Chapter 6 Ecology

Chapter 6 Section 2 Jelena Micic - HA Ecology 5/5/2023 The Organism and its environment All life on earth is carbon based. This means all living creatures are made up of complex molecules built on a framework of carbon atoms. The carbon atom is able to bond readily with other carbon atoms, forming...

Chapter 6 Section 2 Jelena Micic - HA Ecology 5/5/2023 The Organism and its environment All life on earth is carbon based. This means all living creatures are made up of complex molecules built on a framework of carbon atoms. The carbon atom is able to bond readily with other carbon atoms, forming long, complex molecules. The carbon atoms needed to construct these molecules—the building blocks of life—are derived from various sources. The means by which organisms acquire and use carbon represent some of the most basic adaptations required for life. Humans, like all other heterotrophs, gain their carbon by consuming other organisms. However, the ultimate source of carbon from which life is constructed is carbon diox-ide (CO2) in the atmosphere. Not all living organisms can use this abundant form of car-bon directly. Only autotrophs can transform carbon in the form of CO2 into organic molecules and living tissue. Autotrophs can be subdivided into chemoautotrophs and photoautotrophs, depending on how they derive the energy for their metabolism. Chemoautotrophs convert carbon dioxide into organic matter using the oxidation of inorganic molecules (such as hydrogen gas or hydrogen sulfide) or methane as a source of energy. Most chemotrophs are bacteria or archaea that live in hostile environments such as the hydrothermal vents of the deep sea floor (see Chapter 24). Photoautotrophs, the dominant form of autotrophs, use the Sun’s energy to drive the process of con-verting CO2 into simple organic compounds. That process, car-ried out by green plants, algae, and some types of bacteria, is photosynthesis. Photosynthesis is essential for the maintenance of life on Earth. Although all green plants derive their carbon from photo-synthesis, how organisms, from the most minute of flowering plants (members of the duckweed family— Lemnaceae) to the largest of trees, allocate the products of photosynthesis to the basic processes of growth and maintenance varies immensely. These differences represent the diverse outcomes of evolution that allow plants to acquire the essential resources of carbon, light, water, and mineral nutrients necessary to support the process of photosynthesis. In this chapter, we will examine the variety of adaptations that have evolved in plants that allow them to survive, grow, and reproduce across virtually the entire range of environmental conditions found on Earth. First, let us review the process so essential to life on Earth—or as the author John Updike so poetically phrased it, “the lone reaction that counterbalances the vast expenditures of respiration, that reverses decomposition and death.” 6.1 photosynthesis is the Conversion of Carbon dioxideinto Simple Sugars Photosynthesis is the process by which energy from the Sun, in the form of shortwave radiation, is harnessed to drive a se-ries of chemical reactions that result in the fixation of CO2 into carbohydrates (simple sugars) and the release of oxygen (O2) as a by-product. The portion of the electromagnetic spectrum that photosynthetic organisms use is between 400 and 700 nanometers (nm; roughly corresponding to the visible portion of the spectrum) and is referred to as photosynthetically active radiation (PAR). The process of photosynthesis can be expressed in the sim-plified form shown here: 6CO2 + 12H2OSC6H12O6 + 6O2 + 6H2O The net effect of this chemical reaction is the use of six molecules of water (H2O) and the production of six molecules of oxygen (O2) for every six molecules of CO2 that are transformed into one molecule of sugar C6H12O6. The synthesis of vari-ous other carbon-based compounds—such as complex carbo-hydrates, proteins, fatty acids, and enzymes—from these initial products occurs in the leaves as well as other parts of the plant. Photosynthesis, a complex sequence of metabolic reac-tions, can be separated into two processes, often referred to as the light-dependent and lightindependent reactions. The light-dependent reactions begin with the initial photochemical reac-tion in which chlorophyll (light-absorbing pigment) molecules within the chloroplasts absorb light energy. The absorption of a photon of light raises the energy level of the chlorophyll molecule. The excited molecule is not stable, and the electrons rapidly return to their ground state, thus releasing the absorbed photon energy. This energy is transferred to another acceptor molecule, resulting in a process called photosynthetic electron transport. This process results in the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and of NADPH (the reduced form of nicotinamide adenine dinucleo-tide phosphate [NADP]) from NADP+. The high-energy sub-stance ATP and the strong reductant NADPH produced in the light-dependent reactions are essential for the second step in photosynthesis— the light-independent reactions. In the light-independent reactions, CO2 is biochemically incorporated into simple sugars. The light-independent reac-tions derive their name from the fact that they do not directly require the presence of sunlight. They are, however, dependent on the products of the light-dependent reactions and therefore ultimately depend on the essential resource of sunlight. The process of incorporating CO2 into simple sugars be-gins in most plants when the five-carbon molecule ribulose bi-phosphate (RuBP) combines with CO2 to form two molecules of a three-carbon compound called phosphoglycerate (3-PGA). This reaction, called carboxylation, is catalyzed by the enzyme rubisco (ribulose biphosphate carboxylase-oxygenase). The plant quick hate (G3P). The synthesis of G3P from 3-PGA requires both ATP and NADPH—the high-energy molecule and reductant that are formed in the light-dependent reactions. Some of this G3P is used to produce simple sugars (C6H12O6), starches, and other carbohydrates required for plant growth and maintenance; the re-mainder is used to synthesize new RuBP to continue the process. The synthesis of new RuBP from G3P requires additional ATP In this way, the availability of light energy (solar radiation) can limit the lightindependent reactions of photosynthesis through its control on the production of ATP and NADPH required for the synthesis of G3P and the regeneration of RuBP. This photosyn-thetic pathway involving the initial fixation of CO2 into the three-carbon PGAs is called the Calvin–Benson cycle, or C3 cycle, and plants employing it are known as C3 plants (Figure 6.1). The C3 pathway has one major drawback. The enzyme rubisco that drives the process of carboxylation also acts as an oxygenase; rubisco can catalyze the reaction between O2 and RuBP. The oxygenation of RuBP results in the eventual release of CO2 and is referred to as photorespiration (not to be con-fused with the process of cellular respiration discussed herein). This competitive reaction to the carboxylation process reduces the efficiency of C3 photosynthesis by as much as 25 percent. Some of the carbohydrates produced in photosynthesis are used in the process of cellular respiration—the harvesting of energy from the chemical breakdown of simple sugars and other carbohydrates. The process of cellular respiration (also referred to as aerobic respiration) occurs in the mitochondria of all living cells and involves the oxidation of carbohydrates to generate energy in the form of ATP. C6H12O6 + 6O2 S 6CO2 + 6H2O + ATP Because leaves both use CO2 during photosynthesis and produce CO2 during respiration, the difference in the rates of these two processes is the net gain of carbon, referred to as net photosynthesis. Net photosynthesis = Photosynthesis - Respiration Mesophyll cells Chloroplasts The rates of photosynthesis and respiration, and therefore net photosynthesis, are typically measured in moles CO2 per unit leaf area (or mass) per unit time (μmol/m2/s). 