the highest vigor (93 and 95%) and also the highest germination percentage (96%). The improved dark - purple variety Slipea 1 and   white local cultivar Table had the lowest vigor (84 and 85% respectively). Generally, the vigor and germination percentages of the improved released cowpea varieties are higher than that of Tabe except the dark purple Slipea 1. These results are contrary to the findings of Ilunga (2014) who reported that coloured cowpea varieties had higher vigor and germination percentages and therefore keep longer. Similar   trend    was    observed    between     vigor    and germination percentages among the cowpea varieties. These findings are in agreement with McCormack (2004) who also noted that the relationship between vigor and viability is similar except that the vigor declines before viability. Martins et al. (2016) also reported that there is positive correlation between seed vigour and seedling field emergence (Figure 3).

 

 

Speed of germination index (SGI)

The results obtained for speed of germination index (SGI) are similar to those of vigor and germination percentages. The two varieties Slipeas 2 and Slipea 4 with the highest vigor and germination percentages also had the highest speed of germination (Table 3). These results confirm earlier studies by Spears et al. (2002) who also found out that seeds with high vigor can withstand stress during germination and early seedling development longer than low vigor seeds.

 

 

Seedling and root length of cowpea varieties

No significant difference (P = 0. 62) was observed in mean root length whilst seedling length was significant (P= 0.038) among the cowpea varieties at the 5% level of probability. The local variety had the minimum root and seedling length compared to the improved varieties (Figure 4). This could be contributing factors to its low vigor and germination percent. The high yields obtained by the improved cowpea varieties could be attributed to their higher seedling growth and development. Similar observations were made by Igbal (2015) who stated that seedling growth and development determines the final yield as good seedling establishment constitutes the first and foremost step in achieving full potential of crops. The results of the study indicated that the improved cowpea varieties with large seeds and higher 100 seed weight were taller than Local Tabe with small seed size in the initial stages of growth. This could be attributed to the fact that large seeds had the capacity to mobilize storage reserves to growing seedling as was also reported by Ndunguru and Summerfield (1975b). The variation in plant height among varieties may be attributed to  genetic makeup of the varieties. The results are similar to earlier work done by Gan et al. (2003).

 

 

Pearson’s correlation of some seed quality traits of the cowpea varieties

The results of the correlation analysis show that vigor and germination are significantly highly positively correlated (r = 0.92, P< 0.0001). This also confirms earlier report by McCormack (2004) that vigor and germination are strongly correlated. There was also a positive correlation between seed length, width and thickness and 100 seed weight. This indicates high dependence of seed weight on seed length and thickness. No correlation was observed between 100 seed weight and germination (Table 4).

 

 

 

 

 

 

 

 

(ii) The cowpea varieties. Slipea 2 and 4 with the highest

vigor also had the highest germination. This clearly indicates that there is strong correlation between vigor and germination.

(iii) Seed coat damage of cowpea seeds affected vigor and germination of seed. Cowpea varieties with low seed coat damage had the highest vigor and germination.

(iv) Vigor and germination traits in cowpea are influenced by the type of variety.

From the findings of this experiment, the following recommendations can be made

(i) The improved varieties Slipea 2 and 4 are the best in terms of vigor and germination and recommended for good plant establishment in the field.

(ii) Seeds of the local cultivar, Slipea 1 and 5 are prone to seed coat damage and should be handled with care during processing and storage.

(iii) Seeds of the improved varieties Slipea 2 and 4 have higher vigor and could therefore be stored for longer periods than the other varieties.

(iv) Future work should include field emergence test to establish relationship between laboratory germination and field emergence.

 

The authors have not declared any conflict of interests.

 

The authors wish to appreciate Messrs Murray Kaikai and George Puvandi of Njala Agricultural Research Centre Seed Laboratory for assisting in setting up the experiment and collecting data.