ICRAF SOILS WORKING GROUP. NAIROBI, 26-31 MARCH,1979

THB EFFECT OF :JaIl, f/fICROORGANIGMG

ON PLANT PRODUCTIVITY

Y.R.DOMMERGUBG (1 )H.G.DIEM (1 )and F.GANRY

(1) OW3TOM, D.1'. 1 juG, DAKAH, :JBNRGA.L

(2) Ingénieur de Recherche à l'IRAT détaché à

l'ISRA, CNRA, BAMBEY, SRNEGAL

SUMMARY==:::=====

Soil microorganisms favorably or unfavorably affect plant

productivity either indirectly, by actinG upon soil physical

or chemical properties, or directly by interaction with plant

roots.

(1) Benefical or detrimental effects on soil properties con-

cern structure, coating of particles with water-repellent

compounds, redox potential, soil nitrogen status (gains by

N2 fixation, losses through denitrification), availability

of nutrients (especially N,r) and accumulation or elimination

of phyto·toxic inorganic and orgunic c ornpourid e ,

(~) floi l rn i.c r-o o r gu.n.i umo di io c t Ly ll.ffect plan L growth b.y .L1n-

proving or reducing the absorption of nutrients or water up­

take(~~m~'~are w~~l .known such as ecto-or endomycorrhizae;

others are not yet even characterized, such as microorganisms

inrlll~inr: prnt:r·(\·jrl "()(îLr;). 'l'llf',v III/l,V ~1.11\\) l'l'(l,l1lt:I' 1~I'(Jwl.ll-I'I'I·~U-

lating substances or protect the plant against certain patho-

gens.

Manipulation of the soil microflora appears to be highly

desirable, but it is difficult to accomplish. Sorne succesS

has already been achieved with direct inoculation, especially

in the case of N2-fixers

and mycorrhizae. Indirect control

of soil microflora by methods involving classical means, ste­

rilisation, or the application of specifie compounds is possi-

l i lo lU'UV.ld()J tltuL :jU!tll! rcl1ull'èrnentu are I'u.l.Li.Ll.eu , Altering

the soil microflora by acting through the plant is another

promising possibility.

These processes are discussed with special reference to

th!"i r i rnpo r-trm c o n.nrl UCC1lt'(,'UCI.' 111 t.ro p.i cu.]. uo I Lo .

1. INTRODUCTION------------------------------

By now many agronomists would readily agree that soil micro­

organisms affect plant productivity, especially in the tro­

pics. Yet this idea took a long time to catch on, except in

the case of Rhizobium, because microbiologists were mainly

concerned with the physiology of microorganisms which had

been isolated and studied in test-tubes or Petri dishes and

were, therefore, out of their natural environment. Another

reason is that the study of complex systems such as the soil

-plant-microorganism system is more difficult than that of

pure cultures.

~n this paper, we shall consider sorne of the mechanisms by

which soil microorganisms favorably or unfavorably affect

plant growth by altering the soil physical or chemical pro­

perties, or by directly acting upon the plant itself. Since

other contributors have covered the interactions between

plante and mycorrhizae, or N2-fixin

g microorganisme, (Keya,

1979; Redhead, 1979) we shall only briefly mention the role

of those microorganisms, focusing our attention upon other

groups whose influence is still not always recognized. Two

preliminary remarks should be made here which are related

to the unique conditions that prevail in the tropics. The

first is that when soil water content is not limiting, the

temperature is generally high enough to allow much more vi­

gorous microbial activity than in temperate areas. The second

~.~~~ refe~$ to th~ ~hi~~$~h~~~ effect. Since the Qrganic

materials which originate from the plant debris are only to

a slight extent stored as humic compounds and are readily .d·~­

composed, most of the microbial life is located on or around

the root system of plants.

2

3

2. INFLUENCE OF MICROORGANISrJIS ON SOIT.J PROPERTIES

==============================================

2.1. Effect on soil physical properties

The role of microorganisms in the genesis and maintenance of

BQ!l ntnlcture hR8 recently bepn revipwP~ (Hppper, 1975).

Our aim here is merely to emphasièe the importance of this

process in the rhizosphere. It has been demonstrated that

there are more water-stable aggregates in the bhizosphere

than in the non-rhizosphere soil (Harris et al., 1964). Sin­

ce the number of polysaccharide-producing microorganisms is

characteristically higher in the rhizosphere, it can be assu­

mcd that Goil stabilization arounJ the root can, at least to

sorne extent, be due ta the rhizosphere microflora. In tropi­

cal ooile, where most of -the microbial population io conccn­

trateù in the root zone, it would really be definitively

worthwhile to elucidate the relative importance of the root

itself and that of associated microorganisms in soil struc­

ture otabillzBtion. Duch invectigationc chould not bo reo­

tricted to free-living microorganisms, (such as Azotobacter

sp, Beij erinckia indica or Li ')omyces starkeyi, whi.ch are well

known polysaccharide producers), but extended to mycorrhizae

which were reported to be involved in sand aggregation and

dune stabilization in colder climates (Koske et al., 1975).

In contrast to this beneficial activity, microorganisms can

be harmful in two ways : by decomposing the aggregating com­

pOlmùs originating from plants or microorganisms; and by

coating 0011 vartlcules with water-repellent films (Bonu anû

Harris, 1964). By altering the advancing contact angle of wa­

ter with the particles, such films disturb the infiltratmon

of water into the soil, inducing a patchy distribution of

plants and 0. rnur-lce d Lo ao 01' productivi t y , Water r-e peLLeo c y ,

which was mostly attributeù ta basidiomycete hyphae is ~

thought by Griffin (1969) to be of potentially wide impor­

tance, especially in semi-arid conditions.

4

Microorganisms may also alter the soil redox potential. Thus the

growth of aerobic microorganisms, most of which grow at the ex-

pens~ of decaying plant debris, may lead to a reduction in the

redox potential, inducinc pr0~csscS which nr0 deleterious to the

plant (sect. 2.2.3.). Alternatively, photosynthetic aleae might

produce oxygen and raise the red8x potential, thus acting direct-

ly or indirectly u pori the p.Lanc .

