View of The effect of vesicular-arbuscular mycorrhizal inoculation on the growth and root colonization of ten strawberry cultivars

The effect of vesicuiar-arbuscular mycorrhizal inoculation _{on the} growth and ^root colonization ^{of ten} strawberry cultivars

Mauritz^Vestberg

Vestbero, M. 1992.The effect of vesicuiar-arbuscular mycorrhizal inoculation ^on thegrowthandrootcolonization of ten strawberry cultivars. Agric, ^Sci. Finl.

1:527-535.^(Agric. Res. Centre ofFinland,LaukaaRes. andElite^PlantUnit,^SF-41340 Laukaa,Finland.)

Ten strawberry cultivars,fourearly maturing, three latematuringand three “special”

cultivars,wereinoculated with six strains of vesicuiar-arbuscularmycorrhizal (VAM) fungi in apot experiment. ^Growtheffects and colonization of theVAM fungiwere studied. Three strains.Glomus macrocarpum V^3,G.mosseaeRothamsted and G. sp.

V^{4, were} highly efficient, causing significant growth increases in most cultivars.

’Jonsok’ showed the highest mycorrhizal dependency index, 648, and ’Ostara’ the lowest, 269,for themeanresponse of all six fungi. Thefungalstrains which increased shoot growth the most also increased the runner plant formation the most. Early cultivars showedhighercolonization percentages than late maturing cultivars. Sporula- tion of the introduced VAM fungi^{was onaveragemore}abundantin earlyand special cultivars than in late cultivars. Root colonization and strawberry shootdry weight correlatedsignificantly in^mostcultivars,but the correlation between colonization and runner^{formationwasgenerallypoor.}

Key words: VAM,strawberry cultivar, growth response, colonization, mycorrhizal dependency

Introduction

The variation in response of different plant species to vesicuiar-arbuscular mycorrhizal (VAM) infec-

tion is well documented (Powell 1986). Dif- ferences in response^tomycorrhizal infection is also observed amongcultivars, for example in pea (ES-

TAUN etal. 1987), wheat (Bertheau ^etal. 1980,

Stöppleret al. 1990, ^Vierheilig and ^Ocampo 1991), peach (Traquair and Berch 1988) and apple rootstocks (Granger et al. 1983), maize (Hall 1978, Toth et al. 1990), cowpea

(Rajapakse and Miller 1988), peanut (Daft 1991)and sorghum (Raju etal. 1990).^Brylaand Koide (1990) studied the role ofmycorrhizal infec- tion in the growth and reproduction of wild ac- cessions and cultivars oftomato.

In strawberry, the cultivar effect_was studied by Robertson et al. (1988), who found first-year greenhouse plants of strawberry always tobe _non- mycorrhizal, but colonization of plants growing in fumigated foundation (2nd year) and certified fields (3rd year) tobe highly variable between cultivars.

However,inaglasshousetrial,all strawberry culti- Agile.^Sei.Fint. 1 (1992)

vars were infected equally (Robertson et al.

1988). InMexico, Chavez and Ferrera-Cerrato (1990) observed differencesin strawberry cultivar responses toVAM fungal colonization.

The objective of this study was^todetermine the effect of six VAM strains onten strawberryculti- varscommercially grown in Finland.Differencesⁱⁿ response betweenearly maturing, late maturing and special cultivars were also evaluated. The experi- mentwascarriedout in_aglasshouse.

Material^and methods Cultivars

Ten cultivarswerestudied. Eight of these belonged to the species Fragaria ^x ananassa Duch. One cultivarwas aF. Virginiana Duch. hybride (’Alaska Pioneer’) and_one cultivar_{was a}F. _vescaL. (’Min- ja’). Withrespecttomaturity theyweredivided into early, late and “special” cultivars.’Jonsok’, ^’Zefyr’,

’Kristina’ and ’Mari’were early cultivars. The cul- tivars ’Bounty’, ’Hiku’ and ’Senga Sengana’ were regarded as late cultivars for Finnish growingcon- ditions. The special cultivars studiedweretheever- bearer ’Ostara’, as well as ’Alaska Pioneer’ and

’Minja’.

