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REVISTA TRIPLOV
de Artes, Religiões e Ciências
Nova Série | 2010 | Número 11
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FRANCISCO CARRAPIÇO(1),
GENEROSA TEIXEIRA(2) &
M. ADÉLIA DINIZ(3)
Azolla as biofertiliser in Africa.
A challenge for the future
In: Revista de Ciências Agrárias, 23 (3-4):
120-138, 2000 |
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DIREÇÃO |
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Maria Estela Guedes |
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Índice de Autores |
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Série Anterior |
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Nova
Série | Página Principal |
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SÍTIOS ALIADOS |
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TriploII - Blog do TriploV |
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TriploV |
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Agulha Hispânica |
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Arditura |
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Bule,
O |
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Contrário do Tempo, O |
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Domador de Sonhos |
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Jornal de Poesia |
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ABSTRACT
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Azolla
is a small-leaf floating fern, which contains an endosymbiotic community
living in the dorsal lobe cavity of the leaves. The presence in this
cavity of a nitrogen-fixing filamentous cyanobacteria -Anabaena
azollae - turns this symbiotic association into the only fern-cyanobacteria
association that presents agricultural interest by the nitrogen input
that this plant could introduce in the fields. In this work we review
the applications and future challenges of the use of Azolla as
biofertiliser in Africa. In this continent, agriculture is the most
important sector of economy and it employs 75 % of the labour force. The
dwelling of fossil fuel reserves and the increasing costs of commercial
nitrogen fertilisers implicate finding other alternatives, such as the
use of biofertilisers, like the Azolla-Anabaena symbiotic system.
This plant is quite spread in the African continent. The taxonomy of
Azolla is reviewed and the results of the cooperation project
between Portugal and Guinea-Bissau for the use of this aquatic fern as
green manure on rice cultivation are analysed. Finally, we focus the
importance of the use of nitrogen-fixing organisms, like Azolla,
which could help effectively developing countries to improve a more
sustainable agriculture, without the risk of problems associated with
the adverse effects of chemical fertilisers on long term soil fertility,
soil productivity and environmental issues. |
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RESUMO |
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Azolla
é um pteridófito aquático que contém uma comunidade endossimbiótica
vivendo na cavidade do lobo dorsal das folhas. A presença nesta cavidade
duma cianobactéria filamentosa fixadora do azoto atmosférico -Anabaena
azollae -confere a esta associação simbiótica grande interesse como
biofertilizante em agricultura pela incorporação de azoto nos terrenos
em que é utilizada. Neste trabalho são revistas as aplicações e futuros
desafios do uso de Azolla como fertilizante natural em África.
Neste continente, a agricultura é o sector mais importante da economia e
emprega 75% da força laboral. O desgaste das reservas de combustíveis
fósseis e o aumento do custo dos fertilizantes azotados de origem
química implica encontrarmos alternativas, como o uso de
biofertilizantes, nomeadamente o sistema simbiótico Azolla-Anabaena,
o qual apresenta uma larga distribuição no continente Africano. A
sistemática do pteridófito do género Azolla é revista e
analisados os resultados do projecto de cooperação entre Portugal e a
Guiné-Bissau para o uso desta planta como biofertilizante na cultura do
arroz. Por fim, é realçado o papel dos organismos fixadores do azoto
atmosférico, como é o caso de Azolla, no âmbito do
desenvolvimento duma agricultura sustentável, sem o risco dos problemas
associados aos efeitos adversos dos fertilizantes químicos na
fertilidade e produtividade do solo a longo prazo, bem como em questões
do foro ambiental. |
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INTRODUCTION |
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Agriculture is
the most important sector of economy in Africa and it employs 75 % of
the labour force of the continent. Two main agricultural systems domain
this activity: a major traditional subsistence sector and a minor modern
economical one. Sometimes those types can coexist in several countries.
The traditional sector employs the majority of African’s rural
population and is characterised by small and fragmented farms, little
use of technology or fertiliser, high reliance on human labour, low
yields, infrequent surpluses, and an emphasis on staple crops such as
corn, rice, sweet potatoes, peanuts and other high-starch foods (Grolier
Multimedia Encyclopedia, 1999). It is in this type of traditional sector
that the problem of chemical fertilisers can be a restraining factor for
agricultural development and crops’ production increase. The peasants
subsistence type of agriculture prevents the existence of the necessary
funds, namely to buy those chemicals and fuel and frequently contribute
to the shortage of food (Dias and Carrapiço, 1991).
