Adaptations for fog and dew harvesting in cactus spines

Water plays a crucial role for life on earth. In regions with water scarcity especially in deserts plants and animals have evolved intriguing ways of capturing and storing water for survival. Particularly interesting adaptations to cope with water scarcity can be observed in the cactus family: among the most important features are thick, waxy surfaces that cover the plant body, storage of water in stems and roots (known as succulence), high water use efficiency due to a particular acid metabolism (CAM), often a small growth form, and the development of spines and hairs instead of leaves. Presumably the most characteristic features of cacti are their pointy spines, which are commonly known as defense against animals. Interestingly, some species of cacti have spines with the additional function of harvesting fog and dew, thus complementing water uptake by the roots. For this process, the surface properties and the internal structure of the spines play a key role and determine the interaction with water droplets. Therefore, the aim of this project is to understand how the spine tissue develops and how the chemical composition contributes to fog and dew harvesting. In addition, the different pathways of water transport will be investigated systematically in different cacti from fog deserts and areas with dew formation. In order to gain insights into spine structure and water transport mechanisms, different techniques from materials science, botany and plant physiology will be employed. In a broader context, plant surfaces are interesting both for our understanding of nature and for technical applications, as plant surfaces exhibit a wide range of functions and properties (self -cleaning, hydrophilic, hydrophobic, etc.). In the past, this diversity was often used as bio-inspiration for technical applications and also offers great potential in the future.

Water plays a crucial role for life on earth. In regions with water scarcity - such as deserts - plants and animals have evolved intriguing ways of capturing and storing water for survival. Particularly interesting adaptations to cope with water scarcity can be observed in the cactus family: among the most important features are thick, waxy surfaces that cover the plant body, allowing to store water in stems and roots (known as succulence), high water use efficiency due to a particular acid metabolism (CAM), often a small growth form, and the development of spines and hairs instead of conventional leaves. The latter is presumably the most characteristic features of cacti: almost everyone knows how pointy and fierce cactus spines can be. Therefore, they are commonly known as defense against animals. However, this does not necessarily apply to all cacti, since some cacti develop porous and flexible spines. Due to their porous structure, a widely accepted belief was that porous spines collect fog water directly by capillary imbibition. However, in this project, we demonstrated that (in Turbinicarpus alonsoi, as a representative species), water-impermeable tissue at the spine base prevents direct transport of water into the living plant tissue. Instead, a dual process is responsible for increasing fog water collection: first, the curved spines initially imbibe fog water, which causes them to swell and straighten, and second, a thin liquid film then forms on the spine and runs off along the plant surface down to the roots. Our work is the first to describe hygro-morphing in cactus spines and explains how it enhances the capacity of cacti to collect fog water. For this process, the surface properties and the geometry of the spines play a key role. These insights are highly relevant for developing bio-inspired, shape-morphing materials and fog harvesting devices, which offers great potential for technological applications.