Báo cáo khoa học: "First observations on the root morphology and symbioses of 21 major tree species in the primary tropical rain forest of French Guyana"

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Nội dung Text: Báo cáo khoa học: "First observations on the root morphology and symbioses of 21 major tree species in the primary tropical rain forest of French Guyana"

Original article First observations on the root morphology and symbioses of 21 major tree species in the primary tropical rain forest of French Guyana M Béreau J Garbaye 1 INRA, Station de Recherches Forestières, Groupe Régional de Guyane, BP 709, 97387 Kourou CEDEX, French Guyana; 2 INRA, Centre de Recherches Forestières de Nancy, 54280 Champenoux, France 15 June 1993; 2 December (Received accepted 1993) Summary — A wide diversity of root morphology and symbiotic associations have been catalogued in 21 tree species in the tropical rain forest of French Guyana. Some unusual structures are described, such as nodular short roots containing hyphal coils. Endomycorrhizal symbiosis prevails in this type of forest. Parasitic nematodes (Meloidogyne sp) on young seedling roots of Dicorynia guianensis and Pelto- gyne venosa probably interfere with their regeneration. tropical rain forest / Caesalpinioideae / root type / symbiotic status / French Guyana Résumé — Premières observations sur la morphologie des racines et les symbioses racinaires de 21 espèces principales d’arbres en forêt tropicale humide primaire de Guyane française. Dans la forêt tropicale humide de Guyane française, une grande diversité de morphologie racinaire et d’associations symbiotiques a été observée sur 21 espèces d’arbres. Certaines structures inhabi- tuelles sont décrites, comme des racines courtes noduleuses contenant des hyphes en pelotons. Les endomycorhizes constituent l’association symbiotique dominante dans ce type de forêt. Des galles à nématodes (Meloidogyne sp) fréquentes sur les racines des semis de Dicorynia guianensis et de Peltogyne venosa jouent probablement un rôle dans la régénération de ces espèces. forêt tropicale humide / Caesalpinioideae / morphologie racinaire symbiose / Guyane française
in the experimental plots of Paracou (Bariteau INTRODUCTION - and Geoffroy, 1989). The soils here are devel- oped on migmatite and shales (Boulet and Brunet, primary tropical rain forest of French The 1983); Guyana has been extensively studied for along the Anse de Sinnamary road sandy on a - its structure and regeneration dynamics detritic coastal flat. (Riera et al, 1990; Sabatier and Prevost, Table I lists the 21 tree species observed. 1990). Most previous studies have con- They were chosen from 13 families (among the 68 cerned the part of the stands above ground, in French Guyana), which include 61% of the and little attention has been paid to the inter- species, and for their economic importance (more actions between trees within the soil. This than half provide valuable timber). Some species are economically interesting or putatively ecto- should be an important part of any modelling mycorrhizal, according to their taxonomic posi- of the spatial distribution of species and their tion (the sub-family Caesalpinioideae in the Legu- regeneration patterns. More generally, very minosae) and to other studies in Africa (eg, little is known about the rooting habits and Alexander and Högberg, 1986; Fassi and Moser, symbiotic status of trees in the neotropics 1991).The Caesalpinioideae sub-family is the (Souza Moreira et al, 1992). dominant group of trees in the Guyanas, in diver- sity, density and biomass (Puig et al, 1990). The of different types of root The occurrence species Dicorynia guianensis, Peltogyne venosa, and parasitism seems to be par- symbioses Sclerolobium melinonii, Eperua falcata and E ticularly relevant in this respect, because grandiflora, Hymenea courbaril were thus more symbionts determine different modes of extensively sampled at the seedling stage (> 100 nutrient mobilization and uptake, and par- individuals). asites can alter the efficiency of roots. More- Sampling was performed all year long, on over, the spatial distribution and dispersive either roots of mature trees (roots were traced mechanisms of microbial symbionts or par- from the base of the tree) or seedlings at different asites might contribute to the complexity of distances from the corresponding mother trees. those mechanisms in trees. Fine roots were carefully washed and observed fresh under the stereomicroscope. Putative ecto- This paper presents some preliminary mycorrhizas were examined following the methods descriptive results showing the wide diver- described by Ingleby et al (1990). Hand-cut sec- sity of root morphology and their symbiotic tions and crushed roots were observed unstained or parasitic status in this type of rain for- under the light microscope (500 x). Part of the est. We also discuss some unusual struc- fine roots were cleaned and stained according to tures. the technique described by Phillips and Hayman (1970) for the observation of endomycorrhizas with the light microscope (120 x - 500 x). MATERIALS