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Seeing what happens inside plants

Farmers like to be able to control the growth of their crops as much as possible in order to optimize the yield. A group of researchers led by Dr. Claus Buschmann from the Karlsruhe Institute of Technology (KIT) is able to look deep into leaves and fruit of vines and other plants. The researchers use reflectance and fluorescence data to deduce information about the plants’ photosynthetic activity, maturity and stress symptoms. One of the group’s objectives is to develop devices that enable measurements to be carried out quickly and easily in the field. The physiological monitoring of fields and forests is already being carried out on an experimental basis from a small distance (from tractors and planes), but might at some stage in the future also be possible from satellites.

PD Dr. Claus Buschmann from the Karlsruhe Institute of Technology (KIT) © private

Photosynthesis is essential for life on earth. The conversion of sun energy into carbohydrates and other energy-rich metabolites by green algae and plants enabled animal organisms to develop and is at the beginning of any food chain that ends with ourselves as humans. The major focus of Dr. Claus Buschmann and his group of researchers at the Institute of Botany at the Karlsruhe Institute of Technology (KIT) is the complex biochemical processes in the leaves of plants. Carrying out basic research is not just the group’s objective, but also the means to an end. This is because the methods used are excellent tools for both environmental research as well as for the agricultural industry. “As fluorescence image analysis and reflectance measurements enable us to look deep into the physiological processes of plant leaves or fruit, we can measure parameters that are also highly interesting for farmers,” said Buschmann.

Energy can provide a lot of information about what happens inside a leaf or fruit

The leaves of plants are full of chlorophyll, a molecule that absorbs sunlight and uses its energy to synthesize carbohydrates from CO2 and water. The absorption of sunlight temporarily elevates electrons into a higher energy level; the electrons give off energy when they return to their ground energy level. This energy is either used for biochemical processes (e.g. photosynthesis) or “wasted” in the form of fluorescent light. It is assumed that the higher a plant’s photosynthesis rate, the less energy is given off in the form of fluorescence. “We can measure the fluorescence with a spectrometer and derive from this the plant’s photosynthesis rate at a given point in time,” said Buschmann going on to add “the more fluorescence we can detect, the lower a plant’s photosynthesis rate, and vice versa.”

The photo shows a layer of green circles; a wide arrow points into the layer and four slender arrows pointing away from it.<br /> <br />
The light energy that enters a plant leaf is converted into different types of energy, including heat and fluorescent light, or given off as reflectance. © Dr. Claus Buschmann

There are also other molecules inside plant cells that can emit fluorescent light, including secondary plant substances such as ferulic acid. These substances are important constituents of the plant metabolism and the intensity of their fluorescence can provide researchers with insights into the physiological stage that a plant cell in a leaf or in fruit has reached. Buschmann and his team also use devices that can measure reflectance spectra which result from the emission of wavelengths that are not absorbed inside a biological sample. These spectra can provide the researchers with information on the concentration of metabolites that are specific for a certain rate of photosynthesis, a fruit’s degree of maturity or the activity of the stress defence machinery. The spectrometers used by Buschmann and his team are excellent monitoring tools. The tissue of leaves and fruit is a heterogeneous optical sample in which light is distributed by way of scattering, reflectance and refraction. Buschmann is organizing the third Leaf Optics workshop in autumn 2012, which will bring together scientists from Europe, the USA and Japan.

Two vine leaves; left: green vine leaf; right: vine leaf with different blue colours.
Vine leaf infected by a fungus (left: photo; right: fluorescent image) © Dr. Claus Buschmann

Growth control from satellites?

As basic researchers, Buschmann and his colleagues also measure what happens inside plants that grow in the shade of other plants, and hence are only exposed to green light. Working alongside researchers from the Technical University in Budapest, the researchers from Karlsruhe are concentrating on the further development of devices, making them manageable, easy to operate and enabling feedback-assisted measurements. Such measurement processes continuously adapt the irradiated light, which is used to make a sample fluoresce, for example, to the fluctuating rate of photosynthesis of the sample, thereby ensuring an optimal and energy-efficient irradiation intensity. In addition to solving basic research issues, the researchers are also carrying out two large projects in cooperation with the agricultural industry. The major focus of these projects is on barley, sugar beet and vines.

Handheld device used for the measurement of reflectance and fluorescence of grapes in the field. © Dr. Claus Buschmann
The researchers use a highly sensitive camera to acquire relevant fluorescence data from barley and sugar beet. They anticipate that the comprehensive and non-invasive monitoring of physiological parameters over the plants’ entire life cycle will help them to optimize the yield, both under laboratory conditions as well as on the field. The vine project is also focused on the physiological alterations inside leaves and fruit. How much sugar is produced in which tissue and when has the optimal concentration of specific acids and flavouring substances been reached? As part of the cooperative project, the researchers from Karlsruhe are working on the optimization of a handheld spectrometer that wine-growers will be able to use to monitor all the important parameters in their vines.
The researchers from Karlsruhe are interested in other things besides the aforementioned down-to-earth research. “In future, we foresee the possibility of exploring large-scale growing areas from a long way away, for example using satellites,” Buschmann said. Spectrometry from right out in the universe? Is this really possible? It is definitely rendered much more difficult from such a long distance because plant tissue produces a very weak fluorescent signal. In addition, sunlight is omnipresent and would interfere with the signals. The researchers from Karlsruhe hope to be able to contribute to solving this problem. And in addition to all these projects, Buschmann is also working on the generation of bioenergy from artificial photosynthesis systems. “Many researchers would like to be able to artificially reconstruct the photosynthesis system. However, it is worth noting that the plant photosynthesis system has taken billions of years to develop,” said Buschmann going on to add "we are closely following the developments and hope to be able to contribute to reconstructing the photosynthesis system at some time in the future."

Further information:

PD Dr. Claus Buschmann
Institute of Botany II
Karlsruhe Institute of Technology (KIT)
Building 30.41
Fritz-Haber-Weg 4
76128 Karlsruhe
Phone.: 0721/ 608 - 44 876
E-Mail: claus.buschmann(at)kit.edu

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/seeing-what-happens-inside-plants