6.2 the light a plant receives affects its photosynthetic activity Solar radiation provides the energy required to convert CO2 into simple sugars. Thus, the availability of light (PAR) to the leaf directly influences the rate of photosynthesis (Figure 6.2). At night, in the absence of PAR, only respiration occurs and the net uptake of CO2 is negative. The rate of CO2 loss when the value of PAR is zero provides an estimate of the rate of respiration. As the Sun rises and the value of PAR increases, the rate of photosynthesis likewise increases, eventually reach-ing a level at which the rate of CO2 uptake in photosynthesis is equal to the rate of CO2 loss in respiration. At that point, the rate of net photosynthesis is zero. The light level (value of PAR) at which this occurs is called the light compensation point (LCP). As light levels exceed the LCP, the rate of net photosynthesis increases with PAR. Eventually, photosynthesis becomes light saturated. The value of PAR, above which no further increase in photosynthesis occurs, is referred to as the light saturation point. In some plants adapted to extremely shaded environments, photosynthetic rates decline as light lev-els exceed saturation. This negative effect of high light levels, called photoinhibition, can be the result of “overloading” the processes involved in the light-dependent reactions. 6.3 photosynthesis involves exchanges between the plant and atmosphere The process of photosynthesis occurs in specialized cells within the leaf called mesophyll cells (see Figure 6.1). For photosynthesis to take place within the mesophyll cells, CO2 must move from the outside atmosphere into the leaf. In terrestrial (land) plants, CO2 enters the leaf through openings on its surface called stomata (Figure 6.3) through the process of diffusion. Diffusion is the movement of a substance from areas of higher to lower concentration. CO2 diffuses from areas of higher concentration (the air) to areas of lower con-centration (the interior of the leaf). When the concentrations are equal, an equilibrium is achieved and there is no further net exchange. Two factors control the diffusion of CO2 into the leaf: the diffusion gradient and stomatal conductance. The diffusion gra-dient is defined as the difference between the concentration of CO2 in air adjacent to the leaf and the concentration of CO2 in the leaf interior. Concentrations of CO2 are often described in units of parts per million (ppm) of air. A CO2 concentration of 400 ppm would be 400 molecules of CO2 for every 1 million molecules of air. Stomatal conductance is the flow rate of CO2 through the stomata (generally measured in units of μmol/m2/s) and has two components: the number of stoma per unit leaf sur-face area (stomatal density) and aperture (the size of the stomatal openings). Stomatal aperture is under plant control, and stomata open and close in response to a variety of environmental and biochemical factors. As long as the concentration of CO2 in the air outside the leaf is greater than that inside the leaf and the stomata are open, CO2 will continue to diffuse through the stomata into the leaf. So as CO2 diffuses into the leaf through the stomata, why do the concentrations of CO2 inside and outside the leaf not come into equilibrium? The concentration inside the leaf declines as CO2 is transformed into sugar during photosynthesis. As long as photosynthesis occurs, the gradient remains. If photosynthesis stopped and the stomata remained open, CO2 would diffuse into the leaf until the internal CO2 equaled the outside concentration. When photosynthesis and the demand for CO2 are reduced for any reason (such as a reduction in light), the stomata tend to close, thus reducing flow into the leaf. The stomata close because they play a dual role. As CO2 diffuses into the leaf through the stomata, water vapor inside the leaf diffuses out through the same openings. This water loss through the sto-mata is called transpiration. As with the diffusion of CO2 into the leaf, the rate of water diffusion out of the leaf will depend on the diffusion gradient of water vapor from inside to outside the leaf and the stomatal conductance (flow rate of water). Like CO2, water vapor diffuses from areas of high concentration to areas of low concentration—from wet to dry. The relative humidity (see Section 2.5, Figure 2.15) inside a leaf is typically greater than 99 percent, therefore there is usually a large difference in water vapor concentration between the inside and outside of the leaf, resulting in the diffusion of water out of the leaf. The lower the relative humidity of the air, the larger the diffusion gradient and the more rapidly the water inside the leaf will diffuse through the stomata into the surrounding air. The leaf must replace the water lost to the atmosphere, otherwise it will wilt and die. 6.4 Water Moves from the Soil, through the plant, to the atmosphere The force exerted outward on a cell wall by the water contained in the cell is called turgor pressure. The growth rate of plant cells and the efficiency of their physiological processes are highest when the cells are at maximum turgor—that is, when they are fully hydrated. When the water content of the cell declines, turgor pressure drops and water stress occurs, ranging from wilting to dehydration. For leaves to maintain maximum turgor, the water lost to the atmosphere in transpiration must be replaced by water taken up from the soil through the root system of the plant and transported to the leaves. You may recall from basic physics that work—the dis-placement of matter, such as transporting water from the soil into the plant roots and to the leaves—requires the transfer of energy. The measure of energy available to do work is called Gibbs energy (G), named for the U.S. physicist Willard Gibbs, who first developed the concept in the 1870s. In the process of active transport, such as transporting water from the ground to an elevated storage tank using an electric pump, the input of energy to the system is in the form of electricity to the pump. The movement of water through the soil–plant–atmosphere continuum, however, is an example of passive transport, a spontaneous reaction that does not require an input of energy to the system. The movement of water is driven by internal differences in the Gibbs energy of water at any point along the continuum between the soil, plant, and atmosphere. The Second Law of Thermodynamics states that the transfer of energy (through either heat or work) always proceeds in the direction from higher to lower energy content (e.g., from hot to cold). Therefore, a gradient of decreasing energy content of the water between any two points along the continuum must exist to enable the passive movement of water between the soil, plant, and atmosphere. The measure used to describe