378
Rev. Bras. Agrometeorologia, v. 13, n.2, p.353-368, 2005
RIBEIRO, R. V., et al. - Leaf temperature in sweet orange plants under field condition: influence of meteorological elements
Recebido para publicação em 14/07/05. Aprovado em 10/11/05.
ISSN 0104-1347
Leaf temperature in sweet orange plants
under field condition: influence of
meteorological elements
Temperatura foliar de laranjeiras em condição de campo:
influência de elementos meteorológicos
Rafael Vasconcelos Ribeiro1, Eduardo Caruso Machado2, Mauro Guida dos Santos3
Abstract: This study had as objective the evaluation of daily and seasonal changes of leaf temperature in relation to
the variation of meteorological elements [global radiation (Qg), air temperature (TAIR) and air vapor pressure deficit
(VPD)] in field-grown citrus plants. These environmental variables were monitored inside citrus orchard, with NorthSouth orientation. Leaf temperature (TLEAF) was measured with infrared thermometers (IR) and thermocouples (TC) in
both the East and West canopy positions, as also done for micrometeorological monitoring. From these measurements,
the difference between leaf and air temperatures (DT) was calculated. The highest leaf temperatures were found in
days with high radiation loading (summer), reaching values about 35.5 (IR) and 39ºC (TC). Accordingly, the highest
DT values were also noticed in summer, being about 3 (IR) and 8ºC (TC). This discrepancy is due to the light exposure
of leaves, i.e. TC measurements evaluated exposed leaves, whereas IR technique measured the temperature of both
exposed and non-exposed leaves. In fact, DT varied between 3 and -3ºC during most of day for IR-based measurements.
Leaf temperatures measured with TC were always higher than those measured with IR in both canopy positions. The
response of leaf temperature to increasing Qg tended to stabilize at values higher than 600 W m-2, whereas the responses
to VPD and TAIR had an increase trend. Considering these last variables, leaf temperature and DT were lower in the East
position in relation to the West one at values higher than 1.5 kPa and 30ºC. This fact is probably related to differential
leaf transpiration when considered the canopy positions, being a question for further studies. Concluding, the highest
TLEAF values were found during afternoon and in summer season, which is related to the daily and seasonal pattern of
available radiation loading. In studies involving the evaluation of TLEAF in citrus plants, it is important to consider the
level of light exposure in leaves. In exposed leaves, changes in TLEAF are directly related to the variation of TAIR, Qg and
VPD, showing more or less data scattering depending on environmental variable considered. In relation to the evaluation
of TLEAF considering both exposed and non-exposed leaves (given by IR technique), it is shown that TLEAF is directly
affected by TAIR. However, changes in TLEAF are more accentuate under low Qg and VPD values, occurring a tendency
of TLEAF saturation only for Qg. Some physiological implications of increased leaf temperature are discussed.
Key words: canopy, Citrus sinensis, infrared thermometry, micrometeorology.
Resumo: Este estudo objetivou a avaliação das variações diárias e sazonais da temperatura foliar em relação à variação
dos elementos meteorológicos [radiação global (Qg), temperatura do ar (TAR) e déficit de saturação de vapor do ar
(DPV)] em laranjeiras em condição campo. Essas variáveis ambientais foram monitoradas no interior do pomar de
citros, com orientação Norte-Sul. A temperatura foliar (TFOLIAR) foi medida com termômetros a infra-vermelho (IV) e
termopares (TP) nas posições Leste e Oeste da copa das plantas, como também realizado no monitoramento micrometeorológico. A partir dessas medidas, foi calculada a diferença de temperatura entre folha e ar (DT). As maiores
1
2
3
Agronomist, MSc., Centro de Pesquisa e Desenvolvimento de Ecofisiologia e Biofísica, Instituto Agronômico, IAC
– APTA/SAA. Avenida Barão de Itapura, 1481, CP 28, 13001-970, Campinas - SP, Brasil. Corresponding author:
[email protected]
Agronomist, DSc., Centro de Pesquisa e Desenvolvimento de Ecofisiologia e Biofísica, Instituto Agronômico, IAC
– APTA/SAA.
