Plant and Soil220: 207–218, 2000.© 2000Kluwer Academic Publishers. Printed in the Netherlands.

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Wheat responses to aggressive and non-aggressive arbuscular mycorrhizalfungi

J. H. Graham1,∗ and L. K. Abbott21University of Florida, Citrus Research and Education Center, Lake Alfred, 33850 USA;2Soil Science & PlantNutrition, Univ. Western Australia, Nedlands, WA 6907 Australia

Received 28 July 1999. Accepted in revised form 12 January 2000

Key words:carbon costs, host-fungus compatibility, parasitism, plant growth depression, root colonization, soilphosphorus supply

Abstract

In southwestern Australia fields, colonization of wheat roots by arbuscular mycorrhizal fungi (AMF) is reduceddue to repeated use of phosphate (P) fertilizers. We predicted AMF that aggressively colonize wheat roots atlow P supply would also aggressively colonize at high P supply, but provide no additional P uptake benefit andreduce growth. Wheat (cv. Kulin) seedlings were non-mycorrhizal (NM) or inoculated separately with 10 isolatesof AMF from wheat-belt soils in a glasshouse experiment. Kojonup loamy sand was supplied with P to providesuboptimal and supraoptimal P for growth of NM wheat in this soil. At low P supply, wheat growth was limitedby P availability. All AMF isolates colonized wheat roots at 14 days after emergence of seedlings. At 42 days,percentage root length colonized (%RLC) was highest for two isolates ofScutellospora calospora,WUM 12(2)and WUM 12(3), followed byGlomussp. WUM 51,G. invermaiumWUM 10(1),Acaulospora laevisWUM 11(4)andGigaspora decipiensWUM 6(1). These isolates, designated as ‘aggressive colonizers’, ranged from 50 to89%RLC. A second group of AMF ranged from 1 to 19%RLC at 42 days. This group, termed ‘non-aggressivecolonizers’, includedAcaulosporaspp. WUM 11(1), WUM 46, and WUM 49 andGlomussp. WUM 44. High soilP supply increased seedling growth 2–3 fold, but reduced%RLC. Grouping of aggressive and non-aggressive AMFbased on colonization rate at high P supply was similar to that at low P. At low P supply, only the two isolatesof S. calosporaincreased wheat growth compared to the NM plant. The remaining aggressive and non-aggressiveAMF reduced growth of wheat at low P, while aggressive colonizers reduced growth at high P. At low P supply,the aggressive colonizers increased shoot P concentration, while at high P, shoot P was not affected by AMF.Growth depression by aggressive colonizers was associated with reduced sucrose concentration in roots. Based onthe negative growth response under low and high P fertility in the glasshouse, AMF could be expected to producenon-beneficial effects on wheat in the field depending on the P status of the soil and the aggressiveness of AMF inthe community.

Introduction

Arbuscular mycorrhizal fungi (AMF) are a major de-terminant of plant growth response in a crop soil.Effective AMF are those that produce the greatest be-nefit in terms of increased P acquisition for the leastcost expressed as C expenditure on mycorrhizas (Gra-ham and Eissenstat, 1994). Species of AMF in the

∗ FAX No: 863 956 4631. E-mail: jhg@lal.ufl.edu

generaGlomusandAcaulosporaare considered moreeffective and are more abundant in many agriculturalsoils than species ofGigasporaandScutellospora, thatmay decrease in abundance under cultivation (Millerand Jastrow, 1992; Siqueira et al., 1989). Obviously,agricultural managements such as tillage, fertilizationand crop rotations exert selection pressure in favorof AMF that are tolerant of such practices (Johnsonand Pfleger, 1992). Although presently a controversialconcept, some researchers have suggested that certain

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decline diseases of crops may be related to a shift frombeneficial to non-beneficial or even parasitic fungi incontinuous crop monoculture under moderate to highfertility conditions (Johnson et al., 1992; Johnson,1993; Schenck and Siqueira, 1987).

