Christina Walters* and Lisa M. HillAbstract*CorrespondenceUnited States Department of Agriculture, Agricultural Research Service, National Seed Storage Laboratory, 1111 South Mason Street, Fort Collins, CO, USA
This special supplement of Seed Science Research contains several papers from Chinese contributors studying the effect of water content and temperature on seed aging. In order to relate water content and temperature relationships to relative humidity, water sorption isotherms of the seeds were constructed at 5, 25 and 45°C. Seeds from the various contributors were shipped to the National Seed Storage Laboratory where the analyses were performed. When sufficient supplies of seeds were available, lipid contents were also determined. Lipid content was linearly related to seed water content at specific relative humidities and temperatures, with correlation coefficients ranging from 0.75 to 0.85.
Introduction
To achieve maximum storage life, seeds should be equilibrated to a critical relative humidity (Roberts and Ellis, 1989; Ellis et al., 1989, 1990, 1992; Vertucci and Roos, 1990,1993; Vertucci et al., 1994). The value of the critical water activity is currently debated. The water content/temperature interactions on seed aging rates are reported in this special issue for over 20 species of seeds from varying cultivars (Chai et al., 1998; Hu et al., 1998; Kong and Zhang, 1998; Shen and Qi, 1998). Water sorption isotherms were constructed for some of these seeds so that the critical relative humidity for storage could be inferred.
Materials and methods
Seeds of wheat (Triticum aestivum cv. Fengkang No. 8), rice (Oryza sativa, Japonica type, cv. Modao 110), millet (Setaria italica cv. Shuilihum) and peanut (Arachis hypogaea cv. 212) were acquired from the National Genebank, Chinese Academy of Agricultural Sciences (courtesy of Hu Xiaorong). Seeds of durum wheat (Tritucum durum cv. 86240), soybean (Glycine max cv. Jidou No. 6), sesame (Sesamum indicum) and flax (Linum usitatissimum) were acquired from Hebei Academy of Agricultural Sciences (courtesy of Chai Jianfang). Seeds of Chinese cabbage (Brassica campestris ssp. pekinensis cv. 87-3), green Chinese onion (Allium fistulosum var. Gaganeum makino cv. Zhangquidacong), cucumber (Cucumis sativus cv Zhongnong No. 5) and pepper (Capsicum frutescens cv. Nongfa) were acquired from the Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences (courtesy of Shen Di). Seeds of asparagus bean (Vigna unguiculata ssp. sesquipedalis cv. Zhi-Jiang 28-2), cauliflower (Brassica oleracea var. Botrytis cv. Holland snowball), Chinese cabbage (Brassica pekinensis cv. New No. 1, heading species; Brassica chinensis cv. Wu-Yue-Man, non-heading species), Chinese chive (Allium tuberosum cv. Jin-Gou), oriental sweet melon (Cucumis melo var. Makuwa Makino cv. Long-Tian No. 1), radish (Raphanus sativus cv. Xin-Li-Mei) and tomato (Lycopersicon esculentum cv. Qiang-Feng) were acquired from Bejing Vegetable Research Centre (courtesy of Kong Xiang-Hui). Rape (Brassica napus), peanut (Arachis hypogaea) and soybean (Glycine max) were acquired from the Bejing Botanical Garden.
Water sorption isotherms were constructed by equilibrating seeds over saturated salt solutions at 5, 25 and 45°C (Vertucci and Roos, 1993). Because seed supplies were limited, the same seeds were equilibrated in successively lower RH chambers, and water contents were determined from the equilibrium weights at each RH. Water contents of seeds were determined gravimetrically and are expressed on a dry weight basis. Dry weights were determined by drying seeds in an oven at 95°C for 5 days.
Lipid contents of seeds were determined when there was sufficient sample to do so. Lipids were quantitatively extracted from seeds by soaking ground seeds in chloroform: methanol (2:1) solution for 10 min (Vertucci and Roos, 1990). The procedure was repeated twice and all soaking solutions combined. The extract was washed with 0.9% NaCl aqueous solution, followed by 2 washes of a 1:1 mixture of 0.9% NaCl in water: methanol solution. The mass of the extracted lipid was measured after the solvent was evaporated off using a rotary evaporator apparatus. The amount of lipid is expressed as a percentage of the dry weight.
