TRABAJO FIN DE MASTER
Application of thermography for the assessment ofvineyard water status
Helga Ochagavía Orbegozo
PROGRAMA DE DOCTORADO ECOSISTEMAS AGRÍCOLAS SOSTENIBLES
Tutor: Javier Tardáguila LasoFacultad de Ciencias, Estudios Agroalimentarios e Informática
Curso 2010-2011
© El autor© Universidad de La Rioja, Servicio de Publicaciones, 2012
publicaciones.unirioja.esE-mail: [email protected]
Application of thermography for the assessment of vineyard water status,trabajo final de estudios
de Helga Ochagavía Orbegozo, dirigido por Javier Tardáguila Laso (publicado por laUniversidad de La Rioja), se difunde bajo una Licencia
Creative Commons Reconocimiento-NoComercial-SinObraDerivada 3.0 Unported.Permisos que vayan más allá de lo cubierto por esta licencia pueden solicitarse a los
titulares del copyright.
Trabajo de investigación del Programa de Doctorado: “Ecosistemas Agrícolas Sostenibles”, Universidad de La Rioja
Author: Helga Ochagavía Orbegozo
Led by: Dr. Javier Tardáguila Laso
Curso académico: 2010-2011
Application of thermography for the assessment of vineyard water status
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Contents
Acknowledgments…………………………………………………………………………………..2
Abstract……………………………………………………………………………………………….3-4
1. Introduction……………………………………………………….…………………….…...5-9
2. Objectives…………………………………………………………………….……....…..10-11
3. Material and methods…………………………………................................12-17
3.1 Experimental layout………………………….…………..13-14 3.2 Thermal imaging…………………………………………...14-15 3.3 Stress indices and references surface temperatures………………………………………………..…...15-16 3.4 Stomatal conductance and stem water potential measurements…………………………………………………..…….16 3.5 Statistical analysis……………..……………..……..….……..17
4. Results……………………………………………………………………………………....18-30
4.1 Methods of extraction of thermal data: Temperatures of several sun exposed leaves versus Temperatures of regions of interest of the canopy ………………………………………………………………………19-20
4.2 Relationship between temperature, stomatal conductance and stem water potential: Timing effect…………..……………………………………..21-24 4.3 Relationship between stress indices, stomatal conductance and stem water potential: …………………..…………..……………………………..……...25-28 4.4 Frequency distributions of temperatures….....29-30
5. Discussion…………………………………………………………………………..….....31-35
6. Conclusions………………………………………………………………………………..36-37
7. References………………………………………………………………………………...38-41
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Acknowledgments
First, thanks to Televitis group (Dr. Javier Tardáguila, Dra. Maria Paz Diago, Javier
Baluja and Juan Antonio Blanco) which gave me the opportunity to work under their
guidance.
Thanks to Dr. Javier Tardáguila to lead this work.
Thanks to Dr. Olga Grant of University of Maynooth, Ireland, to welcome me in her lab.
I feel truly fortunate to work with her during three months. I really appreciate her help
and her infinite patience.
This project was supported by MoDeM_IVM Project: A web-based system for real-time
Monitoring and Decision Making for Integrated Vineyard Management.
My parents who I admire because they always are with me and support me in all my
decisions: Mª Pilar y Jesus.
My relatives which only with their presence make me happy, Lara, Sergio, Imanol,
Mamen, Abel, Eduardo, Naiara y Gonzalo.
My friends who do my life as enjoyable experience: Andrea, Raquel, Clara, Diana,
Daniel, Alberto, Ainhoa, Sonia y Esteban.
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Abstract
The applicability of thermography to assessment of vineyard water status was
determined in grapevines subjected to different water availability. Thermal images of
the sunlit side of canopies (lateral thermography) were compared with images taken
above the vine (zenithal thermography). Canopy temperature was determined either
by averaging the temperature of several individually selected sun-exposed leaves or by
extracting the average temperature of a region of canopy. Wet and dry artificial
‘leaves’ were included in images as references for calculation of the stomatal
conductance index (IG) and the crop water stress index (CWSI). Thermal data were
compared with stem water potential and stomatal conductance of the same vines.
Similar relationships between these physiological measurements and either method of
temperature extraction indicate the two methods are equally useful. Lateral
thermography in the afternoon was more useful than in the morning. Zenithal
temperatures were similarly indicative of vine water status compared to lateral
imaging.