2.2. Influence on soil ch~~ical-Eroperties

2.2.1. Nitrogen gains and losses through biological

processes

2.2.1 .1. Symbiotic N2

fixation

Since this process has already been reviewed (Keya, 1979), the

discussion of this topic here will be restricted to the effect

of limiting factors, an aspect which is often overlooked. Be­

sides the possible inadequacy of native N2-fixin

g micropopula­

tions and the attacks of pathogens, especially nernatodes (Ger­

mani, 1979), four major factors appear to limit symbiotic N2

fixation in the tropics: moisture stress (especially in semi­

arid conditions), soil acidity and associated toxicity, mineral

deficiencies and, in sorne Si+'l~tions, an excess of combined

nitrogen in the soil (Table 1). As long as one factor is oper­

ntine, N2

fixation is low or nil and the input of nitrogen to

the ecosystem negligiblc or inexistant. Two examples will illus­

trate the unfavorable effect o~ limiting factors. These examples

are related to peanut anù result from field-experiments carried

out at the Bambey Experimental Station, in Central Senegal, over

the last 3 years. The first is illustrated by Fig. 1, which

shows that in the arid conditions prevailing in Central Senegal

N2 fixation (measured by the acetylene assay) is closely related

to soil water content. The second example concerns the limiting

effect of inorganic nitrogen. Using the A value method, Ganry

(1976) found that by incrcélsinG' the rate of application of nitro­

gen fertilizer from 15 to GO kG per ha, N2

fixation by peanut

decreased from 52 to 25 kg per ha. In spite of those limitations,

sorne N2-fixinc systeme

5

can remain active, which explains that Casuarina equisetifolia,

a non-Ieguminous nodule-bearing tree, largely used for refores-

of West Africa, was reported to-1year on the Cap-Vert peninsula

tins sandy soils on the coast-1fix as much as 60 kg N2ha

(Dommergues, 1963).

2.2.1.2. Losses through nitrification and denitrification.

The activity of nitrifying bacteria varies considerably accor­

ding to the soil characteristics and to the nature of the vege­

tation. It is weIl known that these bacteria are typically neu­

trophilic. However, that does not Mean that nitrification is

restricted to neutral soils; it means that it is restricted to

neutral micro-habitats. Since such habitats may occur (e.g. in

the vicinity of organic debris) in soils whose overall pH is

acid, nitrification can be very active in such soils. Thus

aoid tropical soils grown with banana, maize, or rain-fed

rice exhibit a high nitrifying activity when ammonium fertili­

zer is applied. (Dommergues et al), 1978; Chabalier, 1978). In

forest soils, nitrification May be hindered by antibacterial

eubat6noea releaaed by the litterj when the foreet ie oloared,

a flush of nitrification usually occurs (Dommergues, 1954).

There is increasing agreement that nitrification is a detri­

mental process Binee it is responsible for two types of nitro­

gen losses : losses through leaching, since nitrate is of an

anionic nature and losses through denitrification (Focht and

Verstraete, 1977). Such losses are highly variable, but they

are seldom lower than 20-30% of nitrogen applied as fertili­

zero The increased cost and shortage of fertilizer nitrogen,

especially in the tropics, must prompt soil microbiologists

to gather more information on factors that could limit nitri-

fication in soils, since this process is presumably more easi­

ly controlled than denitrification. Recents advances in the

field of methodology (especially direct detection of bacteria

in the soil by the fluorescent-antibody technique) promise to

be Most helpful (Schmidt, 1978).

6

2.2.2. Availability of nutrients

2.2.2.1. Nitrogen

#1D tropical soils, amonification is usually very active so

.tbat the potential for the relGase oï ammonium from soil or­

sanic nitrogen is high. Unfortunately, the organic nitrogen

,inpu~s (through N2

fixation, root and litter deposition) into

~he soil are often limited, so that ammonium release is not

high enough to meet the plant requirements. It is not clear

whether nitrate, which is the end product of nitrificatio~ is

more available to plants than ammonium.

2.2.2.2. Phosphorus

Microorganisms,·especially those thriving in the rhizosphere,.

are often thou'ght'":' to be able to increase the phosphate avai-

lable to plante by dieeolving water-ineoluble mineraI phoepha­

te or by mineralizing phosphate from soil organic matter.

Ae far as mycorrhizae are concerned , their role as solubili­

zing·agents has not yet been demonstrated. Other soil micro-

organisms might be involved. In vitro experiments have olear­

ly shown that many common microorganisms, including Pseudomonas,

Aohromobacter, Flavobacteriurl, Steptomyces, and especially

Aspergillus and Arthrobacter can solubilize soil phoephorus

(Lowand Walker, 1959; Hayman, 1975; Barber, 1978). However

sorne authors argue that the increased uptake of phosphate may

not only result from ~ increase of the availibility of phos­

phate, but could also be explained by the effect on plant

growth of stimulating substances synthesized by the micro-

organisme (sect. 3.5.). With regard to organic phosphate, it

is readi~mineralised by plant phosphatases of the root sur­

face. The soil microflora does not seem to increase this pro­

cess significantly,

2.2.2.3. Trace elements

Microbially-induced increases and decreases of available trace1;

elements have been recently discussed (Baber, 1978). Since a

7

('WaZ"iety of microorganisms (as well as plants) synthesize" somef .

~xamic acids known to be powerful chelating agents, it is

DOt eurprising that soil microorganisms play a prominent role

. 1D the .i.aon metabolism of plE.~'lts (Waid, 1 975:. A class ical

exemple of the decreased availability of trace elements is

~hat of manganese. Manganese deficiency of oats was shown to

ooour when the activity of manganese-oxidizing microorganisms

Was too high. Soil fumigation reduced the population of these

microorganisms and eliminated the manganese deficiency symp­

toms (Timonin, 1946).

2.2.3. Soil toxicity

Phytotoxic compounds that may accumulate in soils are of

microbial or plant origine A classical example of phytotoxicity

induced by microorganisms is that of hydrogen sulfide produced

by sulfate-reducing bacteria. The growth and activity of these

bacteria is triggered in the rhizosphere when the following

environmental conditions exist concurrently : active root exu­

dation, soil sulfate content of the rhizospheric soil above a

minimum threshold, and strict anaerobiosis. Accumulation of

hydrogen sulfide can be high enough to lead to the death of

plants (Dommergues et al., 1 s'69. ;Jacq and ROber, 1978). IJlanga­

nese toxicity which occurs in acid soils which are relatively

rich in m6.l1ganese may be z-eLrif'oz-c ed by rhizosphere microorganisms

capable of reducing manganic sources. Partial sterilisation of

Buch soils may prevent toxicity (Barber, 1978).

