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Seed Technology Papers Extension Service (MSU-ES)

April 2021

Methods for Overcoming Seed Dormancy in Rice Methods for Overcoming Seed Dormancy in Rice

James C. Delouche

Nghe T. Nguyen

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Recommended Citation Recommended Citation Delouche, James C. and Nguyen, Nghe T., "Methods for Overcoming Seed Dormancy in Rice" (2021). Seed Technology Papers. 104. https://scholarsjunction.msstate.edu/seedtechpapers/104

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METHODS FOR OVERCOMING SEED DORMANCY IN RICE !J

James C. Delouche and Nghe Thi Nguyen U Mississippi State University

State College, Mississippi

Seed dormancy is a characteristic of many species of Gramineae. The degree and persistence of dormancy appear to be related to the agricultural history of the species. Those species which are of little or no agricultural importance or which have been brought under agricultural management comparatively recently usually manifest a profound, deep­seated, persistent type of seed dormancy. Species which have been under cultivation and subjected to agriculture improvement for a long period generally exhibit only a residual, short-lived seed dormancy. The cereals belong to the latter group.

Seed dormancy in the cereals is largely a varietal characteristic. Seeds of some varieties exhibit a short- I ived, post-harvest dormancy, while others are essentially non­dormant. In no case, however, is dormancy as deep-seated and persistent as it is in some of the native range grasses or even some of the cultivated forage grasses.

The residua I seed dormancy in cerea Is, however, can present prob I ems to seed ana­lysts, particularly when tests have to be made within a few weeks after harvest. Although the need for immediate post-harvest germination tests may be infrequent, specific situations necessitate such tests. A considerable volume of rice seed produced in southern rice-grow­ing areas is exported to Latin American countries. Export schedules usually allow only a 3 to 4 week interval between harvest and shipment. During this short period the seed have to be dried, processed, treated, tested, bagged and approved for certification. Thus, germi­nation tests, which require 14 days, .have to be made on freshly harvested, usually dormant seed. Considerable confusion and delay often result on export shipments since there are at present no effective nor recognized methods for overcoming dormancy in rice seed.

The objective of this study was to develop effective methods for overcoming the dormant condition of rice seed which would not extend the germination test period and which would be adaptable to routine seed testing procedures.

Review of Literature

The Rules for Testing Seed (1) specify an alternating temperature of 20- 30°C., paper media or sand and a 14 day test period for germination tests of rice. An alternate procedure involving flooding of the sand media on the 7th day is provided. The literature on rice seed germination characteristics, particularly as related to the effect of temperature on germination and seedling growth, is voluminous. This review will be concerned only with a few significant papers.

Buenaventura (2) stated that the post-harvest dormancy period of Philippine rice varieties ranged from 0 to 11 weeks and that the persistence and intensity of seed dorman­cy appeared to be influenced by moisture content at harvest. Lin and Tseng (3) reported that it ha.s long been recognized in Taiwan that freshly harvested rice seed are dormant in varying degrees, dependent on the variety. This dormancy, while desirable from a produc­tion standpoint is minimizing "on-ear germination," causes difficulty in germination test­ing of freshly harvested rice seed. They believed that dormancy was closely associated with the lemma and palea and found that soaking the see.d for 10 to 15 minutes in concen­trated sulfuric acid was effective in breaking dormancy.

Mikkelsen and Sinah (4) reported that water soluble substances from rice hulls diffused into the seed and inhibited germination. Crude extracts from rice hulls also in-

1_; Journal Paper No. 1219 ofthe Mississippi Agricultural Experiment Station.

2_; Former AID-Fellow at Mississippi State University from Saigon, Viet Nom.

