Classification of plant moisture conditions using canopy and leaf temperature responses to step changes of incident radiation

Environmental and soil conditions affect plant productivity. Determining and understanding how plants respond to moisture stress may be considered in different ways, but often not easily done. This study focused on measuring canopy and individual leaf temperature response times of selected horticultural and agricultural crops, treated to very dry (moisture stress) and well-watered conditions, and subjected to an incident step radiation change at the top of the canopy, in a controlled environment at a constant air temperature. A light-emitting-diode (LED), grow light system, delivering approximately 400 μEinstein s-1 m-2 of photosynthetically-active-radiation (PAR) was turned on and off to produce light and dark radiation step functions for the tests. Canopy and leaf temperature time series were measured using both an infrared thermometer and a FLIR thermal imaging camera, over a 10-minute elapsed time, respectively. Temperatures of a selected single, unobstructed leaf at the top of the canopy were analyzed with visual and thermal imaging processing. Response data were modeled using mathematical transfer functions and model discovery methods. Both canopy and leaf temperature-time series for dry plants were found to be a first-order response. Well-watered plants were first order followed by second order response phase, possibly indicating stomatal activity. Thermal response times to a step increase of PAR (lights-on), were higher for well-watered plants than for water-deprived plants. (It took longer for the leaves of well-watered plants to heat up). The thermal response times, to a step decrease of PAR (lights off), were lower for well-watered plants than for water -deprived plants. (It took less time for the leaves of well-watered plants to cool off). These results show that leaf thermal response may be used as an indicator of plant water status, and as a potential instrumentation approach