Seed dormancy - Springer Link

Seed dormancy: General survey of dormancy types in seeds, and dormancy imposed hy external agents. By

Lela V. Barton. With 4 figures.

A. General survey of dormancy types in seeds. Two basic kinds of dormancy have been distinguished by BüNNING (1947): ectogenous, influenced by external factors such as light, temperature, water, etc.; and endogenous, conditioned by the internal constitution of the seed. Whether seeds possess either one or both of these types of rest depends upon two main factors, environmental influences and heredity. It should, however, be realized that the two influences are mutually dependent and sometimes cannot be separated.

I. Embryo dormancy. The most genuine case of seed dormancy is when the embryo of the mature seedisnot capable of development even under optimal moisture and temperature conditions because of the presence of some physiological "block". In very many cases, however, the isolated embryo germinates quite readily and produces a normal seedling. In such cases one might conclude that the dormancy of the seed is determined exclusively by the seed coat. This, however, is not always correct. When the dormant seed is subjected to the action of some specific environmental factor, e.g. light, which takes its effect within the embryo, the latter becomes capable of rupturing the coats and germinates. Thus, we must assume that while the resistance to germination may be localized in the coat, the embryo of the dormant seed is at an actively level insufficient for overcoming this resistance. The purpose of this discussion is to provide a brief, comparative survey of the various cases of embryo dormancy; for physiological details the reader is referred to other chapters of this volume 1 •

1. Seeds requiring low-temperature after-ripening. a) Seeds with a single cold-requirement. Different types of embryo dormancy exhibited by varied and numerous plants have been reviewed previously (BARTON and CROCKER 1948, ÜROCKER and BARTON 1953). One of the best-known types is that where germination 1 "Dormancy in seeds imposed by the seed coat" by L. V. BARTON, pp. 727-745; "Temperature and seed dormancy" by P. STOKES, pp. 746-803; and "Light and seed dormancy" by M. EVENARI, pp. 804-847; "Endogenous inhibitors in seed germination and dormancy" by P. F. WAREING, pp. 909-924. See also chapter "Ruhezustände bei höheren Pflanzen, Induktion, Verlauf und Beendigung: Übersicht, Terminologie, allgemeine Probleme" by A. VEGIS, PP· 499-533.

A. Lang (ed.), Differentiation and Development © Springer-Verlag Berlin Heidelberg 1965

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Seed dormancy: General survey of dormancy types in seeds.

is brought about by a low-temperature pretreatment, generally known as "stratification" (BARTON 1939, FLEISCHER 1851). This process consists of placing the seeds in some moist medium, such as granulated peat moss, sand, or soil, and exposing them to temperatures of 1°, 5°, or 10° 1 for from one to several months, depending upon the kind of seed. At the end of the low temperature or afterripening period, the seeds will produce seedlings promptly at a higher temperature, such as the greenhouse. Seeds of water plants as weil as those of land plants are affected. Numerous species with seeds responding to low-temperature after-ripening occur among the conifers, Liliaceae, Iridaceae, Rosaceae, Gelastraceae and some other families. The phenomenon has been known to horticulturists, foresters and agriculturists for several centuries if not longer, the first published account being perhaps in a book by EVLYN in 1664. Some physiological studies appeared in the latter part of the 19th century, but a vigorous and systematic investigation was started only with the work of W. CROCKER and W. E. DAVIS in the period of 1915-1930.

b) Epicotyl dormancy. Certain seeds produce a root readily when exposed to ordinary temperatures for germination, but they fail to produce shoots if kept continuously at those temperatures. In these cases, it is necessary to expose the germinated seed, with the root system beginning to develop, to low temperature (1 ° to 10°) for a while in order to after-ripen the epicotyl or the bud that forms it. A nurober of the lilies (such as Lilium auratum Lindl., L. japonicum Thunb., and L. superbum L.: BARTON 1936b), Viburnum spp. (GIERSBACH 1937b), and the tree peony, Paeonia suffuricosa Haw. (BARTON 1933), exhibit this type of dormancy. All seeds ofthistype have been known as "two-year" seeds. However, they can be made to produce seedlings the spring after harvest by planting them in flats in a greenhouse, where they be should allowed to remaiu until root systems are formed (usually 2 to 6 months). The flats should then be transferred to low temperatures for one-half to four months depending on the species. Practically, these seedlings may be produced by spring or early summer planting. For example, seeds of Lilium auratum planted outside in April or June in the region of Yonkers, N. Y., gave good seedling stands the following spring. The warm months during the summer permitted the emergence of the radicles and the development of the root systems, while the succeeding cold of the winter months broke the epicotyl dormancy. Plantings made as late as August produced very few seedlings the following spring, since the warm period was too short to permit root formation. The flats were kept in a board-covered frame over winter.