Results and discussion
Water sorption isotherms for the various species are presented in Figs 1-4. These isotherms reflect the familiar reverse sigmoidal shape typical of isotherms from seeds (e.g. Vertucci and Roos, 1990; Ellis et al., 1989, 1990, 1992). In some cases, isotherms are not complete because of inadequate seed supply or because seeds had not equilibrated in time to report the results.
The lipid contents of some seeds are given in Table 1. Generally, seeds with high amounts of lipid had low water contents at a given RH and temperature (r2 values for linear regressions ranged from 0.75-0.85 for RH from 5.5-61). This trend is illustrated in Fig. 5 for seeds equilibrated to 31-34% RH at 45, 25 and 5°C.
These isotherms are not meant to be the definitive study of the RH at which seeds were stored in the experiments cited in this supplement. Methods of determining water content varied among the studies. However, the isotherms are useful in interpreting the approximate RH for storage or drying and can be used to identify critical RH. Once the critical water content for storage of a particular seed species is established through experimentation, the relative humidity at that water content can be determined by interpolating isotherms at the storage temperature of the study.
Table 1. Lipid contents of seeds used in this study.
|
Species |
lipid (% dw) |
|
Durum wheat |
13 |
|
Rice |
17 |
|
Millet |
39 |
|
Wheat |
52 |
|
Soyabean |
146 |
|
Rape |
189 |
|
Cucumber |
391 |
|
Peanut |
480 |
|
Sesame |
648 |
The authors wish to thank Drs Hu Xiaorong, Shen Di, Kong Xiang-Hui and Chai Jianfang for contributing seeds, and Dr Zhou Mingde (IPGRI) for coordinating transport of the seeds. In addition, the authors acknowledge the expert technical assistance of Brendan Walsh.
Figure 1. Water sorption isotherms of cereal grains constructed
at 5°C (
),
25°C (¨) and 45°C (·).
Figure 4. Water sorption isotherms of seeds with high oil contents constructed at 5°C (squares), 25°C (diamonds) and 45°C (circles). In the upper left panel, closed symbols represent cucumber seeds (Shen and Qi, 1998), open symbols (5°C only), melon seeds (Kong and Zhang, 1998). In the lower left panel, closed symbols represent peanut cv. 212 (Hu et al., 1998), open symbols, another peanut variety received from the Beijing Botanical Garden.
Figure 5. Relationship between lipid content and water content in seeds of different species (Table 1) equilibrated over a saturated solution of MgCl2 (33.5-31.5% RH) at 5°C (squares), 25°C (diamonds) and 45°C (circles). The lines represent the least-square fit of a linear regressions (r2 values range from 0.78-0.79). Slopes for each line were 0.0013 0.0009 and 0.0008 g HOH/g lipid for 5, 25 and 45°C respectively, and intercepts were calculated at 0.109, 0.085 and 0.072 g HOH/g dw, respectively.
References
Chai, J., Ma, R., Li, L. and Du, Y. (1998) Optimum moisture contents of seeds stored at ambient temperatures. Seed Science Research 8, Supplement No. 1, 23-28.
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. (1992) the low-moisture-content limit to the negative logarithmic relation between seed longevity and moisture content in three subspecies of rice. Annals of Botany 69, 53-58.
Hu, C., Zhang, Y., Tao, M., Hu, X. and Jiang, C. (1998) The effect of low water contents on seed longevity. Seed Science Research 8, Supplement No. 1, 35-39.
Kong, X.-H. and Zhang, H.-Y. (1998) The effect of ultra-dry methods and storage on vegetable seeds. Seed Science Research 8, Supplement No. 1, 41-45.
Roberts, E.H. and Ellis, R.H. (1989) Water and seed survival. Annals of Botany 63, 39-52.
Shen, D. and Qi, X. (1998) Short- and long-term effects of ultra-drying on germination and growth of vegetable seeds. Seed Science Research 8, Supplement No. 1, 47-53.
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.
US Department of Agriculture, 1998
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