View of Breeding of frosthardy rhododendrons

MaataloustieteellinenAikakauskirja Vol. 60:^235—254, 1988

Breeding of frosthardy rhododendrons

M. UOSUKAINEN" and P.M.A. TIGERSTEDT^2*

" Agricultural Research Centre

Healthy Plant Center SF-41340Laukaa, Finland

21 Department

of

of

SF-00710Helsinki, Finland

Abstract. In 1973aRhododendron breeding program was started at the Department of Plant Breeding, University of Helsinki. The^programwasbased^onthe extremely hardyrhododen- dron material that had been naturally selected at Arboretum Mustila. The aimwasto create newfrosthardy cultivarsthat could tolerate minimumtemperaturesbelow —35°Cinmid-winter.

Rhododendron brachycarpum subsp. ligersledtii Nitz. wasusedasthe maternalparentin the majorityofcrosses. AlsoR. smirnowii and R. calawbiensewere usedas sourcesfor winter hardiness.

Totally 148different combinations weredone between species, species and hybrids and between hybrids. The total number of matingswas 496.Over20 000seedlingswereobtained and atotal of 13 752 plantswereplanted infield trialsoneight different test sites between

1975—1979.

After two extremely coldwinters, 1984/85 and 86/87,about60^%of the plants diedor wereseverely damaged.R. brachycarpum subsp. ligersledtiicrosseswith eitherR.smirnowii orR.calawbiense gave the best genetic material for better climatic adaptation. The progenies of thesecrossessurvived without anyseveredamagewhen minimumtemperaturewas aslow as ^37°C.Tissue culture methodsweredevelopedand usedin vegetativepropagationof selected ortets. Bythe end of 1987six clones^havebeen releasedas newcultivars for commercialpropa- gation.

Index words: Rhododendron breeding, R.brachycarpum subsp. ligersledtii, R. smirnowii.

R.calawbiense. selection for cold tolerance

Introduction

Rhododendrons belong to the Ericaceae- (heath) family that consists of about 70 genera and 1900 species (Lawrence 1951). They are widely distributed on acid soils throughout

temperate regions of northern and southern hemispheres and to a lesser extent in the subarctic. A number of ericaceous species coverthe coniferous forest floor of northern JOURNAL OF AGRICULTURALSCIENCE IN FINLAND

Europe including Finland. However the genus Rhododendron is restrictedtoa single species, R. lapponicumL., naturally distributed in the subarctic under open tundra-likeconditions.

The genus Rhododendron comprises about 900 species inthe cool^{and temperateregions} of the northern hemisphere withaconcentra- tion of species, about 700, in the mountains

of_east and southeast Asia.

The genus Rhododendronwas first recog- nised by Linnaeus in Species Plantarum in 1753. Recently there has beena total rever- sion of the genus into eight subgenera. Some of themarefurther split into sections andeven subsections (Cullen and Chamberlain 1980).

In fact, arevision of azaleatypesis currently undertakenand ^{may result} in the division of these rhododendrons into ^{seven sections.}

Rhododendrons have been used as or- namentals in the gardens of central Europe ever since plant explorations began their ^in- troduction from China and otherpartsof the world early in the 19thcentury. A number of famous plant hunting expeditions (Forrest, Rock, Kingdon-Ward and others) in the be- ginning of the^20thcenturyintroduceda large number of_newrhododendron species into^the gardens of Europe, particularly from China and other parts of southeast Asia (Kruss-

mann 1968). This also promoted plant breed- ing efforts to improve the ornamental value and hardiness ^innew hybrids. Species within sections of rhododendrons are easyto cross and new character combinations of growth habits and flower colours can be produced almost ad libitum. Intersectionalcrosses are however often difficultto make and resulting hybrids_are often totally sterile. Sterility may depend_on strictly morphological malforma- tions, but also ploidy levels may vary from diploid todecaploid in different species thus causing hybrids to have genomic imbalance.

For instance crosses between diploids and tetraploids give sterile triploid hybrids. Ploidy level manipulations by colchicine treatments give promising results toproduce newfertile alloploid rhododendrons. Thus more than a hundred years of selection and hybridisation

has produced over 4 800 named »cultivars»

(Salley and Greer 1986) and many _more man made products have been left un- registered. Most of this plant breeding work has been done in the United Kingdom, Unit- ed States and in various countries of central Europe. Thus the released ^cultivarsaregener- ally not hardy in northern Europe.

At present some 50 000 rhododendron plantsare yearly importedtoFinland (Anon.

1986). The imported plants aremainly winter green Rhododendron catawbiense Michaux and deciduous R. japonicum (A. Gray) Sur- ing. The first mentioned is themostcommonly cultivated rhododendron species inFinland.

Onlysomeplants, grown in southern and_cen- tral Finland, ^{have shown to} be of adequate hardiness (Kallio 1966). This presumably de- pends on genetic variation in hardiness be- tween different individuals of the_truespecies in combination withparticularly favourable localgrowing conditions. The average winter minimum temperature that R. catawbiense can stand is —32°C (Cox 1979). The mini- mum temperatures in southern and central Finland have been varying between —3O,6°C

at Hanko to —3B,5°C ^at Jyväskylä (Il-

matieteen laitos 1986). Also following culti- vars are imported and grown regularly:

’Caractacus’, ’Cunningham’s White’, ’Dr.