In these
conditions, which are associated with the dwelling of fossil fuel
reserves and the increasing costs of commercial nitrogen fertilisers it
is necessary to find others economical and technical options, that may
contribute to solve or help this problem. One of these alternatives is
the use of biofertilisers, especially associated with the use of plants
symbiotic systems combined with the nitrogen fixation. It is the case of
the aquatic fern of the genus Azolla, that presents a symbiotic
association with a cyanobacterium -Anabaena azollae -and is quite
spread in this continent.
The aquatic
fern of the genus Azolla is a small-leaf floating plant, which
contains an endosymbiotic community living in the dorsal lobe cavity of
the pteridophyte leaf (Figure 1). This community is composed of two type
of prokaryotic organisms: one species of a nitrogen-fixing filamentous
cyanobacteria -Anabaena azollae Strasb. (described by
Strasburger in 1873) and a variety of bacteria that some identified as Arthrobacter sp. and associate with others showing the presence
of nitrogenase (Costa et al., 1994). In this association, it is
assumed that an exchange of metabolites, namely fixed nitrogen
compounds, occurs from the cyanobiont to the host (Carrapiço and
Tavares, 1989a, b). |
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Fig. 1
– Transversal section of an Azolla leaf (Adapted from Sevillano et al.,
1984). (th – transfer hair; cyanob – cyanobacterium; bact – bacteria;
het – heterocyst; vc – vegetative cell. |
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Filaments of Anabaena azollae are localised in a cavity of the
dorsal lobe of the fern's leaves, where special conditions stimulate
high heterocyst frequency and a vegetative cell differentiation during
leaf development (Carrapiço and Tavares, 1989a). The existence of the
two symbionts inside the Azolla leaf cavity and its relationship
with the fern, namely the metabolites flow between the host and the
symbionts, can be seen as an unique micro-ecosystem with own well
established caracteristics. This association is maintained during all
the life cycle of the pteridophyte. The Anabaena apical colony is
associated with shoot apex lacks heterocysts and, therefore, is unable
to fix nitrogen. In mature leaves, the Anabaena filaments cease
to grow and differentiate heterocysts, which are the site of N2
fixation. Besides the cyanobacteria, a population of bacteria undergoes
a pattern of infection identical to Anabaena and probably is the
third partner of this symbiosis (Wallace and Gates, 1986; Carrapiço and
Tavares, 1989a; Carrapiço, 1991; Forni et al. 1989). The
prokaryotic colony - cyanobacteria and bacteria - are also present in
the sexual structures (sporocarps) of the fern (Carrapiço, 1991). The
cyanobacterium is transferred from the sporophyte to the next generation
via the megasporocarp. A cyanobacterium colony resides between the
megasporocarp wall and the megasporagium one and inoculates the newly
emerging sporophyte plant. A colony of the symbiotic cyanobacteria is
formed near the shoot apex and thus enables symbiosis to be established
within the developing leaf cavities (Watanabe and Van Hove, 1996). The
presence of bacteria in the megasporocarps in association with the
cyanobacteria also suggest a behaviour pattern similar to the
cyanobionts (Carrapiço, 1991). The presence of Anabaena
throughout the life cycle of the fern favours the obligatory nature of
the symbiosis and suggest a parallel phylogenetic evolution of both
partners (Watanabe and Van Hove, 1996).
This symbiotic
association is the only fern-cyanobacteria association that presents
agricultural interest by the nitrogen input that this plant can
introduce in the fields and continents (Moore, 1969; Kannayan, 1986; Van
Hove and Diara, 1987; Shi and Hall, 1988; Wagner, 1997). Historically, Azolla has been used as green manure for wetland rice in northern
Vietnam and central to southern China for centuries (Nierzwicki-Bauer,
1990; Watanabe and Van Hove, 1996). Only after the oil crisis in the
1970s the research and use of this type of association has been
intensified because of the price increase of the chemical fertilisers
and its negative impact in agriculture, namely in the countries of the
third world (Dias and Carrapiço, 1991). Meanwhile, since the
introduction of a market economy system in those countries, the increase
on the supply of chemical fertilisers has reduced the traditional use of Azolla as green manure for rice cultivation, namely in China and
Vietnam (Watanabe and Van Hove, 1996).
A problem
associated with the use of chemical fertilisers is the adverse effects
on long term soil fertility, soil productivity and environmental safety
(Kannaiyan, 1997). A new strategy for increasing rice production,
particular in developing countries should be taken in account for
programmes to utilise the biological fertilisers which will not only
increase the rice productivity, but also improve the long term soil
fertility (Kanniyan, 1997). In these conditions the inoculation of free
living cyanobacteria in the fields is one of the options. The
inoculation increased rice grain yields by an average of 350 kg/ha. When
successful, the inoculation is low-cost technology, but its effect is
erratic and unpredictable (Watanabe and Liu, 1992). More recently, a new
method of using cyanobacteria on ammonia production for rice crop was
developed by Kannaiyan’s team (Tamil Nadu Agricultural University,
India), by means of immobilisation of nitrogen fixing Anabaena
azollae and Anabaena variabilis in solid matrix of
polyurethane and polyvinyl foam that excretes ammonia continuously with
very positive results on rice culture ( Kannaiyan et al., 1996).
