ABSTRACT
Morphological, anatomical and ecophysiological modifications caused by
flooding in the growth and development of Tabebuia avellanedae Lor. ex
Griseb. (Bignoniaceae) were studied and correlated with tolerance to
the excess of water in the soil. Three-month-old plants, grown in a
greenhouse, underwent a period of 56 days of flooding and a
post-flooding period. Photosynthesis rate and growth decreased in T.
avellanedae when flooding duration increased. Though not much
tolerant, plant adapted itself to short flooding periods. This
relative tolerance was probably due to the capacity of T. avellanedae
in developing structures which lessened flooding effects and promoted
internal diffusion of oxygen from the aerial part to the roots, such
as stem fissures, superficial roots and hypertrophied lenticels in the
roots. As there was no great variation in the internal anatomy of T.
avellanedae leaves and roots, metabolic alterations might have helped
in the survival of the species during flooding.
Key words: Inundation, oxygen deprivation, ecological anatomy,
Tabebuia avellaneadae
INTRODUCTION
Riparian vegetation is fundamentally important in diminishing erosion
that silts up the river channels, improving water quality (Lima, 1989)
and providing food and shelter to land and water fauna (Marinho Filho
and Reis, 1989). The study on the development of woody species in
situations of oxygen deficiency in soils, one of the chief
consequences in flooding, is indispensable when the preservation or
reconstruction of riparian vegetation is dealt with mistakes in
reforestation may thus be lessened (Medri and Correa, 1985).
Research on plant strategies on tolerance to flooding may be
interpreted through metabolic (Crawford, 1978) and morph anatomic
(Armstrong, 1979) changes. It is now evident that tolerance is due to
close links between structural and metabolic characteristics
associated with reproductive cycles and strategies (Blom et al.,
1994).
Flooding causes reduction in photosynthesis rates and stomata
conductance (Dreyer et al., 1991; Pimenta, 1998). The closing of
stomata, and thus the reduction of transpiration and the maintenance
of water potential in leaves, may inhibit desiccation. It may also
cause a diminishing in CO2 assimilation with decrease in plant growth
rate during flooding (Ruiz-Sánches et al., 1996). Decrease in biomass
accumulation, lengthening of plants due to flooding (Kolb et al.,
1998; Medri et al., 1998), epinasty, chlorosis (Pimenta et al., 1994),
early senescence (Marques et al., 1996) and abscission of leaves
(Clemens et al., 1978; Yamamoto and Kozlowski, 1987) have been
frequently recorded. Flooded plants may develop hypertrophied
lenticels (Marques et al., 1996; Colli, 1998; Kolb et al., 1998; Medri
et al., 1998), surperficial and adventitious roots (Pimenta et al.,
1994; Lobo and Joly, 1995) and aerenchyma in stems and roots
(Davanso-Fabro et al., 1998; Kolb et al., 1998), improving gaseous
exchange between the plant and the atmosphere (Armstrong, 1978).
Tabebuia avellanedae Lor. ex Grseb. (Bignoniaceae), popularly known as
"ipê-roxo" in Brazil, has been chosen for this research. Little is
known on the species with regard to the ecological aspects of its
development, in spite of its wide distribution and importance in many
habitats. Species occurs from Maranhão (North state) to the southern
region of Brazil and is extremely frequent in the states of Mato
Grosso and São Paulo. It occupies the upper canopy of the
semi-deciduous forest of the Paraná River basin where it grows with
relative abundance. Because of its great durability, the wood is used
for furniture and in house building (Lorenzi, 1992). According to
Reitz et al. (1978), Tabebuia avellanedae is a very important species
for the reforestation of degraded areas: plants are resistant in
replantations, develop well in the open field and when exposed to the
sunshine produce a great number of winged fertile seeds.
The present experiment was a contribution towards a better knowledge
of strategies using by tropical species to adaptated at inundation. It
might also help in projects for the recovery of riparian vegetation
and attempts to answer the following questions:
- Did hypoxia affect the photosynthesis rate in Tabebuia avellanedae?
- What were the morphological, anatomical and ecophysiological changes
caused by flooding and post-flooding in Tabebuia avellanedae?
- What were the consequences of flooding on the growth and development
of the species?
