View of Plant adaptation to temperature and photoperiod

Plant adaptation ^to temperature and photoperiod

Olavi Junttila

UniversityofTromso, N-9037Tromso,Norway

Plantsrespond toenvironmental conditions bothby adaptation and byacclimation. The abilityof the plantsto grow,reproduce and survive underchanging climatic conditions dependsontheefficiency ofadaptation and acclimation. The adaptation ofdevelopmental processesinplants to temperature and photoperiod isbrieflyreviewed. Inannualplantsthisadaptation is related togrowth capacity and tothetiming ofreproduction. In perennial plants growing under northernconditions,adaptation of the annualgrowth cycle tothe local climatic cycleis ofprimary importance. Examples of the role of photothermal conditions inregulation of thesephenological processes aregivenand discussed.^The genetic and physiologicalbases for climaticadaptation in plantsare briefly examined.

Key words: development, flowering,frost resistance, growth

ntroduction

Plants respond toenvironmental conditions by both adaptation and acclimation. Adaptationcon- sists of heritable modifications in structures or functions that increase the probability ofan or- ganism surviving and reproducing in a particu- lar environment. Acclimation refers tononher- itable modifications caused by exposure of an organism to a changing environment, and is based_on the structural and physiological plas- ticity of the plants. Plasticity is_aunique feature of plants that has been suggested^tohave signif- icance_{as an}integral part of the mechanisms by which plants (a) control reproductive effort and (b) capture resources from their environments (Grimeetal. 1986).Some plantcharacteristics.

such as flowerstructures, seed and fruitanato-

my, have very lowplasticity.However, many of the basic characteristics that _are important for plant growth and development and for plant yield, suchas numbers ofmeristems, numbers of various organs,rates of division and expan-

sion, show high plasticity ^(see also Trewavas 1986). Processes such as the breaking of dor- mancy by chilling and induction ofcold harden- ing by low temperature treatmentsare also ex- pressions of the acclimation abilities of plants.

Acclimationhas, ofcourse, a genetic basis and sois linked toadaptation. High plasticity and a high capacity for acclimation can be of major adaptive significance.

Thepresent distribution of higher plants is a reflection ofan evolutionary adaptation toenvi-

©Agricultural and Food ScienceinFinland ManuscriptreceivedFebruary 1996

Vol.5(1996): 251-260.

Junttila, O.: Plant adaptationtotemperatureandphotoperiod ronmental conditions. The first photosynthetic

organisms evolved about 2500 million years ago and free oxygen based respiration developed some2000 million years ago. From this perspec- tive,the appearance offlowering plants 130 mil- lion years ago is ^a rather recent event. The present vegetation of Nordic countries evolved even morerecently, about 12 000-15 000 years ago. In view of these developments, the domes- tication and cultivation of plants have a very short history. Domestication started a human- imposed adaptation, which continues _{at an ac-} celeratedrate in modern plant breeding with its goaltoproduce species and cultivars adapted ^to various environments, growing conditions and purposes. These timeperspectives should be kept in mind in further consideration of plant adapta- tion toachanging environment.

Since their first appearance, flowering plants have experienced changes in theenvironment, long- and short-term changes in the global cli- mate, and climatic changes due to geographic spreading (north-south, maritime-continental).

The large-scale result of environmental adapta- tion in plants is well demonstrated by the vast differences in vegetation types as one moves from theequatorto the poles, orfrom maritime tocontinentalareas. Moreover, year-to-year var- iations in temperature conditions may be of

greater magnitude than expectedtemperature

changes. ^Thus, cultivated plants, especially the annual species, have been, and are, exposed to

strongly changing environments. Our under- standing of plant adaptation is based both on studies of natural plant populations andonplant breeding and experiments with cultivated spe- cies (e.g. Evans ^1993). Experimental studies under controlled conditions, in which various

components of the climatecan be studied sepa- rately and in design interactions, have beena particularly valuablesourceof information (e.g.