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42 ASSOCIATION OF OFFICIAL SEED ANALYSTS

hibited germination of sorghum and wheat seed. When the substances were altered or re­duced (sic) by soaking the seed in a chlorine solut ion or by removal of all or part of the hulls, ger mina t ion was complete and rap id .- Soaking of rice seed in a sodium hypochlorite solution ( 1 ga l. of commercial bleachi-ng solution to 100 gals. of water) prior to water sow­ing is recommende d in California as a means of stimulating more rapid germination and im­pr~ved gr9wth:,of seedlings (4). Ulmald i and co-workers (6) fC?und that soaking dormant rice seed in water at 40° to 60°C. for 40 mi nu tes greatly .increased germ ination. Removal C?f the hulls and excision of part of the endosperm also promoted germination . ·

. Weir (7) observed that seed -dormancy in Arkansas-grown rice (principally Bluebonnet 50 and Nato varieties) was related . to time of harvest, moisture content at harvest, and meth­od ofdrying. Seed harvested at rela ti vely high moisture levels and dried with unheated air exhibi.ted the greatest degree of dormancy. Dormancy was successfuliy overcome by germi­nating the seed at 30° C. constant temperature rather than at an alternat-ing 20 -3_0° C . Pre­drying the seed at 40° C. for 4 days portia lly overcame dormancy in so.me sample's but did not completely elj'!ljnate it. After .a storage period of 3 to 4 weeks in the laboratory, the seed were essentially non-dormant and germinated well under a va~iety of conditions.

Materials and Methods

Seeds from Nato and Bluebonnet 50, the two rice varieties which most often exhibit post-harvest seed dormancy in the Southern area, were used in these studies. One seed lot of each variety was obtained by hand harvesting panicles from certified fields near Tunica, Mississippi, in September, 1963. Seed moisture content was about 15 percent at harvest. Severa I other rice seed lots were used to further eva I uate some of the treatments found to be effective. These were obtained from remnant seed samples of certified rice seed submitted to the Mississippi State Seed Testing Laboratory.

After the samples were harvested and threshed, the seeds were cleaned with a South Dakota blower and hand screens. They were then placed in sealed glass jars and stored at 5° c.

Germination percentages of the seed based on a 21-day test started immediately after cleaning were 3 percent for Nato, and 50 percent for Bluebonnet 50. Tetrazolium tests . made at the same time indicated that the germination potential of each of the two lots was near 100 percent.

. Most of the germination tes·ts, including those made after application of specific treatments, were made in general accordance with specifications in the Rules for Testing Seed (1).· Ge'rmination percentages were determined by planting 4 replicates of 50 seed between moist, fold e d, blue germination blotters at 20- 30°C., except as indicated other­wise . Germination counts were made at intervals of 2 days and test periods ranged from 14 to 40 days depending upon the nature of the experiment. The specific treatments and methods eval~ated as to their effectiveness in overcoming seed dormancy in rice are de­s cr ibed be low.

In the soaking treatments, the seeds were soaked for periods of 0 to 72 hours in water, or in various concentrations of ethylene chlorohydrin, potassium gibberellate, and sodium hypochlorite in aqueous solutions. In all cases, the proportion of seed to water or solution during the soaking period was about 100 seed per 100 mi. After the soaking treatment, the seed were drained, blotted to remove exce.ss water or solution, and then planted for germination as previously describe d. The soaking periods were included as part of the germination test period.

The eff~ct of storage on persistence of dormancy was evaluated under three temper­atures: 7° C., 25° C., and 45° C.

The relation of the hulls (lemma and paleo) and per icarp to dormancy was investi-: gated in certain experiments. The hulls were carefully removed from air-dry seed and the caryopses examined under low magnification. Seeds with visible damage to the pericarp

r '.•

I .

FIFTY-FOURTH ANNUAL MEETING 43

were discarded. In some of the tests, the pericarp of air-dry seed was ruptured under mag­nification along the lateral margin of the scutellum with a sharp needle. A razor blade was used to excise the distal tip of air-dry or imbibed seed.