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Abstract
Se determinó la aplicabilidad de la termografía para evaluar el estado hídrico del
viñedo en cepas sujetas a diferente disponibilidad hídrica. Las imágenes térmicas de la
parte iluminada de la canopy (termografía lateral) fueron comparadas con las
imágenes tomadas en la parte superior de la canopy (termografía cenital). Se
determinó la temperatura de la canopy mediante la temperatura media de diversas
hojas iluminadas seleccionadas individualmente y a partir de la temperatura media de
una región de la canopy. En cada imagen se incluyeron como referencias `hojas´
artificiales húmedas y secas con el fin de calcular el índice de conductancia estomática
(IG) y el índice de estrés hídrico del cultivo (CWSI). Se compararon los datos térmicos
con el potencial hídrico del tallo y con la conductancia estomática de las mismas cepas.
Las relaciones similares entre estas dos medidas fisiológicas con cualquiera de los dos
métodos de extracción de temperatura indican que los dos métodos son igualmente
útiles. La termografía lateral de la tarde fue más óptima que la de la mañana. La
termografía cenital fue similarmente indicativa del estado hídrico del viñedo al
compararla con las imágenes laterales.
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1. INTRODUCTION
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The most important wine regions in the world are located in areas which are
seasonally dry with high evaporative demand and low water availability. In recent
decades, these characteristics are being aggravated by climate change. Alterations in
precipitation patterns and air temperature induce changes in runoff and water
availability. At the regional level, irrigation demand will be increased due to those
climate changes, with water becoming the main factor limiting production (IPCC 2007).
Both grape yield and quality are influenced by water availability (Chaves et al. 2007;
Matthews and Anderson 1988).
Viticulturists are demanding solutions to the changing environmental situation and the
most widespread strategy to deal with these changes is based on more irrigation to
stabilize yield and improve subsequent wine quality (Chaves et al. 2007). Excess
irrigation may result in high canopy density causing shade in grape clusters. This may
lead to lower grape quality, affecting colour and reducing sugar content. Severe
drought, on other hand, may lead to leaf stomatal closure with lowered
photosynthetic rate, which negatively affects some berry quality characteristics
(Chaves et al. 2007). An intermediate solution between the two situations described
above is deficit irrigation, where the crop is given sufficient irrigation to maintain
quality but less than 100% evapotranspiration either throughout the growing season or
in a specific phenological stage. With this irrigation management strategy, a balance
between vegetative and reproductive development needed to improve grape quality is
achieved (Dry et al. 2001).
The measurement of stem water potential is one of the most used methods for
monitoring water stress in the vineyard, as it was an early indicator of water limitation
(Choné et al. 2001). The procedure, however, is destructive and time-consuming and
therefore unsuited to detecting spatial variation in water status within a large
vineyard. Sap flow meters are another tool to determine water stress, but have the
disadvantage of possibly interfering with plant performance (Fernandez et al. 2001).
Dendrometers also allow evaluation of the amount of available water through
continuous measurement of the stem diameter variations (Intrigliolo and Castel 2007),
but they require a complicated installation and they need maintenance. Imaging
techniques, such as thermal and fluorescence imaging can be used to non-
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destructively assess spatial and temporal changes when the plant is stressed (Chaerle
and Van Der Straeten 2000).
Fig. 1 Different strategies to evaluate vineyard water stress. Water potential measurement (a), sap flow
(b), dendrometers (c) and thermography (d).
Thermography allows the visualization of differences in surface temperature from
emitted infrared radiation. This technique relies on the fact that when water is lost
through the stomata, leaf temperature decreases, but when stomata close, leaf
temperature increases (Costa et al. 2010). Thus, leaf or canopy temperatures can be
considered as an indicator of stomatal conductance and hence canopy stress (Jones et
al. 2002). Leaf or canopy temperatures are sensitive to environmental factors. Stress
indices have been developed to remove the impact of environmental variation. The
“Crop Water Stress Index” (CWSI) which was elaborated by Idso et al. (1981) involves
normalization of both the effects of atmospheric humidity and the expected
temperature of a well-watered crop. It was adjusted to use wet and dry reference
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surfaces by Jones et al. (1999). Alternatively, the stomatal conductance index (IG) was
firstly proposed by Jones et al. (1999), and it is proportional to stomatal conductance.
The choice of appropriate references to calculate these stress indices is important.
