Physics of sap ascent: Mechanisms and biomimetics – PHYSAP

First, we start with a detailed observation of embolisms in real leaves and wood at very high frequency, in optics with a high-speed camera operating at several thousand images per second or under fast X-rays. Acoustic recordings in the MHz range will also give precious insight. These unprecedented observations may unravel the still unknown reasons for the abrupt and intermittent propagation of embolisms. Attention will be given to the difference between tree species.
Second, we will reconstruct basic hydraulic networks in elastomer, in order to be able to model the speed of the propagation of the embolism. We will employ microfluidic techniques to craft the networks. As a starting approximation, constrictions in the channels will mimic the pits connecting conduits together. A more precise approximation will be to include membranes with small pores, which will be closer to the real pits. We will also develop biomimetic channels with an integrated flow regulation, in the objective of modelling stomata that regulate evaporation rates on real leaves.
Third, we will address a model of channels closer to real systems, with water under negative pressure inside stiff microchannels made of hydrogel. The propagation of cavitation from channel to channel will be tested when channels are isolated cells. Then we will go towards more realistic microfluidic network, with sap flowing and more complex topologies.

These three experimental approaches will be complemented by a modelling effort to tackle the role of physical and chemical parameters from the conduit scale to the xylem network on the embolism spreading dynamics. The main scientific outcome of the project will be a detailed understanding of the resistance of trees to drought.

This may prove useful for agricultural purposes, and to the modelling of effect of climate change on trees. New biomimetic autonomous devices regulating humidity, or designed for evaporative microfluidics systems, will also be inspired by this project.

-

The present project aims at understanding the physics of the most important network on earth for living animals: the hydraulic network conducting sap towards the leaves, which results in efficient photosynthesis and oxygen production released in the atmosphere. Here we propose to explore the rupture of this hydraulic network after cavitation periods, during strong hydric stress, which are expected to occur in a context of global climate change with more severe drought.
The objective of this ambitious project gathering physicists and biologists is to obtain a physical understanding of the cavitation propagation at the scale of the conduits within plants. Cavitation is a gaseous embolism that tends to spread, stopping sap circulation. But the exact mechanism and dynamics of the propagation of embolism is still not clear.
Three phases will be conducted in parallel to pursue this objective:
First, we start with a detailed observation of embolisms in real leaves and wood at very high frequency, in optics with a high-speed camera operating at several thousand images per second or under fast X-rays. Acoustic recordings in the MHz range will also give precious insight. These unprecedented observations may unravel the still unknown reasons for the abrupt and intermittent propagation of embolisms. Attention will be given to the difference between tree species.
Second, we will reconstruct basic hydraulic networks in elastomer, in order to be able to model the speed of the propagation of the embolism. We will employ microfluidic techniques to craft the networks. As a starting approximation, constrictions in the channels will mimic the pits connecting conduits together. A more precise approximation will be to include membranes with small pores, which will be closer to the real pits. We will also develop biomimetic channels with an integrated flow regulation, in the objective of modelling stomata that regulate evaporation rates on real leaves.
Third, we will address a model of channels closer to real systems, with water under negative pressure inside stiff microchannels made of hydrogel. The propagation of cavitation from channel to channel will be tested when channels are isolated cells. Then we will go towards more realistic microfluidic network, with sap flowing and more complex topologies.
These three experimental approaches will be complemented by a modelling effort to tackle the role of physical and chemical parameters from the conduit scale to the xylem network on the embolism spreading dynamics. The main scientific outcome of the project will be a detailed understanding of the resistance of trees to drought. This may prove useful for agricultural purposes, and to the modelling of effect of climate change on trees. New biomimetic autonomous devices regulating humidity, or designed for evaporative microfluidics systems, will also be inspired by this project.