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Endotrophic mycorrhizas in Nigeria: Species of the endogonaceae and their distribution
[275 ] Trans. Br, mycol. Soc. 69 (2) 275-280 (1977)
Printed in Great Britain
ENDOTROPHIC MYCORRHIZAS IN NIGERIA: SPECIES OF THE ENDOGONACEAE AND THEIR DISTRIBUTION By J. F. REDHEAD* Federal Department of Forestry, Ibadan, Nigeria Morphology of mycelium in roots and soil was similar to that of temperate Endogonaceae. Spores were widely distributed in soils from the moist Lowland Rain Forest to the dry Sahel Zone, being generally most frequent in areas with a marked dry season in which herbaceous growth died back. Spores occurred mostly in the top 15 em of the soil. Several species of the Endogonaceae, previously recorded in U.S.A., Scotland or Australia, have been found as well as several species not described elsewhere. The vesicular-arbuscular (VA) mycorrhizal association is the commonest type of mycorrhizal association and develops on most important crop plants (Gerdemann, 1968). Butler (1939) reviewed the systematic position of the fungi concerned. There were strong indications that species of the Endogonaceae were the causal fungi but it was not until the work of Mosse (1962) that this was generally accepted. Nicolson & Gerdemann (1968) named some VA endophytes occurring in soils as naked single spores or small sporocarps of less than 1 mm diam. More recently, Gerdemann & Trappe (1974) proposed a new classification of the Endogonaceae that divides the family into five genera. Glomus, Gigaspora, Acaulospora and Sclerocystis contain VA endophytes, but the zygosporic genus Endogone contains none. An alternative method of spore identification based on spore anatomy and ontogeny was proposed by Mosse & Bowen (1968). Studies in Nigeria (Redhead, 1968) indicated that the fungal partners forming the vesiculararbuscular association there were also species of the Endogonaceae. The present paper describes the spore types and the distribution of extramatrical spores in the soils of a wide range of Nigerian vegetation types. METHODS AND RESULTS
Quantitative Survey Soils were examined for Endogone spores using the method of wet-sieving and decanting described by Gerdemann (1955). The numbers of spores over 100 pm diam decreased abruptly at depths below 15 em in 500 em" samples of soil taken from a Guinea savanna zone site on the [os Plateau. The mean numbers of spores at various depths "'Present address: Division of Forestry, University Dar es Salaam.
were: at 2 em, 748; at 7'5 em, 1946; at 15 em, 1064; and at 30 em, 55. In a uniform Southern Guinea savanna zone site, at a depth of 7'5 em, the mean number of spores over 100 pm diam was 38'9 per 25 em" sample, with a coefficient of variation of 0'52. A series of 39 samples from the top 15 em of a fallow grass field on the Ibadan University farm gave a mean number of 11'3 spores per 25 em" soil, with a coefficient of variation of 0'36. In subsequent surveys at least 12 samples were pooled to represent anyone site. Surveys were made at approximately 3-monthly intervals in SUt vegetation zones, and in one of these (Northern Guinea savanna) spores were counted under four different kinds of vegetative cover. The results are shown in Table 1.
Mycelium of Endogone was found by direct examination to be widespread and frequent in Nigerian soils, even in soils of the lowland rain forest where extra-matrical spores were few. The fungus did not appear to undergo a resting period, even in the driest Sahel and Sudan savanna zones' soils. The hyphal morphology appeared identical to that described for other areas of the world. Inside the root cortical cells the endotrophic mycelium showed all the typical stages of coiling, arbuscule and vesicle formation. Vesicles obtained by micro-dissection of roots (Fig. 1) were similar to the extra-matrical spores produced by Endogene in the soil (Fig. 2).
Spore types found Many different spore types were found in the survey and in other areas examined. The following descriptions of the principal types are based on criteria proposed by Mosse & Bowen (1968). Sporocarps: Few sporocarps were found enclosed in a peridium and none were similar to those described by Mosse (1956). Occasionally a
276
Endotrophic Mycorrhizas in Nigeria Table 1. Survey of Endogone sporepopulations in N igerian vegetation types
Site Lowland rain forest, 6° 5' N, 5° 50' E
pH 4'5
Month of collection
Spores per 500 em' of soil
Jan. May July Oct.
3° 10 32 24°
Dec. Apr. June Oct.
432 2°5 43° 2328
Jan. May June Nov.
