Yves Goulas

Dualex: a new instrument for field measurements of epidermal ultraviolet absorbance by chlorophyll fluorescence

Dualex (dual excitation) is a field-portable instrument, hereby described, for the assessment of polyphenolic compounds in leaves from the measurement of UV absorbance of the leaf epidermis by double excitation of chlorophyll fluorescence. The instrument takes advantage of a feedback loop that equalizes the fluorescence level induced by a reference red light to the UV-light-induced fluorescence level. This allows quick measurement from attached leaves even under field conditions. The use of light-emitting diodes and of a leaf-clip configuration makes Dualex a user-friendly instrument with potential applications in ecophysiological research, light climate analysis, agriculture, forestry, horticulture, pest management, selection of medicinal plants, and wherever accumulation of leaf polyphenolics is involved in plant responses to the environment.

Fluorosensing of water stress in plants: Diurnal changes of the mean lifetime and yield of chlorophyll fluorescence, measured simultaneously and at distance with a τ-LIDAR and a modified PAM-fluorimeter, in maize, sugar beet, and kalanchoë☆

We modified a PAM fluorimeter for remote detection from 0.5 m to 1 m distance by using a laser diode for excitation. It permitted us to compare directly and simultaneously measurements of relative fluorescence yield to the measurements of lifetime with the τ-LIDAR performed under natural conditions. The existence of a linear relationship, and therefore the equivalence between the lifetime and yield for chlorophyll fluorescence estimation in vivo was confirmed here for several plant types and under both optimal conditions and conditions of water stress. We have also shown that fluorescence lifetime measurements, with the τ-LIDAR, can be used to perform complex fluorescence quenching analysis in fluorosensing, like the one developed in the laboratory for near-contact measurements with PAM-fluorimetry. Water stress effects on fluorescence changes are especially pronounc ed in maize plants due to the C4 type of metabolism and the absence of photorespiration. This would permit the use of the steady state level of fluorescence (Fs, τs) for stress detection, provided that the irradiance is known or is estimated.

Estimation of the chlorophyll fluorescence lifetime of plant canopies: Validation of a deconvolution method based on the use of a 3-D canopy mockup

Abstract Fluorescence or backscattered signals measured over a plant canopy after a picosecond laser shot (nadir viewing) are composed of elementary contributions coming from different illuminated leaves, branches, twigs, and the soil background. They are affected by time delays, depending on their levels within the canopy. The global signals measured are then rather complex and depend on canopy architecture and fluorescence characteristics. Also, an amplitude decorrelation exists between fluorescence and backscattered signals, due to the difference existing between reflectance and fluorescence properties of plant canopy components. For this reason it was necessary to develop a specific method for retrieving the mean fluorescence lifetime of a complex plant canopy under field condition. In a first step, the position and the reflectance characteristics of each reflecting canopy element is determined from the deconvolution of the backscattered signal. In a second step, the parameters obtained are introduced into the fluorescence function in order to determine the mean fluorescence lifetime. The validity of the deconvolution method has been tested for numerous situations, using simulated laser shots on a 3-D canopy mockup. The results obtained show a very good agreement between estimated and input parameters.

Measurement of laser‐induced fluorescence decay and reflectance of plant canopies

A new lidar fluorosensor, for the remote characterization of the geometry and of the physiological status of plant canopies, is described. It is based on the use of a laser delivering very short pulses (<100 ps) and on the temporal analysis of both fluorescence and backscattered signals. In a first step, the deconvolution of the backscattered signal gives the level and the relative area of leaves illuminated by the laser beam. In a second step, this information on the canopy geometry is used to deduce the mean chlorophyll fluorescence lifetime, that is considered as an indicator of the photo‐synthetic activity. Field and laboratory measurements performed with the prototype show that the results obtained agree quite well with classical near‐contact chlorophyll fluorescence measurements and can bring also information on plant canopy geometry.