Roberts ^etal. 1993).The phytotron is an indis- pensable research facility for systematically analysing the effects of climatic factorson plant growth and development, and acombination of phytotron and genetic approaches provides a powerful tool for studies on plant adaptation.

As aresult ofevolution and adaptation, there are250 000 species of flowering plants. Intraspe- cies adaptation has resulted inclimatic, as well as edaphic, ecotypes. Turesson (1922) defined ecotype as a genotypic response to the various environments in which the species is found. Eco- type is also used as a part of_a dine. Environ- mental conditions may change graduallyoverthe geographic range ofa species, and dine is used todescribe agradual change inacharacterover this geographic range.Quantitative, physiologi- calcharacters, such _asresponses to photoperi- od, areparticularly likelytoshowaclinal varia- tion,but this may also apply to morphological features(Jonesand Wilkins 1971).

Plant growth arises from the production of drymatterthrough energy metabolism andmor- phological development. Both of these main processes are notably affected by environmen- tal conditions andaresubjectedtoadaptation and acclimation. The main emphasis here is on de- velopmental adaptations of plants; the adapta- tion of drymatterproduction is limitedtosome briefcommentsonly. In annual plants this is re- latedtotherate of development andto the tim- ing of reproduction. In perennial plants growing under northernconditions,adaptation of phenol- ogy, growth cycle, ^to the annual climatic cycle is of primary importance. As will be shownhere, adaptation of these developmental processes is mainly basedonresponsestophotothermalcon- ditions. The physiological and certain genetic

aspects of adaptationarealso discussed.

Adaptation of dry matter

production ^and photosynthesis

Plant growth depends on photosynthesis and energy metabolism. As pointed ^outin the intro- duction, these basic metabolic processes have a long evolutionary history. Due totheir key role in plant growth, they have attracted a large vol- ume of research in manyfields, plant breeding among them.

The most important climate-related photo- synthetic adaptation is development and evolu- tion of the

C 4 assimilation

system as anadapta- tion to a warmer climate. In his book Evans (1993) concludes his very comprehensive dis- cussion with the following statements: “..the predominant improvementssofar havenotbeen in the efficiency of the major metabolic andas- similatory processes, but in thepatterns of par- titioning and the timing of development... Con- vincing evidence of improvements of the major metabolic and assimilatory processes through plant breeding is lacking, whether for photosyn- thesis,respiration, translocation_orgrowthrate.

Regulatory processes, by contrast, have been profoundly modified”.

Strictly photoperiodic stimulation of dry matterproduction is important in plant produc- tion^at northern latitudes and has been demon- strated in ^many plant species, particularly in high-latitude cultivars of perennial grass species (Hay 1990, Junttila^et al. 1990^a). It has been suggested that this response has adaptive signif- icance for growth under marginal temperature conditions at high latitudes (Hay and Heide 1983). Physiologically this stimulation of dry matterproduction is primarily related to devel- opmental processes promoted byalong day such asleaf development andstemgrowth.

Adaptation ^of developmenta processes

The life cycle ofaplant includes germination, vegetative growth, flowering and seed produc- tion. In addition, toensure survival through un- favourable seasons, plantsmusthave acycle of dormancy and growth. In _aseasonal habitat there isapremiumondoing the right thingatthe right time and the most efficient mechanism for do- ing this is the one most likely to be selected (Jonesand Wilkins 1971).Thus, phenology^- the influence of environmenton ontogeny ^- is the

most important single factor influencing geno- typic adaptation(Roberts et al. 1993). In tem- perate-zonespecies, environmental regulation of phenology is controlled by the photoperiod and temperature. Photoperiod, whichatagiven lati- tude remains rather constantfrom year to year is the mostreliable time signal. Adaptation to photoperiod has thus been marked in both plants and in animals. These adaptationsaresignificant for survival and reproduction. In most temper- ate-zone woody plants, photoperiod gives the signal for cessation of growth and hardening processes, and thesespecies showadine ofpho- toperiodic responses over latitudes (Vaartaja