773 545 672 675
1205 mm
Jan. Mar. June Oct.
120 984 732 296
1165 mm
Jan. Mar. June Oct.
1320 2276 896 296
Jan. June Sept.
1378 1056 1000 1065
Dry semi-deciduous forest, 7° 28' N, 3° 31' E
6'5
Southern Guinea savanna, 7° 50' N, 3° 30' E (grass with shrubs and scattered trees)
6'8
Northern Guinea savanna, 10° 40' N, 7° 20' E (savanna woodland, close canopy with virtually no grass layer)
6'2
Northern Guinea savanna (grass savanna area)
6'1
Northern Guinea savanna (Eucalyptus torelliana, close canopy, no grass layer)
5'5
Northern Guinea savanna (Eucalyptus teretecornis open canopy with some grass)
5'8
Jan. Mar. June Oct.
499 1944 1012 2472
Sudan savanna 110 51' N, 13° 12' E
6'8
Jan. June July Sept.
3936 2535 1144 4160
Jan. June July Sept.
300 835 8642 3520
Sahel savanna 13° 40 ' N, 13° 25' E
8'2
Ann. rainfall, peak month, no. months when rainfall > 25 mm
Mar.
sporocarp contaming a single spore was found (Fig. 3) and a Sclerocystis sp. was regularly seen. Yellow vacuolate spores: Two kinds were found . (1) Yellow, turning black and opaque, attachment simple, occasionally swollen, open channel, sometimes plugged. Spherical, 80-180 pm diam, occurring in spore aggregates of up to 100 or more without peridium (Fig . 4). Commonly found in the Sahel and Sudan savann a zones. Parasitized spores of this type were found which had rad ial canals in the spore wall (Fig. 5) and
2130 mm
July 10 months 1220 mm
June, Sept. 9 months
June, Sept. 10 months
Sept. 7 months
703mm
Aug.
5 months 163mm
Aug. 2 months
appeared identical to those described by Godfrey (1957) . Sometimes the parasitized spore contained smaller spores and occasionally the germ-tube of these internal spores passed through the Endogone spore attachment. Reproduced in pot culture on Na uclea diderrichii (De Wild & Th.Dur.) Merrill, the spore resembles Endogonefasciculata (Thaxter, 1922), Glomus fasciculatus (Gerdemann & Trappe, 1974)· (2) Spherical, 150-200 pm diam, spore base
open (Fig. 6). This spore type may be the same as
J. F. Redhead
Fig. 1. Vesicles dissected from roots of Tectona grandis Linn. f. Fig. 2. Extra-matrical spores on hyphae in rhizosphere of T. grandis roots. Note the similarity to vesicles in Fig. 1. Fig. 3. Sporocarp consisting of a single spore. Fig. 4. Aggregates of yellow vacuolate spores from Sahel savanna. Fig. S. Yellow vacuolate spore showing advanced stage of parasitism. Fig. 6. Yellow vacuolate spore produced in pot-culture on Nauclea diderrichii.
277
Endotrophic Mycorrhizas in Nigeria
.L--J
SO 11m
10
l---J
70 lim
12
Figs. 7-10. Bulbous based spores, Gigaspora spp. 7, with vacuolate cytoplasm possibly, G. gilmorei. 8, Golden yellow with reticulate cytoplasm. Note emerging germ tube. 9, 10, Two other spore types, brown with reticulate cytoplasm. Spores born laterally (9) or apically (10) on suspensor. Fig. 11. Crenulate spore, note the subtending hypha. Fig. 12. Dimpled spore, note the very thin wall and escaping oil droplet.
