Franks & Farquhar (2007) and beyond: The influence of guard and subsidiary cell bio-mechanics on stomatal function

Stomata are the gatekeepers to leaf gas exchange and as such are essential for CO₂ uptake for photosynthesis, water loss for evaporative cooling and ultimately plant growth and productivity. Research has long focused on understanding the mechanisms that govern stomatal aperture adjustments, in particular how changes in turgor pressure in GCs are achieved and how these cause the reversible cell shape distortion that leads to pore opening and closing. Here we discuss the immense impact the paper by Franks and Farquar (2007) has had on this research field, both in the past and more recently, by stimulating new angles of exploration of stomatal mechanics. As new techniques have emerged, the impact of the mechanical advantage of epidermal/subsidiary cells and how this is overcome by GCs has inspired novel studies into the importance of subsidiary to stomatal function, the effect of different cell shapes (dumbbell vs kidney shaped) on stomatal kinetics, the significance of cell geometries and cell complexes, as well as the underlying biochemical cell wall composition responsible for GC deformation. Whilst many questions remain unanswered, we summarize the take-home messages from the key literature on stomatal biomechanics.