the Gibbs energy of water, at any point along the soil– plant–atmosphere continuum is called water potential (C). Water potential is the difference in Gibbs energy per mole (the energy available to do work) between the water of interest and pure water (at a standard temperature and pressure). Plant physiologists have chosen to express water potential in terms of pressure, which has the dimensions of energy per volume, and is expressed in terms of Pascals (Pa = 1 Newton/m2). By convention, pure water at atmospheric pressure has a water potential of zero and the addition of any solutes or the creation of suction (negative hydrostatic pressure) will function to lower the water potential (more negative values). We can now examine the movement of water through the soil–plant–atmosphere continuum as a function of the gradient in water potential. As previously stated, the transfer of energy will always proceed from a region of higher energy content to a region of lower energy content, or in the case of water po-tential, from areas of higher water potential to areas of lower water potential. We can start with the exchange of water be-tween the leaf and the atmosphere in the process of transpiration. When relative humidity of the atmosphere is 100 percent, the atmospheric water potential (Catm) is zero. As values drop below 100 percent, the value of Gibbs energy declines, and Catm becomes negative (Figure 6.4a). Under most physiologi-cal conditions, the air within the leaf is at or near saturation (relative humidity ∼ 99 percent). As long as the relative hu-midity of the air is below 99 percent, a steep gradient of water potential between the leaf (Cleaf) and the atmosphere (Catm) will drive the process of diffusion. Water vapor will move from the region of higher water potential (interior of the leaf) to the region of lower water potential (atmosphere)— that is, from a state of high to low Gibbs energy. As water is lost to the atmosphere through the stomata, the water content of the cells decreases (turgor pressure drops) and in turn increases the concentration of solutes in the cell. This decrease in the cell’s water content (and corresponding in-crease in solute concentration) decreases the water potential of the cells. Unlike the water potential of the atmosphere, which is determined only by relative humidity, several factors deter-mine water potential within the plant. Turgor pressure (posi-tive pressure) in the cell increases the plant’s water potential. Therefore, a decrease in turgor pressure associated with water loss functions to decrease water potential. The component of plant water potential as a result of turgor pressure represents hydrostatic pressure and is represented as Cp. Increasing concentrations of solutes in the cells are associ-ated with water loss and will lower the water potential. This component of plant water potential is termed osmotic poten-tial (Cπ) because the difference in solute content inside and outside the cell results in the movement of water through the process of osmosis. The surfaces of larger molecules, such as those in the cell walls, exert an attractive force on water. This tendency for wa-ter to adhere to surfaces reduces the Gibbs energy of the water molecules, reducing water potential. This component of water potential is called matric potential (Cm). The total water po-tential C at any point in the plant, from the leaf to the root, is the sum of these individual components: The osmotic and matric potentials will always have a negative value, whereas the turgor pressure (hydrostatic pres-sure) component can be either positive or negative. Thus, the total potential can be either positive or negative, depending on the relative values of the individual components. Values of total water potential at any point along the continuum (soil, root, leaf, and atmosphere), however, are typically negative and the movement of water proceeds from areas of higher (zero or less negative) to lower (more negative) potential (from the region of higher energy to the region of lower energy). Therefore, the movement of water from the soil to the root, from the root to the leaf, and from the leaf to the atmosphere depends on main-taining a gradient of increasingly negative water potential at each point along the continuum (Figure 6.4). Drawn by the low water potential of the atmosphere, water from the surface of and between the mesophyll cells within the leaf evaporates and escapes through the stomata. This gradient of water potential is transmitted into the mesophyll cells and on to the water-filled xylem (hollow conducting tubes throughout the plant) in the leaf veins. The gradient of increasingly nega-tive water potential extends down to the fine rootlets in contact with soil particles and pores. As water moves from the root and up through the stem to the leaf, the root water potential declines so that more water moves from the soil into the root. Water loss through transpiration continues as long as (1) the amount of energy striking the leaf is enough to supply the nec-essary latent heat of evaporation (see Section 2.5), (2) moisture is available for roots in the soil, and (3) the roots are capable of maintaining a more negative water potential than that of the soil. At field capacity, water is freely available, and soil water poten-tial (Csoil) is at or near zero (see Section 4.8). As water is drawn from the soil, the water content of the soil declines, and the soil water potential becomes more negative. As the water content of the soil declines, the remaining water adheres more tightly to the surfaces of the soil particles, and the matric potential becomes more negative. For a given water content, the matric potential of soil is influenced strongly by its texture (see Figure 4.10). Soils composed of fine particles, such as clays, have a higher surface area (per soil volume) for water to adhere to than sandy soils do. Clay soils, therefore, are characterized by more negative matric potentials for the same water content. As soil water potential becomes more negative, the root and leaf water potentials must decline (become more negative) if the potential gradient is to be maintained. If precipitation does not recharge soil water, and soil water potentials continue to decline, eventually the gradient between the soil, root, and leaf cannot be maintained, and at that point, the stomata close to stop further water loss through transpiration. However, this closure also results in stopping further uptake of CO2. The value of leaf water potential at which stomata close and net photosynthesis ceases varies among plant species (Figure 6.5) and reflects basic differences in their biochemistry, physiology, and morphology. The rate of water loss varies with daily environmental conditions, such as humidity and temperature, and with the characteristics of plants. Opening and closing the stomata is probably the plant’s most important means of regulating water loss. The trade-off between CO2 uptake and water loss through the stomata results in a direct link between water availability in the soil and the plant’s ability to carry out pho-tosynthesis. To carry out photosynthesis, the plant must open its stomata; but when it does, it loses water, which it must replace to live. If water is scarce, the plant must balance the opening and closing of the stomata, taking up enough CO2 while minimizing the loss of water. The ratio of carbon fixed (photosynthesis) per unit of water lost (transpiration) is called the water-use efficiency. We can now appreciate the trade-off faced by terrestrial plants. To carry out photosynthesis, the plant must open the stomata to take up CO2. But at the same time, the plant loses water through the stomata to the outside air—water that must be replaced through the plant’s roots. If its access to water is limited, the plant must balance the opening and closing of stomata to allow for the uptake of CO2 while minimizing water loss through transpiration. This balance between photosynthe-sis and transpiration is an extremely important constraint that has governed the evolution of terrestrial plants and directly influences the productivity of ecosystems under differing envi-ronmental conditions (see Chapter 20). 