In India, seeds are traditionally extracted from dry fruits only at the time of sowing. The reason is that the seeds maintain viability for a longer period when they are in fruits than the seeds extracted and stored. Seeds maintain a higher germination value (up to six months) when retained in fruits and stored under ambient conditions. However, the storage of fruits for seed purposes is not commercially feasible for large-scale production, as the storage process needs large areas or storage structures and other infrastructural facilities for the maintenance of the seeds. Chilli seed is a relatively poor storer and its storability is influenced by many intrinsic and extrinsic factors.
Factors influencing the storability of chilli seeds
Several pre- and post-harvest factors influence the longevity of chilli seeds during storage. Some of these are discussed here.
Much variability is found among chilli genotypes for their seed storability. It has been observed that few varieties could maintain viability even up to 18 months and some could only last for a few months when stored under ambient conditions. This sort of cultivar difference could be made use of by the breeders while selecting the parents for the breeding programme.
Stage of fruit maturity
A seed attains maximum quality when it reaches physiological maturity. Harvesting seeds before or after physiological maturity would affect the seed quality in storage. The extent of seed deterioration is much lower if the seeds are harvested after physiological maturity rather than before. In Capsicum, fruits are harvested at the red ripe stage and seeds are extracted either immediately from the fleshy fruits or after the fruits have been dried. Usually red chillies are extracted after drying and wet extraction is followed in bell peppers as the fleshy pericarp of bell peppers takes a longer time to dry and is hence susceptible to fruit rotting. It is suggested that bell pepper fruits should not be harvested before the red ripe stage and seeds should remain in the fruit for a short period of post-harvest ripening to achieve maximum seed germination potential. In-situ priming may occur during post-harvest maturation. According to Dhanelappagol et al. (), full red colour fruits in red chilli produce higher quality seeds compared to half red fruits and thus suggest that for seed purposes, fruits be picked when they have fully ripened on the plant. Demir and Ellis have observed highest seed quality, in terms of viability and seedling growth, in the seeds extracted from the fruits 10—12 days after physiological maturity and stored under ambient conditions. In another study, Oiadiran and Agunbiade observed an improvement in seed germination and seedling development from Capsicum seeds aged for six weeks. These studies suggest that Capsicum seeds require a short ripening period before and after the extraction of seeds.
Seed moisture plays an important role in maintaining seed viability during storage. The moisture content in seeds and fruits at the red ripe stage, i.e. the stage of harvesting, is too high and they cannot be stored unless they are dried down to safe moisture levels. In red chilli, the fruits are dried using traditional methods or by using forced air. Traditional methods of drying result in better quality seeds. Further, the fruits should not be dried on red tiles, asbestos sheets or in shade, ail of which result in the reduction of seed quality due to the development of high temperature. However, under shade drying, the seeds can become infected with mycoflora due to the slow drying process.
Seed moisture and storage temperature
Capsicum seed can withstand desiccation tolerance. Some of the earliest work on storage of vegetable seeds was done by Barton (1935) who found that pepper seed germination began to decline significantly at 10.5% moisture after two years of storage at room temperature. Further, the reduction of seed moisture to 5.2% did not change this decline but increased relative seed germination percentages for a given storage time. However, when the same seeds were held at — 5°C germination was not appreciably affected by storage time regardless of moisture content.
Capsicum seeds can be stored for 18—30 months under ambient conditions if they are dried to 6—8% moisture levels. This suggests that Capsicum seeds can be safely stored under ambient conditions provided the initial seed moisture is less than 8%. Storage of seeds in moisture vapour proof containers requires seeds with a lower moisture content. The seed moisture content of 6% is the upper limit for sweet pepper to be stored in airtight containers, while red chilli seeds could be stored for more than two years in 700 gauge polythene bags if the seed moisture was around 6%.
Seed moisture, or the RH of the storage atmosphere, and temperature are the two important factors that have profound influences on seed viability, both independently or in combination. The higher the storage temperature and RH, the higher the rate of seed deterioration in storage and vice-versa. The effect of one factor can substantially be altered by the other factor. This has been observed by Barton and Garman (1946) in bell peppers where the seeds at 10.5% and 5.2% moisture levels behaved differently during storage under ambient conditions but there was not much difference when stored at — 5°C. Similarly, it has also been shown that the freeze-drying of pepper (C. annuum) seeds to a moisture content of 2.5% is superior to 7.3% during storage at 21-25°Cor40°C.
The method of ultra drying is gaining importance as seed longevity can be increased significantly during storage. However, not all kinds of seeds are tolerant to ultra drying. Nutile (1964) working with vegetable seeds observed severe desiccation injury in the case of C. frutescens L. when seed moisture was reduced from 4% to 1%. In the case of C.annuum L., seeds could retain higher viability in long-term storage for 15 years at 6.1% than at 2.6% moisture. Shen and Qi, working on short- and long-term effects of ultra drying on the germination and growth of vegetable seeds, reported the detrimental effects of drying seeds to less than 2% moisture level and concluded that there was an optimum water content of seeds for safe storage. However, Wu et al. () have reported that the ultra drying of pepper seeds greatly improves their longevity. Based on viability and dormancy studies of tabasco pepper seeds, Sundstrom has concluded that the optimum moisture level for low temperature storage is approximately 10%.
In view of these conflicting reports on the ultra drying effects on seed longevity, an in-depth study is very much required to determine the level of drying and its parameters to prolong the storage life of chilli seeds.
Zhang and Kong have reported that sweet pepper seeds can be stored successfully at higher temperatures and medium moisture levels. For long-term storage of seeds, as in the case of germplasm preservation, the usual practice is to store the seeds at a low or ultra low temperature (liquified nitrogen, — 196°C). Sweet pepper seeds can be stored for very long periods at lower temperatures if they are held in airtight containers. In contrast, Belletti et al. () noticed an appreciable reduction in the germination percentage of pepper seeds (C. annuum L.) at a lower moisture level stored in liquid nitrogen. However, storing seeds at low or ultra low temperature is more cumbersome and expensive and there is every possibility of incurring a change in genetic constitution.
At a given storage moisture and temperature, seed longevity may depend on the storage containers. Packaging materials used for storing seeds include aluminium foil, polythene bags of less than 700 gauge, cloth and jute bags with or without polythene lining, paper covers, friction top cans and rigid plastic containers without a gasket. Capsicum seeds stored in moisture proof containers are found to maintain viability for longer periods than those stored in moisture pervious containers, provided the seeds are dried to a moisture level that is safe for such storage.
Storage pests and diseases
In storage, chilli seeds are attacked by several storage fungi, such as Aspergillus spp., Penicillium spp., Rhizopus spp. These storage fungi cause seed discolouration and loss of viability during storage. Storage fungi are active only when the seed moisture is above 14%. This is possible only when the seeds are stored under ambient conditions. Seeds stored under airtight containers are safe from storage fungi because the initial seed moisture is not more than 6—7%. Seed treatment with thiram has no effect on the viability of chilli seeds when stored in moisture impervious polythene bags, whereas the seeds treated with captan maintain germination at a higher level than untreated seeds. Colletotricbum dematium, an important field fungi which causes anthracnose or die-back disease, is known to be seedborne in nature and seed treatment is the only effective measure for controlling this pathogen. Seed treatment with deltan controls this pathogen very effectively. The seed treatment can be done before storage, as it does not adversely affect the viability of seeds under storage conditions. These results suggest that the chilli seed must be dried to a safe moisture level and treated with suitable fungicides to prolong their storage life under ambient conditions.