Increasing phosphorus supply can increase the infection of plant roots by vesicular-arbuscular mycorrhizal fungi

Soil Bid. Biochcm. Vol. 16, No. 4, pp. 419-420. Printed in Greal Britain. All rights reserved

0038-0717/84 $3.00 + 0.00 I(‘ 1984 Pergamon Press Ltd

1984 Copyright

SHORT COMMUNICATION INCREASING PHOSPHORUS SUPPLY CAN INCREASE THE INFECTION OF PLANT ROOTS BY VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI N. S. BOLAN, A. D. ROBSON Department

of Soil Science and Plant Nutrition, University Western Australia 6009 and

Division

of Animal

Australia,

Nedlands,

N. J. BARROW

Production, C.S.I.R.O., Underwood Western Australia 60 I4 (Accepfed

of Western

3 I October

Increasing phosphorus supply frequently decreases the percentage of root length infected by vesicular-arbuscular (VA) mycorrhizal fungi (Ross, 1971; Menge Ed al., 1978). In some instances the effects of P supply in decreasing the proportion of the roots that are mycorrhizal arises from the effects of P supply in stimulating root growth more than the ability of the fungus to infect (Daft and Nicolson, 1969; Abbott and Robson, 1977; Buwalda et al., 1982). In other instances both the percentage root length infected and the length of mycorrhizal roots decreased with increasing P supply (Mosse, 1973). However in most of these studies the effect of P supply on mycorrhizal infection has been examined with only a few rates of P application. In a study of the factors causing growth responses to applied P to be sigmoidal (Bolan ef al., 1983) we observed striking effects of increasing P supply in increasing the formation of VA mycorrhizas by both indigenous and introduced fungi. This note reports those observations. Subterranean clover (Trifolium subferraneum L. cv. Seaton Park) was grown in pots in the glasshouse on the surface soil (O-15 cm; Experiment I) and the subsoil (I S-30cm; Experiment 2) of a virgin earthy sand from Jarrahdale, Western Australia. The soil properties were as follows: pH 5.6 (surface soil) and 5.3 (subsoil) in 10IIIMCaCi,, 0.62pgPg-’ (surface soil) and 0.16pgPg-’ (subsoil) extracted in 0.5 M NaHCO, (Colwell, 1963). In both experiments there was a large number of rates of applied P added as KH,PO,. In Experiment I, VA mycorrhizas were formed by an indigenous fungus. On the basis of the morphology of infection within the roots (Abbott, 1982) the major VA mycorrhizal fungus in this soil was Acaulospora luevis Gerdemann and Trappe. In Experiment 2, Glomus fusciculufum (Thaxter sen~u Gerdemann) Gerdemann and Trappe was introduced using roots as inoculum. The procedure for the production of inoculum, inoculation and the assessment of mycorrhizal infection were as described by Abbott and Robson (1978). Basal nutrients were added in solution at rates sufficient to overcome all nutrient deficiencies. There were 9 plants per 3 kg soil. Plants were harvested 3.5 days after sowing. Full details of experimental procedures are given in Bolan et al. (1983). Adding P increased both root growth and the percentage of root length infected by mycorrhizal fungi in both experiments (Fig. I). At higher rates of P addition there was a decrease in percentage root length infected. For the indige-

Avenue,

Floreat

Park,

1983)

nous fungus (A. heuis) the maximum percentage root length infected was reached at a P rate that gave 6% of maximum shoot growth (Fig. 2). However, maximum weight of mycorrhizal roots (percentage root length infected x weight of roots) was not reached until sufficient P was added to give 66% of maximum shoot growth. In contrast, for the introduced fungus (G. fusciculatum) both maximum percentage root length infected and maximum weight of mycorrhizal 100

(o ) lndlgenous

1.0

fungus

80

08

60

06

; z a

40 0 al t 2 E

20

L G r -5

i 0

I

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I

60

;; Lz 8

40

20 %

,011

0 Phosphate

lnfectlon

I

I

05

10

applied

(g

’

15

P pot-’

1

00

)

Fig. I. Effect of phosphate application on percentage root length infected (0) and dry weight of mycorrhizal roots (A) of subterranean clover for (a) indigenous fungus and (b) introduced fungus. Vertical bars represents LSD at P = 0.05, 0.01, and 0.001 respectively for percentage infection (upper) and dry weight of mycorrhizdl roots (lower).

419

420

Short communications 100

(a)

60

lndlgenous

.I \“.

-

tion. Acknowledgemenfs-We

l’,

%\. \

. I

demonstrating thal isolates of mycorrhizal fungi differ in Ihcir ability to mfcct rools as P supply incrcascs. If the growth response to VA mycorrhizal fungus IS related to the amount of infection, it is desirable to select fungi which reach their maximum infection at higher levels of P applica-

fungus

I

I

I

I

‘\:

‘0

1b)

introduced

I

Abbott L. K. (1982) Comparative arbuscular mycorrhizas formed Journul

qf Boruny

anatomy of vesicularon subterranean clover.