MATERIALS AND METHODS
Seeds of Tabebuia avellanedae were removed from fruit, washed, dried
and left to germinate in recipients with 700 g of wet substratum
consisting of 80% soil and 20% mixture of grated grass, lime and
coffee powder. Germination occurred in a nursery. After two months,
some plants, selected as to their uniformity in size and development,
and the substratum were placed in 4-L plastic pots with soil and sand
in a 3:1 proportion. Experiment started a month after plant
acclimatization. Forty plants were divided into 5 groups to measure
photosynthesis rate, conductance and CO2 internal concentration: field
capacity and plants flooded for 2, 7, 17 or 40 days. Measurements were
undertaken in one of one of the leaflets of the plant's totally
expanded leaf. Photosynthesis portable system LI-6200 and CO2 LI-6250
analyzer were used. During measurements average radiation on leaves
was 1575.1 mmolm-2s-1 and their average temperature was 25,6oC.
On the first day of the experiment the length of the aerial part and
of the main root, number of leaves, leaf area and stems diameter at
2cm from soil (sliding ruler) were evaluated in 16 plants for growth
and development analysis. To determine leaf area, two correcting
values, 0.726 and 0.376, were determined for simple and compound
leaves respectively from previous length measurements, width and area
of 60 leaves by the gravimetric method. Plants were then divided into
leaves, stems and roots and placed in a buffer at 60° C during 72
hours to obtain constant dry weight.
Another 80 plants were divided into three groups: fc (field capacity);
fd (plants in water at 2 cm from soil); fdfc (return of flooded plants
to field capacity on the 28th day). Similar evaluations were
undertaken with 16 plants of each group after 28 and 56 days. New
leaves and number of leaves lost by abscission were taken into
account. Relative growth rate (RGR) for each part of the plant and for
the whole plant was calculated by formula RGR = (ln WDf – ln WDi) / d,
where WDf is final weight of dry matter; WDi is the initial weight of
dry matter and d the number of days of treatment. Net assimilatory
rate (NAR) was calculated by formula NAR = ((WDf –WDi) /LAf – LAi)) x
((ln LAf – ln LAi / 56), where LAf is the final leaf area and LAi is
the initial leaf area. Weekly follow up was undertaken for possible
formation of stem fissures, hypertrophied lenticels, diageotropic
roots and intumescence of stems and roots.
At the end of the experiment the comparative anatomy of T. avellanedae
was analyzed from remaining laminas. 1-cm segments were collected
close to secondary roots of five control plants and of diageotropic
roots of five flooded plants. In the case of leaflets, segments of
five control plants and from five flooded ones from central region of
the leaflet lamina of totally expanded leaves and formed during the
experiment period were collected. Segments were fixed in formol,
acetic acid and alcohol (F.A.A. 70%), dehydrated, diaphanized and
placed in paraplast. Blocks were sectioned transversally at a
thickness of 13 mm. Cuts were colored in basic blue and mounted on
Canadian balsam. Laminas were analyzed by optic microscope for
parameters: percentage of intercellular spaces of the roots cortical
parenchyma and of the leaflets lacunal parenchyma (gravimetric
method); diameter of cells of cortical parenchyma and of the tracheal
elements of the roots xylem and of the cells of the leaflets adaxial
and abaxial epidermis; thickness of roots cortical parenchyma;
thickness of leaflet and of the lacunal and palisade parenchyma. All
microscopic measurements were made with micrometer ocular.
Statistical analysis was based on variance analysis (ANOVA). LSD was
determined by Tukeys test at P£ 0.05. Percentage data were transformed
in arc sine of proportion root before being statistically analyzed.
RESULTS
After 7 days flooding caused reduction of photosynthesis rate in T.
avellanedae. However, there was no significant rate difference among
plants with 7, 17 and 40 days flooding (Fig. 1A). Stomata conductance
diminished proportionately to an increase in hypoxia period (Fig. 1B).
In spite of reduction in stomata conductance, internal concentration
of CO2 didnt show any decreasing trend with an increase of flood
period (Fig. 1C).
Flooding reduced leafs RGR by approximately 30% and roots RGR by
approximately 40%. After re-aeration, growth of root and of the whole
plant were recovered. NAR of flooded T. avellanedae was reduced to
about 32% with recovery after re-aeration (Table 1).
