Báo cáo lâm nghiệp: "Physiological aspects of seed conservation"

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Physiological aspects of seed conservation B. Suszka Institute of Dendrology, 62-035 K6rnik, Poland Introduction some basic facts and processes occurring during the development of seeds when they are still on the mother plant and later, Seeds of most woody-plant species are after separation from it, in the period be- dormant when fully ripe; germination and tween dissemination and formation of seedling formation assumes breaking of seedlings. This period is characterized by this dormancy. Preservation of seed via- breaking of the developmental processes bility by proper conservation generally causing dormancy, followed by over- does not contribute to dormancy breaking. coming this dormancy under conditions After storage, seeds can be as dormant as making the latter possible. This happens they were before storage was started. both when dormancy alone is imposed However, whether dormancy is broken and when it iis a very deep dormancy before, during or after storage - breaking immediate germination and further making of dormancy and conservation of seeds of the seedling impossible. The growth should not be regarded separately. Seed- reasons for these phenomena can differ. ling production is not possible from dor- They are mostly multifactorial. Investiga- mant seeds if both dormancy and the tors concerned with these problems try to tendency to lose viability are not dealt with separate the action and effects of indivi- by proper treatments. We cannot discuss dual factors oir groups of factors, to ex- conservation of seeds leaving their dor- clude in this way interactions with other mancy aside. Storage of non-dormant processes. This is perhaps the only way to seeds is, except for ’recalcitrant’ seeds, conduct such studies but it is obvious that much easier. in reality such distinctions do not exist. The main trends in seed physiology research are riaflected by a concentration Recent trends in seed physiology of studies on the topics enumerated . l below In the last few decades, intensive work 1. Role of seed dehydration in the has been conducted to understand better switch from the developmental program of 1I !1 would like to express my thanks to Prof. S Lewak, lnstilute of Botany, University of Warsaw, for his kind enumera- Institute ! tion of recent trends in seed physiology (items 1-5), and for his comments concerning the results obtained to date.
gene expression to the germination pro- ziriska and Lewak, 1978; Come, 1980/ gram. These studies on the molecular 1981; Bulard and Le Page-Degivry, 1986; level include identification of specific pro- Haliriska etal., 1987). teins synthesized during the operation of 5. Attempts to understand the mode of both programs, among them synthesis of action of environmental factors, such as amylolytic enzymes and (important for the light, temperature and moisture. Cellular understanding of ageing) studies on the membranes are being postulated as the mechanism of lesions and repair of DNA. sites of primary responses, even a kind of Results obtained so far do not contribute ’memory’ is postulated in the membranes to a better understanding of dormancy (Bartley and Frankland, 1985; Probert et (Osborne, 1981; Daussant et al., 1983; al., 1985a, b; Marbach and Mayer, 1985; Symons et aL, 1983; Kermode and Bew- Hilton, 1987). ley, 1985; Cornford et aL, 1987). The studies mentioned above were 2. Hormonal control of metabolic activity conducted on seeds of annual plants during the onset of and release from dor- (cereals, leguminous plants, herbaceous mancy. So far, the roles of ABA (abscisic perennials). Such investigations on seeds acid) and of gibberellins have been re- of woody plants, mostly single species or cognized in the initiation and cessation of cultivars from few genera, even a concern: growth, weakening of endosperm and apple cultivars Golden Delicious and mobilization of reserves, or in counter- Antonovka (Durand et aG, 1975; Tissaoui acting these processes (Webb et al., and Come, 1975; Isaia and Bulard, 1978; 1973a, b; Karssen et al., 1983; Symons Lewak, 1984; Bulard, 1985; Haliriska et et al., 1984; Schopfer and Plachy, 1985). al., 1987), Acer (Nikolaeva, 1967; Szczot- The proposal to call abscisic acid ’dormin’ ka and Tomaszewska, 1979; Pinfield and shortly after recognizing its importance for Dungey, 1985; Szczotka, 1988), Corylus the hormonal control of some processes in (Bradbeer, 1968, 