H.C. Dresselhuys’, ’F.D. Godman’, ’Nova Zembla’ and ’Scarlet Wonder’. The frost tolerance of their buds varies between —23 °C and —32°C (Salley and Greer 1986).

It can be concluded that imported species and cultivars are generally not adequately adaptedto the Finnish climate. Plants suffer particularly from cold spells below —3O°C that damage flowerbuds^orsometimesvegeta- tive buds or whole shoots. The incentive ^to launch_a major rhododendron breeding pro- gram in Finland is basedon three major facts.

Firstly, it has been well proven, that import- ed material is not adequately adapted to the climate. Secondly, it has been ^{shown in ar-} boretumand garden trials that there arespe- cies of rhododendron that_canstand eventhe coldest of winters in Finland.Thirdly, erica-

ceousspeciesaregenerally well suitedto grow on the acidic forest soils and particularly on the organic peat soils in Finland.

The breeding program wasstarted in 1973.

It was based on the extremely hardy rhododendron material that during 40 years had been grown and naturally selectedatAr- boretum Mustila, southeastFinland,60 deg.

44 min. N. lat. and 26 deg. 29 min. E. long (Tigerstedt 1986). This collectionwas estab- lished by Dr. C.G. Tigerstedt who had ^aclear notion of the importance of seed originsand variability in hardiness between plants of a population. Thus introductionswere,when- everpossible, made in the form ofanumber ofindividualsrepresenting the species. In the years 1930—1950 Mr. B. Schalin, city gard- ner in Helsinki, established a number of rhododendron plantations in the Helsinki city parks and on his private property near Hel- sinki (Schalin 1953).At the Brödtorp plant nurseries, Mr. B. Knape started nursery production of rhododendrons and azaleas in 1953 (Knape 1984). Thus in 1966 Kallio reported that 40 different species and cultivars had been grown in Finland. These plantations have shown that there are a number of rhododendron species that adapt fairly well^to the climate in the south-central part of the

country.

The aim of the program was toproduce superior species hybrids and particularly to find desirable recombinants in populations of crosses between species and hybrids.

Frosthardiness has invariably been the num- beronetrait selectedfor,but in additionalot of effort has beenput onselection for differ- entgrowth habits of thebush, including foliar characteristics. A wholespectrum of flower colours from pure whiteto deep red has also been a collective target including the details of flower morphology and inner colour and feature of the corolla.

Long term testingatArboretum Mustila has shown that R. brachycarpum subsp. tigersted- lii Nitz. isan exceptionally hardy species (Fig.

1). This ^{species was} named after Dr. C.G.

Tigerstedt, who introduced itto the world in

1935. That year he received aseed source originating from the Kongo-San mountains in eastern-central ^{Korea. Plants withstood}

—43,5°Catthe arboretum in February 1940 (Tigerstedt 1986). This specieswas the only tall, ^evergreen species that survived without anyvisibledamage and flowered normally af- ter the extreme cold winters 1984/85 and

1986/87 (Uosukainen 1988). At Anjalan- koski, the official weather station ^{of the} Finnish MeteorologicalInstitute,^about20 km from thearboretum, ^—39.3°C wasrecorded close to the ground in January 1985 and

—39,B°C at the same spot in January 1987 (Ilmatieteenlaitos

1985 a,

The exceptionally good adaptation of the

»Tigerstedt» rhododendron is well document- ed in its ability to produce natural regenera- tion _at Arboretum Mustila. Some of the natural seedlings have growntolarge flower- ing shrubs. Infact,R. brachycarpum has been the only introduced rhododendron species abletoreproduce generatively. At Arboretum Mustila there are several observations made on coniferous trees, indicating that the ulti- mate proof of good adaptation canbe con- cluded from how well the generative cycle, in- cluding meiotic division and formation of pollen, functions in thenewenvironment. The ability _toregenerate naturally also indicates that the introduced species has foundan eco- logical niche in its new environment.

Some other species of rhododendron are worth special mentioning for being hardy at Arboretum Mustila. The true form of R.

brachycarpum, R. metternichii, and R. smir- nowii are completely hardy under normal winter conditions.However, severe damage is caused by temperature around —35°C. Ac- cording _to Cox (1979) they should stand about —26°C. Also _socalled Seidel-hybrids have been hardy at Mustila. Rudolf Seidel used R.smirnowii,R. catawbiense and R. ar- boreum in his hybrids produced in Germany atthe beginning of this century (Krussmann

1968). Particularly R. arboreum contributed

deep red flower colours to his hybrids but it was also a source of frost sensitivity.

These species and hybrids are not general- ly available in commercial plant nurseries be- cause of their unsatisfactory ornamental properties _or difficulties in their vegetative propagation. Their flower coloursvary from pink toalmost pure white and the inflores- cenceis often small_orloose. Also their often tall and loose growth habit causesproblems already during the nursery cultivation. In ad- dition the plants areoften slow flowering,re- quiring several vegetative years before flower- ing commences.