For those reasons, Azolla use is
yet a real option as a green fertiliser, especially in the developing
countries that presents a low cost of labour force. In industrial
countries, exploited for a more developing environmentally-friendly
agricultural system (Watanabe and Van Hove, 1996), namely in particular
segments of this important economical activity. Others uses of the
Azolla-Anabaena system are now in progress, like the control of aquatic
weeds, the use as animal feed and the concentration of mineral elements,
namely its use as biofilter in industrial and domestical effluents
(Watanabe and Van Hove, 1996; Costa et al, 1994, 1999; Lejeune et al.,
1999; Tel-Or, personal communication). |
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Azolla
TAXONOMY |
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Despite its
long history of agricultural use, Azolla taxonomy is
controversial and our knowledge on the subject is still limited, which
probably means that a new global revision of the taxon is required. In
this paper we have used, what we think it is a coherent taxonomic
classification (Saunders and Fowler, 1993), associated with the new
research and progress data on the field, namely the studies using
germplasm collections (Watanabe and Van Hove, 1996).
The word
Azolla has a Greek origin. It results from the agglutination of two
words, azo and ollyo, which means killed by drought
(Moore, 1969; Ashton and Walmsley, 1976). Indeed, it has a true
predicative value indicating that these plants are very sensitive to
water deficit.
Azolla
is a genus of aquatic ferns, mainly found in tropical and warm temperate
regions. It can be observed in quiet waters, ponds, ditches, canals and
paddy fields. Such areas may be seasonally covered by a mat of Azolla
associated with other free-floating plants such as species of Lemna,
Pistia, Salvinia, Trapa, Wolffia and mud rooting
species of Cerathophyllum, Ludwigia, Neptunia and
Polygonum.
In Azolla the free-floating aquatic habit is exhibited by a
complete adaptation of the whole sporophyte, including its reproductive
structures. There is an extreme small plants present a branched stem
that floats horizontally on the water surface, with single or
fasciculate pendulous roots and alternately imbricate millimetric
2-lobed leaves. They are monoecious plants that possess dimorphic
sporocarps, whose micro and megaspores develop in a leptosporangiate
way.
These ferns
present a great disparity in their holomorphologies, being the most
phenotipically influenced. This is another reason for the problems of
Azolla systematics which are very ancient and evident in the
different classifications proposed through the years. The genus has been
established in 1783 by Lamarck and has been placed in the Salviniaceae
along with the genus Salvinia (Svenson, 1944). Since then, it has
been placed in the Marsileaceae, by R. Brown, in 1810 (Svenson, 1944)
and again in the Salviniaceae by eminent systemats such as Sadebeck, in
1902, Lawrence, in 1951 and Benson, in 1957 (Ashton and Walmsley, 1984).
The first to consider Azolla in the monotypic family Azollaceae
was Wettstein, in 1903 (Bonnet, 1957) but only with Reed, in 1954, such
proposal has begun to be accepted and followed (Ashton and Walmsley,
1984).
The living
species of Azolla are divided into two subgenera: Azolla
and Rhizosperma (Mey.) Strasb. (Ashton and Walmsley, 1984). Some
other authors use the taxonomic level section instead of subgenera
(Moore, 1969; Saunders and Fowler, 1992). In both cases this separation
is based in the floats number and in the massulae trichomes. All species
with three floats in the megasporocarp and arrow-shaped glochidia belong
to the subgenus Azolla. The specific classification, usually
accepted, recognises five species integrated in the subgenus Azolla
which are: A. caroliniana Willd., A. filiculoides Lam.,
A. mexicana Presl., A. microphylla Kaulf and A. rubra
R. Br. All these taxa have a notorious morphologic resemblance.
Subgenus
Rhizosperma includes the species with nine floats in the
megasporocarp and absent glochidia or with internal massulae trichomes.
It comprises two species: A. subsp. africana (Desv.) R. M.
K. Saunders and K. Fowler, A. pinnata subsp. asiatica (Desv.)
R. M. K. Saunders & K. Fowler and A. pinnata subsp. pinnata.
Some authors considered a third species in this subgenus, A.
imbricata Nak. (Nakai, 1925), but it might be a synonymous of A.
pinnata (Moore, 1969; Ashton and Walmsley, 1984) and, for others, it
might be a synonymous of A. pinnata subsp. asiatica
(Saunders and Fowler, 1992).