Vine leaves used as references have similar radiometric and aerodynamic properties to
the canopy studied and therefore are more suitable than wet and dry white filter
paper references (Jones et al. 2002). However, Grant et al. (2007) showed that the use
of individual wet and dry leaves as references to calculate stress indices might not be
suitable for whole canopies and differences of time between spraying the wet leaves
and taking the image may cause errors. For these reasons, it was decided to explore an
alternative to the use of wet and dry reference leaves. A new approach to the use of
references surfaces is presented in this paper. This alternative approach uses two
artificial leaves composed of platinum, one of which is covered in a black cotton which
continually absorbs water from a small reservoir. This prevents drying out of the
artificial wet leaf and removes the need to keep spraying the wet reference. The
references are placed under the same environmental conditions as the plant canopy of
interest and thus are exposed to the same solar radiation, air temperature, relative
humidity and air movements.
Recently, thermography is being used from mobile or aerial platforms in order to
expand the area of view and to remotely assess the water stress of several crops. In
cotton, a thermal camera was mounted at a height of a 5 m above the ground,
increasing the field of view at canopy level (Cohen et al. 2005). In olives trees, Ben-Gal
et al. (2009) used two cameras (thermal and visible cameras) mounted on a truck-
crane about 15 m above the canopy. A similar approach was applied by Möller et al.
(2007) in grapevine. The resolution obtained permitted differentiating between leaves
and soil, and distinguishing sun-exposed leaves from shaded leaves.
In another vineyard, Jones et al. (2009) made thermal and visible images from a
balloon at a height of 80 m.
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Fig. 2 Thermal and digital images of vineyard taken from a balloon (Jones et al. unpubl. data)
A vineyard is not a continuous crop, vines are planted in rows, and canopy width is
usually within the range 30-50 centimeters for VSP trellis system (vertically shoot
positioned vines). Generally, leaves face into the row rather than upwards. Most
published data from thermal imaging of grapevine relates to images taken facing the
rows (lateral thermography) (Grant et al. 2007; Jones et al. 2002; Loveys et al. 1999),
but whether images taken from above the canopy can capture information of vine
status and crop stress equally well still needs to be explored. Thus, it is important to
compare thermal images from above the canopy (zenithal thermography) and thermal
images taken facing the vine row (lateral thermography) for the determination of
vineyard water status.
One of the problems that thermometry presents is the separation of temperatures of
leaves of interest from non- leaf temperatures (temperatures of soil, sky, trunk…).
With the development of thermal imaging along with increasingly sophisticated image
analysis software this is no longer a problem. Different approaches have been
employed to exclude non-leaf temperatures (Giuliani and Flore 2002; Leinonen and
Jones 2004).
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2. OBJECTIVES
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The main goal of this study was to explore zenithal and lateral thermography for
determining vine (Vitis vinifera L.) water status under field conditions. Therefore, the
relationships between canopy temperatures, or indices derived from these
temperatures and stomatal conductance and water potential were determined. In
addition:
i. The time of the day for acquisition thermal images were explored.
ii. The extraction of canopy temperatures in image analysis, were compared from
several sun exposed leaves and from a region of interest of the canopy which
contain an area of vine leaves.
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3. MATERIALS AND METHODS
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Experimental layout
The field experiments were carried out in 2010 in a commercial vineyard, located in
Tudelilla, La Rioja, Spain. An experimental plot (42° 18' 21.00''N, 2° 7' 18.58'' W) of
Tempranillo grapevine (Vitis vinifera L.) grafted in 2002 onto 110R rootstock and
trained to vertically shoot positioned (VSP) system and spur-pruned to 12 nodes per
vine was studied. The vines had a between-row and within-row spacing of 2.6 1.2 m
respectively. The orientation of rows was NE-SW. Shoot trimming was performed once
in June.
The shallow soil at the site had a light clay texture and low fertility. The climate of this
area was Mediterranean and semiarid, with hot summers and average annual rainfall
of 400 mm, with very scarce precipitation during the summer.
Fig. 3 Commercial vineyard where the field experiments were carried out, located in Tudelilla, La Rioja,
Spain
Four different irrigation regimes were applied to develop a large range of water status
in the vineyard: Rain-fed (non-irrigated) (RF), standard irrigation (SI), moderate
irrigation (MI) and full irrigation (FI). Irrigation was applied every day from 1 July until
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15 September and frequency varied from 1, 2 and 3 hours per day for standard
irrigation, moderate irrigated and full irrigation, respectively. Drip irrigation was
applied with pressure-compensated emitters of 2.0 l h-1, separated 0.80 m in the vine
rows. Each irrigation regime was tested in four experimental rows. Five vines per
irrigation regime were selected randomly within two central rows, marked and used as
experimental plants for measurements.
Thermal imaging
Thermal images were taken with a thermal camera (ThermaCAM P640, FLIR Systems,
Sweden) that operates in the wavebands 7.5-13 µm, has a thermal resolution of 0.06°C
and accuracy of ± 2°C and produces pictures with spatial resolution of 640 × 480 pixels.