The comparison of climatic constraints in various places where rhododendrons are grown show thatinFinland, in northern Scan- dinavia and in East Baltic, rhododendrons meet the marginal limits of their survival (Cox 1979, Galle 1985). In south-central Finland weather conditionsareextremely vari- able from yeartoyear duetofluctuations be- tweenamaritimeand continental climatic pro- file. Night frosts in early June and again in mid August, heat and drought spells in the summerand cold spells in the winter demand a very high adaptive tolerance of successful plants. This is particularly the case when

Fig. I. Rhododendron brachycarpum subsp. tigersted- tii isatall white flowered bush. It has been used asthe maternalparentin_themajorityof^crosses inthe breeding program.

Fig.2. Meristem tipculture methodsweredevelopedand used for clonal propagation of the selectedor- tets.

Fig. 3. The^crossesbetweenR.b. subsp. tigerstedtii and R. smirnowii resulted invery winter hardy off- springs.The Fl-hybridsweremedium tall bush- eswithafine growthhabit,attractive foliageand pinkishwhite flowers.

Fig. 4. Fl-hybridsfrom the crossbetween R.b. subsp.

tigerstedtiiandR.yakushimanumaredense, _low bushes. They have pubescent leaves and young shoots and whiteorpinkish flowers.

growing conifersorevergreenrhododendrons.

They may suffer heavily from desiccation in March—April when the solar radiation is al- ready strong, evaporation from leaf surfaces is considerable,^but water uptake is virtually nil ^duetofrozen ground.

The taxonomic and systematic classification used in this presentation needs special com- menting. Latin names and names of the cul- tivars have been taken from treatises by Stevenson 1930, Nitzelius 1970, Cox 1979, Encke et al. 1980 ^Salley and Greer 1986 and Krussmann 1968. There has beena ^great confusion in the classification of the Japanese

Rhododendron species in Subsection Pontica.

The confusion ^concerns R. degronianum, R.

metternichii and R. yakushimanum. Accord- ing_{to recent} papers, thesespeciesareall either subspecies _or varieties of R. degronianum (Chamberlain and Doleshy 1987). General- ly the older classification has been used in this paper, because of thedifficultiesin determin- ing thecorrect namesof the used parental in- dividuals. Thus the paternal individuals used in thecrossescarry thenamesthat wereinuse in the rhododendron plantations from where the pollen was collected.

Fig.5. The matings between aspeciesandahybrid,such asbetweenR.b. subsp. tigerstedtii andR./or- restavar.repens-hybrid, gave veryvariable off- springs.

Fig. 7. The first named cultivar ’Elviira’ originates from thecross R.b. subsp. tigerstedtiix R.forrestii

var.re/ww-hybrid. ^Itisa low,dense bush with red buds and flowers.

Fig. 6. Open pollinatedseedwasa^{very good}source^for variation and selection. The mortality among young plants and the number of weak individu- alswere very high particularlyinthe offsprings ofso called Seidel-hybrids.

Fig. 8. In the extremely cold winters, 1984/85 and 1986/87,differencesinwinterhardiness between hybrid progenies became clearly visible. Inthe picturetaken from the field trial atMikkeli,_the frostbitten line inthe middle resulted from the crossR. b. subsp. tigerstedtii xR. arboreum.

Parental breeding material

All crosses were made on mother plants selected among the hardiest species, hybrids and cultivars at Arboretum Mustila. Totally 53 mothers, ^{belonging to} 8 species and 27 hybrids were used.

The hardiest species and hybridsatArbore- tumMustilawere initially used_aspollinators.

Also less hardy species ^at Mustila, ^but with morecolourful flowering andmoredecorative foliage and growth habitwereused as polli- nators. Pollenwas also procured from Bengt Schalin’s garden. In 1975^new pollensources wasbrought in from three rhododendron col- lections in south Sweden (Alnarp, Christinelundand Sofiero)andfrom the Roy- al Botanical Garden of Copenhagen in Den- mark. In 1976 pollenwas procured from the rhododendron collections of the Proefstation voor de Boomkwekerij at Boskoop in Hol- land. Totally 114 individuals belonging to 23 species and 48 hybridswere usedasfathers in the crosses.

It is generally very easytohandle rhododen- dron pollen due to the fact that it can be preserved viable ^{even at} room temperature from several months uptoawhole year. Thus species flowering at very different timescan readily be hybridised. The total list of paren- tals used^{in thecrosses}isgiven in appendix

1.

Methods Time table

The duration of different stages of the breeding program are illustrated in Table 1.

The programwasstarted in 1972 with prelimi- narycrosses. On the bases ofthis orientation the first large scale hybridisation effort was made in 1973. The whole crossing plan was completed within six ^{years. Progenies were} raised from 1974to 1980 simultaneously de- velopingan optimal nursery technique. Sub- sequent field trials were established 1975^— 1980. First selections in the trialsweremade in 1978 and this work phase is planned tobe

completed by 1989. A research program for optimizing micropropagation of rhododen- drons ^wascarried ^outin 1980—1984and sub- sequently applied to cloning of selected hybrids. Thus the cloning of selectedortets wasstarted in 1982 and this work phase ises- timated ^to take nine ^years. The ^planting of clonal field trials_was started in 1983 and the work is planned tobe finished by 1990. The first cultivarwasnamed in 1986and the clonal selection for newcultivars is estimatedto last from 1985 to 1995. Thus the breeding pro-

gram proper is estimated to take about 24 years.