Phytotoxic compounds of plant origin are responsible for dimin

.ishing plant growth when they are not decomposed. Many examples

of such toxic effects have been described by Rice (1974) in his

book on allelopathy. Recently, investigations carried out at the

Agronomie Research Center of Bambey in Central Senegal showed

that sorghum roots contained phytotoxic compounds which, in

Borne circumtances, could significantly reduce the yield of

subsequent crops, especielly sorghum. When sorghum is grown

once in a two-course rotation (peanut-sorghum) instead of once

in a four-course rotation (green manure-peanut-sorghum-peanut)

8

yields are severely depressed. Such a deleterious effect

(known as soil sickness) is induced by the accumulation in

the soil of a phytotoxic compound after the first crop. The

phytotoxic compound, hich is specifically inhibitory to sor­

ghum, remains in the soil aS long as environmental conditions

prevent its biodegradation by soil microorganisms. Since such

unfavorable oonditions may prevail in sandy soils for soven)

to eieht monthe ~, the phytotoxic compounds are still

present when sorghum is resown too soon after its last crop­

ping. It should be pointed out that while soil sickness does

occur in sandy soils containing kaolinite-type clays and

showing a poor microbial activity, no symptoms are noted in

vertisols, which contain montmorilloni~e-type clays and where

microorganisms are significantly more active. In vertisols,

the sorghum rhizosphere is spontaneously colonized by a rich

soil microflora comprising s~rains that oan actively dooomposB

the phytotoxic compound (Dommergues, 1978b).

Another example of phytotoxicity, which is of importance in

forestry is relnted:.to:· the failure of Grevillea

robusta regeneration in Australia. Seedlings of this species

were reported to be killed by sorne water-transferable factor

associated with the roots of parent trees. The resulting

regulation of population in Q. robusta, is thought to explain

the maintenance of floristic diversity in complex tropical

rain forests (Webb et ~., 1967).

3. DIRECT EFFECT ON THE PLANT=================~========

·As a root grows throue~ soil,it encounters diverse components

of soil microflora and it is directly affected by the activity

of soil microorganisms. Rhizoplane and rhizosphere populations

affect the host plant in mariy we.ys 'out there is now increasing

eyidence that the most important effects of microorganisms on

plant growth concern the mod Lf'd ca.u Lori of plant nutrition and

water uptake, the production of growth-regulating substances

and the protection of roots agaimd; pathogens.

3.1. Modification of plant nutrition and water uptake

by mycorrhizae

The best example of the role of microorganisms aS regulating

agents of plant nutrition is illustrated by mycorrhizal aBSo­

ciations, the plant's'mai~ response to nycorrhizal infection

being the increased nptalce of r..utrients, especially of phospho­

rus. Mineral nutrition of plants aD stimulated by ectomycorrhi­

zae has been weIl treatod by Dowen (19739 and the effects of

vesicular-arbuscular mycorrhizee (VAM)bave been reviewed by

Tinker (1 975) Redhead (1 979) :-,--::d. O-~'!::9rs.

Many theories have already been proposed to explain theinc~CaBed

uptake of phosphorus by ectomycorrhizal reots (Bowen,

1 973). Sorne of them conld a.ppLy 't o VAM since Gerdemann (1 968)

considers that the function cf VAM may also be very similar to

that of the ectomycorrhizae.

It includes the formation of more efficient nutrient-absorbing

structures than non-mycorrhizal roo~s. The extensive etrande of

extramatrical hyphae in VM1 may also explore a much greater vo~-

. ume of soil than non-infected roots, as pyphae' of ectomycorrhi­

zal fungi. The postulate of a longer duration of mycorrhizas than

non-mycorrhizal roots as absorbillg organe ae etatad by Bowen and

Theodorou (Bowen, 1973) for eCTootrophic mycorrhizao should also

apply to VAM (Gerdemann, 1 960: ?l.J_cheugh evidence is still lacking.

Another interesting facet of the blology of mycorrhizae is rela­

ted to the behaviour of in:fec'~ed z-oo-t e under low water regimes

in the soil. Apart from a high temperature which is the

1 a

common characteristic, tropical soils are quite different from

one another in water content because there is a wide range of

.011 textures and climatesin the tropics.

In sandy soils and is the semi-arid regions plants are often

eubjected to a relatively long period of water stress. A most

~teresting question is whether soil water supplies could be

.1mproved by mycorrhizae. The physiology of water absorption by

~oorrhizae has been little studied but sorne investigations

have indicated a greater drought resistance in a number of

~oorrhizal seedlings (Bo.wen, 1 973).

ID 1971, Safir et al. indicated that VAM could probably decrease

; t the resistance to water transport in soybean. But later

,,(a.fir .21 al., 1 972) they concluded that increased plant growth~

~ater_streSBed conditions wes due to the improvement of phos-

. rus nutrition. Recently Menge et al.(1 978) reported that my-~.,. --

. bizal infection enabled avocado plants to resist transplant

ok, suggesting that mycorrhizae could improve water uptake;:'r ~he host-plant. Drought resistance of rnycorrhizal plants may

be related to the grater exploration of soil by extensive hyphal

growth, but also to large differences between infected and non-

infeoted roota in their own biology. As stateà by Cromer (nowon,

1973), mycorrhizal roots of Pinus radiata seemed to renew growth

more quk:kly than non infected roots when they are subjected to

severe water stress. Another interesting hypothesis on the rela­

~ionship between soil water regime and mycorrhizal infection is

given by Sieverding (Moawad, 1978) who found that the amount of

water used to produce 19 of dry matter was much lower in mycor­

r.bizal than in non-mycorrhizal plants growing in dry soil ferti-

In.Od with C"5(P04)30H :. (Tab~B 2), Acoording to Moawad, Siever­

~te findings may simply be due to the better utilization of

f~ter by p1ants growing in phosphorus deficient eoils. If we wiehr~~ explain the greater drought resistance of plants, the theory

~ water oonsumption economy as stated aboya eeeme to be more

~.'usible and more attractive than the principle of increased

iPtake or transport of water in plants (Safir et al., 1 971 ) •

1 1

'.2. Mycorrhizae under tropical conditions

The impact of mycorrhizal symbiosis in the growth of tropical

plants has been recently discussed by Bowen (1 978) and Black

(1978). Black noticed that the number of tropical plants asSo­

ciated with ectomycorrhiz€e appears to be very limited as com­

red to the wide range of ectomycorrhizal plants in the temperate

region. The only crop recorded with ectomycorrhizae is Pinus

(Redhead, 1978). Inventories and other informations concerning

ectomycorrhizal forest trees are given in Alewis and Abeynayake

(1978).

As for endomycor~~izae, although sorne " families

. . such as Casuarinaceae, Chenopodiaceae,

yrticaceae are devoid of VAM (Khan, 1974), most tropical plant

species of economic importance are infected : cocoa, tobacco,

ootton, corn, sweet potato, peanut, sugar cane, sorghum, rubber,

~ea, citrus and many species of timber trees (Redhead, 1978).

Spores of VAM are widely distributed in Nigerian soils from the

moist lowland forest to the dry Sahel and Savanna regions (Redhead,

1977). A few olive specimens (Olea cuspidata) may also be infected

by both ectomycorrhizal and endomycorrhizal fungi. Significance

of mycorrhizal symbiosis in the cultivation of olives in Pakistan

bas been discussed by Khan and Saif (1973).