Glasshouse studies predict AMF that aggressivelycolonize roots and stimulate plant growth at low Psupply (Abbott and Robson, 1981), will also aggress-ively colonize at high P supply, but may provide noadditional P benefit and reduce growth (Graham etal., 1996). Wilson and Trinick (1983) first used theterm ‘aggressiveness’ to refer to the ability of AMF tocompete with other fungi for colonization space in theroot. In contrast, Graham et al. (1996) defined aggress-iveness as the rate of root colonization in the absenceof competing AMF. Rate of colonization is a usefulpredictor of growth response to increased P uptake byindividual AMF from low P soil (Abbott and Robson,1981). Fungi that aggressively colonize roots com-monly occur in the field (Abbott and Robson, 1991;Brundrett, 1991), but their relevance to mycorrhizaleffectiveness remains to be determined (Abbott andGazey, 1994). Carbon cost analysis of citrus (Grahamet al., 1996; Peng et al., 1993) and observations ofseveral crops in the field reveal that effectiveness ofAMF from managed crop soils ranges from mutual-istic to parasitic depending mostly on the soil P supply(Johnson et al., 1997). In moderate to high P soils,growth of field-grown citrus supports the hypothesisthat within communities of AMF there exist aggress-ive fungi capable of colonization sufficient to producecarbon cost without off-setting benefit (Graham andEissenstat, 1998).

Given the pervasive over-application of P to cropsoils, decreasing input of P fertilizer is most often citedas the approach to increase symbiotic effectiveness.However, such changes in fertilizer management mustbe based on knowledge of the functional attributes ofAMF species and populations. These attributes are:1. rate and development of colonization of roots (Ab-bott and Robson, 1981); 2. proliferation of externalhyphae in relation to P acquisition (Jakobsen et al.,1992); 3. carbon cost to support growth and mainten-ance of the mycorrhizal root (Peng et al., 1993). Rootsof wheat become colonized by AMF to different ex-tents depending on climate, soils, cropping practicesand fertilizer history. In Australian wheat fields, col-onization of wheat roots by AMF is reduced due torepeated use of phosphate fertilizers (Ryan et al.,1994;C. Blackburn, L. K. Abbott and A. D. Robson, un-published data). The impact of AMF on wheat growth

under high P fertility is not established. However,Thompson (1987) showed that reduction in mycor-rhizal colonization by fallowing had a minimal effecton growth of wheat in cracking clay soil with mod-erate levels of bicarbonate extractable P (10–16 mgkg−1 soil) from the northeastern area of the Australianwheat belt.

The primary objective of this study was to eval-uate the rate and extent to which AMF isolated fromsouthwestern Australia soils colonize wheat roots. Thesecondary objective was to determine how this col-onization affected plant growth, P nutrition and car-bohydrate status of roots at soil P availability similarto levels that have accumulated after fertilization ofwheat fields.

Materials and methods

Soil conditions

Unfertilized Kojonup loamy sand with a pH of 6.0(1 mM CaCl2) and bicarbonate extractable P of4.0 mg.kg−1(Colwell, 1963), was collected fromnative bush near Clackline, Western Australia. Soilwas passed through a 1.4 mm sieve to removecoarse organic matter and then pasteurized withaerated steam for 1 h on two consecutive daysat 70 ◦C. Nutrients were mixed with dry soil atthe following rates (mg.kg−1dry soil): K2SO4, 71;CaCl2, 71; MGSO4

.7H2O, 20; MnSO4.H20, 10;

ZnSO4.7H2O, 5; CuSO4

.H2O, 2; CoSO4.7H2O, 0.35;

NaMoO4.2H2O, 0.18 to prevent nutrient deficiencies

in wheat except for P and N. Nitrogen was suppliedevery 2 weeks at 37.5 mg kg−1 as NH4NO3. Phos-phorus was amended with soil at 10 and 60 mg.kg−1

P, as KH2PO4. The two rates of P provided 4 and42 mg.kg−1extractable P for either suboptimal andmaximal growth of non-mycorrhizal (NM) wheat.Phosphorus supply levels were based on the extract-able P range from southwestern wheat-belt fields (20–45 mg.kg−1) and on the extractable P in Kojonuploamy sand that produced a beneficial response toAMF (approx. 8–10 mg kg−1) in a preliminary glass-house trial of wheat (Triticum aestivumcv. Kulin).

AMF and colonization potential calibration

Ten isolates of the four major genera of AMF,Glomus,Acaulospora, GigasporaandScutellospora, were ob-tained from agricultural soils, revegetated land and

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Table 1. Description of arbuscular mycorrhizal fungi (AMF) studieda

AMF species Isolate no. Origin

Acaulospora laevis WUM 11(4) Badgingarra, WA, low pH, under pasture

A. laevis WUM 11(1) Mt. Barker, WA, low pH, under wheat

Scutellospora calospora WUM 12(2) Badgingarra, WA, low pH, under native bush

S. calospora WUM 12(3) Badgingarra, WA, low pH, under pasture

Gigaspora decipiens WUM 6(1) Harvey, WA, clover pasture

Glomus invermaium WUM 10(1) Merredin, WA, capeweed/clover

Glomussp. WUM 44 Jarrahdale, WA, sandy loam, pH 6.0, revegetated

Acaulosporasp. WUM 46 Jarrahdale, WA, sandy loam, pH 4.8, Jarrah forest

Acaulosporasp. WUM 49 Jarrahdale, WA, sandy loam, pH 5.0, undisturbed heathland

Glomussp. WUM 51 Capel, WA, sand, pH 5.8, under pasture

aAll fungi in pot culture at the University Western Australia, Soil Science and Plant Nutrition.

