RELAÇÕES HÍDRICAS INTERNAS DO UMBUZEIRO SOB CONDIÇÕES SEMI-ÁRIDAS
INTERNAL WATER RELATIONS OF THE UMBU TREE UNDER SEMI-ARID CONDITIONS1
JOSÉ MOACIR PINHEIRO LIMA FILHO2
ABSTRACT -A study was conducted at the Embrapa Semi-Árido, Petrolina-PE,
Brazil, with the aim of understanding the mechanism by which the umbu tree
(Spondias tuberosa Arr. Cam.) maintains its diurnal internal water balance in
dry and wet seasons. The results obtained were based on the measuring of leaf
water potential and its components using the pressure chamber and hygrometric
chambers / microvoltmeter. Under dry conditions, the lowest values on water
potential and osmotic potential were observed around 8:00 h, reaching,
respectively, ¾0. 97 MPa and ¾1.17 MPa, resulting a turgor pressure of 0.2 MPa.
The lowest turgor pressure occurred at 16:00 h when 0.16 MPa was detected but
full recovery was not observed by the end of the light period. During the
raining season, however, the lowest water potential was ¾1.55 MPa at 14:00h and
-1.57 MPa at 14:00 h giving a turgor pressure of 0.02 MPa. Recovery was fast
and, by the end of the day, plant water status was similar to the value
observed at predawn. These results suggest the umbu tree presents two
strategies for maintaining a favorable internal water balance, regarding the
environmental situations studied. Under dry conditions the diurnal balance
would be maintained at the expenses of water stored in the tubers and by
restricted transpiration. During the rainy season, the diurnal water balance
may have been mediated by a short term osmotic adjustment as judged by the
observed afternoon range between water potential and osmotic potential.
Index terms: water potential, osmotic potential, turgor pressure, osmotic
adjustment, Spondias tuberosa.1
RELAÇÕES HÍDRICAS INTERNAS DO UMBUZEIRO SOB CONDIÇÕES SEMI-ÁRIDAS
RESUMO -Realizou-se um estudo na Embrapa-Centro de Pesquisa Agropecuária do
Trópico Semi-Árido (CPATSA), Petrolina-PE, objetivando identificar os
mecanismos, através dos quais o umbuzeiro (Spondias tuberosa Arr. Cam.) mantém
o balanço hídrico interno durante as estações de seca e chuvosa. Os resultados
obtidos basearam-se em observações do potencial hídrico e de seus componentes,
utilizando-se da câmara de pressão e câmaras higrométricas / microvoltímetro.
Sob condições de seca, os valores mais baixos de potencial hídrico e potencial
osmótico foram observados em torno das 8 h, atingindo, respectivamente ¾0,97
MPa e ¾1,17 MPa, resultando em uma pressão de turgor de 0,2 MPa. A pressão mais
baixa ocorreu às 16 h, atingindo 0,16 MPa. Entretanto, a recuperação hídrica
não foi observada, até o final dia. Durante a estação chuvosa, os valores de
mais baixos de potencial hídrico foram obtidos às 14 h , quando foram
detectados, respectivamente ¾1,55 MPa. Neste momento, o potencial osmótico
atingiu ¾1,57 MPa , culminando com uma pressão de turgor de 0,02 MPa.
Entretanto, até o final do dia, a condição hídrica da planta foi similar à
observada no início do dia. Estes resultados sugerem que o umbuzeiro apresenta
duas estratégias para manter, durante o dia, um balanço hídrico interno
favorável, dentro das condições ambientais estudadas. Sob condições de
sequeiro, o balanço seria mantido através da utilização da água armazenada nas
túberas e uma baixa transpiração. Durante a estação das chuvas, o balanço
hídrico pode ter sido mediado por um ajuste osmótico, a julgar pelas variações
observadas à tarde entre níveis de potencial hídrico e potencial osmótico.
Termos para indexação: Potencial hídrico, potencial osmótico, pressão de
turgor, Spondias tuberosa.