c) Double dormancy. Gonvallaria majalis L. and Smilacina racemosa (L.) Desf. seeds show epicotyl dormancy of a rather specialized type in that the period at low temperature must be given, not merely after root production, but after the shoot has started to develop and has broken through the cotyledonary sheath (BARTON 1942). Exposure to low temperatures at earlier developmental stages is without effect in breaking epicotyl dormancy. Furthermore, low temperature pretreatment of the moist seeds increases the root production from seeds of Gonvallaria and is essential to root formation in Smilacina when plantings are made in soil in the greenhouse. These facts point to a double dormancy. An initial treatment of the moist seeds at low temperature is required to after-ripen the root, followed 1 Throughout this review, temperatures are given in degrees centigrade (O C).

Embryo dormancy.

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by a period at high temperature during which the root system grows and the shoot develops until it has broken through the cotyledonary sheath. A second period at low temperature is then necessary to after-ripen the epicotyl, followed by a second period at high temperature during which the first green leaves appear above soil. This double dormancy, overcome by two separate cold periods, one before and the other after germination to form a root, is also characteristic of Trillium grandiflorum (Michx.) Salisb., T. erectum L., and Caulophyllum thalictroides (L.) Michx. (BARTON 1944).

d) Dwarfs from non-after-ripened embroys. Many of the seeds which are favorably affected by low temperature pretreatment exhibit only a partial dormancy of the embryo. When these embryos are removed from the coats and placed in germinators, they do grow, but very slowly and the tops produced by such seedlings are dwarfed (FLEMION 1959). This condition has been reported for the Japanese rase tree (Rhodotypos kerrioides Sieb. & Zucc.), peach (Prunus persica Batsch), apple (Malus spec.), and hawthorn (Crataegus spp.), all of which belong to the rose family. If grown at 20° or above, they remain dwarfs for six months to a year an a half, after which one or more buds usually start to grow in a normal manner. On the other hand, if, at any time, the dwarfs are exposed to 5° or lower for two months, the secondary dormancy is overcome and vigorous growth follows promptly upon removal to a higher temperature.

2. Light-dependent seeds. A second large group of seeds are those in which germination is affected by light. Lack of light may prevent or delay germination in some seeds. The first case of this type was described by ÜASPARY in 1860. Seeds of this type will not sprout if covered with soil to a depth which excludes alllight. In other seeds, germination is, on the contrary, prevented or delayed by light, as first reported by HEINRIOHER in 1903. These seeds will remain dormant if shallow plantings keep them exposed to too much light. The phenomenon of light effects in seed germination has been called "photoblastism" and the two response types "positively" and "negatively photoblastic" seeds, respectively. The difference between the two types is, however, a quantitative rather than a qualitative one. The spectral dependence in both is the same, near-red light (maximal effectiveness about 6,600 A) having a promotive action and far-red light (maximal effectiveness about 7,350 A) an inhibitory one, the two actions being mutually reversible. However, while positively photoblastic seeds have a relatively high near-red sensitivity the negatively photoblastic ones have a relatively high far-red sensitivity. Therefore, white light in the former acts as near-red, in the latter as far-red. The response to light may furthermore depend greatly on such factors as age of the seed, temperature, presence of certain solutes, etc. and may in fact under some conditions be positive and under others negative. While low-temperature after-ripening is usually a sine qua non requirement for germination and even isolated embryos are incapable of normal development, dormancy in light-dependent seed seems frequently to be less absolute and profound. In many seeds, light dependency is evident only under particular conditions of temperature, etc., and germination even under these conditions may be, to some extent at least, obtained by other means, e.g. a temperature change, a period of cold, or certain chemical treatments. There are, however,

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Seed dormancy: General survey of dormancy types in seeds.

seeds, e.g., those of the Gesneriaceae (FIGDOR 1907) and of Lepidium virginicum (TooLE et al. 1955a, b), which do not germinate at all in the absence of the proper light regime, although the other measures may further promote germination. Very many light-sensitive seeds, whether positively or negatively photoblastic, germinate promptly and normally once the seed coat is removed or broken. However, in some species this does not seem the case, i.e. the dormancy condition is .determined exclusively within the embryo itself (e.g., in a strain of Oucumis melo and in Nigella arvensis; AxENTIEV 1930).