Inoculum

The inoculawereproduced by growing the VA-my- corrhizal fungi in maize (FI hybrid ’North Star’) rootsfor 103 days, from 24 Aprilto7 August, 1989, in aglasshouse. Asteam sterilized (three timeson successive days) sandwasusedasgrowth substrate for the inoculum. The substratewaslimedtogivea pH of approx 6.0,and fertilized with2 g 1

1

^bone

meal (Daft and Nicolson 1966). Phosphorus-free Hewitt solution_wasapplied_{once a} week. The ino- cula, ^a mixture of infected maizerootpieces with adheringsand, hyphae and sporeswere left todry and storedat⁺6°C untiluse. The strains and their

origin, the number of spores and thepercentage of rootinfectionwere as^follows;

Inoculum Strain

VAM-%spores/g

Name Origin

1.Glomus mossae, Kent,UK Rothamsted

4 126

2.Glomus sp.V^4, Kärsämäki, FIN

”thin-walled white”

48 1

3.G. macrocarpum Saarijärvi, FIN

V 3

30 ^< 1 4.G.intraradices V2oAnjala, FIN

5.G. sp.V2l/88, Pelkosenniemi,

"thin-walled white” FIN

27 ^< 1

49 9

< 1 38

6.G.mosseaeVIlb Nilsiä, FIN

Experiment

The experimentwasstarted in a glasshouseon 27 and 28 May, 1990, and it lasted 91 days. The growth substratewas asteamsterilized (1 honthree successive days) sand basically limed with 5 g f

1

dolomite lime topH 6, and fertilized with2 g 1

1

bone meal (Daft and Nicolson 1966). The sub- stratewas stored open foratleasttwoweeks after thesteamsterilization. Micropropagated strawberry plantlets were planted in 2.5 dl plastic Vefipots.

Theinoculum,(1 ml for each plant) which had been stored for 10months, ^wasplaced into the planting hole priortoplanting. After planting the_{pots were} placed on top ofan approx. 2 cm layer of wet vermiculite. During the experiment, the vermiculite substrate_was watered and aphosphorus-free He- witt solution (0.1%) was applied. The experiment was of_a split-plot design. During the experiment, the temperature in the glasshouse varied between +2O°C and +39°C.

Observations

Growth response atthe end of the experimentwas evaluated by the following criteria:

- shoot dry weight including motherplant andrun- ners

- number of runners/mother plant

- number ofrunnerplants/mother plant

Agric. Sei.Fint. 1⁽¹⁹⁹²⁾

Mycorrhizal dependency (MD) of strawberry cultivarswasdeterminedasthe ratio of dry weight of inoculated plants touninoculated plants multi- plied by 100. Thepercentage of infectedroot was determined by the gridline intersect method (GtO-

vannetti ^& Mosse 1980). The abundance of sporesin strawberrypots wasestimated from a50 ml_root and soil sample. The sample was washed through^a0.5 and^a0.074^mmsieve and theamount of spores detectedonthe sieveswasevaluated_{on a} rating scale of 0-5asfollows:

O⁼nospores

1⁼very few spores (1-5/50 ml substrate) 2⁼few spores (6-20/50 ml substrate)

3 ⁼ _amoderate number of spores (20-several hun- dreds/50 ml substrate)

4⁼spores abundantly 5⁼spores veryabundantly Results

Shout growth

Significantgrowth responses duetoVAM inocula- tion were observed in all cultivars (Fig. 1). Three strains, G. macrocarpum

V 3

^{(3), G. mosseae (1)}

Rothamsted and G. sp.

V 4

^{(2), were on average}

superiortotherest of the strains. G. macrocarpum

V 3

(3) increased shoot dry weight the mostin all cultivars.

