Quantitatively, plants are the only relevant primary producers on earth. Thus, they constitute the base for all life on the planet. However, production is restricted to the light phase, because light provides energy for reduction of CO2. During the night, plants themselfes consume reduced carbon to fuel maintenance of their organs.
The figure on the right shows a diurnal profile of CO2 exchange of aerial organs in nmol/h*g fresh weight of an Arabidopsis thaliana plant during vegetative growth. The light phase starts at 8:00 am (480 min) and lasts until 4:00 pm (960 min). It becomse clear that during the light phase, more carbon is aquired than what is lost during the night. Thus, the aerial organs can export reduced carbon to supply sink organs like, e.g., the root. It is not easy to measure root respiration, because in soil, roots are associated with micro organisms that make a strong contribution to respiratory activity. We have developed a special technique to measure root respiration in an aeroponic system, and this allows us to study the full carbon balance of a plant over diurnal cycles.
During the light phase, plants produce carbohydrates that serve as building blocks for biomass and provide energy. Green leaves produce more carbohydrates than they need for their own metabolism, and they export the surplus to heterotrophic organs: young, growing leaves, roots that provide water and minerals, and flowers that asure reproduction.
Organs that export carbohydrates are called "sources", while those that import are called "sinks". Sources and sinks closely cooperate: only what is available can be consumed for growth and maintenance; but how much is produced also depends on demand.
We investigate, how supply and demand are regulated: do the sources dictate what will be available, or do they have to produce, what is needed?
In the relationship between sinks and sources, transport processes in the phloem, but also storage compounds play important roles. We investigate, if efficient storage of sugars in plant cells could stimulate photosynthesis. With respect to sugar storage, subcellular compartmentation is very important: while the large vacuole constitutes a nearly exhaustless reservoir, e.g., for fructans, storage of starch in the plastids can restrain photosynthesis. Nevertheless, in most plants, starch is the dominating carbon store. We investigate, how plants the detrmine the optimal amount of starch they produce to optimize diurnal change of light and dark phases.
Because carbohydrates function as transport as well as storage compounds, their diurnal dynamics are highly complex. To study organization of carbohydrate metabolism, we therefore use mathematical methods that allow simulation of non-intuitive processes.
A further level of complexity is given by the fact that carbohydrates also also involved in osmo-regulation of the cell. Thus, their concentrations are strongly influenced by environmental factors such as low temperature: during cold acclimation, large amounts of soluble sugars are accumulated that have protective functions. These environment interactions build another focus of our research.
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