Cucurbit Genetics Cooperative Report 4:13-16 (article 7) 1981
James Nienhuis and R. L. Lower
University of Wisconsin, Madison, WI 53706
One technique commonly used to clean cucumber seeds is to scrape the placental tissue and seed from the seed cavity and allow natural fermentation to occur for a given length of time. At temperatures between 15 and 21°C sufficient fermentation will usually occur within 3 to 6 days (1). However, the effect of the duration of fermentation on the germinability of cucumber seeds is not well documented. In tomatoes, Lower and Cadregari (4) found differences in varietal response to the duration of fermentation. After 12 days of fermentation the percent germination in one variety decreased from 95 to 20%, whereas in another variety germination remained in excess of 90%.
An additional potential problem in seed germination is that some or all seeds of several crop species are dormant at harvest (3). In adapted Cucumis sativus cultivars fresh seed dormancy, though seldom a serious problem, can be overcome by removal of the seed coat, infusion of any of several growth regulators, or dissipated during storage (2, 5, 6, 7).
The objective of this study was to observe the effects of the duration of fermentation and storage time on the rate and percent germination of cucumber seed at optimal (25°C) and suboptimal (15°C) temperatures.
Materials and Methods: Mature fruit were harvested from a random mated heterogenous population of adapted cucumber lines which were field grown at the Hancock Experiment Station, Hancock, WI. Seeds and pulp were scraped from the seed cavity of more than 200 fruit and bulk fermented at 25°C. The fermenting material was stirred daily and bulks of seed were sampled over a 12-day period. The seed samples were washed to float off debris, dried on a screen overnight, and then placed in storage. The storage room conditions were variable, and the temperature ranged from 15 to 25°C; this simulated normal storage conditions of seed used in our breeding program. Germination was tested by removing samples of seed from storage at specified intervals of time and placing 50 seeds on filter paper in 90 mm petri dishes with 10 ml of water. The treatment combination represented a 6x7x2 factorial set, i.e. 6 fermentation times (0, 1, 2, 4, 8, and 12 days), 7 storage times (0, 1, 2, 4, 8, 20, and 30 weeks), and 2 germination temperatures (15 and 25°C). The experimental design was randomized complete block with two replications (petri dishes) per treatment. Germination readings were taken at 3, 6, 9, and 12 days after planting. The percent germination after 12 days and the germination rate (mean number of days to germinate of those seed which germinated) were calculated. Germination was defined as radicle emergence of 5 mm or longer. For the analysis of variance arcsine transformation was performed on percentage data. Significant main effects were subjected to further analysis to fit the appropriate regression model.
Results: Main effects of fermentation. The pH of the fermenting material increased from 4.0 at day 0 to 4.5 after eight days of fermentation. The effect of fermentation duration on the percent germination gave contrasting results at 15 and 25°C. The percent germination at 25°C remained above 97% over the first four days of fermentation and then decreased to 83% after 12 days of fermentation (Fig. 1). The percent germination at 15°C varied about a mean of 3% regardless of the duration of fermentation time (Fig. 1). The effect of fermentation duration on the germination rate at 15 and 25°C was similar. The rate of germination at 25°C was stable at three days over the first four days of fermentation and then slowed to six days after 12 days of fermentation (Fig. 2). The rate of germination at 15°C slowed linearly from 7.5 days initially to 10 days after 12 days of fermentation (Fig. 2).
Main effects of storage time. The seed was stored from September 1980 to May 1981; during that time seed moisture was reduced from 7.3 to 5.0%. The effect of the duration of storage time on the percent and rate of germination gave contrasting results at 15 and 25°C. The percent germination at 25°C remained above 915 regardless of storage time (Fig. 3). The percent germination at 15°C increased from 1 to 18% after 30 weeks of storage (Fig. 3). The rate of germination of 25°C accelerated from 4.1 days initially to 3.2 days after eight weeks of storage, and then slowed to 3.9 days after 30 weeks of storage (Fig. 4). The germination rate at 15°C varied about mean of 8.9 days regardless of storage time (Fig. 4).
Figure 1. Effect of fermentation on % germination of cucumber at 15°C and 25°C
Figure 2. Effect of fermentation on germination rate of cucumber at 15°C and 25°C
Figure 3. Effect of storage time on % germination of cucumber at 15°C and 25°C
Figure 4. Effect of storage time on germination rate of cucumber at 15°C and 25°C
Discussion: Fermentation for up to four days at 25°C adequately separated seeds from pulp for cleaning, without adversely affecting the viability of cucumber seeds from this population. However, excess fermentation beyond four days resulted in reduced germination and a slower rate of germination at an optimal temperature (25°C).
The initial accelerated rate of germination at 25°C and the increased percent germination at 15°C with storage time indicate continued physiological changes in cucumber seeds during storage. Whether these changes simply reflect a response to decreased seed moisture prior to imbibition or dissipation of low temperature germination inhibitors with age or some other phenomena remains to be tested.
The results of this study provide information useful in understanding occasional erratic germination of cucumber seeds which have been subjected to different processing methods. In addition, the results focus attention on how the effects of processing, handling, and storage of cucumber seeds can alter their ability to germinate at both optimal and suboptimal temperatures. These effects will warrant increased attention in the planning and interpretation of future seed germination experiments.
Literature Cited
- Hawthorne L. R. and L. H. Pollard. 1954. Cucurbitaceae. pp. 206-207. In: Vegetable and Flower Seed Production. Blakiston Co., Inc., New York.
- Heit, C., R. W. Robinson and W. Mishanec. 1978. Dormancy of Cucumis species. Cucurbit Genetics Coop. Rpt. 1:36.
- Justice, O. L. and L. N. Bass. 1978. Seed Dormancy. pp. 20-21. In: Principles and Practices of Seed Storage. Ag. Handbook No. 506, USDA/ARS, Washington, D.C.
- Lower, R. L. and C. H. Cadregari. 1965. Effects of fermentation on germination of tomato seed. Veg. Imp. Newsletter 7:12.
- Nelson, J. M. and G. C. Sharples. 1980. Effect of growth regulators on germination of cucumber and other cucurbit seed at suboptimal temperatures. HortScience 15:253-254.
- Shifriss, O. and W. L. George. 1965. Delayed germination and flowering in cucumbers. Nature 208:424-425.
- Watts, V. M. 1938. Rest period for cucumber seeds. Proc. Amer. Soc. Hort. Sci. 36:652-654.