native bush representative of wheat belt soils in south-western Australia (Table 1). To compare wheat re-sponses to several AMF, it was necessary to verifyand calibrate the colonization potential of each isol-ate prior to the study. Pot cultures of the 10 fungipropagated on subterranean clover (Trifolium subter-raneumcv. Seaton Park) in Lancelin sand (Gazeyet al., 1992) were assayed on subterranean clover atthree inoculum densities: 10, 25 and 75 g pot culturesoil per 0.5 kg Kojonup loamy sand and screened onwheat at 75 g per 0.5 kg. Phosphorus supply (20 mgkg−1 P added as KH2PO4) was low for subterraneanclover and moderate for wheat. Nitrogen was suppliedas NH4NO3 for wheat and rhizobial inoculation forN2-fixing subterranean clover. The trial was conduc-ted in the glasshouse from July to August 1996 with∼600µmol.m−2. s−1 photosynthetically-active radi-ation (PAR) and a controlled soil temperature of 20◦C.Seedlings were harvested at 28 days after emergenceand percentage root length colonized (%RLC) was de-termined by clearing and staining, and measurementin roots of hyphae, arbuscules or vesicles dependingon the AMF by the line intersect method (Giovanettiand Mosse, 1980).

The 10 isolates colonized wheat and produced20–40%RLC on subterranean clover after 28 days atthe intermediate and high inoculum densities. An in-oculum density of 100 g per 1.5 kg soil was chosenfor further characterization of symbiotic effectivenessin the final experiment.

Plant growth conditions and harvest

Pre-germinated wheat seedlings were transplantedinto non-inoculated Kojonup loamy sand or soil in-

oculated with each of 10 AMF contained in pots of1.5 kg soil. Non-mycorrhizal (NM) plants receiveda microbial extract passed through a 38µm sieveof the composite of soil from the 10 inoculant fungi(Table 1). Mycorrhizal treatments were exposed to asimilar combined extract of the 10 AMF. The glass-house experiment was conducted from September toNovember 1996 (spring) under a light intensity of∼800µmol.m−2. s−1 PAR and controlled soil temper-ature conditions (20◦C). The experimental design wascompletely randomized with 2 P levels, 11 AMF (in-cluding NM) and three harvests at 14, 28, and 42 daysafter seedling emergence. There were three replicateseedlings each harvest per AMF and P treatment.

Plant growth and other parameters were determ-ined at each of three harvests. Seedlings were analyzedfor dry weight (70 ◦C, 24 h) and percentage rootlength colonized (%RLC). Leaf nutrient concentrationof P, K, Ca, Mg, Fe, Zn, Mn, Cu was analyzed byinductively coupled plasma atomic emission spectro-scopy after acid digestion of dried and ground tissue.Root starch and sucrose were analyzed in ground roottissue as previously described (Graham et al., 1996)and expressed on a percent dry weight basis. Plantgrowth and carbohydrate responses to AMF were ex-pressed relative to the NM response by calculatingthe mycorrhizal response as [AM dry weight/NM dryweight] - 1. A three-factor analysis of variance (AMF,P supply, harvest) for each parameter was performedusing the general linear models procedure (SAS Insti-tute, Cary, NC USA). The%RLC of groups of AMFwas compared by orthogonal contrast analysis atP≤ 0.05. Comparisons of parameters within groupsof AMF were performed at each harvest time using

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Figure 1. Percentage root length colonized (%RLC) and mycorrhizal response of wheat grown at low and high P supply at 14, 28 and 42 daysafter inoculation with six aggressive arbuscular mycorrhizal fungi (AMF) from southwestern Australia soils. Mycorrhizal response is calculatedas [AM dry weight response/non-mycorrhizal response] – 1. Significant differences (P≤0.05) in %RLC among AMF at 42 days are indicatedby unlike letters following each curve. For mycorrizal responses, the bars represent +/− 1 standard error of 3 replications. See Table 1 foridentification of the AMF.

Student-Newman-Keuls multiple range test atP ≤0.05.