Growth in height of shoot was not affected (Table 2). After 28 and 56
days of experiment, there was a decrease in roots length of flooded
plants. Recovery occurred after re-aeration.
Production of new leaves was smaller in flooded plants and without
significant recovery after re-aeration period (Fig. 2A). There was no
difference in abscission of leaves between treatments (Fig. 2B).
Flooding caused a reduction in leaf area of T. avellanedae without any
sign of recovery in re-aeration of plants (Fig. 2C).
During the first 28 days of the experiment, flooding stimulated an
increase in the stems diameter (Fig. 3).
From the second week, stem fissures started to develop in the flooded
plants of T. avellanedae, and the sponge tissue of stems were exposed
(Fig. 4). Flooding stimulated intumescence and the formation of
hypertrophied lenticels in the roots of many plants, with exposure of
sponge tissue (Fig. 4). At the end of the experiment, system of
flooded plants roots were less branched than those of controls. Traces
of necrosis were also present (Fig.4).
From the 20th day, the formation of diageotropic roots at the water
surface was registered in flooded plants. They were much branched and
of a whitish color (Fig. 5).
Taking into consideration all parameters evaluated in the internal
anatomy of T. avellanedae, the only one to show a significant
variation was that of the diameter of the leaflets lower epidermis
cells (Table 3). When similar regions of secondary roots of plants in
field capacity and of diageotropic roots of flooded plants were
analyzed, a higher number of absorbent hairs were found in the latter
(Fig. 6).
DISCUSSION AND CONCLUSION
A decrease in photosynthesis rate of T. avellanedae in proportion to
the duration of flooding suggested that the species might be impaired
in its development if exposed to long flood period. Decrease in
photosynthesis had already been registered in Campomanesia xanthocarpa
in flooding soil (Pimenta, 1998). Fall in photosynthesis in T.
avellanedae could be related to closing of stomata, since stomata
conductance tended to be lower in proportion to the increase of the
inundation period. Many authors have suggested that stomata closing
could be a response to fall in water potential due to decrease of
absorption by roots and consequently dehydration of leaf was prevented
(Ruiz-Sánches et al., 1996). However, in some cases, Quercus rubra,
for example (Dreyer et al., 1991), stomata closing occurred even when
water potential was maintained. This fact suggested that other factors
would be contributing towards the reduction of stomata conductance.
These might be low soil oxygen concentration (Ruiz-Sánches et al.,
1996) and the accumulation of ABA in leaves during hypoxia, which
acted on the guard cells (Zhang and Davies, 1987).
The fact that in T. avellanedae internal concentration of CO2 did not
decrease in spite of a reduction in stomata conductance suggested that
other factors besides the closing of stomata also could be causing
photosynthesis reduction. Dreyer et al., (1991) and Pimenta (1998)
reported decrease of internal concentration of CO2 and suggested that
photosynthesis limitation could be sometimes attributed to the closing
of stomata.
This happened together with other factors and interfered in CO2
assimilation. Photosynthesis reduction may be caused by biochemical
limitations such as decrease of Rubisco activity (Liao and Lin, 1996).
Fall in photosynthesis rate of T. avellanedae might be still a
consequence of the reduction of nutrient absorption. During inundation
a fall in energy levels of the root cells occurs owing to low
availability of oxygen for the respiratory metabolism. Maintenance of
cell may be interrupted and nutrient acquisition would be impaired
(Armstrong et al., 1994). Ascending flow of nutrients may also de
reduced during hypoxia because of reduction of transpiration by the
stomata closing (Collin-Belgrand et al., 1994).
Downton et al. (1988) suggested that conventional measuring techniques
of intercellular CO2 concentration were not adequate to evaluate to
which point photosynthesis decrease was due to the stomata closing.
Since this technique showed a non-uniform closing of stomata,
intercellular CO2 concentration would be overestimated. With more
precise methods, cases in which a reduction in photosynthesis was not
attributed to stomata closing, could be related to a decrease in
stomata conductance. Anyway, more detailed studies were necessary on
the photosynthesis properties of the leaf so that suitable conclusions
could be reached (Dreyer et al., 1991).
Decrease in photosynthesis and stomata conductance of T. avellanedae
during flooding were probably responsible for lowering of RGR of
leaves and principally of roots. Besides, a certain quantity of
photoassimilated factors may have been displaced for the formation of
structures, such as diageotropic roots. They contribute for a lower
accumulation of the biomass of the different organs during flooding.