In 1993, using
cladistic, Saunders and Fowler proposed another supraspecific
classification. They consider that the differences that divide A.
nilotica from all the other Azolla species are enough to
establish a new subgenus. That subgenus, denominated Tetrasporocarpia
(sporocarps grouped in four), includes only A. nilotica. Subgenus
Azolla, is then divided into two sections, Azolla and
Rhizosperma, the last one conglobating just A. pinnata, this
with 3 subspecies.
Table 1 presents the last classification proposed by Saunders and Fowler
(1993) and followed by Diniz and Carrapiço (1999). |
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Table 1 -Synopsis
of the classification of Azolla, proposed by Saunders and Fowler,
1993. |
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Azolla
LIFE CYCLE |
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Azolla
usually reproduces vegetatively by fragmentation of the abscision layer,
at the base of each branch. Sexual reproduction is not very common and
seems to be influenced by environmental factors, namely several stress
conditions.
Sporocarps are visible in
pairs (in A. nilotica in four) in the place of the first leaf
lower lobe, of a sporophyte branch. They occur in pairs of either
microsporocarps, megasporocarps or one of each.
Mature microsporocarps are
globular and enclose numerous stalked microsporangia, each one with 32
or 64 microspores, divided into 3 to 10 complex structures, called
massulae. In subgenus Azolla each massulae shows characteristic
arrowlike projections, the glochidia. In subgenus Rhizosperma
glochidia do not exist or there are filiform internal trichomes. At
maturation microsporangia releases massulae which may be dragged
underwater. Microspores germinate inside the massulae and flagellated
antherozoids move through the gelatinised massulae to fertilise oospores
within the archegonia (Braun-Howland and Nierzwicki-Bauer, 1990).
Megasporocarps are much smaller than microsporocarps and have two
distinct parts. The upper is filled by the so called floats, 3 in
subgenus Azolla and 9 in subgenus Rhizosperma. In its
lower portion, each megasporocarp contains only one megaspore. This
germinates under water and a prothallus is formed producing
chegonia. Antherozoids will
fertilise the only egg cell existing in each archegonium and the zygote
formation occurs within the megaspore apparatus below the water surface
and will give rise to a new sporophyte (Braun-Howland and Nierzwicki-Bauer,
1990).
Azolla
is worldwide distributed and man influence is the main responsible for
the dispersion of these plants through the globe. During the last two
decades, the use of Azolla in rice cultivation has resulted in
the introduction of new species to areas where they are not indigenous.
New World species are now widespread in Asia and Africa, and often they
had eliminated the natural populations of Azolla pinnata
(Watanabe and Van Hove, 1996). Azolla filiculoides was introduced
in the late 1970s into northern China where it is now a well adapted
species (Watanabe and Van Hove, 1996). Before that action some species
were endemic in the following areas: A. caroliniana, in
eastern United States (Svenson, 1944); A. filiculoides, from
Alaska to the southern South America (Svenson, 1944); A. mexicana,
from west coast of the United States, into Mexico and Central America;
A. microphylla, from tropical and sub-tropical zones of the
American continent (Moore, 1969); A. nilotica, in Nile basin
(Moore, 1969); A. pinnata, in tropical Africa, southern Africa
and Madagascar (Moore, 1969).
A new revision of Azolla
world distribution, as previously reported by Moore (1969) and Lumpkin
and Plucknett (1980), now needs a global revision (Watanabe and Van
Hove, 1996).
At the
present state of our knowledge, A. caroliniana has been
observed in Central and South America, in the east of the Andes and
reached western Europe (Lumpkin, 1987) and has also been recently
introduced in Egypt in the Nile Delta (Yanni et al., 1994). A.
filiculoides is still in western America and it has been introduced
in South Africa as well as in western Europe, China, Japan and southern
Australia and New Zealand (Lumpkin and Plucknett, 1980; Lumpkin, 1987). A. mexicana has tolerated little expansion from his indigenous
area and A. microphylla has been confined to the Galapos Islands
(Lumpkin, 1987). A. nilotica is still an African species and can
be found in eastern Africa,
from Egypt to South Africa. A. pinnata, besides tropical Africa,
is also distributed in Australasia and Southeast Asia. According to
Saunders and Fowler (1992), geographical zones: A. pinnata subsp.
africana, A. pinnata subsp. pinnata and A.
pinnata subsp. asiatica.