In different soils of the arid and semi-arid regions, it is pro­

bable that mycorrhizal associations play an important part in the

crowth and drought-resistance of a number of plants because of

~eir ability to regulate uptake of nutrients and soil water.

Unfortunately, little is known about the mycorrhizal response of

plants usually growing unde~r~onditions. Studies of mycorrhizal

effects in these regions of the world would be of great practical

interest particularly in the case of afforestation with plant

epecies that usually are transplanted. In our laboratory, obser­

vations of AZadirachta indica rooto, a trce WhOB0 8ro~th iB wid&-

'spread in dry sandy soils in Senegal, indicate that most roots,

if not all, are infected with VAM (Fig. 2). It is significant to

note that Azadirachta indica is able to grow vigorously in non­

fertilized Boils and in arid conditions.

12

,.,. Effect of VAM infection on legume-Rhizobium symbiosie

Aocording to a number of papers, VAM also occurs iB many tro­

pical legumes of economic importance e.g. peanuts, cow-pea,

~cropti].ium. atropurpureum, Stylosanthes spp. (Sanni, 1976;

Possingh~o ~t al., 19711 Graw and Rehm, 1977). As legumes have

been shown to require high levels of phosphate for nodulation,

it is likely that mycorrhizal infection may affect the nodula­

~ion process and also even N2

fixation. Many authors have alre­

ady explored this facet of mycorrhizal response in legumes

(Crush, 1974; Islam et al., 1976; Mosse et al., 1976; Smith

and Daft, 1977). Rooently, in an oxoellent Qseay on the role

of mycorrhizae in legume nutrition on marginal soils, Mosse

(1977) summarized our knowledge on this subject and reported

evidence that endomycorrhizal inoculations associated with the

eupply of rock phosphate stimulated growth and nodulation of

many ~egume&.

On the mechanism of legume stimulation by mycorrhizae, Mosse

wrote that the principal cause is undoubtedly increased assi­

milation of phosphate, but mycorrhizae may have other secondary

affects possibly of a hormonal nature.

3.4. Microbially induced proteoid roots

Mycorrhizal associations are not the sole process which stimula­

tés the phosphorus nutrition of host plants. Despite resistance

of aeveral lupin species to mycorrhizal infections, they are,

nevertheless, able to grow in sandy soils which are highly defi­

oient in phosphorus. The ability of lupin to absorb soil phos­

phate has been attributed to the formation of clusters of root­

1ets in lovalized parts of the lupin root system. These clusters

of rootlets resemble the dense clusters known as proteoid roots

which have been described in the family of Proteaceae by Purnell

(Trinick, 1977). Other proteoid roots ha~~' also been recorded by

Lamont on Viminaria juncea and by Malajczuk on Kennedia (Trinick,

1977). It h&s now been shown that proteoid roots play an impor­

tant role in phosphorus nutrition of plants due to their increa­

sed absorbing ability as compared with normal roots (Jeffrey,

1967; Malajczuk and Bowen, 1974).

13

Acoording to the literature published, very few plant species

form proteoid roots. In Senegal, one of the authors (R.G.D.)

observed that rootlet clusters similar to proteoid roots can be

found in Casuarina ~uisetifolia usually growing in sandy and

deficient soils. In the cluster , lateral rootlets are so nume­

rous that they resemble fingers (Fig. 3). It is interesting to

note that, c. equisetifolia does not seem sensitive to mycorrhi­

zal infection, so that proteoid roots may be an alternative for

this plant to take up phosphate from deficient soils. Investi­

gations are now in progress in our laboratory to demonstrate the

effects of these root formations on the physiology of C. esuise­

tif01ia. Although the mechanisms of the initiation of proteoid

roots are not clear, sorne inoculation experiments indicate that

proteoid roots may be induced by microorganisms colonizing the

root surface (Malajczuk and Bowen, 1974).

'.5. Effect on plant growth of phytohormone. -producing

microorganisms

As microbial numbers and activity are more intense in the rhizos­

phere than in soil, it hardly seems conceivable that the develop­

ment of such rhizosphere microflora would not directly affeot the

development of the roots. Microbial effects may be detrimental to

root growth for example roots of tomato, subterranean clover and

radiate pine were stunted by the presence of soil microo~ganisms

(Rovira and ~ic Dougall, 1967). However, particular attention has

been paid to the benefical effect exerted by rhizosphere inhabi­

tants. It has been a long tœme since typical rhizosphere bacteria

such as Arthrobacter, Pseudomonas and Agrobacterium were found to

he able to produce substances promoting plant growth (Krasilnikov,

1958) •

Ectomycorrhizal fungi also provide the host plant with phyto­

hormones and growth-regulating B vitamins (Slankis, 1973).