6.5 the process of Carbon uptake differs for aquatic and terrestrial autotrophs A major difference in CO2 uptake and assimilation by aquatic autotrophs (submerged plants, algae, and phytoplankton) versus terrestrial plants is the lack of stomata in aquatic autotrophs. CO2 diffuses from the atmosphere into the surface waters and is then mixed into the water column. Once dissolved, CO2 reacts with the water to form bicarbonate is that some aquatic species can also use bicarbonate as a car-bon source. However, the organism must first convert it to CO2 using the enzyme carbonic anhydrase. This conversion can oc-cur in two ways: (1) active transport of bicarbonate into the cell followed by conversion to CO2 or (2) excretion of the enzyme into adjacent waters and subsequent uptake of converted CO2 across the membrane. As CO2 is taken up, its concentration in the waters adjacent to the organism decline. Because the dif-fusion of CO2 in water is 104 times slower than in the air, it can easily become depleted (low concentrations) in the waters adjacent to the organism, reducing rates of uptake and photo-synthesis. This constraint can be particularly important in still waters such as dense seagrass beds or rocky intertidal pools. 6.6 plant temperatures reflect their energy Balance with the Surrounding environment Both photosynthesis and respiration respond directly to varia-tions in temperature (Figure 6.6). As temperatures rise above freezing, both photosynthesis and respiration rates increase. Initially, photosynthesis increases faster than respiration. As temperatures continue to rise, the photosynthetic rate reaches a maximum related to the temperature response of the enzyme rubisco. As temperatures continue to rise, photosynthetic rate declines and respiration rate continues to increase. As temperatures rise further, even respiration declines as temperatures reach critical levels. The temperature response of net photo-synthesis is the difference between the rate of carbon uptake in photosynthesis and the rate of carbon loss in respiration. This reaction is reversible, and the concentrations of CO2 and bicarbonate tend toward a dynamic equilibrium (see Section 3.7). In aquatic autotrophs, CO2 diffuses directly from the waters across the cell membrane Action aquatic plants do so primarily by convection. Evaporation occurs in the process of transpiration. Recall from chapter three that the phase change of water from a liquid to a gas evaporation requires an input of thermal energy 540 calories or 2260 jewels per gram G. Of water. As waters transpires from the leaves of plants to the surrounding atmosphere through the stomata, the leaves lose thermal energy and their temperature declines through evaporative cooling. See section 3.2. The ability of terrestrial plants to dissipate heat by evaporation is dependent on the rate of transpiration. The transpiration rate is in turn influenced by the relative humidity of the air and by the availability of water to the plant. See section 6.3. Convection is the transfer of heat energy through the circulation of fluids, Chapter 2, whereas conduction is the transfer of thermal energy through direct contact between two objects. For convection to occur, the surface of the leaf must first transfer thermal energy between the adjacent molecules of air or water through conduction. The direction of this conductive exchange depends on the difference between the temperature of the leaf and the surrounding air. If the leaf temperature is higher than that of the surrounding air there. Is a net transfer of heat from the leaf to the surrounding air. Thermal energy is then transported from the air adjacent to the surface of the leaf to the surrounding air through the process of convection, the circulation of fluids. The transfer of heat from the plant to the surrounding. Environment is influenced by the existence of the boundary layer, which is a layer of still air or water adjacent to the surface of each leaf. The environment of the boundary layer differs from that of the surrounding environment, air or WHR, because it is modified by the diffusion of heat, water and CO2 from the plant surface. As water is transpired from the stomata, the humidity of the air within the boundary layer increases, reducing further transpiration. Likewise, as thermal energy heat is transferred from the leaf surface to the boundary layer, the air temperature of the boundary layer increases, reducing further heat transfer from the leaf surface. Under still conditions, no air or water flow, the boundary layer increases in thickness, reducing the transfer of heat and materials, water and CO2 between the leaf and the atmosphere, or water, wind or water flow functions to reduce the size of the boundary layer. Allowing for mixing between the boundary layer and the surrounding air or water and re establishing the diffusion or temperature gradient between the leaf surface and the bulk air. Leaf size and shape also influence the thickness and dynamics of the boundary layer and therefore the ability of plants to exchange heat through convection. Air tends to move more smoothly laminar flow over a larger surface than a smaller one, and as a result the boundary layer tends to be thicker and more intact and larger leaves. Deeply low beliefs and small compound leaves, figure 6.7 tend to disrupt the flow of air, causing turbulence that functions to reduce the boundary layer and increase the exchange of heat and water. The relative importance of evaporation and convection to the maintenance of leaf temperatures. Dissipation of heat is dependent on the physical environment. In locations where water is available, such as regions of high mean annual precipitation, most of the dissipation of heat can occur through transpiration, evaporation as planned soap, and stomata to support the uptake of CO2. As conditions become drier, however, transpiration becomes limited and the average leaf size of species decreases, enhancing heat loss through convection. See figure 6.18 B. 6.7 Constraints imposed by the physical environment have resulted in a wide array of plant adaptations We have explored variation in the physical environment over Earth's surface, the salinity, depth and flow of water, spatial and temporal patterns in climate, precipitation and temperature, variations in geology. And soils part one. in all but the most extreme of these environments autotrophs harness the energy of the sun to fuel the conversion of CO2 into glucose in photosynthesis. To survive, grow and reproduce, plants must maintain a positive carbon balance, converting enough CO2 into glucose to offset the expenses of respiration. Photosynthesis, respiration. to accomplish