Pollination

Most of the hybrids wereproduced bycon- trolled hand pollination. Flower buds were emasculated just before opening and isolated in terylene bags, specially designedfor tree breeding work (Duraweld Ltd., U.K.). Polli- nationwas done after 7—14 days when the stigma had opened and whenadrop of sticky nectarrevealed its readiness toacceptpollen.

Insome casesseed after open pollination was collected as it was found ^that atthe arbore- tum, pollination between species flowering

simultaneously often takes place. Totally 496 cross batches were obtained in the breeding program (Appendix 2).

Hybrid progenies

Seedswere sownand progenieswereraised using methods described by Uosukainen (1976). The hybrid populations wereplanted atsevenlocations in southern and central Fin- land (Appendix 3). No plantationsweredone north of 65 deg. N.lat.,^whichwasconsidered to be the northern limit of garden rhododen- drons. In thewest east direction, ^the cli-

matebecomes graduallymorecontinental as the influence of the western Golf-stream gradually decreases and the influence of the vast land masses to the east of Finland in- creases. These facts weretaken into account

inselectionof experimental sites for the winter

Table I. Timetable of the breedingprogram from the first ^crossestothe release ^ofnewcultivars.

Duration ofstage

Year Stageof program 0 123456789

1972 Preliminary work 0

1973 Crosses 1

1974 Raising of progeny 2

1975 Plantingof progeny tests 3

1976 1977

1978 Selection of progeny 4

1979

1980 Research program inmicropropagation ^. 5 1981

1982 Cloning of ortets 6

1983 Plantingof clone tests 7

1984

1985 Clone selection ⁸

1986 ^New cultivars ⁹

1987 1988 1989 1990 1991 1992 1993 1994 1995

Total tenure of breeding^program: 24 ^years.

hardiness trials. Also the hybrid populations planted _were classified in hardiness groups avoiding the planting ofsensitivegroups inthe

most severe test locations.

The experimental sites also represented varying edaphic conditions. Soiltypesvaried from glacial moraine and podsolic woodland soils to drained sphagnum ^swamp. All sites had_aconiferous_crowncanopy of Scots Pine orNorwaySprucetoprevent direct solarradi- ation.

Selection criteria

Themostimportant selection criterionwas adaptation of hybrid plants to climaticcon- straints. This involves several single traits, such aslowtemperaturetolerance, growth ini- tiation and cessation periods and drought resistance. The duration of field testing was determinedto 10—15 years dependingonthe occurrenceof critical »bottleneck»years(Ti-

gerstedt 1970). In addition following charac- ters were considered in selection:

Flowering characters: colours, ^flower and inflorescens shape, durability of flower clusters and their weatherresistance, flower- ing period, first flowering, flowering propen- sity.

Foliage characters: colour, ^position, shape, morphology, ageing and their resistance to nutrient disorders.

Growth habit: creeping, shrublike, ^tree- like, compact or loose.

Phenology: vegetative and generative cy- cles.

General plant health.

Ageing of plants.

Cloning

of

In 1981 a research project was started in ordertomatch each selected hybridto anop- timal meristem tip culture method (Uosukai- 241

nenand Niskanen 1985). This was necessary asit_was found, thatgenotypesreacted differ- entlytohormones and nutritionalcomponents

in the culture medium. The selection and vegetative propagation (Fig. 2) of the hybrids is illustrated in figure 9. The vegetative propa- gation terminology used (ortet ramet clone) is adopted fromtree breeding (Zobel and Talbert 1984). From each ortet 10—20 ramets wereplanted for clone testing in Hel- sinki, ^{atÖverby, at}Kaarina, ^atMäntsälä, ^at Arboretum Mustila, ^at Jyväskylä and at Haapamäki (Appendix 3.). These clonal tri- als were started in 1983.

Results

The total number of matings in the breed- ing program was 496. Therewere 442 cross- pollinated batches that consisted of 148 differ- entcombinationsbetween species, species and hybrids and between hybrids (Table2 and Ap- pendix 2). In addition there^were30 selfpolli- natedbatches, 14controls (no pollination) and

10 openpollinated seed batches. (Table 3).

More than 20 000 seedlings were obtained from theseed batches^{and about25%of them} died during the first year. There were also plenty of week individuals that did not sur- vive transplanting outdoors.

Between 1975—1979a total of 13 752 plants wereplanted in field trialson eight different test sites. By 1982, 37 ^% of these plants had died (Table 4) due ^to differentreasons; un- favourable planting sites, replantings, theft and failure ofcare during the establishment of field trials. During the first 5—9 years af- terplanting, poor winterhardinesswasseldom the actual ^reason for mortality. Selection in the trials was startedabout three years after planting, whenmost plants had reached the height of 75cmand thus their shootswere no longer sheltered bysnow coverin mid-winter.

