Photosynthesis: Physiological and Ecological Considerations PDF

9 Photosynthesis: Physiological and Ecological Considerations T he conversion of solar energy to the chemical energy of organic compounds is a complex process that includes electron trans- port and photosynthetic carbon metabolism (see Chapters 7 and 8). This cha...

9 Photosynthesis: Physiological and Ecological Considerations T he conversion of solar energy to the chemical energy of organic compounds is a complex process that includes electron trans- port and photosynthetic carbon metabolism (see Chapters 7 and 8). This chapter addresses some of the photosynthetic responses of the intact leaf to its environment. Additional photosynthetic responses to different types of stress will be covered in Chapter 24. When dis- cussing photosynthesis in this chapter, we are referring to the rate of net photosynthesis, the difference between photosynthetic carbon assimilation and loss of CO2 via mitochondrial respiration. The impact of the environment on photosynthesis is of broad inter- est, especially to physiologists, ecologists, evolutionary biologists, climate change scientists, and agronomists. From a physiological standpoint, we wish to understand the direct responses of photosyn- thesis to environmental factors such as light, ambient CO2 concentra- tions, and temperature, as well as the indirect responses (mediated through the effects of stomatal control) to environmental factors such as humidity and soil moisture. The dependence of photosynthetic processes on environmental conditions is also important to agrono- mists because plant productivity, and hence crop yield, depends strongly on prevailing photosynthetic rates in a dynamic environment. To the ecologist, photosynthetic variation among different environ- ments is of great interest in terms of adaptation and evolution. In studying the environmental dependence of photosynthesis, a central question arises: How many environmental factors can limit photosynthesis at one time? The British plant physiologist F. F. Blackman hypothesized in 1905 that, under any particular condi- tions, the rate of photosynthesis is limited by the slowest step in the process, the so-called limiting factor. The implication of this hypothesis is that at any given time, photosynthesis can be limited either by light or by CO2 concentration, for instance, but not by both factors. This hypothesis has had a marked influence on the approach used by plant physiologists to study photosynthesis— that is, varying one factor and keeping all other environmental 246  Chapter 9 (A) (B) Epidermis Palisade cells Spongy mesophyll Epidermis 100 µm Leaf grown in sun Leaf grown in shade Figure 9.1 Scanning electron micrographs of the leaf (columnlike) cells are much longer in the leaves grown in anatomy of a legume (Thermopsis montana) grown in dif- sunlight. Layers of spongy mesophyll cells can be seen ferent light environments. Note that the sun leaf (A) is below the palisade cells. (Courtesy of T. Vogelmann.) much thicker than the shade leaf (B) and that the palisade conditions constant. In the intact leaf, three major meta- We will start by examining the capture of light, and bolic processes have been identified as important for pho- how leaf anatomy and leaf orientation maximize light tosynthetic performance: absorption for photosynthesis. Then we will describe how Rubisco capacity leaves acclimate to their light environment. We will see that the photosynthetic response of leaves grown under Regeneration of ribulose bisphosphate (RuBP) different light conditions also reflects the ability of a plant Metabolism of the triose phosphates to grow under different light environments. However, Graham Farquhar and Tom Sharkey added a funda- there are limits to the extent to which photosynthesis in a mentally new perspective to our understanding of pho- species can acclimate to very different light environments. tosynthesis by pointing out that we should think of the For example, in some situations photosynthesis is lim- controls on the overall net photosynthetic rates of leaves ited by an inadequate supply of light. In other situations, in economic terms, considering “supply” and “demand” absorption of too much light would cause severe problems functions for carbon dioxide. The metabolic processes if special mechanisms did not protect the photosynthetic referred to above take place in the palisade cells and system from excessive light. While plants have multiple spongy mesophyll of the leaf (Figure 9.1). These biochemi- levels of control over photosynthesis that allow them to cal activities describe the “demand” for CO2 by photo- grow successfully in constantly changing environments, synthetic metabolism in the cells. However, the rate of there are ultimately limits to what is possible. CO2 “supply” to these cells is largely determined by dif- Consider the many ways in which leaves are exposed fusion limitations resulting from stomatal regulation and to different spectra (qualities) and quantities of light subsequent resistance in the mesophyll. The coordinated that result in photosynthesis. Plants grown outdoors are actions of “demand” by photosynthetic cells and “supply” exposed to sunlight, and the spectrum of that sunlight by guard cells affect the leaf photosynthetic rate as mea- will depend on whether it is measured in full sunlight or Plant Physiology 6/E Taiz/Zeiger sured by net CO Sinauer 2 uptake. Associates under the shade of a canopy. Plants grown indoors may In the following Morales sections we will focus on how naturally Studio receive either incandescent or fluorescent lighting, each occurringTZ6E_09.01 variation in lightDate and03-17-14 temperature influences of which is spectrally different from sunlight. To account photosynthesis in leaves and how leaves in turn adjust for these differences in spectral quality and quantity, we or acclimate to such variation. We will also explore how need uniformity in how we measure and express the light atmospheric carbon dioxide influences photosynthesis, an that affects photosynthesis. especially important consideration in a world where CO2 The light reaching the plant is a flux, and that flux can concentrations are rapidly increasing as humans continue be measured in either energy or photon flux units. Irradi- to burn fossil fuels for energy production. ance is the amount of energy that falls on a flat sensor of known area per unit time, expressed in watts per square Photosynthesis Is Influenced meter (W m –2). Recall that time (seconds) is contained by Leaf Properties within the term watt: 1 W = 1 joule (J) s –1. Quantum flux, or photon flux density (PFD), is the number of incident Scaling up from the chloroplast (the focus of Chapters 7 and quanta (singular quantum) striking the leaf, expressed in 8) to the leaf adds new levels of complexity to photosynthe- moles per square meter per second (mol m –2 s –1), where sis. At the same time, the structural and functional proper- moles refers to the number of photons (1 mol of light = 6.02 ties of the leaf make possible other levels of regulation. × 1023 photons, Avogadro’s number). Quanta and energy  Photosynthesis: Physiological and Ecological Considerations    247 Total solar energy Figure 9.2 Conversion of solar energy into carbohydrates by a leaf. (100%) Of the total incident energy, only 5% is converted into carbohydrates. (Figure 9.2). The reason this percentage is so low is that a major percentage of the light is of a wavelength either too short or too long to be absorbed by the photosynthetic Nonabsorbed wavelengths pigments (Figure 9.3). Furthermore, of the photosyntheti- (50% loss) cally active radiation (400–700 nm) that is incident on a leaf, a small percentage is transmitted through the leaf and some is also reflected from its surface. Because chlo- 50% rophyll absorbs strongly in the blue and red regions of the spectrum (see Figure 7.3), green wavelengths are the ones Reflection and transmission (15% loss) most dominant in the transmitted and reflected light (see 35% Figure 9.3)—hence the green color of vegetation. Lastly, a percentage of the photosynthetically active radiation that Heat dissipation (10% loss) is initially absorbed by the leaf is lost through metabolism and a smaller amount is lost as heat (see Chapter 7). 