In an Africa map (Figure 2)
we present, in a more detailed way, the distribution of the Azolla
species in that continent, including the results of some Portuguese
prospecting expeditions, as well as some herbarium material kept in
Centro de Botânica, Lisbon (LISC) and from references mentioned bellow.
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Figure 2
– Distribution of the Azolla species in Africa. |
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To achieve
botanical descriptions of Azolla African species we consulted the
following works: Schelpe (1970); Schelpe and Diniz (1979); Schelpe and
Nicola (1986); Tardieu-Blot (1964 a, b); Franco (1971); Almeida (1986);
Yanni et al. (1994). Some new data are also introduced from our
own research.
AZOLLA Lam. Azolla Lam., Encycl. Méth., Bot. 1: 343 (1783). Genus
description as for the family.
According to species
distribution, the true African Azolla taxa are A. nilotica
and A. pinnata subsp. africana. As mentioned before, A.
filiculoides and A. caroliana has been introduced into this
continent.
Key to the African species:
1 - Elongate plants up to 5 cm long; megasporangia with 3 floats;
massulae with external arrowlike glochidia at the apex
------------------- 3
-Deltoid plants or, if
elongate, with more than 6 cm long; megasporangia with 9 floats;
massulae with internal filiform trichomes and absent glochidia ---------------2 |
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AZOLLACEAE
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Free-floating aquatic plants
with pinnate branched rhizomes and single or fasciculate roots, node
inserted. Alternately imbricate 2-lobed leaves, with an aerial
chlorophylous upper lobe and a no chlorophylous submerged lower lobe.
Sporocarps with thin walls, developed at the branch first leaf;
microsporocarp with numerous microsporangia each one with its own
pseudo-cellular structure, called massulae, where microspores germinate;
megasporocarp with a single megasporangium involving a single megaspore
with 3 or 9 floats in distal apex, similar to massulae. Female
gametophyte submerged. It is a single-genus family.
2 - Deltoid plants up to 2
cm long; single roots or up to 3 roots per node; sporocarps in 2;
internal filiform trichomes
--------------------------- 1. A. pinnata
-Elongate plants with more
than 6 cm; fasciculate roots; sporocarps in 4; internal filiform
trichomes and absent glochidia
---------------------------------2. A. nilotica
3- Massulae
with external arrowlike glochidia at the apex, rarely septate
---------------------------------------------------------------------------------3. A. filiculoides
-Massulae with external
arrowlike glochidia at the apex, often septate
---------------------------------------------------------------------------------
4. A. caroliniana
1. Azolla pinnata
R. Br. Prodr. Pl. Nov. Holl.: 167 (1810).
Type from Australia. Subsp. africana (Desv.) R. M. K. Saunders and K. Fowler in Bot. J. Linn.
Soc. 109: 351 (1992). Type from West Africa. (Figures 3 and 4).
Azolla africana Desv. in Mém. Soc. Linn. Paris 6, 2: 178
(1827). Type from West Africa. Azolla guineensis Schumach. in
Kongel. Dansk. Vid. Selsk. 4: 236 (1829). Type from West Africa.
Azolla pinnata var. japonica (Franch and Sav.) French and
Sav. in Enum. Pl. Jap. 2, 2: 612 (1878) pro parte. Type
from Japan. Azolla pinnata var. africana (Desv.) Baker in
J. Bot. 25: 101 (1886). Type from West Africa. Azolla japonica
sensu Nak. in Bot. Mag. (Tokyo) 39: 184 (1925) non
Franch. and Sav. pro parte.
Deltoid plants with
horizontal rhizome, minutely papillate, up to 2 cm long and 0.2 mm in
diameter; single roots or up to 3, hairy, up to 3.5 cm long. Leaves
2-lobed with the upper lobe imbricate, up to 1.1 mm long, papillate,
with a chlorophylous central portion, hyaline border with 3-4 cells
layers, with acute apex; lower lobe similar in size in the stem ventral
face in the place of the lower lobe of the first leave, covered by the
upper lobe, spherical, about 1.7 mm in diameter, with numerous stalked
microsporangia; microspores massulae with internal filiform trichomes;
ovoid megasporocarp up to 0.8 mm long with a prominent dark brown apex
with a single granular megaspore surmounted by 9 floats; granular perine
surface; tuberculate excrescences.
Distribution: Occurs in tropical Africa, from Gambia to Senegal, to east
up to Kenya and Tanzania and southwards up to Natal, Transvaal and
Botswana. Also in Madagascar. In ponds and quiet rivers. It has been
introduced in Egypt (drainage canals of the Nile Delta) between 1977-80
for agricultural purposes |
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Figure 3
– Azolla pinnata subsp. africana (Desv.)