The frosthardiness of the breeding materi- al was tested for the first time during the winter 1984/1985 and again two years later during in 1986/1987. Thesetwo winters were in many test sites the coldest in this century

Table 2. Number of different combinations and num- ber ofcrosses(batches inparentheses),which were made inthe breeding ^program.

Paternal

Maternal Species Hybrid Total

Species 41 51 92

(174) (98) (272)

(62 intra specific) (112 inter specific)

Hybrid 27 29 56

(108) (62) (170)

Total 68 80 148

(282) 060) (442)

Table3. Number of self pollinated, control andopenpol- linated seed ^batches.

Typeof pollination

Self Controls Open Total

�� ^*«*

Species Hybrids

23 3 32

7 6

8

7 22

Total 30 14 10 54

* Self pollination ⁼the emasculated and isolated flow- erswith pollen from the sameindividual.

** Controls ⁼emasculation,isolationor noisolation, no pollination.

*** Open pollination⁼without emasculation and con- trolled handpollination.

Table 4. Number of planted hybrids inthe field trials and thepercentageof survival after establish- ment (1982) and after the first and second

»bottleneck» winters (1985 and 1987).

Number Survival^% of plants

°!

J P'am_.ed

1982 1985 1987 Hybrids

Testsites

Mustila 713 61 46 42

1 600 68 31 26

2 917 74 57 51

HelsinkiI Helsinki 2 Vähämäki Piikkiö

751 94 47 40

3 129 69 59 54

211 92 84 80

2 475 72 57 41

1 995 13 5 1,5

Lappeenranta Mikkeli Oulu

13 752 Total

63,5 46,2 39,3 Mean^%

Helsinki I ⁼Kaivopuisto.

Helsinki 2 ⁼Haaga.

242

(Table 5). During these winters 30 ^% of the breeding materialwas either severely damaged or killed (Table 4). In winter 1986/87 par- ticularly, therewas severe root damage due to thinsnow coverageduring the lowest tem- perature period. In January 1987 the snow cover was only 10—20 cm in southern and central Finland (Ilmatieteen laitos 1987).

Therewere large sib-group differences be- tween the offsprings from different mother

species and cultivars. Winter survivalwas in- variably superior in the offsprings of R.

brachycarpum subsp. tigerstedtii. Particular- ly hardywereits progenies after crossing with R. smirnowii. These hybrids survived 100^% on all testsites and over 80^% had _novisible damages (Fig. 3). Even the flower buds toler- ated —36°C without damage. Also »Tiger- stedt» progenies aftercrosses with R. cataw- biense and its variety album ’Glass’ gave very hardy FI-hybrids. InFlelsinki, all individuals of these crosses survived, 22 ^% without damage, 56 ^% slightly damaged and 22 ^% withseveredamagesontheirleaves,^{buds and} branches.However, the reciprocal crossesin- dicateda strongmaternal effect for hardiness inherited fromthe »Tigerstedt» rhododendron mother.

The matings between »Tigerstedt» and a number of other species gave somewhat less hardy offsprings than the particularones men- tioned above. Crosses with R. metternichii and R. yakushimanum gave offsprings (Fig.

4) thatsurvived —37°Cbut mostly with slight or even severe damages. All crosses with R.

williamsianum, R. wardiiorR. orbiculareas fathers gave offsprings with poor winter hardiness. Plantswereoften killed withintwo or three years after planting in test fields.

The matings between two hybrids or be- tween aspeciesand ahybrid gave extremely variable offsprings (Fig. 5). There was large variation both in morphology, flowering characteristics and in frost tolerance. Mortal- ity was generally high in recombinant plant populations but very interesting material could be selected from such groups forfurther^clone testing.

Table^5. The minimumtemperaturesatseven weather stations near the test sites (ILMATIETEEN LAITOS 1985(a,b) and 1987).

Weather Lowest Minimum Minimum

stations recorded temp. temp.

min.temp. °C °C

°C 1984/85 1986/87

Mustila —37,3 —33,2 —37,3

Piikkiö —36,7 —36,7 —34,5

Helsinki 1 —34,3 —26,5 —34,3

Helsinki 2^(—52) —35,9 —31,8 —35,9

Lappeenranta —38,3 —31,0 —36,8

Mikkeli —42,4 —34,7 —36,8

Oulu (—53) —40,1 —36,5 —37,5

Mustila: Weather station of Anjalankoski.

Helsinki 1:Weather station of Kaisaniemi.

Helsinki2;Weather station of Helsinki— Vantaa airport.

Openpollinated seed, ^{collected at}the Ar- boretum Mustila rhododendron valley,was a verygoodsource for variation and selection.

The mortalitywasvery high particularly in the offsprings of R. smirnowii and the so called Seidel-hybrids(Fig. 6). Inmanycases the off- springs died already as seedlings duetofun- gal diseases, ^{such as} Botrytis cinerea. Some of these recombinant populations died total- ly in the field within the first five years.

Among the survivingrecombinants, beautiful flower colours and shapes were often_com- bined with satisfactory winter hardiness.