J. F. Redhead the previous one but a black stage has not been observed and the oil vacuoles are generally larger. Found in the Gambari Forest Reserve, Ibadan Division, in the rain forest zone; reproduced in pot culture on Tectona grandis Linn. f. and Nauclea diderrichii. Bulbous based spores (Gigaspora spp.), Six different kinds were found: (1) Spherical, 350-450 Jlm diam, with oil vacuoles of very different sizes (Fig. 7). Similar to the bulbous vacuolate spore type described by Mosse & Bowen (1968); possibly G. gilmorei (Gerdemann & Trappe, 1974). Found in Northern Guinea zone savanna. (2) Colourless to whitish, spherical with fine open pore, 180-280 Jlm diam, cytoplasm with many small even-sized oil globules (reticulate) occurring singly. Often associated with smaller brown spores in pot cultures. (3) Golden yellow, spherical, 300 Jlm diam, occurring singly (Fig. 8). Similar to G. gigantea (Nicol. & Gerd.) Gerdemann & Trappe (1974) and to a bulbous reticulate spore described by Mosse & Bowen (1968). Found at Miango, Jos Plateau. (4) Brown, almost spherical, 3°0-4°0 Jlm diam, attached laterally on suspensor with prominent side branch, cytoplasm reticulate (Fig. 9). (5) Darker brown than type 4, spore attached to suspensor apically, side branch on suspensor abortive, cytoplasm reticulate (Fig. 10). (6) This previously unrecorded species of unusual interest has been described by Old, Nicolson & Redhead (1973). Spores appear black with a distinctive multilayered wall in which the outer layers form a reticulate network. In some respects the wall resembles that of the 'honey-coloured sessile' spore described by Mosse (1970). Crenulate spore. Black, spherical, 100-180 Jlm diam (Fig. 11). Found at Miango, Ios Plateau at 1200 m elevation in the Guinea savanna zone. Spores like these were described by Mosse & Bowen (1968). They did not form VA mycorrhiza but a fungus forming similar spores in culture developed ectendotrophic mycorrhiza on Pinus resinosa (Wilcox, Ganmore-Neumann & Wang, 1974)· Dimpled spore: Attachment simple, spherical, 150-180 Jlm diam, thin walled (Fig. 12). Only recorded from the Guinea savanna zones. A somewhat similar spore (yellow punctate) was found by Koske (1975) in Australian sand dunes.
279
unexpected because most rootlets and microorganisms are in the top layers of the soil. The variability in the spore distribution was more surprising and confirms the need for adequate sampling in any study of spore populations. The survey of Endogone spores indicated the world-wide distribution of several species and genera. The reason for. this wide distribution is not obvious because Endogone spores are so large and appear to have no special mechanism for dispersal other than by small animals which might eat them. The world-wide distribution may be accounted for by the great antiquity of this group of fungi (Nicolson, 1975). Spore populations showed a trend towards increasing frequency from moist forest to the driest savannas (Table 1) with some fluctuations and unexpected results. There appeared to be no reason for the high October count in the dry semi-deciduous forest nor for the low January count in the Sahel savanna. Further sampling, covering a series of successive years, would be necessary to adequately correlate spore populations with climatic conditions. Soil in a lowland rain forest nursery, with vigorously growing, strongly mycorrhizal Nauclea and Khaya seedlings, contained no spores at all in most samples. In such moist forest areas of Nigeria growth occurs throughout the year, but in the savanna areas the herbaceous growth dies back during the dry season. In Guinea savanna the grasses are mainly perennial; they die back above ground and sprout again from their roots. The Sudan and Sahel savanna grasses are mainly annuals and they die off completely during the dry season. This seasonal die-back can be expected to produce a similar effect on spore numbers to that observed when arable crops such as maize, barley and wheat are harvested. Pot cultures (Mosse, 1956) proved a successful way of building up spore populations and several distinct Endogone species, found only in savanna soils, easily formed associations on the indigenous lowland rain forest species Nauclea diderrichii and the exotic Tectona grandis. Several introduced conifers had previously been observed to form vesicular-arbuscular mycorrhizal associations readily with locally occurring fungi (Redhead, 1974). These observations confirm the wide host range of many Endogone sp. reported in temperate regions (Mosse, 1975).
REFERENCES
DISCUSSION
The observation that Endogone spores were most frequent in the top 15 em of the soil was not
BUTLER,
E.
J. (1939). The occurrences and systemic
position of the vesicular-arbuscular type of mycorrhizal fungi. Transactions of the British Mycological Society U, 274-301.
