The African eggplant (Solanum aethiopicum L.) is one of the most commonly consumed fruit vegetable in Ghana and West Africa. It is considered the third most cultivated and consumed after tomato, onion and before okra in Ghana (Horna et al., 2007; Osei et al., 2010). Seeds that are borne in fleshy fruits are subjected to several postharvest handling practices which do affect the quality of seeds positively or negatively. Two of such important practices are seed extraction and drying methods.

Several seed extraction methods such as wet, dry, natural fermentation, chemical fermentation, and mechanical extraction have been used to extract seed from fruit vegetables such as tomato (Demir and Samit, 2001), egusi melon (Ogbonna and Odo, 2011), cucumber  (Chethan et al., 2013) and eggplant (França et al., 2013; Rahman et al., 2015). These various methods had varied effects on seed quality among different species. Silva et al. (1982) observed that natural fermentation gave a lower germination than acid treatment fermentation in tomato. Demir and Samit (2001) also observed the same results. For eggplant, França et al. (2013) observed no significance difference among seed extraction methods used on first germination count. A reduced percent seed germination was observed when fermentation duration was extended to 48 h either with acid or without. In cucumber, seeds extracted with natural fermentation resulted in highest germination (94.5%) and seed vigour compared to alkali or acid extraction methods (Chethan et al., 2013). These studies suggest that each species has an ideal extraction method for maintaining maximum seed quality. There is few studies reported on how natural fermentation, a common practice by smallholder farmers influence the seed quality of the African eggplant (Solanum aethiopicum L.).

After seed extraction from fleshy fruits, the next important step in maintaining the quality of seed is drying. Seed moisture content (MC) plays critical role in determining the longevity of seed in several vegetables (Wang et al., 2001). Generally, seeds are harvested at high moisture content and need to be dried before storage with a careful attention to the rate and extent of post-harvest drying (Babiker et al., 2010). Botey et al. (2021a) reported a seed moisture content of 42 - 46% when fruits were harvested at the recommended harvest stage of 60-76 days after anthesis in African eggplant. While most vegetable seeds can withstand drying to extend their storage life with low moisture content (Wang et al., 2001), this may differ with species.

This suggests that not all kinds of drying methods will suit equally well under given set of conditions in retaining viability and vigour of seeds, hence each crop should be evaluated accordingly. The extent to which these various drying methods reduce seed moisture and influence the physiological quality of African eggplant seeds is however not known. The present study sought to evaluate the physiological quality of African eggplant seeds subjected to different natural fermentation and drying methods.

A first experiment was carried out to evaluate the effect of extraction methods on the physiological quality of seeds. The trial was conducted at the University of Eldoret, Department of Seed, Crop and Horticultural Sciences, Kenya. Fifty fruits of two cultivars (cv. Oforiwa and Kpando) were harvested 70 days after anthesis (DAA) and processed same day or less than 48 h to avoid possible after-ripening effect. For seed extraction, ten fruits were carefully cut individually with sharp knife and seeds were extracted immediately by hand from the pulp or put in a plastic container containing distilled water placed in an ambient condition for 6, 12, 24  and   48 h   for   fermentation.   Well-formed   seeds    after   the fermentation periods separated from the pulp and remained at the bottom of the container while immature seeds which were floating were discarded. Seeds were thoroughly rinsed with running water. After each natural fermentation period, the seed moisture content was determined for each treatment using the oven method at 105°C ± 3 for 24 h on fresh wet basis (Brasil, 2009). All the seeds after each treatment were dried under shade for 48 h. Seed quality tests were evaluated as follows:

Seed dry weight: Sub-samples of seeds extracted from each treatment were dried in oven at 30°C for 24 h after which seed weight by weighing four replicates of 100 seeds on an analytical balance to two decimal places and the mean results expressed in grams.

Germination test: Four replications of 50 seeds for each treatment were used. Seeds were evenly spread on a two layer Whatman paper moistened with distilled water and placed in a petri dish. The petri dishes were then incubated at 30/20°C in an 8/16 h alternating light/dark periods as recommended by Botey et al. (2021a). Germination was conducted for 14 days. Daily germination was recorded until the 14^th day. Results were expressed as a mean percentage of normal seedlings for each lot.

First germination count: This was done together with the germination test by counting the number of normal seedlings identified on 7^th day after sowing.