1988), Fraxinus (Sond- growing plants and in seeds reflects best heimer and Galson, 1966) and some other the excessive hopes connected earlier species. with these studies. The majority of woody-plant species 3. Metabolic changes while overcoming from the temperate zone produces dor- dormancy and initiating germination, main- mant seeds. Dormancy of seeds of some ly mobilization of reserves and energy species (Pinus sylvestris, Betula alba) was metabolism. Results of studies on the role recognized very late because it is over- of an alternative (CN-resistant) electron come by a very short period of action of transport pathway are still controversial. red light or a light including the red band of The involvement of the oxidative pentose the spectrum (Nyman, 1963; Junttila, phosphate pathway in the control of dor- 1976). mancy is evident, but still far from being Some species, among them such impor- completely understood (Lewak et aG, tant ones as beech and oaks native to 1975; Lewak and Rudnicki, 1977; Esashi Europe, in addition to some coniferous et al., 1979; Roos, 1980; Pradet, 1982; species, produce viable seeds at long time K6hler and Hecker, 1985). intervals, sometimes every 5-10 yr and 4. Correlations between various seed these intervals seem to increase as a organs. Important interactions were de- consequence of air pollution, even in the tected between various seed structures in case of species such as Picea abies, their protective (inhibitory), nutritional (nu- which produces cones more frequently trient supply) and regulatory roles (Wy- (Chalupka and Giertych, 1973). Despite
short-term storage but especially long- importance of proper solutions for the term conservation of seeds of coniferous practical plant production, investigations species and of the much less understood on the effective and genetically harmless broadleaves, especially shrubs. This is all treatments for breaking dormancy, in- more urgent because some ecotypes the cluding storage, especially long-term stor- whole species are in danger of age, are conducted by only a few research or even disappearing. 1-t is very seldom that seeds groups concentrating their efforts on these of woody-plants are stored in gene banks. problems (Tyszkiewicz, 1949; Rohmeder, Even when they are, this happens often 1951, 1953; Holmes and Buszewicz, without a well-based knowledge of germi- 1956; Sch6nborn, 1964; Buszewicz, 1967; Vlase, 1969; Machanicek, 1973; Suszka, nation. Only a very few research centers exist where ic!ng-term studies on these 1974; Bonnet-Masimbert and Muller, 1975, topics are sysi:ematically conducted. Sel- 1977; Bonner, 1978; Suszka and Tylkow- dom are the even more laborious studies ski, 1980; Suszka and Kluczyriska, 1980; conducted on long-term storage of non- Muller and Bonnet-Masimbert, 1985; Mul- dormant and/or deep-dormant seeds of ler, 1986; Bonner and Vozzo, 1987; Susz- woody plants, where the whole complex of ka, 1982, 1988; Wang, 1982) and some of problems concerning storage, breaking of longer active. them are no dormancy, germination and formation of Of high importance for the elaboration of seedlings under controlled conditions, and proper methods of seed handling under the latter also in the nursery, is investi- controlled conditions in the period be- gated simultaneously. tween seed collection and seedling emer- Special difficulties are connected with gence are the following: effective methods investigations on the storage of seeds with of estimation of potential germinability of a high moisture content, which cannot be deep-dormant tree-seeds (viability tests dehydrated below a usually high threshold developed already in the 20’s or 30’s level. The clear separation of all seeds (Nelubov, 1925; Piskarev, 1937; Flemion, into categories of ’orthodox’ and ’recalci- 1938; Krzeszkiewicz, 1939; Tyszkiewicz, trant’ (Roberts, 1975) proved very helpful 1939; Lakon, 1950; Lakon and Bulat, here. Results of the abovementioned stu- 1952; Bulat and Lindenbein, 1961 ), dies are of high interest, not only for prac- methods of X-ray testing (Simak, 1957; tical nursery producers but also for seed Simak and Kamra, 1963; Belcher, 1974; banks and other institutions interested in on frost tol- Chavagnat, 1984), studies seed reserves. This is all the more impor- of seeds in connection with their erance tant because it has become evident that hydration level (Zakhariev and Tsonev, some species of woody plants are endan- 1958; Sch6nborn, 1964) and studies on gered by total extinction