Agronomist, DSc., Departamento de Botânica, Universidade Federal de Pernambuco, UFPE.
50670-901, Recife - PE, Brasil.
379
Rev. Bras. Agrometeorologia, v. 13, n.3, p.378-388, 2005
temperaturas foliares foram verificadas em dias com alta carga radiante (verão), alcançando valores ao redor de 35,5
(IV) e 39ºC (TP). Valores elevados de DT foram também observados no verão, chegando a 3 (IV) e 8ºC (TP). Essa
discrepância é devida à exposição luminosa das folhas, i.e. medidas de TP avaliaram folhas expostas, ao passo que a
técnica de IV mediu a temperatura tanto de folhas expostas como não-expostas. De fato, DT variou entre 3 e -3ºC
durante a maior parte do dia para as medidas baseadas em IV. A temperatura foliar medida com o método de TP foi
sempre maior do que a medida com a técnica de IV em ambas posições da copa das plantas. A resposta da temperatura
foliar ao aumento de Qg tendeu a estabilizar em valores superiores a 600 W m-2, enquanto que as respostas ao DPV e
TAR apresentaram tendência crescente. Considerando essas últimas variáveis, a temperatura foliar e DT foram menores
na posição Leste em relação à posição Oeste em valores superiores a 1.5 kPa e 30ºC. Esse fato está possivelmente
relacionado à transpiração foliar diferencial quando consideradas as posições na copa, sendo essa uma questão para
futuros estudos. Concluindo, os maiores valores de TFOLIAR são observados durante a tarde e na estação de verão, o que
é relacionado ao padrão diário e sazonal de energia radiante disponível. Em estudos envolvendo a avaliação de TFOLIAR
em citros, torna-se importante considerar o nível de exposição das folhas à luz. Em folhas expostas, as mudanças de
TFOLIAR são diretamente relacionadas à variação de TAR, Qg e DPV, mostrando maior ou menor dispersão dos dados,
dependendo da variável ambiental considerada. Em relação à variação de TFOLIAR considerando ambas folhas expostas
e não expostas (dada pela técnica IV), é mostrado que TFOLIAR é diretamente afetada por TAR. Entretanto, as mudanças
em TFOLIAR são mais acentuadas em baixos valores de Qg e DPV, havendo tendência de saturação de TFOLIAR apenas para
Qg. Algumas implicações fisiológicas da elevação da temperatura foliar são discutidas.
Palavras-chave: Citrus sinensis, copa, micrometeorologia, termometria a infra-vermelho.
Introduction
The fluctuation of meteorological elements
affects citrus growth, productivity (REUTHER,
1977; ORTOLANI et al., 1991; PAULINO &
VOLPE, 2001) and fruit quality (ALBRIGO, 1992;
VOLPE et al., 2000), being citrus plants subjected
to both seasonal and diurnal variation of
environmental variables. Under tropical or
subtropical conditions, high air temperature is
normally found during summer season, whereas low
temperatures are verified during winter season. This
seasonal pattern is mostly caused by energy
availability (ANGELOCCI, 2002; PEREIRA et al.,
2002), i.e. incoming solar radiation. As
evapotranspiration and rain are also regulated by
available energy and atmospheric movements, air
relative humidity is another meteorological element
with seasonal and diurnal variation that affects citrus
physiology (KHAIRI & HALL, 1976a).
As there are large changes of environmental
elements throughout the year, physiological
responses are expected in order to maintain plant
temperature around an optimum range. Considering
the energy dissipation by physiological mechanisms,
the main process involved in the maintenance of leaf
or canopy temperature is the transpiration (NOBEL,
1999). Under well-irrigated conditions, most of
available energy will be used in evapotranspiration
(70-80% of net solar radiation). However, this does
not take place under low soil water availability, being
the energy used in heating of air, plant and soil
(PEREIRA et al., 2002).