25% The anatomy of the leaf is highly specialized for light absorption. The outermost cell layer, the epidermis, is typ- Metabolism (20% loss) ically transparent to visible light, and the individual cells are often convex. Convex epidermal cells can act as lenses and focus light so that the amount reaching some of the 5% chloroplasts can be many times greater than the amount Carbohydrate of ambient light. Epidermal focusing is common among herbaceous plants and is especially prominent among tropical plants that grow in the forest understory, where units for sunlight can be interconverted relatively easily, light levels are very low. provided that the wavelength of the light, l, is known. The energy of a photon is related to its wavelength as follows: hc 100 0 E= l Percentage of transmitted light Percentage of reflected light where c is the speed of light (3 × 108 m s –1), h is Planck’s 80 20 Reflected light constant (6.63 × 10 –34 J s), and l is the wavelength of light, usually expressed in nanometers (1 nm = 10 –9 m). From this 60 40 equation it can be shown that a photon at 400 nm has twice Absorbed light the energy of a photon at 800 nm (see WEB TOPIC 9.1). 40 60 When considering photosynthesis and light, it is appro- priate to express light as photosynthetic photon flux den- Transmitted light 20 80 sity (PPFD)—the flux of light (usually expressed as micro- moles per square Plant Physiology 6/Emeter per second [μmol m–2 s–1]) within Taiz/Zeiger Sinauer Associates 0 100 the photosynthetically active range (400–700 nm). How 400 500 600 700 800 Morales Studio much light is there onDate TZ6E_09.02 a sunny day? Under direct sunlight 04-23-14 Wavelength (nm) on a clear day, PPFD is about 2000 μmol m–2 s–1 at the top of a dense forest canopy, but may be only 10 μmol m–2 s–1 at the bottom of the canopy because of light absorption by Photosynthetically the leaves overhead. active radiation Leaf anatomy and canopy structure maximize light absorption Figure 9.3 Optical properties of a bean leaf. Shown here are the percentages of light absorbed, reflected, and trans- On average, about 340 W of radiant energy from the sun mitted, as a function of wavelength. The transmitted and reach each square meter of Earth’s surface. When this reflected green light in the wave band at 500–600 nm gives sunlight strikes the vegetation, only 5% of the energy is leaves their green color. Note that most of the light above ultimately converted into carbohydrates by photosynthesis 700 nm is not absorbed by the leaf. (After Smith 1986.) 248  Chapter 9 Below the epidermis, the top layers of photosynthetic radiation that reaches leaves lower down in the canopy. cells are called palisade cells; they are shaped like pillars Leaves that are shaded by other leaves experience lower that stand in parallel columns one to three layers deep (see light levels and different light quality than the leaves Figure 9.1). Some leaves have several layers of columnar above them and have much lower photosynthetic rates. palisade cells, and we may wonder how efficient it is for However, like the layers of an individual leaf, the structure a plant to invest energy in developing multiple cell layers of most plants, and especially of trees, represents an out- when the high chlorophyll content of the first layer would standing adaptation for light interception. The elaborate appear to allow little transmission of incident light to the branching structure of trees vastly increases the intercep- leaf interior. In fact, more light than might be expected tion of sunlight. In addition, leaves at different levels of the penetrates the first layer of palisade cells because of the canopy have varied morphology and physiology that help sieve effect and light channeling. improve light capture. The result is that very little PPFD The sieve effect occurs because chlorophyll is not penetrates all the way to the bottom of the forest canopy; uniformly distributed throughout cells but instead is almost all of the PPFD is absorbed by leaves before reach- confined to the chloroplasts. This packaging of chloro- ing the forest floor (Figure 9.4). phyll results in shading between the chlorophyll mol- The deep shade of a forest floor thus makes for a chal- ecules and creates gaps between the chloroplasts where lenging growth environment for plants. However, in many light is not absorbed—hence the reference to a sieve. shady habitats sunflecks are a common environmental Because of the sieve effect, the total absorption of light by feature that brings high light levels deep into the canopy. a given amount of chlorophyll in chloroplasts of a pali- These are patches of sunlight that pass through small sade cell is less than the light that would be absorbed by gaps in the leaf canopy; as the sun moves, the sunflecks the same amount of chlorophyll were it uniformly dis- move across the normally shaded leaves. In spite of the tributed in solution. short, ephemeral nature of sunflecks, the photons in them Light channeling occurs when some of the incident comprise nearly 50% of the total light energy available light is propagated through the central vacuoles of the during the day. In a dense forest, sunflecks can change palisade cells and through the air spaces between the the sunlight impinging on a shade leaf by more than ten- cells, an arrangement that results in the transmission of fold within seconds. This critical energy is available for light into the leaf interior. In the interior, below the pali- only a few minutes, and in a very high dose. Many deep- sade layers, is the spongy mesophyll, where the cells are very irregular in shape and are surrounded by large air spaces (see Figure 9.1). The large air spaces generate many interfaces between air and water that reflect and refract H2O vapour the light, thereby randomizing its direction of travel. This phenomenon is called interface light scattering. Light scattering is especially important in leaves Daylight because the multiple refractions between cell–air inter- faces greatly increase the length of the path over which Relative PPFD photons travel, thereby increasing the probability of absorption. In fact, photon path lengths within leaves are commonly four times longer than the thickness of the leaf. Canopy Thus, the palisade cell properties that allow light to pass through, and the spongy mesophyll cell properties that are conducive to light scattering, result in more uniform light absorption throughout the leaf. Green In some environments, such as deserts, there is so Blue Red much light that it is potentially harmful to the photosyn- Far red thetic machinery of leaves. In these environments leaves 400 500 600 700 800 often have special anatomic features, such as hairs, salt Wavelength (nm) glands, and epicuticular wax, that increase the reflec- tion of light from the leaf surface, thereby reducing light absorption. Such adaptations can decrease light absorp- Photosynthetically tion by as much as 60%, thereby reducing overheating active radiation and other problems associated with the absorption of too Figure 9.4 Relative spectral distributions of sunlight at much solar energy. the top of a canopy and in the shade under the canopy. At the whole-plant level, leaves at the top of a canopy Most photosynthetically active radiation is absorbed by absorb most of the sunlight, and reduce the amount of leaves in the canopy. (After Smith 1994.)  