The thermal camera also provided digital colour images (RGB). Lateral images in the
morning were obtained at 10:00 h and lateral images in the afternoon at 16:30 h on 4
and 5 September 2010. Zenithal images were taken at 14:30 on 4 September 2010.
Lateral images were taken from the sun-exposed side of the canopies at 1.5 m from
the canopies, and zenithal images of the top of the canopies were captured from a
truck-crane at 1.5 m above the canopies.
Fig. 4 Lateral photograph (a) and thermal image (b) of non-irrigated grapevine canopies.
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Fig. 5 Zenithal photograph (a) and thermal image (b) of non-irrigated grapevine canopies.
Five replicate plants were imaged per irrigation regime. All images were analyzed with
ThermaCAM Researcher Pro 2.9 software (FLIR Systems, Sweden). For each image
analyzed, the background reflectance temperature required for the calculation of
object temperatures was estimated as the radiative temperature of a crumpled
aluminum foil sheet placed in the same position as the object being viewed, with
emissivity set at 1.0. Emissivity for measurements of leaves and plant canopies was set
at 0.96 according to Jones (2004). By comparison of the thermal and RGB images of
each vine, four fully exposed leaves per vine were selected to obtain average
temperatures. In addition, in each image, a region of interest (ROI) of the canopy of
213 × 160 pixels approximately that included an area of vine leaves was selected and
average temperatures and frequency distributions of pixels in that ROI of the canopy
were calculated.
Stress indices and references surface temperatures
Wet and dry references temperatures were used for the derivation of stress indices,
which do not required environmental information. An evaporimeter (EvapoSensor,
Skye Instruments Ltd, Powys, UK) was used to provide artificial leaves which act as wet
and dry references. Wet and dry references were used to simulate leaves with open
and fully closed stomata, respectively. The artificial leaves were composed of black
metal (platinum), 5 cm long × 1 cm wide and 0.5 cm thick. The wet artificial leaf was
maintained wet by means of a wick of black cotton which continuously absorbs water
from a small reservoir, which was filled with distilled water. The evaporimeter was
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placed on handmade holders with the artificial leaves facing the same direction as the
canopy of interest.
The temperatures of these references were obtained from the thermal images and
were used in conjunction with canopy temperature to calculate the thermal indices.
The crop water stress index (CWSI) was calculated as:
CWSI= (Tcanopy─Twet) / (Tdry─Twet) (Idso et al. 1981)
and the stomatal conductance index (IG) was calculated as
IG= (Tdry─Tcanopy) / (Tcanopy─Twet) (Jones et al. 1999),
where Tcanopy was the mean temperature of the leaf area of the experimental plant, Tdry
was the temperature of the dry reference and Twet was the temperature of the wet
reference.
Stomatal conductance and stem water potential measurements
Stomatal conductance (gs) of two leaves per vine located within the area captured in
the thermal image on five vines per irrigation regime, was measured immediately after
each lateral thermal infrared image using a gas-exchange system (LC pro+, ADC, USA),
at 10:00 in the morning and 16:30 in the afternoon.
Stem water potential (Ψstem) was measured at midday (14:00, solar noon) using a
Scholander-type pressure chamber (model 600, PMS Instrument Company, USA) on
two leaves per vine, on five vines per irrigation regime. These physiological parameters
were measured on sun-exposed fully mature main leaves.
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Statistical analyses
The coefficient of determination (R2) and the significance of the correlations was
tested, using InfoStat software (Professional Edition 2010, Córdoba, Argentina), to
determine the relationship between canopy temperatures, stress indices and the
different physiological parameters.
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4. RESULTS
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Methods of extraction of thermal data: Temperatures of several sun exposed leaves
versus Temperatures of regions of interest of the canopy
The different methods of extraction of thermal data in the image analysis were
compared for lateral images in the morning, lateral images in the afternoon and for
zenithal images on 4 September (Fig. 6).
Strong relationships were found between the two methods of extraction (Fig. 6), but
average temperatures of regions of interest of the canopy were cooler than average
temperatures of several sun exposed leaves, indicating that selection of regions of
interest of the canopy might contain shaded leaves, which decrease average
temperatures, whilst that selection of several sun-exposed leaves presented higher
temperatures than shaded leaves.
The relationships between canopy temperatures extracted from both methods of
extraction, both stress indices (IG and CWSI) calculated from those canopy
temperatures and both physiological parameters (stomatal conductance and water
potential) will be described in the next sections.