1954, Dormling etal. 1968, Heide 1974,Håb- jprg 1978).In woody plants the critical photope- riod for cessation of growth is relatively little affected by temperature (Heide 1974, Junttila 1980). Temperature cannevertheless substantial- ly modify the process of growth cessation. Anal- yses of the phenology of growth cessation un- der natural conditions reveal a photoperiod x temperature interaction (Koski 1985). Lankinen (1986), among^others, has demonstrated_a lati- tudinal dine in critical photoperiod for diapause response in Drosophila strains.

In herbaceous plants photoperiodic respons- es arerelated first of alltoflowering but alsoto overwintering. Plant species arecommonly clas- sified into categories of long day, short day and day neutral plants. Daylength responses, how- ever,also vary within species.

Summerfield and his colleagues have carried out detailed studies on regulation of flowering in several crop species and conclude that the time when _a crop flowers is determined almost ex- clusively by the genetically controlled responses

to daylength and temperature (Summerfield et al. ^1991,Roberts ^etal. 1993). On the basis of experiments under controlled conditions in whichtemperature and photoperiod could be in- dependently manipulated, they have developed quantitative models that arereliable enough to predict when particulargenotypeswill flower in any environment.

Adaptation ^to photoperiod and temperature for flower induction has also been shown intem- Vol. 5(1996):251-260.

Junttila, O.: Plant adaptationto temperatureand photoperiod

Table 1. Primary inductionrequirements forflowering in sometemperateperennial grasses.(Modified from Heide 1994).

Inshortdays^(< 12^h) Inlong days^(> 16h)

Temperature Exposure Temperature Exposure

Species (°C) (weeks) (°C) (weeks)

Poa pratensis 3-18 6-10 3-12 8-12

Alopecurus pratensis 6-18 6 6-15 6-8

Dactylisglomerata 9-21 8-10 0-3 20-

Festuca pratensis 3-15 16-20 3-12 18-20

perateperennial grass species (Heide 1994). The majority of such species havea dual induction requirement for flowering. Primary induction, which is brought about by lowtemperatureand/

or short photoperiod, and secondary induction atlong photoperiod and moderateor high_tem- peratures. For primary induction, low tempera- tureand short photoperiod cansubstitute for each other, and this combination provides extra in- surance for effective induction, even under changing temperature conditions. As shown in selected species in Table 1,temperature limits for both primary and secondary inductionare relatively wide and thus the induction process is notvery sensitivetominor variationsorchang- esin temperature.Table 1 also demonstrates dif- ferences among species, but there is also some variation within species. Generally, primary in- duction is reached earlier bygenotypes of_more northern than those of southern origin, and the critical photoperiod for secondary inductionvar- ies from 9-10 h in Mediterraneanecotypes to morethan 16 h in high latitudeecotypes(Heide 1994). Among common pasture species, timo- thy ⁽Phleumpratense L.) is _an obligatory long day plant without any chilling requirement. High temperatures, however,haveaninhibitory effect on flowering in timothy and northern origins appeartobemoresensitivetothis inhibition than southern origins (Heide 1982).

Likewise in strawberry, flower induction is controlled by interaction between photoperiod and temperature,and the degree of dependence on a short photoperiod increases with rising tem- perature (Heide 1977).Therearesignificant dif-

ferences between cultivars. Somecultivars,e.g.

Jonsok andGlima, are abletoform flower buds even under continuous light attemperatures of 12°Corlower(Heide 1977). In plants with this type of response tophotoperiod and tempera- ture,elevatedtemperatureconditions combined with the long days of high latitudes can reduce flowering. Similarly, in Poa nemoralis, a long day plant, the number of panicles per plant is reduced withan increase in temperaturefrom 9

to21°C (Heide 1986).In timothy, temperatures

above 15°C reduce flowering, particularly in cultivars witha northern origin (Junttila and Schjelderup 1984).

