WESTERN A rborist Botany for arborists: Energy and trees Dr. Kevin T. Smith and Dr. A. James Downer Energy capture glucose. Glucose is converted into free-living soil microorganisms. This The sun bathes the earth in waves of other sugars, such as sucrose for transformed solar energy then be- radiated energy. The waves of radia- translocation throughout the plant. comes part of the soil matrix and the tion occur along the electromagnetic These sugars are collectively known living web of soil organisms. In this spectrum that includes microwaves, as photosynthate. Other plant parts way, trees energize the environment radio waves, visible light, and infra- such as young branches or cortical around them. Even in death, trees red heat. The visible portion of that cells just beneath the epidermis or are a source of energy. As trees shed spectrum is the rainbow formed by thin bark can also photosynthesize leaves, flowers, branches, and roots, sunlight passing through a prism. (Fig. 2). In regions with very short or when the stem itself dies or falls, Solar energy is increasingly used to growing seasons, such as subarctic the energy bound in the plant parts power our homes, offices, and busi- forests and arid deserts, stem and becomes an energy source for sapro- nesses. Trees and other green plants branch photosynthesis may provide phytic bacteria and fungi. As plant have been using solar energy for a significant share of total energy parts decompose, they provide food many millions of years. Plants use captured. as well as habitat for many inverte- that radiant energy to make and Photosynthate moves to the fur- brates and other animals. break chemical bonds that provide thest root tip and eventually the soil The cellular machinery that per- the energy needed for growth and immediately around the roots, feed- forms photosynthesis is in chlo- survival. Much of the solar radiation ing beneficial mycorrhizal fungi and roplasts — small green structures that reaches the earth’s atmosphere is reflected or diffused. Less than 0.1% is available for plants. Of the light that actually reaches the leaves, Photosynthesis is the pathway for about 1.5% is used to fuel plant growth. the capture of energy and the bulk Leaves capture solar energy in the process of photosynthesis that of the physical matter in trees and converts radiant energy into chemi- cal energy (Fig. 1). The end prod- other green plants. uct of photosynthesis is the sugar Figure 1. Closeup of the leaf surface of Acer saccharum Figure 2. Photosynthesis in bark furrows of Ceiba speciosa (sugar maple) (floss silk tree). Fall 2019 28 WESTERN A rborist within the photosynthetic paren- use of these compounds to grow, re- use their sugars for metabolism, but chyma cells that comprise much of produce, and survive. in the soil as leaves and woody de- the leaf volume. Parenchyma cells Photosynthesis is the pathway for bris are decomposed by the micro- are relatively small, thin-walled, the capture of energy and the bulk of bial community, releasing bound and roughly uniform in size. Many the physical matter in trees and oth- energy. Humus, the organic matter parenchyma cells in the living tis- er green plants. There are two parts, that remains in the soil following sues lack chloroplasts, and function with the first involving the capture decomposition, resists further deg- to store energy, produce hormones, of solar energy to split the chemical radation, and may persist in the soil and defend against pathogens, in- bonds of water (H2O) into hydrogen for long periods. Humus serves to sect pests, and mechanical damage. and oxygen, and to transfer that solar increase soil water-holding capacity Within the chloroplasts are high- energy into short-lived high energy and to store essential potassium, cal- ly ordered membrane structures that chemical compounds (Fig. 3). The cium, and magnesium. support various parts of the photo- second part uses those high energy The three dominant elements synthetic process. The collection of compounds to split carbon from at- (carbon, hydrogen, and oxygen), photosynthetic paren- comprising about 98% chyma within the leaf of a tree’s dry weight, is the mesophyll. The Carbon dioxide enter during photosyn- photosynthetic cells in Water thesis. Carbon, which the upper or palisade High Energy makes up about half mesophyll are tightly compounds of that weight, is ob- packed in an upright ATP, NADPH tained from the carbon orderly array and are dioxide in the atmo- primarily concerned Light-Dependent Light-Independent sphere. Almost half of with light interception. Energy capture Carbon fixation the remaining weight The cells in the lower is composed of oxygen spongy mesophyll layer and hydrogen, derived are much less ordered ADP+PI, NADP from water. Just as es- and contain open spac- sential for the living es between some of the Low Energy tree, but required in cells. They are primar- compounds much smaller amounts, ily concerned with gas are nitrogen (about Oxygen