either as a conse- the mutual interactions between various quence of diseases (Ulmus) or merciless thermal conditions causing germination or utilization in earlier times (Taxus) or by the induction of a new dormancy in seeds most recent consequences of air pollution (Haut, 1938; Crocker and Barton, 1931, (Abies alba in Central Europe). The recent 1953; Nikolaeva, 1967, 1977; Suszka, and rapidly spreading and so feared forest 1976; Edwards, 1986). New phenomena decline (’Waldsterben’) concerns not only resulting from the pollution of air, soils and coniferous species, such as pine, spruce waters contribute to a decrease in seed and larch, but also beech and oak forests and further extension of time production weakened in East and Central Europe in intervals between seed years. All this the early 80’s by drought, frost and insect makes it more important not only to have
invasions, and attacked finally by plant subjected earlier to warm stratifica- were diseases caused by fungi and viruses. tion under natural conditions (e.g., Fraxi- nus excelsior’) or not (e.g., Acer pla- tanoides). These modes of pretreating So far, stratification in a moist medium has been the most promising method of cannot be applied to stored seeds. Seeds of woody plants intended to storage must breaking dormancy of woody-plant seeds. be completely ripe at collection time and Low temperature above the freezing point traditional treatments assuring often good in autumn, winter and early spring and the elevated temperature in summer in the results for ’green’ collected seeds are inef- fective for late collected ripe seeds, which soil or under more artificial conditions must furthermore be dried for storage and assured a more or less effective prepara- tion of seeds for germination and the dry-stored. For pretreating seeds under controlled conditions, new methods have emergence of seedlings, whether they to be developed, for each species sepa- needed a cold period only or a cold one rately, because even seeds of various preceded additionally by a warm one. species belonging to the same genus can When dormancy of seeds has to be brok- differ substantially in their biological en under controlled conditions, the practi- characteristics and their physiological cal experience collected through centuries condition. These differences decide the by nurserymen and gardeners is practical- choice of the most useful method of pre- ly less useful, because it is based mostly sowing treatment. This can be illustrated on early, sometimes ’green’ collected and with the example of two common ge- therefore non-dried seeds, sown very Fraxinus and Acer. often even before winter, whether they nera - j j - -- - -. - ___L___._- > - >, - j - , - - - j-
application to seeds of species de- its research directions Promising manding not o!nly a cold but also a warm- followed-by-coldtreatment. A satisfactory treatment of dormant ‘ortho- Presowing solution to this problem would permit dox’ seeds without any medium (naked either a reduction of the dimensions of stratification) (Fig. 1) stratification chambers and as a conse- quence a serious reduction in the cost of their construction and maintenance for at Because of the large volume of the least some important tree and shrub spe- seed/stratification medium mixture, new cies, or else filling the unchanged cham- methods are being developed now based bers with 4 times more seed material. on the already known method of stratifica- tion without any medium, the so called ’naked stratification’ of imbibed seeds, applied mostly for chilling of seeds of coni- Conservation or dormant or non-dormant fers. Our method of stratification without ’orthodox’ seeds (Figs. 2 and 3) medium resembles that of ’priming’ non- dormant vegetable seeds (Heydecker, The ’classic’ way of storing seeds charac- 1973/1974) with osmotically active solu- terized by deep dormancy when ripe in the tions of PEG (polyethylene glycol), as- suring the retention of a constant level of case of the ’orthodox’ ones depends upon drying the seeds to a low level and in plac- moisture content in the treated seeds. The ing them for shorter or longer periods in latter method has been applied (Simak, 1976) to ’priming’ of non-dormant seeds of sealed non-corrosive and chemically neu- Scots pine. For beechnuts, we have de- tral containers at a low temperature. Seeds of temperate zone species should veloped a method based on the same be stored at a temperature just above the principle and it could be called ’priming’ freezing point or, even better, below it. For without PEG. Seeds placed in the moist long-term storage, temperature in the stratification medium imbibe as much range between -5 and -20°C should be moisture as can be taken at the given temperature and the physiological state of preferred, for short-term storage a temper- ature of -3 to --5°C is sufficient. After stor- the seeds themselves. When working age, the seeds should be defrosted and without the stratification medium, the prepared for germination by a stratification seeds should be brought to and after- treatment, depending upon the species. wards kept at a level of moisture content high enough for their proper after-ripening, When the stratified seeds start to germi- nate, they should be sown in the nursery but simultaneously so low that it inhibits germination until the content is increased or in plastic tunnels or tents. In order to have the termination of stratification and to a level of hydration permitting germina- the sowing date coincide, it is necessary tion and seedling formation. In this way, the volume necessary for stratification of to know the necessary period of pretreat- seeds mixed with the stratification medium ing the seeds either by a cold only (1:3, v/v) can be reduced to 1/4, i.e., to the stratification or by a more complex warm- followed-by-cold treatment. Because there volume of the seeds only. The possibility of applying this method to other species of is no possibilii:y to adapt methods of pre- treating seeds of one species to seeds of woody-plants is now being studied intensively in the seed laboratories in another one, even when it belongs to the Nancy-Champenoux and in Kornik, as is same genus, seed physiologists must
Acer pseudoplatanus. Even mant seeds of study the demands of seeds of each spe- ’orthodox’ seeds of Fagus sylvatica or separately. In the case of species with cies Abies alba aind of some other species a large area of natural distribution, this has to be done even for single ecotypes were earlier regarded as belonging to ’recalcitrant’ seeds. ’Orthodox’ seeds are from certain regions or from different ele- difficult to store because their high mois- vations. Data concerning storage, espe- ture content has to be retained to assure cially long-term storage, of dormant ’ortho- seed viability. For each species, individual seeds of many broadleaf and dox’ methods of storage have to be elaborated. coniferous woody-plant species are still Great difficulties are created in the case of lacking and intensive investigations are seeds of ’recalcitrant’ species by their necessary. often enormously high respiratory activity resulting from the high hydration level of Conservation of dormant and non-dor- the living tissues. This activity can easily mant ’recalcitrant’ seeds (Fig. 4) cause overheating of seeds when stored under conditions insulating them from the cooler outer air. The high moisture content To this category belong completely non- is also responsible for the low frost resis- dormant seeds like those of Quercus tance of such seeds, making utilization of robur or Acer saccharinum, semidormant low subfreezing temperature below -3°C seeds of Quercus rubra but also deep-dor- impossible. Thanks to the efforts of a few investigators (Holmes and Buszewicz, 1956; Messer, 1960; Suszka and Tylkow- ski, 1980, 1982; Muller and Bonnet-Ma- simbert, 1984; Bonner and Vozzo, 1987), solutions exist already permitting storage of ’recalcitrant’ seeds of at least some species over 2-3 seasons. However, these methods should be improved and elaborated for species still not investiga- ted. Conservation of already pretreated seeds (Figs.1 and 5) In the last few years, attempts have been made to reverse the sequence of opera- tions connected with storage and breaking of dormancy of deep-dormant seeds. This could be achieved by pretreating them first, shortly after collection, without al- lowing them to lose the high initial viability and high moisture content, and to store them afterwards at low temperature. There are two ways to do this after dor- mancy has been broken. The first way can