Photosynthesis: Physiological and Ecological Considerations    249 shade species that experience sunflecks have physiological the sun only on clear days, and they stop moving when a mechanisms for taking advantage of this burst of light cloud obscures the sun. In the case of intermittent cloud when it occurs. Sunflecks also play a role in the carbon cover, some leaves can reorient as rapidly as 90° per hour metabolism of densely planted crops, where the lower and thus can catch up to the new solar position when the leaves are shaded by leaves higher up on the plant. sun emerges from behind a cloud. Solar tracking is a blue-light response (see Chapter Leaf angle and leaf movement 16), and the sensing of blue light in solar-tracking leaves can control light absorption occurs in specialized regions of the leaf or stem. In spe- The angle of the leaf relative to the sun determines the cies of Lavatera (Malvaceae), the photosensitive region is amount of sunlight incident on it. Incoming sunlight can located in or near the major leaf veins, but in many spe- strike a flat leaf surface at a variety of angles depending cies, notably legumes, leaf orientation is controlled by a on the time of day and the orientation of the leaf. Maxi- specialized organ called the pulvinus (plural pulvini), mum incident radiation occurs when sunlight strikes a leaf found at the junction between the blade and the petiole. perpendicular to its surface. When the rays of light deviate In lupines (Lupinus, Fabaceae), for example, leaves consist from perpendicular, however, the incident sunlight on a leaf of five or more leaflets, and the photosensitive region is in is proportional to the angle at which the rays hit the surface. a pulvinus located at the basal part of each leaflet lamina Under natural conditions, leaves exposed to full sun- (see Figure 9.5). The pulvinus contains motor cells that light at the top of the canopy tend to have steep leaf angles change their osmotic potential and generate mechanical so that less than the maximum amount of sunlight is inci- forces that determine laminar orientation. In other plants, dent on the leaf blade; this allows more sunlight to pen- leaf orientation is controlled by small mechanical changes etrate into the canopy. For this reason, it is common to along the length of the petiole and by movements of the see the angle of leaves within a canopy decrease (become younger parts of the stem. more horizontal) with increasing depth in the canopy. Heliotropism is another term used to describe leaf Some leaves maximize light absorption by solar track- orientation by solar tracking. Leaves that maximize light ing; that is, they continuously adjust the orientation of interception by solar tracking are referred to as diaheliotro- their laminae (blades) such that they remain perpendicu- pic. Some solar-tracking plants can also move their leaves lar to the sun’s rays (Figure 9.5). Many species, includ- so that they avoid full exposure to sunlight, thus minimiz- ing alfalfa, cotton, soybean, bean, and lupine, have leaves ing heating and water loss. These sun-avoiding leaves are capable of solar tracking. called paraheliotropic. Some plant species, such as soybean, Solar-tracking leaves present a nearly vertical posi- have leaves that can display diaheliotropic movements tion at sunrise, facing the eastern horizon. Individual leaf when they are well watered and paraheliotropic move- blades then begin to track the rising sun, following its ments when they experience water stress. movement across the sky with an accuracy of ±15° until sunset, when the laminae are nearly vertical, facing the Leaves acclimate to sun and shade environments west. During the night the leaves take a horizontal posi- Acclimation is a developmental process in which leaves tion and reorient just before dawn so that they face the express a set of biochemical and morphological adjust- eastern horizon, ready for another sunrise. Leaves track ments that are suited to the particular environment in (A) (B) Figure 9.5 Leaf movement in a sun-tracking plant. (A) Ini- ing of a pulvinus, found at the junction between the lamina tial leaf orientation in the lupine Lupinus succulentus, with and the petiole. In natural conditions, the leaves track the no direct sunlight. (B) Leaf orientation 4 hours after expo- sun’s trajectory in the sky. (From Vogelmann and Björn 1983, sure to oblique light. Arrows indicate the direction of the courtesy of T. Vogelmann.) light beam. Movement is generated by asymmetric swell- 250  Chapter 9 which the leaves are exposed. Acclimation can occur in Effects of Light on Photosynthesis mature leaves and in newly developing leaves. Plasticity is the term we use to define how much adjustment can in the Intact Leaf take place. Many plants have developmental plasticity to Light is a critical resource that limits plant growth, but respond to a range of light regimes, growing as sun plants at times leaves can be exposed to too much rather than in sunny areas and as shade plants in shady habitats. The too little light. In this section we will describe typical ability to acclimate is important, given that shady habi- photosynthetic responses to light as measured by light- tats can receive less than 20% of the PPFD available in an response curves. We will also consider how features of a exposed habitat, and deep-shade habitats receive less than light-response curve can help explain contrasting physi- 1% of the PPFD incident at the top of the canopy. ological properties between sun and shade plants, and In some plant species, individual leaves that develop between C3 and C4 species. The section will conclude with under very sunny or deep shady environments are often descriptions of how leaves respond to excess light. unable to persist when transferred to the other type of habitat. In such cases, the mature leaf will abscise and Light-response curves reveal a new leaf will develop that is better suited for the new photosynthetic properties environment. You may notice this if you take a plant that Measuring net CO2 fixation in intact leaves across vary- developed indoors and transfer it outdoors; after some ing PPFD levels generates light-response curves (Figure time, if it is the right type of plant, a new set of leaves will 9.6) In near darkness there is little photosynthetic carbon develop that are better suited to high sunlight. However, assimilation, but because mitochondrial respiration con- some plant species are not able to acclimate when trans- tinues, CO2 is given off by the plant (see Chapter 12). CO2 ferred from a sunny to a shady environment, or vice versa. uptake is negative in this part of the light-response curve. The lack of acclimation indicates that these plants are spe- cialized for either a sunny or a shady environment. When 25 plants adapted to deep-shade conditions are transferred Light- into full sunlight, the leaves experience chronic photoin- limited Carboxylation-limited Photosynthetic CO2 assimilation (µmol m–2 s–1) hibition and leaf bleaching, and they eventually die. We 20 will discuss photoinhibition later in this chapter. Sun and shade leaves have contrasting biochemical and 15 morphological characteristics: Shade leaves increase light capture by having more total chlorophyll per reaction center, a higher ratio 10 of chlorophyll b to chlorophyll a, and usually thinner Initial slope = Quantum yield laminae than sun leaves. 