AND METHODS The occasional presence of sporocarps of putative ectomycorrhizal higher basidiomycetes noted. was The observations were carried out in 3 sites that are relatively close to each other (ca 20 km apart, centred around Sinnamary, 5° 20’ N, 52° 50’ W, RESULTS 110 km west of Cayenne on the Atlantic coast). These sites are representative of the wide diver- sity of soil conditions and forest structure pre- general architecture of the fine roots The vailing in the region and are found: varies greatly in both diameter and branch- Saint Elie track in the ECEREX along the - ing, from 0.3 mm and a very dense mat of (Sarrailh, 1984). The soils here research zone fine roots for D guianensis and P venosa developed on the Bonidoro shales (Boulet, are (fig 1),to 1 mm and a poorly branched sys- 1990);
excelsa, Qualea rosea, P venosa (fig 1). tem for E falcata (fig 2) and E grandiflora. Typical bacterial nodules of the Rhizobium The length distribution of fine roots is also or Bradyrhizobium type, containing bacte- variable, from very short roots (0.5-5 mm, fig ria and infection threads, found on 3 2) on long axes (10 cm and over with no were Caesalpinioideae (D guianensis, R specio- long laterals) for the 2 Eperua sp, to the sum, fig 3; S melinonii, (fig 4) and the 2 evenly branched systems of Sterculia
referred to below as ’polymorphous short Papilionoideae examined (Andira coriacea roots’. A dense accumulation of such roots and Bocoa prouacensis). These nodules belonging to Eperua sp was found in the were either spheroid (S melinonii, fig 4) or litter around the base of neighbouring trees. lobed (other species), with colours ranging This suggests that these roots are particu- from off-white to brown. However, no nitro- larly well adapted to this special ecological gen fixation test was performed and we have niche with high water and nutrient fluxes no proof that these nodules are functional due to stem flow, litter accumulation and and effective. mineralization. Typical vesicular-arbuscular endomycor- Another unusual feature found rhizas with entry points, intercellular was on Caesalpiniaceae (D guianensis, R specio- 3 mycelium and arbuscules, and/or vesicles sum, and Vamericana). Small (0.5-1 mm in (fig 5) were found in the fine long roots of diameter), jet-black, smooth and glossy, all the species examined. Another endo- spherical or slightly elongated nodules were mycorrhiza-like structure, with intracellular directly attached to the long roots (fig 9). hyphal coils instead of arbuscules, with or The presence of a stele connected to the without vesicles, was found on D guianensis main vascular system of the long roots indi- (fig 6), P venosa, Platonia insignis, Sym- cates that nodules are transformed short phonia globulifera, Ocotea rubra, S excelsa, roots, like the previously described ’poly- R speciosum, and Brosimum rubescens. morphous short roots’ (fig 10). They differ in Many of the Caesalpinioideae tree their dense and smooth black surface and species examined (eg, D guianensis, P by the fact that their parenchyma contains venosa and E falcata) had brown, swollen, no hyphal coils or vesicles. short roots, which were more or less The fine roots of D guianensis, P venosa branched, and very similar to smooth ecto- and occasionally other species were some- mycorrhizas at the first glance (fig 2). How- times covered by a loose white or cream- ever, sections revealed that none of these coloured fungal mantle, often associated short roots (out of several hundred trials) with hyphal strands (fig 11).However, the were ectomycorrhizas. In contrast, they dis- absence of a Hartig net indicates that these played a dark outer layer of cells with thick- structures are only non-symbiotic peritrophic ened walls and a voluminous cortical associations. As these associations are usu- parenchyma with isodiametric, thin-walled ally observed on the surface of dead leaves, cells (fig 7). When stained with the Philips or are found growing through dead leaves in and Hayman (1970) technique, the litter occupied by the white mycelia of parenchyma contained conspicuous decomposers, it is probable that they are endomycorrhiza-like structures with vesi- caused by these saprophytic fungi (although cles and coils. The formation of the thick the root tissues are clearly alive). outer layer occurs very rapidly after the apparition of the short root, close to the The seedlings of 2 Caesalpinioideae elongating tip of the long root. The most species (P venosa and more particulary D spectacular diversity was found on Voua- guianensis) always had galls (fig 12) due capoua americana and on the 2 Eperua to an endoparasitic nematode identified as species (E falcata and E grandiflora) with Meloidogyne sp (fig 13). These parasitic shapes ranging from spheres to cones, structures mostly occupied the superficial roots growing in the litter from the collar of clubs and beaded short roots (fig 2). In the plants, which were severely affected by these species, the outer layer was also thicker than in D guianensis and P venosa the infection (reduced number and length of roots). (fig 8). This types of structure will be