INTRODUCTION
The umbu tree (Spondias tuberosa Arr. Cam.) belongs to the Anacardiceae family
which grows naturally in the ¨Caatinga¨ areas throughout the semi-arid
Northeast Brazil. During the dry season, the plant shed their leaves to avoid
transpiration. The flowering process is initiated while under drought, when the
first leaves appear at the base of the inflorescence. However, leaf flushing is
enhanced after the first rains. The survival of the species in such hash
environment is assured by a specialized root system bearing tubers, whose
function is to store water, minerals and other solutes. According to Lima
(1994), the tubers can reach 20.0 cm of diameter and can be found from 10.0 cm
to 30.0 cm depth. These resources are used during the dry season for
maintaining plant normal metabolism and the initiation of the flowering
process. Its fruits, rich in carbohydrates and ascorbic acid, are consumed ¨in
natura¨ or worked into preserves, sweets and beverages of pleasant taste (Mors,
1994). The gathering of fruits is a very important activity for complementing
the familiar income of the small farmers (Mendes, 1990; Cavalcante et al.,
1996). Due to its social and economic importance, the umbu tree has been
studied in relation to dispersion (Santos, 1990), propagation (Oliveira et al.,
1989; Nascimento et. al, 1993), morphology (Braga, 1976), root anatomy (Lima,
1994) and fruit chemical composition (Narain et al, 1992). Regarding its
physiological behavior and interaction with the environment, very few reports
are found in the literature. In this context, Ferri & Laboriau (1952) and
Ferri (1953), studied the water balance of this specie during the dry season,
based on stomatal behavior. They observed that maximum transpiration occurred
between 7:00 h and 9:00 h. More recently Lima Filho & Silva (1988), using
sophisticated instrumentation showed that the umbu tree exerts a rigid control
of transpiration through the stomata, even under good soil moisture conditions,
assuring a significant water economy. Despite these considerations, there is a
need for new insights on plant water status under distinct environmental
situations naturally experienced by the species. Therefore, our objective was
to study the water relations of the umbu tree based on the monitoring of leaf
water potential and its components in order to understand the mechanism by
which this species maintains its internal water balance under dry and wet semi-
arid conditions.
MATERIAL AND METHODS
The experiment was conducted at the Research Center for the Semi-Arid Tropic
(Embrapa Semi-Árido) during the dry and wet seasons of 1994/1995. Data were
collected on four trees grown under natural conditions at the end of the dry
season of 1994 (October) and during the wet season of 1995 (March) after an
accumulated precipitation of 510 mm. Leaf water potential was monitored using a
pressure chamber (PMS Instruments, USA) of the type described by Scholander et
al. (1965 ) on sun exposed leaves located at the canopy midpoint. The osmotic
potential was observed with the C-52 sampler chamber connected to the
microvoltmeter HR-33T (Wescor, USA). Leaf discs were sampled and stored in 2.0
ml disposable syringe and frozen in liquid nitrogen for 10 seconds. After
thawing under laboratory conditions the samples were pressed and the osmotic
potential of the exuded sap measured in the dew point mode. Turgor pressure was
then estimated by difference between leaf water potential and osmotic
potential, as suggested by Turner (1981). Irradiance, air temperature and
humidity and precipitation were obtained with respective sensors connected to
the LI-1000 datalogger (Licor, USA). All data were monitored from 5:00 h to 18:
00 h during three cloudless days, on both seasons.