Obviously, the plant heating above determined
temperature threshold has physiological
consequences such as reduced photosynthetic
activity and probably impaired plant growth and
production (KHAIRI & HALL, 1976b; ALBRIGO,
1992), depending on the exposure time (VU, 1999),
growth temperature regime (RIBEIRO et al., 2004)
and phenological stage in which it happens
(REUTHER, 1977; ORTOLANI et al., 1991). In
addition, plant heating is affected by the leaf
morphology and sky nebulosity (ANGELOCCI,
2002).
Two additional aspects that should be
considered when studying perennial tree species are
the canopy portion exposed to solar radiation as well
as the differences of temperature between exposed
leaves and overall plant canopy. Among the various
practical applications of knowing the leaf
temperature variation throughout a year and during
a day, it may be pointed out the modeling of plant
growth and development (ACOCK & ACOCK,
1991) and studies involving the crop water stress
index for citrus plants based on plant temperature
(SEPASKHAH & KASHEFIPOUR, 1994).
380
RIBEIRO, R. V., et al. - Leaf temperature in sweet orange plants under field condition: influence of meteorological elements
Hence, the objective of this study was to
characterize the daily changes of leaf temperature
considering the seasonality of environmental
elements and measurements of temperature in
individual leaves and plant canopy. In addition, some
physiological consequences of increased leaf
temperature are discussed in the clarity of plant
growth and related processes.
Material and Methods
The experiment was conducted in a citrus
orchard located at the Centro APTA Citros
‘Sylvio Moreira’, Cordeirópolis, SP, Brazil
(22°32’S; 47°27’W; alt. 639 m). The climate is
subtropical with an average annual rainfall of
1366 mm and mean monthly air temperature
varying between 23.8ºC (maximum in summer)
and 17.8ºC (minimum in winter).
The citrus orchard was 15-year old, being
constituted by sweet orange plants (Citrus
sinensis L. Osb.) var. ‘Valência’ grafted on
mandarin ‘Cleopatra’ (Citrus reticulata Blanco)
rootstocks and planted with spacing of 8x5 m in
the North-South orientation. Mineral nutrition
(macro and micronutrients) was practiced
periodically, and pest and disease control was
done when necessary to prevent/reduce any
undesirable interference on measurements.
Plants were under natural environmental
conditions, i.e. they were non-irrigated and
exposed to natural variations of solar radiation,
air temperature and humidity, and rain.
Diurnal measurements of leaf temperature
(T L E A F , ºC) were taken with two infrared
thermometers (IR) model IRT 4000-4GL
(Everest Interscience, Tucson, AZ, USA)
positioned at 0.2 m from a plant canopy in both
the East and the West positions at a height of 2
m (middle of plant canopy). The temperature data
sampled by the IR sensors in intervals of 1 s were
averaged in each 15 min and recorded in a
datalogger model CR7 (Campbell Scientific,
Logan, UT, USA). In addition, leaf temperature
was also measured with a Ni-CrNi thermocouple
(TC) built in a leaf clip holder (2030-B, Walz,
Effeltrich, Germany) of a portable fluorometer
model PAM-2000 (Walz). Measurements were
taken in the abaxial leaf surface of five plants
throughout the daylight period in intervals of
approximately 1 h. It is important to note that
TC measurements were conducted in fully
expanded, mature and exposed leaves.
Meteorological elements were also recorded
in both East and West sides of plant canopy at
the same position in which IR measurements
were taken. The environmental variables
monitored were: global radiation (Qg, W m -2),
air temperature (TAIR, ºC), and air vapor pressure
deficit (VPD, kPa). Qg was assessed by using a
pyranometer model LI-200 (LICOR, Lincoln NE,
USA), whereas TAIR and VPD were monitored
with an aspired psycrometer (cooper-constantan
thermocouples) built according to MARIN et al.
(2001). The temperature difference between leaf
and air (DT, ºC) was calculated using both IR
and TC measurements.
