The relationship of potato leaf temperatures to air temperatures as affected by overhead irrigation, soil moisture and weather.
Automatically recording thermocouple sensing elements clipped to potato leaves and thermocouple needles inserted into the midrib of the leaf showed that under normal weather conditions the temperatures of upper crop canopy leaves exceeded those of the macro-climate during the day, whereas during the night their temperatures were lower. Differences in temperature during the day were primarily determined by the evapotranspiration rate which was closely linked to the soil moisture regime. Under conditions of extreme evaporative demand, with soils at or near field capacity, evaporative cooling seemed to be sufficient to keep potato leaves below the ambient air temperatures. Overhead sprinkler irrigation caused a drop in leaf temperatures, up to 10-12°C at 1200 h. The effect of the timing of irrigation on leaf temperatures was most pronounced during the morning and at 1200 h. Irrigation in late afternoon and early evening had a much smaller effect, but at night, leaf temperatures were at wet bulb temperature values. This was not the case with irrigations earlier in the day, where the upper crop canopy leaves dried before nightfall. RB.