Results

All treatment factors, AMF, P supply and harvest, sig-nificantly affected and interacted with root coloniza-tion (Table 2). At low P supply, AMF colonized wheatroots at 14 days after emergence, the earliest stage

of seedling development evaluated (Figures 1 and 2).Isolates ofS. calospora, WUM 12(2) and WUM 12(3),had already produced abundant external hyphae andauxiliary cells at this early stage of colonization. At42 days,%RLC was highest for the two isolates ofS.calospora,WUM 12(2) and WUM 12(3), followed byGlomussp. WUM 51, G. invermaiumWUM 10(1),Gi. decipiensWUM 6(1) andA. laevisWUM 11(4)(Figure 1). These six isolates, designated as ‘aggress-

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Figure 2. Percentage root length colonized (%RLC) and mycorrhizal response of wheat grown at low and high P supply at 14, 28 and 42 daysafter inoculation with four non-aggressive arbuscular mycorrhizal fungi (AMF) from southwestern Australia soils. Mycorrhizal response iscalculated as [AM dry weight response/non-mycorrhizal response] – 1. Significant differences (P≤0.05) in%RLC among AMF at 42 days areindicated by unlike letters following each curve. For mycorrizal responses, the bars represent +/− 1 standard error of 3 replications. See Table 1for identification of the AMF.

ive colonizers’, ranged from 50–89%RLC. Orthogonalcontrast analysis separated (P ≤ 0.05) the aggressivecolonizers from a second group of AMF that rangedfrom 1 to 19%RLC after 42 days (Figure 2). Thisgroup, identified as ‘non-aggressive colonizers’, in-cluded Acaulosporaspp. WUM 11(1), WUM 46,WUM 49 andGlomussp. WUM 44. The%RLC ofonly one isolate in this group,A. laevisWUM 11(1),increased with time.

High soil P supply reduced%RLC, but did notprevent colonization by any of the fungal isolates. Al-though there was a highly significant (P ≤ 0.001) Pinteraction with individual AMF (Table 2), groups ofaggressive and non-aggressive colonizers were sim-ilar to those at low P supply (Figures 1 and 2). Theaggressive AMF ranged from 4 to 10%RLC after42 days, and no significant differences in coloniza-tion occurred among these isolates. Among the non-

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Table 2. Main effects and interactions of colonization by 10 arbuscular mycorrhizal fungi (AMF) (seeTable 1) on percentage root length colonized, total plant dry weight, nutrient concentration and rootcarbohydrate status of wheat grown at high and low P supply (P) and harvested at 14, 28 and 42 daysafter plant emergence (HVST) compared to non-mycorrhizal wheat

Source Root length Total dry Leaf P Leaf N Root Root

colonized (%) weight (g) (%) (%) starch (%) sucrose (%)

P ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗AMF ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ NS ∗∗∗HVST ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗P∗AMF ∗∗ ∗∗∗ ∗∗∗ NS NS ∗∗P∗HVST ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ∗∗∗AMF∗HVST ∗∗∗ ∗∗∗ ∗∗ ∗ NS ∗∗∗P∗AMF∗HVST ∗∗∗ ∗∗∗ ∗ ∗∗ NS ∗

Significance of F:∗∗∗ = 0.001,∗∗ = 0.01,∗ = 0.05, NS = non-significant.

aggressive isolates,A. laevis WUM 11(1) was theonly isolate whose colonization was not significantly(P ≤ 0.05) less than the aggressive group by 42 days(Figure 2).

Plant growth responses to root colonization

All treatment factors significantly affected total dryweight (Table 2). High P supply increased plantgrowth 2–3 fold after 28 days compared to low Pplants (data not shown). At low P supply, colonizationby aggressive and non-aggressive AMF as groups pro-duced significant negative growth responses of wheat(mycorrhizal response = [AM response/NM response]– 1) at 14 and 28 days (Figures 1 and 2). By 42days, the two isolates ofS. calospora,WUM 12(2)and WUM 12(3), significantly (P ≤ 0.05) increasedtotal dry weight of seedlings compared to the NMplant, while Glomussp. WUM 51 andGi. decip-iens WUM 6(1) significantly (P ≤ 0.05) decreasedgrowth (Figure 1). Among the non-aggressive colon-izers, onlyGlomussp. WUM 44 significantly (P ≤0.05) depressed growth after 42 days (Figure 2).