Detailed discussion about the direct effects of bacteria on

root growth by the production of plant growth reg~lating fac­

k~rs can be found in many reviews (Krasilnikov, 1958; Katne1­~!M""8OD, 1965; Brown, 1975). The influence of ectomycorrhizal hor-

~~~8 on the development of roots of the host plant has BlsoIl ~.t. ..

14

been amply demonqtrated in Slankis (1973). However, careful ~~'.

scrutiny of the literature dealing with increased plant growth

resulting from interactions between soil microorganisms and

plants lead us to remark that when plants are artificia1ly inoc-

:ulated with a particular microorganism known for a determined

biological activity (e.g. N2

fixation; phosphorus solubiliza­

tion), stimulation of plant growth often appeared to be putative-

:ly attributed to the effect of this specifie activity, although

it may simply be due to the production of phytohormones by the

sarne microorganism. Three examples found in different fields

reinforce this point of view :

(1) Thirty years ago, Gerretsen (1948) thought that the increased

.: growth of plants in sterilized sand containing insoluble

phosphate compounds Was due to inoculation with solubilizing

phosphate bacteria. This conclusion is now criticized by Brown

(1975) who attributes the improved growth reported by Gerretsen

to the production of gibberellin-like substances ,by the bacteria

used for inoculation.

(2) Increases in plant growth and crop yields have often been

recorded after inoculation with Azotobacter, but it is now well

known that these effects are caused, not by fixation of signi­

ficant amounts of N2,

but by the production of small amounts of

highly active growth-promoting substances by the bacteria (Brown,

1975). Similarly, inoculation with Azospirillum brasiliense, a

free living N2-fixing bacterium in vogue nt present, can also

induce increased plant growth. Table 3 shows that inoculating

rice with a non N2-fixin

g bacterium stimulated plant growth even

more actively than A. brasiliense. Moreover, since inoculation

with Azoapir;i,llum E{enerf\lly dOflA not nignifioantly improve N2fixation, Gasking and Hubbell (1978) and Tien et~. (1979)

suggested that the effect of Azosp~i+~ inoculation on plant

growth could be due to growth-stimulating substances produced

by this bacterium, as in the case of Azotobacter.

(3) In some experiments mf biological control, root disease of

wheat associated with Rhizoctonia solan~·.was reduced and grain

yield increased by seed inoculation with bacteria and actinomy­

oetes. Merriman ~ al. (1974) sugggested that the yield increases

1 5

•

-.. t prir,arily due to plant growth-stimulating factors

~o ~he biological control of root disease.f,t,

j~1';'.6. Improvement of plant resistance to infection

rather

Discussion will be restricted to the control of pathogens through

the improvement of plant resistance by symbiotic microorganisms

or microorganisms more or less loosely associated with the roots,

which is only one aspect of the vast problem of biological con­

trol. Two types of mechanisms may be involved in the type of

con~rol studied here.

3.6.1. Protection by mechanical barriers

In his review, Marx (1975) indicatcd that if pine .roots were

:"a8sociated with Leucopaxillus cerealis var. piceina to form

-;eotomycorrhizas, they became rcsistant to infections caused by

euch pathogenic fungi as Phytophthora cinnamomi. Many mechanisms

oould be involved to explain the protective role of ectomycorrhi­

.&1 pine roots. Apart from the explanation that antibiotic pro­

~duction inhibits fungal pathogens (Marx, 1975), the fungal man-fI'

~~l. of ectomycorrhiza~ also creates effective mechanical barriers

:~acainst penetration by R. cinnamomi. There was further evidence

~~ fungal mantles formed by non-antibiotic-producing ectomy­

r·oorrhizal fungi also protected roots from pathogenic root infec­1;.

is also suggested that endophytic mycorrhizae may pro­

(Wj lhelm, 1 9~·3). In this case, there is no

~"1ons. ItRI~;vide plant protectionl''' r

mPhYsical barrier, but early territorial occupation of living root

H'tissues by the endophyte is anothur principle applicable for bio-1

~10lical oontrol.

3.6.2. Protection by chemicnl response

X~ is well known that ,in. ·thè Pl'e'sence of saprophytic microflora,

~ plants produce a multitude of compounds, especially the so­

oa11ed phytoalexins, which can play a role in root diseas~ resis­

tance. Most have' been identified in aerial portions, but i t is

16

1ikely that the same compounds cen also be formed in the root

system (Paxton, 1975). For instance, pisatin, the well known

phytoalexin of the pea plant, occurs in the roots as well as

in most other parts of the plant and has a wide spectrum of

antibiotic activity. Strawberry roots also produce phytoalexins

in response to Phytophthora fra~ariae infections (Mussell and

Staplee, 1971).

17

4. MANIPULATING THE SOIL MICROFLORA===================================

lince the major part of the soil population in tropical condi­

tions is made up of the rhizosphere microflora, and since the

Z'himosphere microflora must be viewed as a component of the

,~o1e soil-plant-atmosphere system (Dommergues, 1978a), the

[8011 microflora could predictably be manipulated, not only by

;tibectly acting u.pori the microorganisms, but also- indirectly

,~ acting upon the soil and the plant. Actually, direct manip­~

r~ation of the soil microflora has been achieved by inoculation

lpractioes, sterilization and application of specifio inhibitors

or specifie substrates. Indirect manipulation of the soil-plant­

.~mosphere system has been achieved by classical or non-conven­

~ional soil management practices, or by acting upon the plant

oomponent itself.

4.1. Inoculation

In spite of the fact that root colonization by non-pathogenio

microorganisms is still poorly undGrstood, soil miorobiologiste

and agronomists have been trying for' many years to alter the

rhizosphere microflora by introduci~g selected microbial strains,

either by coating seeds with an inoculum, or by placing the ino­

oulum into the sail close ta Dhe seed or the seedling.

4.1.1. Inoculation of plants with symbiotic microorganisms

The value of legume inoculation is well recognized, provided :

that the strain used is highly effective and efficient in its

symbioeis with the selected legume cultivar; that it is good

colonizer of the roots and is able to compete with any native

root mioroorganism , and that the proper environmental prereq-

.uisites are fulfilled. However,legume inoculation by classical

Methode ie not alwaye fully eatiefactory (see sect. 2.2.2.).

