ORIGINAL PAPER
Water relations and chlorophyll fluorescence responses of two
leguminous trees from the Caatinga to different watering regimes
Bruna D. Souza Æ Marcos V. Meiado Æ
Bruno M. Rodrigues Æ Mauro G. Santos
Received: 3 March 2009 / Revised: 6 July 2009 / Accepted: 27 August 2009 / Published online: 19 September 2009

Franciszek Go´rski Institute of Plant Physiology, Polish Academy of Sciences, Krako´w 2009
Abstract Leguminous species, Piptadenia moniliformes
(Benth.) and Trischidium molle (Benth.) H. E. Ireland, both
prevalent in the Caatinga vegetation, were submitted to
varying watering regimes under greenhouse conditions.
In experiment I, 60-day-old P. moniliformes plants were
maintained under suspended irrigation for 12 days.
Assessment on day 12 of drought revealed that leaf relative
water content decreased to 40% and stomatal conductance
and transpiration were also strongly diminished. Apparent
electron transport rate (ETR) and photochemical quenching
(qP) values were reduced by water deficit treatment com-
pared to controls, while non-photochemical quenching
(NPQ) increased; however, the basal values were recovered
in moisturized plants when analyzed after 48 h of rewa-
tering. In experiment II, T. molle plants were watered once
(1 9), 3 (3 9) or 5 times (5 9) per week, up to day 65 after
emergence. Chlorophyll a, chlorophyll b and carotenoid
contents were reduced in the 3 9 and 5 9 watering treat-
ments. Photosystem II maximum efficiency (Fv0/Fm0), ETR
and qP values strongly decreased when drainage frequency
and NPQ values were increased. Observation verified that
chlorophyll fluorescence is a suitable tool for evaluating
the developmental characteristics of the arboreal legumi-
nous species studied. Analysis of the data obtained suggest
that plant tolerance to the dry climate conditions of the
Caatinga ecosystem is directly associated with fast physi-
ological adaptation to water deficit, by accumulating
biomass in the root system in detriment to the shoots. The
data presented contribute to further understanding the
developmental and physiological mechanisms that enable
plant adaptation to dry climates and, particularly, to the
unique dry environmental conditions of the Caatinga
region.
Keywords Arid environmental
Drought tolerance

Stomatal conductance
Transpiration
Introduction
The Caatinga, a dry climate tropical forest in the north-
eastern region of Brazil, is the fourth largest vegetation
type in the country, after the Amazonian forest, the Cer-
rado, and the Atlantic forest (MMA 2002). It covers
734,478 km2 and comprises a vegetation characterized by
trees and branched underbrush, with deciduous foliage in
the dry season, as well as peculiar species, such as bro-
meliads and thorned cactus (Queiroz et al. 2005). The
Caatinga scrub vegetation occupies well-defined seasonal
areas with low rainfall levels (e.g., 500–750 mm year-1)
irregularly distributed throughout the year (3–5 months)
and with annual temperatures averaging 23–27C (Mach-
ado et al. 2006). Rainfall irregularity causes hydric deficit
and leads to drought during a large part of the year (for
details, see Sampaio 1995). Recently, the Caatinga was
recognized as one of ‘earth’s last wild places’ and classi-
fied as one of the 37 ‘wilderness areas of the world’ (Gil
2002). In spite of its high biological importance, knowl-
edge of the Caatinga ecosystem is still limited.
Pluviometric irregularity in the Caatinga associated with
other factors, such as high temperatures and high light
intensities, induce expressive levels of evaporation.
Communicated by J. Zwiazek.
