Extending Seed Viability in Groundnut (Arachis hypogaea L.) under Ultra-dry Moisture and Vacuum Storage

Groundnut or peanut (Arachis hypogaea L.) is an important leguminous oilseed crop commonly called as ‘poor man’s nut’ widely cultivated in the tropical and sub-tropical conditions. It is the third most vital vegetable protein source in the world and fourth important source of edible oil crop. India is the leading country in groundnut cultivation and ranks first in area and second in production in the world. Besides high oil content, the crop is valued for its edible seeds. During 2021-22, 101 lakh tonnes was produced in India with the productivity of 1863 kg ha^-1 (agricoop.nic.in). Quick loss in seed viability during storage is an important issue in groundnut seed production. In which, the seeds deteriorate rapidly due to accumulation of specific iso-esterases during ageing process (Aung and McDonald, 1995). Therefore, the availability of quality seed at the time of sowing is a common problem in groundnut cultivation.
       
The longevity of seed is mainly depends on the amount of water it contains and the temperature in which it is stored. In general, the lower the moisture content, the longer the viability. Seed longevity is doubled for every one per cent decrease in seed moisture content or 5^oC decrease in temperature. Therefore, seed storage life can be enhanced considerably by lowering both moisture and temperature. However, moisture content is the key factor that can be lowered for successful seed storage.
       
Ultra-dry storage, also called low moisture content storage, is a technique for decreasing seed moisture content to below 5-6 per cent using different methods and then storing hermetically at ambient, but preferably lower temperatures. Some studies have confirmed that low moisture content storage can not only be used to maintain the quality of seeds, but also improve their storability (Li et al., 2008). Further, ultra-dry seed storage at ambient temperature not only improves their longevity and vigour but also assures to be a cost-effective technique for germplasm conservation (Guang-Hua  et al., 1998). For example, soybean seed deterioration can be decreased at ambient temperature by the ultra-dry treatment compared to non-ultra-dry wherein, the deterioration was higher during storage at room temperature than cold room (Ali et al., 2020).
       
The ability of seeds to withstand severe desiccation is reliant on the drying rate, which has been revealed to affect seed survival after drying (Bailly et al., 2001). Li and An (2006) opined that the water content of seed is a key factor for storage at ambient temperature and 3.5% seem to be the best water content for ultra-dry seeds in Ammopiptanthus mongolicus. Ultra-dry seeds with a moisture content of 3.9% can be effectively used to preserve Handeliondendron bodinieri seeds at low temperature storage for enhanced seed vigour, seedling growth and development (Xie et al., 2021). Similarly, Perez-Garcia ​ et al. (2007) found that the seed preservation through silica gel and low temperature i.e. -5C^o and -10^oC has proved highly efficient for Brassicaceae. Since the groundnut seed possess very poor storability, it can be stored for the period of upto 6-8 months. This creates the problems in seed supply, field stand and germplasm conservation. Therefore, the present study was conducted to extend the storability of the seeds by ultra-dry method. 

The experiment was conducted in the Department of Seed Science and Technology, Seed Centre, Tamil Nadu Agricultural University, Coimbatore, India during 2020-23. The freshly harvested groundnut var. CO 6 pods were collected and decorticated for seed (kernel) storage. The kernels were thoroughly cleaned and graded for uniformity in size. Then, the kernels were dried to different moisture contents viz., 3.5, 5.0 and 6.0% by using zeolite beads. During which, the 200 g zeolite beads were put in the bottom of the desiccator for one kilogram of kernels and placed a wire mesh on it. Then, the kernels filled in porous bag were placed on the wire mesh by avoiding direct contact with beads and the desiccator was closed air tight by applying vaseline. The kernel samples were drawn once in two days for the assessment of moisture content. Also, the kernels placed in the desiccator were turned regularly during sampling for uniform drying of moisture and closed immediately. After attainment of required moisture contents, the dried kernels were vacuum packed in aluminium foil pouches and stored at four different temperatures viz., -20, -5, 5^oC and ambient condition.
       
The stored kernels were evaluated for its viability and vigour at bimonthly intervals by drawing the samples stored at different temperatures. The germination test of this seeds was conducted by placing 400 seeds in four replications with 100 seeds each in sterilized sand medium (ISTA, 2013) and evaluated on final count day i.e. on 10^th day. Seedling length was measured in randomly selected ten seedlings during the evaluation of the germination test. The vigour index was calculated by multiplying the germination percentage and seedling length (Abdul-Baki and Anderson, 1973).
       
Electrical conductivity of seed leachate was measured in the seeds placed for storage (Presley, 1958). In addition, Scanning Electron Microscopy (SEM) images of the kernels stored with different moisture contents at various temperatures were taken by using the SEM image analyzer (Model: FEI QUANTA 250) at the laboratory hub of the Department of Nano Science and Technology, Tamil Nadu Agricultural University, Coimbatore for examining anatomy of the seeds.
       
The data collected were subjected to statistical analysis and the critical difference values were calculated at 5 per cent probability level (Gomez and Gomez, 1984).

