Temperature and Light Effects on Germination Behaviour of African Eggplant (Solanum aethiopicum L.) Seeds

The African eggplant (Solanum aethiopicum L.) is one of the most commonly consumed fruit vegetable in Ghana and other West African countries. In both quantity and value, the crop is third after tomato and onion and before okra (Osei et al., 2010). Unfortunately, the long neglect of this crop by formal crop improvement programmes except in breeding has resulted in absence or lack of formal seed system for the production and supply of quality seeds. Farmers therefore, rely largely on saved and recycled seeds for planting with observed erratic germination behaviour and poor seedling establishment.
       
A preliminary study of the African eggplant seeds obtained from farmers and breeder sources recorded a wide variation in percentage germination under ambient conditions (25±2°C). The germination percentage (normal seedlings) ranged from 0% to 25% while fresh seeds ranged between 53% and 87% (Botey, 2019, unpublished data). This observation could be attributed to several factors that influences germination. Previously, there have been reports on seed dormancy in cultivated varieties of eggplant (Solanum melongena) and Solanum torvum, which are close relatives of Solanum aethiopicum (Yogeesha et al., 2006; Cutti and Kulckyzynski, 2016). This type of dormancy could be either due to seed immaturity at harvest or external stimuli such as temperature and light.
       
Among the various germination factors, temperature is the most prominent environmental factor regulating growth and development of plants (Ghaderi et al., 2008). The temperature at which the maximum germination and emergence occurs tends to differ among crops and within species. Some germinate well under constant temperatures while others do well under alternating temperatures. Kamgar (2009) reported that germination of Solanum nigrum seeds was maximum at constant temperatures of 26°C and 30°C under 16/8 hours photoperiod. It was however, concluded that alternating temperatures of 20/12°C and 30/12°C gives a greater seed germination. Similar reports by Finch-Savage and Leubner-Metzger (2006) for five Solanum species concluded that optimum germination occurs between 28-33°C. In Solanum torvum seeds, maximum germination (86-95%) occurred when seeds were incubated at alternating temperatures of 20/30°C under 16 hours of light and 8 hours of dark respectively (Cutti and Kulckyzynski, 2016).
       
At low temperatures (5-10°C) seed germination was inhibited for Solanum lycopersicum and Solanum nigrum but responded to germination when temperatures increased to 15°C and above (Abdel et al., 2016; Dong et al., 2019). Torres-Gonzalez (2019) also recorded low germination at lower temperatures in studying Solanum betaceum and S. quitoense. Solanum lycopersicum seeds however, exhibited thermo dormancy, with no seed germination at 40°C, similar to an observation for S. nigrum at 35°C and 40°C (Dong et al., 2019). Wilcox and Pfeiffer (1990), had earlier also demonstrated the effect of temperature on time to germinate (days). They reported that eggplant (Solanum melongena) took between 8 days when germination temperatures decreased from 24°C to 12.3°C. Low temperatures of 10°C and 15°C delayed by 160 and 33.3 times to achieve 50% seed germination in several common bean genotypes (Lamichaney et al., 2021).
       
The requirement of light for the germination of seeds of certain plant species prevents germination in places and times not favourable for seedling establishment (Fenner and Thompson, 2005). The light requirement of such seed acts as a mechanism that determines where and when germination takes place and it is important for survival of the plant species concerned, as it prevents stored seed reserves from being depleted. Some seeds germinate equally well in light and darkness, whilst others germinate better under only light or darkness (Teuton et al., 2004). Ochuodho and Modi (2005) reported that both light influenced seed germination of Cleome gynandra and temperature and this differed between the seed lots of same species. Seeds subjected to ≥ 12 h day^-1 at 20°C continuously significantly (p<0.001) reduced germination compared to those exposed to ≤ 8 h day^-1. Thus, it the requirement of these two important factors for germination of especially neglected crops becomes imperative.
       
