The assessment of water deficit stress in plants using optical measurement methods

This dissertation examined the possibility of detecting water deficit stress (WDS) in plants using optical signals collected from leaves. Three theoretical approaches were investigated in general, one of which was investigated in more detail.; This study examined if spectrally resolved chlorophyll fluorescence yields unique signatures in response to the development of WDS, but none of the chlorophyll fluorescence ratios investigated exhibited a clear dependency.; This study also examined if the principle of pressure shifting of spectral absorption bands can be used for the non-invasive measurement of turgor pressure in leaf cells. No plausible trend could be found for the 650 nm leaf absorption band, and for the 1450 nm leaf absorption band.; A third principle examined involves correlation of light absorption within leaves to leaf-water-content using water absorbing wavelengths. This study showed that the equivalent water thickness (EWT) of leaves appears to be the independent variable governing leaf water indices when applying a dual wavelength method measuring reflectance or transmittance. This study investigated the influence of several extraneous variables on such leaf water indices.; This study further examined the dynamics of various plant parameters during the development of WDS in several species. It was found that the near infrared water index based on reflectance (RIRWI) correlated to the relative water content of leaves qualitatively, but decreased late during the time course of WDS. Leaf thickness decreased earlier than the RIRWI, possibly indicating the loss of turgor pressure. Comparing the EWT and overall leaf thickness, it was concluded that it is often the airspaces within leaves that collapse before leaf cells start to dehydrate substantially. For the experimental conditions given, the near infrared water index based on transmittance (TIRWI) was found to increase substantially between about 15 hours and 100 hours before leaves started to dehydrate. A WDS index including the EWT and leaf thickness exceeded an alarm limit in all species examined before signs of plant desiccation became apparent to visual inspection.