this a plan must acquire the essential resources of light, co2, water and mineral nutrients as well as tolerate other features of the environment that directly affect plants p affect plants roses. Such is the temperature. Salinity and pH. Although often discussed and even studied as though they are independent of each other, the adaptation exhibited by plants through these features of the environment are not independent for reason relying to the physical environment and the plants themselves. Many features of the physical environment that are left directly influenced plants, processes are independent, for example. The light temperature and the moisture environment are all linked through a variety of physical processes. The amount of solar radiation not only influence the availability of light PAR required for photosynthesis, but also directly influence the temperature of the leaf and its surrounding. In addition, air temperature directly affects. Relative humidity are key feature influencing the rate of transpiration and evaporation of water from the soil. For this reason we see as correlation in the adaptation of plants to variation of these environmental factors. Plantation to dry sound environment must be able to deal with higher demand of water association associated with a higher temperature and the lower relative humidity. And they tried to have characteristics such as the smaller leaves and increased production of roots. In other cases, there are trade-offs in the ability of plants to adapt. UH adapted to limitation is supposed to multiple environmental factors, particular resources. One of the most important of these trains involves acquisition of above and below ground basis. Allocating carbon to the production of the user understands increases the plant has access to the resources of the light and CO2, but in the expense of allocating carbon to produce of roots. Likewise, all keeping carbon through the production of ROW increases access to water and soil motions by limiting carbon allocation to the production of leaves. To set off characteristic adaptations and allow the plants to successfully survive, grow and reproduce under one set of environmental conditions inevitably limits the ability to equally well under different environmental conditions. We explore the consequences of this simple premise in the following section. 6.8. Species of plants are adapted to different light environments. The amount of solar radiation reaching Earth's surface warriors diurnally, seasonally and geographically. However, major factors influencing the amount of light PAR. A plant receiving is presence of other plants through shading. Older the amount of light that the riches and individual plant varies continuously as a function of the area of leaves above it. Plants live on one or two qualitatively different light environments, sun and shade. Depending on whether they are overtopped by other plants, plants have evolved to possess a range of physiological and morphological adaptations that allow individuals to survive, grow and reproduce in. These two different lighting environments. Plant species adapt to high light environments are called shade intolerant species and sun adapted species. Plant species adapted to low light environments are called shade tolerant species and or shade adapted species, shade tolerant and shade intolerant species. Word where variety of phenotypical characteristics is represented dictations to the sun and shade environment. One of the most fundamental differences between shade intolerant and shade tolerant plants species lies in their patterns of photosynthesis. In response to varying levels of light availability, shade tolerant species tend to have a lower light saturation point and lower maximum rate of photosynthesis then shade intolerant species. The differences relate on the part to lower cell concentration of photosynthetic enzyme Rubisco found on shade tolerant plants. Plants must expand a large amount of energy and nutrients produce Rubisco and another component of photosynthetic apparatus initiated environment lower light. Not the availability for Bisco to catalyze the fixation of CO2. Limits the rate of photosynthesis. Shade Tolerant Shade Adaptive Species. Produce less rubisco's result, by contrast producing of chlorophyll delight harvesting pigment. He leaves. Often, often increases. To reduce energy cost of producing Rubisco and other compounds involved in photosynthesis. Lower rate of leaf respiration because of LCP in value of PAR necessary to maintain photosynthesis. The rate that exactly offsets the loss of CO2 in respiration. Now it's photosynthesis of 0. The lower rate of respiration can be offset by lower rate of photosynthesis requiring less light. The result a lower LCP initiate tolerance species. However, the same reduction of enzyme concentration that is associated with lower rates of respiration limits the maximum rate at which photosynthesis can occur. Where light is abandoned. Bothering the pot light saturation point and maximum rate of font to the lower maximum rate of 40 synthesis inevitably results in a lower rate of net carbon gain and growth rate by shade tolerant species as compared to shade intolerant species when growing under highlight levels. The varieties of photosynthesis, respiration, and the growth rate that characterizes plant species adapted to different light environments are illustrated in the work of a plant ecologist, Peter Reich, in colleges of University of Minnesota. They examine characteristics of nine. But. Of nine three species that inhabit the cool temperate forests in northeast North America. For real forest, the species differ wildly in shade tolerance from very tolerant or shading conditions to very intolerant seeding. Of the nine species were grown in the greenhouse and measured of maximum net photosynthesis photosynthesis rate at light saturation. Lice respiration rate and relative growth rate. Were made over the course of the experiment. Species adapted to lower light icons, environments, shade tolerance are characterized by lower maximum rates of net photosynthesis, leaf respiration, and relative growth rates than our species adapted to highlight environment shade tolerant. The difference in the photosynthetic characteristics between shade tolerant and shade tolerant species influencing not only rates of net carbon gain and growth, but also relatively the ability of individuals to serve in a low light environment. The relationship in illustrated in the work of Caroline Augsberger in the University of Illinois. She conducted a series of experiments designed to to examine the influence of light availability on seeding, survival and growth for variety of tree species, both shade tolerant and shade tolerant, that inhabit the tropical rainforest of Panama. Augsberger grew 3 seeding of each species under two level of light availability. These two treatments mimic the condition found either under a shaded environment or condition forest canopy shade treatment, or in the higher light environment in openings or gas or canopy caused by the death of large trees. She continued to experiment for a year, monitoring the survival and growth seeding on weekly basis. president the result for two contrasting species shade tolerant and shade intolerant the shade tolerant species mercyland balsem showed little difference in survival and the growth rates over the the shades. Or the sunlight conditions in contrast the