B. D. Souza
M. V. Meiado
B. M. Rodrigues

M. G. Santos (&)
Departamento de Botaˆnica, Universidade Federal de
Pernambuco, 50670-901 Recife, Pernambuco, Brazil
e-mail: mauroguida@yahoo.com.br
123
Acta Physiol Plant (2010) 32:235–244
DOI 10.1007/s11738-009-0394-0

Consequently, the Caatinga vegetation has specific adap-
tation strategies, such as deciduous leaves and modifica-
tions in the anatomy and morphology of aerial parts of the
plants. The main survival strategy is their high water use
efficiency within the typical scarce rainy periods (Mansur
and Barbosa 2000; Silva et al. 2003a, 2008). Water avail-
ability in the soil is an important requirement for deter-
mining plant diversity, because it affects plant growth and
development (Silva et al. 2003b). Thus, water shortage in
the Caatinga soil during its long drought seasons seems to
be a major factor interfering in the success of seedling
development, growth and survival in this environment.
The leguminous species are among the prevalent arbo-
real plant species in the Caatinga. Piptadenia moniliformes
(Benth.), belonging to the subfamily Mimosoideae, and
Trischidium molle (Benth.) H. E. Ireland are widely dis-
tributed species in the region (Go´mez-Aparicio et al. 2004).
Both species grow as trees in the Caatinga sandy soils
and in the Cerrado, a typical ecosystem of the central region
of Brazil (Queiroz 2007). The Cerrado climate is typical of
the more humid savanna regions of the world, with an
average precipitation of 800 ± 2,000 mm year-1 over 90%
of the area, a severe dry season during the southern winter
(approximately, April to September) and annual tempera-
tures varying between 18 and 28C (Dias 1992).
Reduction in soil water availability leads to low plant
water potential. Consequently, as one of the first plant
responses to avoid excessive transpiration, the leaves lose
turgescence, the stomata are closed (Kramer and Boyer
1995) and cell elongation is paralyzed. When the stomata
close under water deficit, gas exchanges between the
leaf and the atmosphere are reduced, leading to low
intercellular CO2 concentration (Tang et al. 2002). Thus,
the diffusion of CO2 to chloroplasts is diminished and so is
the net CO2 assimilation rate (Chaves et al. 2002), with
consequent negative feedback in photochemical efficiency
(Baker and Rosenqvist 2004; Ribeiro et al. 2008; Santos
et al. 2006).
Studies on chlorophyll a fluorescence have shown that
the photosystem II (PSII) itself is not directly affected by
water deficit (Cornic and Massacci 1996; Souza et al.
2004), or is only affected under severe drought conditions
(Baker and Rosenqvist 2004). The photosynthetic rate is
the primary sink for energy derived from photochemistry,
whereas photorespiration and nitrogen assimilation are
relatively small sinks, particularly in developed leaves.
Measurements of O2 and CO2 exchange indicate a close
relationship between the true rate of O2 evolution from
PSII and the net rate of CO2 fixation (Edwards and Baker
1993; Santos et al. 2006). In the Caatinga forest, during the
rainy season, the vast majority of plants develop leaves,
flowers and fruits. During this period, the vegetation is
submitted daily to high light intensities, high temperatures
and low relative air humidity, as well as continuously well-
drained soils. Thus, during the Caatinga rainy season,
plants undergo strong stomatal control. Further studies on
photosynthetic activity and water deficit tolerance in plants
from the Caatinga vegetation could contribute to improving
current understanding of the physiological mechanisms
that enable their adaptation to dry climate and, more spe-
cifically, to the dry environmental conditions of this unique
ecosystem.
The objective of this study was to evaluate water rela-
tions and chlorophyll fluorescence in plants of the Caatinga
arboreal species, P. moniliformes and T. mollis, submitted
to varied water availability under greenhouse conditions.
Materials and methods
Plant material and growth conditions
Seeds of Piptadenia moniliformes Benth. and Trischidium
molle (Benth.) H. E. Ireland (Ireland 2007) were collected
near adult plants in the Catimbau National Park, located
in Buı´que, Pernambuco State, Brazil (083702300S;
370902100W). Selected seeds were placed on a double layer
of filter paper on Petri dishes (90 mm) and incubated in a
germination chamber at 25C for a photoperiod of 12 h.