R. M. K. Saunders
and K. Fowler. |
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Figure 4 – Scanning electron micrograph of a megasporocarp of Azolla
pinnata subsp. africana, with prominent apex and a granular megaspore
perine surmounted by the floats (c-cap; f- float; p- perine). |
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2. A.
nilotica Decne.
ex Mett. in Kotschy and Peyr., Pl. Tinn.: 54, t. 25 (1867). Type
from Central Africa.
Elongate plants with
horizontal rhizome or slightly ascending, pubescent, up to 35 cm long
and 2 mm in diameter; numerous roots in 5 or more fascicles, hairy, up
to 15 cm long. Leaves 2-lobed, usually with plane upper lobes and not
imbricate, up to 2 mm long, broadly elliptic with a papillate
chlorophylous central portion surrounded by broad hyaline margins, with
acute or seldom obtuse apex; the lower lobe similar but smaller than the
upper lobe. Microsporocarps and megasporocarps grouped in 4, either all
megasporocarps, either all microsporocarps or mixed together, being
initially enveloped by an hyaline ovoid membrane; microsporocarp up to 1
mm in diameter, spherical, with numerous stalked microsporangia;
microspores massulae with internal filiform trichomes and eventually
absent glochidia; megasporocarps up to 0.3 mm long, with a dark apex,
with a single megaspore surmounted by 9 floats; slightly granular perine
surface; sparse and small spiniform excrescences, specially near the
distal pole.
Sudan to
Zimbabwe and Mozambique, up to the Zaire basin. In quiet waters and in
lazy rivers.
3. Azolla filiculoides
Lam., Encycl. Méth., Bot. 1: 343 (1783). Type from South America.
This species
differs from A. pinnata by presenting elongate plants, single
node roots, leaf lobes with obtuse apex, unicellular papillae, more than
two hyaline border cells, the glochidia have 0, 1 or 2 septa and the
megaspore surmounted by 3 floats. The perine surface has raised
hexagonal markings tied by the ends and microspore massulae with
external arrowlike glochidia.
Distribution: it
occurs in western North, Central and South America. It has been
introduced in South Africa (Free State and Cape Province) as well as in
temperate western Europe and tropical and temperate China, Japan,
Australia and New Zealand. In quiet waters, ponds and paddy fields. It
has been introduced in Egypt (drainage canals of the Nile Delta) between
1977-80 for agricultural purposes. In South Africa, the uncontrolled
growth of this fern is now a serious problem that in numerous water
bodies, especially in the Free State, form a thick mat that put in risk
the life in those ecosystems.
4. Azolla caroliniana
Willd., Sp. Pl. 5 (1): 541 (1810). Type from North America.
This
species is similar to Azolla filiculoides from which it differs
by presenting leaf lobes with acute apex, bicellular papillae,
microspore massulae with external arrowlike usually septate glochidia,
collar and megaspore perine surface densely covered with filosum.
Distribution: it
occurs in eastern North, Central and South America. It has been
introduced in temperate western Europe and in Egypt (drainage canals of
the Nile Delta) paddy fields. |
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Azolla
AS GREEN MANURE |
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The use of this fern as
biofertiliser in Africa has been mainly implemented by ADRAO
(Association pour le Développement de la Riziculture en Afrique de
l’Ouest) and FAO (Food and Agriculture Organisation) in several Western
African countries during the 1980s by a project co-ordinated by the
Catholic University of Louvain, Belgium, with the support of the ADRAO
field station located in St. Louis, Senegal (Diara et al., 1987;
Van Hove and Diara, 1987; Van Hove, 1989). The goal of this project was
to use the potential of Azolla as symbiotic nitrogen-fixing
system for rice cultivation and to develop new agricultural techniques
as well as testing several Azolla strains more adapted to local
conditions. Despite some constraints, the results of this project were
positive, not only for using Azolla as green manure, but also as
feed for animals and namely through the development of an integrate use
of Azolla with rice and animals farming, like fishes, pigs and
ducks (Van Hove, 1989; Lejeune et al., 1999).
Another region where
Azolla was used is Egypt. From 1977 to 1980, soil microbiologists of
the Agriculture Research Center, introduced three Azolla species:
A. pinnata, A. caroliniana and A. filiculoides for green
manuring of rice (Yanni et al., 1994). Two main procedures for
Azolla application were used. Either the fern was grown on flooded
field for 2-3 weeks and incorporated in the soil by ploughing often two
weeks before rice plantation, or a dual culture method was used in which Azolla was added during the week after rice transplanting and the
propagated Azolla incorporate in the soil after a temporary water
drainage. The environmental and climatic conditions in Egypt indicates
that the second method was the more adequate (Yanni et al.,
1994). The results were better when Azolla was incorporated in
soil rather when left floating during the all the rice growth season.