280
Endotrophic Mycorrhizas in Nigeria
GBRDBMANN, J. W. (1955). Relation of a large soilborne spore to phycomycetous mycorrhizal infections. Mycologia 47, 619-"632. GBRDBMANN, J. W. (1968). Vesicular-arbuscular mycorrhiza and plant growth. Annual Review of Phytopathology 6, 397-418. GBRDBMANN, J. W. & TRAPPB, J. (1974). The Endogonaceae in the Pacific Northwest. Mycologia Memoir 5, 76 pp. GODFREY, R. M. (1957). Studies of British species of Endogene, II. Fungal parasites. Transactions of the British Mycological Society 40, 136-140. KOSKE, R. E. (1975). Endogene spores in Australian sand dunes. CanadianJournal of Botany 53, 668-672. MOSSB, B. (1956). Fructifications of an Endogone species causing endotrophic mycorrhiza in fruit plants. Annals of Botany, London 20, 349-362. MOSSB, B. (1962). The establishment of vesiculararbuscular mycorrhiza under aseptic conditions. Journal of General Microbiology 27, 509-520. MOSSB, B. (1970). Honey-coloured, sessile Endogone spores. III. Wall structure. Archiv fur Mikrobiologie 74, 146-159. MOSSE, B. (1975). Specificity in VA mycorrhizas. In Endomycorrhizas (ed. F. Sanders, B. Mosse and P. B. H. Tinker), pp. 469-484. London: Academic Press.
MOSSE, B. & BOWEN, G. D. (1968). A key to the recognition of some Endogone spore types. Transactionsof the British MycologicalSociety 51,485-492. NICOLSON, T. H. (1975). Evolution of vesiculararbuscular mycorrhizas. In Endomycorrhizas (ed. F. Sanders, B. Mosse and P. B. H. Tinker), pp. 25-34. London: Academic Press. NICOLSON, T. H. & GERDEMANN, J. W. (1968). Mycorrhizal Endogone species. Mycologia 60, 313325. OLD, K. M., NICOLSON, T. H. & REDHEAD, J. F. (1973). A new species of mycorrhizal Endogone from Nigeria with a distinctive spore wall. New Phytologist 72, 817-823. REDHEAD, J. F. (1968). Mycorrhizal associations in some Nigerian forest trees. Transactions of the British Mycological Society 51, 377-387. REDHEAD, J. F. (1974). Aspects of the biology of mycorrhizal associations occurring on tree species in Nigeria. Ph.D. thesis, University of Ibadan, Nigeria. THAXTER, 'R. (1922). A revision of the Endogoneae. Proceedings of the American Academy of Arts & Sciences 57, 291-350 • WILCOX, H. E., GANMORE-NEUMANN, R. & WANG, C. J. K. (1974). Characteristics of two fungi producing ectendomycorrhizae in Pinus resinosa, Canadian Journal of Botany 52, 2279-2282.
(Accepted for publication 27 January 1977)
Printed in Great Britain
ENDOTROPHIC MYCORRHIZAS IN NIGERIA: SPECIES OF THE ENDOGONACEAE AND THEIR DISTRIBUTION By J. F. REDHEAD* Federal Department of Forestry, Ibadan, Nigeria Morphology of mycelium in roots and soil was similar to that of temperate Endogonaceae. Spores were widely distributed in soils from the moist Lowland Rain Forest to the dry Sahel Zone, being generally most frequent in areas with a marked dry season in which herbaceous growth died back. Spores occurred mostly in the top 15 em of the soil. Several species of the Endogonaceae, previously recorded in U.S.A., Scotland or Australia, have been found as well as several species not described elsewhere. The vesicular-arbuscular (VA) mycorrhizal association is the commonest type of mycorrhizal association and develops on most important crop plants (Gerdemann, 1968). Butler (1939) reviewed the systematic position of the fungi concerned. There were strong indications that species of the Endogonaceae were the causal fungi but it was not until the work of Mosse (1962) that this was generally accepted. Nicolson & Gerdemann (1968) named some VA endophytes occurring in soils as naked single spores or small sporocarps of less than 1 mm diam. More recently, Gerdemann & Trappe (1974) proposed a new classification of the Endogonaceae that divides the family into five genera. Glomus, Gigaspora, Acaulospora and Sclerocystis contain VA endophytes, but the zygosporic genus Endogone contains none. An alternative method of spore identification based on spore anatomy and ontogeny was proposed by Mosse & Bowen (1968). Studies in Nigeria (Redhead, 1968) indicated that the fungal partners forming the vesiculararbuscular association there were also species of the Endogonaceae. The present paper describes the spore types and the distribution of extramatrical spores in the soils of a wide range of Nigerian vegetation types. METHODS AND RESULTS
Quantitative Survey Soils were examined for Endogone spores using the method of wet-sieving and decanting described by Gerdemann (1955). The numbers of spores over 100 pm diam decreased abruptly at depths below 15 em in 500 em" samples of soil taken from a Guinea savanna zone site on the [os Plateau. The mean numbers of spores at various depths "'Present address: Division of Forestry, University Dar es Salaam.