5 Light compensation point Sun leaves increase CO2 assimilation by having more (CO2 uptake = CO2 evolution) rubisco and can dissipate excess light energy by hav- 0 ing a large pool of xanthophyll-cycle components (see Chapter 7). Morphologically they have thicker leaves Dark respiration rate and a larger palisade layer than shade leaves (see Fig- –5 ure 9.1). These morphological and biochemical modifications are 0 200 400 600 800 1000 associated with specific acclimation responses to the PPFD (µmol m–2 s–1) amount of sunlight in a plant’s habitat, but light quality can also influence such responses. For example, far-red Figure 9.6 Response of photosynthesis to light in a C 3 light, which is absorbed primarily by photosystem I (PSI), plant. In darkness, respiration causes a net efflux of CO2 is proportionally more abundant in shady habitats than from the plant. The light compensation point is reached in sunny ones (see Chapter 18). To better balance the flow when photosynthetic CO2 assimilation equals the amount of of energy through PSII and PSI, the adaptive response CO2 evolved by respiration. Increasing light above the light compensation point proportionally increases photosynthe- of some shade plants is to produce a higher ratio of PSII sis, indicating that photosynthesis is limited by the rate of to PSI reaction centers, compared with that found in sun electron transport, which in turn is limited by the amount plants. Other shade plants, rather than altering the ratio of of available light. This portion of the curve is referred to as PSII to PSI reaction centers, add more antenna chlorophyll light-limited. Further increases in photosynthesis are even- to PSII to increase absorption by this photosystem. These tually limited by the carboxylation capacity of rubisco or the changes appear to enhance light absorption and energy metabolism of triose phosphates. This part of the curve is transfer in shady environments. referred to as carboxylation-limited.  Photosynthesis: Physiological and Ecological Considerations    251 At higher PPFD levels, photosynthetic CO2 assimilation photosynthesis. When corrected for light absorption, the eventually reaches a point at which CO2 uptake exactly slope of this linear portion of the curve provides the maxi- balances CO2 evolution. This is called the light compen- mum quantum yield of photosynthesis for the leaf. Leaves sation point. The PPFD at which different leaves reach of sun and shade plants show very similar quantum yields the light compensation point can vary among species and despite their different growth habitats. This is because the developmental conditions. One of the more interesting basic biochemical processes that determine quantum yield differences is found between plants that normally grow are the same for these two types of plants. But quantum in full sunlight and those that grow in the shade (Figure yield can vary among plants with different photosynthetic 9.7). Light compensation points of sun plants range from pathways. 10 to 20 μmol m–2 s –1, whereas corresponding values for Quantum yield is the ratio of a given light-dependent shade plants are 1 to 5 μmol m–2 s–1. product to the number of absorbed photons (see Equation Why are light compensation points lower for shade 7.5). Photosynthetic quantum yield can be expressed on plants? For the most part, this is because respiration either a CO2 or an O2 basis, and as explained in Chap- rates in shade plants are very low; therefore only a lit- ter 7, the quantum yield of photochemistry is about 0.95. tle photosynthesis is necessary to bring the net rates of However, the maximum photosynthetic quantum yield CO2 exchange to zero. Low respiratory rates allow shade of an integrated process such as photosynthesis is lower plants to survive in light-limited environments through than the theoretical yield when measured in chloroplasts their ability to achieve positive CO2 uptake rates at lower (organelles) or whole leaves. Based on the biochemistry PPFD values than sun plants. discussed in Chapter 8, we expect the theoretical maxi- A linear relationship between PPFD and photosyn- mum quantum yield for photosynthesis to be 0.125 for thetic rate persists at light levels above the light com- C 3 plants (one CO 2 molecule fixed per eight photons pensation point (see Figure 9.6). Throughout this linear absorbed). But under today’s atmospheric conditions (400 portion of the light-response curve, photosynthesis is ppm CO2, 21% O2), the quantum yields for CO2 of C3 and light-limited; more light stimulates proportionately more C4 leaves vary between 0.04 and 0.07 mole of CO2 per mole of photons. In C 3 plants the reduction from the theoretical maxi- 32 mum is caused primarily by energy loss through photo- respiration. In C4 plants the reduction is caused by the 28 additional energy requirements of the CO2-concentrating Photosynthetic CO2 assimilation (µmol m–2 s–1) Atriplex triangularis mechanism and potential cost of refixing CO2 that has dif- 24 (sun plant) fused out from within the bundle sheath cells. If C3 leaves are exposed to low O2 concentrations, photorespiration is 20 minimized and the maximum quantum yield increases to about 0.09 mole of CO2 per mole of photons. In contrast, if 16 C4 leaves are exposed to low O2 concentrations, the quan- tum yields for CO2 fixation remain constant at about 0.05 12 to 0.6 mole of CO2 per mole of photons. This is because the carbon-concentrating mechanism in C4 photosynthesis 8 Asarum caudatum eliminates nearly all CO2 evolution via photorespiration. (shade plant) At higher PPFD along the light-response curve, the photosynthetic response to light starts to level off (see 4 Figures 9.6 and 9.7) and eventually approaches saturation. Beyond the light saturation point, net photosynthesis no 0 longer increases, indicating that factors other than inci- –4 dent light, such as electron transport rate, rubisco activity, 0 400 800 1200 1600 2000 or the metabolism of triose phosphates, have become lim- PPFD (µmol m–2 s–1) iting to photosynthesis. Light saturation levels for shade plants are substantially lower than those for sun plants Figure 9.7 Light-response curves of photosynthetic car- (see Figure 9.7). This is also true for leaves of the same bon fixation in sun and shade plants. Triangle orache (Atri- plant when grown in sun versus shade (Figure 9.8). These plex triangularis) is a sun plant, and wild ginger (Asarum caudatum) is a shade plant. Typically, shade plants have levels usually reflect the maximum PPFD to which a leaf a low light compensation point and have lower maximum was exposed during growth. photosynthetic rates than sun plants. The dashed line has The light-response curve of most leaves saturates been extrapolated from the measured part of the curve. between 500 and 1000 μmol m–2 s –1, well below full sun- (After Harvey 1979.) light (which is about 2000 μmol m–2 s –1). An exception to 252  Chapter 9 40 40 Atriplex triangularis (sun plant) Photosynthetic CO2 assimilation (µmol m–2 