Accelerated ageing test: Accelerated aging was conducted in plastic boxes measuring 14 x 12 x 4.5 cm. A fiber mesh screen was suspended inside the plastic box on which seeds were evenly distributed to form a thin layer. Approximately 2.0 g of seeds from each treatment was used. To create the desired humidity condition for the test, 40 ml of water was added to each plastic box. The boxes were covered and maintained in an incubator at 42°C for 72 h, after which seeds were submitted to the germination test as previously described. Evaluations were performed seven days after sowing and the results expressed as mean percentage of normal seedlings for each lot.

Experiment II: The objective was to evaluate the seed physiological quality of African eggplant cultivars as influenced by drying method. Seeds were extracted from 5 harvested fruits of each cultivar at the same maturity stage as in the first experiment. Extracted seed were then immediately washed under running water and subjected to the various drying methods of sun/24 h; shade/24 h; shade/48 h; desiccant (silica gel)/24 h; shade/24 h + 30°C oven (24 h); 30°C/24 h; 35 °C/24 h; 45°C/24 h; 50°C/24 h and 60°C/24 h. The seed moisture content was measured after the different treatments following the same methodology as previously described in the first experiment. The seed physiological quality was measured by first germination count as an indication of seed vigour and percentage seed germination. First germination count was recorded after 7 days of sowing. Final seed germination percentage was calculated after day 14 as number of seeds germinated against total number of seeds sown and expressed as a percentage.

Design and statistical analysis

Experiment I: A completely randomized design in a factorial arrangement was used for this experiment. Cultivar and fermentation durations constituted the factors. Treatments were replicated four times. The African eggplant cultivar were cv. Oforiwa and cv. Kpando. The fermentation durations were 0, 6, 12, 24 and 48 h.  The  data  collected  were  subjecting  to  analysis of variance (ANOVA) and treatment means compared using the Tukey test at 5% significant level.

Experiment II: Data was similarly analyzed in a two cultivar x eight drying methods factorial in a completely randomized design (CRD) with four replications. Data were subjected to normality tests (Shapiro-Wilks) before analysis of variance (ANOVA) and means compared using Tukey’s test at the 5% significant level.

The effects of fermentation duration on seed moisture content and seed dry weight of the cultivars of African eggplant is presented in Figure 1. As indicated in Figure 1, the analysis of variance of the data gave significant results (p < 0.05) for the individual effects of duration of natural fermentation and cultivar in seed moisture, first count, and accelerated ageing except on germination percentage (Table 1). The interactive effects were significant for all studied variables hence their individual effects is not further discussed.

These percentages were not significantly different when seeds were fermented up to 48 h in both cultivars. However, when seeds were subjected to sub-optimal conditions through accelerated ageing (AA %) (Table 1), seeds fermented up to 12 h proved resilient with 80 - 82% seed germination in both cultivars. In normal eggplant (Solanum melongena L.), Chethan et al. (2013) observed a higher seed vigour index I and II in cucumber seeds subjected to natural fermentation for 24 h. This suggests that fermentation improves seed vigour in African eggplant although the period is less than those reported for eggplant. Rahman et al. (2015) also reported similar results of an improved seedling vigour when eggplant was extracted by the wet method which is similar to natural fermentation. The authors observed a higher seedling vigour index in wet seeds extraction than those extracted without fermentation (Rahman et al., 2015).

Fermentation period did not improve seed germination percentage (Table 1). However, germination capacity differed between the cultivars (Table 1). The difference was only observed among the cultivars, attributable to genetic difference. Results by Franca et al. (2013) for eggplant (S. melongena) on the contrary showed a significant decrease in percentage germination when seeds were fermented up to 48 h.  In the case of tomato, Sabongari and Aliero (2004) reported an improvement in germination when seeds were soaked with maximum germination occurring after 24 h but declined when this period was extended to 36 h. The decreased observed at longer fermentation periods could be due to reduced oxygen content (anoxia) in the water resulting in lower germinative capacity. Cardoso et al. (2001) also reported an increase in germination percentage when yellow passion fruits were subjected to fermentation. These varied results suggest that each crop species behave differently to fermentation. It has been reported that each species has an ideal fermentation period without causing anoxia. Lopes et al. (2001) reported 72 h for pomegranate and Franca et al. (2013) recommended 24 h for eggplant. From this study, the African eggplant seeds can be extracted after harvesting at the optimum time without fermentation. For the purpose of improving seed vigour, up to 12 h of fermentation would be ideal. 