At high P supply, mycorrhizal response was alsonegative, but the significance of the response differedfor the groups of aggressive and non-aggressive col-onizers (Figures 1 and 2). At 14 and 28 days, theaggressive colonizers significantly (P≤ 0.05) reducedtotal dry weight (Figure 1), while non-aggressive col-onizers depressed growth only at 14 days after emer-gence (Figure 2). By 42 days at high P supply, wheatroots became constrained by pot volume such thatcomparisons of treatment effects on seedling growthwere no longer valid.

Nutrient and CHO responses to AMF isolates

Supply of P, AMF and harvest significantly affectedleaf P and N concentrations (Table 2). As plants grew,there was dilution of leaf nutrient concentrations (Fig-ures 3 and 4). At low P supply, leaf P concentrationsdropped below the sufficiency level (0.4%) by 28 days.At low P supply, most of the aggressive colonizersmaintained a higher shoot P concentration comparedto P-deficient, NM wheat (Figure 3). From this groupof isolates,S. calosporaWUM 12(3) and WUM 12(2)significantly (P ≤ 0.05) increased P status at 28 daysandGlomussp. WUM 51 andA. laevisWUM 11(4) at42 days compared to NM plants. At high P supply, leafP status of wheat seedlings was more than adequateand not affected by AMF. Hence, there was a signific-ant interaction of P supply with mycorrhizal treatment(Table 2, Figures 3 and 4).

Although treatment factors affected leaf N concen-tration (Table 2), N did not limit growth at low or highP supply (Figures 3 and 4). Other nutrients analyzedwere in the sufficiency range for optimal growth ofwheat except Mn which accumulated to toxic levelsin plants inoculated withAcaulosporaspp. WUM 49at high P supply (data not shown). The Mn toxicitymay have been responsible for the sudden develop-ment of chlorosis in leaves and the highly negativegrowth response to this isolate at 42 days (Figure 2).

There appeared to be negative responses of bothstarch and sucrose concentration in 14-day-old rootscolonized by AMF at both low and high P supply, butthe effect of AMF was only significant (P≤ 0.05) forsucrose (Table 2, Figures 5 and 6). In high P plants,aggressive colonizers significantly (P≤ 0.05) reducedsucrose concentration in roots at 28 days. At low P

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Figure 3. Shoot phosphorus and nitrogen concentration in shoots of wheat grown at low and high P supply at 14, 28 and 42 days after inoculationwith six aggressive arbuscular mycorrhizal fungi (AMF) from southwestern Australia soils compared to the non-mycorrhizal control response.At 28 and 42 days, the bars represent +/− 1 standard error of 3 replications (reps are pooled at 14 days). See Table 1 for identification of theAMF.

supply, aggressive colonizers significantly (P≤ 0.05)decreased sucrose in roots at 42 days.

Discussion

Arbuscular mycorrhizal fungi have been assumed tohave either a beneficial or neutral effect on crop pro-duction in fertilized agricultural fields. The resultsof this glasshouse study of AMF from southwesternAustralian soils provide a different perspective for

management of mycorrhizas on wheat, and perhapsother crops grown in high P fertility soils. Field sur-vey of wheat soils predicted that AMF would havethe ability to colonize wheat roots at early stagesof crop development (Ryan et al., 1994; Blackburnet al., unpublished data). This observation was con-firmed in that all 10 AMF colonized wheat roots tosome extent by 14 days after seedling emergence.Studies of colonization of clover and citrus (Abbottand Robson, 1981; Graham et al., 1996) predicted

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Figure 4. Shoot phosphorus and nitrogen concentration in shoots of wheat grown at low and high P supply at 14, 28 and 42 days afterinoculation with four non-aggressive arbuscular mycorrhizal fungi (AMF) from southwestern Australia soils compared to the non-mycorrhizalcontrol response. At 28 and 42 days, the bars represent +/− 1 standard error of 3 replications (reps are pooled at 14 days). See Table 1 foridentification of the AMF.

that isolates ofGlomus and other genera of AMFwith relatively high colonization rates would have agreater tendency to increase P uptake at limiting Psupply than non-aggressive colonizers. The majorityof the aggressive colonizers increased shoot P statusof P-deficient wheat but positive growth response, ifpresent, was marginal. Non-aggressive AMF were in-effective for increasing P acquisition and growth ofwheat seedlings. Furthermore, as predicted from be-havior of Glomusspp. on citrus seedlings in high P

soil (Graham et al., 1996), AMF maintained the ag-gressiveness phenotype on P-sufficient wheat. Severalaggressive isolates depressed early growth of wheat athigh P supply. Aggressive AMF originated from soilsin native plant communities as well as agriculturalsoils. Thus, aggressive behavior of AMF occurs evenin the absence of selection pressure created by agri-cultural management. These management factors maynot necessarily be a prerequisite for parasitic behaviorof AMF as previously postulated (Johnson, 1993).

