New paper describing the use of the Aquation White light-emitting Reflectance-Absorptance-Transmittance meter or (White-RAT). The earlier Blue-RAT and later version White-RAT enable the direct convenient measurement of leaf absorptance by measuring the proportion of light transmitted through a leaf and that reflected off the leaf. Absorptance is then calculated and displayed on the meter. The published study used this instrument to assess a range of terrestrial plant leaves. These directly-measured absorptance values, along with measurements of effective quantum yield of chlorophyll fluorescence (see Aquation Chlorophyll Fluorometers), can be used to determine Electron Transport Rate, a fundamental physiological parameter that describes photosynthetic activity.
PAM (Pulse Amplitude Modulation) fluorometers can be used to estimate the Electron Transport Rate (ETR) (mol (e–) m-2 s-1) from photosynthetic yield determinations, provided the absorptance (Abtλnm) of the photoorganism is known. The standard assumed value used for absorptance is 0.84 (Leaf Absorptance Factor, AbtF). We describe a Reflectance Absorptance Transmittance (RAT) meter for routine experimental measurements of the actual absorptance of leaves. The RAT uses a red-green-blue (RGB) LED diode light source to measure absorptances at wavelengths suitable for use with Pulse Amplitude Modulated (PAM) fluorometers and InfraRed Gas Analysers (IRGA). Results using the RAT are compared to Abtλ nm spectra using a Taylor integrating sphere on bird’s nest fern (Asplenium nidas), banana, Doryanthes excelsa, Kalanchoe daigremontiana and sugarcane. Parallel venation has no significant effect upon Abt465 nm in banana, Doryanthes, a Dendrobium orchid, pineapple and sugarcane but there was a slight difference in the case of the fern Asplenium nidus. The average Abt465 nm(≈ 0.96)and Abt%625 nm (≈ 0.89) were ≈14%and 6% higher than the standard value (AbtF = 0.84).The PAR-range Abt%400-700 nm was only ≈ 5% higher than the standard value (≈ 0.88) based on averaged absorptance from the blue, green and red light data and from where the RGB-diode was used as a ‘white’ light source. In some species absorptances at blue and red wavelengths are quite different (e.g. Water Lily). Reflectance measurements of leaves using the RAT would also be useful for remote sensing studies.
Ritchie, R.J. and Runcie, J.W. 2014. A portable reflectance-absorptance-transmittance meter for photosynthetic work on vascular plant leaves. Photosynthetica 52(4): 614-626