Shortly after germination, they were individually trans-
ferred to 40 plastic pots, each one containing 500 g of soil
from the Caatinga, which was collected near adult plants.
No fertilization was applied in the experiments and the
plants were irrigated daily with 50 ml of distilled water.
The plants were grown for 60 days in the greenhouse,
with mean temperatures varying from 25 to 27C. Two
experiments were conducted, each using one of the legu-
minous species studied and submitted to varied water
supply. Completely expanded leaves of approximate
physiological age were evaluated in both experiments.
Experiment I
On day 60 after emergence, 20 P. moniliformes plants were
submitted to water deficit treatment (stress) by suspending
irrigation for 12 days, while in 20 control plants, daily
irrigation was continued. At the end of day 12, the plants
were rewatered and 24 and 48 h later they were evaluated
for relative water content (RWC), stomatal conductance,
transpiration, chlorophyll a fluorescence, length of shoot
and root system and weight of dry biomass.
Experiment II
After emergence, T. molle plants were separated into three
groups containing 20 plants each. They were then watered
236 Acta Physiol Plant (2010) 32:235–244
123

once (1 9), 3 (3 9) or five times (5 9) per week for
65 days. On day 66, chlorophyll and carotenoids contents
were quantified and biometric variables and chlorophyll a
fluorescence were also assessed.
Relative water content
Relative water content in the plants was assessed according
to Barrs and Weatherley (1962), on the day on which
irrigation was suspended (day 0) and on days 3 and 12
during irrigation suspension and recovery. Five leaf discs
per leaf were immediately weighed (fresh mass, FM). To
obtain the turgid mass (TM), leaf discs were floated (except
where noted) in distilled water inside a closed Petri dish.
During the imbibition period, leaf samples were weighed
periodically, after gently wiping the water from the leaf
surface with tissue paper. Unless otherwise noted, the Petri
dishes were maintained under dim light (approximately
20 lmol m-2 s-1) and under naturally fluctuating tem-
perature conditions in the laboratory. At the end of the
imbibition period, leaf samples were placed in a preheated
oven at 80C for 48 h, to obtain the dry mass (DM). All
mass measurements were made using an analytical scale
with a precision of 0.0001 g. Values of FM, TM and DM
were used to calculate RWC, using the equation: RWC
(%) = [(FM-DM)/(TM-DM)] 9 100.
Stomatal conductance and transpiration
Stomatal conductance (gs) and transpiration (E) were
determined on days 3 and 12 of irrigation suspension using a
steady-state porometer, model LI-1600 (LI-COR, Inc.,
Lincoln, NE, USA), in which the null point was set near the
actual humidity value in the greenhouse. Five plants with
completely expanded leaves were randomly chosen for
evaluation within the drought treatment and among the
controls. Evaluations were performed every hour throughout
the day to obtain the daily curves. At the moment of each leaf
evaluation, the LI-1600 porometer was also used to deter-
mine the air (Tair) and the leaf (Tl) temperatures, photosyn-
thetic active radiation (PAR) incident on the plant and the
relative air humidity (UR%). On day 3, air temperatures
varied from 26 to 36C and on day 12, from 25 to 36.5C.
PAR values at the beginning of the morning and at 10:00 a.m.
were determined as 15.3 and 1,517 lmol m-2 s-1, respec-
tively, on day 3 and 25.0 and 1,415 lmol m-2 s-1 on day 12
of irrigation suspension. The greatest values of relative
humidity were registered in the mornings, ranging from 43 to
67% on day 3 and from 45 to 76% on day 12. Stomatal
conductance values were close to 0 on day 12, when rewa-
tering was initiated. At 24 and 48 h after rewatering, the
plants were evaluated once during the period of highest
transpiration demand.