With this technique the researchers could save half of the effects of
nitrate and nitrite ions in water resources (Yanni et al., 1994).
The use of molybdate to the inoculated rice field can enhance
contribution of Azolla to rice performance. The Azolla
species used in these field assays was A. caroliniana collected
from water drainage canals.
Two main problems appear to
exist in the Azolla Egyptian project. The first one is the
foregoing results indicating that the high labour cost needed for
incorporation of Azolla is one of the most important constraints
in all the project. The second one was and it is associated with the
problem of Azolla escape during the field assays. This situation
has occurred, namely during the field trials in season 1985, when
Azolla escaped, undesignedly, to nearby drainage canals. Now,
Azolla is present on most stagnant water of drainage canals in Nile
Delta (Yanni et al., 1994). The uncontrolled growth of this fern
is now a serious problem that in some places form a thick mat that put
in risk the fish life present in those ecosystems and turn into a
mechanical obstruction of irrigation and drainage of the canals.
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Azolla
IN GUINEA-BISSAU |
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The Republic of Guinea-Bissau is a small West African country, located
on the Atlantic coast between Senegal and Guinea (Figure 5), with an
area of 36,125 Km2 and a population of 1 million inhabitants, with
850,000 persons working on agriculture. Most of the country is located
on the African mainland, but also includes numerous offshore islands,
most of them part of the Bijagós archipelago. The country is relatively
flat, with low-lying areas subject to flooding. It includes a coastal
plain, a somewhat higher interior plateau centered on Bafatá and a
higher plateau near the border with Guinea (Boé region), where the land
rises to 244 m. The coastal plain is made up of marshes and swamps and
rain forests. Vegetation in the interior of the country is composed by
is normally heavy from May to October and the dry season extends from
November to April (Diniz and Carrapiço, 1999).
The country’s coast is
heavily indented by river estuaries and inlets. One of the more
important rivers is the Geba that crosses the country from Northeast to
West. This river has its spring in Senegal and its mouth in the
Guinea-Bissau, together with the Corubal River, forming the Bissau
estuary. In the eastern regions of these rivers, namely in the Geba,
develops the aquatic fern Azolla pinnata subsp. africana.,
that forms, in the dry season, wide dark red carpets covering the
river’s water (Diniz and Carrapiço, 1999) (Figure 6). This fern presents
a vegetative cycle during several months of the year (August to March)
and a sporulation period between November and March (Carrapiço (coord.) et al., 1996). |
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Fig. 5
- Distribution of Azolla pinnata in the Republic of
Guinea-Bissau. |
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Fig. 6 – Geba river
(Guinea-Bissau) almost covered
by a thick mat of Azolla pinnata.
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Guinea-Bissau is mainly an agrarian country and its economy is strongly
influenced by the chemical fertilisers prices, which leads to a
limitation of the farmer’s buying power, since most of them practices a
livelihood agriculture. Rice culture is an important agrarian activity
in the country and rice is one of the main feeding sources of the
population (Dias and Carrapiço, 1991). This crop represents 65% of the
country’s cereal production and their consumption per capita is
estimated to be 167 kg/year. The difficulties they have to overcome to
get these chemical fertilisers, that the country has to import from
abroad, as well as the fragility of many of their soils, demands the
study and the adoption of new solutions that, with the help of the local
potentialities, would adapt to the crop’s techniques practised by the
guinean farmers and would enable to keep the necessary crop productivity
(Carrapiço (coord.) et al., 1996).
During six years (1989-1995)
a co-operation global project for the use of this aquatic fern as green
manure on rice yield and improve the use of this biological system (Carrapiço
(coord) et. al., 1996).
The Azolla and rice
experiments were developed in Contuboel (25 km from Bafatá) where the
agronomic field station from National Institute for Agrarian Research (INPA)
is located. Azolla pinnata was collected in the Geba river and
maintained in the field station in small basins using superphosphate and
furadan granules. The preparation of the field and the Azolla
compost was done by the method suggested by Kannaiyan (1986) and also
developed by ADRAO. The rice variety used during the years 1989, 1990,
1991 and 1995 was the BG 90-2 and the assays were done during the wet
season (Figure 7). Different amounts of Azolla compost (7000
Kg/ha or 14000 Kg/ha), used 1 week before rice transplanting, were
compared with urea fertiliser (87 kg N/ha). Some other experiments were
conducted using a mixture of Azolla compost and urea -Azolla
7000 Kg/ha + 43.5 kg N/ha (urea) and Azolla 14000 Kg/ha +
43.5 kg N/ha (urea). All assays were compared with a blank treatment (no
chemical nitrogen and no Azolla inoculation). The use of
Azolla as compost was an option to avoid the problems associated
with the uncontrolled growth of this fern and it was also chosen for its
easy way to be used by the farmers.