survival and the growth rate of the shade intolerant species Ceiba Pentandra, we're dramatically reduced under shade conditions. These observed differences are direct result of the differences in the adaptation relating to photosynthesis and the carbon allocation discussed previously. The higher rate rate of light saturated photosynthesis results in a higher growth rate of the shade intolerant species in the highlight environment. This associated high rate of leaf respiration and LCP, however, reduced rates of survival in the shaded environment. By week 20 of the experiment and individual had died. In countries the shade tolerant species were able to survive in the low light environment the low rates of leaves respiration that. The light saturation for the SO this is at the low for the low LCP. However limit rate of growth even in high light environment. In addition to the differences in photosynthesis and the growth rate, shade tolerant and shade tolerant species also exhibit the differences in pattern of life morphology. The ratio of surface area, measured in centimeters squared to weight for a leaf is called a specific leaf area. The value of SLA represents the surface area, or a leaf produced per gram of biomass allocated to the production of leaves. Shade tolerant species typically produce leaves. The greatest specific live area. These differences in leaves structure effectively increases the surface area for the capture of the light, the limiting resource per unit of biomass allocated to the reproduction of LEAPS. Mark Abrams and Mark Cubiscan, Pennsylvania State University examined the leaf morphology of 31 tree species inhabiting a forest of central Wisconsin. these researchers measured both SLA and lift thickness for individuals of each species growing influence sunlight and in the shaded conditions compared to open, sunny conditions. The increased surface area of lives in the shaded. A Functions to increase the font to synthetic surface area, partially offsetting the reduction rates of photosynthesis. The decor tomy in adaptation between shade tolerant and shade intolerant species reflects the fundamental traits of between characteristics that enable a species to maintain higher rates of net photosynthesis and growth under highlight condition and the ability to continue survival and growth under low light conditions. The differences in biochemistry, Physiology, and leaf morphology exhibited by shade tolerant species reduce amount of light required to survive in growth. However, these same characteristics limit their ability to maintain high rates of net photosynthesis and growth, with light levels high in contrast. Planted up to higher level environments, shade intolerant species can maintain a high rate of net photosynthesis and grow under highlight conditions, but that expense of continuing photosynthesis grows and survival under stated conditions. 6.9 The link between water demand and the temperature influences plants adaptation. As with Delight, environment arranged adaptation has evolved into racial plants in response to variation in precipitation and soil moisture. As we saw in the previous discussion at Town's position, however, the demand for water is linked to temperature as air temperature rises as saturation. Paper pressures will likely rise, increasing the gradient of the water vapor between the inside of the leaf and the outside of the air. Subsequently the rate of transpiration. As a result, the amount of water required by plants to offset losses for transpiration will likewise increase with temperature. Plant exhibit both acclimation and developmental plasticity. Both forms of phenotypic plasticity. In response to changes in water availability and demand on a variety of timescales when atmosphere or soil is dry, plants response by partially closing stomata. And opening them for shorter period of time. In early two years of water stress, a plant closes. A stomata during the hottest part of the day, where relatively remained is low, it resumes normal activity in afternoon. as water becomes. scarcer the plants opens its stomata all in the cooler more humid conditions in the morning closing a stomata reduces the loss of water through Transpiration But also reduces CO2 diffusion into the leaf And this dissipation of heat through evaporative cooling. As a result of photosynthesis rates declining leaf temperatures may rise. Some plant species such as the Evergreen rhoden drones respond to moisture stress by an inward curling of leaves. Other showing a wilted appearance caused by a lack of truger in the leaves. Leaf curling in wilting allows leaves to reduce water loss and heat gain by reducing the surface area exposed to solar radiation. Prolonged moisture stress inhibit the production of chlorophyll causing the leaves to turn yellow or later in the summer the exhibit premature autumn coloration. As condition worsened the deciduous trees by prematurely shed their leaves all these ones dying first. Such premature shading can result in dieback of twigs and branches. Plants also exhibit developmental plasticity in response to variation in availability of water to meet demand of the transpiration. During development plants respond to low soil water availability by increasing the allocation of carbon to the production of roads while decreasing the production of leaves. By increasing the production of roads the plants can explore larger volume and depth of soil for extracting water the reduction of live area decreases amount of solar radiation the plant intercepts as well as the surface area that is losing water through transpiration. The combined effect is to increase the uptake of water per unit of leaf area while reducing the total amount of water that is lost to the atmosphere through transpiration. The decline in leaf area with decreased water availability is actually a combined effect of reducing allocation of carbon to the production of leaves And changes in leaves morphology. the leaves of plants grown under reduced water condition tend to be smaller and thicker lower species leaf area then those of individual growing in a more massive wet environments. on an evolutionary timescale await area of adaptations has evolved in plant species in response to variations in the availability of water or relative to demand period in some species of plants referred to as C4 plants and Cam plants are modified form of photosynthesis has evolved them increases water use efficiency in warmer or drier environments. The modification involves an additional step in the conversion fixation of CO2 into sugars. In C3 plants the capture of light energy and transformation of CO2 into sugars occur in mesophyll cells. The product of auto synthesis is move into the vascular bundles part of the plant transport system where they can be transported to other parts of the plant. In contrast plant possessing the C4 photosynthetic pathway have a leaf anatomy difference from that of sea triplets C4 plants have two distinct types of photosynthetic cells the massive fuel cells and bundle sheath cells. the bundle sheath cells around the vein of vascular bundles C4 plants divide photosynthesis between mesophyll and the bundle of sheath cells. In C4 plants CO2 reacts with phosphoenolpyruvate (PEP), 3 carbon compound with mesophyll cell. this is in contrast to initial reaction with RuBP in C3 plants. This reaction is catalyzes by enzyme PEP carboxylase, producing oxaloacetate (OAA), as the initial