18

!he value of ec~omycorrhizal inocula~ion if ~he proper environ­

.en~al condi~ions are me~ (e.g. Hacskaylo, 1972; Marx and Krupa,

1978) is also generally acknowledged. Inocula~ion by endomyoor-

rhizae is curren~ly a~ ~he experimen~al s~age excep~ in special

~.itua~ions (sec~. 4.2.1.). Preliminary repor~s sugges~ ~ha~ larger

'responses are more likely in ~ropical regions ~han in ~empera~e

regions, because Qf higher ~emperatures and ~he na~urally low­

phosphorus level of soils (Hayman, 1978).

aecen~ experiments carried out in the nor~hern coastal area of

Senegal showed ~ha~ inoculating Casuarina equisetifolia with

orushed nodules improved the plant grow~h markedly (Dubreuil

and Andeque, personal communica~ion). Fur~her inves~iga~ion on

the endophy~e of Casuarina is needed in order ~o improve ~he

CUrren~ method of inoculation, which is obviously hazardous

sinoe crushed nodules used as inoculurn May carry pathogens.

4.1 .2. Inoculation of plan~s wi~h microorganisms

which are not stric~ly symbiotic

Whereas ~echniques of inoculation with typicallY symbio~ic micro­

organisme (e.g. Rhizobium) are already in use in ~he field, or

could predic~ably be in use in ~he near fu~ure (e.g. endomycor­

rhizae), ~echniques of inocula~ion with loosely symbio~ic or non­

symbio~ic microorganisms (e.g. rhizosphere N2

fixers or phosphate­

80lubilizing bac~eria) cannot ye~ be safely recommended.

;~e first at~empts at using N2-fixin

g rhizosphere bacteria to~fiDoou1a~e grasses or cereale were made as early as 1902 (Ruben-

~, 1963). Since tha~ date many experiments have been performed,

:,t firs~ wi~h ~z~~2bac~~L or BeijerinolEl~ and l&ter with ~­

Apiri11um (e.g. Smith et al., 1976; Dobereiner, 1978). Yield

have generally been inconsis~en~.

Field-experiments with phosphate-solubilizing bacteria (espe-

oially Baoillue mega~herium) did no~ show any consis~en~ effec~

on plant yield. According to Barber (1978), "this lack of

response is not really surprising for two ressons.

19

Pirstly, since a considerable proportion of soil phosphorus is

present in organic compounds and up to 90% of the rhizosphere

microflora are capableof~prodUoingphosphatases,the introduction

of other organisms, which would have to compete for available

oarbon sources, is unlikely to cause any increase in the supply

of phosphate to plants. Secondly, the inoculum used, Bacillus

megatherium ~. phosphatic~ is a spore-forming bacterium and

such organisms grow far less ream.ly in the rhizosphere than do

other types of bacteria".

When stimulation of plant growth consecutive to inoculation by

N,..···fixers or phosphate-solubilizing bacteria was observed, it

"'oould not be explained by N2 fixation, ~or by an increase of

phosphate solubilization. The stimulation of plant growth prcb-

ably resulted, at least in part, from the effect of growth

substances produced by the microorganisms added with the inocu-

lum, a.e already mentioned (sect. 3. -5") . In spi te of sorne

recent improvements in the preparation of the inoculum itself

(Dommergues et al., 1979) or in the introduction of mixed cul­

tures (Dommergues et al., 1978), there would seem to bé no easy

solution to the difficulties which arise when attempting to

inoculate non-sterile soils.

4.1 .3. Inoculation of soil containing phytotoxic residues

Soil sickness can result from the presence of plant residues in

the soil,especially root litter co,ntaining phytotoxic substances

(sect. 2.2.3.), Inoculating such soils with microorganisms which

actively decompose the root litter appears to be a promising

approach to curing these soils. Thus inoculating a ferrallitic

sandy soil which contained phytotoxic root debris with Enterobao­

ter cloacae restored soil fertility (Table 4). Phytotoxic suh­

stsno~~,pre-existing in plant residues·or formed during decompo­

sition can possees a broad spectrL~m of e~fects which are injuri­

".:ous to the roots and stems of pl':l.nts (Tousson and Patriok,

1963). Such a deleterious effect Bould probably be reduced by

Boil inoculation with proper microbial strains.

20

4.2. Soil sterilization and application of specifie compounds

4.2.1. Soil sterilization

If sterilization by heating, irradiation and drying is used in

certain circumstances, sterilization is often achieved by fumi-

sation with such chemicals as chloroform, carbon-sulfide methyl­

bromide or chloro-picrin. Such treatments often improve plant

growth even in the absence of pathogens (Wilhelm, 1966; Rovira,

1976). This beneficial effect can be attributed to differer.t ..

Causes: chemical modifications, especially increase of NH4~

oontent, flush of organic matter decomposition, including dead

microorganisms (Anderson and Domch, 1978), elimination of nitri­

fying bacteria, which are particularly vulnerable to fumigation

'(Jenkinson and Powlson, 1976), and re-colonization of soil by

non-pathogenic microorganisms, especially pseudomonads, which

are thought to stimulate plant growth (Ridge, 1976).

Soil sterilization prior to inoculation with mycorrhizae appears

~o be most helpful in special situations (Lamb and Richards,1978).

Among these are fumigated nursery soils where severe stunting of

oitrus was reported; inoculation with vesicular-arbuscular mycor­

'rbizae appcared to be the best method to overcome this stunting

(Lamb and Richards, 197f.; Timmer a~d Leyden, 1978; Hayman, 1978).

4.2.2. Specifie inhibitors

Among the different specifie inhibitors that have been studied

(e.s. Anderson and Domch, 1975), nitrification inhibitors have

received much attention becausB of their possible use in the

field 1. Besides agronomie practices reported in sect. 4.2.1.,

inhibitors such as 2-chloro-6- (trichloromethyl)-pyridine have

been successfully used to inhibit nitrification, thus increasing

the efficiency of nitrogen fertilizers by reducing deni­

~rification and leaching of the nitrate ion. Unfortunatoly,

especially in tropical conditions, the inhibitor is readily

decomposed by the soil microflora so that nitrification occurs

before the plant requirements for nitrogen are at their peak.

21

er reason for the restricted use of nitrification inhibitors

priee. However sorne inexpensive substitutes have been

such aS neem cake (cake mo.de out of the seeds of

rachta indica), but this material is not as effective as

oro-6 (trichloromethyl)-pyridine (Prasad and de Datta1978).

4.2.3. Specifie substr.ates

stimulation of a given component of the microflora can be

adding a specifie substrate to the soil. A classi­

example is that of the selective multiplication of actino­

