Wasserstatusüberwachung an Nutzpflanzen mittels THz-Spektroskopie

The basic principle of the non-destructive and non-invasive measurement of a plant's leaf water content by the means of THz and sub-THz radiation is based on the high contrast in absoption between liquid water and the leaves' other components in this frequency range. Thus the amplitude of the signal that is transmitted trough the leaf is strongly correlated to its water content. While the measurement itself is contacless, it can be convenient to fix the leaf in a sample holder that keeps the leaf in a defined position. To conduct long-term measurements on agricultural crop plants a laboratory setup was constructed which can be used for experiments with several plants at the same time. In this setup a fiber-coupled THz time domain spectrometer is used with the THz emitter and detector and the THz optics intergrated as a compact measurement head which is mounted on the motorized arm of a goniometer. The plants under examination are placed in a circular arrangement so that they can be reached by the automated movement of the measurement head. Using this setup measurements on several different crops were performed. In contrast to hand-operated experiments a continuous and comparably high rate of one measuement per plant per hour is achieved. Thus, a detailed view on changes in leaf water content is possible, revealing for example changes between day and night. By logging the weight of the plants' pots during the experiment the leaf water content can be seen in conjunction with the plants' water supply. The comparison between rye and oat shows a significant difference regarding the minimum amount of water needed by the plants before they start to dry out resulting in an increase of the transmitted THz signal. When such a stressed plant is rewatered, it may or may not recover. If it does, the amplitude of the THz signal will return back to its initial level. If the leaf is not able to recover any more, it will dry out despite of the irrigation until nearly the complete THz signal is transmitted through it. The experiments show that a correlation exists between the water content that is determined by the THz measurements at the moment of rewatering and the ability of the leaf to recover. In the case of corn the observations made during the experiments in this regard can be divided into three groups. Depending on how far the drought of the leaves has proceeded they are either able to recover completely, only temporarily or not at all. Similar observations are made with soy plants. However, these are only found to either recover completely or not at all. Therefore, a threshold can be found that decides whether the recovery of the leaf after rewatering will be likely or not. For determining the leaf water content as a volumetric or gravimetric fraction, a model based on an effective medium theory can be used describing the leaf as a mixture of water, solid material and air. By fitting this model to the measured THz spectra the fractions of these three components can be determined. The comparison to gravimetric measurements which were conducted in parallel shows that this method leads to realistic results. Another application of THz spectroscopy in the field of agriculture is the quality control of seeds. Measurements with sugar beet seeds in a THz time domain spectrometer were performed and based on the results of these measurements a good prediction rate of whether a seed contains an emryo that is needed for the seed to sprout or not is achieved. A great reduction of the overall costs and complexity of a THz time domain spectrometer can be achieved, when the modelocked fs-laser is replaced by a simple multimode laser diode. Measurements on plants under drought stress which were conducted with such a THz quasi time domain setup are in good agreement with the measurements with the fiber-coupled THz time domain setup. Aditionally, based on the THz quasi time domain spectrometer a comparably compact and portable setup that can be used in the field was built. In order to perform measurements not only on one leaf but on a larger part of a plant longer wavelengths in the sub-THz frequency range can be used. In a measurement series at a frequency of 35 GHz barley plants under drought stress were compared to a well-irrigated control group. Both groups could be clearly distinguished in the results of the measurements. The measurement setup is constructed in a way that captures not only the radiation that is transmitted trough the plant but also the parts which are scattered on the plant’s surface. From the measured values the water content of the plants can be calculated as the absolute amount of water in gramms as wells as the gravimetric fraction in percent. To sum up, the different experiments that are described above show that, depending on the intended use, several different possibilities exist how radiation in the THz and sub-THz frequency range can be used for water status measurements on agricultural crop plants.