The biomechanics of turgor pressure

If you ever forget to water your houseplant, you may find its leaves getting soft and droopy - if you water it again in time, the leaves may stiffen, spring back up, and resist gravity. During this recovery, plant cells absorb water and build up an intracellular pressure, called turgor pressure, similar to inflating a balloon. Turgor pressure is an intrinsic component of plant physiology, and its biomechanical role as the 'hydroskeleton' is generally appreciated either statically in structural stability, like leaves resisting gravity, or dynamically in rapid motions, like Venus flytrap snapping, Mimosa closing, or stomatal opening. Slow but non-static processes like plant cell expansion also rely on turgor pressure, and it has been increasingly realized that turgor pressure and water fluxes in plant tissues play active roles in plant growth and morphogenesis. But where does turgor pressure come from, and how does it interact with other biomechanical properties of a plant cell? This primer aims to answer these questions by taking a brief tour from osmosis, to pressure vessel theory, then plant cell rheology. Although this primer is centered around intracellular pressure in plant cells, the biomechanical concepts are generally applicable to other organisms like bacteria and fungi, and even animal cells, which do not have cell walls, but are either embedded in stiff extracellular matrix (ECM) or are contracting (see the following section). To explain some of these concepts, we included a few equations, most of which are not hard to derive at all. We encourage readers to check the included examples, try for themselves, and look up derivations in suggested further reading materials.