30, 485-189.

Abbott L. K. and Robson A. D. (1977) Growth stimulation of subterranean clover with vesicular-arbuscular mycorrhizas. Australian Journal of Agriculrural Reseurch 28,

fungus

2

for proved-

REFERENCES

Australiun 60

thank Dr L. K. Abbott

ing mycorrhizal inoculum and advice on techniques for inoculation and assessing mycorrhizal infection. One of us (N.S.B.) gratefully acknowledges the award of a University of Western Australia Research Studentship.

.

639-649.

0

l-1 0 %

I

I

I

I

20

40

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60

Maxlmum

dry

wt

I 100

of shoot

Fig. 2. Relationship between percentage maximum dry weight of shoots and percentage roots length infected by (a) indigenous fungus and (b) introduced fungus. roots were reached at a P rate that gave 622, of maximum shoot growth. There may also be important differences between our two experiments in the number and distribution of infective propagules of mycorrhizal fungi. In Experiment I the indigenous fungi were distributed throughout the entire soil whereas in Experiment 2 the inoculant fungus was localized in a band. The importance of these differences to the differences in the effect of P on infection is not clear. Our observations extend more limited data from our previous work where increasing the supply of P has increased the percentage of the root length infected by VA mycorrhizal fungi as well as increasing root growth (Abbott and Robson, 1977; Pairunan et al., 1980; Same ef al., 1983). In most of these studies, P application stimulated mycorrhizal infection only when it alleviated a severe deficiency. In the present experiment the P levels which increased mycorrhizal infection of plant roots did not alleviate complctcly the P deficiency for plants. This indicates that the increase in percentage root length infected with increasing P supply is not through the direct effect of P supply on root growth; but either through the direct effect of P supply on the growth of the fungus itself or through an indirect effect on the fungus mediated by the plant metabolism altered by P supply. Same et al. (1983) consider that the most likely explanation of the effect of P supply in increasing infection is that the growth of VA mycorrhizal fungi is limited at low P supply. If that is the case, it is likely from our observation that the growth of the introduced fungus (G. fasciculurum) is more limited by low P supply than is the growth of the indigenous fungus (A. lueuis). Our results also extend the observations of Mosse (1977) and Jasper el al. (1979) in

Abbott L. K. and Robson A. D. (1978) Growth of subterranean clover in relation to the formation of endomycorrhizas by introduced and indigenous fungi in a field soil. New Phyrologisf 81, 575-585. Bolan N. S., Robson A. D. and Barrow N. J. (1983) Plant and soil factors including mycorrhizal infection causing sigmoidal response of plants to applied phosphorus. Plum and Soil. 73, 187-201. Buwalda J. C., Ross G. T. S., Stribley D. P. and Tinker P. B. (1982) The development of endomycorrhizal root systems. The mathematical analysis of effects of phosphorus on the spread of vesicular-arbuscular mycorrhizal infection in root systems. New Phytologisr 92, 391-399. Colwell J. D. (1963) The estimation of the phosphorus fetilizer requirements of wheat in southern New South Wales by soil analysis. Australian Journal of Experimenlal Agriculture and Animal Husbandry 3, 19&197. Daft M. J. and Nicolson T. H. (1969) EfTect of Endowone mycorrhiza on plant growth.‘ 11. influence of soluble phosphate on endophyte and host in mycorrhizal maize. New Phylologist 68, 945-952. Jasper D. A., Robson A. D. and Abbott L. K. (1979) Phosphorus and the formation of vesicular-arbuscular mycorrhizas. Soil Biolog), & Biochemisfr.v II, 50 I-505. Menge J. A., Steirle D., Bagyardj D. J.. Johnson E. L. V. and Leonard R. T. (1978) Phosphorus concentration in plant responsible for inhibition of mycorrhizal infection. Nena Phyrologisr 80, 575-578. Mosse B. (1973) Plant growth responses to vesiculararbuscular mycorrhiza. IV. In soils given additional phosphate. Nen, Phyrologist 72, 127-136. Mosse B. (1977) Plant growth response to vesiculararbuscular mycorrhiza. X. Response of S~ylosun/he.s and maize inoculation in unsterile soils. Necc Phytologisl 78, 277-288.

Pairunan A., Robson A. D. and Abbott L. K. (1980) The electiveness of vesicular-arbuscular mycorrhizas in increasing growth and phosphorus uptake of subterranean clover from phosphorus sources of different solubilities. NEH. Plrvfologisl 84, 327-338. Ross J. P. (1971) EtTect of phosphorus fcrtihzation on ylcld of mycorrhizal and non-mycorrhizal soybean. Ph.v/opathology 61, 140@1403. Same B. I., Robson A. D. and Abbolt L. K. (1983) Phosphorus, soluble carbohydrates and endomycorrhizal infection. Soil Biology & Biochemisfry IS, 593-597.