Growth decrease in different parts of the plant in terms of the
accumulation of dry matter has typically been reported in studies on
flooding (Marques et al., 1996; Kolb et al., 1998; Medri et al.,
1998). Wiedenroth (1993) and Armstrong et al. (1994) reported that the
growth decrease of some organs during flooding could be a strategy to
economize energy and keep metabolism of the most affected regions.
Thus, the survival of plant was dependent of equilibrium in the
distribution of photoassimilated factors among the various parts of
the plant. RGR reduction in leaves of T. avellanedae may still have
been caused by a decrease in nutrient availability. According to
Collin-Belgrand et al. (1991) and Armstrong et al. (1994) this may
occur during hypoxia.
While flooding affected photosynthesis, NAR decrease in flooded plants
shows a lowering of efficiency of plants as assimilating systems. This
might have led to a reduction in their growth. Results in net
assimilation similar to those for T. avellanedae were also registered
by Dreyer et al. (1991) and Pimenta (1998).
Results for the development of T. avellanedae showed that species
maintained growth of stem in extension during flooding. The roots,
however, had pronounced growth impairment. Similarly to what occurs in
soils with low oxygen concentration due to flooding, the roots are the
first organs affected by hypoxia and generally their growth is greatly
impaired (Vartapetian, 1993).
Decrease in the production of new leaves in flooded plants of T.
avellanedae caused a diminishing of growth in leaf area. Consequently,
this resulted in NAR lowering during flooding. Decrease may have also
been a consequence of the reduction in nutrient flow towards the
aerial part and a redirection of displacement of photoassimilated
factors for the production of diageotropic roots. Reduction in leaf
production during flooding has also been registered by Pimenta (1998)
in Campomanesia xanthocarpa.
Increase in the stems diameter of T. avellanedae gave a spongy aspect
to the stem at the height of the water lamina. Probably hypertrophy
was caused by the formation of aerenchyma. Increase in stem diameter
is a typical answer to soil flooding and has been registered by many
authors (Pimenta et al., 1994; Lobo and Joly, 1995; Medri et al.,
1998). Aerenchyma formed in flooding facilitates internal diffusion of
oxygen and allows atmosphere gases to be transported to the submersed
parts of the plant (Wiedenroth, 1993). It have been recently verified
that in some species stem hypertrophy may have been caused by an
increase in exchange activity or by an increase in the number and size
of xylem fibers (Yamamoto et al., 1995).
Stem hypertrophy of T. avellanedae provoked fissures. Together with
hypertrophied lenticels in the roots, this might have been an
important factor in the survival of plants during flooding. These
fissures, also verified by Joly and Crawford (1982) in Sebastiania
klotzchiana and by Davanso-Fabro et al. (1998) in Sesbania virgata,
may have the same role of hypertrophied lenticels. According to Hook
and Scholtens (1978), the latter would be involved in the increase of
gas exchange between the atmosphere and the plant.
Another factor that could have an importance in the survival of the
species under analysis was the development of diageotropic roots.
Growth decrease and death of roots in the deepest layers of soil and
the establishment of diageotropic roots may be a strategy in energy
economy (Schlüter et al., 1993). The new roots at the more aired soil
surface would be more efficient in O2 transportation and water and ion
absorption (Armstrong, 1978). Lobo and Joly (1995) verified that in
Talauma ovata flood-induced diageotropic roots had a higher rate of O2
consumption and release of CO2. It shows a more active respiratory
metabolism than in other roots. Roots produced during flooding would
be also substituting the deteriorated roots during hypoxia in
producing vegetal hormones such as cytocynin and gibberellin. The
probable hormonal production by diageotropic roots in T. avellanedae
would avoid early abscission of leaves during flooding. Thus, the
survival of plant does not depend on a system of tolerant roots but on
the production capacity of new roots substituting the deteriorated
ones during flooding (Collin-Belgrand et al., 1991). In their study of
hypoxia in T. avellanedae, Messias et al. (1986) verified that species
produced many adventitious roots. This has not been registered in the
present experiment. Phenotype variation may probably be due to genetic
variations among populations of different locals.