The actual temperature during the growth season atnorthern latitudes is generally lower than the optimum temperatures for both intro- duced and native plants, provided that adequate water and nutrients_areavailable.Temperature requirements for,and temperature effects on, the^rateof development for various developmen- tal processes can differ between species, eco- types and cultivars. Intraspecific genetic varia- tion in perennial grass species with respect to growth responses to temperaturehas been dem- onstrated. Mediterranean and Scandinavian ori- gins of Dactylis glome^rataL., Loliumperenne L., and Festuca arundinaceae Schreb., forex- ample, differ in theirtemperature responses (Cooper 1964,Eagles 1967). Mediterranean races generally have lowertemperatureminima for growth than Scandinavian races, and they have lesssteep growth responsecurvestorising

temperature than do northern origins (Wareing 1979). Our studies on white clover ⁽Trifolium

repens L.) have shown similar responses, with genotypes of southern origin starting to grow at lowertemperatures than genotypes from northern Norway (Svenning etal. unpub- lished).

Although examples of the specific effects of fluctuatingtemperatures areknown(Went 1957), manyplants generally respondtothe dailymean temperature. Recent studiesonwheatcanbe tak- en as an example. Slafer and Rawson (1995) showed that developmentalrates in wheatwere determined by mean temperatureandnotby day/

night amplitudes. Linear relationships between mean temperatures and rates of development during various growth phases _werefound for all the cultivars studied, but the temperature re- sponses of the various cultivars differedsignifi- cantly (Slafer and Rawson 1995). Timothy isan example ofa species in which flowering is en- hanced by diurnal temperature fluctuations as compared with constant temperatures (Junttila

1985).

When the photothermal requirements of a genotype for floweringareknown from experi- mentalstudies,the datacanbe plottedonphoto- thermographs for various localities. Predictions of flowering behaviourcan then be made under corresponding climatic conditions. Robertsetal.

(1993) have provided such examples for vari- ouscultivars of soyabeans, and Heide(1994) has used climate-photothermstoillustrate the climat- ic adaptation of primary induction of flowering in Bromus and Poa. Such exercises canbe very useful for discussions_onbehaviour of plantspe-

cies andgenotypesunder changing climatic con- ditions.

Adaptation tovarious germination tempera- tures has also been demonstrated inanumber of species (Thompson 1973). Typically, species from high latitudes and high altitudes seem ^to have high optimum temperatures for germina- tion. and germination is markedly restricted ^at lowertemperatures (Junttila 1976

a,

₁₉₇₆b). The specific requirements for germination tempera- tureprovide very clear examples of adaptation totemperatureconditions. Such adaptationsare necessary to synchronize the germinationproc-

esswith local growthconditions, and germina- tion mustbe timed^to periods that provide the highest probability of the young seedlings sur- viving and developing. These temperature re- quirements for germination arerelated to seed dormancy, and normally change drastically dur- ing the breaking of dormancy. Thus, in many casesthetemperaturerequirement for seed ger- mination has _a highly plastic character, and is strongly modifiedby environmental conditions.

Survival adaptation

^-

frost resistance

The ability to survive the winter is a basic re- quirement for perennial and winter annual plants growing ^at northern latitudes. Adaptations that

promote winter survival include timing and^rate of hardening, maximum level ofhardiness,and timing andrateof dehardening. In woody plants, photoperiod is asignal for the initiation of hard- ening, but generallytemperature conditions play amajor role in the regulation of frost resistance.

Although the maximum level of hardinesscan be_alimitingfactor,the main problemsareoften linkedtothecorrectsynchronization of harden- ing and dehardening with the localtemperature cycle. These processes have been studiedexten- sivelyoverthe years, and manyaspects ofregu- lation and control of cold hardiness are now known. In herbaceous plants, the hardening-de- hardening cycle is primarily controlled by tem- perature and is anexample of physiological ac- climation; resistance to low temperature im- proves when thetemperature starts tofall,and the process is reversed when the temperature starts ^to rise (Levitt 1980). The effect of pho- toperiodonfrost resistance in herbaceous plants is not as clear, although in _some species, e.g.

whiteclover,hardening is enhanced by short and dehardening by long photoperiod (Junttila etal.