s–1) Photosynthetic CO2 assimilation (µmol m–2 s–1) 30 30 Grown at 920 µmol m–2 s–1 Forest canopy PPFD (sun) 20 20 Shoot 10 10 Grown at 92 µmol m–2 s–1 Individual PPFD (shade) needles 0 0 0 500 1000 1500 2000 2500 0 500 1000 1500 PPFD (µmol m–2 s–1) PPFD (µmol m–2 s–1) Figure 9.8 Light-response curve of photosynthesis of a Figure 9.9 Changes in photosynthesis (expressed on a sun plant grown under sun versus shade conditions. The per-square-meter basis) in individual needles, a complex upper curve represents an A. triangularis leaf grown at a shoot, and a forest canopy of Sitka spruce (Picea sitchensis) PPFD level ten times higher than that of the lower curve. as a function of PPFD. Complex shoots consist of group- In the plant grown at the lower light levels, photosynthesis ings of needles that often shade each other, similar to the saturates at a substantially lower PPFD, indicating that the situation in a canopy where branches often shade other photosynthetic properties of a leaf depend on its grow- branches. As a result of shading, much higher PPFD levels ing conditions. The dashed red line has been extrapolated are needed to saturate photosynthesis. The dashed portion from the measured part of the curve. (After Björkman 1981.) of the forest canopy trace has been extrapolated from the measured part of the curve. (After Jarvis and Leverenz 1983.) this is well-fertilized crop leaves, which often saturate above 1000 μmol m–2 s –1. Although individual leaves are photosynthetic apparatus (Figure 9.10). There are several rarely able to use full sunlight, whole plants usually con- routes for energy dissipation that involve nonphotochemical sist of many leaves that shade each other. Thus, at any quenching (see Chapter 7), the quenching of chlorophyll given time of the day only a small proportion of the leaves fluorescence by mechanisms other than photochemis- are exposed to full sun, especially in plants with dense try. The most important example involves the transfer Plant Physiology 6/Eof the Taiz/Zeiger Plant Physiology 6/E Taiz/Zeiger canopies. The rest leaves receive subsaturating pho- of absorbed light energy away from electron transport Sinauer Associates Sinauer Associates ton fluxes Morales that come from sunflecks that pass through Studio toward Moralesheat Studioproduction. Although the molecular mecha- gaps in the leaf canopy, TZ6E_09.08 Datediffuse light, and light transmit- 03-17-14 nisms are not yet fully TZ6E_09.09 understood, Date 03-17-14 the xanthophyll cycle ted through other leaves. is an important avenue for dissipation of excess light Because the photosynthetic response of the intact plant energy (see WEB ESSAY 9.1). is the sum of the photosynthetic activity of all the leaves, only rarely is photosynthesis light-saturated at the level the x anthophyll cycle The xanthophyll cycle, of the whole plant (Figure 9.9). It is for this reason that which comprises the three carotenoids violaxanthin, crop productivity is usually related to the total amount of antheraxanthin, and zeaxanthin, establishes an ability to light received during the growing season, rather than to dissipate excess light energy in the leaf (see Figure 7.33). single-leaf photosynthetic capacity. Given enough water Under high light, violaxanthin is converted to antherax- and nutrients, the more light a crop receives, the higher anthin and then to zeaxanthin. Both of the aromatic rings the biomass produced. of violaxanthin have a bound oxygen atom. In antherax- anthin only one of the two rings has a bound oxygen, Leaves must dissipate excess light energy and in zeaxanthin neither does. Zeaxanthin is the most When exposed to excess light, leaves must dissipate the effective of the three xanthophylls in heat dissipation, and surplus absorbed light energy to prevent damage to the antheraxanthin is only half as effective. Whereas the level  Photosynthesis: Physiological and Ecological Considerations    253 70 Figure 9.10 Excess light energy in relation to a light- response curve of photosynthetic oxygen evolution in a Photosynthetic O2 evolution (µmol m–2 s–1) 60 shade leaf. The broken line shows theoretical oxygen evolu- tion in the absence of any rate limitation to photosynthesis. At PPFD levels up to 150 μmol m –2 s –1, a shade plant is able 50 to use the absorbed light. Above 150 µmol m –2 s –1, however, photosynthesis saturates, and an increasingly larger amount 40 of the absorbed light energy must be dissipated. At higher PPFD levels there is a large difference between the fraction 30 Excess of light used by photosynthesis versus that which must be light energy dissipated (excess light energy). The differences are much 20 greater in a shade plant than in a sun plant. (After Osmond 1994.) 10 Photosynthetic oxygen evolution they are occasionally exposed to sunflecks. Exposure to 0 200 400 600 just one sunfleck results in the conversion of much of the PPFD (µmol m–2 s–1) violaxanthin in the leaf to zeaxanthin. The xanthophyll cycle is also important in species that remain green during winter, when photosynthetic rates of antheraxanthin remains relatively constant throughout are very low yet light absorption remains high. Unlike in the day, the zeaxanthin content increases at high PPFD the diurnal cycling of the xanthophyll pool observed in and decreases at low PPFD. the summer, zeaxanthin levels remain high all day dur- In leaves growing under full sunlight, zeaxanthin and ing the winter. This mechanism maximizes dissipation of antheraxanthin can make up 40% of the total xanthophyll- light energy, thereby protecting the leaves against photo- cycle pool at maximum PPFD levels attained at midday oxidation when winter cold prevents carbon assimilation. (Figure 9.11). In these conditions a substantial amount of excess light energy absorbed by the thylakoid membranes chloroplast movements An alternative means of can be dissipated as heat, thus preventing damage to the reducing excess light energy is to move the chloroplasts so photosynthetic machinery of the chloroplast (see Chapter that they are no longer exposed to high light. Chloroplast 7). Leaves that grow in full sunlight contain a substan- movement is widespread among algae, mosses, and leaves tially larger xanthophyll pool than do shade leaves, so of higher plants. If chloroplast orientation and location are they can dissipate higher amounts of excess light energy. controlled, leaves can regulate how much incident light is Nevertheless, the xanthophyll cycle also operates in plants absorbed. In darkness (Figure 9.12A and B), chloroplasts that grow in the low light of the forest understory, where gather at the cell surfaces parallel to the plane of the leaf so that they are aligned perpendicularly to the incident light—a position that maximizes absorption of light. 100 Under high light (Figure 9.12C), the chloroplasts move Plant Physiology 6/EViolaxanthin Taiz/Zeiger to the cell surfaces that are parallel to the incident light, Sinauer Associates thus avoiding excess absorption of light. Such chloroplast Light 2000 Xanthophylls (mmol [mol Chl a + b]–1) Morales 80 Studio rearrangement can decrease the amount of light absorbed TZ6E_09.10 Date 04-23-14 by the leaf by about 15%. Chloroplast movement in leaves is a typical blue-light response (see Chapter 16). Blue PPFD (µmol m–2 s–1) 1500 light also controls chloroplast orientation in many of the 60 lower plants, but in some algae, chloroplast movement is controlled by phytochrome. In leaves, chloroplasts move along actin microfilaments in the cytoplasm, and calcium 1000 40 regulates their movement. Figure 9.11 Diurnal changes in xanthophyll content as a 20 500 Zeaxanthin function of PPFD in sunflower (Helianthus annuus). As the + amount of light incident to a leaf increases, a greater pro- Antheraxanthin portion of violaxanthin is converted to antheraxanthin and 0 0 zeaxanthin, thereby dissipating excess excitation energy 0600 1200 1800 and protecting the photosynthetic apparatus. (After Dem- Time of day mig-Adams and Adams 1996.) 