By the end of 1987 about 80 individuals from hybrid populations had been selectedto be used as ortets for micropropagation. The ramets were planted in clonal trials for fur- ther selection. By the end of 1987a total of 2 410 ramets from35 ortetshave been plant- ed in the field for repeated clone testing. Six hybrid clones have been released asnamed cul- tivars to commercial production. There are two red flowered hybrids, ’Elviira’ (Fig. 7) and ’Hellikki’, two hybrids with pink flow- ers, ’Flaaga’ and ’Universitas Helsingiensis

350’ andtwowhite hybrids, ’St. Michel’ H42 and ’P.M.A. Tigerstedt’.

Discussion

Evergreen rhododendrons _are typically 243

244

plant species that prefermaritime climates.^In Europe, rhododendron breeding and use is concentrated _to Ireland and the _U.K., the Benelux and coastal northwest Germany. In North America thePacific ^Northwest is ^the optimal_areawithsomeadditional_{areas on}the east coast. In Asia, Japan is the center of breeding and usealthough they haveconcen- trated their efforts _onthe deciduous azaleas.

Extremely low wintertemperatures, below

—35°C, under conditions of littlesnow cover- age isparticularly detrimentalto flower buds of rhododendrons. Critical lowtemperatures forflowerbud damage may vary within spe- cies depending on the seed origin. Justas in conifers, adifference ofone degree latitude or 100 m altitude may be critical for frost hardiness. Thus it is obviously somewhatun- critical ^to classify species of rhododendrons as hardy or not hardy in _acertain location without simultaneously noting the seed origin, including the specific site from where seed was initially procured.

Particular attention must here be givento the seed origins of Rhododendron brachycar- pum. Thereseem^tobe controversial opinions

about the taxonomic classification of this spe- cies and somescientists have questioned the

justification _{to separate} it into subspecies (Chamberlain and ^Doleshy 1987). From a strictly population genetic _aspect,itmay well be that this species varies gradually (clinal variation) within its natural distribution in Japan and Korea. Thus the subspecies tiger- stedtii^{may well}represent theextremeend of adine where hardiness and evenmorpholog- ical traitsdeviateconsiderably from the typi- cal R. brachycarpum.

The fact that there is considerable genetic variation in hardiness of different seedsources of a species is worth special consideration from the plant breeder. In fact, there may even be variation in hardiness between in- dividuals belonging to thesame seed source.

Thus the breeder mustapriori select the bas- icmaterial for hybridization from seedsources originating from climatically marginal low-

temperature areasof the species. Secondly, the

breeder may considerably improve hardiness breeding by using parental material^{that has} already been naturally selected for winter hardiness ina newcritical environment. This was the ^{case in} the ^present rhododendron breeding program, where the parental materi- al,particularly _onthe maternalside, ^{has been} naturally selected in the arboretum for peri- ods upto40 years. Particularly hybrid popu- lations,^{where both}parents had been through selectionatArboretum Mustila exhibitedout- standing climatic adaptation. Much hardiness wasusually lost by using imported pollen from ornamentally interesting plants that had grown in betterclimatic conditionsin southern Scandinavia _or central Europe.

Critical low temperatures also damage the vegetativebud, although its hardiness is usual- ly greater than that of the flower bud. Ulti- mately low temperatures maydamageor kill annual shoots thus totally destroying rhododendron plants. This oftenoccurs only after plants have grown above thesnow cover, to aheight of 50—100cminsouth ^central Finland (Fig. 8).

In additiontothe direct effects of low ^tem- perature onbud _orshoot damage, evergreen rhododendrons may, asmentionedabove, in- directly suffer from lowtemperatures due_to frozen ground and subsequent desiccation.

This »sunburn-effect» ^{usually occurs in late} winterorearly spring when the ground is still frozen,but solarradiation is alreadyintensive, particularly duetothestrongreflection from the white ^snow around the plants. This par- ticular condition is typical of the Finnish cli- mate. Infact theFinnish language hasa spe- cial word (ahava) for this kind of »sunburn- effect». It is particularly fatal to evergreen rhododendrons that growonsouth slopesun- der heavy solar radiation but it may even damage indigenous evergreen conifers.

From the description above we conclude, that rhododendron field trials following hybrid breeding must continueat least until the test plants reach a height well over the snow cover. This may requireatestperiod of at least 10^yearsfor high-bush ^types,while this

question may be uninteresting for procumbent growth types.

However, observations at Arboretum

Mustila haveindicated, ^{that each} 15-year peri- od during thiscenturyhas experienced atleast onecritical year for determining woody plant

Fig. 9. Selectionof ortetsinhybrid progenies, micropropagationof ramets, planting of clone tests and the release ofnew cultivars.

hardiness. In the rhododendron programwe have been^{luckyto havetwo} such years with oneyearinterval, but generallya 10—15 year testperiod is mandatory sothat progenytest- ing may withreasonable precision determine hardiness of new cultivars.

Breeding for extreme hardiness is inevita- bly a very long-term enterprise. It actually startswith the geographical selection ofmar- ginal seed sources ofwild species. The seed sources arethen grownas plant populations in the experimentalarea(Arboretum) where- by natural conditions further select hardy in- dividuals within the population. This phase of

»passive breeding» should take the population throughatleastone critical »bottleneck» year at a stage when the plants are already far above snow coverage. The rhododendrons used in the breeding program atArboretum Mustilawereintroduced in the 1930

s

Hybrids produced in the breeding program havenowbeen in field trials for B—l38—13 years and breeder selection is still goingon. Select- ed and cloned material has beenout in repli- cated field trials (clone tests) for about 5 years.