exchange. Sugar 1%) and the remaining dozen elements (col- Energy flow and ele- lectively about 1%). In Figure 3. Sketch of photosynthesis showing the two sets of reactions ment cycling a simplistic way, the (circles), the entry of solar energy (bolt), the uptake of water and Let’s take a closer look heat given off by the carbon dioxide, and the production of oxygen and sugar. at the flow of energy burning of leaves or and mineral elements wood demonstrates in trees as well as the living world. mospheric carbon dioxide (CO2) and the energy contained within those Of the 92 chemical elements that nat- to recombine the carbon with the plant parts. The residual ash after urally occur on earth, 16 are proven oxygen and with the hydrogen from complete combustion contains the to be essential for all flowering water to eventually form a molecule mineral elements that were not vola- plants, including trees. A few other of glucose sugar (C6H12O6). Energy is tilized by the heat of burning, most elements have been proposed, but stored in the chemical bonds, partic- of these derived from soil. are not yet accepted as essential by ularly the bonds that link carbon to the consensus of biological chemists. carbon. Oxygen released from H2O Metabolism and biosynthesis Advanced physics teaches us that during photosynthesis diffuses out The sugar from photosynthesis is ultimately, energy and matter are of the leaf into the atmosphere. Res- the ultimate feedstock for all the interconvertible. For living systems, piration is the collection of biochemi- structural and energy needs of it’s convenient to think of matter as cal pathways that release energy trees. Those needs are met through the physical arrangement of chemi- stored in those chemical bonds and metabolism, the chemical processes cal elements, and energy as the prop- cycles CO2 back to the atmosphere. that occur within living organ- erty to perform the work of moving Some carbon compounds are se- isms to sustain life. Traditionally, matter. For living systems, energy is questered for long periods in wood the term primary metabolism re- contained within the bonds of chem- and organic matter in soil. Respira- fers to processes of normal growth, ical compounds. Organisms make tion occurs not only in trees as they development, and reproduction. 29 Fall 2019 WESTERN A rborist Secondary metabolism refers to ad- with attached nitrogen. Enzymes are lenges, energy is diverted from nor- ditional chemical pathways with catalysts that help make and break mal growth processes and applied specific functions including tree chemical bonds and transport ions to produce plant hormones (natural defense. Both primary and second- and other materials across mem- growth regulators), phenols (such as ary metabolism are essential for tree branes. Much of the nitrogen in trees tannins) and terpenes (the primary survival. The metabolic shifts result is contained in enzymes. The precise constituents of pine resin). Phenols from “turning on” or “upregulating” structure of enzymes are determined are most abundant in broad-leaved portions of the genetic program to by the genetic program, contained trees while conifers rely primarily change the amounts and specific en- within living cells. The instructions on terpenes. Both groups, though, zymes produced. Metabolism con- of the genetic code are recorded in produce these two classes of com- structs all parts of the tree through chains of nucleic acids. They, too, are pounds. When tree energy reserves biosynthesis. The major products of comprised of a sugar backbone en- are low, production of secondary biosynthesis are carbohydrates, pro- riched with phosphorous and nitro- metabolites for induced tree defense teins, lipids, and nucleic acids. gen. may be sluggish or non-existent, The transformations of sugar Another group of metabolites increasing the vulnerability of the for structural growth and energy derived from sugar are lipids. Lip- tree to pests, pathogens, and abiotic needs all involve chemical catalysts ids include fats and are generally stress. known as enzymes. Enzymes often insoluble in water. Lipids perform require small amounts of mineral important gatekeeper roles in cell Evergreen and deciduous solar elements such as phosphorous and membranes and cell walls. Lipids collectors magnesium as cofactors. Through are involved in both normal growth Leaves are collectively referred to as enzymatic activity, photosynthate is and induced tree defense through foliage. Conifers and many tropical converted into other simple sugars the formation of waterproofing lay- trees are typically evergreen and re- and more complex carbohydrates. ers (suberization) in bark, leaves, tain their foliage throughout the year. Carbohydrates include simple sug- stems, and roots. Broadleaved trees, with some excep- ars (monosaccharides such as glu- In mature trees, the bulk of the tions, have deciduous foliage which cose and fructose), disaccharides dry weight is composed of wood.