Chenglian Hu, Yunlan Zhang, Mei Tao, Xiaorong Hu* and Chaoyu JiangAbstract*CorrespondenceInstitute of Crop Germplasm Resources, 30 Bia Shi Qiao Road, Chinese Academy of Agricultural Sciences, Beijing 100081, China
The effects of water content and temperature on the viability of seeds in storage were studied using four crop species: wheat (Triticum aestivum), rice (Oryza saliva, Japonica type), millet (Setaria italica) and peanut (Arachis hypogaea). Seeds were dried from about 10% to about 2% water content then packaged in airtight containers and stored at 45°C, room temperature (22.5±7.5°C) and 0°C for 2.5-4 years. At 45°C, there was a clear optimum water content at 2% for peanut, 4% for millet, 4.5% for rice and 5% for wheat. At lower storage temperatures, there was a detrimental effect on longevity when seeds were stored at extremely low water contents, but aging rates at higher water contents were negligible. Thus the presence of an optimum water content was less clear for seeds stored at room temperature and 0°C. Observations of reduced longevity in extremely dry seeds suggest that genebank curators must be careful not to over-dry seeds.
Keywords: genebank, germplasm, seed storage, seed aging, seed viability, temperature, water content, millet, peanut, rice, wheat, Arachis hypogaea, Oryza saliva, Setaria italica, Triticum aestivum.
Introduction
Conservation of germplasm in seed storage facilities necessitates techniques that prolong seed life spans. It is generally accepted that the water content and temperature at which seeds are stored are the most important factors affecting seed longevity. However, the effects of extremely low water content on seed longevity and the possible interaction of water content and temperature are disputed (Ellis et al., 1989, 1990, 1991, 1995; Hong and Ellis, 1996; Vertucci and Roos, 1990, 1993; Vertucci et al., 1994). Ellis and colleagues maintain that the water content achieved by equilibrating seeds at 20°C and 10-13% relative humidity is critical to seed longevity (Ellis et al., 1989, 1990, 1991, 1995; Hong and Ellis, 1996). At water contents above the critical value, seed longevity is a function of log water content. At water contents less than the critical value, seed longevity is unaffected by water content (i.e. there is no benefit or detriment to drying) (Ellis et al., 1989, 1990, 1991, 1995; Hong and Ellis, 1996). On the other hand, Vertucci and colleagues reported a detrimental effect of drying seeds to low water contents (Vertucci and Roos, 1990), and suggested that there was an optimum water content for storage. Based on their experiments at 35°C, they suggested that this optimum corresponded to a relative humidity between 19 and 27% at the storage temperature (Vertucci and Roos, 1990).
The idea that there was an optimum relative humidity for storage also led Vertucci and colleagues to propose that the optimum water content for storage changed as a function of temperature (Vertucci and Roos, 1993; Vertucci et al., 1994). Because water content at a given relative humidity increases as temperature decreases, these authors proposed that the optimum water content for seed storage would increase as the storage temperature was lowered. Ellis and colleagues expressed concern that the critical water content might change with storage temperature (Ellis et al., 1989), but they felt that it would be a minor effect, and experiments storing lettuce and sunflower at temperatures between 35 and 65°C showed no evidence of a temperature effect (Ellis et al., 1995).
The purpose of this study was to confirm the existence of a critical water content for seed longevity in several species, and to determine whether drying to water contents below this value had no effect on longevity or actually shortened seed life spans. Given the possibility of an optimum water content, these experiments were also designed to address whether the value of the optimum changed with temperature.
Materials and methods
Aging rates were studied in seeds of wheat (Triticum aestivum cv. Fengkang 8), rice (Oryza sativa, Japonica cv. Modao 110), millet (Setaria italica cv. Shuilihun), and peanut (Arachis hypogaea cv. 212). Seeds were harvested by the Chinese Academy of Agricultural Sciences, Beijing in 1991. The initial water content of seeds ranged from 9.8% for wheat to 4.4% for peanut. Initial germination rates of seeds ranged from 93 to 99% (Table 1).
Table 1. Water contents and germination percentages of seeds before and after drying. Values in parentheses represent standard deviation of the mean of four (grains) and three (peanut) replicates
|
Water content |
Germination percentage |
Experiment initiated |
|||
|
Crop |
Initial |
Lowest |
Before drying |
After drying |
|
|
Wheat |
9.8 |
2.2 |
99 (1.0) |
93 (1.4) |
Jun 1992 |
|
Rice |
8.8 |
2.0 |
99 (1.3) |
90 (4.6) |
Jul 1992 |
|
Millet |
9.4 |
0.85 |
95 (1.9) |
51 (3.0) |
Aug 1992 |
|
Peanut |
4.4 |
1.0 |
93 (2.3) |
85 (6.1) |
Jul 1992 |
Once the drying phase of the experiment was completed, the percentage germination of seeds at each water content was evaluated. Seed packages were then placed in an incubator at 45°C, in the laboratory at room temperature (22.5±7.5°C), or in a refrigerator at 0°C. Percentage germination was assayed after 12-48 months of storage depending on the species. Prior to being germinated, seeds were hydrated slowly by placing them at 40-60% RH for 7 days and then in an incubator at 80-90% RH for 2 additional days. Germination tests were conducted according to ISTA rules (ISTA, 1985). Seeds were placed on the top of two pieces of wet paper and incubated at 20°C (wheat), 25°C (millet, peanut) or 28°C (rice) for 7 or 10 days. Samples of 100 (grains) and 50 (peanut) seeds were used in each germination assay, and germination assays were replicated four times (grains) or three times (peanut) for each treatment.