The above measurements were taken in
different times throughout the year, permitting
the evaluation of TLEAF in days with discrepant
energy availability. The meteorological
conditions of evaluation times are shown in Table
1, being the data recorded in an automatic
weather station (Centro Integrado de
Informações Agrometeorológicas, CIIAGRO –
IAC/APTA/SAA) located 500 m far from the
experimental area. From those data, it was
possible to compare the meteorological
conditions in the interior of citrus orchard with
the external environment.
Results and Discussion
Meteorological conditions
Days with different energy availability were
chosen for evaluating leaf temperature changes.
Daily global radiation varied from 31.4 (summer)
to 11.93 MJ m-2 d-1 (autumn), affecting directly the
air temperature (Table 1). In fact, maximum air
temperatures were higher than 30ºC between
December-04 and March-05, whereas the minimum
temperatures were found in 16-Sep-04. In Southeast
Brazil, the winter season (from June to September)
is characterized by low relative humidity and low
energy availability as well. The lower air temperature
during the winter is induced by less energy
availability, which is about 40% lower when
381
Rev. Bras. Agrometeorologia, v. 13, n.3, p.378-388, 2005
comparing June and February, in the State of São
Paulo (PEREIRA et al., 2002).
The transition between summer and winter
(i.e. autumn and spring seasons) had intermediate
levels of energy and humidity, as observed in
evaluation dates of October, November, March, and
April. As expected, the summer season (from
December to March) had the highest evaporative
demand, when ETo was higher than 5 mm d-1. This
is in accordance to the highest energy availability
during summer in the South hemisphere. Non
substantial changes were verified in daily mean wind
velocity among evaluation dates, with u2 varying
between 0.79 and 1.86 m s-1 (Table 1), which could
induce a significant and differential plant response
in a citrus orchard. However, the influence of wind
velocity is well apparent in ETo values (Table 1).
Based on differences in meteorological
conditions, we decided to show the daily variations
of environmental elements inside the studied citrus
orchard in the following dates: JD 296, 349, 48 and
119 (Figure 1). These days had different temporal
variations of nebulosity and different available
energy, determining the diurnal course of TAIR and
VPD as consequences (Figure 1). High incoming
radiation during the morning is expected to occur in
the East position, whereas the West position receives
solar energy predominantly during evenings. This
is true when there is low or none nebulosity, as
observed in JD 349 and 48 (Figure 1D,G). In fact,
the instantaneous solar energy (Qg) in summer
season (JD 349 and 48) was about the maximum
possible value in tropical regions, i.e. 1100 W m-2
(Figure 1D,G). On the other hand, the high
nebulosity did not cause significant differences in
available energy between canopy positions (Figure
1A,J).
When comparing canopy positions, differences
in TAIR and VPD were observed only in JD 349 and
48. It is important to consider that these small
discrepancies between the East and the West
positions in those variables may not represent
differential responses in citrus plants (Machado et
al., 2005), mainly under field condition. Higher TAIR
at the East position in relation to the West one was
only verified during the morning in JD 349, which
caused increase of VPD until the early evening
(Figure 1E,F).
Regarding the meteorological conditions in
the citrus orchard and in the weather station, we
observed that TAIR and Qg values are similar during
most of daytime (Figure 2A,B). Some data scattering
Table 1. Meteorological conditions* of the evaluation dates in Cordeirópolis, SP, Brazil.