Chlorophyll fluorescence
Chlorophyll a fluorescence was assessed in both experi-
ments using a modulated plant fluorometer (IMAGING-
PAM, Heinz Walz GmbH, Germany). In experiment I,
assessments were performed every other day during the
12 days of water deficit and 24 and 48 h after rewatering,
in five plants from each treatment. In experiment II, chlo-
rophyll a fluorescence was measured on day 66 of treat-
ment. The measurements were always performed on the
leaf, avoiding the veins and edges. Each value generated by
a plant is the mean of five points taken on the same leaf.
Measurements for maximal (Fm) and basal (Fo) fluores-
cence yields were made on dark-adapted (20 min) leaves,
whereas steady-state (Fs) and maximal (Fm0) fluorescence
were assessed in light-adapted leaves (van Kooten and Snel
1990). The maximum fluorescence yield (Fm) was attained
during a 2.5-s saturation pulse (18,000 lmol m-2 s-1),
following the measurement of Fo under weak, continuous
illumination, with an actinic light of 5 lmol m-2 s-1.
Variation in chlorophyll a fluorescence was determined
in both dark-adapted (Fv = Fm - Fo) and light-adapted
(DF = Fm0-Fs) states. Fo is the basal fluorescence yield
after excitation of photosystem I by far-red light. In both
experiments, the potential (Fv/Fm) and effective (DF/Fm0)
quantum efficiency of PSII, photochemical [qP = (Fm0-Fs)
/(Fm0 - Fo0)] and non-photochemical fluorescence
quenching [NPQ = (Fm - Fm0)/Fm0] and apparent electron
transport rate (ETR = DF/Fm0 9 PPFD 9 0.5 9 0.84)
were calculated according to Schreiber et al. (1994). For
the ETR estimates, the 0.5 value was used as the fraction
of energy excitation distributed to PSII and 0.84 as the
fraction of light absorption.
Biometric assays
Ten plants randomly chosen within each treatment and
among the controls were evaluated for weight of dry bio-
mass, length of shoot and root system, area of leaf and
number of leaves, all of which are variables that could
undergo modification due to the time duration of the
treatments.
Photosynthetic pigments
Approximately, 200 mg of fresh leaves were homogenized
with 80% acetone and total chlorophyll was extracted
according to the method described by Lichtenthaler and
Wellburn (1983) with minor modifications. The spectro-
photometric quantification of chlorophylls a and b and total
carotenoids was determined by analyses of their absorbance
at 664, 646 and 470 nm wavelengths, respectively. The
amount of each photosynthetic pigment was expressed in
Acta Physiol Plant (2010) 32:235–244 237
123

milligrams per gram of fresh weight and estimated using the
following equations, where A means absorbance: Chl a
(mg l-1) = 12.21. A664—2.81. A646; Chl b (mg l
-1) =
20.13. A646—5.03. A664; Carotenoids (mg l
-1) = (1,000.
A470—3.27 [Chl a] - 104 [Chl b])/227.
Statistical analysis
The data were submitted to analysis of variance and when
differences were detected, the means were contrasted by
the Student–Newman–Keuls test, adopting a level of sig-
nificance of 5% (P \ 0.05).
Results
Experiment I
Relative water content and growth
At the end of day 12 after suspending irrigation, the RWC
in the treated plants was 50% lower than in the controls,
indicating that severe water deficit had an effect on the
treated plants (Table 1). Shoot growth in the control plants
was twofold greater on average than in the treated plants
(Table 1). Regarding the length and biomass of the root
system, no significant differences were observed between
the treated plants and controls. On day 12 of irrigation
suspension, observation verified that the growth of the root
system was greater than that of the shoot.
Stomatal conductance (gs) and transpiration (E)
Stomatal conductance (gs) and transpiration (E) varied
similarly on days 3 and 12 of irrigation suspension and on
both days they were lower in the treated plants than in
controls and greater between 9:00 a.m. and 2:00 p.m.
(Fig. 1a–f) (P \ 0.05). Differences in gs and E between
treated plants and controls increased on day 12 of drought.
On day 3, the transpiration rate was 71% higher among
controls than among treated plants and increased to 87% on
day 12 of water deficit (Fig. 1c, f).