For a rapid and uniform germination to occur, the seed must be placed in an optimum environmental condition such as temperature, moisture and in some cases light (Fenner and Thompson, 2005). There is currently less information on the temperature and light requirement for the African eggplant seeds and how they interact to influence their seed germination pattern under controlled conditions. This study investigated the effect of temperature on the germination pattern and the influence of light in interaction with temperature on seed germination of African eggplant (Solanum aethiopicum L.) under controlled conditions.

The experiment was conducted at the Seed Physiology Laboratory, Department of Seed Crop and Horticultural Sciences, University of Eldoret, Kenya from March to September 2019.
 
Plant material
 
Seeds of two cultivars of African eggplant grown in Ghana namely cv. Oforiwaa and cv. Kpando were obtained from CSIR-Crops Research Institute, Ghana. These are seeds that have been improved and currently been proposed for released into the seed system. The seeds were produced in 2018/2019 cropping season and about 4 weeks old in cold storage when they were collected for the experiment.
 
Seed weight and seed moisture content determination
 
The thousand seed weight (TSW) was determined from a sample of eight replicates of 100 seeds using an analytical balance and mean weight expressed in grams and multiplied by 10 to obtain thousand seed weight. The seed moisture content was determined using the low constant temperature oven method at 105± 3°C for 24 hours with two replicates of 2g of seeds (Brasil, 2009). Seed moisture content was expressed as a percentage of fresh weight basis (fwb).
 
Seed germination and temperature
 
Four replicates of 25 seeds were placed in petri dishes on two layers of Prat Dumas (90 mm) filter papers, moistened with 5 ml distilled water. Additional water was added to filter paper as and when necessary to keep the filter paper moist. The seeds were incubated in growth chamber (WTB binder BD 400) at constant and alternating temperatures of 15, 20, 25, 30, 35, 20/30, 30/25 and 35/20°C at 8/16 hours of light/dark. Seeds were observed daily for germination for a period of 14 days and considered germinated when the radicle had protruded at least 2 mm from the testa.
 
Seed germination and light
 
To investigate the effects of light exposure periods on germination, seeds were incubated at temperatures of 15, 20, 25, 30/20 and 35/20°C under three different light exposure periods: 24 hours dark (L1) and 24 hours full light (L2) and 8/16 hours of light/dark alternating (L3), for a period of 14 days. The light source was 6 tubes (100cm) of TL-D 36W (Philips EcoBright®).
 
Determination of seed germination pattern
 
Germination capacity (G%) was determined as the proportion of germinated seeds in relation to the total number of seeds sown in the petri dishes. Other seed quantitative traits such as mean germination time (MGT), time to 50% germination (T₅₀), germination index (GI) and mean daily germination (MGD) were calculated using the advanced germination measurement tool developed by Khalid (2018). The relationship dynamics among these seed quantitative traits were measured by Pearson correlation using Statistical Tool for Agricultural Research (STAR).
 
Data analysis
 
The experimental design was a completely randomized design. Variables expressed in percentages were arcsine transformed before subjected to statistical analysis. Data was checked for normality using Shapiro-Wilk test of normality, followed by analysis of variance (ANOVA). Mean separation was carried out using Tukey with a significance level of 5% (a=0.05). The Statistical Tool for Agricultural Research (STAR) was used for analysing the data.

Effect of constant temperatures on seed germination (G%) of Solanum aethiopicum cultivars
 
The total germination of cv. Oforiwa was significantly (p<0.001) higher than that of cv. Kpando regardless of constant and alternating temperature conditions during germination (Fig 1). However, under constant temperatures (Fig 1A), germination was maximum (74%) at 20°C and 25°C (76%) while lower temperatures of 15°C and higher temperatures of 30°C and 35°C gave the least percent seed germination ≤15% for cv. Oforiwa. No germination was recorded at lower (15°C) constant and higher (35°C) temperatures for cv. Kpando (Fig 1C).
 