Experimental Results

Soaking Treatments-- The experimental material was initially dormant. Under standard test conditions germination of the Nato and Bluebonnet 50 seed was 5 and 53 per­cent, respectively, after a 40-day test period. Light, prechill treatments, various alternat­ing and constant germination temperatures, and potassium nitrate and gibberellin solutions applied as moistening agents had little effect on germination of the seed.

All the soaking treatments, water, 0.1 percent ethylene chlorohydrin and 0.25 per­cent sodium hypochlorite, increased germination of the rice seed (Table 1). The deeply dormant Nato seed generally required longer soaking periods for promotion of maximum ger­mination than did the Bluebonnet 50 seed. The soaking treatment in water was the least effective of the three treatments, and its effectiveness related to temperature. A 24 hour soak in water at 40°C. completely overcame dormancy of Bluebonnet 50 seed, while a 72-hour soaking period at the same temperature was required for the Nato seed. The 0.1 per­cent ethylene chlorohydrin soaking treatment was most effective. At 40°(., soaking periods of only 6 and 24 hours, respectively, were required to promote complete germination of Blue­bonnet 50 and Nato seed. Soaking the seed in the ethylene chlorohydrin solution for over 24 hours at 40°C. was injurious. The 0.25 percent sodium hypochlorite treatment was about as effective as the ethylene chlorohydrin treatment on the Bluebonnet 50 seed, but somewhat less effective on the deeply dormant Nato seed.

The data in Table 1 represent average germination percentages after a 40-day test period. Thus, the influence of the various treatments on rapidity of germination is not evi­dent. In order to further characterize the effects of soaking, data on germinative responses of the Nato seed to selected temperature-time treatment combinations over the first 15 days of the 50 day period are presented in Table 2.

For the 20°(. and the 72 hour temperature-time combination, the sodium hypochlorite treatment stimulated most rapid germination. The most rapid germination at the other two temperature-time treatment combinations occurred in the ethylene chlorohydrin treated seed. Germination of seed soaked in 0.1 percent ethylene chlorohydrin at 40° C. was essentially complete after only 12 days.

Since the water soak treatment was quite effective in promoting germination of the moderately dormant Bluebonnet 50 seed, its effectiveness was further evaluated on ei..ght samples of certified Bluebonnet 50 seed submitted for testing about 1 month after harvest. Although none of the samples were very dormant, germination of the water-soak treated seed was much more rapid and somewhat higher than that of those germinated under standard test conditions (Table 3).

Mechanical Treatments

~. It has been suggested that germination of rice seed is inhibited by substances lo-cated in the lemma and paleo (3, 4). A series of mechanical treatments were conducted to eval·uate this hypothesis. They were as follows: (a) the seeds were carefully hulled; (b) the seeds were hurled and the pericarp ruptured with a fine-pointed needle along the edge of the scutellum; (c) the seeds were carefully hulled and 1 mm. of the distal end was excised; (d) a very small portion of the lemma over the embryo was removed; and (e) a very small portion of the lemma over the embryo was removed, and the pericarp was ruptured along the edge of the scutellum with a needle inserted through the opening in the lemma. After the various operations were performed, the seeds were planted between moist blotters.

The most effective treatments were those involving rupture of the pericarp near the ·edge of the embryo (Table 4) . . This was true whether the seeds were completely hulled

44 ASSOCIATION _OF OFFICIAL SEED ANALYSTS

Table l. Effect of soaking solution, soaking period, and soaking temperature on emergence of Nato and Bluebonnet 50 rice seed. Figures percentage germination after 40 days.

