In vivo photoacoustic spectra of herbicide-treated bean leaves

Summary Photoacoustic spectra of control leaves and herbicide-treated bean leaves ( Phaseolus vulgaris L. cv. Fori GS) were taken with a modulation frequency of 22 Hz. The excised leaves were floated in water (water control) or in a 10 −4 M herbicide solution. The action of PS II inhibitors of the urea type (diuron) and of the phenol type (3-bromo-4-hydroxy-5-nitro-benzonitrile) were compared to that of the non-selective herbicide, paraquat. After 5 h of incubation the paraquat-treated leaves were characterized by a reduced carotenoid content, lower chlorophyll fluorescence yield and a lower variable portion in the fluorescence kinetics. After treatment with the PS II inhibitors, diuron and benzonitrile, the pigment content was not changed; the chlorophyll fluorescence yield was increased, whereas the variable fluorescence was decreased. The three types of chemicals induced a decrease of the in vivo photoacoustic signal in the entire spectral range from 380 to 720 nm, which was paralleled by a decline in photosynthetic function as seen from low Rfd-values. This decrease in the photoacoustic signal was more pronounced in the red than in the blue spectral region. The photoacoustic signal is partially determined by a pulsed, photosynthetic oxygen evolution. This is seen from the addition of continuous white light, which leads to a similar decrease of the photoacoustic signal as the herbicide treatment since the homogeneously produced oxygen no longer induces a photoacoustic signal. The results are discussed in view of the application of photoacoustic spectroscopy for the in vivo investigation of physiological effects of photosynthetic herbicides.