254  Chapter 9 (A) Darkness (B) Weak blue light (C) Strong blue light Figure 9.12 Chloroplast distribution in photosynthesizing where they can absorb maximum amounts of light. When cells of the duckweed Lemna. These surface views show the the cells are irradiated with strong blue light (C), the chloro- same cells under three conditions: (A) darkness, (B) weak plasts move to the side walls, where they shade each other, blue light, and (C) strong blue light. In (A) and (B), chloro- thus minimizing the absorption of excess light. (Courtesy of plasts are positioned near the upper surface of the cells, M. Tlalka and M. D. Fricker.) leaf movements Plants have also evolved responses toinhibition is caused by the diversion of absorbed light that reduce the excess radiation load on whole leaves dur- energy toward photoprotective heat dissipation—hence ing high sunlight periods, especially when transpiration the decrease in quantum yield. This decrease is often tem- and its cooling effects are reduced because of water stress. porary, and quantum yield can return to its initial higher These responses often involve changes in the leaf orienta- value when PPFD decreases below saturation levels. Fig- tion relative to the incoming sunlight. For example, helio- ure 9.13 shows how photons from sunlight are allocated tropic leaves of both alfalfa and lupine track the sun but at to photosynthetic reactions versus being thermally dis- the same time can reduce incident light levels by folding sipated as excess energy over the course of a day under leaflets together so that the leaf laminae become nearly favorable and stressed environmental conditions. parallel to the sun’s rays (paraheliotrophic). These move- Chronic photoinhibition results from exposure to high ments are accomplished by changes in the turgor pressure levels of excess light that damage the photosynthetic sys- of pulvinus cells at the tip of the petiole. Another com- tem and decrease both instantaneous quantum yield mon response is mild wilting, as seen in many sunflow- and maximum photosynthetic rate. This would happen ers, whereby a leaf droops to a vertical orientation, again if the stress condition in Figure 9.13B persisted because effectively reducing the incident heat load and reducing photoprotection was not possible. Chronic photoinhibi- transpiration Plant Physiologyand 6/E incident light levels. Many grasses are Taiz/Zeiger tion is associated with damage to the D1 protein from the able to Associates Sinauer effectively “twist” through loss of turgor in bul- reaction center of PSII (see Chapter 7). In contrast to the Moralescells, liform Studio resulting in reduced incident PPFD. effects of dynamic photoinhibition, the effects of chronic TZ6E_09.12 Date 03-17-14 photoinhibition are relatively long-lasting, persisting for Absorption of too much light weeks or months. can lead to photoinhibition Early researchers of photoinhibition interpreted When leaves are exposed to more light than they can use decreases in quantum yield as damage to the photosyn- (see Figure 9.10), the reaction center of PSII is inactivated thetic apparatus. It is now recognized that short-term and often damaged in a phenomenon called photoinhibi- decreases in quantum yield reflect protective mecha- tion (see Chapter 7). The characteristics of photoinhibition nisms (see Chapter 7), whereas chronic photoinhibition in the intact leaf depend on the amount of light to which represents actual damage to the chloroplast resulting from the plant is exposed. The two types of photoinhibition are excess light or a failure of the protective mechanisms. dynamic photoinhibition and chronic photoinhibition. How significant is photoinhibition in nature? Dynamic Under moderate excess light, dynamic photoinhibition photoinhibition appears to occur daily, when leaves are is observed. Quantum yield decreases, but the maximum exposed to maximum amounts of light and there is a cor- photosynthetic rate remains unchanged. Dynamic pho- responding reduction in carbon fixation. Photoinhibition  Photosynthesis: Physiological and Ecological Considerations    255 (A) Favorable environmental conditions Figure 9.13 Changes over the course of a day in the alloca- 2000 tion of photons absorbed by sunlight. Shown here are con- (full trasts in how the photons striking a leaf are either involved sunlight) in photochemistry or thermally dissipated as excess energy under favorable conditions (A) and stress conditions (B). (After PPFD (µmol m–2 s–1) Demmig-Adams and Adams 2000.) 1000 is more pronounced at low temperatures, and becomes chronic under more extreme climatic conditions. 0 Effects of Temperature on Photosynthesis in the Intact Leaf (B) Environmental stress conditions Photosynthesis (CO2 uptake) and transpiration (H2O loss) 2000 (full share a common pathway. That is, CO2 diffuses into the sunlight) leaf, and H2O diffuses out, through the stomatal opening regulated by the guard cells. While these are independent PPFD (µmol m–2 s–1) processes, vast quantities of water are lost during pho- tosynthetic periods, with the molar ratio of H 2O loss to 1000 CO2 uptake often exceeding 250. This high water-loss rate also removes heat from leaves through evaporative cool- ing, keeping them relatively cool even under full sunlight conditions. Transpirational cooling is important, since photosynthesis is a temperature-dependent process, but 0 the concurrent water loss means that cooling comes at a Dawn Noon Dusk Time of day cost, especially in arid and semiarid ecosystems. Absorbed photons Leaves must dissipate vast quantities of heat Photons dissipated The heat load on a leaf exposed to full sunlight is very Photons involved in photochemistry high. In fact, under normal sunny conditions with moder- ate air temperatures, a leaf would warm up to a danger- ously high temperature if all incident solar energy were absorbed and none of the heat was dissipated. However, Energy input Heat dissipation this does not occur because leaves absorb only about 50% of the total solar energy (300–3000 nm), with most of the Long-wavelength absorption occurring in the visible portion of the spec- Sunlight radiation (radiative absorbed heat loss) trum (see Figures 9.2 and 9.3). This amount is still large. by leaf The typical heat load of a leaf is dissipated through three processes (Figure 9.14): Radiative heat loss: All objects emit long-wave radia- tion (at about 10,000 nm) in proportion to their tem- Plant Physiology 6/E Taiz/Zeiger perature to the fourth power (Stephan Boltzman equa- Sinauer Associates Convection Morales Studio from leaf to air tion). However, the maximum emitted wavelength is TZ6E_09.13 Date 04-23-14 to cool leaf inversely proportional to the leaf temperature, and leaf (sensible heat temperatures are low enough that the wavelengths loss) emitted are not visible to the human eye. Figure 9.14 Absorption and dissipation of energy from sunlight by the leaf. The imposed heat load must be dis- Evaporative sipated in order to avoid damage to the leaf. The heat load cooling from water loss (latent is dissipated by emission of long-wavelength radiation, by heat loss) sensible heat loss to the air surrounding the leaf, and by the evaporative cooling caused by transpiration. 