Date
JD
Qg
(MJ m-2 d-1)
Tm
Tmax
(oC)
Tmin
RH
(%)
ETo**
(mm d-1)
u2
(m s-1)
16-Sep-04
260
14.17
18.84
26.02
11.66
63.77
2.90
1.120
22-Oct-04
296
17.42
21.52
28.74
14.29
79.89
3.50
1.522
19-Nov-04
324
16.08
22.46
27.20
17.71
83.11
3.47
1.419
14-Dec-04
349
31.41
22.96
30.13
15.79
67.50
6.63
1.864
17-Feb-05
48
25.52
24.61
34.63
14.58
76.68
5.46
1.265
29-Mar-05
88
21.71
22.17
30.01
14.33
76.31
4.19
1.208
29-Apr-05
119
11.93
20.09
26.27
13.91
83.36
2.22
0.793
02-Jun-05
153
13.29
20.05
26.55
13.55
81.22
2.42
1.143
* Data recorded by an automatic weather station located 500 m far from the experimental area. JD = Julian day; Qg =
global radiation; Tm, Tmax, Tmin = daily mean, maximum and minimum air temperature, respectively; RH = daily
mean relative humidity; u2 = daily mean wind velocity at 2 m of height.
** ETo = reference evapotranspiration by the Penman-Monteith method (ALLEN et al., 1998).
382
RIBEIRO, R. V., et al. - Leaf temperature in sweet orange plants under field condition: influence of meteorological elements
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circle) and the West (closed circle) canopy positions (2-m height) in the following dates: 22-Oct-04 (Julian day 296, AC); 14-Dec-04 (Julian day 349, D-F); 17-Feb-05 (Julian day 48, G-I); and 29-Apr-05 (Julian day 119, J-L). Citrus
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Figure 2. Relationship between meteorological elements measured inside a citrus orchard and in an automatic weather
station located 500 m far from the experimental area. Data shown represent measurements of global radiation (Qg, A), air
temperature (TAIR, B) and air vapor pressure deficit (VPD, C) taken at the East (open circle) and the West (closed circle)
canopy positions (2 m height). Citrus orchard is oriented in the North-South direction (Cordeirópolis, SP, Brazil).
shown in Figure 2A is due to the attenuation of solar
energy caused by parallel planting rows (NorthSouth orientation) of citrus orchard. So, Qg values
in the weather station were higher than those ones
observed inside citrus orchard, which happened
during early morning and late evening. In addition,
vapor pressure deficit was lower inside citrus orchard
than values found in the weather station in about
19.8-17.3%, depending on the canopy position
(Figure 2C). This fact suggests a wetter atmosphere
inside citrus orchard than in the weather station,
which is probably related to citrus transpiration.
383
Rev. Bras. Agrometeorologia, v. 13, n.3, p.378-388, 2005
Diurnal course of leaf temperature
High TLEAF values were verified in days with
high radiation loading (i.e. JD 349 and 48), when
leaf temperatures measured with IR and TC
techniques reached values about 35.5 and 39oC,
respectively (Figure 3). Despite measurement
technique, the maximum temperatures were
observed during afternoon in the West position in
days with low or none nebulosity, whereas the East
position showed higher TLEAF during morning. In
cloudy days (JD 296 and 119), non-significant
differences in leaf temperature between canopy
positions were detected.
The differences between TLEAF taken with IR
and TC techniques were probably induced by the
leaf exposure. As IR method gives an average
temperature of plant canopy, evaluating both
exposed and non-exposed leaves, the recorded
values are normally lower than those ones taken with
the TC method, which evaluated only exposed
leaves. It is reasonable to assume that the highest
TLEAF values will be found in those leaves, which
represent less than 20% of total leaf area in a crosssection of an orange tree canopy (COHEN &
FUCHS, 1987). As exposed leaves reached TLEAF
values higher than 39ºC, impairment of
physiological activity related to CO2 fixation is
expected to occur (RIBEIRO et al., 2004).
The optimum temperature for citrus growth is
between 25 and 31ºC, occurring growth impairment
at temperatures below 13ºC and above 36ºC
(REUTHER, 1973), whereas the highest leaf
photosynthesis is observed around 25ºC under
normal air CO2 concentration (RIBEIRO et al.,
2004). Under increasing temperature, photosynthesis
is initially affected by reduction of stomatal
conductance (due to high leaf-to-air vapor pressure
difference); and afterwards (>35ºC) by decrease of
both leaf mesophyll CO2 conductance (VU, 1999)
and carboxylation efficiency (RIBEIRO et al., 2004).