Results
The influence of water content on longevity of seeds stored at 45, 22.5±7.5 and 0°C was studied using four crop species. Seeds with initial water contents of about 4-10% were dried to water contents as low as 1% (Table 1). Germination percentages were reduced during the drying process (Table 1). This detrimental effect was apparent during the final drying stage in wheat, rice and millet when water contents were reduced from 3 to 2% (wheat, 96±1 to 93±1% germination and rice, 96±1 to 90±5% germination) or from 1.8 to 0.85% (millet, 92±3 to 51±3% germination).
There was a progressive reduction in germination percentage for seeds at all water contents stored at 45°C (Fig. 1). The decline in germination percentage was fastest for seeds stored with higher water contents (9.8% for wheat, 7 and 8.8% for rice, 9.4 and 5.8% for millet, 4.4% for peanut) and lower water contents (2.2 and 3% for wheat, 2 and 3% for rice, 0.85 and 1.8% for millet, 1% for peanut). This result shows that there is an optimum water content for storage at 45°C, and aging rates are greater if seeds are stored at water contents above or below the optimum. The value of the optimum varied with species, from 2% for peanut, 4% for millet, 4.5% for rice, to 5% for wheat (Fig. 1).
As in the storage experiment at 45°C, germination rates for seeds stored at room temperature and 0°C declined faster in seeds stored at lower water contents (£4% for wheat, rice and millet and 1% for peanut) compared with seeds stored at higher water contents (Fig. 2). The time-course data clearly show a progressive reduction in germination of extremely dry seeds, signifying that the detrimental effect was a result of seed aging and not an artefact imposed by imbibitional damage. Unlike the experiment at 45°C, there was no apparent decline in germination percentage for seeds stored at the highest water contents (Figs 2 and 3). Because deterioration of high-water-content seeds was negligible, the existence of an optimum water content was not clear.
Discussion
The results of this study clearly show that drying seeds to very low water contents has a detrimental effect on seed longevity. The enhanced loss of germination when seeds were stored at very low water contents was apparent at all storage temperatures studied (45-0°C), but was slower at lower temperatures (Figs 1-3). These results imply that ultra-dry storage technology (FAO/IPGRI, 1994) must be used with caution. Germplasm curators must be careful to store seeds at the optimum water content and not to over-dry seeds.
As has been shown previously (Ellis et al., 1989, 1990,1995; Vertucci and Roos, 1990), the optimum (or critical) water content for storage appears to be species-dependent, with oily seeds such as peanut having a lower optimum than starchy seeds such as grain crops (Fig. 1). The water contents determined here for optima at 45°C (2, 4, 4.5 and 5% for peanut, millet, rice and wheat, respectively) correspond extremely well with critical water contents reported by Ellis and colleagues (1.95, 4, 4.43 and 5.26, respectively) for seeds stored at 65°C (Ellis et al., 1989, 1990). However, a major difference in the results of these two experiments is that detrimental effects of over-drying were not observed by Ellis and colleagues.
The results presented here suggest that drying seeds to very low water content does not compensate for the lack of refrigeration (FAO/IPGRI, 1994), and may in fact be counter-productive to efforts to preserve germplasm.
Acknowledgements
This work was co-sponsored by IPGRI and the Chinese Government. The authors wish to express appreciation to Professor Zhou Mingde for her advice, and to Dr Christina Walters for assistance in editing the paper.
References
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1989) A comparison of the low-moisture-content limit to the logarithmic relation between seed moisture and longevity in twelve species. Annals of Botany 63, 601-611.
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.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1991) Seed moisture content, storage, viability and vigour. Seed Science Research 1, 275-279.
Ellis, R.H., Hong, T.D. and Roberts, E.H. (1995) Survival and vigor of lettuce and sunflower seeds stored at low and very low moisture contents. Annals of Botany 76, 521-534.
FAO/IPGRI (1994) Genebank Standards. Rome, Food and Agriculture Organization of the United Nations/ International Plant Genetic Resources Institute.
Hong, T.D. and Ellis, R.H. (1996) A protocol to determine seed storage behaviour, in IPGRI Technical Bulletin No. 1. Rome, International Plant Genetic Resources Institute. ISTA (1985) International rules of seed testing. Seed Science and Technology 13, 299-355.
Vertucci, C.W. and Roos, E.E. (1990) Theoretical basis of protocols for seed storage. Plant Physiology 94, 1019-1023.
Vertucci, C.W. and Roos, E.E. (1993) Theoretical basis of protocols for seed storage II. The influence of temperature on optimal moisture levels. Seed Science Research 3, 201-213.
Vertucci, C.W., Roos, E.E. and Crane, J. (1994) Theoretical basis of protocols for seed storage III. Optimum moisture contents for pea seeds stored at different temperatures. Annals of Botany 74, 531-540.
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