RESULTS AND DISCUSSION
The environmental variables recorded during physiological data acquisition are
presented in Fig._1. During the dry season, irradiance was higher in the
morning but lower in the afternoon than during the wet season as a result of
differences in solar inclination between seasons. However, the highest value
occurred at noon, reaching around 4.5 Wm-2, in both periods. Despite the small
difference in solar radiation, air temperature and vapor pressure deficit were,
higher during the dry period. Early in the morning, temperature and vapor
pressure deficit were, respectively, 25 oC and 0.59 KPa in the dry and 22.5 oC
and 0.36 KPa in the wet period .The highest values were observed around 14:00 h
, reaching 37 oC and 2.35 KPa in the dry and 32 oC and 1.62 KPa in the wet
period, indicating that plants were under higher evapotranspiratory conditions,
mainly during the dry period. Due to the environmental differences between
seasons, the water relations of umbu plants were significantly affected . In
fact, the statistical data analysis detected that the interaction measurement
time x periods was significant (P< 0.01) for all variables studied. Under dry
conditions, leaf water potential and its components presented a very small
variation during the day (Fig._2) . Early in the morning, leaf water potential
declined from -0.73 MPa at predawn to about -0.97 MPa at 8:00 h, meaning a 0.24
MPa difference. From this point a smooth recovery was observed even when the
environment was already conductive to greater water loss. Thus, at the end of
the day, leaf water potential was ¾ 0.76 MPa, almost reaching the value found
at predawn. This situation may have been caused by the fact that the umbu tree,
early in the morning, closes their stomata in response to drought, resulting a
drastic decline on plant water loss (Lima Filho & Silva, 1988). Plant
recovery may have been sustained by water stored in the tubers.
The osmotic potential data followed similar hourly trend, although lower points
had been detected. Values were around -1.15 MPa from predawn to 8:00 h,
increasing until -0.95 MPa, by the end of the day. As it can be seen (Fig._2),
the diurnal range between plant water potential and osmotic potential was
rather small and the difference between these variables was narrowed toward the
end of the light period. This may mean that during the day there was no solute
accumulation in the leaf tissue but a decrease in its concentration due to
dilution by water. However, sincethe osmotic potential was always lower than
the water potential, cell turgor pressure was maintained ( Tyree & Jarvis,
1982). The maximum pressure occurred at predawn when it reached 0. 41 MPa. From
then on, a linear decline was observed until 0.16 MPa at 16:00 h, meaning a
0.25 MPa pressure drop. At 18:00 h, turgor pressure was still around 0.19 MPa,
indicating that recovery to predawn values would probably be attained at night.
Because the evapotranspiration conditions during the dry season were more
stressing (Fig_1), it was expected the umbu tree to develop a more stable
internal water balance during the raining season. However, leaf water potential
presented a threefold drop during the day as it declined from - 0.5 MPa at
predawn to -1.55 MPa around 14:00h. (Fig.3). In this case, the increasing leaf
density may have enhanced plant water loss, not matched by root absorption,
causing the water potential to drop significantly. Despite this fact, plants
recovered about 1.0 MPa from 14:00 h to 18:00 h. The osmotic potential data
decreased less steeply from -1.23 MPa at predawn to ¾1.58 MPa at 16:00 h. By
this time, leaf water potential had already recovered, suggesting an increase
in solute concentration in the leaf tissue. This situation may have been caused
by a transient osmotic adjustment due to the uptake of additional inorganic
salts, or from the accumulation of organic solutes as it naturally occurs (Taiz
and Zeiger, 1991). As it can be seen ( Fig._3), the range between water
potential and osmotic potential was wider early in the morning and during late
afternoon, but significantly reduced around 1400 h. Consequently, turgor
pressure, which was about 0.74 MPa at predawn, drastically declined to 0.02 MPa
at 1400 h. Full recovery was attained at 1800 h, when pressure reached 0.82
MPa, meaning a 0.08 MPa above the predawn values.
CONCLUSIONS
1- During the dry period the umbu tree presents a more stable internal water
balance as judged by the diurnal variation observed for leaf water potential
and its components. This situation may be attributed to plant control of
transpiration under drought and to the presence of tubers in the root system,
as a reservoir for water and solutes;
2- During the raining season, despite the better environmental condition
observed, the increasing leaf density may have enhanced plant water loss,
causing an unfavorable internal water balance at a time of higher
evapotranspiration demand. However, the maintenance of lower osmotic potential
while plant water potential had already recovered restored the balance and
suggests that the umbu tree may express a short term osmotic adjustment.
ACKNOWLEDGEMENTS
We are thankful to Mr. Genival Nunes Ferreira for his valuable help, during
data collection.
