Understanding the mechanisms of systemic signaling in dynamic photosynthesis

Plants grown in natural environment and greenhouses frequently experience changes in irradiance. When a shaded leaf is suddenly exposed to a high irradiance, leaf photosynthesis gradually increases rather than immediately increases to a steady-state level at high irradiance (a process termed as photosynthetic induction). When irradiance drops back to a low level, photosynthesis experiences a sharp decrease, due to slow relaxation of non-photochemical quenching (NPQ), and then slightly increases to a steady-state level at low irradiance (the post-illumination phase). Both the time needed to reach full induction at high irradiance and the time needed for NPQ relaxation upon an irradiance decrease lead to potential losses in carbon fixation. Previous studies have shown that pre-illuminating leaves in the upper part of a plant enhances the speed of photosynthetic induction in leaves at lower parts of the plant, which is a response relevant with systemic signaling (Hou et al., 2015). Systemic signaling allows plant to quickly adapt to environmental changes and has been widely studied in research topics such as plant responses to biotic and abiotic stresses. However, the role of systemic signaling in dynamic photosynthesis remains an under-explored topic. Previous studies suggested that a higher speed of photosynthetic induction caused by pre-illuminating upper leaves of the plant (or the whole plant) was relevant with a higher increasing rate of stomatal conductance (gs) of the target leaf upon an irradiance increase (Hou et al., 2015; Shimadzu et al., 2019). Despite the faster increase of gs, intercellular [CO2] dropped to a lower level at the beginning of light increase in pre-illuminated plants, indicating that the fast activation of Rubisco may also play a role in speeding up photosynthetic induction. However, it is not yet clear how biochemical and stomatal responses during photosynthetic induction determine the faster induction speed caused by systemic signaling, and how systemic signaling affects the post-illumination phase. This project aims to answer three research questions:

  1. Does systemic signaling induce a faster induction rate of Rubisco carboxylation rate (Vcmax)?
  2. Is there an anatomical constraint on the faster gs increase induced by the systemic signaling? For example, would species with large stomata (e.g. chrysanthemum) show less enhancement in gs increase, compared with species with small stomata (e.g. cucumber)?
  3. Is systemic signaling involved in the post-illumination phase? For example, could pre-illumination upper leaves cause a faster stomatal closure (for avoiding excessive water loss) and faster relaxation in NPQ?

Used skills

Performing climate chamber experiments using a gas exchange system (LI‐6800 or LI-6400) on cucumber and chrysanthemum, including measurements on photosynthesis and stomatal conductance in both induction and the post-illumination phases, chlorophyll fluorescence and dynamic A/Ci curves. Taking stomatal imprints samples and measuring stomatal size and density.

Interested in doing a BSc or MSc thesis at HPP? Please contact the HPP student coordinator Katharina Hanika.