product. The OAA is then rapidly transformed into the 4 carbon molecules of malic and aspartic acid, from which the name C4 photosynthesis is derived. These organic acids are then transported to the bundle sheath cells. These enzymes break down the organic acids to form CO2, reversing the process that is caried out in mesophyll cells In the bundle sheath cells, the CO2 is transformed into sugars using the C3 pathway involving RuBP and rubisco. The extra step in the fixation of CO2 gives C4 plants certain advantages. First, PEP does not react with oxygen, as does RuBP. This eliminates the inefficiency that occurs in the mesophyll cells of C3 plants when rubisco catalyzes the reac-tion between O2 and RuBP (photorespiration), leading to the production of CO2 and a decreased rate of net photosynthesis (see Section 6.1). Second, the conversion of malic and aspartic acids into CO2 within the bundle sheath cells acts to concen-trate CO2. The CO2 within the bundle sheath cells can reach much higher concentrations than in either the mesophyll cells or the surrounding atmosphere. The higher concentrations of CO2 in the bundle sheath cells increase the efficiency of the reaction between CO2 and RuBP catalyzed by rubisco. The net result is generally a higher maximum rate of photosynthesis in C4 plants than in C3 plants. To understand the adaptive advantage of the C4 pathway, we must go back to the trade-off in terrestrial plants between the uptake of CO2 and the loss of water through the stomata. Resulting from the higher photosynthetic rate, C4 plants exhibit greater water-use efficiency (CO2 uptake/H2O loss; see Section 6.4). That is, for a given degree of stomatal opening and asso-ciated water loss in transpiration, C4 plants typically fix more carbon in photosynthesis. This increased water-use efficiency can be a great advantage in hot, dry climates where water is a major factor limiting plant growth. However, it comes at a price. The C4 pathway has a higher energy expenditure because of the need to produce PEP and the associated enzyme, PEP carboxylase. The C4 photosynthetic pathway is not found in algae, bryophytes, ferns, gymnosperms (includes conifers, cycads, and ginkgos), or the more primitive flowering plants (angio-sperms). C4 plants are mostly grasses native to tropical and subtropical regions and some shrubs characteristic of arid and saline environments, such as Larrea (creosote bush) and Atriplex (saltbush) that dominate regions of the desert south-west in North America. The distribution of C4 grass species in North America reflects the advantage of the C4 photosynthetic pathway under warmer and drier conditions (Figure 6.15). The proportion of grass species that are C4 increases from north to south, reaching a maximum in the southwest. In the hot deserts of the world, environmental conditions are even more severe. Solar radiation is high, and water is scarce. To counteract these conditions, a small group of desert plants, mostly succulents in the families Cactaceae (cacti), Euphorbiaceae, and Crassulaceae, use a third type of photo-synthetic pathway—crassulacean acid metabolism (CAM). The CAM pathway is similar to the C4 pathway in that CO2 initially reacts with PEP and is transformed into four-carbon compounds using the enzyme PEP carboxylase. The four-carbon compounds are later converted back into CO2, which is transformed into glucose using the C3 cycle. Unlike C4 plants, however, in which these two steps are physically separate (in mesophyll and bundle sheath cells), both steps occur in the mesophyll cells but at separate times (Figure 6.16). CAM plants open their stomata at night, taking up CO2 and converting it to malic acid using PEP, which accumulates in large quantities in the mesophyll cells. During the day, the plant closes its stomata and reconverts the malic acid into CO2, which it then fixes using the C3 cycle. Relative to both C3 and C4 plants, the CAM pathway is slow and inefficient in the fixation of CO2. But by opening their stomata at night when temperatures are lowest and relative humidity is highest, CAM plants dramatically reduce water loss through transpiration and increase water-use efficiency. In addition to adaptations relating to modifications of the photosynthetic pathway, plants adapted to different soil moisture environments exhibit a variety of physiological and morphologi-cal characteristics that function to allow them to either tolerate or avoid drought conditions. Plant species adapted to xeric conditions typically have a lower stomatal conductance (lower number and size of stomata) than species adapted to more mesic conditions. This results in a lower rate of transpiration but also functions to decrease rates of photosynthesis. Because of the higher diffusion gradient of water relative to CO2, the reduc-tion in stomatal conductance functions to increase water-use efficiency. ant species adapted to drier conditions tend to have a greater allocation of carbon to the production of roots rela-tive to aboveground tissues (greater ratio of roots to shoots), particularly leaves. This pattern of carbon al-location allows the plant to explore a larger volume and depth of soil for extracting water. The decline in leaf area in more xeric environments is actually a combined effect of reduced allocation of carbon to the production of leaves and changes in leaf morphology (size and shape). The leaves of plant spe-cies adapted to xeric conditions tend to be smaller and thicker (lower specific leaf area; see Section 6.9) than those of species adapted to more mesic environments (Figure 6.18). In some plants, the leaves are small, the cell walls are thickened, the stomata are tiny, and the vascular system for transporting wa-ter is dense. Some species have leaves covered with hairs that scatter incoming solar radiation, whereas others have leaves coated with waxes and resins that reflect light and reduce its absorption. All these structural features function to reduce the amount of energy striking the leaf, enhance the dissipation of heat through convection and thus, reduce the loss of water through transpiration. In tropical re-gions with distinct wet and dry seasons, some species of trees and shrubs have evolved the characteristic of dropping their leaves at the onset of the dry season (see Section 2.6). These plants are termed drought deciduous. In these species, leaf senescence occurs as the dry season begins, and new leaves are grown just before the rainy season begins. Although the decrease in leaf area and corresponding increase in biomass allocated to roots observed for plant spe-cies adapted to reduced water availability functions to reduce transpiration and increase the plant’s ability to acquire water from the soil, this shift in patterns of allocation has conse-quences for plant growth. The reduced leaf area decreases carbon gain from photosynthesis resulting in a reduction in plant growth rate 6.10 plants exhibit Both acclimation and adaptation in response to Variations in environmental temperatures As sessile organisms, terrestrial plants are subject to wide varia-tions in temperature on a number of spatial scales and timescales. As we discussed in Chapter 2, at a continental to global scale, temperatures vary with latitude (see Section 2.2). At a local to regional scale, temperatures vary with elevation, slope, and aspect. Seasonal changes in temperature are influenced by both latitude and position relative to the coast (large bodies