Seed storage is one of the major constraints in groundnut because of the presence of high oil in the seeds which ranges from 40 to 56% (Dean et al., 2009). Also, it comprises of about 80% unsaturated fatty acids and 20% saturated fatty acids. In case of unsaturated fatty acids, 42% is constituted by mono unsaturated fatty acids, i.e., oleic acid whereas, 37% by poly unsaturated fatty acids, i.e., linoleic acid (Wang et al., 2015). Thus, the groundnut with high linoleic acid is susceptible to oxidation, leading to unpleasant smell, taste and short shelf life. In addition, the thin and delicate seed coat in groundnut leads to little protection from mechanical damage and slight protrusion of the radicle tip makes the seed vulnerable to damage. Thus, the groundnut seeds are stored along with the shell i.e. pods are stored as such with the seeds to protect from external factors (Navarro et al., 1989). In which, in-shell seeds are better protected from mechanical injuries during handling and invasion of moulds and fungi during storage. Nevertheless, pod storage requires more storage space and also expensive compared to shelled seeds especially when the pods stored under controlled conditions of temperature and relative humidity. Conversely, controlling the deterioration is a major task in the shelled seeds because the seeds are much more sensitive to external environments that cause loss of quality. Also, it is important to perceive better methods and techniques to improve the shelf-life of the shelled seeds by controlling the conditions and environments that cause quality deterioration.
       
Thus, the results from the present study revealed that the germination of groundnut kernels were significantly influenced by moisture contents and storage temperatures. In general, the kernels have recorded decline in germination during storage in irrespective of the seed moisture content and storage temperature. However, kernels with 5.0% moisture content, vacuum packed and stored at -20^oC, -5^oC and 5^oC have maintained the germination above Indian Minimum Seed Certification Standards (>70%) for the period upto 20 months when compared with other treatments (Table 1). Further, the 5.0% moisture content kernels stored at -20^oC and -5^oC would have maintained the 70% germination upto 24 months. However, the kernels with 5.0% moisture content and stored at ambient condition have maintained the required germination (72%) for the period of 6 months and thereafter, hasty reduction was recorded. Similarly, kernels with 6.0% moisture content and stored under ambient condition have showed analogous reduction in germination and recorded 68% in four months storage. But, requisite germination (70%) in 6.0% moist kernels was recorded when they were stored at -20^oC and -5^oC. Conversely, the kernels dried with 3.5% moisture content were not able to withstand the moisture loss and recorded rapid reduction in germination below IMSCS even at four to six months period. Similar results of greater loss in viability in seeds conditioned to very low moisture contents were reported earlier (Jain et al., 2016). Correspondingly, Rao et al., (2002) studied that groundnut seeds hermetically stored at room temperature (23-25^oC) with low moisture content (3.6-4%) could retain high germination. Under cold room condition, seed quality differences in groundnut cultivars were minimal and maintained high quality up to 6 months when compared with ambient condition (Phyo et al., 2004). Nathalia et al., (2020) reported that increasing the water content in groundnut seed reduces the germinative potential and thus, storage of seeds in polyethylene terephthalate packaging with 8% water content showed better results for preserving the seeds. Therefore, groundnut seed quality can be influenced mainly by storage conditions, temperature and relative humidity besides the pest and disease incidence. Desheva (2016) predicted the time of safe storage for peanuts and it was 10 to 20 years for long-term storage conditions with low moisture content of 5±2% in hermetically closed containers at -18^oC. Similarly, Hong et al., (2005) found that the sub-zero storage temperature helped to maintain the long-term storability of the groundnut seeds in laminated aluminium foil packets. In another study, it was found that the groundnut seeds dried up to 4% moisture content with secondary refrigeration (15^oC and 15% RH) retain viability considerably for longer periods and replacing air with vacuum further enhanced seed longevity (Sastry et al., 2007). Ramanadane et al., (2024) found that kernel storage in groundnut is possible if kernels are pre-treated with double spectrum fungicide and stored in super grain bags or HDPE bags. Further, soybean, a problematic seed, could be stored for a period of eight months with better germination and vigour with seed treatment of wood ash @ 5 g/kg in aluminium foil container (Vaghasiya et al., 2023), which showed that the aluminium foil storage has better storage life as recorded in the present study.


       
Similarly, the vigour was reduced with the increase in the storage period irrespective of the seed moisture content and storage temperature. However, the reduction was lesser in the kernels stored at -20^oC, -5^oC and 5^oC with 5.0% seed moisture content (Table 2). The kernels stored at ambient condition with higher moisture have recorded fast reduction in seed vigour due to rapid deterioration. Therefore, the kernels with optimum moisture content of 5.0% have potential to store at lower temperatures. The higher moisture as well as oil in the seed poses the issue of rapid lipid peroxidation during the deterioration process. Chen et al., (2011) found that the ultra-dry storage can improve the storability and membrane permeability along with the increase in the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD).

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Additionally, electrical conductivity of the seed leachate has showed increasing trend with respect to increase in the storage period. In which, the kernels stored with 5.0% moisture content and stored at -20^oC, -5^oC and 5^oC have recorded lesser leachate when compared to other moisture contents and storage temperatures (Table 3). The kernels stored with 6.0% moisture and stored at ambient condition have showed maximum electrical conductivity because of higher leachate of electrolytes from the seed. This might be due to the lipid peroxidative process and further breaking of cell structures which leads to the escape of the cellular constituents into the solutes. Also, the minimum required moisture is essential for functioning of the cellular organelles. In this case, the kernels dried to 3.5% moisture content has recorded with damages in the seed coat and cells of the embryonic axis (Fig 1) which leads to more electrolytes leachate. Also, slightly increased electrolyte leakage was observed initially in the seeds dried to 3.5% moisture content rather 5.0 and 6.0% moisture contents which may be due to cellular damages with more moisture loss. However, the intact seed coat and cells were noticed in the seeds dried to 5.0% moisture content and stored at lower temperatures (Fig 2).

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The groundnut kernels can be stored well with 5.0% moisture content at controlled conditions for the period of 20 months. Therefore, because of the better viability, much reduced space required for storage, insignificant differences in regeneration interval and transportation convenience, conservation of decorticated seeds would be highly cost-effective under the controlled environmental conditions, as compared to pod storage.

The author declare that there is no conflict of interest.