1990^{b,Eagles 1994).}Generally,plants of south- ern origin reachalower level of maximum har- Vol. 5(1996);251-260.

Junttila^,().:Plantadaptation totemperatureandphotoperiod diness than the corresponding northern geno-

types, and therearealso differences in therates of hardening and dehardening. According to Eagles (1994),_aUK cultivar of timothy dehard- ened in response toelevatedtemperatureunder

short-day and long-day conditions, whereas a cultivar from northern Norway showedamarked daylength requirement, with dehardening being enhanced by a long photoperiod. A short pho- toperiod enhanced hardening of white clover genotypes from different latitudes, but the re-

sults didnotindicate any significant adaptation tophotoperiod (Junttila etal.

1990

^b).

The range ofeffectivetemperatures for hard- ening is so wide that arise in temperaturedur- ing the autumn and the spring ofafew degrees from thepresent level wouldnot necessarily be harmful for overwintering of cultivatedorwild plants atour latitudes.However, if the climate change results in abrupttemperature variations during the hardening and dehardening periods, such conditions might have deleterious effects on genotypes in which resistance is sensitive _to temperature. Studies _on both perennial grass species and white clover indicate that popula- tions from northern coastal areas, where _snow and temperatureconditions vary throughout the winter,often showahigh degree of stability and strongwinterresistance. At suchlocations,plants are, still today, exposed ^to such highly variable conditions.

Genetic aspects ^of adaptation

By definition, adaptation is a genetic process based on natural orhuman-imposed selection.

Selection during domestication ofourcropplants has resulted ingenotypeswithreduced seed dor- mancy, reduceddependence onphotoperiod for flowering, and enhanced allocation ofresources to the harvested parts of the plant (e.g. Evans 1993). Much of this development has happened without any deeper understanding of physiolog- ical _or molecularnature of the processes in-

volved. Several of the main crop speciesare now well characterized genetically, and present de- velopment in this field is very rapid. Most of the characteristics discussed above are considered to be quantitative. Photoperiodic responses in woody plants show a quantitative inheritance (Eriksson etal. 1978, Hummeletal. 1982, Junt- tila 1982, Junttila and Kaurin 1985). Similarly, frost resistance in El generation from a cross betweenafrost hardy and lessahardygenotype shows aclear intermediate inheritance(Rpsnes et al., unpublished results).

Quantitative

^char-

acters,e.g.photoperiodic responses, may still be based_on relatively few genes,_as shown by Pa- terson etal.(1995) for sorghum, rice and maize.

In these species, despite 65 million years of reproductive isolation, the quantitative loci for photoperiodic responses correspond closely. The ability of many cultivated cereals ^to flower in the long days of the temperate summer may largely be the result of mutations _atasingle_an- cestral locus (Paterson etal. 1995). In Chymo-

myza(a Drosophilidae) the critical photoperiod for diapause in El population is intermediateto that ofparentpopulations and the photoperiodic response is controlled by ^few,perhaps _{no more} thantwoloci(Riihimaaand Kimura 1989).

Frost resistance isaquantitative trait control- led by several genes (Levitt 1980), and intraspe- cific variation in frost resistance has been dem- onstrated in several cultivated species ^(seee.g.

