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PBIO*3110 – Crop Physiology Lecture #10 Fall Semester 2008 Lecture Notes for Thursday 9 October Leaf Photosynthesis IV – Leaf Net Photosynthesis How is leaf photosynthesis affected by 1. incident PPFD? 2. temperature? 3. CO2 concentrations? Learning Objectives 1. Know how the response of leaf photosynthesis to incident PPFD can be mathematically modeled, and understand the physiological significance of the different parameters of the photosynthetic response to irradiance curve. 2. Be able to explain how leaf acclimation, species adaptation, and different environmental factors affect the photosynthetic response to irradiance. 3. Be able to describe how the quantum efficiency of photosynthesis is affected by different environmental parameters, and how it varies between C3 and C4 photosynthetic types. Introduction In the previous three lectures, we reviewed the process of photosynthesis at the chloroplast level, considered the fundamental biochemical differences between the C3, C4 and CAM photosynthetic pathways, and then saw how the interaction of CO2 supply (diffusion) and demand (carboxylation) interact to determine the leaf photosynthetic rate. In this lecture, we will examine how photosynthetic rates of individual leaves respond to different aspects of their physical environments, and how these responses differ among C3 and C4 crop species.
1 Photosynthetic Response to Irradiance The PRI Curve The photosynthetic response of an individual leaf to incident solar radiation can be described by a photosynthetic response to irradiance (PRI) curve. In general, as incident light increases from zero to a moderate level, leaf photosynthesis increases in an approximately linear fashion. As the incident light increases further, the photosynthetic rate begins to level off, until finally it reaches a plateau, the light­saturated level. Note that the response of a leaf to irradiance will vary with:
· light environment under which the leaf developed
· plant species
· leaf age When considering photosynthetic response to irradiance, the most meaningful measurement of light is the photosynthetic photon flux density (PPFD). This is defined as the number of photosynthetically­active photons incident per unit area per unit time. The most common units for PPFD are µmol photons m ­2 s ­1 . Left: Example of how the leaf photosynthetic response to irradiance acclimates to the growth conditions. Right: Adaptation of the PRI curve according to the light environment to which a species is adapted. (From Björkman, 1981)
2 As we have already discussed, PPFD can be considered a "count" of the number of incident photons, as opposed to a measurement of the energy of the photons. This is appropriate, since every photon in the PAR absorbed by the leaf has the same effect on photosynthesis, regardless of its wavelength or energy. Careful examination of the figures above reveals that the PRI curve does not pass through the origin; instead, it has a negative intercept on the y­axis. This indicates that in darkness, photosynthesis is negative; i.e. CO2 is released by the leaf, rather than being absorbed by the leaf. This release of CO2 is due, of course, to leaf respiration. It is important to realize that respiration occurs even in the light, so that the apparent photosynthesis that we measure in the light is actually the balance between respiration and photosynthesis. This apparent (i.e., measurable) photosynthesis is also called the net photosynthesis, or net CO2 assimilation (AN). For C3 leaves, AN = AG – RP ­ RD. AG is the gross photosynthesis rate; that is, the actual rate of carboxylation. RP is the rate of photorespiration. This is zero in the dark, but increases along with AG as PPFD increases. RD is the rate of “dark” respiration. For any given leaf under any given set of environmental conditions, there is a PPFD level where respiration and gross photosynthesis are exactly equal (i.e., net photosynthesis is zero); this PPFD level is the x­intercept of the PRI curve, and is called the light compensation point (LCP). Shade Leaves and Sun Leaves The degree to which PRI is affected by the light environment under which the leaf developed is highly species dependent. Many species will produce leaves that are well adapted to either sun or shade environments, depending on the prevailing PPFD levels. The figure below shows cross­ sections of "sun leaves" and "shade leaves" from sugar maple. What advantage is there to "shade leaves", when sun leaves obviously exhibit higher maximum photosynthetic rates? The answer lies in the balance between photosynthesis and respiration. Leaves which are adapted to sun environments are generally thicker (i.e., have lower specific leaf area, SLA), and must therefore expend more energy per unit leaf area to maintain the living tissue of the leaf. At low PFPD a sun leaf may actually be below its light compensation point, and will therefore experience net respiration (i.e., net carbon loss). Shade leaves have lower maintenance respiration costs per unit area, and therefore have lower light compensation points. This allows them to achieve positive net photosynthetic rates under relatively low PPFD conditions.
