Canopy temperature and it measurement by the infrared
thermometer in selection for drought resistance
Since
a major role of transpiration is leaf cooling, canopy temperature and its
reduction relative to ambient air temperature is an indication of how capable
is transpiration in cooling the leaves under a demanding environmental load.
The relationships between canopy temperature air temperature and transpiration
is not simple, involving atmospheric conditions (vapor pressure deficit, air
temperature and wind velocity), soil (mainly available soil moisture) and plant
(canopy size, canopy architecture and leaf adjustments to water deficit). These
variables are considered when canopy temperature is used to develop the Crop
Water Stress Index (CWSI)
which is gaining use in scheduling irrigation in crops.
However,
in plant breeding and selection for drought resistance the interest is in
finding genotypes that maintain transpiration, gas exchange and therefore a
lower canopy temperature as compared with other genotypes under the same field
conditions. Relatively lower canopy temperature in drought stressed crop plants
indicates a relatively better capacity for taking up soil moisture and for
maintaining a relatively better plant water status by various plant onstitutive or adaptive traits (see Blum, 2009). This capacity, as expressed in relatively
lower canopy temperatures, was correlated with final yield under stress or
other parameters drought resistance in terms of yield such as various plant
yield indices under stress. Canopy temperature was also found to be negatively
correlated with relative water content (RWC) across diverse wheat and rice
genotypes. Canopy temperature is also affected by the relative amount of
desiccated and dead leaf in the canopy and thus it was found to be positively
correlated with ‘leaf death score’ which is a visual rating of drought stress
in selection work.
Since
the initial development of the method by Blum
et al. (1982) about 33 studies (by the end of 2010) confirmed the utility
of this selection method (see references below).
Canopy
temperature is measured remotely by the infrared thermometer (IRT) (see
photograph of one of the earlier instruments; see the detailed theory – a large pdf file). Canopies emit
long-wave infrared radiation as a function their temperature. The IRT senses
this radiation and convert it to an electrical signal, which is displayed as
temperature (photo of one of the earlier models). Various instruments are
available today from different suppliers.
Using the thermometer properly is crucial to obtaining reliable data. Following
are the most important points in the protocol of using the IRT in breeding
nurseries.
1.
The correlation
between canopy temperature and plant water status becomes stronger as plant
water status is reduced. Therefore, measurements should be made under
well-developed drought stress, typically when most of the materials in the
nursery present some leaf wilting or leaf rolling at
2.
The thermometer has
a fixed angle of view (around 2-5 degrees, depending on the model). The size of
the measured target area therefore depends on the distance between the
thermometer and the target. The target must consist only of canopy leaves. Any
other object in the target area, such as soil surface or inflorscences will
result in a temperature reading which does not represent the optimal target,
namely leaf canopy. Soil is generally hot and cereal inflorescence is much
warmer than leaves because it hardly transpires. Therefore, screening by canopy
temperature measurements under drought stress can be done only during the
vegetative growth stage, after full ground cover has been attained and prior to
inflorescence emergence. In certain large-leaf crops, such as sunflower, maize
or sorghum it is difficult to view a canopy made only of leaves. The target
area usually consisted of dark areas or stem parts between leaves. These
situations do not provide a good option for selection with the IRT, unless the
thermometer can view only leaves. A protocol for the selection by measuring
individual leaves with the IRT has not been yet established. Considering all of
the above, the operator holding the thermometer can manipulate his distance,
position and angle with respect to the viewed plot so as to optimize the target
view. This position, however, must be maintained with all plots measured.
3.
Since the assessment
of plant stress by canopy temperature within a breeding population is relative,
atmospheric conditions during measurements should be relatively stable. Cloudy
or windy conditions should be avoided. Especially difficult is transient
cloudiness which has an immediate effect on leaf temperature.
4.
The thermometer
should not be exposed to heat unnecessarily, such as letting it lie in the sun.
The thermometer can be harmed by direct solar radiation entering its lens.
Readings should be made with the sun at the back of the operator, basically
similar to the rule in photography. This should be taken into account when the
nursery layout is planned.
5.
The nursery should
contain a running check (control) cultivar, every 10 to 100 genotypes,
depending on the case. Canopy temperature of the running check provide a basis
for assessing site variability and offer a means for normalizing data against
this variability.
6.
Finally, several
measurements should be taken during a drying cycle and depending on the data
and the stress level the final evaluation can be based on average data or data
from a selected date.
Data analysis
The
corollary of these measurements in selection work is that the breeder is
interested in large differences so that he can reduce the population reliably
into the most desirable materials. Experience shows that if work is performed
carefully as outlined above, about 10 to 20C can be the
least significant difference (at 5%). If stress is sufficient and atmospheric
demand for transpiration is high, genotypes may differ by up to 50
to 100C on a given day, depending on the crop and the nature of the
population.
For
each date of measurements data can be processed in three forms: actual
temperatures, temperature of the genotypes as percent of the mean temperature
of the block; and temperature of the genotypes as percent of the temperature of
the nearest running check. ‘Nearest neighbor’ statistical analysis is also
appropriate in this case and it is available as part of some statistical
software packages, such as ‘AGRIBASE’ (at http://www.agronomix.mb.ca/).
Remember,
no method of data analysis will compensate for failing to observe the correct
measurement protocol.
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(The
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