Soaking Temp. Soaking 2eriod {hours2 solution oc. 6 12 24 48

Nato rice

Water 20° 10 36 300 23 77 40° 1+8 92

Sodium 20° 85 93 hypochlorite (0.25%) JOO 82 87

40° 82 96

Ethylene 20° 55 83 chlorohydrin (0.1%) 30° -- 87 97

40° 52 68 99 86

Not treated 5%

Bluebonnet 50 rice

Water 20° -- 71 98 30° -- -- 88 97 40° 96 98

Sodium 200 98 99 hy pochlorite (0.25%) 30° 98 98

40° 98 98 98 97

Ethylene 200 98 98 chlorohydrin (0.1%) 30° -- 98 98

40° 99 99 98 89

Not treated 53%

72

81 80 95

92 86 92

95 97 68

98 96 98

97 98 98

97 97 87

FIFTY-FOURTH ANNUAL MEETING

Table 2. Rapidity of germination of Nato rice seed following soaking treatments in water, 0.1 percent ethylene chlorohydrin, and 0. 25 percent sodium hypochlorite under three temperature­time combinations.

Soaking Days after 2lanting treatment 0 6 9 12 15

2 0° C • - 7 2 hrs . Water 0 3 9 13 50 Sod. hypochlor. 0 8 75 88 92 Et. chlor. 0 7 64 75 95

30° C. - 48 hrs. Water 0 8 16 22 42 Sod. hypochlor. 0 22 78 84 85 Et. chlor. 0 10 86 92 97

4 oo C • - 2 4 hrs : Water 0 8 13 21 25 Sod. hypochlor. 0 44 63 75 80 Et. chlor. 0 78 94 96 98

Table 3. Effect of a water soak treatment on rate of germination and percentage germination of Bluebonnet 50 rice seed samples.

Days After Lot No. Treatment Plantil}g_ l 2 3 4 5 6 7 8

Standard 6 54 48 46 48 51 46 51 50 test 14 80 87 84 88 85 84 76 85

Water soak 6 84 83 81 80 82 88 82 84 40° c. - 24 hrs. 14 87 88 88 88 89 90 88 91

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46 ASSOCIATION OF OFFICIAL SEED ANALYSTS

Table 4. Germination percentages of Nato rice seed subjected to various mechanical treatments.

Days Treatment 4 6 8 12 16 20

Hulls intact 0 2 4 5 5 5

Hulls removed 6 16 18 28 30 34

Hulls removed, 72 98 pericarp nicked along scutellar margin

Hulls removed, 12 20 28 44 60 distal end of seed severed

Portion of lemma 2 8 20 28 28 28 removed over embryo

Portion of lemma 28 84 98 removed over embryo, pericarp nicked

- - - ·

Table 5. Influence of storage condition on loss of dormancy in Nato rice seed.

Storage condition Time

Weeks 0 1 2 3 5 7 9 11

25° c. 3 10 24 46 67 75 88 97 45° c. 3 43 80 90 95

Months 0 1 2 3 5 7 9 11

7° c. 3 3 4 6 12 14 18 26

30

5

35

72

30

13

96

13

36

Table 6.

Treatment

FIFTY-FOURTH ANNUAL MEETING

Percentage germination of remnant dormant Nato rice seed from 80 day blotter tests after sealed storage for periods up to 90 days at room temperature.

Time (days) 80 + 0 80 + 30 8 + 60 80 + 90

Standard test 3 9 27 30

24 hr. soak 98 98 98 98 in 40° C. water

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before rupturing the pericarp, or only a small portion of the lemma (less than 5 percent of the total hulls) was removed. Excision of the distal tip of the hulled seed was next .in effective­ness. When the seeds were only hulled, or a small opening was made in the lemma over the embryo, germination was slow and reached on'ly 30-35 percent in 30 days.

Storage Treatment

Seeds of the Nato variety were stored at 7° C., 25° C. and 45° C. in open bottles for periods up to 13 months. Storage temperature had a pronounced effect on rate of loss of dormancy (Table 5). The dormancy period persisted for 5 weeks at 45° C. and for 11 weeks at 25° C. At 7° C., dormancy was lost very slowly; the seed germinated only 36 percent after 13 months.

