Narrow-band Reflectance Imagery Compared with ThermalImagery for Early Detection of Plant Stress

Abstract

Summary A field experiment compared plant stress detection by narrow-band reflectance and ratio images withthermal infrared images. Stress was induced in a mixed stand of 5 year old loblolly pine ( Pinus taeda L.) and slash pine {Pinus elliottii Engelm.) by a soil application of diuron (DCMU) on 22 August followed by bromacil on 19 September, 1994. Herbicide-induced stress was first indicated on 24 and 26 September by significant (p⪯0.05) decreases in photosynthesis and the ratio of variable to maximum fluorescence (F v / Fm ), respectively. Stress was first detected remotely on 5 October by 694 ± 3 nm reflectance imagery and its ratio with reflectance at 760 ± 5 nm (p⪯0.05). This reflectance increase was detected at least 16 days prior to the first visible signs of damage, as quantified by the CIE color coordinate u', that occurred between 21 and 26 October. Reflectance images at 670 + 5 nm, 700 ± 5 nm and 760 ± 5 nm first detected stress on 21 October, 12 October and 20 December, respectively. Canopy temperature as indicated by imagery in the 8 to 12 μm band never differed significantly between herbicide-treated and control plots. This resulted from the close coupling of leaf temperatures with air temperature, and the tendency of wind and environmental moisture to equalize temperatures among treatments. The high sensitivity to stress of reflectance imagery at 694 ± 3 nm supports similar conclusions of earlier work, and indicates that imagery in the 690 to 700 nm band is far superior to thermal imagery for the early and pre-visual detection of stress in pine.