etes in a soil amended by chitin, Streptomycetes, and to a

',••er extent Nocardia, constituting the bulk of the chitin­

.~ oomposing microflora (Alexander, 1961).

ther example is that of the solubilization of rock-phosphate

F Thiobacilli· These chemoautotrophic bacteria are introduced.'IDto the soil together with sulphu.r which is oxidized to Bulphu-

tric acid, thus dissolving the phosphate (Swaby, 1975).

4.3. Fertilization and soil management

lnooulation ev en with opacifie rni~roorganisrns, especially

Rhizobium, is unsuccessful when one of the environmental limiting

~ .... factors listed in Table 1 is still operating. Therefore,

improvement of the environmental conditions is a prerequisite

which can be achieved by different soil management practices,

such as irrigation, liming, application of organic amendments

or slow-release fertilizers. The benefical effect, of liming is

illustrated by Table 5 (Expt. 1) which reports on a study of

soybean nodulation in a ferrallitic acid soil from Casamance,

Senegal. The increased nodulation was attributed to the elimi­

nation of Mn and Al toxicity by liming. Table 5 (Expt. 2),

shows that the application of organic matter even at low rates

(400 kg of peat per ha) favorably affected the growth and nodu­

lation of soybean. This last r-e au.Lb confirms those obtnined .. '

by Dart et al., 1973 with Vig~ mungo and y. radiata. Nei­

ther species grew well in a nitrogen free sand-grit mixture.

22

:But adding 10;6 of Kettering loam by volume improved growth and~~

rnodulation. When added loam had been previously ignited at~(

_450°C for 4 h to remove soil organic matter, plant growth wae

'poor and the plants eventually died.

The combination of liming, ploughing and farm-yard manure

application was reported to significantly increase peanut

yields in Central Senegal, probably through increasing N2

fixa­

tior ('Wey and Obrrtori , 1 978) .

Bince N2

fixation is not always active enough to meet the

legume's requirements, it is necessary to use nitrogen fertilizers.

But it is known that such applications inhibit N2

fixation

_(sect. 2.2.1.1.). To prevent this inhibition in legumes, Hardy

.t al. (1973) suggested the use of other forms of nitrogen

fertilizers which do not inhibit N2

fixation, while providing

the plants with the complementary nitrocrcn roquired for thoir

growth. Such new forms of chemical fertilizers,which they desig­

nated as compatible fertilizers, could also be recommended for

use. The possibility, though promising,has not yet been serious­

ly explored.

Nitrification can be controlled by such classical methods as

split application of ammonium fertilizers, localization in mud

balls (International Rice Research Institute, 1978), or banding,

which inhibits nitrification due to the high concentration effect

on nitrifying bacteria (Wetselaar et al., 1972; Myers, 1978). The

use of slow-release fertilizers is also recommended to avoid the

harmful effects of nitrification (Fochts and Verstr~cte, 1977).

4.4. Manipulation of the plant component of the eoil-plant-

microorganisms system

4.4.1. Rotations

Introducing a specific'crop in the rotation system has been

used successfully as a basis- of the biological control of sorne

pests. Thus in Florida, soils infested by nematodes pathogenic

to tomato, are cured by growing a grass, Digitaria decumbens,

2'. after the tomato crop (Salette, personal communication). Crop

rotation is often the best method of control of soil-borne phy­

topathogenic fungi in cereals (see Baker and Cook, 1974). The

possibility of increasing populations of microorganisms benefi­

oial to plants through proper crop rotation was suggested by

Krasilnikov (1958) but has not yet oeen exploited. Though crop

sequence to manipulate the microhilogical balance is a promising

approach, investigations in that field will probably be diffic~t

to initiate and develop because of the large variability of cli­

mate and soil conditions.

4.4.2. Plant breeding

Genetic variability in plants responding to Rhizobium infection

is well known. This variability could be used as a basis for the

breeding programs of legumes. The future of this approach was

envisioned a.e follows by Holl and La Rue (1974).. "Plant genes

controlling fixation do occur, and experience shows that we oan

obtain informative and useful variants. There is no obvious rea-

son why symbiotic fixation cannot be increased by genetic means.

We oan envisage cultivars which nodulnte early in ha:t·sh soil

oonditions, fix dinitrogen, even in the presence,"of high Boil

nitrate levels, and continue ~ixing throughout t~eir life. It

appears that ~ixation may be limited by the supply o~ photosyn­

thate to the roots. Increased fixation may then require greater

photosynthesis,decreased photorespiration, delayed lodging, or

~ess pod-nodule competition for carbon". Two examples may serve

as an illustration of such a promising approach which has not

yet been seriously exploited. The first concerns the nodulation

of peanut. Comparing the time course of nodule dry weight of

three peanut cultivars grown at the same time in identical con­

ditions (Lior soil, Central Senegal, 1977), Germani (1979) found

that the maximum nodule weight of two of them was much higher

than that of the third (Fig. 4). However, such results should be

interpreted with caution since diiferences in nodule woight are

also ••••••

24

observed from one year to another. Thus the maximum nodule weight

of cv. 55-437 which was only 70 mg in 1977 could reach 100 mg

in ~he sarne soil during more humid years (1973 and 1975) and

even more than 200mg during an even more humid year (1974)

(Wey and Oba~on, 1978). The other example is rela~ed to soybean.

In West African soils, certain soybeen cultivars, such as Mala­

yan, are readily nodulated by na~ive Rhizobium of the cow-pea

group, whereas other cultivars, such as Bossier, a high yielding

cv. from the USA, require inoculation with the specifie Rhizobium

japonicum strains. Selection

of high-yielding soybeans which could nodulate with native

Rhizobium of the cow-pea group would allow the developmen~ of

this crop in Africa without requiring any inoculation, since

na~ive Rhizobium of the cow-pea group are common in most African

soils (International Institute of Tropical Agriculture, 1979).

~. 1. Uctho~s ta control Lho cffects of euvLronmen t a I

fat=lors limiling sym1)iotic Il2

fixation

factors

~11 acidity anù toxicity

~neral deficiencies,

~pecially phosphorus dcfi.ciency

.So11 inorganic nitrogcn

[Iathugcns

Mctho~s of control

- Irrigation

- Search for drought-rcsisting

cv. of legumes anù drought­

