Diurnal trends in wheat canopy temperature, photosynthesis, and evapotranspiration

Remote sensing technology to study vegetative canopies by measuring optical, thermal, and microwave signatures provides optimism for frequent characterization of plant performance in the field. Ground truth data are required in the calibration and the validation of satellite data. The objective of this work was to measure canopy temperature (CT), canopy photosynthesis (CER), and evapotranspiration (ET) of spring wheat (Triticum durum Desf. "Aldente") irrigated at two levels. A large portable field chamber (volume of 3.25 m³ and an area of 2.67 m²) enclosed the canopy, and CO₂ and water vapor concentrations were measured at 2-s intervals for 60 s with an infrared gas analyzer operated in differential mode. Canopy temperature was measured with an infrared thermometer (4° FOV; 0.15 m dia spot size) mounted on the outside front corner of the chamber aimed 3 m directly in front of the chamber and represents the mean of 15 measurements. Other estimates of CT were obtained from a fixed infrared thermometer at the weather station and from low flying aircraft. Diurnal measurements were completed on 10 April 1989 when the wheat was in midflowering (Zadoks scale 65). Diurnal trends in CER showed differences between fully irrigated and deficit-irrigated plots. The small differences in ET and CT between irrigated and deficit-irrigated plots suggested minimum plant stress on deficit-irrigated plots, at least during the early part of the day. The cumulative daytime photosynthesis showed the irrigated plots averaged 28.9 g CO₂ m^-2 while the deficit-irrigated plots showed a 52% reduction in CER. The CT ranged from about 7°C near sunrise through a maximum of 27°C and then decreased to 15°C at sundown. The average cumulative daytime chamber ET was 8.41 mm and 7.69 mm for the fully and deficit-irrigated plots, respectively, which compared with reference ET of 8.91 mm and aircraft estimate of 8.93 mm. Diurnal trends in CT and ET showed a stronger association and were similar for both irrigated and deficit-irrigated plots.