Concepts for Understanding Fruit Trees

Preface

In addition to helping fruit growers, fruit tree enthusiasts, students and fellow scientists better understand how fruit trees work so they can manage and study them more effectively, I hope that, by reading this book, the reader gains a greater appreciation of the physiological, structural, ecological and evolutionary capacity of trees to cope with, and adapt to, short- and long-term changes in their environment. This book is the culmination of 40 years of research on trees, analysis of other scientists’ research, and teaching university students and fruit growers about fruit tree physiology and tree fruit production.

Although scientists often emphasize the complexity of biological systems—and they are indeed complex when considering details of their genetics, biochemistry, molecular and cellular biology, physiology and ecology—the thesis of this book is that there are a few unifying concepts that make much of the general behavior and responses of fruit trees to the environment or management fairly easy to understand and predict. Presenting those unifying concepts is the primary goal of this book. Thus, this book is not meant to provide exhaustive details about any individual species of fruit trees or detailed descriptions of the genetics, biochemistry, molecular and cellular biology, physiology or ecology of fruit trees. This type of information is found in numerous pomology textbooks. A recent textbook that I would recommend for those details is Principles of Modern Fruit Science (Sansavini, S., Costa, G., Gucci, R., Inglese, P., Ramina, A., et al. (eds), 2019, published by the International Society for Horticultural Science, Leuven, Belgium).

Many of the examples presented in this book are focused on stone fruit, specifically peach trees, because that was the primary species I was focused on for much of my career. However, I believe that all the general concepts presented in this book pertain to virtually all temperate deciduous fruit tree species, recognizing that specific patterns of behavior will vary by species and cultivar. Most of the concepts also probably pertain to evergreen fruit tree species with some adjustments of sensitivities to environmental signals that trigger seasonal activities; however, this is beyond the scope of this book.

As a person who believes in a creator, I count it a great privilege to have had the opportunity to spend the majority of an academic career studying the nature of the fruit trees as biological systems that are flexibly adapted to acclimate and adapt as required by nature and/or fruit tree managers. Not only did my career provide the opportunity to study the complexity of amazing biological systems, it also afforded the opportunity to work with numerous creative, enthusiastic, dedicated and industrious people of quality. I am indebted to many colleagues whom I have had the pleasure of working with over the past four decades. There are too many to name them all; however, I would be remiss if I did not mention a few who had particular impacts on my research and the formulation of many of the ideas that are contained in this book. These include Jim Doyle and Kevin Day, two of the most knowledgeable, practical pomologists in the University of California system; Carolyn DeBuse and Sarah Castro, pomology research associates; Steve Weinbaum and R. Scott Johnson, pomology faculty colleagues at the University of California, Davis (UC Davis); Eric Walton, Edelgard Pavel, Yaffa Grossman, Mat Berman, Gaston Esparza, Luis Solari, Darcy Gordon, Katherine Pope, Anna Davidson and Claudia Negron, graduate students in my program; Joan Girona, Isaac Klein, Adolfo Rosati, Jordi Marsal, Boris Basile, Gerardo Lopez and Sergio Tombesi, visiting scholars to my laboratory; and Mitchell Allen, Przemek Prusinkiewicz, Romeo Favreau and David Da Silva, postdoctoral scholars and collaborators involved in recent fruit tree modeling work.

Although I did not set out to write a book that summarized my academic research career, much of this book has turned out to be essentially that. The first four chapters mainly deal with how trees take up and assimilate resources (solar energy, carbohydrates, water and nutrients) as well as the general structure of trees. The information in these chapters is largely available in numerous other pomological or tree physiology textbooks. The subsequent chapters that address the distribution and use of these resources for growth over multiple seasons contain novel information and approaches to understanding tree behavior that are based mainly on research conducted with numerous colleagues during my career; this information is available in primary research articles but not yet available in textbooks on the subject.

In its essence, biology is the study of how living organisms cope with the constraints placed on them by the physical and chemical laws of the physical world in which they must survive and thrive. Thus, biology is essentially the discovery of biological solutions to solving problems posed by the physical world and other living organisms with which they coexist. This book focuses on concepts for understanding how a select group of biological organisms (fruit trees) are adapted to cope with the major constraints imposed on them by the physical world. In this context I choose to think of trees as problem solvers. Fruit trees are faced with multiple problems posed by physical limitations of their environment, such as variations in temperature, water, light and nutrient availability, and I try to identify some strategies that plants (especially fruit trees) have for solving those limitations in this book. However, in studying and thinking about plants and the problems they must solve, it is important to keep in mind that biology can’t change the laws of physics and chemistry, it can only develop strategies to deal with them.

Although scientists often emphasize the complexity of biological systems—and they are indeed complex when considering details of their genetics, biochemistry, molecular and cellular biology, physiology and ecology—the thesis of this book is that there are a few unifying concepts that make much of the general behavior and responses of fruit trees to the environment or management fairly easy to understand and predict. Presenting those unifying concepts is the primary goal of this book. Thus, this book is not meant to provide exhaustive details about any individual species of fruit trees or detailed descriptions of the genetics, biochemistry, molecular and cellular biology, physiology or ecology of fruit trees. This type of information is found in numerous pomology textbooks. A recent textbook that I would recommend for those details is Principles of Modern Fruit Science (Sansavini et al., 2019).

