Transpiration in plant canopies occurs for 2 primary reasons:
1) as a consequence of stomatal opening for the uptake of CO2 for photosynthesis.
2) to keep the canopy cool and avoid heat damage.
Under moisture deficits, plant canopies can increase in temperature as the plant undergoes partial stomatal closure to maintain hydration. Persistently high canopy temperatures can lead to significant stress and the subsequent decrease in growth and yield.
Scientists at CSIRO have found that if cotton canopy temperatures exceed 28°C for 4.45 hours per day then this can lead to a significant reduction in yield (Conaty 2010).
To demonstrate the effect that watering can have on canopy temperatures, an experiment was conducted on sorghum growing at the University of Queensland, Gatton Campus (Figure 1). Two treatments were in effect: an irrigated treatment where the sorghum was well-watered; and a rain-fed treatment. Click on this link to see a YouTube video on the experimental set-up:
Ten days of canopy temperature and rainfall data, as measured by an Apogee Infrared Temperature (IRT) sensor connected to a TSM Temperature Sensor Meter (Figure 2) and an AWS Automatic Weather Station, is shown in Figure 3. The first 5 days of data show the rain-fed sorghum had significantly higher canopy temperatures during the day. Following a large rainfall event, canopy temperatures of the irrigated and rain-fed crops converged.
These IRT data demonstrate the importance of monitoring canopy temperature in maintaining crop health and growth.
Conaty, WW (2010). TEMPERATURE-TIME THRESHOLDS FOR IRRIGATION SCHEDULING IN PRECISION APPLICATION AND DEFICIT FURROW IRRIGATED COTTON. PhD Thesis, The University of Sydney.