Thesetestsareplanned _tocontinue until 1995 (Table 1).

Considerable time-saving can be achieved in the breeding program by effective ^{use of} meristematic cloning. Field trials of new hybrids must also be given proper time (10—15 years) to obtain reliable hardiness selection and also to select for ornamental

values. However, meristematic cloning of pos- sible^cultivarcandidates ^{(ortets)canbe start-} ed during the field trial periodsothat materi- al is available for replicated clonal testingonce theselection decision has been made.Alsoop- timal cloning methods can be workedoutin- dividually for each cultivar candidatesothat they are available when cultivar decision is made.

Deliberate breeding for climatic adaptation, such as hardiness, ^is a long process, often covering halfa centuryto beon a firm_genet- icresourcebasis. ^{In the}rhododendron ^breed- ing program described here, the crosses be- tweenR. brachycarpum subsp. tigerstedtii and either R. smirnowiiorR. catawbiense gave the best genetic material for the future effortsto

breed for better climatic adaptation.

Acknowledgements.We wish particularly toexpressour gratitude toMrs. Sylvi Lehtonen for her valuableas- sistance, professional skill,enthusiasm and personal de- votion to thisprogram.

Wearegrateful toArboretum Mustila for allowing us tousethe living collection of rhododendrons for the breed- ing program.

We wishalso to^expressourgratitude tothe Academy of Finland and to the horticultural research foundation of Nikolai and Ljudmila Borisoff for financing this researchprogram.

Wearealso grateful to the City Park Department of Helsinki, the City Park Department of Mikkeli, the Department of Horticulture at the Agricultural Research Centre, the Research Farm Kotkaniemi of Kemira Oy, the University of Oulu and toMr.Niilo Karhu. They have allgivenus land for^the field trials of^thehybridproge- nies. Without their help inplanting and careing of the test fields the establishment of the field trials in their presentextentwould not have been possible.

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247

SELOSTUS

Talvenkestävien alppiruusujen jalostus M. Uosukainen^1* ja P.M.A. Tigerstedt^2*

1* Maatalouden tutkimuskeskus Tervetaimiasema

41340 Laukaa

2* Kasvinjalostustieteen laitos Helsingin yliopisto

00710 Helsinki

Alppiruusut⁽Rhododendron sp.) kuuluvat Ericaceae- eli kanervakasvien heimoon ja sukuun kuuluu lähes900, etupäässä aasialaista lajia. Alppiruusujen jalostuskäyn- nistyi Keski-Euroopassa ja Pohjois-Amerikassa1800-lu- vun loppupuolella.Kohta sata vuotta kestäneen jalos- tustyön tuloksena on yli ₄800 nimettyä lajiketta rekisteröityvirallisesti. Suuresta lukumäärästä huolimatta ei näiden lajikkeiden joukosta löydy Pohjois-Euroopassa hyvin menestyviä lajikkeita. Heikosta talvenkestävyydestä huolimatta tuodaan Suomeen vuosittain noin50 000alp- piruusuntainta Keski-Euroopan taimitarhoista.

Helsingin yliopiston kasvinjalostustieteenlaitoksella käynnistettiinvuonna 1973alppiruusujen(Rhododendron sp.) jalostusohjelma, joka perustuiMustilan arboretumin kestävään alppiruusuaineistoon. Erityisen kestäväksi alp- piruusuksiosoittautui korealainen kookas,valkeakukkai- nen laji R. brachycarpum subsp. tigerstedtiiNitz. eli mustilanalppiruusu. Erinomaisen talvenkestävyytensä vuoksi tämä laji valittiin jalostusohjelman keskeiseksi lajiksi. Pyrkimyksenä oli yhdistää mustilanalppiruusun talvenkestävyys jamuiden sitä arempien lajien ja lajik- keiden koristeellinen kasvutapa ja kaunis kukinta. Mui- takeskeisiä lajeja ohjelmassa olivatR. calawbiense,R.

mellernichii, R. smirnowii jaR. yakushimanum.

Risteytysohjelmassa käytettiin äiteinä kaikkiaan ⁵³ Mustilan arboretumissa kasvavaa alppiruusuyksilöä.^Ne kuuluivat 8 lajiin ja27niistä oli hybridejä. Pölyttäjinä käytettiinkaikkiaan 114yksilöä, joistaosaoli Mustilan arboretumista tai muista suomalaisista alppiruusukokoel- mistä jaosasiitepölystähaettiinRuotsista,Tanskasta ja Hollannista. Pölyttäjien joukossa oli 23 lajia ja 48 hybridiä. Käytetyt hybridit on lueteltu liitteessä I.

Risteytysohjelmassa tehtiin kaikkiaan 148 erilaista lajien, lajien ja hybridien ja hybridien välistä risteytystä.

Niiden lisäksi tehtiin30itsepölytyserääsekä 14kontrol- lierää, joissa emaskuloitua kukintoa ei pölytetty.