 
The results is consistent with other solanum species. Kamgar (2009) observed maximum seed germination for S. nigrum at constant temperatures of 26 and 30°C. Similarly, Finch-Savage and Leubner-Metzger (2006) recorded optimum germination for five solanum species at 28-33 constant temperatures. The maximum germination (74%) recorded for cv. Oforiwa (Fig 1A) at 20°C is consistent with that of Solanum ptychchanthum (Zhou et al., 2005) and S. betaceum (Mavi and Uzunoðlu, 2020). At lower temperatures, no germination occurred in both cultivars. This is consistent with other reports for Solanum lycopersicum and Solanum nigrum, where seeds failed to germinate at low temperatures (5-10°C) (Abdel et al., 2016; Dong et al., 2019). This observation confirms that the African eggplant (Solanum aethiopicum L.) as related to the commonly cultivated relative (Solanum melongena L.) is a warm tropical crop and requires relatively warmer environment for germination, characterized by slow germination rates (Chen and Li, 1996).
       
At constant higher temperatures, seeds exhibited thermo inhibition, especially in cv. Kpando. This could be attributed to the inhibitory effects of protein denaturing at higher temperatures (Maguire, 1973) or embryo immaturity as reported for eggplant (Yogeesha et al., 2006). Saeidnejad et al., (2012) also asserted that such observed differences could be due to genetic variability among the seeds. The inability of seeds to germinate at higher than optimal temperatures is attributed to a condition called thermo inhibition (Hills and van Staden, 2003). The current study corroborates with studies of other solanaceous crops. Abebe (1993) observed that seeds of Solanum lycopersicum seeds did not germinate at high temperatures of 35°C but germinated when temperatures reduced to 25°C or 30°C. Abdel et al., (2016) later concluded that the optimal temperature for tomato is 25°C for maximum germination. Similarly, Solanum nigrum failed to germinate at higher temperatures of 35 and 40°C (Dong et al., 2019).
 
Effects of alternating temperatures on seed germination (G %) of Solanum aethiopicum cultivars
 
The current study further showed that seed germination increased significantly (p<0.01), when subjected to alternating temperatures (Fig 1B and D) for both cultivars. The highest seed germination were 95% and 76% for cv. Oforiwa and cv. Kpando respectively at alternating temperatures of 30/20°C. This suggests that African eggplant germinates well under alternating temperatures of 30/20°C, which simulates the tropical temperatures. This result concurs with reports of Chen and Li (1996) and Ullio (2003) that seed germination in Solanum melongena ranges from 20-32°C with the highest germination at 27-30°C. Similarly, Cutti and Kulckyzynski (2016) reported maximum germination (86-95%) for S. torvum at 20/30 under 16/8 hours light and dark periods. Torres-Gonzalez (2019) and Dong et al., (2019) concluded that alternating temperatures of 25/15°C and 30/20°C, gives maximum germination for Solanum betaceum and Solanum nigrum respectively. In a non-dormant seed, temperature alternations may accelerate germination by regulating the balance of growth inhibitors and promoter hormones (Copeland and McDonald, 2001; Ferreira and Borghetti, 2004). This suggests that alternating temperature is effective in increasing the germination percentages of most seeds than constant temperatures (Govindaraj et al., 2017).
 
Effect of light exposure and its interaction with temperature on seed germination
 
The seed germination of cv. Oforiwa was essentially indifferent to light although there was some minor negative effect when germinated at 20°C under full darkness (Fig 2A). This concurs with the report that many cultivated species are indifferent to light to germinate (Miranda et al., 2017). This result is consistent with the germination behaviour of S. nigrum, where light had no effect on seed germination at certain constant and alternating temperatures (Dong et al., 2019). Seed germination of cv. Kpando however, was significantly (p<0.01) affected when placed under full darkness (Fig 2A) or limited light (Fig 2C) at constant temperatures (20°C and 25°C). Germination improved at alternating temperatures (30/20°C or 35/20°C) and full light (Fig 2B) or limited light (Fig 2C).
 

