Discovery of a new active matter process that controls plant-atmosphere coupling

The leaves of plants dominate the exchange of water and carbon dioxide with the atmosphere over land. Their ability to regulate this exchange defines their productivity and resilience. To date, physiologists have recognized only a single point of control of this exchange, namely, via the opening and closing of stomata in the epidermis. Here, I will describe experiments with a new fluorescent nanoreporter of chemical potential of water that has allowed us to interrogate the constitutive properties relative to water transfer of the leaf mesophyll, the tissue that carries water from the xylem vessels to the sites of evaporation beneath the stomates. I will present: i) our discovery that this living tissue undergoes a massive loss of conductivity to water that leads to significant undersaturation inside the leaf, ii) the manner in which this process serves as a previously unrecognized point of control for water evaporation (transpiration), and iii) a model of multiphase transport processes in the mesophyll that guides our interpretation of our observations. I will show that the conductance of mesophyll tissue to water is under active biological regulation: a phenomenon that is captured by a pseudo 1-D representation of mass transfer with a single non-dimensional number describing the onset of extreme loss of hydraulic conductivity and emergence of non-stomatal control of transpiration.