The African eggplant seeds attain physiological maturity at high moisture contents ranging from 46- 52% (Botey et al., 2021b). Therefore, the seeds should be extracted and dried early enough to obtain better quality seeds. The various methods employed for drying seeds of African eggplant resulted in varied levels of moisture content of seeds. Oven drying of seeds at 35, 45 and 50°C for 24 h reduced seed moisture content significantly from initial 47% seed moisture content to 3.4 and 6.6% compared to other types of drying (Figure 2).  Oven drying temperatures applied in this study were able to remove 85.5 to 93% moisture within 24 h of drying. Shade drying removed the lowest moisture content (80.4%) followed by desiccant drying (81.7%). The other drying methods reduced seed moisture down to 8.3 and 9.2% except shade drying for 48 h in cv. Kpando (6.1%).

This is consistent with the observation made by Franca et al. (2013) for eggplant, when the reduction in moisture content was greater for oven drying temperatures. Oven drying was able to dry seeds quickly at higher temperatures because the capacity of air to remove moisture depends on the surrounding temperature and humidity. The higher the temperature, the lower the humidity and the greater the moisture removal capacity of the surrounding air. Seeds dried in the open (sun or shade drying) have the tendency to reabsorb moisture from the surrounding air due to its hydroscopic nature. Ravi et al. (2007) also reported similar results for chilli seeds dried under sun (8.74%) and shade (9.58%). Chilli seeds dried using oven at 35°C gave the lowest seed moisture content of 8.59% (Ravi et al., 2007).

Taking into consideration the recommendation by George (2000) that 6% seed moisture content is suitable for  eggplant  seeds  storage  and  a  general observation that vegetable seeds stored longer at seed moisture content below 8% (Ells et al., 2020), it can be suggested from this study that oven drying at 35-50°C for 24 h can reduce African eggplant seeds to these required moisture levels. The other methods such as desiccant or shade drying period may be extended to obtain similar moisture levels.

With the exception of the first germination count, the different drying methods caused minimal effect on seed germination percentage as all seeds for both cultivars germinated above 70% (Figure 3). Seeds dried in oven at low temperatures of 30 and 35°C had comparable germination values ranging from 85 to 96% as the other types of drying methods. Ravi et al. (2007) reported similar results (83 to 90%) for chilli (Capsicum annuum L.) seeds when subjected to sun, shade and oven drying. When oven temperatures were elevated to 45 and 50°C, seed germination for cv. Kpando reduced by 16.2 to 21%. Cultivar Oforiwa on the other hand reduced by 26% when temperature was increased to 45°C.  This suggest that although oven drying was able to dry seeds quickly and efficiently, high temperatures may cause damage to seed viability and vigour. This results corroborate with Siddique and Wright (2003) who reported a small decrease in germination percentage when peas (Pisum sativum L.) seeds were dried between 40 and  60°C.  Similarly,  seed physiological quality decreased at 60 and 70°C drying temperatures for Adzuki beans (Vigna angularis) (Resende et al., 2012). Drying sweet sorghum at 40°C negatively affects germination (Ullmann et al., 2015) and above 40°C also affects soybean seed quality (Hartmann Filho et al., 2016). This observation is consistent with the view by Marcos-Filho (2005), that fast and excessive drying can affect seed viability.

According to Franca et al. (2013), the effect of drying on seed physiological quality may be observed immediately after drying or during storage as a latent effect. In the case of eggplant, drying did not affect seed germination immediately and after storing for 6 months (França et al., 2013). This could be as a result of the relatively low temperatures (38°C) applied by the authors and the low moisture contents of seeds (4-6%) prior to germination test. The effect of higher drying temperatures is most damaging when seed moisture content is high (Almekinders and Louwaars, 1999). In the present study, the seed moisture contents prior to germination were low. Any possible negative effect could be latent during storage as observed by Araújo et al. (2000). It concluded that drying at temperatures of 50 and 60°C and storing at higher relative humidity were harmful to seed quality of sweet corn (Araújo et al., 2000).  For drying of vegetable seed,     temperature   not   exceeding   35°C   has   been recommended by Harrington (1972).

In the present study, drying at 30°C for 24 h is recommended.

African eggplant seeds do not require natural fermentation during extraction for enhanced seed germination. Fermentation for up to 12 h can however improve seed vigour. All drying methods were able to reduce seed moisture content for storage and maintained seed physiological quality. Oven drying methods however, is recommended for drying seeds meant as it was able to reduce moisture to the recommended moisture level (6%) for  long  term  storage  of  seeds. It is recommended that the latent effect of these two important methods on the seed physiological quality are studied.

The authors have not declared any conflict of interests.