       
This germination behaviour of cv. Kpando could be a temperature or light imposed dormancy (Baskin and Baskin, 1998). It further suggests that seed germination in African eggplant requires an interaction of light and temperature. At alternating temperatures (15/5°C and 20/10°C), germination was greater in darkness (75.5 and 93%) than in light/dark for Solanum nigrum (Dong et al., (2019). Similarly, Ochuodho and Modi (2005) observed that Cleome gynandra seeds lots failed to germinate or were lower <10% under 20°C with 24 h light but improved significantly under alternating temperatures of 30/20°C. Zhou et al., (2005) also observed that seed germination of S. physalifolium was not sensitive to photoperiod and germinated well under 14 hour photoperiod or continuous darkness at 30°C. This suggests that light effects on seed germination depends on the surrounding temperatures. Thus, seed germination in most solanum species will occur at favourable temperature regimes irrespective of the presence or absence of intermittent exposure to light as observed in the present study.
 
Effect of temperature and light on mean germination time (MGT) and time to reach 50% germination (T₅₀)
 
The results of this study indicated a highly significant (α=1%) relationship between temperature and light on mean germination time (MGT) and time to reach 50% germination (T₅₀) (Table 1). The data showed that, under constant lower temperatures (15°C) at full light or alternating light/dark periods, seeds of both cultivars took more days to reach 50% germination (T₅₀) and complete germination (MGT) Table 1. Seeds took 8.3 to 8.5 days to complete germination (MGT) under 15°C but gradually reduced to (5-7 days) when temperature increased to 20°C or 25°C.
 

       
This result concurs with earlier observations by Wilcox and Pfeiffer (1990) that Solanum melongena seeds and pepper seeds took between 18 to 44 days to complete germination when temperature decreased from 16.7 to 14.5°C respectively. At higher temperatures of 21.1 to 24°C however, germination completed in 7-8 days (MGT). Simon et al., (1976) has earlier reported similar trends for cucumber seeds where the time required for 50% of seeds to germinate increased to 14 weeks at about 14°C or below. The rate of seed germination (the reciprocal of MGT) is reported to usually increase as the temperature increase (Al-Ahmadi and Kafi, 2007) attributed to the reactivation processes occurring within the imbibing seeds (Hills and van Staden, 2003). Thus, the temperature effect observed in this study suggests that at lower temperatures, the rate of metabolic activities was retarded or enzymatic activities were inhibited (Kamaha and Magure, 1992; Thygerson et al., 2002). This low temperature condition slows down the diffusion process, which causes a disruption of imbibition and escape of solutes from seeds, which are critical in the protrusion of the radicle, hence delaying germination.
 
Correlation dynamics among seed quantitative parameters of the African eggplant
 
Seed germination percentage represents the number of seeds germinated within a specified period under favourable conditions such as suitable temperature, adequate moisture and in some seeds light as observed for African eggplant in this study. In this study, all the quantitative parameters measured significantly (p<0.01) related to seed germination percentage (Table 2). While mean germination time (MGT), correlated significantly with percent seed germination, giving an indication of the time taken for a seed lot to germinate, it was not strong (-0.481**) as this measure fails to account for the time spread and uniformity of germination (Kader, 2005). Germination index (GI) and mean daily germination (MDG) however, strongly correlated to seed germination 0.934** and 0.979** respectively (Table 2). Mean daily germination indicates the percentage of filled-seed germinating at the end of the test period divided by the number of days of the test (Diavanshir and Pourbeik, 1976). GI in the other hand from this study appears to be the most comprehensive measured parameter as it combines both the germination percentage and its speed in terms of spread and duration (Kader, 2005). These parameters are significant in giving an indication of the seed vigour and stress resistance of the African eggplant seeds studied (Kader and Jutzi, 2001).
 

Temperature affected germination of seeds of African eggplant. Seeds fail to germinate or <10% under low temperatures (15°C) or higher constant temperatures (30°C and 35°C). The highest percentage germination under constant temperatures was recorded at 25°C (76%) and 20°C (74%). The interactive effect of temperature and light exposure during incubation period also improved germination particularly under alternating temperatures and constant full light exposure. Lower temperatures also delayed the time to complete germination (MGT) and time to acquire 50% germination (T₅₀). Maximum seed germination (76%-95%) for the African eggplant was observed when seeds were subjected to alternating temperatures of 30/20°C under alternating 16/8 h light/dark periods. This condition also reduced the time to complete germination and reach 50% germination to 4-6 days.