CPla has as- sociated with it peripheral light-harvesting complexes that are absent from the CPI com- plex. Electron micrographs revealed that thylakoid membranes form structures like stacks of pancakes, called grana, linked together by single thylakoids, the stromal lamellae. In fact, grana can be seen through a microscope as bright spots of chlorophyll fluorescence within chloroplasts Anderson Grana are more prominent in shade plants, having greater numbers of thylakoids per granum than those from sun leaves.
The closely packed thylakoids in the grana exclude protein complexes that protrude into the stroma. The adenosine triphosphatase ATPase complex was found only on stroma exposed lamellae Miller and Staehelinl Subsequently, physical separation of grana and stro- mal membrane fractions revealed lateral heterogeneity in the distribution of the photosystems Andersson and Anderson Photosystem II was enriched in the grana while photosystem I was depleted.
Complexity continues to be added because each photosystem consists of at least two types. The reaction center of photosystem I contains about 90 chlorophylls and is surrounded by a single layer of 8 light-harvesting complex I pigment-proteins Lhcal to 4 , Boekema et al. Together these dimerise to form a larger photosystem II complex that has been resolved by elec- tron microscopy Boekema et al.
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The outer trimeric complexes bind with varying stoichiometry depending on the packing structure analyzed. The variable stoichiometry exists at multiple levels. Firstly, the thylakoids are fluid and dynamic, so that the complexes are forever jostling about and interact- ing with different neighbors. Secondly, lateral heterogeneity means that a given reaction center complex can have a different apparent antenna size depending on its location, due to the way it associates with nearby complexes. This in turn alters the likelihood of interactions between complexes, with the antenna size of photosystem II decreasing as stacking decreases.
Fourthly, the relative amounts of the complexes depend on growth conditions, principally the amount of light. Un- der low irradiance, a shade-type phenotype is observed. It can also be observed within a leaf, where the gradient in light results in sun- and shade-type chloroplasts being present in palisade and spongy mesophyll cells, respectively Terashima and 2. Chloroplast to Leaf 21 Inoue Surprisingly, this did not affect the forma- tion of grana stacks Andersson et al.
This does not rule out the involve- ment of LHCll in thylakoid membrane stacking, because the plants compensated for the loss of Lhcbl and Lhcb2 by increased expression of Lhcb5, which formed a novel alternative trimeric LHCll complex Ruban et al. Diagrams of thylakoids tend to misrepresent them because they generally portray an idealized electron transport chain with one of each complex. By combining quan- titative measurements of each component of chloroplast thylakoids with estimates of their size, it is possible to calculate the average proportion of membrane surface area covered.
An example is given for spinach in Table 2. Two thirds of the mem- brane surface is covered by pigment-protein complexes on average. The existence of appressed and stroma exposed membranes means that the proportions of surface area given in Table 2. Freeze fracture analysis under the electron microscope has allowed visualization of some of the complexes in vivo.
Photosystem II complexes in grana membranes oc- cur with a density of to Staehelin and van der Staay The formation of grana creates regions in the thylakoid membrane that have different compositions of the protein complexes. Physical separation of photo- system II and photosystem I between appressed and non-appressed regions al- tered the way captured light could be transferred Andersson and Anderson More recently, Albertsson , has proposed that cyclic electron trans- port occurs at the stroma exposed thylakoids. Relative stoichiometries between the major components of spinach thylakoids and the proportion of the membrane surface they occupy Kirchhoff et al.
Quantum yield does however vary with wavelength McCree The wavelength dependence has been interpreted to reflect the relative absorption spectra of the two photosystems Evans , but the precise distribution be- tween all the complexes awaits the development of new methods. Leaf Tissue Optics Most leaves are sufficiently pigmented such that they absorb most of the light within the photosynthetically active region of the spectrum to nm.
Erom the standpoint of maximizing photosynthetic rate at the level of the whole leaf, it would be advantageous to match the internal distribution of absorbed light energy with CO 2 supply and photosynthetic capacity Evans , Evans and Vogelmann Presumably, these quantities depend upon leaf anatomy, ori- entation, environment, and growth conditions Smith et al.