In ’Minja’, all isolates used causeda significant growth increase. In ’Senga Sengana’ and ’Mari’, onlytwo outof six strains increased growth signifi- cantly. The restof the cultivars lay between these extremes(Fig. 1).

The group ofthe three less efficient strains tended to stimulate the growth of special cultivars in par- ticular. G. intraradices V2O (4) increased growth significantly in all the specialcultivars,^{inoneof the} late cultivars (’Hiku’) but innoneof the early culti- vars. Glomus sp. V2l/88 (5) increased growth sig- nificantly only in the early cultivar ’Kristina’ and in the special cultivar ’Minja’, while G. mosseae(6)

VI lb didsoin the early cultivar’Jonsok’,and in ali the special cultivars (Fig. 1).

’Jonsok’ had the highest mycorrhizal dependency index (MD), 648, and ’Ostara’ the lowestMD,269, for themeanresponse of all six fungi. When MD wascalculated for themostefficientstrain,^G.mac- rocarpum

V 3

(3), it varied from 920 in ’Hiku’ _to 366 in ’Mari’. When the least efficient VAM strain was used, the MD varied from 400 in ’Kristina’ to

138 in ’Hiku‘ and ’Ostara’ (Table 1).

Table 1.Mycorrhizal dependencyof tenstrawberrycultivars.

Mycorrhizal dependency(MD)

Cultivar Most Least

efficient efficient Averageof strain strain 6strains Early

’Jonsok’ 829 373 648

’Zefyr’ 522 229 358

’Kristina’ 614 400 470

’Mari’ 366 196 281

Late

’Hiku’ 920 138 626

’Senga Sengana’ 414 278 309

’Bounty’ 618 200 471

Special

’Ostara’ 319 138 269

’Alaska Pioneer’ 757 294 553

’Minja’ 475 252 278

Runner formation

With regard torunner plant production thestraw- berry cultivars used in the experiment could be divided into three groups. The cultivars ’Hiku’,

’Mari’, ’Bounty’ and ’Kristina’ produced only a minimum amount ofrunner plants during the_ex- perimental period. ’Senga Sengana’, ’Zefyr’, ’Jon- sok’, ’Ostara’ and ’Alaskan Pioneer’ produced ä moderate number ofrunnerplants, while ’Minja’

produced_runner plants abundantly. The results of

’Minja’ onlyarepresented here.

In ’Minja’, the three strains⁽

G.

macrocarpumV

3

(3), G.mosseaeRothamsted (1) and G. sp.

V 4

⁽²⁾⁾

Agric. Sei.Finl. 1⁽¹⁹⁹²⁾

Fig. 1.The effect ofVAMinoculationonshootdry weightand root colonization of tenstrawberrycultivars.

Potexperiment in^theglasshouse. Early maturing, latematuringand”special” cultivarsareindicatedbyletters A, Band C,respectively. For inoculation treatments (0⁼uninoculated, 1-6⁼inoculated) seematerial and methods. Numbers inside bars indicate root colonization percentages. Withincultivars, bars of shootdry weightsmarked with different letters differ significantly atP⁼0.05.

Sei.Finl. 1⁽¹⁹⁹²⁾ Aghc.

which caused the highest shoot growth increase, increased significantly also the numbers ofrunners and numbers ofrunnerplants per mother plant (Fig.

2). However, also G. mosseaeVI lb (6) increased significantly_runnerand_runnerplant production.

Root colonization and speculation

All cultivars tested showed mycorrhizal coloniza- tion. Only intwohost-fungus combinations,i.e. for strain V2l/88 (5) in ’Kristina’ and’Hiku’, no^root colonization occurred (Fig. 1). G.sp.

V 4

^{(2) caused}

the highest rootinfection,which varied from42%

in ’Hiku’ to 83^% in ’Senga Sengana’. The early cultivars hadonaverageahigher colonization per- centage than the late cultivars (Fig. 1, Table 2).

