Influence of ambient light and temperature on laser-induced chlorophyll fluorescence measurements

Chlorophyll fluorescence analysis is a powerful tool for the study of spatial and temporal heterogeneities of photosynthetic performance and has been successfully used at the laboratory scale. Field applications, however, are confronted with the challenges presented by fluctuating environmental factors, particularly varying light and temperature conditions. The influence of ambient light and temperature on the measurement signal of the laser-induced two-wavelength chlorophyll fluorescence remains both controversially and largely not well understood. Yet, a thorough understanding of this issue is essential for exploiting the large potential of this measurement method for precision agriculture as well as for precision phenotyping in plant breeding where the influence of ambient light and temperature is inevitably present. To fill this gap in our knowledge, we therefore investigated this area under both field and controlled laboratory conditions. Field measurements of laser-induced two-wavelength chlorophyll fluorescence under variable light conditions show a linear influence of direct sunlight on the ratio of F ₆₉₀/F ₇₃₀, whereas the ratio is unchanged under diffuse sunlight. We describe these measurement results in terms of a physical model of light-matter interaction in plant leaves. In particular, the results appear to derive from energetic saturation effects of the upper plant layers induced by sunlight, which result in larger mean depths of fluorescence emission induced by additional exciting light (i.e. laser light), in connection with the different scattering properties of the two different fluorescence light wavelengths (690nm and 730nm) on their way to the surface. The same model also provides an explanation for the temperature dependence of the fluorescence intensity ratio and is well suited to deal with the effects of both variable light and temperature conditions under field conditions.