[Cactus thorns as water-absorbing organs]

Revisiting an ecophysiological oddity: Hydathode-mediated foliar water uptake in Crassula species from southern Africa

Hydathodes are usually associated with water exudation in plants. However, foliar water uptake (FWU) through the hydathodes has long been suspected in the leaf-succulent genus Crassula (Crassulaceae), a highly diverse group in southern Africa, and, to our knowledge, no empirical observations exist in the literature that unequivocally link FWU to hydathodes in this genus. FWU is expected to be particularly beneficial on the arid western side of southern Africa, where up to 50% of Crassula species occur and where periodically high air humidity leads to fog and/or dew formation. To investigate if hydathode-mediated FWU is operational in different Crassula species, we used the apoplastic fluorescent tracer Lucifer Yellow in combination with different imaging techniques. Our images of dye-treated leaves confirm that hydathode-mediated FWU does indeed occur in Crassula and that it might be widespread across the genus. Hydathodes in Crassula serve as moisture-harvesting structures, besides their more common purpose of guttation, an adaptation that has likely played an important role in the evolutionary history of the genus. Our observations suggest that ability for FWU is independent of geographical distribution and not restricted to arid environments under fog influence, as FWU is also operational in Crassula species from the rather humid eastern side of southern Africa. Our observations point towards no apparent link between FWU ability and overall leaf surface wettability in Crassula. Instead, the hierarchically sculptured leaf surfaces of several Crassula species may facilitate FWU due to hydrophilic leaf surface microdomains, even in seemingly hydrophobic species. Overall, these results confirm the ecophysiological relevance of hydathode-mediated FWU in Crassula and reassert the importance of atmospheric humidity for some arid-adapted plant groups.

Survival in desert: Extreme water adaptations and bioinspired structural designs

Deserts are the driest places in the world, desert creatures have evolved special adaptations to survive in this extreme water shortage environment. The collection and transport of condensed water have been of particular interest regarding the potential transfer of the underlying mechanisms to technical applications. In this review, the mechanisms of water capture and transport were first summarized. Secondly, an introduction of four typical desert creatures including cactus, desert beetles, lizards, and snakes which have special adaptations to manage water was elaborated. Thirdly, the recent progress of biomimetic water-collecting structures including cactus, desert beetles, and lizards inspired designs and the influence of overflow on water collection was demonstrated. Finally, the conclusions were drawn, and future issues were pointed out. The present study will further promote research on bioinspired water management strategies.

Passive water harvesting by desert plants and animals: lessons from nature

Fresh water sustains human life and is vital for human health. For some of the poorest countries, 1 in 10 people do not have access to safe and easily accessible water sources. Water consumption by man continues to grow with an increasing population. The current supply of fresh water needs to be supplemented to meet future needs. Living nature provides many lessons for water harvesting. It has evolved species which can survive in the most arid regions of the world by passively collecting water from fog and condensation of water vapour in the night. Before the collected water evaporates, species have mechanisms to transport water for storage or consumption. These species possess unique chemistry and structures on or within the body for collection and transport of water. Among the high diversity of species surviving in deserts, only a handful of species have been studied. Based on lessons from nature, bioinspired water harvesters can be designed. In this paper, an overview of various desert plants and animals is given and known water harvesting mechanisms of some are presented. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology (part 3)'.

Foliar water uptake: Processes, pathways, and integration into plant water budgets

Nearly all plant families, represented across most major biomes, absorb water directly through their leaves. This phenomenon is commonly referred to as foliar water uptake. Recent studies have suggested that foliar water uptake provides a significant water subsidy that can influence both plant water and carbon balance across multiple spatial and temporal scales. Despite this, our mechanistic understanding of when, where, how, and to what end water is absorbed through leaf surfaces remains limited. We first review the evidence for the biophysical conditions necessary for foliar water uptake to occur, focusing on the plant and atmospheric water potentials necessary to create a gradient for water flow. We then consider the different pathways for uptake, as well as the potential fates of the water once inside the leaf. Given that one fate of water from foliar uptake is to increase leaf water potentials and contribute to the demands of transpiration, we also provide a quantitative synthesis of observed rates of change in leaf water potential and total fluxes of water into the leaf. Finally, we identify critical research themes that should be addressed to effectively incorporate foliar water uptake into traditional frameworks of plant water movement.

Making a point: shared mechanics underlying the diversity of biological puncture

A viper injecting venom into a target, a mantis shrimp harpooning a fish, a cactus dispersing itself via spines attaching to passing mammals; all these are examples of biological puncture. Although disparate in terms of materials, kinematics and phylogeny, all three examples must adhere to the same set of fundamental physical laws that govern puncture mechanics. The diversity of biological puncture systems is a good case study for how physical laws can be used as a baseline for comparing disparate biological systems. In this Review, I explore the diversity of biological puncture and identify key variables that influence these systems. First, I explore recent work on biological puncture in a diversity of organisms, based on their hypothesized objectives: gripping, injection, damage and defence. Variation within each category is discussed, such as the differences between gripping for prey capture, gripping for dispersal of materials or gripping during reproduction. The second half of the Review is focused on specific physical parameters that influence puncture mechanics, such as material properties, stress, energy, speed and the medium within which puncture occurs. I focus on how these parameters have been examined in biology, and how they influence the evolution of biological systems. The ultimate objective of this Review is to outline an initial framework for examining the mechanics and evolution of puncture systems across biology. This framework will not only allow for broad biological comparisons, but also create a baseline for bioinspired design of both tools that puncture efficiently and materials that can resist puncture.

Superhydrophobic and superhydrophilic plant surfaces: an inspiration for biomimetic materials

The diversity of plant surface structures, evolved over 460 million years, has led to a large variety of highly adapted functional structures. The plant cuticle provides structural and chemical modifications for surface wetting, ranging from superhydrophilic to superhydrophobic. In this paper, the structural basics of superhydrophobic and superhydrophilic plant surfaces and their biological functions are introduced. Wetting in plants is influenced by the sculptures of the cells and by the fine structure of the surfaces, such as folding of the cuticle, or by epicuticular waxes. Hierarchical structures in plant surfaces are shown and further types of plant surface structuring leading to superhydrophobicity and superhydrophilicity are presented. The existing and potential uses of superhydrophobic and superhydrophilic surfaces for self-cleaning, drag reduction during moving in water, capillary liquid transport and other biomimetic materials are shown.