Xiang-Hui Kong* and Hai-Ying ZhangAbstract*CorrespondenceBeijing Vegetable Research Centre, PO Box 2443, Beijing 100081, China
This investigation aimed to determine whether ultra-dry storage improves the longevity of vegetable seeds. Seeds of asparagus bean (Vigna unguiculata), cauliflower (Brassica oleracea), Chinese cabbage (Brassica pekinensis and B. chinensis), Chinese chive (Allium tuberosum), cucumber (Cucumis sativus), oriental sweet melon (Cucumis melo var. Makuwa Makino), radish (Raphanus sativus) and tomato (Lycopersicon esculentum) were dried from water contents ranging from 12-5% (depending on species) to about 1.5%, and stored at 40 and 20°C for 52 months. Three different drying strategies were used: drying over silica gel, in a freeze-drier and in an oven at 50°C. While comparable water contents were achieved using all methods, it took only 2 days to dry seeds to about 1.5% using the freeze- and heat-drying methods, and about 3 weeks using silica gel as a desiccant. After 52 months of storage, significant deterioration was observed in all samples stored at 40°C. At this temperature, an optimum water content for storage was observed at about 2-3% water in most species, except for asparagus bean which had an optimum at about 6%, and Chinese chive which showed no evidence of an optimum. Some deterioration occurred in seeds stored at 20°C and it was most apparent at the highest water contents. From these results, we conclude that drying seeds enhances longevity regardless of the drying method as long as seeds are not over-dried.
Keywords: germplasm, seed longevity, seed storage, temperature, water content, freeze drying, silica gel drying, heat drying, ultra-dry, Vigna unguiculata, Brassica oleracea, Brassica pekinensis, Brassica chinensis, Allium tuberosum, Cucumis sativus, Cucumis melo, Raphanus sativus, Lycopersicon esculentum.
Introduction
Germplasm facilities in China and elsewhere are seeking low-cost alternatives to refrigerated storage of seeds (FAO/IPGRI, 1994; IBPGR, 1992). The idea of ultra-drying seeds to water contents less than 5% was introduced as a means to reduce or eliminate the need to store seeds at sub-ambient temperatures (IBPGR, 1992; Ellis et al., 1989, 1990; FAO/IPGRI, 1994; Zheng, 1994).
The potential benefit of ultra-drying to seed longevity must be weighed against the potential risk of damaging seeds by extreme drying. It has been recognized that drying seeds to very low water contents may reduce longevity (Nutile, 1964; Nakamura, 1975; Woodstock et al., 1976). Thus an optimum water content for storage may exist, and the value is likely to be dependent on species (Woodstock et al., 1976). It is also possible that the method of drying affects seed longevity. Many studies that used silica gel to dry seeds to low water contents showed no evidence of reduced storage life when seeds were dried to very low levels (Ellis et al., 1989,1990; Zheng, 1994). However, onion, pepper and parsley seeds aged faster if freeze dried to water contents less than 4, 3 and 5%, respectively (Woodstock et al., 1976,1983).
The purpose of this research was to determine whether drying seeds to very low water contents improved seed life spans, to establish the water content at which there was a limit to the beneficial effects of drying, and to evaluate the efficacy of different drying methods.
Materials and methods
Seeds of asparagus bean (Vigna unguiculata Walp ssp. sesquipedalis L. cv. Zhi-Jiang 28-2), cauliflower (Brassica oleracea L. var. Botrytis cv. Holland snowball), Chinese cabbage (Brassica pekinensis (Lour.) Rupr. cv. New No. 1, heading species and B. chinensis L. cv. Wu-Yue-Man, non-heading species), Chinese chive (Allium tuberosum Rottler cv. Jin-Gou), cucumber (Cucumis sativus L. cv Jin-Yan No. 4), oriental sweet melon (Cucumis melo L. var. Makuwa Makino cv. Long-Tian No. 1), radish (Raphanus sativus L. cv. Xin-Li-Mei) and tomato (Lycopersicon esculentum L. cv. Qiang-Feng) were harvested in 1991. Initial water contents of seeds ranged from 12% (asparagus bean) to 5.2% (cauliflower) and represented the equilibrium moisture contents for seeds in open storage in the Beijing area in the summer (26°C, 75% RH). Initial germination percentages were greater than 95% for all accessions except asparagus bean (60% ±4), Chinese chive (86%±3) and melon (90%±2).
A range of seed water contents was achieved by drying seeds with different desiccating agents for different periods. All seeds were dried from their original water content to water contents that were 2-3% less than the original value by storing them over activated silica gel (silica gel: seed mass ratio = 4:1) for 1 day (cauliflower, B. chinensis and cucumber), 5 days (Chinese chive, B. pekinensis and melon) and 8 days (asparagus bean, radish and tomato). Seeds were then divided into three aliquots. The first aliquot of seeds was freeze dried (TFD-550-5 freeze-drier, Tokara Seisakusho Co. Ltd) for 1 and 2 days. Comparable water contents were achieved in the second aliquot by drying seeds in a 50°C oven for 1 and 2 days. Water contents were matched in the third aliquot by drying seeds over silica gel (silica gel: seed = 4:1) at room temperature (23°C) for 2 and 3 weeks. The silica gel was regenerated daily during drying. Seeds were also dried to a constant weight over silica gel after 4 weeks. After each drying period, seeds were packaged in foil-laminate bags (Hua-Dun Plastics Co., Beijing) and stored at room temperature until all drying treatments were completed. Water contents were determined gravimetrically and are expressed on a fresh weight basis. Dry weights were measured after seeds were heated to 103±2°C for 17±1 h (ISTA, 1993).