were: at 2 em, 748; at 7'5 em, 1946; at 15 em, 1064; and at 30 em, 55. In a uniform Southern Guinea savanna zone site, at a depth of 7'5 em, the mean number of spores over 100 pm diam was 38'9 per 25 em" sample, with a coefficient of variation of 0'52. A series of 39 samples from the top 15 em of a fallow grass field on the Ibadan University farm gave a mean number of 11'3 spores per 25 em" soil, with a coefficient of variation of 0'36. In subsequent surveys at least 12 samples were pooled to represent anyone site. Surveys were made at approximately 3-monthly intervals in SUt vegetation zones, and in one of these (Northern Guinea savanna) spores were counted under four different kinds of vegetative cover. The results are shown in Table 1.
Mycelium of Endogone was found by direct examination to be widespread and frequent in Nigerian soils, even in soils of the lowland rain forest where extra-matrical spores were few. The fungus did not appear to undergo a resting period, even in the driest Sahel and Sudan savanna zones' soils. The hyphal morphology appeared identical to that described for other areas of the world. Inside the root cortical cells the endotrophic mycelium showed all the typical stages of coiling, arbuscule and vesicle formation. Vesicles obtained by micro-dissection of roots (Fig. 1) were similar to the extra-matrical spores produced by Endogene in the soil (Fig. 2).
Spore types found Many different spore types were found in the survey and in other areas examined. The following descriptions of the principal types are based on criteria proposed by Mosse & Bowen (1968). Sporocarps: Few sporocarps were found enclosed in a peridium and none were similar to those described by Mosse (1956). Occasionally a
276
Endotrophic Mycorrhizas in Nigeria Table 1. Survey of Endogone sporepopulations in N igerian vegetation types
Site Lowland rain forest, 6° 5' N, 5° 50' E
pH 4'5
Month of collection
Spores per 500 em' of soil
Jan. May July Oct.
3° 10 32 24°
Dec. Apr. June Oct.
432 2°5 43° 2328
Jan. May June Nov.
773 545 672 675
1205 mm
Jan. Mar. June Oct.
120 984 732 296
1165 mm
Jan. Mar. June Oct.
1320 2276 896 296
Jan. June Sept.
1378 1056 1000 1065
Dry semi-deciduous forest, 7° 28' N, 3° 31' E
6'5
Southern Guinea savanna, 7° 50' N, 3° 30' E (grass with shrubs and scattered trees)
6'8
Northern Guinea savanna, 10° 40' N, 7° 20' E (savanna woodland, close canopy with virtually no grass layer)
6'2
Northern Guinea savanna (grass savanna area)
6'1
Northern Guinea savanna (Eucalyptus torelliana, close canopy, no grass layer)
5'5
Northern Guinea savanna (Eucalyptus teretecornis open canopy with some grass)
5'8
Jan. Mar. June Oct.
499 1944 1012 2472
Sudan savanna 110 51' N, 13° 12' E
6'8
Jan. June July Sept.
3936 2535 1144 4160
Jan. June July Sept.
300 835 8642 3520
Sahel savanna 13° 40 ' N, 13° 25' E
8'2
Ann. rainfall, peak month, no. months when rainfall > 25 mm
Mar.
sporocarp contaming a single spore was found (Fig. 3) and a Sclerocystis sp. was regularly seen. Yellow vacuolate spores: Two kinds were found . (1) Yellow, turning black and opaque, attachment simple, occasionally swollen, open channel, sometimes plugged. Spherical, 80-180 pm diam, occurring in spore aggregates of up to 100 or more without peridium (Fig . 4). Commonly found in the Sahel and Sudan savann a zones. Parasitized spores of this type were found which had rad ial canals in the spore wall (Fig. 5) and
2130 mm
July 10 months 1220 mm
June, Sept. 9 months
June, Sept. 10 months
Sept. 7 months
703mm
Aug.
