Thermal infrared imaging data were acquired using a CAT S60 smartphone equipped with a FLIR Lepton longwave infrared micro thermal camera module (https://www.catphones.com/ (accessed on 5 November 2019)).
- Images had a resolution of 320 × 480 pixels.
- Two images per plot were taken to cover the complete plot surface.
- Images overlapped at the center of the plot, where a fastened piece of crumpled aluminum foil (40 cm<sup>2</sup>) and a leaf covered with petroleum jelly used as a dry reference could be included in both images.
>###### Sampling time
>- Images were acquired at the visible inflorescence stage of development for all plots in both the FI and RI treatments in the afternoon of 46 and 47 days after sowing.
>- Images were acquired in the afternoons of 48, 49 and 50 days after sowing for all plots of blocks 2 and 5, 1 and 4, and 3 and 6, respectively.
>- At 47 days after sowing, all plots were also imaged in the late morning when the leaves were still wet with morning dew.
- The derived temperature data were used as a wet reference.
- Images were processed in R using the ‘Thermimage’ package [1].
- Settings for air temperature and relative humidity at the time of imaging were obtained from the collected environment data.
- Emissivity was set to 1 in the conversion of image data for measuring the mean temperature of the crumpled aluminum foil, which represented the reflected temperature. This was then applied together with an emissivity value of 0.96 to obtain the leaf temperature data.
- An ImageJ macro was used to semi-automatically determine regions of interest (ROIs) in the images and corresponding temperature data for the piece of aluminum foil, the dry reference leaf and patches of sunlit, exposed soil. The temperature data of these ROIs were then excluded from the image to obtain the mean leaf temperature data.
- Thermal index 1 (TI1) was calculated as follows:
$$TI1 = dTwet.m − dTm$$
The dTwet.m is the mean of the difference between the temperature of the wet leaves per plot at 47 days after sowing and the ambient air temperature at the time of imaging. The dTm is the difference between the mean leaf temperature per plot and the ambient air temperature at the time of imaging [2,3].
References:
[1] Tattersall, G.J. Thermimage: Thermal Image Analysis. Available online: https://CRAN.R-project.org/package=Thermimage
[2] Maes, W.H.; Steppe, K. Estimating evapotranspiration and drought stress with ground-based thermal remote sensing in agriculture: A review. J. Exp. Bot. 2012, 63, 4671–4712.
[3] Perich, G.; Hund, A.; Anderegg, J.; Roth, L.; Boer, M.P.; Walter, A.; Liebisch, F.; Aasen, H. Assessment of multi-image unmanned aerial vehicle based high-throughput field phenotyping of canopy temperature. Front. Plant Sci. 2020, 11, 150.