Therefore, one should take care when considering
the photosynthetic contribution of exposed leaves
to crop growth and production. Probably, these
leaves are the first ones to show reductions in
photosynthetic rates under constraint conditions (e.g.
drought, excessive radiation energy, extreme high
or low temperatures). Accordingly, MEDINA et al.
(2002) reported a large reduction in photoinhibition
of photosynthesis in exposed citrus leaves by
decreasing the incident solar energy during summer.
Differences in measurements of TLEAF taken
with IR and TC techniques were higher in conditions
of high energy availability (Figure 3C,E). Although
JD 119 had shown low available energy (Table 1),
there were no clouds causing decrease of the
incoming solar energy between 11 and 14 h, when
the highest TLEAF values were measured in exposed
leaves (Figure 3G). This caused a large discrepancy
between IR and TC measurements.
The difference between TLEAF and TAIR is a
meaningful tool to evaluate the heat dissipative
capacity in plants as well as their physiological
adaptation under certain environmental conditions
(SOUZA et al., 2004; SOUZA & RIBEIRO, 2005).
However, a clear discernment should be done when
studying plant species with large canopies, where
leaves have different light exposure. In TC
measurements (exposed leaves), DT (TLEAF-TAIR)
reached 8ºC during the early evening of JD 349
(Figure 3D), whereas the maximum DT measured
with IR technique was about 3ºC during the early
morning of JD 48 (Figure 3F). During most of
daylight period, DT varied between 3 and -3ºC when
considering the IR technique (Figure 3B,D,F,H).
Nevertheless, DT estimated from TC measurements
were positive during most of daylight period,
indicating leaf heating of exposed leaves.
SYVERTSEN & LLOYD (1994) pointed out that
DT in citrus plants can vary about 8-10ºC in warmsubtropical climates. We believe that these values
are probable referring to exposed leaves, as observed
in this study (Figure 3).
High DT values were found during morning at
the East position and during afternoon at the West
position. Both DT estimated with IR and TC
measurements showed the same pattern when there
was high available energy (Figure 3D,F). However,
non-significant differences between canopy
positions were verified in a cloudy day (Figure 3B),
when DT and TLEAF measured with TC and IR
techniques were less discrepant (Figure 3A,B).
Interestingly, the East position showed higher DT
than the West one, in a day with high concentration
of water vapor (fog) during the early morning
(Figure 1L), regardless of the measurement
technique (Figure 3H).
384
RIBEIRO, R. V., et al. - Leaf temperature in sweet orange plants under field condition: influence of meteorological elements
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Figure 3. Diurnal course of leaf temperature (A,C,E,G) and difference between leaf and air temperatures (B,D,F,H)
measured with infrared thermometers (IR, circles) and thermocouple (TC, triangles) in citrus plants under field condition.
Data shown represent measurements taken at the East (open symbols) and the West (closed symbols) canopy positions
(2-m height) in the following dates: 22-Oct-04 (Julian day 296, A-B); 14-Dec-04 (Julian day 349, C-D); 17-Feb-05
(Julian day 48, E-F); and 29-Apr-05 (Julian day 119, G-H). Citrus orchard is oriented in the North-South direction
(Cordeirópolis, SP, Brazil). Each point represents one replication in IR measurements and the mean value of five
replications ± SE in TC measurements. IR thermometers were positioned 0.2 m from plant canopy.
Probably, low transpiration rates caused high
DT values found during early morning in the East
position (Figure 3D,F), being this physiological
process the main heat dissipative strategy adopted
by plants (NOBEL, 1999; SOUZA et al., 2004;
SOUZA & RIBEIRO, 2005). After the onset of plant
transpiration due to the increase of evaporative
demand (Figure 2F,I), plant cooling was induced and
DT values remained negative during the hottest
hours of day (Figure 3D,F). On the other hand, high
DT values during evening were noticed in the West
position, remaining between 0 and 1ºC (Figure 3D,F)
or even negative (Figure 3B,H) depending on the
available energy. This fact indicates the great cooling
capacity of citrus plants, suggesting that leaf
transpiration is an important process leading to a
high energetic efficiency and avoiding constraint
temperatures (TLEAF above 35ºC). It is important to
consider that such process is dependent of adequate
soil water levels to be effective against leaf heating.