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Fig. 6 Correlation between average canopy temperatures of several leaves and average canopy temperatures of an area of vine leaves on 4 September (A, B, C) (n=17-20) for lateral canopy in the morning (A), lateral canopy in the afternoon (B) and zenithal canopy (C)
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Relationship between temperature, stomatal conductance and stem water potential:
Timing effect
Average canopy temperatures of the region of interest of the canopy were not
significantly correlated with either stomatal conductance (Fig. 7A) or stem water
potential (Fig. 7B) for the lateral canopy in the morning on 4 September. However, in
the thermal data measured on 5 September, average canopy temperatures of the
regions of interest of the canopy were highly significantly correlated with stomatal
conductance (Fig. 7E) and water potential (Fig. 7F) for this time of thermal image
acquisition.
When the extraction of average canopy temperatures was carried out from selection
of several sun- exposed leaves, canopy temperature again was not significantly
correlated with stomatal conductance or stem water potential on 4 September during
the morning (Fig. 7C and 7D, respectively). Nevertheless, on 5 September during the
morning, the correlations between average canopy temperatures (extracted from a
selection of several leaves) and both physiological parameters (stomatal conductance
and stem water potential) were strongly significant (Fig. 7G and 7H, respectively). In
this date, the relationships between average canopy temperatures of a ROI of lateral
canopy in the morning with both physiological parameters was stronger than the
correlations obtained when average canopy temperatures of several sun exposed
leaves was used.
Average canopy temperatures of a region of interest of the canopy and stomatal
conductance were highly significant for both lateral canopies in the afternoon and for
zenithal canopy on 4 September (Fig. 7A). Moreover, both lateral temperature in the
afternoon and zenithal temperature were negatively correlated with stem water
potential (Fig. 7B). Stem water potential exhibited values between ─0.7 MPa and ─1.7
MPa, indicating that some vines were not under water deficit (≥─1.0 MPa) and other
vines showed intense water deficit (≤─1.6 MPa). Strong correlations between average
canopy temperatures of ROI of the canopy and both physiological parameters (gs and
Ψstem) were also found on 5 September in the afternoon (Fig. 7E and 7F, respectively).
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Fig. 7 Correlations between stomatal conductance, gs (A, C ,E , G) or stem water potential, Ψstem (B, D, F, H) measured on 4 (A, B, C, D) and 5 (E, F, G, H) September 2010 and canopy temperatures (n=17-20). Canopy temperatures represent the average temperature of a region of interest (ROI) of the canopy of 213 × 160 pixels, approximately (A, B, E, F) and of several sun exposed leaves (C, D, G, H). In each graph, R
2 values from top to bottom respectively correspond to lateral canopy temperature in the afternoon (
open circles), zenithal canopy temperature ( triangles) and lateral canopy temperature in the morning ( filled circles).
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Stronger coefficients of determination were found between canopy temperature and
stomatal conductance/water potential from lateral canopy in the afternoon than
zenithal canopies (Fig. 7A and 7B). Lateral canopy temperatures in the afternoon were
higher than both lateral canopy temperatures in the morning and zenithal canopy
temperatures. In the afternoon, lateral canopy temperatures reached 38°C, whereas
lateral canopy temperatures in the morning and zenithal canopy temperatures peaked
at 28°C and 33°C, respectively. The relationship between canopy temperature and
stomatal conductance and stem water potential was similar for lateral canopy in the
afternoon and zenithal images (Fig. 7A and 7B):
Canopy temperature = ─0.03 gs + 36.2 (lateral in the afternoon)
and
Canopy temperature = ─0.03 gs + 32.7 (zenithal)
and
Canopy temperature = ─5.51stem + 26.7 (lateral in the afternoon)
and
Canopy temperature = ─4.50stem + 24.7 (zenithal) (Fig. 7A and 7B),
where canopy temperature is measured in °C and gs in mmol m-2 s-1 and stem in MPa.
Average canopy temperatures in lateral canopies in the afternoon obtained from
several sun exposed leaves were significantly correlated with stomatal conductance
and with stem water potential on 4 September (Fig. 7C and 7D, respectively). During
the afternoon, higher coefficients of determination were found in the regressions that
employed average canopy temperatures of several sun exposed leaves on 4
September.
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However, no significant correlation was presented between the average canopy
temperatures of several leaves and either stomatal conductance or stem water
potential on 5 September during the afternoon (Fig. 7G and 7H, respectively). For this
date, the correlations were higher when average temperatures of a region of interest
of the canopy were used.