II. Hard coats and emhryo dormancy. Among seeds requiring low-temperature after-ripening, an increasing number are being found to possess seed coats which are impermeable to water. Since dormancy is broken at low temperatures only after the seeds have imbibed water, it is necessary first to remove the coat effect and then pretreat at low temperature in order to after-ripen the embryo. The coat restriction can be removed by treating with concentrated sulfuric acid or by mechanical abrasion before placing the seeds in a moist medium at low temperature for breaking the dormancy of the embryo. Degeneration of the coats can also be effected by bacteria and fungi when the seeds are planted in a moist medium at a warm temperature (20° to 30°) (PFEIFFER 1934). The length of time necessary for this action varies from 30 to 120 days depending upon the species. For example, seeds of Orataegus crus-galli L. should be planted and kept at a temperature of about 25° for 120 days and then transferred to 5° for 180 days after which seedlings will appear above ground upon transfer again to a warm temperature (FLEMION 1938). The initial period in the moist medium may be eliminated if the dry seeds are treated for two hours in concentrated sulfuric acid before planting. Among other seeds with both impermeable coats and dormant embryos are those of Oornus canadensis L. and Taxus cuspidata Sieb. & Zucc. (BARTON 1939), and Symphoricarpus racemosus Michx. (FLEMION 1934). For the bearberry (Arctostaphylos uva-ursi [L.] Spreng.) (GIERSBACH 1937 a) neither a period at high temperature nor sulfuric acid alone is sufficient to render the seed coats permeable, butthebest results follow the use of both of these methods (H 2 S04 treatment for 2 to 4 hrs., depending on whether entire nutlet stones, stone pieces, or single seeds are used; followed by 30 to 60 days in a moist medium at 25°). Seeds of this category, i.e., possessing both impermeable coats and dormant embryos, can be handled commercially by planting in the spring or early summer in a temperate climate. Seedlings will appear the following spring.

111. After-ripening in dry storage. 1. Occurrence. Some seeds which fail to germinate are dormant only in the sense that they require certain, specific conditions for sprouting. Seeds of a surprising number of plants are dormant when they are freshly harvested, the dormancy being measured by their requirement of such special conditions. A period of dry storage is usually sufficient to prepare these seeds for germination; in many, a period of low or of high temperatures has the same effect. The first report of such behavior is due to AT'I'ERBERG (1899, 1907); a thorough analysis was made by GASSNER (1910b) in Paspalum dilatatum. ÜROCKERandBARTON (1953; see Table 1) have listed 41 species of plants whose seeds after-ripen in dry storage together with the length of the dormancy period and methods for

After-ripening in dry storage.

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Table I. Some seeds which after-ripen in dry storage, indicating special treatments found effective in germination of fresh seeds, length of the dry storage period necessary for after-ripening, and author and date of the report. (From ÜROCKER and BARTON 1953.) Dry storage

Treatment'

Seed

Ambrosia trifida .

50 3 months

Agrostis tenuis. Aira flexuosa .

KN0 3 ; alternating temperature Light

Astrebla lappacea

KN0 3

Asystasia gangetica. ( Avena)l oats.

Barbarea verna

several months to 1-2 years

DAVIS (1930a)

14-15 months

Drying at 35° j

!

Light and KN0 3 at 20°-300

12 months 135 days

AKAliHNE (1947)

2-6weeks 9 months

G. T. RARRINGTON (1923) FoY (1932) ÜROSIER (1946)

1-2 months

several months 12 months

TooLE and TooLE (1940) TooLE and TooLE (1939) TooLE and TooLE (1939) TooLE and TooLE (1939) ÜARLETON (1936) RUNYON (1930)

4 weeks

ÜDLAND (1938)

7 years

ANDREWS (1946)

1 year

TooLE and TooLE (1941) TooLE and ToOLE (1941) KEARNS and TOOLE (1939)

Brassica campestris

7 months

Brassica juncea

1 month

Brassica nigra.

7 months

Oosmos, Orange Flare

29° or more

Oovillea. (Oucurbitaceae)l, Cucumbers, pumpkins, muskelons, watermelons. Oyperus rotundus

Leave in fruit until overripe H 2S04 15 min

Daucus pusillus .

Alternating temperatures

I BARTON (1936a)

7-8 months

Festuca capillata.

3° 4-8 weeks + 20°-40° 30 2-4 weeks + 20°-35° 5° 7 days; K..~0 3 ; light

Festuca elatior var. arundinacea .

5° 7 days

3-4weeks

Festuca rubra strains .

5° 7 days

(Gossypium)l cotton.

