Thermal images processing

At a recording framerate of 60 FPS, a treatment replication’s length of 25 m and an average speed of 5 km/h, the number of thermal frames captured per each replication was approximately 1080. In order to reduce the amount of redundant information, not every frame from the recordings was used in the processing of the images—only those frames having no overlapping were thus used for the calculation of different thermal statistics. If a treatment block of 25 m length is covered by 1080 frames, and each frame displayed approximately a horizontal length of 1.07 m, one out of 46 consecutive frames would have no overlapping. Thus, each treatment replication consisted of roughly 23 frames, and only the 14 middle ones were used since they comprised the 15 middle plants of the replication (the plants in which the water status measurements were taken).

To support the thermal images segmentation and develop different canopy temperature indices, T_wet and T_dry reference temperatures were acquired using an evaposensor (Skye Instruments, Llandrindod Wells, UK) having two artificial leaves: a dry one (dry reference) and another one covered with a black cotton wick and receiving continuous water absorption for the wet reference [36]. Reference temperatures were acquired once per each measurement day.

To remove the influence of the sky, soil and fruiting zone in the top and bottom part of the thermal images respectively, the middle section of the thermal images was selected, named as area of interest, and used for the calculation of the different statistics and thermal indices (Fig 3). The area of interest had a size of 320 × 135 pixels, from a total image resolution of 320 × 256 pixels.

The middle section between the green bars was defined as the area of interest. Total image resolution was 320 × 256 pixels, while the area of interest had a size of 320 × 135 pixels. The temperature values are expressed in degrees Celsius.

The average temperature (T_canopy), the median and the standard deviation were the statistics extracted from the thermal frames. Additionally, two thermal indices, the crop water stress index (CWSI) and the conductance index (I_g) were calculated, using the reference temperatures, as follows, according to [37] and [6], respectively:

FLIR ResearchIR 4.40 (FLIR^® Systems, Inc., Billerica, MA, USA) and MATLAB 8.5.0 (The MathWorks Inc., Natick, MA, USA) were used for the processing of the thermal images.