256  Chapter 9 Sensible heat loss: If the temperature of the leaf is 40 higher than that of the air circulating around the leaf, Photosynthetic CO2 assimilation Tidestromia oblongifolia, C4 the heat is convected (transferred) away from the leaf Hot desert to the air. The size and shape of a leaf influence the 30 amount of sensible heat loss. (µmol m–2 s–1) Latent heat loss: Because the evaporation of water Atriplex glabriuscula, C3 20 Cool coastal requires energy, when water evaporates from a leaf (transpiration), it removes large amounts of heat from the leaf and thus cools it. The human body is cooled by 10 the same principle, commonly known as perspiration. Sensible heat loss and evaporative heat loss are the most important processes in the regulation of leaf temperature, 0 and the ratio of the two fluxes is called the Bowen ratio: 10 20 30 40 50 Leaf temperature (°C) Sensible heat loss Bowen ratio = ____________________ Evaporative heat loss Figure 9.15 Photosynthesis as a function of leaf tempera- ture at normal atmospheric CO2 concentrations for a C3 In well-watered crops, transpiration (see Chapter 4), and plant grown in its natural cool habitat and a C4 plant growing hence water evaporation from the leaf, is high, so the in its natural hot habitat. (After Berry and Björkman 1980.) Bowen ratio is low (see WEB TOPIC 9.2). Conversely, when evaporative cooling is limited, the Bowen ratio is high. For example, in a water-stressed crop, partial stomatal clo- sure reduces evaporative cooling and the Bowen ratio is increased. The amount of evaporative heat loss (and thus the same species are grown at different temperatures and the Bowen ratio) is influenced by the degree to which sto- then tested for their photosynthetic response, they show mata remain open. photosynthetic thermal optima that correlate with the Plants with very high Bowen ratios conserve water, but temperature at which they were grown. That is, plants of consequently may also experience high leaf temperatures. the same species grown at low temperatures have higher However, the temperature difference between the leaf and photosynthetic rates at low temperatures, whereas those the air does increase the amount of sensible heat loss. same plants grown at high temperatures have higher Reduced growth is usually correlated with high Bowen photosynthetic rates at high temperatures. The ability to ratios, because a high Bowen ratio is indicative of at least adjust morphologically, physiologically, or biochemically partial stomatal closure. in response to changes in the environment is referred to as plasticity. Plants with a high thermal plasticity are capable There is an optimal temperature of growing over a wide range of temperatures. for photosynthesis Plant Physiology Changes 6/E Taiz/Zeigerrates in response to temper- in photosynthetic Sinauer Associates Maintaining favorable leaf temperatures is crucial to plant ature play Morales an important role in plant adaptations to differ- Studio growth because maximum photosynthesis occurs within a ent environments and TZ6E_09.15 contribute Date 04-23-14 to plants being produc- relatively narrow temperature range. The peak photosyn- tive even in some of the most extreme thermal habitats. thetic rate across a range of temperatures is the photosyn- In the lower temperature range, plants growing in alpine thetic thermal optimum. When the optimal temperature for areas of Colorado and arctic regions in Alaska are capable a given plant is exceeded, photosynthetic rates decrease. of net CO2 uptake at temperatures close to 0°C. At the The photosynthetic thermal optimum reflects biochemi- other extreme, plants living in Death Valley, California, cal, genetic (adaptation), and environmental (acclimation) one of the hottest places on Earth, can achieve positive components. photosynthetic rates at temperatures approaching 50°C. Species adapted to different thermal regimes usually have an optimal temperature range for photosynthesis Photosynthesis is sensitive to both high that reflects the temperatures of the environment in which and low temperatures they evolved. A contrast is especially clear between the When photosynthetic rates are plotted as a function of C3 plant Atriplex glabriuscula, which commonly grows in temperature, the temperature-response curve has an cool coastal environments, and the C4 plant Tidestromia asymmetric bell-type shape (see Figure 9.15). In spite of oblongifolia, from a hot desert environment (Figure 9.15). some differences in shape, the temperature-response curve The ability to acclimate or biochemically adjust to tem- of photosynthesis among and within species has many perature can also be found within species. When plants of common features. The ascending portion of the curve rep-  Photosynthesis: Physiological and Ecological Considerations    257 resents a temperature-dependent stimulation of enzymatic Quantum yield (mol CO2 per absorbed quantum) 0.10 activities; the flat top is the temperature range that is opti- mal for photosynthesis; and the descending portion of the curve is associated with temperature-sensitive deleterious 0.08 effects, some of which are reversible while others are not. C3 plants What factors are associated with the decline in photo- synthesis above the photosynthetic temperature optimum? 0.06 Temperature affects all biochemical reactions of photosyn- C4 plants thesis as well as membrane integrity in chloroplasts, so it is not surprising that the responses to temperature are com- 0.04 plex. Cellular respiration rates increase as a function of tem- perature, but they are not the primary reason for the sharp decrease in net photosynthesis at high temperatures. A 0.02 major impact of high temperature is on membrane-bound electron transport processes, which become uncoupled or 0.00 unstable at high temperatures. This cuts off the supply of 10 15 20 25 30 35 40 reducing power needed to fuel net photosynthesis and leads Leaf temperature (°C) to a sharp overall decrease in photosynthesis. Under ambient CO2 concentrations and with favor- Figure 9.16 Quantum yield of photosynthetic carbon fix- able light and soil moisture conditions, the photosyn- ation in C3 and C4 plants as a function of leaf temperature. thetic thermal optimum is often limited by the activity of Photorespiration increases with temperature in C 3 plants, rubisco. In leaves of C3 plants, the response to increasing and the energy cost of net CO2 fixation increases accord- temperature reflects conflicting processes: an increase in ingly. This higher energy cost is reflected in lower quantum carboxylation rate and a decrease in the affinity of rubisco yields at higher temperatures. In contrast, photorespira- for CO2 with a corresponding increase in photorespiration tion is very low in C4 plants and the quantum yield does not show a temperature dependence. Note that at lower tem- (see Chapter 8). (There is also evidence that rubisco activ- peratures the quantum yield of C3 plants is higher than that ity decreases because of negative heat effects on rubisco of C4 plants, indicating that C3 photosynthesis is more effi- activase at higher [>35°C] temperatures; see Chapter 8.) cient at lower temperatures. (After Ehleringer et al. 1997.) The reduction in the affinity for CO2 and the increase in photorespiration attenuate the potential temperature response of photosynthesis