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Infrared thermal camera can be used to measure plant canopy temperature, which can be used as an indirect proxy for transpiration. Following parameters can be calculated from the IR imagery technique:

  • Estimation of Canopy Temperature (CT)
  • Estimation of Canopy Temperature Depression (CTD)
  • Estimation of Index of Canopy Conductance (Ig)

Estimation of Canopy Temperature (CT)

Thermal images are obtained with an infrared camera IR FlexCam S (Infrared solutions, USA) with a sensitivity of 0.09ºC and accuracy of ± 2%. Measurement of CT is indirectly related to transpiration rate (TR; i.e. canopy conductivity). The thermal images can be taken in indoor (growth chamber) or outdoor (field) environments. The software SmartView 2.1.0.10 (Fluke Thermography, supplied with the camera) is used for the analysis of the thermal images and the estimation of canopy temperatures (Zamman-allah et al 2011). Within the SmartView2.1.0.10 platform, we can output a histogram reflecting the distribution of RGB pixels value summarizing the different temperature values found across the thermal image. The temperature distribution of the canopy is expected/hypothesized to follow a normal distribution. Therefore, any pixel that diverts from that normal distribution is considered not to be part of the canopy and is then attributed to the background. A temperature threshold is then fixed between the canopy and the background (therefore it is convenient to have background contrasting with the canopy), and then allows to exclude all the pixels that are part of the background. The estimation of the plant canopy temperature is then the weighted average of the canopy pixels (Zaman-Allah et al. 2011a).

Estimation of Canopy Temperature Depression (CTD)

The Canopy Temperature Depression [CTD] is calculated as the difference between the air temperature and the canopy temperature.

Estimation of Index of Canopy Conductance (Ig)

The Index of Canopy Conductance (Ig) is used as an indirect estimation of the absolute canopy conductance (Jones, 1999).

Data Analysis

Canopy Temperature (CT) can be estimated using the following formula:

Finally, using the data of the distribution of thermal image pixels of canopy temperatures, an average canopy temperature is calculated as:

TCanopy= Sum [(Ti* Pxi)/Pxt)

Where Pxi is the number of pixels for a given temperature Ti, and Pxt is the total number of pixels for range of temperatures covering the whole canopy (Zaman-Allah et al. 2011a and Belko et al. 2012).

Canopy Temperature Depression (CTD) can be estimated using the following formula:

CTD = Tair – TCanopy

where Tair is temperature of air recorded from a temperature recorder (Gemini Tiny Tag Ultra 2 TGU-4500 Data logger), which is placed at the crop canopy level.Tcanopy is temperature of canopy measured by IR camera.

Index of Canopy Conductance (Ig) can be estimated using the following formula

Ig is calculated using canopy and detached wet and dry leaf temperatures

Ig = (TDry leaf -TCanopy)/(TCanopy -TWet leaf)

where TWet leaf and TDry leaf are temperatures of wet leaf and non-transpiring leaf surface, respectively. TWetleaf is measured on detached green leaves soaked in water for about 10 min and then on which the temperature is measured with the camera. TDryleaf is measured on the same detached leaves previously dried in an oven at 60 °C for 48 hrs. These canopy temperatures were measured on six leaflets from six contrasting genotypes outdoors at the end of the experiments. The averages of TWetleaf and TDryleaf were calculated and applied in the Ig formula for all genotypes (Zaman-Allah et al. 2011a and Belko et al. 2012).

For more details, please see the following publications:

  • Chickpea-Zaman-Allah et al. 2011a
  • Chickpea-Zaman-Allah et al. 2011b
  • Cowpea-Belko et al. 2011
  • Cowpea-Belko et al. 2012
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