On the relationship between infra-red and
thermocouple measurements
The measurement of TLEAF using TC technique
was always equal or higher than those measurements
with IR technique (Figure 3). When comparing these
methods, TC tended to overestimate TLEAF in both
canopy positions (Figure 4). In fact, this
overestimation was higher in the East position. The
difference between IR and TC measurements were
in part due to the light exposure of evaluated leaves,
i.e., TC measurements evaluated exposed leaves,
where as IR techniques measured the temperature
of both exposed and non-exposed leaves.
Influence of meteorological elements on leaf
temperature
Both T LEAF measured with IR and TC
techniques showed similar responses to increasing
Qg (Figure 5A,C). There was a tendency of TLEAF
385
Rev. Bras. Agrometeorologia, v. 13, n.3, p.378-388, 2005
East
TC=0.89*IR+3.38
36 R=0.93, N=81, P<0.0001
40
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UDH 24
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DH 16
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TC=0.98*IR+1.49
R=0.93, N=81, P<0.0001
12
12
16
20 24 28 32 36
R
/HDIWHPSHUDWXUH,5 &
40
Figure 4. Relationship between leaf temperature
measured with infrared thermometers (IR) and
thermocouple (TC) in citrus plants under field condition.
Measurements were taken at the East (open circle) and
the West (closed circle) canopy positions (2-m height).
Citrus orchard is oriented in the North-South direction
(Cordeirópolis, SP, Brazil).
stability at Qg values higher than 600 W m-2 when
considered IR measurements (Figure 5A), presenting
reduction in data scattering. Besides showing high
TLEAF, exposed leaves had an increasing response to
Qg increase, not showing a defined ceiling even at
1100 W m-2 (Figure 5C).
Stomatal aperture is probably saturated at Qg
values of about 300 W m-2 (» photosynthetic photon
flux density of 600 mmol m-2 s-1), which could be
sufficient to permit adequate transpiration rates
(SYVERTSEN & LLOYD, 1994; MACHADO et al.,
2005) and plant cooling as consequence, i.e.
maintenance or decrease of TLEAF. However, TAIR and
VPD vary simultaneously to Qg, causing a synergetic
effect on stomatal physiology. At high evaporative
demand, decreases in stomatal conductance of acid
lime plants have been ascribed to the interaction of
above environmental variables (ANGELOCCI et al.,
2004).
In relation to the effects of VPD on TLEAF,
similar response patterns were also verified in IR and
TC measurements (Figure 5E,G). A rapid increase
was noticed when VPD changed from 0 to 0.5 kPa,
showing a positive linear relation between 0.5 and
2.3 kPa (Figure 5E). It is known that the main effect
of high VPD on gas exchange of citrus plants is via
stomatal mechanism (HALL et al., 1975; SINCLAIR
& ALLEN Jr., 1982; ANGELOCCI et al., 2004).
Reduction in stomatal conductance is verified at VPD
values higher than 1.5 kPa, being this response an
adaptive response to prevent plant dehydration
(SINCLAIR & ALLEN Jr., 1982; MACHADO et al.,
2002, 2005). Stomatal closure also causes plant
heating due to decreases of transpiration, leading to
increase of TLEAF, as evidenced by the linear increase
of TLEAF at VPD values higher than 1.0 kPa (Figure
5E). This fact is probably more pronounced in
exposed leaves (Figure 5G), where the VPD threshold
for stomatal closure may be different.