The experiences carried out
on rice fertilisation (Figure 8) during those four years, show that the
best results were obtained using only a chemical fertiliser, with an
average production of 3158 Kg/ha of this crop. The second best
production, 2946 Kg/ha, was obtained using a mixture of Azolla
and urea, in the proportions of Azolla 14000/ha
+ 43.5 KgN/ha (urea) but a
quite close result, 2835 Kg/ha was achieved using the mixture Azolla
7000 Kg/ha + 43.5 KgN/ha (urea). The production of the first mixture
represents a yield of 180 % (expressed in function of the control
assay), a value not far from the 193 % obtained with urea only.
Considering the mixture Azolla 7000 Kg/ha +
43.5 KgN/ha (urea) the
yield, 173% is quite similar to that mentioned above but represents a 50
% Azolla saving. |
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Fig. 7
– A rice field assay at the Contuboel Center,
during the year of 1995 |
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The global data was
submitted to a one-way ANOVA analysis. It shows that the several
fertilisers mixtures used were responsible for distinct rice yields (F=
6.09; p= 0.001) and the obtained results were consistent all along the
experimental years (data was not a significant parameter F= 1.28; p=
0.176).
In spite of the best results
being obtained with urea, as it was expected, the prices of this
chemical fertiliser and other productive factors strongly disagree with
the exclusive use of it on rice fertilisation and suggest the use of
Azolla compost combined with chemical fertiliser (Azolla 7000
Kg/ha + 43.5 kg N/ha) for increasing rice yield at low cost in this
country. Considering the four years assays, we can say that the use of
7000Kg/ha Azolla compost has an effect equivalent to 43.5 Kg/ha
of nitrogen, that is
94.5 Kg/ha of urea, with a
production increase for the rice variety studied (BG-90-2) of 73 %
compared with the control, in which the wet season is concerned. It
means a 50% saving of the chemical fertiliser without significant loss
on the rice yield production. |
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Fig. 8
– Average production (kg/ha) and yield (compared to control assay) of BG
90-2 rice variety relative to different fertilizers mixtures.
Experimental data from GuineaBissau’s wet season during four years
(1989, 1990, 1991 and 1995) was considered. |
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Despite the
positive results obtained in the last recent years by nitrogen-fixing
organisms in the field crop productivity, namely with the Azolla-Anabaena
symbiosis, the gradual introduction of a more aggressive market economy
in many African countries, the breakdown of the traditional agriculture
way of living in some of them, namely with the population migration to
the cities, associated with the pressure of the international industry,
could lead some governments to increase the use of chemical fertilisers
as the only solution for the problems of crop production and food
shortage. We believe that it is help developing countries to improve a
more sustainable agriculture, without the risk of problems associated
with the adverse effects of chemical fertilisers on long term soil
fertility, soil productivity and environmental issues. This strategy,
including the creation of an Azolla research and development
centre based in Africa suggested by Lejeune et al., (1999) can
also help to fix the population and integrate it in new developing
regional programmes that not only increase the crop productivity, but
can also improve new applications of this fern in others economical
sectors of activity. |
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ACKNOWLEDGEMENTS
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The authors are
grateful to the Calouste Gulbenkian Foundation (FCG) and the National
Board for Scientific and Technical Research (JNICT/FCT/MCT), Portugal,
for its financial support in the Azolla Project with
Guinea-Bissau and also to the President and the Azolla team of
Guinea-Bissau’s National Institute for Agrarian Research (INPA).
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(1) Francisco Carrapiço. Centro de
Biologia Ambiental, Departamento de Biologia Vegetal, Faculdade de
Ciências da Universidade de Lisboa, Edifício C2, Campo Grande, 1749-016
Lisboa, Portugal (E-mail:
F.Carrapico@fc.ul.pt);
(2) Generosa Teixeira Centro de
Biologia Ambiental, Faculdade de Farmácia da Universidade de Lisboa, Avª
das Forças Armadas, 1649-019 Lisboa, Portugal (E-mail:gteixeira@ff.ul.pt)
& (3) M. Adélia Diniz. Centro de
Botânica, Instituto de Investigação Científica Tropical, Trav. Conde da
Ribeira 7-9, 1300-142 Lisboa, Portugal (E-mail: cbotn@iict.pt) |
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© Maria Estela Guedes
estela@triplov.com
PORTUGAL |
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