3 Of course, another potential advantage of shade leaves is that they require less initial energy input per unit leaf area developed by the plant. That is, shade leaves allow the plant to maximize interceptance of available radiation, with the lowest possible metabolic cost, both initially and in terms of subsequent maintenance respiration. The trade­off is that high PPFD levels, should they occur, will not be effectively utilized. Cross­sections of sugar maple leaves grown under high (a), moderate (b) or low (c, d) PPFD conditions. (From Hanson, 1917) Equations to Describe the PRI Curve An equation used to describe a biological phenomenon should:
· provide an excellent fit to the experimental data
· be as simple as possible, and
· be defined by parameters that are physiologically meaningful. A number of different equations have been used successfully to describe photosynthetic responses to irradiance. Unfortunately, none is perfect. Some equations fit the experimental data
4 very well and are biologically meaningful, but are very complex. Others are simpler, but their parameters can not be defined in biological terms. In this course, we will usually describe the PRI using a hyperbolic function, which is both simple and meaningful. Its main limitation is that it does not always provide a good fit to the data at intermediate PPFD levels. The net photosynthetic rate of a leaf (AN, in µmol CO2 m ­2 leaf s ­1 ) is described as: where PPFDa is the PPFD absorbed by the leaf, f is the initial slope of the PRI curve at low PPFD, Amax is the light­saturated value for photosynthesis net of RD , and RD is the respiration rate in darkness. In general, low values of RD and high values of f are most important for productivity in low PPFD environments. In high PPFD environments, high values of Amax are most critical. Another equation that is often used to describe PRI curves is given below. At first glance it looks much more complex, but if you look at it more closely you will see that it actually contains only one additional parameter:
f × PPFD + A max -
AN = { ( f × PPFD + A max 2 Q
) 2 - 4 Q × f × PPFD × A max }
- R D The additional parameter in this second equation is the Q, which represents the “convexity” of the curve – that is, how sharply it makes the transition from the initial linear phase to the final plateau phase. It typically has a value between 0.7 and 1, with 1 modelling an instant, sharp transition (such a PRI curve with Q = 1 is known as a Blackman response). This second model overcomes the main limitation of our first model – that is, it does a much better job of modelling the photosynthetic response to irradiance at intermediate and high PPFD levels. The initial slope of the PRI curve, f, is called the quantum efficiency. It represents the light efficiency of photosynthesis (CO2 fixed per photon absorbed) under PPFD­limited conditions. The light­saturated rate of photosynthesis, Amax represents the maximum rate of CO2 fixation when PPFD is saturating – its value is determined by a number of factors affecting diffusion of CO2 into the leaf and its use by the Calvin cycle. The convexity parameter, Q, is less well understood, but it tends to be higher (more abrupt) in shade leaves than in sun leaves. Specifically, the convexity is high when the capacity for electron transport (light reactions) is high relative to the capacity for carboxylation (dark reactions)
5 Left: PRI (net assimilation) curves modelled using f = 0.05, Amax = 30 µmol m ­2 s ­1 and various values of the convexity parameter, Q. Right: Gross CO2 assimilation vs. PPFD for young willow leaves grown under low or high PPFD (200 or 1400 µmol m ­2 s ­1 ), old willow leaves grown under low PPFD, and mature leaves grown in the field under natural PPFD (adapted from Ogren, 1993). PRI curves for willow leaves grown under low (left) or high (right) irradiance. In each case, the three curves are for different external CO2 concentrations: 120 ppm (diamonds), 340 ppm (circles) or 900 ppm (squares). At any given CO2 concentration, convexity of the curve is always greater for the low­light grown leaves. (From Ogren, 1993)
6 Effects of Leaf Internal CO2 Concentration and Temperature Leaf Internal CO2 Concentration Since CO2 is a substrate for photosynthesis, AN is sensitive to the CO2 concentration inside the leaf. As we have seen in the previous lecture, leaf internal CO2 concentration will always be less than the ambient CO2 concentration when net photosynthesis is positive, since CO2 is consumed by photosynthesis and its replacement by diffusion into the leaf is restricted by the stomatal resistance. Once inside the leaf, the CO2 must dissolve in the cell water and diffuse in the liquid phase to the stroma of the chloroplast where it is actually used by photosynthesis. Availability of CO2 in the chloroplast stroma is be determined by the stomatal resistance to diffusion in the gas phase, and by diffusion of CO2 within the leaf in the liquid phase. Recall that, like the PRI curve, the CO2 response curve does not pass through the origin. The CO2 concentration that results in zero net photosynthesis is called the CO2 compensation point. At this point, leaf internal CO2 is so low that it restricts photosynthesis to the extent that gross photosynthesis is exactly in balance with leaf respiration, including photorespiration. Recall also that the initial slope of the curve occurs in the “CO2 limited” phase, and is determined primarily by the concentration of RubisCO. The upper plateau (CO2­saturated, RuBP­limited phase) is determined by the capacity of the regeneration phase of the Calvin cycle.