’Mari’ and ’Jonsok’, both early cultivars, had on average (of 6 VAM isolates) the highest^rootinfec- tion levels, ^{42.44% and} 42.09%, respectively.

’Hiku’, alatecultivar,had the lowest average VAM

infection,22.71%. Of the specialcultivars, ^’Alaska Pioneer’ had also a high average root infection which exceeded 40%. Due ^to a spread of VAM fungi, uninoculated plants were also infected in many instances.

Out of six VAMstrains,four sporulated under the prevailing conditions. The G.mosseaestrain from Rothamsted (1) sporulated in all cultivars and G.

macrocarpum (3) sporulated in all cultivars but

’Senga Sengana’ and ’Minja’. The highest average sporulationwasfound in ’Mari’ and ’Alaska pion- eer’. The average sporulation indexwas higher in the early maturing (0.88) and special (0.78) than in the late maturing (0.50) cultivars (Table 2).

Correlations

The strawberry shoot dry weights correlated signi- ficantly with VAM colonization in many cultivars (Table 3). A high correlationwasfound in the cul- Fig. 2.The effect ofVAMinoculationonthe number ofrunnersand the number ofrunnerplantsper mother plant incultivarMinja.Potexperiment intheglasshouse.For inoculation treatments (0⁼uninoculated, 1-6⁼ inoculated),_see material and methods. Bars ofrunners and number of^runnerplants,respectively,^marked

with different letters differsignificantlyatP⁼0.05.

Agric. Sei.Fin!. 1 (1992)

Table2.The effect ofVAMinoculationonrootcolonization and sporulationof tenstrawberrycultivarsas an average of six VAM isolates. Colonization percentages followed by different letters differ significantly atP⁼0.05.

Mean of6 VAM^isolates Strawberrycultivar Root Sporulation

colonization,^% 0-5 Early

’Jonsok 42.09^a 0.75

’Zefyr’ 31.38^ba 0.83

’Kristina’ 22.97^b 0.75

’Mari’ 42.44“ 1.17

Late

’Hiku’ 22.71^b 0.42

’Senga Sengana’ 34.86^ba 0.50

’Bounty’ 26.12^b 0.58

Special

’Ostara’ 27.97^b 0.58

’Alaska Pioneer’ 41.13“ 1.17

’Minja’ 30.90^ba 0.58

Table3. Significanceof correlation coefficients (r) of linear plots between percentage root colonization and shoot dry weight, the number of runners/motherplant and number of runnerplants/mother plant in 10strawberry cultivars. N ⁼ 35/cultivar. NS ⁼not significant. ^{*, **,} and ^*** indicate significances atthe 10%, 5%and 1% levels,respectively.

Correlationbetween root colonization and Number Number of

Shoot of runner

dry runners/ plants/

Cultivar weight plant plant

Early

’Jonsok’ ^*** NS ^**

’Zefyr’ ^{** *** **}

’Kristina’ ^*** NS NS

’Mari’ ^* NS NS

Late

’Hiku’ ^*** NS NS

’Senga Sengana’ NS NS NS

’Bounty’ ^{* NS NS}

Special

’Ostara’ ^{*** * *}

’Alaska Pioneer’ NS NS NS

’Minja’ ^{** *** NS}

tivars ’Jonsok’, ’Hiku’, ’Ostara’ and ’Kristina’, a moderate correlation in thecultivars ’Zephyr’ and

’Minja’, aweak correlation in the cultivars ’Mari’

and ’Bounty’, and_no correlation in ’Senga Senga- na’ and ’Alaskan Pioneer’. In contrast to this,run- nerformation and^rootcolonization correlated only occasionally in _a few cultivars. The numbers of runners per plant correlated significantly with VAM colonization in ’Zefyr’ and ’Minja’, and the number ofrunner plants per mother plant didso in

’Zefyr’ and ’Jonsok’.