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Figure 5. Response of nonstructural carbohydrates (starch and sucrose) in root tissues of wheat grown at low and high P supply at 14, 28 and42 days after inoculation with six aggressive arbuscular mycorrhizal fungi (AMF) from southwestern Australia soils. Mycorrhizal response isthe AM [starch or sucrose]/non-mycorrhizal [starch or sucrose] – 1. At 28 and 42 days, the bars represent +/− 1 standard error of 3 replications(reps are pooled at 14 days). See Table 1 for identification of the AMF.

Growth depression of citrus seedlings by aggress-ive colonizers has been explained by the linkagebetween early and rapid colonization and greater Ccosts associated with the formation and maintenanceof the symbiosis (Peng et al., 1993). As predictedfrom citrus studies (Graham et al., 1996), aggress-ive colonizers decreased the sucrose status of wheatroots presumably because more CHO was utilizedfor colonization construction and maintenance. How-ever, non-aggressive colonizers of P-deficient wheat

did not respond as predicted by balance of C costwith P benefit. Non-aggressive AMF were poor col-onizers at low P, yet depressed growth of wheat by 28days after seedling emergence. In P-sufficient wheat,the non-aggressive phenotype was maintained, but theisolates did not depress growth. The negative responseof wheat to non-aggressive AMF at low P could notbe linked to greater utilization of CHO in roots asdemonstrated for aggressive colonizers.

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Figure 6. Response of nonstructural carbohydrates (starch and sucrose) in root tissues of wheat grown at low and high P supply at 14, 28and 42 days after inoculation with four non-aggressive arbuscular mycorrhizal fungi (AMF) from southwestern Australia soils. Mycorrhizalresponse is the AM [starch or sucrose]/non-mycorrhizal [starch or sucrose] – 1. At 28 and 42 days, the bars represent +/− 1 standard error of 3replications (reps are pooled at 14 days). See Table 1 for identification of the AMF.

Growth depression of modern wheat lines in lowP soil by Glomus spp. was first reported by Het-rick et al. (1992). They found a positive relationshipbetween growth response and higher colonization by10 Glomusspp. from North America and Europe forthe land race of wheat, cultivar, ‘Turkey’. However,the modern cultivars, ‘Newton’ and ‘Kanzler’, re-sponded negatively to inoculation with almost all ofthe Glomus isolates. No relationship was observed

between extent of root colonization by AMF andgrowth depression of the modern wheat lines.

The rate and extent that wheat, citrus and othercultivars and species of plants become colonized isgenetically regulated (Graham and Eissenstat, 1994;Hetrick et al., 1992; 1993; Smith et al., 1992). Ingenetically unimproved species of citrus, regulationof colonization has been functionally linked with theC availability to the fungus during the colonizationprocess (Graham et al., 1997). Studies by Hetrick et

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al. (1991) suggest that extent of colonization of nat-ive grasses is related to root fibrousness and linkedto mycorrhizal dependency. This relationship may nolonger be maintained in highly bred wheat cultivarsthat are selected for yield response and disease resist-ance when grown under high fertility regimes (Hetricket al., 1993).

Toth et al. (1990) suggested from their study ofmaize that cultivars containing genes for resistanceto fungal pathogens and other diseases are less com-patible with AMF depending on the breeding line.Recently, Douds et al. (1998) described an AMF byhost interaction for two cultivars of alfalfa (Medicagosativa) and Paspalum notatum. Pot culture inocula-tions with G. intraradicesandGi. margaritashowednormal colonization progress and sporulation ofG.intraradiceson P. notatumand alfalfa, but reducedcolonization and sporulation byGi. margaritaon thealfalfa cultivars compared to the grass species.In vitroinoculation of root organ cultures of alfalfa elicited ahypersensitive-like response toGi. margaritathat con-trasted with normal colonization byG. intraradices.Roots colonized byGi. margaritabecame necrotic andcontained elevated phenolics and isoflavanoids. Thedata support the existence of specificity between AMFand genetically improved plant cultivars, but not inunimproved species likeP. notatum.

Hetrick et al. (1993) demonstrated that mycor-rhizal dependency is least in the AABB tetraploid andthe AABBDD hexaploid genomes of wheat. In ourstudy, the negative responsiveness of cultivar Kulin,a hexaploid AABBDD bread wheat, supports theconcept of the linkage of certain alleles with low de-pendency. Based on the negative growth responses ofthese wheat genotypes, Hetrick et al. (1993) concludedthat colonization of wheat at the earliest stages shouldnot necessarily be considered beneficial and perhapseven detrimental to crop growth.