5 months 163mm
Aug. 2 months
appeared identical to those described by Godfrey (1957) . Sometimes the parasitized spore contained smaller spores and occasionally the germ-tube of these internal spores passed through the Endogone spore attachment. Reproduced in pot culture on Na uclea diderrichii (De Wild & Th.Dur.) Merrill, the spore resembles Endogonefasciculata (Thaxter, 1922), Glomus fasciculatus (Gerdemann & Trappe, 1974)· (2) Spherical, 150-200 pm diam, spore base
open (Fig. 6). This spore type may be the same as
J. F. Redhead
Fig. 1. Vesicles dissected from roots of Tectona grandis Linn. f. Fig. 2. Extra-matrical spores on hyphae in rhizosphere of T. grandis roots. Note the similarity to vesicles in Fig. 1. Fig. 3. Sporocarp consisting of a single spore. Fig. 4. Aggregates of yellow vacuolate spores from Sahel savanna. Fig. S. Yellow vacuolate spore showing advanced stage of parasitism. Fig. 6. Yellow vacuolate spore produced in pot-culture on Nauclea diderrichii.
277
Endotrophic Mycorrhizas in Nigeria
.L--J
SO 11m
10
l---J
70 lim
12
Figs. 7-10. Bulbous based spores, Gigaspora spp. 7, with vacuolate cytoplasm possibly, G. gilmorei. 8, Golden yellow with reticulate cytoplasm. Note emerging germ tube. 9, 10, Two other spore types, brown with reticulate cytoplasm. Spores born laterally (9) or apically (10) on suspensor. Fig. 11. Crenulate spore, note the subtending hypha. Fig. 12. Dimpled spore, note the very thin wall and escaping oil droplet.
J. F. Redhead the previous one but a black stage has not been observed and the oil vacuoles are generally larger. Found in the Gambari Forest Reserve, Ibadan Division, in the rain forest zone; reproduced in pot culture on Tectona grandis Linn. f. and Nauclea diderrichii. Bulbous based spores (Gigaspora spp.), Six different kinds were found: (1) Spherical, 350-450 Jlm diam, with oil vacuoles of very different sizes (Fig. 7). Similar to the bulbous vacuolate spore type described by Mosse & Bowen (1968); possibly G. gilmorei (Gerdemann & Trappe, 1974). Found in Northern Guinea zone savanna. (2) Colourless to whitish, spherical with fine open pore, 180-280 Jlm diam, cytoplasm with many small even-sized oil globules (reticulate) occurring singly. Often associated with smaller brown spores in pot cultures. (3) Golden yellow, spherical, 300 Jlm diam, occurring singly (Fig. 8). Similar to G. gigantea (Nicol. & Gerd.) Gerdemann & Trappe (1974) and to a bulbous reticulate spore described by Mosse & Bowen (1968). Found at Miango, Jos Plateau. (4) Brown, almost spherical, 3°0-4°0 Jlm diam, attached laterally on suspensor with prominent side branch, cytoplasm reticulate (Fig. 9). (5) Darker brown than type 4, spore attached to suspensor apically, side branch on suspensor abortive, cytoplasm reticulate (Fig. 10). (6) This previously unrecorded species of unusual interest has been described by Old, Nicolson & Redhead (1973). Spores appear black with a distinctive multilayered wall in which the outer layers form a reticulate network. In some respects the wall resembles that of the 'honey-coloured sessile' spore described by Mosse (1970). Crenulate spore. Black, spherical, 100-180 Jlm diam (Fig. 11). Found at Miango, Ios Plateau at 1200 m elevation in the Guinea savanna zone. Spores like these were described by Mosse & Bowen (1968). They did not form VA mycorrhiza but a fungus forming similar spores in culture developed ectendotrophic mycorrhiza on Pinus resinosa (Wilcox, Ganmore-Neumann & Wang, 1974)· Dimpled spore: Attachment simple, spherical, 150-180 Jlm diam, thin walled (Fig. 12). Only recorded from the Guinea savanna zones. A somewhat similar spore (yellow punctate) was found by Koske (1975) in Australian sand dunes.