rcsisting ahizobium

- Stimulating VA mycorrhizal

infection

- Liming

A~dition of organic matter

- ALidition of phosphorus

- Stimulating VA mycorrhizal

infection

- Split application of

nitrogen fertili~ers

- Slow-release nitrogen

fertilizers

- Use of compatible fertili~ers

- Search for legwnes \'li th c.J,

lower capacity for nitrate

assimilation

- Chcmical, biological or

integratcd cOntrol

- Crop rotations

'I'ub l o 2. W':lter con sump t Lou (expr o s scd in ml/g Jry wcight) by

EupaLuriullI oJorûLuln I,. a nd 'l'aycLes ercctù L. at; two

Lov e Ls o I' soi1 W.:lt~r cou t.un t, (ÙO urul 20%) (1) und with

two l' [oruis (a tt.c r :lodwdLl, 1978).

NM 1208 1207 2860 4112

ri 1237 1177 1574 1436

NM 1073 1005 2563 3397

r-1 923 1060 1180 1424

inoculateJ wi~h VA mycorrhiza

80% (I) 20% (I)sos (1) 20% (I)'1'realmcnL

ritp,;cies

~r.:ltum

"

l)rcctûrr.:IJ: Non

.•: Inoculatcù with VA mycorrhiza

~oil W.:ltcr I::ontont cxprcsscu as ~ aVûilablc water.

~e ~n~nc..a ~ i~la~ ~ A 'ir~ ..' __

bacterial s~ain (?e=~.) and a ~ixt~e 0= both s~ains

parts or roots of 17 day-old rice seedlings grown in a

or a mixture of the same soil and coarse sand (Exp~.2)

anpublishe~ data, 1979).

__s_e~ 7~ ~in"'i:::1

upon the dry weight 0= aeria:

sterile alluvial soil (EX?~.l)

(Gauthier and Rina~jo,

Dr~l weight 0= aerial parts (::tg) ~ry weiqht 0= roots (mg)

Control Sp 7 Pect. Sp7 + Pec~. ~ontrol Sp 7 ?e~~. Spi' + ?ect.

EXDt. 1 62 12S xxXn.c:.. ,.. ~ 48 84 xXX

n.o.. n •.:l •• ...... ~ .Expt. 2 173 196 xXX

203xXX 233x XX 13ô 149 161x 187x X

n.d. non decermineë.

x,XX,xxx value significantly different =ro~ control, P=.J5, P=.~l, P=.OOl respectively.

Tne pectinolytic strain (Pect.) .did not fix N2.

'l'au1e 4. Ln ï' ~uence or so i I inoculation with EntcrobActor ~1;oto9

on, the growth .:1I1U ni trogen content of sorghua growR iD

a phytotoxic or non-phytotoxic soil. (Burgos, Tho.1B,

University of Nancy, 1979)

Phytotoxic soil

Non-phytotoxic

No inoculation Inoculation soil

(control)

li parts

~ht (cm) 30 65 60

ili(Jhl, plant) 0.5 . 3.9 2.7

al 1-1 content

Jr plant) 13.9 413.7 4.7

tt i CJ h t

plant) \J.u 8.l 1.3

Table 5. Effcct·of aùdition of caco3

.o r sterile peat on nodul~tlon

and dry weight of acrial parts of soyuean cv. Ch1ppcW4(Iloureau, unpulJlisheù data, 1979)

--

fxPt. 1

'Itrol

aCO

lo6k9 per ha)

1

It. 2

:entrol

rit1400 kg per ha)

1

pH

4 '.0 a

7.0 b

4.0 a

4.0 a

Nodul.e

numlJer

(pcr plant)

14 a

39 b

213 cl

41 b

Noùule

dry weight

(mg per plant)

42 a

112 b

44 a

88 b

Dry wei(jht

of ùerial

(mg par plan t)

3.38 a

3.78 ù

2.16 a

3.03 b

One plant per pot containing 5 k~ of sail froln Sefa Rcsearch Station,

s eneqa l , All plants were inoculated with. IrnL of a 3-ùay o Ld culture

of Rhizobiu~1\ japonicum G2Sp (108 lJactcria per m.l) ; Observations were

made when plants were 6 wecks old. In each cxperiment, numbers in

columns nat having the same lctter diffet, P =.05.

FIGUR~ CAPTIONS

Fj~. 1. Variations of acptylene reduaing activity (ARA per plant)

of fif>ld-grown peanut and of soil water content throughout

the t->p.anut growth cycle as observed in 1977 ut the Bamb.ey

Experimental Station,Centra.l Senegal (Ducerf. 1978).

FIg. 2. Azadirachta indica roote infected ·wi.th VA mycorrhizae.

Fi g •.}. Clueter of root lets (proteoid roote) of Caeuarina equieeti-

foLi n grow i nI{ in 8 sandy BO i l (SanAgal).

Pige 4. Timu couree .of nodule dry weight of peanut expreeeed as mg

par plant. A : cv. 28-206 and GH 119-20; B : cv. 55-437.

Al L d u Ln w('r{~ c oLl.nc t ed 'In lq77 ut Patu e , Central Senegal

(ci p rman i, 1 97 q ) •

~la..

..J

o6 (J)

Z

o

5 ~1­e:(

~4 UJ

C)e:(1-

3 ~oCt:tu

'} CL

\l\,\1 \

l ,1 \l ,, ,1 1l 'l '4 \1 \, \1 \1 \,,, ~/\

l " : \v \ l ,\1 ll

,,,\

~\\,

\

~\

80 ·100

DAYS AFTER SOWING

60

1"1\

" 11,/ 1 1 ~

(\ " 1\l ,1 \( \ i~/\

: \ ) "1 V \1

pod ifillinq

4020

11\1 \, \1 .. -,, \1 1l ,

1 \l ,1 \, 1

: \, \1 \1 \, \. 1, ~

1

'''''pw :, l , 1

"\ 1\ ,, 1, :

l 1........ 1

, 1\ ,\ ,~

u

!l,l,1)lu

'0110

1

B

..-'---T ... ------·--r---.----r------ 160 80 '100 120

......-

1+'c 300_CO-C.

C)

E- 200......cC)

·cü~

100CI)-:::J"C0Z

0. 1~o

A

Age of the plant ( day s j

A 1.~:j(ANj)I~H, M. (lO(Jl). l n t r-od u c tl o n to o o i I mi c r-o b i o l o gy , John WïJ(',y,'

Nnw Yo r-k , 4'12 l'P.

AI,WI:;, Il.1'. d(~ llnd AB~~YNAYAK(~, K. (1 ~'(B) • ln: Proe. InternatJonal

'NorkHhop on 'froId ca 1 Mycorrhiza Heoearch. Kumars d ,

::ip.ptembnr lq'TH. IP~). Stockholm. 135-1?5.

ANDI~IW()N, .r . l' .1';. ami ]lOM~)CH, K .11, (1 07B). :30il Bit)l. Hiochem .. 10'.

HAKIm. K."'. IInd COOK. H.,I. ( 1 q74 ) • Aiological control of' plant

fll1thr>f~l'nn. V.H. 1·'r·N.... man. ~lu.n ~'rnncloc('). 433 pp.

( 1 Cl'TH) • ln: Inte['action6 b .. twcp.n non-pathosenic soil

microorgl1ni~mo and planta. (Y.H. Dommergues and

"V L' !~i~)• r , • 1\ ru pu ,l, S • ~nnpvjer, Amnter·dum. 131-162 •

HI,ACK. H. ( 1 I)'{H ) • ln: l'roc. lnl~~rnatjonal Workohop on Tropical

BONII. fi.ll.

Myeordl i zu. Htw('Hrclh. KUlnua.i. Beptt'~mbl'r 1978, IFS,

~it;ockho1rn. '15-B6.

BONIJ, H.II. und IJAHHI:i ••J.H. (1 1164 ) . ~tr. J. ~50il HfHl. , 2,111-

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l.ond o n , 1~1-1l)·T.

BOWBN, G.D. (1978). Ina Proc. International Workshop on Trppi 0a1

MYcorrhiza Research. Kumaei, September 1978, IFS,

Stookholm, 31-71.

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Fixation. (R.W.F. Hardy and W.S. Silver Eda.) Wiley

and Bons, New York, 557-576.

,.~-.

CHABALIBR, P.P. (1978). Ina Proc. Workshop on Nitrogen Cyoling in

West Atrican Agrosystems. Ibadan, December 1978 (in preaa~

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DART, P., DAY, J., IBLAM, R. ar-d DOBERElNER, J. (1973). Inl Sypbio­

tie Nitrosen Fixation in Plants. (P.S. Nutman Ed.)

Ca~bridge University Press. 361-384.

DODBRElNER, J. (1977). Ina Proc. Advisory Group Meeting op the

Potential Use n1_J~~~~~n thu Study or Biolo,ieel

Dinitrosen Fixation.. Vienna, November 1977 (in press).

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DOMMBRGUBS, Y.R. (1977). In: Proc. ~Ao/sIDA Regional Workshop on

Atrica on Crianto Recycling in Agriculture. Buea

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Boil Microorganisme and Plants. (Y~R. Dommergues and

B.V. Krupa Ede.) Elsevier, Amsterdam, 1-36.

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Soil Microorganisme and Plants. (Y.R. Dommergues and

S.V. Krupa Ede.) Elsevier, Amsterdam, 443-458.

DOMMEIWUltJU, y .R •• CO~1H1mMONT, H•• HBèK, G. and Or.r.AT, C. (1 ~·6o).

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