of water; see Section 2.7), whereas diurnal (daily) changes in temperature occur everywhere. These patterns of temperature variation are consistent and predictable, and evolution has resulted in a variety of adaptations that enable plants to cope with these variations. When examined across a range of plant species inhabit-S. South America N. South America Australia Central America different thermal environments, Tmin, Topt, and Tmax (see Figure 6.6) tend to match the prevailing environmental temperatures. Species adapted to cooler environments typically have a lower Tmin, Topt, and Tmax than species that inhabit warmer climates (Figure 6.19). These differences in the temperature response of net photosynthesis are directly related to a variety of biochemical and physiological adaptations that act to shift the temperature responses of photosynthesis and respiration toward the prevailing temperatures in the environment. These differences are most pronounced between plants using the C3 and C4 photosynthetic pathways (see Section 6.9). C4 plants inhabit warmer, drier environments and exhibit higher optimal temperatures for photosynthesis (generally between 30°C and 40°C) than do C3 plants (Figure 6.20). This is in large part because of the higher Topt for PEP carboxylase as compared to rubisco (see Section 6.9). Although species from different thermal habitats exhibit different temperature responses for photosynthesis and respira-tion, these responses are not fixed. When individuals of the same species are grown under different thermal conditions in the labo-ratory or greenhouse, divergence in the temperature response of net photosynthesis is often observed (Figure 6.21). In general, the range of temperatures over which net photosynthesis is at its maximum shifts in the direction of the thermal conditions under which the plant is grown. That is to say, individuals grown under cooler temperatures exhibit a lowering of Topt, whereas those individuals grown under warmer conditions exhibit an increase in Topt. This same shift in the temperature response can be observed in individual plants in response to seasonal shifts in temperature (Figure 6.22). These modifications in the tempera-ture response of net photosynthesis are a result of the process of acclimation—reversible phenotypic changes in response to changing environmental conditions (see Section 5.4). In addition to the influence of temperature on plant carbon balance, periods of extreme heat or cold can directly damage plant cells and tissues. Plants that inhabit seasonally cold envi-ronments, where temperatures drop below freezing for periods of time, have evolved several adaptations for survival. The ability to tolerate extreme cold, referred to as frost hardening, is a genetically controlled characteristic that varies among spe-cies as well as among local populations of the same species. In seasonally changing environments, plants develop frost harden-ing through the fall and achieve maximum hardening in winter. Plants acquire frost hardiness—the turning of cold-sensitive cells into hardy ones— through the formation or addition of pro-tective compounds in the cells. Plants synthesize and distribute substances such as sugars, amino acids, and other compounds that function as antifreeze, lowering the temperature at which freezing occurs. Once growth starts in spring, plants lose this tol-erance quickly and are susceptible to frost damage in late spring. Producing the protective compounds that allow leaves to survive freezing temperatures requires a significant expendi-ture of energy and nutrients. Some species avoid these costs plants are termed winter deciduous, and their leaves senesce during the fall. The leaves are replaced during the spring, when conditions are once again favorable for photosynthesis. In con-trast, needle-leaf evergreen species—such as pine (Pinus spp.) and spruce (Picea spp.) trees—contain a high concentration of these protective compounds, allowing the needles to survive the freezing temperatures of winter. Although evolution has resulted in an array of physiologi-cal and morphological mechanisms that enable plant species to adjust to the prevailing environmental temperatures, these adaptations have a cost. Most mechanisms (particularly bio-chemical) for both acclimation and adaptation to temperature involve trade-offs between performance at higher temperatures and performance at lower temperatures. For example, shifts of enzymes and membranes (both acclimation and adaptation) to low temperatures generally result in poor performance (or maladaptation) to high temperatures, that is, shifts in Tmin are associated with a corresponding shift in Tmax. In addition, reductions in Topt are typically associated with a decline in maxi-mum rates of net photosynthesis and growth. 6.11 plants exhibit adaptations to Variations in nutrient availability Plants require a variety of chemical elements to carry out their metabolic processes and to synthesize new tissues (Table 6.1). Thus, the availability of nutrients has many direct effects on plant survival, growth, and reproduction. Some of these ele-ments, known as macronutrients, are needed in large amounts. Other elements are needed in lesser, often minute quantities. These elements are called micronutrients, or trace elements. The prefixes micro–and macro–refer only to the quantity of nutrients needed, not to their importance to the organism. If micronutrients are lacking, plants fail as completely as if they lacked nitrogen, calcium, or any other macronutrient. Of the macronutrients, carbon (C), hydrogen (H), and oxygen (O) form the majority of plant tissues. These elements are derived from CO2 and H2O and are made available to the plant as glucose through photosynthesis. The remaining six macronutrients— nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)—exist in vary-ing states in the soil and water, and their availability to plants is affected by different processes depending on their location in the physical environment (see Chapters 3 and 4). In ter-restrial environments, plants take up nutrients from the soil. Autotrophs in aquatic environments take up nutrients from the substrate or directly from the water. The rate of nutrient absorption (uptake per unit root) de-pends on concentrations in the external solution (soil or water; Figure 6.23a). As the availability (concentration) of nutrients at the root surface declines, the rate of absorption declines, which eventually results in a decline in tissue nutrient concentrations (Figure 6.23b). In the case of nitrogen, the decrease in leaf concentrations has a direct effect on maximum rates of photosynthesis (Figure 6.23c) through a reduction in the production of rubisco and chlorophyll (see Section 6.1). In fact, more than 50 percent of the nitrogen content of a leaf is in some way involved directly with the process of photosynthesis, with much of it tied up in these two compounds. In response to reduced nutrient availability, carbon is al-located to root growth at the expense of shoot growth, resulting in an increase in the ratio of roots to shoots (Figure 6.24). plasticity and allows the plant to compensate for the decrease in nutrient absorption per-unit root by increasing the root area and soil volume from which nutrients are absorbed. Despite the shift in patterns of carbon allocation to compensate for reduced nutrient availability, decreased rates of photosynthesis and re-duced allocation to leaves