Enhancement of Germination of Oryza sativa L. (Rice) Seeds using Solar Concentrators

For any kind of work, the beginning is very important. A good start most probably results in a better ending as well. In living beings (animals, plants) also, the initial stage is very important. Germination is “the process in which seed embryo starts growing, which leads to the development of seedling” (Jhade, 2019). Seed germination is the most crucial stage in the plant development process. Physiological and biochemical changes that happen during germination directly affect seedling survival, vegetative growth and ultimately yield and quality (Ali et al. 2017; George and Rice, 2016).
 
The vigour, optimal growth and development of plants are dependent on solar radiation within the required spectrum for plant growth (Kumari et al. 2017). Therefore, the Sun is the main energy source of the earth (www.dnr.louisiana.gov). Solar radiation provides the energy for all the metabolic processes in plants (Campilo et al. 2012). All those metabolic processes consist of biochemical reactions. Almost all the biochemical reactions are catalyzed by enzymes (Chaudhury, 2010). If a higher amount of energy is given, the rate of biochemical reactions increases when there are adequate substrates. A previous study by Sritharan et al. (2019) revealed that when solar energy is concentrated at the early growth stage of tomato (Solanum lycopersicum L.) plants, it enhances seed germination, shows higher plant growth, early flowering, early yield and reduced transplanting shock.
 
The most important variable affecting seed germination is the temperature (Milbau et al. 2009). Every species has a minimum and a maximum temperature range where germination can occur. Germination cannot be taken place below and above these extreme temperatures. Botey et al. (2021) reported low germination of Solanum aethiopicum L. seeds at lower (15^oC) and higher (35^oC) temperatures. Gibberellins promote germination. Temperature directly influences on upregulation of genes that control the production of gibberellins. Germination occurs when the embryo elongates and the radical protrudes from the seed coat. This process is facilitated by enzymes. Enzymes degrade endosperm tissue and rupture the seed coat. Chemical signaling induced by temperature regulates the production of enzymes (Finch-Savage and Leubner-Metzger, 2006). Furthermore, Amiri et al. (2017) revealed that the combination of different light qualities and gibberellic acid affect on morphological characteristics of plants.
 
Solar concentrators can be used to get a higher amount of solar energy to the specified surface area. Those are devices that allow the collection of sunlight from a large area and focus it on a small receiver (Muhammad-Sukki et al. 2010). There are different types of solar concentrators such as parabolic concentrators, spherical concentrators, hyperboloid concentrators, Fresnel lens concentrators, quantum dot concentrators, etc. Out of these concentrators, spherical solar concentrators are more useful as larger areas are always exposed to sun rays, can concentrate solar radiation coming from any direction and are less affected by changes in the position of the sun (Devaraj et al. 2016).
 
Many studies indicate that higher solar radiation increases the growth and yield of plants (Deng et al. 2015; Barmudoi et al. 2016). Therefore, the provision of artificial light is becoming more popular in areas where solar radiation is less. But, instead of artificial light, it would be very beneficial to give higher energy to the plants with freely available solar radiation with solar concentrators. But it would be very costly to use solar concentrators to cover a large crop field. Nursery management is very important for any crop as it facilitates better care and attention during the early growth stage. Since the early growth stage is very important for any crop, it can be expected to have a boost of growth rate by concentrating solar energy only at the early growth stage of plants, especially during the germination process. Therefore, if the concentration of solar energy is done only at the nursery stage of the plants, it would be more practical and cost-effective.
 
Rice is one of the most important crops in the world. It is the most popular food crop in the developing world and the staple food of more than half of the world’s population (ricepedia.org). Rice plays a major role in the agriculture sector of Sri Lanka being the staple food. It occupies 34% of the total cultivating area in Sri Lanka (https://doa.gov.lk/rrdi/index.php). Rice contributes 1.6% of the country’s GDP Wijesinghe et al. (2015). It is projected that the demand for rice will increase by 1.1% per year and to meet this task, rice production should increase at a rate of 2.9% per year (https://doa.gov.lk/rrdi/index.php). Rice production can be increased in two ways; increasing the cultivating area and increasing the yield per unit area. Since the availability of arable land becoming lower, the challenge is to increase yield per unit area. Thus, the increment in the production of rice is very important. Also, rice is grown in nurseries to give better care and ultimately to increase the yield. Therefore, in this research study, the germination of rice seeds under two different radiation levels was studied compared to direct sunlight to evaluate the possibility of accelerating the germination.
 

The experimental setup of the research study was established at the site of Meewathura farm, Faculty of Agriculture, University of Peradeniya. This research study was conducted from January to March 2020. The Spherical Solar Concentrator Refracted through Glass Water Media (SSC) designed and fabricated by Sritharan et al. (2019) was placed at the experimental site for concentrating solar radiation (Fig 1). Afterward, the refracted beam of solar radiation at different heights (25, 30, 45, 60 and 90 cm) below the SSC was studied. Solar power was measured using Solar Power Meter (TES 1333R). Then, 60 cm and 45 cm below the SSC were selected for further experiments. BG 252 rice variety was selected and dry seeds (without soaking) were placed in nursery trays (306 seeds per nursery tray) filled with coir dust. Each nursery tray was kept under the SSC and open environment (exposed to direct sunlight). Experimental setup 1 consisted of 60 cm below the SSC (SSC 60 cm) and open environment whereas Experimental setup 2 consisted of 45 cm below the SSC (SSC 45 cm) and open environment. Germination was observed and the percentage was calculated by using the below equation (Eq. 01). Soil surface temperature was measured using an IR thermometer (AS530).

