Humidity-driven shape morphing enhances fog harvesting in porous cactus spines

Cacti develop spines instead of conventional leaves, which often serve as mechanical defence against herbivores. However, some cactus species grow porous and flexible spines, suggesting fundamentally different functions. Here we demonstrate the mechanism of fog harvesting in the porous spines of Turbinicarpus alonsoi, a cactus native to central Mexico. Surprisingly, we discovered that the spines are highly hygroscopic and straighten when exposed to fog, leading to increased fog water collection rates. Experiments and numerical simulations confirm that straightening is driven by swelling-induced pressure in the cell walls of the spine tissue. Swelling results from capillary imbibition of fog water and predominantly generates expansion in the transverse plane, which causes the pre-curved spines to straighten. Despite their porosity and hygroscopicity, the spines prevent direct absorption of fog water into the living cortex due to the presence of a suberin-rich tissue layer at the spine base that instead promotes surface runoff towards the roots. Our work suggests that hygro-morphing emerges from distinct structural, biochemical and geometric adaptations of cactus spines, and enables a fine modulation of the flow dynamics on the surface of spines. We conclude that increasing plant water supply from fog by shape morphing may provide an adaptive advantage for survival of the species in a hot, semi-arid region with frequent fog formation. Significance statementIn many arid regions of the world, fog is a critical source of fresh water. This has shaped plant evolution in striking ways. For cacti with porous spines, a widely accepted belief is that they collect fog water directly by capillary imbibition. However, here we demonstrate that, in Turbinicarpus alonsoi, water-impermeable tissue at the spine base prevents direct transport of water into the living cortex. Instead, a dual process increases fog water collection: the curved spines initially imbibe fog water, which causes them to swell and straighten, and a thin liquid film then forms on the spine and runs off along the plant surface down to the roots. Hygro-morphing spines therefore enhance the capacity of cacti to collect fog water.