During these studies it was observed that remnant dormant seed from germination tests of up to 80 days duration were firm and free from decay at the end of the period. Some of these seed were collected, sealed in a small bottle without drying, and stored at room temperature for 90 days. The moisture content of the seed during the storage period was about 32 percent. The results of germination tests made at 30-day intervals during the storcge period are given in Table 6. After a total time of 170 days at 32 percent moisture and temperatures averaging about 25° C., the seed germinated 98 percent when given the water soak treatment.

Discussion

Dormancy in rice seed is manifested as a complete inhibition of germination or as an increase in the time required for germination. It is transitory, persisting for only a few weeks after harvest under the usual conditions of post-harvest storage. Apparently, considerable "after-ripening" or loss of dormancy occurs during harvesting, drying and processing.

Samples of rice seed submitted for testing seldom contain more than 10 to 20 percent dormant seed. In the Mississippi area, dormancy problems are almost exclusively associated with seed of the Bluebonnet 50 and Nato varieties. Dormancy problems usually arise in con­nection with seed scheduled for export, since such seed often have to be in transit within a month after harvest. During this period the seed have to be dried, processed, treated, bagged, tested and approved for certification. Certification ogene ies, recognizing the difficulty in meeting this schedule, permit germination tests requiring 14 days, to be made on bulk seed, i.e., unprocessed seed. The minimum germination standard for certification of rice sampled in bulk in Mississippi is 85 percent. Ten to 15 percent dormant seed combined with the usual 5 to 10 percent dead seed or abnormal seedlings are sufficient to render rice seed un-

48 ASSOCIATION OF OFFICIAL SEED ANALYSTS

acceptable for certification on the bas is of a first test. Retests or tests on samples of pro­cessed seed usually eliminate the problem, but the additional time required is often not available. Thus, the first germination test must be based on a presumption of the presence of dormancy and an appropriate dormancy-breaking treatment properly applied.

No special provisions for germination testing of dormant rice seed are incorporated in the Rules for Testing Seed (1). This has led to development and use of a variety of tech­niques for breaking dormancy in laboratories concerned with testing rice seed. None of these .nethods has proved to be entirely satisfactory.

The problem outlined above, although largely confined to rather unusual circumstances in this country, is often very real and of considerable significance in rice-growing countries in Southeast Asia and the Far East. Two crops of rice are frequently grown "back to back" with only a 1 to 3 months intervo I between them. When seed of the first crop are used for seeding the beds for the second crop, seed dormancy can then pose a serious problem re­sulting in slow emergence and non-uniformity of seedlings for transplant.

The results obtained in this study on ·deeply dormant rice seed clearly demonstrate the facility with which the dormant condition can be -overcome. Soaking treatments of rice seed in water, 0.1 percent ethylene ch lorohydr in, and 0.25 percent sodium hypochlorite under appropriate temperature-time combinations, completely overcomes the dormant condition without increasing the germination test period. Although soaking treatments in ethylene chlorohydrin or sodium hypochlorite solutions appear to be necessary for maximum effec­tiveness on deeply dormant seed, the water soak treatment for 24 hours at 40° C. is s-uffi­cient for promotion of complete germination of samples exhibiting the slight degrees of dormancy likely to be encountered in seed testing. Germination tests of even deeply dor­mant seed can be completed in only 4 to 6 days by rupturing the pericarp along the margin of the embryo of individual seeds.

Although this study was not designed to specificaHy investigate the mechanism of dormancy in rice seed, some general observations con be adequately supported by the data. The suggestion (3, 4) that germination inhibitors located in the hulls ore involved in dor­mancy does not appear to be applicable to the varieties used here. Careful removal of the lemma and paleo did not overcome dormancy. Germination percentages were increased by hulling, but this response con probably be attributed to breaks in the pericarp incidental to removal of the hulls. When the seeds were carefully hulled and the pericarp was ruptured near the embryo, germination was prompt and complete. Rupturing the per icorp through a small opening in the lemma over the embryowas equally effective in overcoming dormancy. In this treatment about 5 percent of the total hull was removed without disturbing the inti­mate contact of the remainder of the hull with the seed.