Intercellular cortical spaces in T. avellanedae in roots and in leaves
could be merely constitutive since their formation was not affected by
flooding. In their study on species of riparian vegetation with
various tolerance degrees, Smirnoff and Crawford (1983) defined as
tolerant those species with more than 10% porosity of roots in drained
conditions, while they might or not increase quantity during flooding.
Even in some tolerant species, porosity of roots might not react to
flooding.
No variation was detected in thickness of root cortex of T.
avellanedae in flooded plants and those in field capacity. Percentage
of intercellular spaces and diameter of cortical cells were equal for
both groups. Whereas increase in diameter of cortical cells as a
consequence of flooding has been registered in Spathodea campanulata
(Medri and Correa, 1985), reduction in diameter of these cells was
recorded in Sesbania virgata (Davanso-Fabro et al., 1998).
The fact that the diageotropic roots of flooded T. avellanedae were
covered with more hairs than plants in field capacity suggested that
an increase in area absorption might be occurring, with greater
efficient absorption of water and ions. High concentrations of
ethylene may act as stimulator in hair formation in the root of many
species such as in the genera Lactuca (Abeles et al., 1992) and
Arabidopsis (Dolan et al., 1994). This occurs too in the colon of
Chorisia speciosa, since ethylene levels are raised when the plant
undergoes flooding (Giloni, personal communication).
If flooded T. avellanedae survived during the experiment, possibly
owing to their ability in producing photosynthesis, it showed that the
species, although not high tolerant to flooding, might adapt itself to
short flooding periods. Relative tolerance could be corroborated by
ability of T. avellanedae to develop the necessary structures to ease
flooding effects and thus facilitating the internal diffusion of
oxygen from the aerial section to the roots. Further, as T.
avellanedae did not undergo pronounced variations in leaf and root
internal anatomy, metabolic changes probably helped in the survival of
species during flooding.
Tolerance of species may vary from a few days to some months and
depends on the time of the year, the plants stage of development,
degree and duration of flooding (Crawford and Brändle, 1996). Caution
is necessary when verifying the real meaning of tolerance especially
in tropical species. Rogge et al. (1998) studied the respiratory
metabolism of flooded roots of tolerant and intolerant tropical
species. These authors didnt register patterns of answers that would
differentiate the two groups. Conclusion confirmed that there was a
great variety of mechanisms of flood tolerance. Since the "ipê-roxo"
roots were not directly affected by flooding, selection occured in the
initial stages of the plants development. More studies are necessary
on the factors that influence its establishment in the field.
ACKNOWLEDGEMENTS
Present research is part of a project entitled "Aspects of Fauna and
Flora in the Tibagi River Basin" supported by the Universidade
Estadual de Londrina, Intermunicipal Agreement for Environmental
Protection of the Tibagi River Basin, COPATI and KLABIN Fabricadora de
Papel e Celulose and FINEP. It had also the support of CAPES and
NUPELIA (Research Nucleus in Limnology, Ichthyology and Aquiculture)
of the Universidade Estadual de Maringá.
RESUMO
Foram estudadas as modificações morfo-anatômicas e ecofisiológicas que
o alagamento provoca em Tabebuia avellanedae Lor. ex Griseb.
(Bignoniaceae), correlacionando-as com sua capacidade de tolerar o
excesso de água no solo. Plantas com três meses de idade, crescendo em
casa de vegetação, foram submetidas a 56 dias de alagamento e a um
período de pós-alagamento. A taxa fotossintética e o crescimento de T.
avellanedae decresceram conforme aumentou o tempo de alagamento, mas
mesmo não sendo altamente tolerante, a espécie foi capaz de se adaptar
a curtos períodos de alagamento. Esta relativa tolerância
provavelmente se deve à capacidade apresentada por T. avellanedae de
desenvolver estruturas que amenizam os efeitos do alagamento,
promovendo a difusão interna de oxigênio da parte aérea para as
raízes, tais como rachaduras caulinares, raízes superficiais e
lenticelas hipertrofiadas nas raízes. Como não houve acentuada
variação na anatomia interna de folhas e raízes de T. avellanedae,
alterações metabólicas provavelmente auxiliaram na sobrevivência da
espécie durante o alagamento.
hecho por: Willson A Mendoza C
c.i:16.959.604
CRF
FUENTE:http://www.scielo.br/scielo.php?pid=S1516-89132002000300016&script=sci_arttext&tlng=en
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