under ambient CO2 concen- trations. By contrast, in plants with C4 photosynthesis, thesis, with changes particularly noticeable as tempera- the leaf interior is CO2-saturated, or nearly so (as we dis- tures vary. Figure 9.16 illustrates quantum yield for pho- cussed in Chapter 8), and the negative effect of high tem- tosynthesis as a function of leaf temperature in C3 plants perature on rubisco affinity for CO2 is not realized. This and C4 plants in today’s atmosphere of 400 ppm CO2. In is one reason that leaves of C4 plants tend to have a higher the C4 plants the quantum yield remains constant with photosynthetic temperature optimum than do leaves of C3 temperature, reflecting low rates of photorespiration. In Plant Physiology 6/E Taiz/Zeiger plants (see Figure 9.15). the C3 plants Sinauer the quantum yield decreases with tempera- Associates At low temperatures, C 3 photosynthesis can also be ture, reflecting Morales Studio a stimulation of photorespiration by tem- limited by factors such as phosphate availability in the TZ6E_09.16 perature Date 04-23-14 and an ensuing higher energy cost for net CO2 chloroplast. When triose phosphates are exported from fixation. the chloroplast to the cytosol, an equimolar amount of The combination of reduced quantum yield and inorganic phosphate is taken up via translocators in the increased photorespiration leads to expected differences chloroplast membrane. If the rate of triose phosphate use in the photosynthetic capacities of C3 and C4 plants in hab- in the cytosol decreases, phosphate uptake into the chlo- itats with different temperatures. The predicted relative roplast is inhibited and photosynthesis becomes phos- rates of primary productivity of C3 and C4 grasses along a phate-limited. Starch synthesis and sucrose synthesis latitudinal transect in the Great Plains of North America decrease rapidly with decreasing temperature, reducing from southern Texas in the United States to Manitoba in the demand for triose phosphates and causing the phos- Canada are shown in Figure 9.17. This decline in C4 rela- phate limitation observed at low temperatures. tive to C 3 productivity moving northward closely paral- lels the shift in abundance of plants with these pathways Photosynthetic efficiency is temperature-sensitive in the Great Plains: C4 species are more common below Photorespiration (see Chapter 8) and the quantum yield 40°N, and C 3 species dominate above 45°N (see WEB (light-use efficiency) differ between C3 and C4 photosyn- TOPIC 9.3). 258  Chapter 9 High any experienced on Earth in the last 2 million years. Most extant plant taxa are therefore thought to have evolved in a low-CO2 world (~180–280 ppm CO2). Only when one C3 superior looks back about 35 million years does one find CO2 con- C4 carbon gain centrations of much higher levels (>1000 ppm). Thus, the Relative carbon gain geologic trend over these many millions of years was one of decreasing atmospheric CO2 concentrations (see WEB C4 superior C3 carbon gain TOPIC 9.5). Currently, the CO2 concentration of the atmosphere is increasing by about 1 to 3 ppm each year, primarily because of the burning of fossil fuels (e.g., coal, oil, and natural gas) and deforestation (Figure 9.18C). Since 1958, when C. David Keeling began systematic measurements of CO2 in the clean air at Mauna Loa, Hawaii, atmospheric Low CO2 concentrations have increased by more than 25%. By 25 30 35 40 45 50 55 60 2100 the atmospheric CO2 concentration could reach 600 Latitude (°) to 750 ppm unless fossil fuel emissions and deforestation are diminished (see WEB TOPIC 9.6). Figure 9.17 Relative rates of photosynthetic carbon gain CO2 diffusion to the chloroplast predicted for identical C 3 and C4 grass canopies as a func- tion of latitude across the Great Plains of North America. is essential to photosynthesis (After Ehleringer 1978.) For photosynthesis to occur, CO2 must diffuse from the atmosphere into the leaf and to the carboxylation site of rubisco. The diffusion rate depends on the CO2 concentra- Effects of Carbon Dioxide on tion gradient in the leaf (see Chapters 3 and 6) and resis- tances along the diffusion pathway. The cuticle that covers Photosynthesis in the Intact Leaf the leaf is nearly impermeable to CO2, so the main port of We have discussed how light and temperature influ- entry of CO2 into the leaf is the stomatal pore. (The same ence leaf physiology and anatomy. Now we will turn our path is traveled in the reverse direction by H 2O.) CO2 dif- attention to how CO2 concentration affects photosynthe- fuses through the pore into the substomatal cavity and sis. CO2 diffuses from the atmosphere into leaves—first into the intercellular air spaces between the mesophyll through stomata, then through the intercellular air spaces, cells. This portion of the diffusion path of CO2 into the and ultimately into cells and chloroplasts. In the presence chloroplast is a gaseous phase. The remainder of the diffu- of adequate amounts of light, higher CO2 concentrations sion path to the chloroplast is a liquid phase, which begins support higher6/E Plant Physiology photosynthetic Taiz/Zeiger rates. The reverse is also at the water layer that wets the walls of the mesophyll cells true: low Sinauer CO2 concentrations can limit the amount of pho- Associates and continues through the plasma membrane, the cytosol, Morales Studio tosynthesis TZ6E_09.17 in C3 plants. Date 03-17-14 and the chloroplast. (For the properties of CO2 in solution, In this section we will discuss the concentration of see WEB TOPIC 8.8.) atmospheric CO2 in recent history, and its availability for The sharing of the stomatal entry pathway by CO2 and carbon-fixing processes. Then we will consider the limita- H 2O presents the plant with a functional dilemma. In tions that CO2 places on photosynthesis and the impact of air of high relative humidity, the diffusion gradient that the CO2-concentrating mechanisms of C4 plants. drives water loss is about 50 times larger than the gradient that drives CO2 uptake. In drier air, this difference can be Atmospheric CO2 concentration keeps rising much larger. Therefore, a decrease in stomatal resistance Carbon dioxide presently accounts for about 0.040%, or through the opening of stomata facilitates higher CO2 400 ppm, of air. The partial pressure of ambient CO2 (ca) uptake but is unavoidably accompanied by substantial varies with atmospheric pressure and is approximately 40 water loss. Not surprisingly, many adaptive features help pascals (Pa) at sea level (see WEB TOPIC 9.4). Water vapor counteract this water loss in plants in arid and semiarid usually accounts for up to 2% of the atmosphere and O2 regions of the world. for about 21%. The largest constituent in the atmosphere Each portion of the CO2 diffusion pathway imposes a is diatomic nitrogen, at about 77%. resistance to CO2 diffusion, so the supply of CO2 for pho- Today the atmospheric concentration of CO2 is almost tosynthesis meets a series of different points of resistance. twice the concentration that prevailed over the last 400,000 The gaseous phase of CO2 diffusion into the leaf can be years, as measured from air bubbles trapped in glacial ice divided into three components—the boundary layer, the in Antarctica (Figure 9.18A and B), and it is higher than stomata, and the intercellular spaces of the leaf—each  Photosynthesis: Physiological and Ecological Considerations    259 (C) 400 390 (B) 400 380 380 (A) 370 400 Atmospheric CO2 concentration (ppm) 360 360 350 340 350 300 320 250 340 300 330 200 150 280 320 100