Average canopy temperatures extracted of several leaves corresponding zenithal
canopy on 4 September presented high correlations with stomatal conductance and
with stem water potential (Fig. 7C and 7D, respectively). Similar coefficients of
determination (R2) were found for the two methods of extraction. Overall, neither
method of extraction appears to be preferable to the other.
As expected from these results, average temperatures of lateral canopy in the
afternoon and average temperatures of zenithal canopy were significantly correlated
for both methods of extraction of canopy temperatures, showing similar coefficients of
determination (R2) (Fig. 8).
Fig. 8 Correlation between zenithal canopy temperature and lateral canopy temperature in the afternoon on 4 September (n=15-19) obtained from two different methods of extraction: average canopy temperatures of several sun exposed leaves ( filled circles) and average canopy temperatures of a region of interest of canopy ( open circles).
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Relationship between stress indices, stomatal conductance and stem water potential
In the morning of 4 September IG and CWSI were not significantly correlated either
with stomatal conductance (Fig. 9A and 9C, respectively) or stem water potential (Fig.
9B and 9D, respectively). However, statistically significant correlations between both
stress indices and both physiological parameters were found on 5 September with the
extraction of temperature from a region of interest of the canopy (Fig. 10A, 10B and
10C), with the exception of the relationship between CWSI and water potential (Fig.
10D). IG and CWSI obtained from several sun exposed leaves which were within the
lateral canopy in the morning were not significantly correlated either with stomatal
conductance (Fig. 9E and 9G, respectively) or stem water potential on 4 September
(Fig. 9F and 9H, respectively). However, IG and CWSI indices from the average
temperature of several sun exposed were significantly correlated with both
physiological parameters (gs and Ψstem) on 5 September in the morning (Fig. 10E, 10F,
10G and 10H, respectively). For this date, generally, the correlations were higher when
average temperatures of region of interest of canopy were used, compared to the
average temperature of several sun exposed leaves.
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Fig. 9 Correlations between stomatal conductance, gs (A, C, E, G) or stem water potential, Ψstem (B, D, F, H) and IG (A, B, E, F) and CWSI (C, D, G, H) on 4 September 2010 (n=15-19). Canopy temperatures represent the average temperature of a region of interest (ROI) of the canopy of 213 × 160 pixels, approximately (A, B, C, D) and of several sun exposed leaves (E, F, G, H). In each graph, R2 values from top to bottom respectively correspond to lateral canopy temperature in the afternoon ( open circles), zenithal canopy temperature ( triangles) and lateral canopy temperature in the morning ( filled circles).
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IG obtained from lateral temperatures in the afternoon from a ROI of the canopy
showed a significant correlation with stomatal conductance on 4 September (Fig. 9A).
For the same day, significant correlation was exhibited between IG obtained from
zenithal temperatures and stomatal conductance (Fig. 9A). Furthermore, IG indices
calculated from both lateral and zenithal temperatures in the afternoon presented a
significant correlation with stem water potential (Fig. 9B). CWSI indices calculated with
both lateral and zenithal temperatures in the afternoon from a region of interest of the
canopy showed strong correlations with stomatal conductance and with stem water
potential (Fig. 9C and 9D, respectively). The relationship between both thermal indices
(IG and CWSI) and both physiological parameters (gs and Ψstem) was strongly significant
during the afternoon on 5 September (Fig. 10A, 10B, 10C and 10D) when a region of
interest of canopy was extracted.
During the afternoon, for both dates, significant correlations were found between both
thermal indices (IG and CWSI) derived from the temperature of several sun exposed
leaves and both physiological parameters (gs and Ψstem) (Fig. 9 and 10 respectively). It
was not clear that either method of temperature extraction from the images was
preferable to evaluate vine water status in the image analysis on 4 September during
the afternoon. However, the correlations were higher when average temperatures
were obtained from a ROI of canopy on 5 September.
The stress indices (IG and CWSI) obtained from zenithal canopy temperatures with
average temperatures of several sun exposed leaves presented significant correlations
with stomatal conductance and with water potential on 4 September (Fig. 9). Neither
extraction method appeared preferable for detection of vine water status using the
stress indices.
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Fig. 10 Correlations between stomatal conductance, gs (A, C, E, G) or stem water potential, Ψstem (B, D, F, H) measured, IG (A, B, E, F) and CWSI (C, D, G, H) on 5 September 2010 (n=17-18). Canopy temperatures represent the average temperature of a region of interest (ROI) of the canopy of 213 × 160 pixels, approximately (A, B, C, D) and of several sun exposed leaves (E, F, G, H). In each graph, R
2 values from
top to bottom respectively correspond to lateral canopy temperature in the afternoon ( open circles) and lateral canopy temperature in the morning ( filled circles).