Thorough drying

Digitaria ischaemum Digitaria sanguinalis .

Guayule ( Parthenium argentatum) .

+ KN0 3

ANDERSEN (1944) NELSON and MAcLAGAN (1935) MYERS (1942)

Removal of scales; 40° for 8 days

( Avena)l oats. ( Avena)l Vicland oats

Author

5 months

1-2months 1 month

KEARNS and TOOLE (1939) KEARNS and TooLE (1939) SrMPSON (1935)

6 months-1 yr. BENEDICT and Puncture seed coat; RoBINSON (1946) 4° 3 weeks; washin water 18 hrs., then soak for 2 hrs. in calcium hypochlorite 1 Scientific name assigned by ÜROCKER and BARTON, common names only appeared in original article. 2 Temperatures in degrees centigrade (° C).

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survey of dormancy types in seeds.

Table 1. (Continued.) Seed

(Hordeum)l barley. (Hordeum)l barley. (Hordeum)l barley. (Hordeum)l barley. Impatiens balsamina ( Lactuca)1 lettuce . Lepidium lasiocarpum Lepidium virginicum . N icotiana rustica. Oenothera odorata (Oryza)l rice . Peltandra virginica. Perilla ocimoides. (Phleum)l timothy (Poa)l Kentucky blue grass. ( Poa) 1 Canada blue grass ( Sorghastrum )1 J ohnson grass.

Sporobolus asper . Sporobolus cryptandrus . Stipa viridula . Streptanthus arizonicus . (Trifolium)1 red clover. (Trifolium)l subterranean clover . Trifolium subterraneum .

Treatment'

Remove hulls

Dry storage

2-6weeks

FoY (1932)

3-9months

VINES (1947)

3-5 months

G. T. RARRINGTON (1923) ÜROSIER (1946) KROEGER (1941) WHARTON and FRAZIER (1939) BARTON (1936a) ToOLE and TooLE (1940) lliLBKAYA (1936)

7 months

TAKIGUTI (1930a)

Remove scales; 40° 8 days 5° 2-4 days 5° 2 weeks High temperature storage

4-6months

10°-30°

12 months

Light or KN03 20°-30°

+

Drying at 15 or 21° 30 min. KN03

Author

4 months

2

+ weeks

6 weeks

BURGESS (1936)

Remove pericarp and soak in water

HART (1928)

Remove coats

TAKIGUTI (1930b)

Light and KN0 3 and 20°-30°

E. H. TOOLE (1939)

Store at 400 Store at 40°

HITE (1923) HITE (1923)

Remove hulls and H 2 S0 4 4-5months 2-3min; 24hrs. H 2 0 2 ; alternating temperatures; 50% 002 7 months Prechill 14 days and KN0 3 4 + years KN03 and light; H 2 S0 4 2 min. 4 months 1-2 years Alternating temperatures 4-6months

G. T. RARRINGTON (1916/17)

Low temperature 3 days

(Triticum)l wheat.

Prick grain, cool temperature

(Triticum)lt wheat (Triticum)l wheat.

5° 2-4 days

V. K. TOOLE (1941) V. K. TOOLE (1941) McALISTER (1943) BARTON (1936a) MARTIN (1945)

3 months

WooDFORDE (1935)

12 months

LoFTUS HILLs (1944)

1-2 months

BYTSCIDKIDNA (1929) ÜROSIER (1946)

40° 8 days

G. T. RARRINGTON (1923) 1 Scientific name assigned by ÜROCKER and BARTON, common names only appeared in original article. 2 Temperatures in degrees centigrade (° C).

overcoming it. It has been found generally that the dormancy period is shorter in seeds produced during dry than during wet seasons.

705

After-ripening in dry storage.

