What Information Does Canopy Temperature Provide about Plant Water Use?.

Canopy temperature is both a useful indicator of crop water status for irrigation scheduling and a potential tool for phenotyping efforts. Consequently, handheld canopy temperature measurements have become a standard tool for assessing crop water status. A first order approach works well, where considerable differences in canopy temperature relate to severe stomatal closure and indicate that a genotype is in a much more severe state of drought than other genotypes. This study aims to determine whether second order approaches to analyzing canopy temperature data can provide information on physiological mechanisms in wheat. Measurements made on two wheat mapping populations, using a handheld IR sensor, were compared to continuous measurements made using a custom array of twenty IR sensors measuring every 5 seconds for the latter half of the growth season. Variation in stomatal conductance due to varying VPD and soil water status lead to measurable differences in canopy temperature, but due to high variation were practically only interpretable when temperature differences were extreme. The major cause of variation in canopy temperature was differences in the height of a genotype relative to its neighbors; an experiment using copper plates indicated that this effect was due to canopy to atmospheric coupling, not genotypic differences. The magnitude of height related differences in temperature was of the same order as the absolute differences between genotypes, greatly complicating analyses. An analysis of the response of canopy temperature to VPD measured using the continuous IR sensors was used to design a protocol for measurement of VPD responses using a handheld IR sensor.