- 29 -
Frequency distributions of temperatures
Selecting a ROI of canopy allowed analyzing the frequency distribution of
temperatures in the selected area. In all irrigation regimes, lateral canopy
temperatures in the morning were cooler than lateral canopy temperatures in the
afternoon and zenithal canopy temperatures. Highest temperatures were presented in
lateral canopies in the afternoon. As expected, there was a variation between
irrigation regimes, with rain-fed (RF) showing the highest temperatures while full
irrigation (FI) showing the lowest temperatures. These differences of temperature
between irrigation regimes were not clearly observed in lateral canopies in the
morning, which showed similar mean and variability (Fig. 11).
Frequency distributions of pixel temperatures within canopy showed different patterns
in different irrigation regimes. Generally, a higher range of distribution of pixel
temperatures within canopies was found for lateral canopy temperatures in the
morning (CV=9.49-11.58%), indicating more variation. For all irrigation regimes, the
lowest ranges of distribution of pixels temperature within canopies were presented in
zenithal canopy temperatures. Negative kurtosis was found for all irrigation regimes of
the lateral part of the canopy in the morning and in the afternoon, with the exception
of non irrigated (RF) regime in the afternoon i.e. the distribution was flattened
(platykurtic) in comparison with a normal distribution; all irrigation regimes in the
zenithal canopy and in the non irrigated (RF) regime for the lateral canopy in the
afternoon showed above 0 values of kurtosis i.e. the distribution is leptokurtic (higher
peak in comparison with normal distribution) (Fig. 11). The skewness values for all
irrigation regimes showed slight asymmetry, with higher values in the zenithal part of
the canopy.
When the distribution of pixel temperatures was compared between irrigation
regimes, it was observed that full irrigation (FI) presented smaller variation of thermal
distribution than other irrigation regimes.
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Fig. 11 Frequency of pixel temperatures obtained within a region of interest (ROI) of the canopy in five vines in each water regime (rain-fed, non-irrigated -RF-, standard irrigation -SI-, moderate irrigated -MI- and fully irrigated -FI-) for lateral canopy in the morning (lateral AM), lateral canopy in the afternoon (lateral PM) and zenithal canopy (zenithal) on 4 September.
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5. DISCUSSION
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Extraction of temperatures
The main problem generated in the extraction of thermal data from images is the need
to separate non-leaf material and background soil from leaf material. Different
approaches to the elimination of background temperatures have been explored.
Giuliani and Flore (2000) used a custom-written program based on thermal histograms
to process the digital images and exclude background temperatures. Jones et al. (2002)
discussed the use of reference surfaces (dry and wet) as limits to remove non-leaf
material from histograms. Other approaches required the analysis of individual pixels
in each image, such us the addition of the mean temperature of the reference image
to each pixel in the analyzed image, the transformation of each pixel in 8-bit gray-scale
image from an equation, the separation of canopy pixel from soil pixel by thresholds
related with air temperature (Alchanatis et al. 2010; Leinonen and Jones 2004; Meron
et al. 2010).
In our case, two approaches were explored, the use of average temperatures of
several sun exposed leaves and average temperatures of a ROI of the canopy. The use
of average temperatures of several sun exposed leaves involved checking the visible
image corresponding to each thermal image in order to select different leaves
manually.
Generally, analysis of the thermal data obtained on two measurement dates on
different parts (lateral and zenithal) of the canopy suggested that stronger correlations
with physiological data were presented when thermal data were obtained from a ROI
of the canopy. Average temperatures of a ROI were lower than average temperatures
of several leaves, supporting the idea that selection of a ROI of the canopy might
contain shaded leaves that decrease the average temperature of the area. The higher
correlations with stomatal conductance/water potential for a ROI of the canopy than
several sun exposed leaves might be explained by the sensitivity of canopy
temperature to stomatal conductance for sunlit canopy. There was less variability of
temperatures between shaded leaves than between sun-exposed leaves (Jones et al.
2002). However, Leinonen and Jones (2004) in a later study showed that the
temperature variance of the sunlit leaves was lower than shaded leaves. These results
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were obtained from a method that requires the manipulation of individual pixels,
allowing the calculation of temperature distribution of sun exposed leaves and shaded
leaves separately. This could explain the contradictory results obtained by Jones et al.
(2002), where temperature distribution were extracted manually from large areas of
the canopy.