It is reasonable to assume that any plant
physiological response to changes in environmental
conditions happens in order to maintain the plant
growth and development. This physiological plasticity
was reported in citrus plants, in which changes in
quantum efficiency of exposed leaves happened in
order to maintain high values of apparent electron
transport rate and adequate level of heat dissipation
by non-photochemical processes (RIBEIRO et al.,
2005). The maintenance of canopy temperature, given
by IR measurements, close to TAIR (Figure 5I) is an
indication of TLEAF regulation by plants themselves.
Even in exposed leaves (Figure 5K), a close
relationship between TAIR and TLEAF was maintained.
In spite of environmental elements, higher DT
values were observed in exposed leaves, ranging
from -2.5 to 8ºC (Figure 5D,H,L) and being positive
most of daylight period. Non-significant differences
were found between canopy positions when
considered TC measurements. The smaller DT
amplitude in IR-based estimations (Figure 5B,F,J)
is in accordance to the assumption that exposed and
non-exposed leaves were evaluated. At high values
of Qg, VPD and TAIR, the DT values in the East
position were lower than ones in the West side
(Figure 5B,F,J). This suggests that the East position
has higher heat dissipative capacity than the West
one during the warmest hours of day (11 to 15 h),
when there are the highest Qg, VPD and TAIR values
(Figure 1). Together with photosynthesis,
transpiration is an essential factor determining citrus
orchard productivity, being its spatial distribution
in canopy an important aspect to be considered
386
RIBEIRO, R. V., et al. - Leaf temperature in sweet orange plants under field condition: influence of meteorological elements
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Figure 5. Leaf temperature (A,E,I,C,G,K) and difference between leaf and air temperatures (B,F,J,D,H,L) measured with infrared
thermometers (A,E,I,B,F,J) and thermocouple (C,G,K,D,H,L) as functions of instantaneous global radiation (A-D), air vapor
pressure deficit (E-H) and air temperature (I-L) in citrus plants under field condition. Data shown represent measurements taken
at the East (open circles) and the West (closed circles) canopy positions (2-m height). Citrus orchard is oriented in the NorthSouth direction (Cordeirópolis, SP, Brazil). Each point represents one replication in IR measurements and the mean value of five
replications ± SE in TC measurements. IR thermometers were positioned 0.2 m from plant canopy.
(COHEN & FUCHS, 1987; COHEN et al., 1987).
As plant cooling is directly related to transpiration,
we can argue that the East side probably had higher
transpiration rate than the West position. However,
further studies for evaluating this hypothesis should
be done.
Conclusions
The highest TLEAF values are found during
afternoon and in summer season, which is related to
the daily and seasonal pattern of available radiation
loading. In studies involving the evaluation of TLEAF
in citrus plants, it is important to consider the level
of solar radiation exposure in leaves due to the high
heterogeneity of citrus canopy in relation to leaf age
and position. In exposed leaves, changes in TLEAF
are directly related to the variation of TAIR, Qg and
VPD, showing more or less data scattering
depending on environmental variable considered. In
relation to the evaluation of TLEAF considering both
exposed and non-exposed leaves (given by the
infrared technique), it is shown that TLEAF is directly
affected by TAIR. However, changes in TLEAF are more
387
Rev. Bras. Agrometeorologia, v. 13, n.3, p.378-388, 2005
accentuate under low Qg and VPD values, occurring
a tendency of TLEAF saturation only for Qg.
Acknowledgements
The authors are grateful to Drs. Ricardo F.
Oliveira and Luiz R. Angelocci (ESALQ/USP,
Brazil) for helpful discussions during the
experimental planning. Thanks to Drs. Marcos A.
Machado and Orivaldo Brunini (IAC/APTA/SAA,
Brazil) for research facilities; and José Zanetti Jr.
for the excellent field assistance. This work was
supported by grants from the Fundação de Amparo
à Pesquisa do Estado de São Paulo – FAPESP
(R.V.R.), Conselho Nacional de Desenvolvimento
Científico e Tecnológico – CNPq (E.C.M.), and
Coordenadoria de Aperfeiçoamento de Pessoal de
Nível Superior – CAPES (M.G.S.).
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