these fluxes are concentrated, for instance, Table II presents the distribution of the different structures described in the Legu- around the base of other trees. minosae (all other observed species had No ectomycorrhiza were found, even on only vesicular-arbuscular endomycorrhizas). Caesalpinioideae, a tree group proposed as potentially ectomycorrhizal in the litera- Africa, genera such as Gilbertio- ture. In DISCUSSION AND CONCLUSIONS dendron, Afzelia and Dalbergia have often to be been reported ectomycorrhizal (eg, Khasa et al, 1990; Fassi, 1991). In South The different tree species in the tropical rain America (Venezuela), Moyersoen (1991) forest of the French Guyana obviously dis- found typical ectomycorrhizas only on Ald- play a wide range of strategies in exploring ina kunhardtiana (Papilionoideae, the soil and using its water and mineral Swartieae), and none on Eperua sp, which resources. Some, for example Dicorynia is consistent with results. Thus, ecto- guianensis, form dense mats of thin and our mycorrhizal symbioses seem to be less fre- highly branched roots, intercepting fluxes quent in Venezuela and the Guyanas than in of low intensity on large areas, whereas other tropical rain forests, in spite of the fact others, such as Eperua spp, send long but that the soil fertility is particulary low in these poorly branched roots to microsites where
of Allen and Allen (1981) and Faria et al and thatectotrophy is commonly regions, (1989). Souza Moreira et al (1992) recently adaptation to low fertility proposed as an published the first report of nodulation in conditions (Janos, 1983). Observations of the genus Dicorynia. Our results confirm sporocarps of putative ectomycorrhizal fungi in Amazonia seem to be in contradiction this finding and the presence of nodules on with this conclusion (Singer and Araujo, D guianensis. 1979, 1986). However, these authors did The systematic occurrence of nematode not put forward any direct proof that ecto- galls due to Meloidogyne sp on young mycorrhizas were actually present on the seedlings of D guianensis and P venosa roots. It is also important to note that in our suggests that this parasite probably plays studies in French Guyana, no local factor a major role in the seedling mortality, and is responsible for the rarity of ectomycor- thus in the regeneration dynamics of these rhizas. Introduced pines and eucalyps in species. Indeed, D guianensis is one of the nearby plantations were massively ecto- species with the highest mortality rate in the mycorrhizal. Our results are thus consistent experimental plots in Paracou (Schmitt and with the remark by Fassi and Moser (1991), Bariteau, 1990). according to which plant formations with dominant ectomycorrhizal species do not appear to constitute climatic climaxes in the ACKNOWLEDGMENTS neotropics. All the sampled roots had arbuscular The authors wish to thank the CIRAD-Forêt in endomycorrhizas, which are thus by far the Kourou for making its experimental sites avail- most dominant symbiotic association in this able for this work, the staff of the INRA Forest type of rain forest. Some tree species also research Station in Kourou for initiating MB to had endomycorrhizas with intracellular forest research, P Planquette for his help in pho- tography, J Anaïs and P Quénéhervé for identi- hyphal coils. In Eperua sp it is not known if fying the nematodes, and A Patient and E this type of symbiosis is host-specific or not, Louisanna for their valuable technical assistance. but it is clearly associated with structures atypical for Angiosperms in some Caesal- pinioideae species. What we called ’poly- REFERENCES morphous short roots’ are somewhat similar (mostly those with a nodular shape) to the roots of Podocarpaeceae and Araucari- Alexander IJ, Högberg P (1986) Ectomycorrhizas of trop- ical angiospermous trees. New Phytol 102, 541-549 aceae (Gymnosperms) described by Baylis ON, Allen EK (1981) The Leguminosae. Univer- Allen et al (1963) and Baylis (1972). They have a sity of Wisconsin Press/Macmillan Publishing Com- thickened outer cortex and endomycorrhizal pany, Madison, WI infection (coil-type in our case) in the swollen Bariteau M, Geoffroy J (1989) Sylviculture et régénéra- inner cortex. tion naturelle en forêt guyanaise. Rev For Fr 16, 309-323 The black glossy nodules, which were Baylis GTS (1972) Fungi, phosphorus and the evolu- found on R speciosum, V americana only tion of root systems. Search 3, 257-258 and D guianensis, are unusual and their ori- Baylis GTS, McNabb RFR, Morrisson TM (1963) The gin and function remain unknown. mycorrhizal nodules of Podcarps. Trans Br Mycol Soc 46, 378-384 Bacterial nodules of Rhizobium or Boulet R (1990) Organisation des couvertures Bradyrhizobium occur on several genera pédologiques des bassins versants ECEREX. of Cesalpinioideae belonging to the Hypothèses sur leur dynamique. In: Mise en valeur Cassieae and Caesalpinieae, but not on de l’Ecosystème Forestier Guyanais (INRA-CTFT Detarieae. This is consistent with the data Paris, ed), 15-45
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