The effect of water content on seed aging rates was evaluated by placing packets of seeds dried to different water contents in incubators at 20 and 40°C and assaying viability periodically over 52 months of storage. Germination percentage was measured after seeds were placed between damp filter paper at 20°C (asparagus bean, chive, cauliflower, Chinese cabbage and radish) and 25°C (melon, cucumber and tomato) for 7-14 days (ISTA, 1993). To avoid possible imbibition injury, dried seeds were exposed to 100% RH at room temperature for 2-4 days prior to imbibition.
Results and discussion
The purpose of these experiments was to develop methods to dry seeds and then to determine the interaction between water content and drying method on seed aging rates. Seeds were dried to comparable levels by freeze drying, heating at 50°C and drying over activated silica gel at room temperature. Initially, seeds were dried using silica gel at rates ranging from about 2.6 to 0.2% water per day, depending on species (representative data in Fig. 1A). Drying rates were maintained if seeds were transferred to an oven at 50°C or to a freeze-drier, but were reduced if drying continued over silica gel (Fig. 1A). Neither the water content to which seeds were dried nor the method of drying affected the germination percentage: all seed lots maintained germination percentages close to the initial (undried) value (Fig. 1B).
Figure 1. Change in water content (A) and germination percentage during drying (B) for seeds stored over silica gel at room temperature (solid symbols) or freeze dried or placed in an oven at 50°C (open symbols). Time-courses are given for asparagus bean (squares), Chinese chive (circles) and cauliflower (diamonds), but are representative of the other species studied.
As has been shown previously (e.g. Ellis et al., 1989, 1990; Nutile, 1964; Nakamura, 1975; Woodstock et al., 1976, 1983; Zheng, 1994), the water content at which seeds were stored had a significant effect on germination percentage (Figs 2 and 3). Germination percentages were significantly reduced in all samples stored at 40°C after 52 months (Fig. 2). The reduction was most severe for the undried and driest samples. Germination of undried seeds declined to 0% within 16 months of storage at 40°C for all species except Chinese cabbage (non-heading species). Reductions in germination percentage were also greater in seeds stored at water contents of <4% (asparagus bean), £1.5% (radish, cauliflower, Chinese cabbage (both species) and tomato) or £1% (cucumber and melon) (Fig. 2). Deterioration of extremely dry samples was detected after 16 months in some samples and to a greater extent after 52 months in most samples, suggesting that damage was a result of aging and not of imbibitional injury. Thus optimum water contents for storage were detected at 40°C in all species except Chinese chive, and values ranged from about 5-6% (asparagus bean), 1.5-3% (radish, cauliflower, Chinese cabbage (both species), tomato) and 1-2.5% (cucumber, melon). The values of these optima are similar to critical water contents presented for related species stored at 65°C - 5.6% for asparagus bean (Ellis et al., 1990) and 2.8% for Brassica spp at 65°C (Ellis et al., 1989).
Reductions in germination occurred in some samples stored at 20°C for 52 months, but to a lesser extent than in samples stored at 40°C (Fig. 3). Germination was significantly reduced in undried cucumber, Chinese chive, cauliflower, Chinese cabbage (heading species), melon and tomato seeds after 52 months of storage at 20°C (Fig. 3). Evidence for reduced longevity is also apparent in severely dried samples of asparagus bean, melon and tomato. Because differences among treatments cannot be distinguished in most species studied within the 52 months of the storage experiment at 20°C, it is premature to draw firm conclusions about maximum longevity or optimum water contents for storage.
Within optimum moisture ranges, there appeared to be no relationship between drying method and seed germination after 52 months of storage (notice cluster of points at each moisture level in Figs 2 and 3). Differences in germination rates among treatments appeared to be more a result of experimental variation than a consistent effect of drying. For example, maximum germination in radish seeds was observed in the freeze-dried samples stored at 40°C and in the silica gel-dried samples stored at 20°C. The faster aging rates in seeds dried for 4 weeks over silica gel are more probably a result of the very low moisture contents achieved, than of prolonged exposure to silica gel. Reduced longevity was also observed in seeds dried to very low water contents by freeze drying for more than 2 days (Woodstock et al., 1976).
Summary
The results from these experiments demonstrate that seeds stored at 20-40°C must be dried to prolong shelf life. There are several ways that seeds can be dried, and there appears to be no particular difference in longevity when seeds are dried by either exposure to silica gel, freeze drying or heating to 50°C, as long as the seeds are not over-dried. Advantages of freeze drying and heating treatments arise from the speed at which seeds can be dried (10 times faster than with silica gel), and the cost of drying (less energy may be required to heat seeds at 50°C for 2 days than to regenerate silica gel daily for 2-3 weeks by heating at 100°C).
Acknowledgements
This research was supported by the Beijing Municipal Natural Science Foundation.
References
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Ellis, R.H., Hong, T.D., Roberts, E.H. and Tao K.-L. (1990) Low moisture content limits to relations between seed longevity and moisture. Annals of Botany 65, 493-504.
FAO/IPGRI (1994) Genebank Standards. Rome, Food and Agriculture Organization of the United Nations/ International Plant Genetic Resources Institute.
IBPGR (1992) Ultradry seed storage, p. 40 in Annual Report 1991. Rome, International Board for Plant Genetic Resources.
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Nakamura, S. (1975) The most appropriate moisture content of seed for their long life span. Seed Science and Technology 3, 747-759.
Nutile, G.E. (1964) Effect of desiccation on viability of seeds. Crop Science 4, 325-328.
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Woodstock, L.W., Maxon, S., Paul, K. and Bass, L. (1983) Use of freeze-drying and acetone impregnation with natural and synthetic antioxidants to improve storability of onion, pepper and parsley seeds. Journal of the American Society for Horticultural Science 108, 692-696.
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© CAB INTERNATIONAL, 1998
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