The use of a ROI of the canopy to extract thermal data might present limitations, as
the stomata tend to be more closed in shaded leaves, hence smaller ranges of
stomatal conductances and temperature can be expected (Jones et al. 2002), but also
might present advantages for automated image processing (Jimenez-Bello et al. 2011).
Also, ROI analysis appeared to be a more objective method than to manually select
several sun exposed leaves. Furthermore, the ROI method can provide the frequency
distribution of temperatures of this selected ROI.
Time of imaging
During the morning, the relationships between thermal data obtained and both
physiological parameters were not significant on 4 September. However, they were
highly significant on 5 September. These contradictory results indicate that morning
measurement might not be accurate to detect vineyard water status. This might be
caused by great changes in the climatic conditions when thermal images were taken in
the morning hours (data not shown). When thermal images were obtained in the
afternoon hours, climate conditions were more stable. On the two dates (4 and 5
September) high correlations were found between thermal data in the afternoon and
stomatal conductance and stem water potential. These results demonstrate that time
of imaging affects the utility of thermal imaging for evaluation of water stress in the
field. In this study, afternoon imaging was more reliable to assess vineyard water
status.
Stronger correlations with physiological data were always found with thermal data
obtained from lateral canopies in the afternoon, compared to lateral canopies in the
morning or zenithal canopies. Furthermore, higher temperatures were found for the
- 34 -
lateral canopies in the afternoon. This may be because in the zenithal part of the
canopy, most leaves were not facing the sun at the time of measurement, whilst leaf
angle in the lateral canopy was directed towards the sun, resulting in higher leaf
temperatures. Fuchs (1990) pointed out that water stress is better monitored if
thermal images are taken in the same direction as the sun and an angle similar to the
solar zenith angle. In order to decrease the effect of individual leaf angle in the canopy
temperature, average temperatures of several leaves were taken. Average
temperatures of several leaves have previously been observed to be more useful to
note the effects caused by different water regimes (Grant et al. 2007).
When the correlations between physiological parameters and average temperatures of
lateral canopy in the afternoon and zenithal canopy were compared, zenithal
thermography appeared to be as effective as lateral thermography in the detection of
water stress. This demonstrates that aerial thermal imaging could be a feasible tool to
assess vineyard water status.
Temperature distribution
The distribution of thermal data differed between different times of thermal imaging
and between irrigation regimes. Temperature distribution of ROI of the canopy might
be a good indicator of water stress. Smaller variation of thermal distribution within in
canopies of fully irrigated vines (FI) was found compared to canopies of stressed vines.
This findings support the idea of Fuchs (1990) that temperature variation increased as
stomata close. Hence, temperature variation was higher in a stressed plant than in a
non-stressed plant. Similarly, Clawson and Blad (1982) found that the variation of
thermal distribution is less in non-stressed corn plants compared to stressed plants.
On the other hand, Grant et al. (2007) observed that there were not greater variation
of grapevine leaves’ temperature within stressed vines as compared to non-stressed
vines. These contradictory results with respect to Fuchs´ idea could be explained by
the non-random distribution of leaf angles within grapevine canopies. Where stomata
are open, stomatal conductance is the predominant determinant of leaf temperature,
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but when stomata are closed, leaf angle has the biggest impact on the temperature of
individual leaves in a canopy: hence as stomata close, variation in leaf angle has an
increasingly important impact on variation in temperature within a canopy (Fuch
1990). The present results, however, particularly for lateral canopies in the afternoon,
suggest that even in grapevine canopies there can be sufficient variation in leaf angle
to result in increased variation in temperature distribution within a canopy when
stomatal conductance is low.
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6. CONCLUSIONS
- 37 -
The main conclusions from this work are:
a) Two different methods of extraction of temperature from the images
were suitable for detection of vine water status: averaging the
temperature of several individually selected sun-exposed leaves or by
extracting the average temperature of a region of canopy. However, the
use of the average temperature of a region of canopy presents great
advantages as image processing could be automated, and it allows the
frequency distribution of canopy temperature to be obtained.
Additionally, extracting the average temperature of a region of canopy
is less subjective than averaging the temperature of several individually
selected sun-exposed leaves.
b) Strong correlations between canopy temperatures (or thermal indices)
and stomatal conductance and stem water potential were observed.
These results indicate that thermal imaging can be used as a non-
invasive technique to determine grapevine water status under field
conditions and therefore it could be applied for irrigation scheduling.
The most favourable time to acquire thermal images in this study was
during the afternoon.
c) Zenithal temperatures were similarly indicative of vine water status
compared to lateral imaging. These results suggest that aerial imaging
may be suitable for monitoring water status in grapevines.
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