Discussion

Differences in response of cultivars^toVAM inocu- lation have been demonstrated innumerousstudies.

In ^extreme instances, cultivars within one plant species may response toVAM inoculation in vary- ing degrees, from highly increasing to decreasing response (Bertheau et al. 1980). Plant species which are normally mycorrhizal have even been made non-mycorrhizal by mutation (Due et al.

1989).However, lack of differences in cultivarre- sponsetoVAM inoculation has also been reported (Grangeretal. 1983,Traquair^&Berch 1988).

In the present study, all Finnish commercial strawberry cultivars responded positively toVAM inoculation during the prevailing experimental con- ditions in the glasshouse. However,^thereweredif- ferences in response to fungal strains, which

stressesthe importance of screening for mycorrhi- zal efficiency in VAM research. Of sixstrains,^three were highly efficient, causing significant growth responses rather uniformly in all the cultiv-ars.

However, ^a group of three other strains were less efficient,which resulted in differentpatterns of_re- sponsein the cultivars. This group of strains tended toincrease shoot growth of strawberry themostin the special cultivars, including the semi-wild ’A- laska Pioneer’ and ’Minja’ cultivars. This indicates a difference in response in wild or semi-wild_vs cultivatedgenotypes of strawberry toVAM infec- tion. Such differences in response have been found also by Koide etal.(1988) and ^Bryla and Koide

Agric. Sei.Finl. 1⁽¹⁹⁹²⁾

(1990) inoatsandtomato.However, contrarytoour study, they found that the cultivars_{were as a}group moreresponsive to mycorrhizal infection than the wild accessions.

Mycorrhizal dependency (MD) is used as an indexto compare thereceptivity of different plant species or cultivars to VAM fungi (Gerdemann 1975). The MD can vary from none to complete dependency. _However, the MD of a host _can be altered byanumber offactors suchassoiltype,soil phosphorus content, mycorrhizal species, etc.

(MENGEetaI. 1978,Azcon^&Ocampo1981).This wasalso demonstrated in thepresentstudy, where the MD index for strawberry cultivars variedcon- siderably when calculated for themostor for the least efficient VAM strain. The MD ofthe cultivars varied from648 in ’Jonsok’to281 in’Mari’,^butno distinct differences_were observed between early, late _or special cultivars when the MD was calcu- latedas anaverage for all sixVAM strains. Itcanbe concluded that all strawberry cultivars tested show a moderate mycorrhizal dependency under low- phosphorus conditions.

A number of studies have been conductedtofind outthe_reasons for differences in MDatthe cultiv- arlevel. ^Baylis(1975) reported that the length of roothairs and the thickness ofrootscandetermine the MD level ofaplant speciesorcultivar. ^Menge etal. (1978) suggested that the number of feeder rootsin citrus cultivars might correlate with MD. In another study with citrus, ^{Graham and Syvert-} sen (1985) showed that the leaf P concentration, fineness ofroots, growthrate,hydraulic conductiv- ity, transpiration and COz assimilationrate were linkedtoMD. In cowpea,^Rajapakseand Miller

(1988) observed that the average length of fine rootswasnegatively correlated with MD.However, only27 ^% of the variability in MDwas explained by this variable.Therefore,they conclude thatroot morphology didnotappeartodetermine the MD in cowpea.

Early strawberry cultivars showed_on average a higherroot colonization and speculation than late cultivars. The _reason for this remains unknown, because _{no root} growth parameters were deter-

mined in thepresent study. However, it is _awell- known fact that cultivars differ from each other withrespect to root morphology. Alsorootexuda- tion has been foundtovary greatly between cultiv- ars. Azcon and ^Ocampo (1981) found that the absence of mycorrhizal infection in some wheat varietieswas associated with lack of sugar exuda- tion from therootsrather than with the sugarcon- tentof the_roots.In their study, VA infection ledto adecrease in the reducing and total sugarcontentof root extracts, and this effect correlated with the degree of mycorrhizal infection.