be realized the by drying non-dor- already from the subfamilies Pomoidae species mant, highly imbibed seeds to a low mois- and Prunoidae of the Rosaceae family ture content level, followed by dry and (Crocker and Barton, 1931; Haut, 1932; cold storage in sealed containers (like the Stepanov, 1955; Visser, 1956; Decourtye ’classic’ way). The other solution is to and Brian, 1967; Kaminski and Rom, freeze the moist stratification medium with 1973). These data are contradictory but the already pretreated seeds and to sow indicate for seeds of some species that the seeds or the seed/medium mixture seed populations with at least some seeds after a period of cold storage followed by already after-ripened can be dried without defrosting of the medium. Some data are serious decrease of their viability and available from the 60’s and 70’s (Vanesse, without an induction of secondary dorman- 1967; Hedderwick, 1968; Barnett, 1972) cy. The contradiction lies in that some drying of already stratified, previously authors believe that seeds treated in this on dormant Pinus ponderosa and Pseudo- manner should be restratified, while other tsuga menziesii seeds, and data from the authors deny this necessity. This problem early 50’s (Rohmeder, 1951) on drying of has been thoroughly studied by us (Grzes- artificially highly hydrated non-dormant kowiak and Suszka, 1983) in the model of Pinus sylvestris and Picea abies seeds. Prunus avium seeds in intact stones and it Other data, from the early 30’s up to the was found that the most favorable date of early 80’s, concern dormant seeds of coni- sowing coincides with the start of germina- fers (Edwards, 1982, 1986) or of fruit tree tion. The already stratified stones when
seed lots. Other sources of differences, dried to 10% moisture content can be even between seed lots of the same spe- stored up to 1 yr but germinability cies are connected with provenances, decreases to half of its initial level. Posi- seed years, characteristics of individual tive results were obtained in our studies trees from the same stand or even the for beechnuts (Suszka, 1975). Beechnuts conditions of pollination. can be dried after chilling without medium at a controlled hydration level, extended until all germinable seeds are ready to germinate but still do not germinate. When Acknowledgments dried in a forced air stream at a moderate temperature they can be stored like other presented in the figures originate The data ’orthodox’ seeds, e.g., at -10°C for 100 exclusively from investigations conducted in the days without any decrease of germinabili- Dendrology in K6rnik. Institute of ty. Our attempts to extend the storage pe- riod up to 1 yr did not assure positive effects, however, before treatment, the seeds had already been dry-stored at a References low temperature for more than 1 yr. When fresh seeds were used for such a treat- Barnett J.P. (1972) Drying and storing loblolly ment, germinability remained high even pine seeds reinduces dormancy. Tree Plant. after nearly 6 yr of storage (Muller and 1 Notes, 23, 10-11 Bonnet-Masimbert, 1982). These authors M.R. & Frankland B. (1985) Effects of Bartley phytochrome-controlled germination produced also report promising results for seeds of by far-red irradiation of seeds before and during Fraxinus excelsior and Prunus avium. rehydration. J. Exp. Bot 36, 149-158 When these first results will be confirmed Belcher E.W. Jr. (1974) Specialized use of X- by studies on broader seed material, com- rays in seed analysis. Proc. X-ray Symp. 7-8 mercial storage of already pretreated Nov. 1974, Macon, GA, USA. pp. 41-44 seeds deprived of their dormancy can per- Bonner F.T. (1978) Handling and storage of haps become reality. hardwood seeds. Proc. Second Symp. South- east Hardwoods pp. 145-152 Positive solutions to conservation of Bonner F.T. & Vozzo J.A. (1987) Seed biology already pretreated seeds would (at least and technology of Quercus. Gen. Tech. Rep. for some species) in properly equipped SO-66. New Orleans, LA. USDA For. Serv. stations or facilities enable seedmen or South. For. Exp. Stn. 1-21 seed trade companies to deliver to nurse- Bonnet-Masimbl3rt M. & Muller C. (1975) La conservation des faines est possible. Rev. For. ries seeds ready to germinate after Fr. 27, 129-138 sowing. Which species could be adapted Bonnet-Masimbert M. & Muller C. (1977) De to this seed strategy must be thoroughly nouveaux espoirs pour la conservation des studied. glands. Bull. Tech. Office Nat. For. 9, 47-55 Introduction of new methods of seed Bradbeer J.W. (1968) The role of endogenous inhibitors and gibberellins in the dormancy and handling necessitates the building of new germination of Corylus avettana L. seeds. facilities or an adaptation of the old ones. Planta 78, 266-276 In such facilities, well-trained personnel, Bradbeer J.W. (1988) In: Seed Dormancy and backed by a laboratory providing the Germination. Blaackie, Glasgow, pp. 1-144 necessary data, would be able to treat Bulard C. (1985) Intervention by gibberellin and each seed lot separately, according to the cytokinin in the release of apple embryos from individual life history of each lot and thus dormancy: a reappraisal. New Phytol. 101, 241 - 249 determine individual differences between