279
unexpected because most rootlets and microorganisms are in the top layers of the soil. The variability in the spore distribution was more surprising and confirms the need for adequate sampling in any study of spore populations. The survey of Endogone spores indicated the world-wide distribution of several species and genera. The reason for. this wide distribution is not obvious because Endogone spores are so large and appear to have no special mechanism for dispersal other than by small animals which might eat them. The world-wide distribution may be accounted for by the great antiquity of this group of fungi (Nicolson, 1975). Spore populations showed a trend towards increasing frequency from moist forest to the driest savannas (Table 1) with some fluctuations and unexpected results. There appeared to be no reason for the high October count in the dry semi-deciduous forest nor for the low January count in the Sahel savanna. Further sampling, covering a series of successive years, would be necessary to adequately correlate spore populations with climatic conditions. Soil in a lowland rain forest nursery, with vigorously growing, strongly mycorrhizal Nauclea and Khaya seedlings, contained no spores at all in most samples. In such moist forest areas of Nigeria growth occurs throughout the year, but in the savanna areas the herbaceous growth dies back during the dry season. In Guinea savanna the grasses are mainly perennial; they die back above ground and sprout again from their roots. The Sudan and Sahel savanna grasses are mainly annuals and they die off completely during the dry season. This seasonal die-back can be expected to produce a similar effect on spore numbers to that observed when arable crops such as maize, barley and wheat are harvested. Pot cultures (Mosse, 1956) proved a successful way of building up spore populations and several distinct Endogone species, found only in savanna soils, easily formed associations on the indigenous lowland rain forest species Nauclea diderrichii and the exotic Tectona grandis. Several introduced conifers had previously been observed to form vesicular-arbuscular mycorrhizal associations readily with locally occurring fungi (Redhead, 1974). These observations confirm the wide host range of many Endogone sp. reported in temperate regions (Mosse, 1975).
REFERENCES
DISCUSSION
The observation that Endogone spores were most frequent in the top 15 em of the soil was not
BUTLER,
E.
J. (1939). The occurrences and systemic
position of the vesicular-arbuscular type of mycorrhizal fungi. Transactions of the British Mycological Society U, 274-301.
280
Endotrophic Mycorrhizas in Nigeria
GBRDBMANN, J. W. (1955). Relation of a large soilborne spore to phycomycetous mycorrhizal infections. Mycologia 47, 619-"632. GBRDBMANN, J. W. (1968). Vesicular-arbuscular mycorrhiza and plant growth. Annual Review of Phytopathology 6, 397-418. GBRDBMANN, J. W. & TRAPPB, J. (1974). The Endogonaceae in the Pacific Northwest. Mycologia Memoir 5, 76 pp. GODFREY, R. M. (1957). Studies of British species of Endogene, II. Fungal parasites. Transactions of the British Mycological Society 40, 136-140. KOSKE, R. E. (1975). Endogene spores in Australian sand dunes. CanadianJournal of Botany 53, 668-672. MOSSB, B. (1956). Fructifications of an Endogone species causing endotrophic mycorrhiza in fruit plants. Annals of Botany, London 20, 349-362. MOSSB, B. (1962). The establishment of vesiculararbuscular mycorrhiza under aseptic conditions. Journal of General Microbiology 27, 509-520. MOSSB, B. (1970). Honey-coloured, sessile Endogone spores. III. Wall structure. Archiv fur Mikrobiologie 74, 146-159. MOSSE, B. (1975). Specificity in VA mycorrhizas. In Endomycorrhizas (ed. F. Sanders, B. Mosse and P. B. H. Tinker), pp. 469-484. London: Academic Press.
MOSSE, B. & BOWEN, G. D. (1968). A key to the recognition of some Endogone spore types. Transactionsof the British MycologicalSociety 51,485-492. NICOLSON, T. H. (1975). Evolution of vesiculararbuscular mycorrhizas. In Endomycorrhizas (ed. F. Sanders, B. Mosse and P. B. H. Tinker), pp. 25-34. London: Academic Press. NICOLSON, T. H. & GERDEMANN, J. W. (1968). Mycorrhizal Endogone species. Mycologia 60, 313325. OLD, K. M., NICOLSON, T. H. & REDHEAD, J. F. (1973). A new species of mycorrhizal Endogone from Nigeria with a distinctive spore wall. New Phytologist 72, 817-823. REDHEAD, J. F. (1968). Mycorrhizal associations in some Nigerian forest trees. Transactions of the British Mycological Society 51, 377-387. REDHEAD, J. F. (1974). Aspects of the biology of mycorrhizal associations occurring on tree species in Nigeria. Ph.D. thesis, University of Ibadan, Nigeria. THAXTER, 'R. (1922). A revision of the Endogoneae. Proceedings of the American Academy of Arts & Sciences 57, 291-350 • WILCOX, H. E., GANMORE-NEUMANN, R. & WANG, C. J. K. (1974). Characteristics of two fungi producing ectendomycorrhizae in Pinus resinosa, Canadian Journal of Botany 52, 2279-2282.
(Accepted for publication 27 January 1977)