Most people who observe fruit trees and wonder how or why they behave in specific ways—such as growing very upright or more spreading, producing many flowers and small immature fruit only to drop most of the fruit later on, growing more on their sunny side than their shady side, etc.—ascribe such behavior to the trees as a whole. That is the wrong approach to understanding tree functioning and behavior. Trees are not in control of what they do. The control of what trees do and how they function lies in the individual organs that make up the tree, not in the tree as a whole. The genetic code of a tree only indirectly determines the habit, structure and behavior of a tree by defining the behavioral and functional limits of the organs, tissues and cells that make up the tree. Unlike animals, which have a mechanism for collective control of the whole organism (a nervous system), trees (and plants in general) can be more appropriately considered as collections of semi-autonomous organs that are dependent on one another for resources (water, energy (carbohydrates) and nutrients) but control their own destiny.

An excellent illustration of this principle is what has been described as a five-in-one fruit tree that is marketed to home gardeners. Such trees are attractive to home gardeners because they allow the production of different fruits (such as an early-ripening peach, a late-ripening peach, a nectarine, a plum and an apricot) all on a single tree. Such trees are produced by bud-grafting each of these fruit types on to a single rootstock. When this five-in-one tree grows, each branch representing the individual fruit types maintains the characteristics of those fruit types as if they were growing as individual trees. The structures produced by their growth (shoots, flowers, fruit, leaves, bark, etc.) and the timing of their development (bloom, leaf-out, pattern of shoot and fruit growth, time of fruit maturity, etc.) remain consistent with behaviors of the same varieties as individual trees. Therefore, the control of a tree cannot reside in the tree as a whole but trees must be viewed as collections of semi-autonomous organs that are collectively dependent on each other for resources to sustain their life. This brief book will explain how this view of fruit trees can lead to an integrated understanding of how fruit trees work.

Pomology (the science of growing fruit) has usually been approached by describing different types of fruit trees, their patterns of behavior and how they can be managed to efficiently produce. Tree physiology has generally been approached by describing the structural aspects and behavior of parts of trees and especially the perennial aspects of tree physiology, in addition to details about photosynthesis; respiration; nutrient uptake; biochemistry; water, nutrient and carbohydrate transport; etc., and then attributing the coordination of these processes and the behavior of the parts of the tree to the activities of plant hormones. However, there is rarely an attempt to address the fact that plant hormones are merely signaling molecules and there is little explanation for what or how the signals are initiated or controlled. Thus, there has been relatively little emphasis on trying to develop a set of unifying concepts for understanding what controls how fruit trees work.

I believe there is a general set of integrative concepts for understanding the overall physiology of temperate deciduous fruit trees. The emphasis here is on overarching principles rather than detailed descriptions of tree physiology or differences among the numerous species of fruit trees. Most of what is presented pertains mainly to temperate deciduous fruit trees but some aspects may also pertain to evergreen fruit trees or even trees that grow naturally in unmanaged situations.

Fruit trees are biological systems that have been engineered by nature and managed by humans to harvest solar energy to grow and produce fruit. Although most of our fruit trees have been genetically bred and selected by humans for specific characteristics that make them horticulturally valuable, it is important to remember that most of their physiological traits have been selected by nature to enhance their survival and reproduction in natural settings. In the process of developing strategies for dealing with the limitations of their physical world to survive and thrive on land, there are many issues to confront or problems to solve. It is instructive to continually analyze fruit trees from the viewpoint of understanding the problems they face for surviving and thriving on land, the adaptations they have for dealing with those problems and how they can be optimally managed for desired results.

In general, green plants are nature’s original biologically based solar-powered organisms. After the Industrial Revolution and until relatively recent advancements in modern sustainable energy development, much of the energy used to sustain life on earth has been derived from plant-based solar energy collection. The solar energy cells in green land plants are primarily housed in specialized structures (chloroplasts) within leaf cells and leaves are specially constructed to provide a controlled aqueous environment suitable for the function of those solar cells. Furthermore, the primary purpose of plant architectural structure (trunk, branches, stems and shoots) is to bear and display leaves so that they are exposed to solar energy (the sun’s rays). Thus, the productivity of any crop is a function of two things: the efficiency of resource (solar energy, water and nutrients) capture and assimilation; and the efficiency of distribution and use of those assimilated resources. Interestingly, while much research has been focused on understanding and enhancing the efficiency of resource capture and assimilation by plants, most agricultural advances have been based on manipulating resource distribution and use. This probably reflects the fact that it is hard to imagine a scenario in nature or in managed agriculture where anything but optimal resource capture and assimilation would be advantageous and selected for. Whereas there are many situations in managed agriculture where optimal distribution and use of resources in an anthropocentric monoculture may differ from what might be advantageous, and naturally selected for, in a competitive natural environment (e.g. plant stature, specific fruit characteristics, time of harvest, etc.). For this reason this book will only provide a cursory description of factors affecting the efficiency of resource capture and assimilation and the bulk of the focus will be placed on understanding how assimilated resources are distributed in fruit trees and how trees can be managed to optimize distribution of resources for horticultural gain.

Understanding the scheme of assimilate distribution within fruit trees allows for a more complete understanding of how fruit trees function and how horticultural manipulation of fruit trees can influence the growth of various organs, as well as how specific organ growth can be