Our observations extend previous studies to focuson response of one modern wheat cultivar in high Pavailability soils that are prevalent in southwesternAustralia and elsewhere in the world. While growthdepressions by aggressive fungi under high P condi-tions are probably due in part to C cost without P ac-quisition benefit, negative responses to non-aggressiveAMF may be also attributable to interactions relatingto host-fungus incompatibility like those described foralfalfa (Douds et al.,1998). If so, then reduction offertilizer P supply to modern wheat lines to bring costand benefit into balance and increase reliance on themutualistic behavior of the symbiosis is questionable.

Modern wheat lines may require re-selection underlow input agricultural conditions before reducing Psupply in soils that will increase colonization by AMFperhaps without commensurate crop benefit.

Alternatively, cultural practices to reduce earlyseason colonization rate by indigenous fungi may havepositive impact on wheat vegetative and reproductivegrowth under high P fertility conditions as previouslysuggested (Graham and Eissenstat, 1998). This pre-diction is based on studies of maize in the midwesternNorth America. Field trials have repeatedly demon-strated increases in maize growth and yield up to1000 kg.ha under high P fertility (>50 kg P.ha) ifthe soils are conventionally tilled to reduce coloniza-tion by AMF compared to no-till crop practice (Gavitoand Miller, 1998; McGonigle and Miller, 1996;Vivekanandan and Fixen, 1991). Positive growth re-sponse to tillage is observed despite tillage-inducedreduction in mycorrhizal-mediated uptake of P be-cause the soil P is being supplied for luxury uptake.For wheat grown in relatively high P availability soilsin New South Wales Australia, wheat responses totillage and soil fumigation are consistently positive.‘Biological factors’ are implicated in the tillage re-sponse, rather than negative factors associated withno-till conditions such as higher bulk density of soilor pathogen pressure that might limit growth (Chanand Mead, 1992). Future studies will focus on link-ing agronomic practices, such as tillage, that mitigatesearly season colonization by AMF (Gavito and Miller,1998), with consequent crop responses to the con-trol of colonization (Graham and Eissenstat, 1998;Johnson et al., 1997).

Acknowledgements

Florida Agricultural Experiment Station JournalSeries No. R-07017. JHG was generously supportedby a fellowship from the Australian Grain Researchand Development Corporation (GRDC). The authorsappreciate the technical assistance of S. Mercer andD. Drouillard.

References

Abbott L K and Gazey C 1994 An ecological view of the formationof VA mycorrhizas. Plant Soil 159, 69–78.

Abbott L K and Robson A D 1981 Infectivity and effectivenessof vesicular-arbuscular mycorrhizal fungi: Effect of inoculumsource. Aust. J. Agric. Res. 1, 631–639.

218

Abbott L K and Robson A D 1991 Factors influencing the occur-rence of vesicular-arbuscular mycorrhizal fungi. Agric. Ecosys.Environ. 35, 120–150.

Brundrett M 1991 Mycorrhizas in natural ecosystems. Adv. Ecol.Res. 21, 171–313.

Chan K Y and Mead J A 1992 Tillage-induced differences in growthand distribution of wheat roots. Aust. J. Agric. Res. 43, 19–28.

Colwell J D 1963 The estimation of the phosphorus fertilizer re-quirements of wheat in southern NSW by soil analysis. Aust. J.Agric. Animal Husb. 3, 190–197.

Douds D D, Galvez L, Becard G and Kapulnik Y 1998 Regulationof arbuscular mycorrhizal development by plant host and fungusspecies in alfalfa. New Phytol. 138, 27–35.

Gavito M E and Miller M H 1998 Early phosphorus nutrition,mycorrhizae development, dry matter partitioning and yield ofmaize. Plant Soil 199, 177–186.

Gazey C, Abbott L K and Robson A D 1992 The rate of developmentof mycorrhizas affects the onset of sporulation and production ofexternal hyphae by two species ofAcaulospora. Mycol. Res. 96,643–650.

Giovanetti M and Mosse B 1980 An evaluation of techniques formeasuring vesicular-arbuscular mycorrhizal infection in roots.New Phytol. 84, 489–500.

Graham J H and Eissenstat D M 1994 Host genotype and theformation and function of VA mycorrhizae. Plant Soil 159,179–185.

Graham J H and Eissenstat D M 1998 Field evidence for carbon costof citrus mycorrhizas. New Phytol. 140, 103–110.

