Preface

Vascular plants have inhabited Earth's surface for about 400 million years. They lead the evolutionary race by all accounts,¹  and their importance to the terrestrial ecosystem, geology, and human civilization requires no emphasis. Yet, plants remain mysterious when observed from the vantage point of a physicist or an engineer. How do organisms made from just a few soft polymers, minerals, and water perform such striking physical and chemical feats? The remarkable relationship between structure, deformation, flow, and function in plants is the topic of this book.

A myriad of reasons has been put forward to justify the scientific study of mechanics and fluid flow in plants. In most instances, they fall within the four categories proposed by T. J. Pedley:²  (1) physiology and ecology: understanding how plants work and interact with other living organisms and the environment; (2) stress: understanding how strong fluctuations impact plants; (3) pathology: discerning the origins and development of diseases; (4) bioengineering: exploring synthetic changes to the plant genome. It is evident that scientists working in these diverse research fields will be motivated by different research questions and applications of their work. However, the essential fluid and solid mechanical principles are clearly the same for all. Exploring the physics of both equilibrium and extreme cases as they relate to the mechanical integrity and transport capacity of plants is therefore worthwhile.

From a societal perspective, it is appears that plants will become more (or at least not less) significant to humans in the coming decades and centuries. To this end, substantial resources are being invested into research programmes with the goal of improving plant productivity, water and fertiliser efficiency, disease resistance, and so on. The critical question, however, is how best to achieve this goal. On the one hand, our understanding of molecular processes in plants is steadily advancing, and many transformation tools have become more widely available. Also, certain key activities, such as photosynthesis or starch biosynthesis, are being elucidated with ever greater accuracy. On the other hand, the strong focus in research and funding on molecular biology has yet to achieve a Moon-shot jump in productivity. The reason for this is, of course, as of yet unknown. One cannot help wonder, however, if pursuing a complementary approach to understand the mechanics of plants at mesoscopic and microscopic scales is worthwhile. Such an approach would bridge the intellectual gap between the action of genes and proteins and the biophysical or biochemical processes with which they are ultimately associated. It would also help us better understand plants’ uniquely distributed organismal architecture. However, this portfolio of research questions are, we believe, too often overlooked.

This book is written in an effort to place soft matter physics, which naturally focuses on mesoscopic and microscopic scales, in the context of modern plant science. As such, our objective is twofold: first, we aim to introduce physicists to a myriad of fascinating and important phenomena in plants. Second, we seek to highlight the benefits of using reasonably simple models to describe tangled biological systems. Physics applied to biology has often involved bringing new instrumentation to tackle biological questions. However, we do not believe this should be the sole focus. Physics represents a way of thinking that can shed new light on biological questions:³  the desire to find the simplest possible description, with a few essential ingredients, of complex phenomena is a unique perspective that has important advantages (and yes, limitations as well). Equally important is the fact that biology can inspire physicists to address new questions in physics, e.g., related to transport networks or efficient design strategies.

Having established the need for a concise introduction to soft matter in plants, we were first to recognize our own inability to provide such an overview. We are therefore extremely grateful to our editor Michelle Carey who permitted us to commission chapters from some of the leading researchers working in the field. Apart from two introductory chapters written by us, their work comprises the main body of this book, and should be recognised as such.

The book begins with a general introductory chapter giving the basic physical concepts needed for describing important processes in plant biophysics and biomechanics, such as water transport, growth or plant movements (Chapter 1). It is then followed by six independent chapters covering a wide range of applications and scales, from cell and tissue physics to engineering applications. Chapter 2 focuses on fluid–structure interaction phenomena in plants, in connection with signaling, vascular transport or intracellular flows. Chapter 3 is devoted to the mechanics of growth and morphogenesis, a central topic in modern biology. Starting from the seminal work of Lockhart on single plant cell growth, it provides the basic theoretical tools and concepts for modeling growing plant tissues. Chapter 4 provides an overview of the rich physics of water stress and cavitation, as found in the vascular system of plants during sap ascent, with particular emphasis on cavitation in plant-relevant confined and deformable environments. The book then moves underground to explore the fascinating world of plant roots (Chapter 5). This chapter details how soil affects the physics of root growth and the feedback of roots on the mechanical properties of the soil. Root penetration in the soil can be seen as a particular case of invasive growth – a topic of broad importance in biology and related to reproduction, nutrition and disease transmission. Chapter 6 discusses the experimental strategies developed to investigate the biomechanics of invasive growth at the cellular level in plants and fungi. The book closes with a discussion on biomimetic applications of plant movements, focusing on passive movements induced by humidity change in botanical and artificial systems (Chapter 7).

Kaare H. Jensen and Yoel Forterre