The results of this studycannotbe directlyextra- polated to field conditions, ^{where a much more} complex _system of microorganisms, indigenous VAM fungi, nutrients and soil animals prevails.

However, in strawberry fields of low soluble P and low incidence of indigenous VAM fungi (such fields do occur), strawberry cultivars can be ex- pectedto react to inoculationby differentarbuscu- lar mycorrhizal fungi inasimilarmanner tothat of thepresent potexperiment. In conclusion, however, the cultivar responsetomycorrhization under field conditions needs further study.

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ManuscriptreceivedJuly 1992 Mauritz Vestberg

AgriculturalResearch Centre ofFinland Laukaa Research and Elite Plant Unit SF-41340Laukaa,Finland

Agric. Sei.Fin!. 1⁽¹⁹⁹²⁾

SELOSTUS

Vesikkeli-arbuskcli mykorritsasieniymppäyksen vaikutus kymmenen mansikkalajikkeen kasvuun ja mykorritsainfektioon

Mauritz^Vestberg

Maatalouden tutkimuskeskus

VA-mykorritsasieniymppäyksen vaikutusta kymmeneen Suomessaviljeltävään mansikkalajikkeeseen tutkittiin astia- kokeessa. Kaikkien lajikkeidentaimet olivatMTTK:nLau- kaan tutkimus- ja valiotaimiasemalla tuotettuja mikrolisät- tyjä pikkutaimia. Kasvualustana käytettiin luujauholla (2 g l"¹⁾ lannoitettua steriiliä hiekkaa. Taimille annettiin myös kerran viikossafosforivapaataHevvittin ravinneliuosta.

Koeperustettiin 27.-28.toukokuuta 1990 jasekesti 91 vuo- rokautta,Lajikkeet jaettiinaikaisiin (’Jonsok’, ’Zefyr’, ’K- ristina’ ja ’Mari’), myöhäisiin (’Bounty’, ’Hiku’ ja ’Senga Scngana’)ja erikoislajikkeisiin (’Ostara’,’Alaskanpioneeri’

ja ’Minja’). Sienikantojenlukumääräolikuusi, viisisuoma- laistajayksiulkomainen.Erilajikkeille laskettiin myös my- korritsariippuvaisuusindeksi (MRI), jokasaatiin jakamalla ympätyn taimenkuivapainoymppäämättömän taimen kuiva- painolla jakertomalla tulos sadalla.

Vallitsevissa olosuhteissa kaikki tutkitut mansikkalajik- keet hyötyivät VAM-ymppäyksestä, Kolme sienikantaa.

Glomus macrocarpum Suomesta, G.mosseaeEnglannista ja määrittämätönGlomus-\a)\V 4^{Suomesta,olivathyvintehok-}

kaitajalisäsivät merkittävästi mansikalla sekäversonkasvua ettärönsyjonojen ja rönsytaimien tuotantoa. Lajikkeiden välisiä eroja ei mainittavasti ollut. ’Jonsok’ oli keskimäärin enitenriippuvainen mykorritsaymppäyksestä(MRI⁼648)ja

’Ostara’ vähiten (MRI ⁼269). Ymppäyksen seurauksena aikaisilla lajikkeilla oli keskimäärin korkeampi juuriston prosentuaalinen raykorritsainfektio kuin myöhäisillä lajik- keilla. VAM-sienet muodostivat myös itiöitä runsaammin aikaisissaja erikoislajikkeissakuinmyöhäisissä lajikkeissa.

Useimmilla lajikkeilla juuriston mykorritsasienen prosen- tuaalinen runsaus korreloi sienen aiheuttaman versonkas- vunlisäyksen kanssa, mutta rönsyntuotannon kanssa olimer- kitsevää korrelaatiota harvoissatapauksissa.

Agric.Sei.Fin!. 1⁽¹⁹⁹²⁾