Recently Roberts (5) advanced an interpretation for the mechanism of dormancy in rice seed which appears to most closely fit the results of these studies. He hypothesize~ that some oxidation reaction (possibly non-enzymic) is necessary before germination proc­esses con be initiated. Since conditions in the seed ore relatively anaerobic because the seed covering (pericorp) restricts the inward diffusion of oxygen, loss of dormancy is a rel­atively slow process. Any treatment which increases the availability of oxygen for the reaction such as rupturing the pericorp, storage in on oxygen enriched atmosphere or at a high temperature, and provision of alternate hydrogen acceptors to decrease the respiratory competition for oxygen, increases the rote of loss of dormancy.

One other observation should be noted here. Dormont rice seed hove the capability to survive long periods under the most adverse of conditions. Dormont seed remained alive for a year in contact with moist blotters ·at 30°C. Firm, dormont seed removed from moist blotters after 80 days and then sealed in a glass bottle (without drying) for an additional 90 days were still olive at the end of the 170 day period. Moisture content of the seed was obout-32 ·percent. This seems incredible when one considers that non-dormont seed at. a moisture content just below the minimum for -germination (28- 30 percent) will die in a month or less ~t 30°C. It appears that whatever inhib i ts the genninotion processes in dor­mont seed also inhibits the processes of deterioration. Investigation of this phenomenon could lead to extremely sign ificont developments in the area of seed storage and seed dete­

rioration.

FIFTY-FOURTH ANNUAL MEETING 49

Summary

Seed dormancy in rice persisted for only a few weeks after harvest under normal conditions of storage. When the dormant seed were stored at 7° C., however, seed dormancy persisted for over a · year.

Soaking rice seed in water, 0.1 percent ethylene chlorohydrin, and 0.25 percent s~dium hypochlorite at 40°C. for 24 to 72 hours effectively promoted germination of deeply dormant seed lots. The ethylene chlorohydrin treatment appeared to be the most effective. A 24 hour-soak in water at 40°C. was sufficient for promotion of maximum germination in s I ightly dormant seed lots.

Removal of the hulls (lemma and paleo) from rice seed only slightly increased ger­mination percentage of dormant seed. Rupturing t.he pericarp along the margin of the embryo induced rapid and complete germination whether the hulls were removed or not. On the basis of these responses, it is suggested that the mechanism of seed dormancy in rice prob­ably involves a restriction of gaseous exchange imposed by the pericarp-seed coat complex rather than diffusable germinative inhibitors located in the hulls.

Literature Cited

1. Association of Official Seed Analysts. 1960. Rules for testing seeds. Proc. Assoc. Off. Seed Anal. 49 (2): 1-71.

2. Buenaventura, M. R. 1956. Dormancy periods of promising rice varieties. Philippine Agr i. 39:558-570.

3. Lin, C. 1., and S. T. Tseng. 1959. Breaking dormancy of rice grain by sulfuric acid treatments. National Taiwan University, College of Agriculture, Taipei, Taiwan, China. Misc. Pub. No. 11.

4. Mikkelsen, D. S., and M. N. Sinah. 1961. Germination inhibition in Oryza sativa and control by preplanting soaking treatments. Crop Sci. 1:332-335.

5. Roberts, E. H. 1964. A survey of the effects of chemical treatments on dormancy in rice seed. Physiol. Plant. 17:30-43.

6. Uma ldi, D. L., M. B. Barker, and R. C. Dumlao. 1960. A preliminary study on the cancellation of the dormancy period of rice seed. Philippine Agri. 44:279-289.

7. Weir, H. L. 1959. Some observations on the germination of rice seed. Proc. Assoc. Off. Seed Ana I. 49:80-81.