Many seeds which respond to dry-storage after-ripening are capable of germination even when freshly harvested, but only in a narrow range of temperature or other environmental conditions. Thus, fresh barley seeds germinated readily at 10° but not at 15°, freshly harvested achenes of Taraxacum megalorhizon (Forsk.) Hand.-Maz. at 15-20°, fresh seeds of Thlaspi arvense only at 28-30° (ATTERBERG 1899; PoPcov 1935; WEHSARG 1918); afterdry storage or appropriate temperature treatment they are all capable of germinating in a much wider range of temperatures, usually between 5° and 30° approximately. Other seeds, when freshly harvested, are photoblastic but become aphotoblastic in the course of dry storage. BoRRISS (1940) has proposed the term "relative dormancy" for the behavior of such seeds. After-ripening in dry storage or dormancy of freshly harvested seeds, is characteristic of many grasses, including some of our common cereals such as oats, barley, wheat, rye and rice. The period of dry storage needed for afterripening varies from a few days to several months. For this reason, germination tests on fresh seeds may not be reliable unless special methods are used to break the dormancy. Many vegetable and flower seeds as weil as a number of weed seeds also exhibit this type of dormancy. MYERS (1950) points out that such dormancy may last years in wild species but is usually confined to a few weeks in cultivated plants. The persistence of dormancy in seed of some cultivated species of Brassica is described by NuTILE (1952). CHRISTIDIS (1955) has found that dormancy effects in cotton seed may last from 25 to 150 days depending on the date of boll dehiscence, variety and possibly other factors. No differences were found between light and normal irrigation. The greatest amount of seed dormancy in Upland cotton was found in bolls collected on the day of dehiscence (Hsi and REEDER 1953). All evidence of dormancy disappeared when the seeds were taken from bolls which had dehisced 21 to 30 days before they were picked.

2. Special methods for inducing germination. Some methods which have been used for obtaining early germination in freshly harvested seeds are shown in Table 1. In addition, some reports indicate that gibberellin and carbon dioxide treatment may be potent means for overcoming dormancy in such seeds and thus "replacing" the dry-storage after-ripening. Germination induced by gibberellin treatment has recently been described in freshly harvested barley (FISCHNICH et al. 1957) and potato seeds (FrscHNICH and GRIMM 1958), and also in a number of seeds the precise dormancy condition of which was not indicated (species of Luzula, Trollius, Erysimum, Draba, Geranium, Diapensa, Gentiana and Bartsia) while other species of the same genera (Luzula, Draba, Gentiana) failed to respond (KALLIO and PIIROINEN 1959). In barley, promotive effects on freshly harvested seed have also been obtained by treatment with "rindite", a mixture of ethylene chlorohydrin, ethylene dichloride and carbon tetrachloride (FISCHNICH et al. 1958). Carbon dioxide treatment of imbibed seeds has so far been successfully used in small-seeded legumes [Trifolium subterraneum L., 5 other Trifolium species, Medicago hispida Gaertn., Trigonella ornithopoides (L.) DC.; BALLARD 1958, GRANT LIPP and BALLARD 1959]. Treatment at an initial 00 2 concentration of 2.5% by volume was in most cases sufficient to increase the percentage of germination from between 1.5% and 50% to 63.5-100% and to greatly hasten its onset, for example from over 14 days to 2.2 days in a strain of Trifolium arvense L. The effect is not due to.influences on the seed coat as the seeds were scarified before the 00 2 treatment. In Trifolium subterraneum it was shown Handbuch d. Pflanzenphysiologie, Bd. XV/2.

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Seed dormancy: General survey of dormancy types in seeds.

(BALLARD l. c.) that germination is sim.ilarly promoted by the presence of activated charcoal. As there is no indication that these seeds contain an inhibitor which would be absorbed by the carbon, the author assumes that the latter acts by release of co2.

IV. lnduced and secondary dormancy. Dormancy may be induced in non-dormant seeds, or seeds which have been subjected to an optimal dormancy-breaking treatment, by holding them in an imbibed condition under an environmental regime unfavourable for germination. Such "induced" and "secon100 80 Lo;;~~e{d tl':fte;;~ted dary dormancy" has been reported for several different S:: 60 seeds. In general, it can be ~ 4-0 overcome by subjecting the ·~ zo seed to the normal dormancy~ o f--r-h---,---,---,--,-----1f-----+--+---,--t----, breaking treatment, i.e. low~100 Lowerseetl t!pper seetl temperature or an appropri~ 80 ot30°C otJ0°C ate light regime. Sometimes, ~ 60 however, particularly in light~ ft.O and dark- "hardened" seeds, a different treatment, e.g. 20 a low-temperature exposure, 0 l--r'4' - -To--J:+-tJ--'----!--5iT7J--fi+-'fJ-,l0--+[J--g,.-'O-J,'b,-'0---,1 may be necessary, or the 3 6 10 2 40 8 Oxygen,percmf(]!Je by vo/ume seeds have to be dried and Fig. 1. Minimum oxygen required for germination of the intact re-imbibed before the normal cocklebur seed during six days at 21° and 30°. dormancy-breaking treatment (From THORNTON 1935.) can be effectively applied. 100 Among the factors which have Embryoof Embryoot 80 up,oer seetl lowerseed been shown to induce secondot 2!°C ot2!°C 60 ary dormancy are restriction