Bulard C. & Le Page-Degivry M.T. (1986) Edwards D.G.W. (1986) Special prechilling for tree seeds. J. Seed Technol. 10, Phytohormones in seed and fruit development. techniques ln.! Plant Growth Substances. (Bopp E., ed), 151-171 Springer-Verlag, Berlin, pp. 308-314 4 Esashi Y, Wakabayashi S., Tsukada Y. & Satoh S. (1979) Possible involvement of the alterna- Bulat H. & Lindenbein W. (1961) Zwanzig jahre tive respiration system in the ethylene-stimu- erfahrung mit der tetrazoliummethode an lated germination of cocklebur seeds. Plant geholzsamen. Mitt. lnt. Ver. Samenkontr. 26, Physiol. 63, 1039-1043 453-471 Flemion F. (1938) A rapid method for deter- Buszewicz G. (1967) Problems and rewards in mining the viability of dormant seeds. Contrib. processing and storing seed. J. R. Scott. For. Boyce Thompson Inst. 9, 339-351 Soc. 22, 129-134 Grzeskowiak H. & Suszka B. (1983) Storage of W. & Giertych M. (1973) Seed years Chafupka partially after-ripened and dried mazzard (Pru- in Picea abies (L.) Karst. Arbor. Kornickie 18, nus avium L.) seeds. Arbor. Kornickie 28, 261 - 183-186 281 A. (1984) Determination de la quali-- i Chavagnat Halifiska A., Sitiska I. & Lewak S. (1987) t6 des semences d’arbres et arbustes d’orne- Embryonal dormancy in apple seeds is control- ment par la radiographie industrielle aux rayons led by free and conjugated gibberellin levels in X. Hortic. Fr. Dec. 22-24 the embryonic axis and cotyledons. Physiol. C6me D. (1980/1981) Problems of embryonal Plant. 69, 531-534 as exemplified by apple embryo. Isr. dormancy (1932) The influence of drying on the Haut I.C. J. Bot. 29, 145-156 and germination of fruit tree after-ripening Cornford C.A., Black M., Daussant J. & Mur- seeds. Proc. Am. Soc. Hortic. Sci. 29, 371-374 doch Y.M. (1987) a-Amylase production by pre- Haut I.C. (1938) Physiological studies on after- mature wheat (Triticum aestivum L.) embryos. ripening and germination of fruit-tree seeds. J. Exp. Bot. 38, 277-285 Md. Agric. Exp. Stn. Bull. 420, 1-54 Crocker W. & Barton L.V. (1931) After-ripening, Hedderwick G.W. (1968) Prolonged drying of germination and storage of certain rosaceous stratified Douglas fir seed affects laboratory seeds. Contrib. Boyse Thompson Inst. 3, 385- germination. New Zealand For. Serv. Res. 404 Leafl. 19, 1-2 Crocker W. & Barton L.V. (1953) In: Physiology Heydecker W. (1973/1974) Germination of an of Seeds. An Introduction to the Experimental priming of seeds. In: University of idea: the Study of Seed and Germination Problems. Nottingham, School of Agriculture Report, pp. Chronica Botanica Co., Waltham, MA, pp. 116- 50-66 117 7 Hilton J.R. (1987) Photoregulation of germina- Daussant J., Mosse J. & Vaughan J. (1983) In: tion in freshly harvested and dried samaras of Seed Proteins. Ann. Proc. Phytochem. Soc. Bromus sterilis L. J. Exp. Bot. 38, 286-292 Eur. No. 30. Academic Press, New York, pp. Holmes G.D. & Buszewicz G. (1956) Longevity 101-133 of acorns with several storage methods. For. Decourtye L. & Brian C. (1967) Determination Comm. Rep. For. Res. 1955, 88-94 des besoins en froid des p6pins de pomac6es. Isaia A. & Bulard C. (1978) Relative levels of lnterpr6tation des courbes de germination. some bound and free gibberellins in dormant Ann. Amelior. Plant. 17, 375-391 and after-ripened embryos of Pyrus malus cv. Durand M., Thevenot C. & Come D. (1975) ’Golden Delicious’. Z. Pflanzenbiol. 90, 409-414 4 R61e des cotyl6dons dans la germination et la Junttila O. (1976) Effects of red and far-red irra- lev6e de dormance de I’axe embryonnaire de diation on seed germination in Betula verruco- pommier, apr6s traitement par I’acide absci- sa and B. pubescens. Z. Pflanzenphysiol. 80, sique. Physiol. Veg. 13, 603-610 0 426-435 Edwards D.G.W. (1982) Storage of prechilled Kaminski W. & Rom R. (1973) Secondary dor- Abies seeds. Proc. Int. Symp. Forest Tree mancy in stratified peach embryos. Hortic. Sci. Seed Storage IUFRO Working Party S2.01.06 8,401 Seed problems. Petawawa Nat. For. lnst., Karssen C.M., Brinkhorst-Van der Swan D.L.C., Chalk River, Canada, 23-27 Sept. 1980. pp. Breekland A.E. & Kornneef M. (1983) Induction 195-203
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