On day 12 of water deficit, when rewatering was initi-
ated, gs in the treated plants was close to 0 (Fig. 1e). With
regard to hydration recovery, gas exchange measurements
were performed 24 and 48 h after initiating rewatering and
within the period of highest transpiration level (11:00 a.m.
to 12:00 noon). After a 48-h period, the plants submitted to
water deficit did not recover basal gs and E values, when
compared with controls (Table 2). In contrast, after 48 h,
the values of RWC were similar to that of control plants
(Table 1; P \ 0.05).
Chlorophyll fluorescence
In assessments performed on day 2 of irrigation suspension,
the PSII effective quantum yield (DF/Fm0, data not shown)
and apparent ETR did not differ significantly between treated
plants and controls. On day 12 of irrigation suspension, ETR
was diminished, although this variable recovered after 48 h
of rewatering, achieving values superior to the controls
(Fig. 2a). On the other hand, photochemical quenching (qP)
was stable under water deficit from days 2 to 6 and did not
significantly differ from the controls (P \ 0.05). However,
from days 8 to 12 of drought, the values were lower in the
treated plants (P \ 0.05), and 24 h after rewatering, the
values rose to their basal levels (Fig. 2c). Non-photochem-
ical quenching (NPQ) in treated plants was 87% higher than
among controls (Fig. 3d) from day 4 to 12 (P \ 0.05);
however, 24 h after rewatering, NPQ had decreased to
similar levels as the controls (P [ 0.05).
Experiment II
Shoot dry weight values in plants watered five times per
week were 55% lower than in plants watered only once and
37% lower than in those watered three times per week
(P \ 0.05). Plant height and leaf area also differed sig-
nificantly among the treatments (Table 3; P \ 0.05). No
significant differences in the number of leaves per plant
were detected among the treatments. In the 5 9 watering
treatment, the biomass of the root system decreased to
values approximately 70% lower than in the 1 9 treatment.
The root system length was 67% shorter in the 5 9 treat-
ment compared with 1 9 watering (Table 3; P \ 0.05).
Photosynthetic pigments and chlorophyll fluorescence
In T. molle plants, chlorophylls a and b and total carote-
noids contents diminished when watering frequency per
Table 1 Relative water content (RWC%), length and dry weight (of
the shoot and root system) in the control and in drought-treated plants
of Piptadenia moniliformes submitted to 12 days of water deficit by
suspension of irrigation
Variables
RWC
Plants
Control Treated
Day 0 91.54 ± 0.43
Third day 96.38 ± 0.08 81.0 ± 2
Twelfth day 94.64 ± 1.89 40.0 ± 1.27
Biometrics Shoot Root Shoot Root
Dry weight
(mg)
147.43 ± 19 50.31 ± 6 115.02 ± 11 62.57 ± 9.6
Length (cm) 10.69 ± 0.9 15.24 ± 2.3 5.17 ± 0.7 16.37 ± 2.2
Values are mean of ten replications (± SE)
238 Acta Physiol Plant (2010) 32:235–244
123

a principal mechanism of tolerance to water deficit in this
ecosystem, since gs controls the levels of gas exchange.
Under regular hydration conditions, open stomata permit
higher gas exchange, thus enabling efficient vegetative and
reproductive development during the rainy season. In
contrast, rapid closing of the stomata controls transpiration
when sudden short periods of water deficit occur within the
rainy season. In general, the Caatinga species show more
sensitive gs behavior in response to drought stress before
the change in leaf water content is detectable, similar to
that reported for other species (Elsheery and Cao 2008;
Santos et al. 2006; Silva et al. 2008).
Escape and drought tolerance mechanisms are among
the strategies adopted by plants to tolerate water deficit
(Subbarao et al. 1995). In the Caatinga, plants may have to
endure around 8–9 months without rain. During this per-
iod, the plants lose their leaves, which enable them to
overcome the climate period that is unfavorable to their
growth (Sampaio 1995). The remaining 3–4 months com-
prise the rainy season, with about 500–750 mm year-1
rainfall (Nimer 1989), favoring the recovery of vegetative
development, followed by the reproductive period.
