How memory shapes shoot gravitropism*

Tropisms are growth-driven motions by which plants reorient themselves in response to directional stimuli (eg. light or gravity). It has long been known that tropisms obey a dose-response relationship i.e. that plants respond to an integrated history of stimuli. Several recent studies have proposed an updated model of tropisms where the temporal integration behaviour is accounted for by a response function, or memory kernel.

Here, we aim to determine the memory kernel underlying shoot gravitropism. We conduct several kinds of transient gravistimulation experiments on wheat coleoptiles. The resulting data is then used to numerically invert the Volterra integral equation describing plant tropisms with memory.

Our approach reveals the complex shape of the memory kernel, displaying a striking negative lobe. We then show that this specific shape leads to previously unobserved regimes of tropic responses. Finally, we explore the processes underpinning the form of the memory kernel, showing that it depends on the stimulation time and the gravistimulation method.

Altogether, our findings provide a quantitative framework to further investigate behavioral processes in plant tropisms, as well as the microscopic origins of temporal integration.