Hömmö 1994). In wheat, the chromosomes of the sth homoeologous group and chromosomes 7A,28, 4B and 4D carry genes controlling frost resistance and winter hardiness. Chromosomes 5A andSD,which have been implicated themost frequently, appeartocarrymajor genes for frost resistance(Sutka etal. 1994).The locus for frost resistance (Erl) is located on the long arm of chromosome 5A and ^seems tobe completely linkedtothe locus Vrnl controlling the vernali- zation requirement. However, Vrnl and Erl genesare separable and could betargetsfor gene isolation and positional cloning. Further, there appears to be a close linking between Erl and the generegulating abscisic acid(ABA)produc- tion in wheat(Galiba etal. 1994).

Voi5(1996): 251-260.

Larsen (1985) has provided ^an example of the effect of selectiononfrost resistance in Dac- tylis glomerata. Five per centdirectional selec- tion for high resistance significantly increased resistance within only three generations, partic- ularly inaless-hardy cultivar(Unke).Selection in the opposite direction had only _a minor ef- fect.

In this regard, a puzzling phenomenon de- scribed for Norway spruce (Picea abies (L.) Karst.) and Scotch pine (Pinus sylvestris L.) should be mentioned. In repeated experiments, seeds produced from controlled crosses in _a warm climate gave seedlings withan extended growth period and delayed frost hardening as compared with seedlings from similarcrosses made in a cooler climate (Bjprnstad 1981, Johnsen 1989).The mechanisms for these effects arenotknown,butsomeevidencesuggests acli- matic effect (genetic orepigenetic) during pol- lination and fertilization (Johnsenetal.,unpub- lished).Andersen(1971)has shown that the win- tering capacity ofatimothy cultivar canbe sig- nificantly reduced afterafew generations of seed production in a southern location.

Physiological and molecular _aspects of developmental adaptation

Our knowledge of the physiological and molec- ular _aspects of both the environmental and in- ternal regulation of plant growth and develop- mentis rapidly expanding, and the progress made in different fields is frequently reviewed in pro- ceedings and periodicals. Improved methods for molecular mapping, identification of genes, stud- ies onmechanisms of gene regulation, and utili- zation of gene transfer techniques provide us with unique possibilities tomanufacture plants with designed properties, including responses to climatic factors, ifso desired. The following three examples are particularly relevant to the developmental processes discussed above.

Known tobe ofgreatimportance for metab- olism in plantcells, the lipid composition of plant membranes is affected by temperature, and changes in the fatty acid composition ofmem- brane lipidsarerelatedtoaplant's abilitytogrow atlowtemperatures. The role of membrane lip- ids intemperature related growth processes has been well demonstrated withmutantsandtrans- genic plants of Arabidopsis. Plants deficient in desaturase activity, and therefore lacking poly- unsaturated fattyacids, are not ableto grow at low temperatures. Decreased desaturase activi- ty thus turns a cold-tolerant plant intoa cold- sensitive plant (Miquel etal. 1993). The impor- tance of trienoic fatty acid composition in con- ditioning cold tolerance has also been demon- strated in transgenic tobacco(Kodama et al.

1994). Clearly, modification of membrane lip- ids by gene technology provides newpossibili- ties for changing plant responsestotemperature.

The future of this field wasdiscussed atlength in a^recentreview by Gibson^etal.(1994).

Phytochrome is the primary receptor in- volved in light perception for photoperiodic re- sponses, shadeavoidance,proximity perception, seed germination,etc.Higher plants have atleast five different phytochromes. The genes forsev- eral of these phytochromes have already been cloned and the effects of these genes have been studied in transgenic plants (Smith 1994). Both transgenic plants and phytochrome_mutantsshow markedly changed responses to light quality (spectral composition), irradiance and photope- riod, and phytochrome genes can, moreover, strongly modify plant growth and development.

Smith (1992) has discussed the ^prospects for improving cultivated plants by genetic modifi- cations of the phytochrome ^system.