7 Response of AN to leaf internal CO2 in a soybean leaf (data of H.J. Earl). Temperature Since photosynthesis is an enzymatic process, it displays a temperature optimum. Temperature optima for photosynthesis are somewhat species dependent, being higher for species that originate from warmer climates. The following figure shows how atmospheric CO2 and leaf temperature can interact to determine the photosynthetic rate of cucumber leaves. Under normal CO2 concentrations, photosynthesis may be limited by the leaf internal CO2. Increasing the leaf temperature under these conditions does nothing to augment the photosynthetic rate. However, if ambient CO2 is increased to higher than normal levels, then photosynthesis may benefit from higher leaf temperatures. The next figure below shows temperature response of photosynthesis for leaves of several species under normal (18%) and reduced (3%) O2 concentration. At lower temperatures photorespiration is minimal and so O2 concentration is relatively unimportant; as temperature increases, the difference in AN under normal and low O2 increases. The temperature optima for photosynthesis are generally lower for C3 than for C4 crop species. This difference can be explained, at least partially, in terms of the species’ relative susceptibilities to carbon losses through photorespiration.
8 Limitation of photosynthesis in a cucumber leaf by atmospheric CO2 concentration, and by leaf temperature. Circles indicate measurements taken at 20°C; crosses indicate measurements taken at 30°C. (From J. Goudriaan, Wageningen) Effect of temperature on AN for leaves of six C3 plant species. Ambient O2 concentration was either near normal (18%, closed symbols) or reduced (3%, open symbols). From Sage and Sharkey, 1987.
9 Quantum Requirement and Photorespiration Recall that photorespiration is a result of RuBisCO’s affinity for O2; that is, the oxygenation function of RuBisCO competes with the carboxylation function for available ribulose bisphosphate. In C4 leaves, photorespiration is virtually eliminated by raising the CO2 concentration relative to the O2 concentration in the vicinity of RuBisCO (i.e., in the bundle sheath chloroplasts). This means that when conditions are “ideal” for photorespiration (high temperature, low CO2 relative to O2), the initial slope of the PRI curve (f) will be much lower for C3 species than for C4 species. The ratio of quanta (i.e., photons) absorbed to CO2 fixed at low PPFD is known as the quantum requirement (it is simply the inverse of f). A lower quantum requirement indicates a higher light use efficiency (a higher f). In C3 leaves, the quantum requirement is sensitive to both the CO2 : O2 ratio and to leaf temperature. In C4 leaves, the quantum requirement is relatively stable. a) The effect of ambient O2 concentration on the quantum requirement of C3 and C4 leaves (Bjorkman et al., 1970) b) The effect of leaf temperature on the quantum requirement of C3 and C4 leaves (Ehleringer and Björkman, 1977). Remember, the quantum requirement as displayed in the figure above only really has meaning at low PPFD (i.e., when light use efficiency is maximal). However, the basic idea also holds at higher PPFD. That is, in C3 leaves, high temperatures tend to increase the ratio of photorespiration to photosynthesis, causing net photosynthesis to decline, and the light use efficiency to decrease. In C4 leaves, since photorespiration is minimal, photosynthesis continues to increase up to much higher temperatures. Of course, at very high temperatures C4 species also
10 experience a suppression of their photosynthetic rates, but this suppression is caused by factors other than the enhancement of photorespiration. Under conditions where photorespiration is strongly inhibited, quantum efficiency tends to be higher in C3 leaves, since extra energy in the form of ATP is required by the C4 cycle to support the activity of PEP carboxylase. However, under conditions where photorespiration is substantial, C4 plants generally have higher quantum efficiencies. The effect of temperature and leaf internal CO2 partial pressure on the quantum efficiency of photosynthetic CO2 assimilation in a C3 and a C4 plant. Note that most pi values in the left panel are above normal levels (e.g., 40 Pa CO2 = 400 ppm). From Björkman, 1981 Again, the efficiencies discussed above are the maximum efficiencies, i.e., those observed in the initial linear phase of the PRI curve when PPFD is extremely low. Such a measure of efficiency may be of relatively little importance in the context of season­long carbon accumulation by, for instance, a crop canopy, since only a minor fraction of total photosynthesis occurs under such low PPFD conditions.
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