 Germination percentage =

Solar power
 
The variation of solar power with SSC (60 cm) and SSC (45 cm) compared to the open environment is displayed in Fig 2 and 3, respectively. Both solar power under the SSC (60 cm) and SSC (45 cm) were higher compared to the open environment throughout the day. The correlation between cumulative solar power in the open environment and the SSC (60 cm) indicates that solar power under the SSC (60 cm) is 1.2 times higher than in the open environment (Fig 4). Fig 5 shows the correlation of cumulative solar power in a day between the open environment and SSC (45 cm). It indicates that 45 cm below the SSC gives 1.5 times higher solar energy than the open environment. SSC (45 cm) proved 2 times higher solar energy than the open environment from 11 am to 1 pm.  Although it provided higher solar energy, it may not have caused harm as the refracted beam did not concentrate in one place for a longer period. Also, the glass and water media in the SSC might filter harmful radiation. 








 
Temperature
 
Variations of the temperature of the soil surface in the nursery tray within a day in different treatments are shown in Fig 6 and 7, respectively. Although solar power is higher under the SSC, soil surface temperature under both SSC (60 cm) and SSC (45 cm) are lower than open environment throughout the day. Statistical analysis for cumulative temperature in SSC (60 cm) and SSC (45 cm) are displayed in Table 1. That may be due to SSC filter infrared radiation present in the solar radiation or else evaporative cooling may take place at a higher rate as energy is higher under the SSC. Florence et al. (1950) stated different types of glasses can absorb infrared rays in different wavelength regions.







Germination results
 
The seed germination percentage of the treatments with SSC (60 cm) and SSC (45 cm) are given in Table 2. It took 7 days to initiate germination from the sowing as dry seeds were placed without soaking. In each experimental setup, the highest germination percentage could be seen in the SSC compared to the open environment. The germination process involves several biochemical transformations. These transformations are governed by energy. Higher solar energy availability under the SSC might be the reason for higher germination under the SSC. Also, final germination in the SSC (45 cm) is higher than in the SSC (60 cm). Higher solar energy received in the SSC (45 cm) over SSC (60 cm) may be the reason.


         
Temperature is related to energy. Farooq et al. (2005) have observed earlier and synchronized germination as a result of the thermal hardening of indica rice seeds. Farooq et al. (2004) also reported that pre-sowing temperature treatments significantly affect germination and seedling vigor. The reason for the enhancement of germination due to thermal treatment is the pre-enlargement of the embryo (Austin et al. 1969). However, it is contradictory to our results since soil temperatures reduced with the increased influx of energy.  
 
General discussion
 
The reason for this ambiguity may be due to the dispersion of light by the SSC refracted through glass water media most likely towards photosynthetically active radiation (PAR), which is light in the 400 to 700 nanometer wavelength range. Additional support to this hypothesis could be that the ultraviolet spectrum would have been shifted towards longer wavelengths because the recorded energy levels were much higher.  Furthermore, the position of the concentrated light was continuously shifting, thus preventing the scorching of the sprouts. Interestingly, if the intensity of a sun fleck light exceeds the photosynthetic photon flux density (PFD) level of photosynthetic saturation, a part of the extra energy is stored in high-energy metabolite pools and then it can be used for successive CO₂ fixation for a fewseconds (Kirschbaum and Pearcy, 1988; Pearcy et al. 1990). The light intensity of the SSC is much more than the sun fleck effect but may not be limited to the assimilation of CO₂ since the leaf canopy density in the nursery was low.  
 
Nevertheless, constraints may arise like high humidity influencing photosynthetic rate according to Nishimura et al. (2000). The best possible conditions could be to increase CO₂ and reduce humidity in a controlled nursery environment. 
 
It is also important to consider physiological balances during different stages of growth, more so in the initial stages. According to the findings of Yang and Lee (2001), for rice plants to grow, adequate amounts of chlorophylls are acquired to provide sufficient amounts of photosynthetic assimilates and different growth stages need matched amounts of chlorophylls to meet the developmental requirements. The same authors concluded that high temperatures had pronounced negative effects on the growth of rice plants, although chlorophyll content was higher.  In other words, the SSC plants must have reached the fullest potential in maturity for the nursery stage of growth at lower soil temperatures, if there were no constraints on available nutrients.
 

SSC (60 cm) and SSC (45 cm) indicated 1.2 and 1.5 times higher solar power compared to the open environment. But temperature under SSC is significantly lower than the open environment at 5% probability level. Germination percentages were higher in the experiments with the SSC compared to the open environment. SSC (45 cm) reported 86.9% of germination while the open environment displayed 77.5%. Higher solar energy given by SSC enhanced germination and initial growth.