Graham J H, Drouillard D L and Hodge N C 1996 Carbon economyof sour orange in response to differentGlomusspp. Tree Physiol.16, 1023–1029.

Graham J H, Duncan L W and Eissenstat D M 1997 Carbohydrateallocation patterns in citrus genotypes as affected by phosphorusnutrition, mycorrhizal colonization and mycorrhizal dependency.New Phytol. 135, 335–343.

Hetrick B A D, Wilson G W T and Leslie J F 1991 Root architectureof warm- and cool-season grasses: relationship to mycorrhizaldependency. Can. J. Bot. 69, 112–118.

Hetrick B A D, Wilson G W T and Cox T S 1992 Mycorrhizal de-pendence of modern wheat cultivars and ancestors. Can. J. Bot.70, 2032–2040.

Hetrick B A D, Wilson G W T and Cox T S 1993 Mycorrhizal de-pendence of modern wheat cultivars and ancestors: a synthesis.Can. J. Bot. 71, 512–518.

Jakobsen I, Abbott L K and Robson A D 1992 External hyphae ofvesicular-arbuscular mycorrhizal fungi associated withTrifoliumsubterraneumL. 1. Spread of hyphae and phosphorus inflow intoroots. New Phytol. 120, 371–380.

Johnson N C 1993 Can fertilization of soil select less mutualisticmycorrhizae? Ecol. Applic. 3, 749–757.

Johnson N C and Pfleger F L 1992 Vesicular-arbuscular mycorrhizae

and cultural stresses.In Mycorrhizae in Sustainable Agricul-ture. Eds GJ Bethlenfalvay and RG Linderman. pp. 71–99. ASA,Madison, WI.

Johnson N C, Copeland P J, Crookston R K and Pfleger F L1992 Mycorrhizae: possible explanation for yield decline withcontinuous corn and soybean. Agron. J. 84, 387–390.

Johnson N C, Graham J H and Smith F A 1997 Functioning ofmycorrhizal associations along the mutualism-parasitism con-tinuum. New Phytol. 135, 575–585.

McGonigle T P and Miller M H 1996 Mycorrhizae, phosphorusabsorption and yield of maize in response to tillage. Soil Sci.Soc. Am. J. 60, 1856–1861.

Miller R M and Jastrow J D 1992 The role of mycorrhizal fungiin soil conservation.In Mycorrhizae in Sustainable Agricul-ture. Eds GJ Bethlenfalvay and RG Linderman. pp. 29–44. ASASpecial Publication Number 54.

Peng S, Eissenstat D M, Graham J H and Williams K 1993 Growthdepression in mycorrhizal citrus at high phosphorus supply:analysis of carbon costs. Plant Physiol. 101, 1063–1071.

Ryan M H, Chilvers G A and Dumarsq D C 1994 Colonization ofwheat by VA-mycorrhizal fungi was found to higher on a farmmanaged in an organic manner than on a conventional neighbour.Plant Soil 160, 33–40.

Schenck N C and Siquiera J O 1987 Ecology of VA mycorrhizalfungi in temperate agroecosystems.In Mycorrhizae in the NextDecade – Practical Solutions and Research Priorities. Eds DMSylvia, LL Hung and JH Graham. pp. 2–4. Univ. of Florida,Gainesville.

Siqueira J O, Colozzi-Filho A and Oliveira E 1989 Ocurrencia demicorrizas vesiculo-arbusculares em agro e ecossistemas nat-urais do etado de Minas Gerais. Pesqui. Agropecu. Bras. 24,1499–1506.

Smith S E, Robson A D and Abbott L K 1992 The involvement ofmycorrhizas in assessment of genetically dependent efficiency ofnutrient uptake and use. Plant Soil 146, 169–179.

Thompson J P 1987 Decline of vesicular-arbuscular mycorrhizaein long fallow disorder of field crops and its expression inphosphorus deficiency of sunflower. Aust. J. Agric. Res. 38,847–67.

Toth J D, Toth D, Starke D and Smith D R 1990 Vesicular-arbuscularmycorhizal colonization ofZea maysaffected by breeding forresistance to fungal pathogens. Can. J. Bot. 68, 1039–1044.

Vivekanandan M and Fixen P E 1991 Cropping systems effects onmycorrhizal colonization, early growth and P uptake of corn. SoilSci. Soc. Am. J. 55, 136–140.

Wilson J M and Trinick M J 1983 Infection development and in-teractions between vesicular-arbuscular mycorrhizal fungi. NewPhytol. 93, 543–553.

Section editor: M Jones