Regarding the mechanisms of tolerance to drought condi-
tions, plant species tend to maintain their capability for
both water absorption from the soil and avoid transpiration
to maintain a water potential favorable to their metabolism
(Kramer and Boyer 1995). Consequently, in environments
where the rainy season is of short duration, the plants grow
faster. Stomata control prevents excessive water loss
(Schulze 1986) and the development of the root system
increases water absorption from the soil (McCully 1995),
as a form of protection for plants submitted to such con-
ditions (Kramer and Boyer 1995).
After rewatering, a period of 48 h was not sufficient for
P. moniliformes plants to recover basal gas exchange levels
(Table 2). The recovery of gs and E values depends on the
ability of water absorption from the soil and of the degree
of dehydration of the plants (Laffray and Louguet 1990;
Santos et al. 2004, 2006, 2009).
Analysis of the ETR values suggested that the light
collector complex of PSII was not affected (Fig. 2a).
Similar results were previously reported in Myracrodruon
urundeuva submitted to 14 days of drought (Queiroz et al.
2002).
The decrease in qP values may be accompanied by
damage to the photosynthetic complex proteins (Baker and
Rosenqvist 2004); however, the results for P. moniliformes
suggest that such damage did not occur, because the qP
values were recovered after rewatering (Fig. 2b). Bjo¨rkman
and Powles (1984) obtained similar results with Nerium
oleander L., a species native to arid regions, and they
considered that water stress predisposed the leaves to
photoinhibition. However, the recovery of the photo-
chemical activity was very slow after the restoration of
favorable water relations. This fact was not observed in our
study, since the plants showed rapid recovery after rewa-
tering (Fig. 2).
Studies involving orange trees showed that environ-
mental variations, such as increased temperature, induce
changes in the activity of the photosynthetic apparatus,
decreasing the qP values and increasing NPQ (Ribeiro et al.
2003). NPQ values may indicate a capacity for photopro-
tection (Maxwell and Johnson 2000). In P. moniliformes
plants under water deficit, the variation in NPQ values
suggests a capability to protect the photosynthetic system,
which could help the species adapt to warm environments
with low water availability and high light intensities
(Fig. 2c). Such plant adaptation in the Caatinga is essential
to species colonization in the harsh, unfavorable environ-
mental conditions of this ecosystem.
In the Caatinga vegetation, mature plants flourish, pro-
duce and disperse their seeds at the end of the short rainy
season (Barbosa et al. 2003). At the beginning of the
1x 3x 5x
0.0
0.2
0.4
0.6
0.8
Waters turn
qP
A
1x 3x 5x
0
2
4
6
8
B
N
PQ
Fig. 4 Changes in chlorophyll fluorescence parameters of Trischidi-
um molle plants watered once (1 9), thrice (3 9) or five times (5 9)
per week, during 65 days after emergence. a Photochemical quench-
ing (qP), b non-photochemical quenching (NPQ). Values are mean of
four replications (± SE)
Acta Physiol Plant (2010) 32:235–244 241
123

following rainy season, these seeds germinate and new
plants grow rapidly (Silva et al. 2003b; Cabral et al. 2004).
The seedlings grow and develop under low water potential
in the soil, due to its sandy nature and irregular rain fre-
quencies (Nimer 1989). Thus, when the plants are exposed
to excess water they suffer stress.
The values of dry weight, plant height and foliar area
demonstrated that, under the experimental conditions
adopted in this work, the watering frequency of once a week
was the most optimal for the T. molle species (Table 3).