There is increasing evidence that plant hor- monesplay an important role in mediating cli- matic signals into growth responses. Plant hor- mones arealso strongly involved in variousac- climation processes and the control of plasticity of plant growth ^(see also Trewavas 1986). The identification and characterization of various hormonemutantsin differentplant species (Reid and Howell 1995) have provided convincing

Junttila, ^().:Plant adaptationto temperatureandphotoperiod evidence that plant hormonesareimportant_com-

ponents of the adaptation mechanisms of devel- opmental processes. In particular, ABA and gib- berellins (GA) are involved in the climatic reg- ulation of plant growth. ABA is crucial for the regulation ofwater balance in plants (Zeevaart and Creelman 1988). Plants unable ^to synthe- size ABAare notcabable of regulating stomata closure,andarethus highly vulnerabletodrought stress.ABA _can also function as a drought re- lated signal from theroots tothetopof the plants (Davies and Zhang ^1991). The role of ABA in regulating freezingstresstolerance has also been documented in a number of studies (Chen and Gusta 1983,Reaney etal. 1989).

GAs _are involved in the regulation of seed germination, vegetative growth, flowering and seed and fruit growth (Graebe 1987). Several studiessuggestthat biosynthesis of GAs in plants canbe affected by light andtemperature (for ref- erences,seeGraebe 1987, Takahashietal. 1991).

Thus,both photoperiodic responses and certain temperature responses, e.g. vernalization and elongation growth, could be linked to environ- mental regulation of endogenous GAs in plants.

Genes coding for some of the enzymes in bio- synthetic pathways for GAs have recently been cloned (Lange etal. 1994), providing new op- portunities for _more precise analyses of the

functions and roles of GAs in regulating plant growth and development.

In conclusion, higher plants have evolved effective mechanismstoacclimate and to adapt to temperature and photoperiod. The develop- mental processes in plants areespecially adapt- able and plastic, and have been subjectedtoboth natural and human imposed selection.Plant spe- cies and ecotypes adapted to various environ- mental conditions have developed throughevo- lution.Furthermore, plant breeding has brought abouta wide selection of cultivars for various purposes and environments. The capacity for plastic behaviour found in many plants enhances their potential totackle changing environmental conditions. Understanding the genetic and phys- iological control of the mechanisms behind the responses ofplantstoclimatic conditions isnow rapidly expanding.

Despite the complexity of the mechanisms in the climatic regulation of developmental proc- esses in plants, the examples discussed above suggest that molecular biology will eventually be able_toprovide effectivemeanstotailor plants for specific environmental conditions.

Acknowledgements. Thanks aredue to Professor Ronald Robberecht,UniversityofIdaho, Moscow, Idaho,U.S.A., for valuable commentsonthemanuscript.TheNorwegian Research Council isacknowledgedfor financial support.

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SELOSTUS

Kasvien sopeutuminen lämpötilaan ja päivän pituuteen

OlaviJunttila UniversityofTromso, Norja

Kasvien kyky kasvaa, lisääntyä jatalvehtia vaihtu- vissa ilmastollisissa olosuhteissa riippuu suurelta osalta niiden kyvystä sopeutua ympäristöön ^geneet- tisesti(adaptaatio) ja fenotyyppisesti (akklimaatio).

Eläimiin verrattuna kasvien sopeutumiskykyon var- sin hyvä etenkin sellaisten ominaisuuksien suhteen, jotkaovatmerkittäviäkasvulle,kehittymiselle ja kyl- mänkestävyydelle. Artikkelissa käsitellään kasvien sopeutumista lämpötilaan ja päivän pituuteen.^Tällai-

nen sopeutuminen on selvintä kasvien kehitystä ja kehitysrytmiä säätelevien mekanismien kohdalla.

Yksivuotiset kasvit sopeutuvatennenkaikkea kukin- nanja lisääntymisenavulla. Monivuotiset kasvit ^so- peutuvat lisäksi säätelemällä vuotuista kasvu-lepo- rytmiä. Artikkelissa esitetäänesimerkkejäsekäyksi- että monivuotisten kasvien sopeutumisesta, jakäsi-

tellään lyhyesti sopeutumisen fysiologista jageneet- tistä taustaa.