Vapaapölytteisiäeriä oli10.Kaikkiaan risteytysohjelmassa oli ⁴⁹⁶risteytyserää. Risteytystentuloksena saatiin yli 20 000 siementainta,^joistaensimmäisen elinvuoden ai- kana kuoli noin 25 ^%.

Jälkeläiskokeet perustettiin seitsemälle paikkakunnal- le:Elimäelle, Helsinkiin, Piikkiöön,Kotkaniemen Vähä-

mäkeen, Mikkeliin,Lappeenrantaansekä Ouluun. Vuo- sina 1975—79jälkeläiskokeisiinistutettiin kaikkiaan noin

14 000tainta. Vuoteen 1982mennessäoli istutetusta ai- neistosta kuollut37^%eri syistä johtuen.Huono talven- kestävyysoli ensimmäisinä vuosina vain harvoin syynä taimien menehtymiseen; useimmiten syynä oli epäsopiva istutuspaikkatai taimienväärähoito istutustenjälkeen.

Hyvin usein taimiamyösvarastettiin koepaikoilta.

Aineiston valinta käynnistyi, kun taimet olivat noin 75 cm:nkorkuisia, eli niiden latvat olivat lumipeitteen ylä- puolellakeskitalvellakin. Tärkein valintaperuste oli ilmas- tollinen sopeutuminen, kuten pakkasenkestävyys, hallan- kestävyys jakasvukaudenaikaisen kuivuuden kestävyys.

Muita valintaperusteita olivatkukintaominaisuudet,leh- distönkoristeellisuus,pensaankasvutapa,kasvullisten ja suvullisten jaksojen rytmittyminen, kasvienterveysjakas- vien ikääntyminen.

Risteytysaineiston talvenkestävyyttätestattiin kylminä talvina 1984/85 ja 1986/87. Nämä kaksi talvea olivat useimmilla koepaikkakunnilla vuosisadan kylmimmät.

Erityisestitalvella ^1986/87esiintyi poikkeuksellisenrun- saasti juuristovaurioita. Mainittujen kahden kylmän tal- venseurauksena noin30 ^%risteytysaineistostakuoli tai vaurioitui niinpahoin, ettänepoistettiinkoekentiltä.

Risteytysjälkeläistöjenkesken oli huomattavia eroja tal- venkestävyydessä. Mustilanalppiruusun (R. b. subsp.

ligersledlii) jälkeläiset olivat talvenkestävyydeltään sel- västi parhaimmat. Kunseristeytettiin nukka-alppiruusun (R. smirnow'n⁾kanssa saatiin erityisen kestäviä jälkeläis- töjä. Samoin, ^kunpölyttäjänäkäytettiin puistoalppiruu- sua(R. calawbiense), saatiin hyvinkestäviä, joskaanei kovin koristeellisia jälkeläisiä. Resiprookkiset risteytyk- setosoittivat mustilanalppiruusun voimakkaan äidinvai- kutuksen talvenkestävyydenperiytymisessä. Sen sijaansen kookas kasvutapa ei periytynyt voimakkaasti jälkeläis- töön. Risteytyksistä kääpiölajikkeidenkanssa saatiin hy- vin hillittykasvuisia,usein kasvutavaltaan kompakteja hybridijälkeläisiä.

Risteytysaineistostavalittiin kaikkiaan noin80paras- ta hybridi yksilöä tarkempia lajikekokeita varten. Niiden lisäämiseksi kehitettiin meristeemilisäysmenetelmä, jon- ka avulla kloonattiin lajikekokeita varten tarvittavat tai-

met. Lajikekokeita alettiin ^perustaavuonna 1983 seit- semälle paikkakunnalle Etelä- ja Keski-Suomeen. Vuo- den ¹⁹⁸⁷ loppuunmennessä kuusi hybridi-kloonia oli laskettu kaupalliseen lisäykseen. Lajikkeet ’Elviira’ ja

’Hellikki’ olivatpunakukkaisia, ’Haaga’ ja’Universitas Helsingiensis350’ olivat vaaleanpunakukkaisia sekä ’St.

Michel’H42ja’P. M. A.Tigerstedt’olivat valkeakuk- kaisia.

Jalostusohjelmantuloksena ^on Suomeen saatu alp- piruusulajikkeisto, jonkaalhaisten lämpötilojenkestävyys on ainakin —35°C ja osa lajikkeista kestää jopa -40°C:een pakkasiailmanmainittavia vaurioita. Täten

alppiruusujen viljelyaluetta voidaan Euroopassa ja Amerikassa laajentaa sekä pohjoiseenettä ilmastoiiaan mantereisille alueille.

Monivuotisten puuvartisten koristekasvien jalostus vaatii runsaasti aikaa. Alppiruusun jalostusohjelma vie arviolta noin20vuotta. Talvenkestävyyden testauksessa kenttäolosuhteissaonotettavahuomioon se,ettäSuomes- saesiintyykeskimäärin 10—15vuoden väleinns. »pul- lonkaulavuosi»,eli normaalia kylmempi vuosi. Täten jälkeläiskokeitasuunniteltaessa onniiden kesto arvioitava siten,ettäainakin yksi »pullonkaulavuosi»osuukokeen ajaksi.