This result was corroborated by the percentage of total
biomass in the plant root system (Table 3). Caatinga plants
accumulate more biomass in the root system during the first
few months of development, except those growing along
rivers margins. Thus, with a minimum aerial surface sub-
jected to water loss, they have greater chance of surviving
the common water deficit periods during the rainy season,
when these plants are in their reproductive state, as well as
during the long drought season period, when often they do
not possess leaves and fruits (Cabral et al. 2004).
The relative leaf chlorophyll content decreased when
plants were submitted to water stress (watered 3 9 and
5 9 a week). In Phlomis fruticosa L. grown under Medi-
terranean field conditions, the photosynthetic pigments and
RWC of young leaves decreased considerably with the
onset of the dry summer period and were stabilized at low
values during the last two summer months, while leaf
growth was arrested. Corresponding decreases in photo-
chemical efficiency of PSII were negligible, as estimated
by the chlorophyll fluorescence measurements of predark-
ened leaves (Kyparissis et al. 1995). The authors reported
that under stress conditions, the prevention of photoinhib-
itory damage could occur by decreasing chlorophyll con-
tents. Analysis of the present data for the T. molle species
corroborates previous observations in P. fruticosa, showing
diminished photochemical activity under stress conditions
due to threefold and fivefold increases in watering levels
(Table 3; Figs. 3, 4).
In the assessments of water stress effects, chlorophyll
fluorescence is considered to be a suitable variable for the
estimation of inhibition or damage in the PSII electron
transfer process (Bolhar-Nordenkampf et al. 1989), as well
as other environmental stresses (Baker and Rosenqvist
2004; Yang et al. 2009). Moreover, it is a fast, nonde-
structive and sensitive technique, which represents an
important advance in physiological studies and in plant
ecology (Krause and Weis 1991). Photoinhibition is usu-
ally described as a sustained reduction in the quantum yield
of photosynthesis. The severity of photoinhibition is
influenced not only by light intensity, but also by the
superimposition of other environmental stresses, such as
high temperature, water availability or CO2 supply (Ribeiro
et al. 2008; Santos et al. 2009; Souza et al. 2004). Analysis
of the present results indicated that excessive water content
in the soil, over a prolonged period, can diminish plant
growth and the development of T. molle (Table 3).
In the Caatinga, plants are usually exposed to water
deficiency for around 9 months of the year, accompanied
by high temperatures and high light intensities. These
environmental conditions induce low water contents in the
leaves, as well as stomatal closure (Kramer and Boyer
1995). Consequently, CO2 availability decreases and a
concomitant decrease in the photosynthetic rate occurs
(Lawlor and Cornic 2002). Such stomatal effect on the
photosynthetic rate does not impact the primary PSII effi-
ciency (Cornic and Massacci 1996), as verified in several
arboreal plants, including the mango (Elsheery and Cao
2008), apple (Massacci and Jones 1990) and cashew
(Blaikie and Chacko 1998). Throughout almost the whole
year, plants existing in the Caatinga adapt themselves to
low water availability and later to the rainy season, which
includes fast drought periods of short duration due to
shallow soils and irregular rainfalls. Analysis of the data
presented in Table 3 suggest that plants adapted to the
Caatinga environmental conditions have a low tolerance to
high water levels in the soil, which may affect their growth
and development. Excess soil moisture can displace the
oxygen in the sand pore spaces and lead to low ATP pro-
duction and the accumulation of toxic substances under
these anaerobic conditions (Drew and Lynch 1980).
The P. moniliformes and T. molle species showed high
tolerance to water stress. Both species showed stable
maximum photochemistry during the treatment period
(experiments I and II) and rapid recovery in photochem-
istry a few days after rehydration (experiment I), indicating
that both species prefer low water availability in the soil.
The data presented herein contribute to current knowledge
regarding the developmental and physiological mecha-
nisms that enable young plant adaptation to dry climates,
particularly in the Caatinga forest.
Acknowledgments The authors M.V. Meiado and B.R.M. Rodri-
gues are grateful to the Conselho Nacional de Desenvolvimento
Cientı´fico e Tecnolo´gico (CNPq) for the scholarships.
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