Detailed knowl- edge about how internal gradients of absorbed light and photosynthetic capacity influence whole-leaf photosynthetic performance depends upon the ability to measure these gradients, and recent advances have made this possible Vogel- mann et al. With respect to light, leaf anatomy and pigmentation play ma- jor roles in determining light propagation and absorption and there are subtleties that can exert a profound influence on the allocation of absorbed light energy to the photosynthetic tissues of a leaf. Leaf tissue optics is strongly influenced by two optical phenomena: The sieve effect arises from the fact that light-absorbing pigments are not distributed homogeneously throughout the leaf, but rather they are packaged within chloroplasts.
When light travels though a leaf, it may intercept a chloroplast and be absorbed; alternatively, it may miss the chloroplasts and pass through the leaf, hence the reference to a sieve. The net result of the sieve effect is that it decreases that amount of light absorption per unit of chlorophyll and per unit of leaf area Eig. If the photosynthetic pigments were released from the chloroplasts and distributed throughout the leaf, then leaves would absorb more light, even though the total amount of pigment remains constant. It follows that, in determining how much light the individual cell layers absorb within a leaf, it is not a simple matter of measuring the total amount of pigments but rather determining the location and density of chloro- plasts.
Moreover, chloroplasts move in response to how much light they receive and this movement can profoundly influence the internal distribution of absorbed quanta see later section on chloroplast movements. Thus, the sieve effect is not just an optical curiosity but rather it is something that can be used by a leaf to dynamically control the internal distribution of absorbed light energy. Light scattering is a second optical phenomenon that influences the global ab- 2.
Chloroplast to Leaf 23 A graphical portrayal of the sieve effect. The same amount of pigment is pres- ent in cells A to C. The overall transmittance of the cell is Concentrating the pigment further C leads to progressively higher transmittance. The path of light through a leaf can be thought of as a series of deflections between cells and air spaces Fig. Light scattering in this system is independent of wavelength and light is scattered predominantly in the forward direction. These deflections Figure 2. Light scattering and increase in pathlength of light. Light scattering in leaves is caused primarily by reflection between the intercellular air spaces and cells.
This mirror-like reflection deflects light at relatively small angles so that light is scattered mostly in the forward direction. Also, this type of scattering is relatively independent of wavelength. Light scattering counteracts the sieve effect; it increases the amount of light absorption per unit chlorophyll and per unit leaf area. The extent to which light is scattered in a leaf is determined by the number of times that it encounters an air-cell interface.
Total intercellular air space volume is important but so is the three-dimensional geometry of that air space. Given an equal amount of air space, many small pockets of air will potentially scatter light more than a few large air channels. Unlike chloroplast movement, which can al- ter internal light absorption within minutes, that spatial distribution of air space is fixed during leaf development and its three-dimensional geometry is probably irrevocably linked to the type of mesophyll tissue palisade versus spongy and the number of cell layers for the life of the leaf.
Estimates of pathlength en- hancement by scattering in leaves usually range from 2 to 4 Vogelmann However, quantifying the relationship between leaf anatomy and pathlength has been confounded by difficulties in directly measuring pathlength enhancement and how it is related to the three-dimensional organization of the intercellular air spaces.
Because of these methodological limitations, more quantitative research is necessary to evaluate how leaf anatomy may control light absorption through scattering. Optical Properties of Individual Leaf Tissues The epidermis is the first cell layer that light encounters as it strikes a leaf. The epidermis is usually transparent to visible light and it frequently contains screen- ing pigments that filter ultraviolet-B UV-B radiation to nm.
Even though the epidermis is transparent to PAR, it can affect the irradiance within the underlying photosynthetic tissue at the microscopic level because each epi- dermal cell can act as a lens that focuses light. Epidermal focusing is quite com- mon in plants and focal intensifications can reach 20 times incident light levels, although in most leaves, light focusing intensifies the light only severalfold. Whether epidermal focusing serves any adaptive purpose is unknown. It has been suggested that the extreme convex epidermal cell shape of some tropical under- story plants may enhance light capture by minimizing specular reflection of dif- fuse light.
The underlying photosynthetic tissue of many leaves is frequently comprised of tubular palisade cells and then a layer of more randomly arranged irregular- shaped spongy mesophyll. The tubular shape may facilitate the penetration of light be- cause these cells have a central vacuole, which serves as a transparent channel that appresses the chloroplasts against the cell periphery. The elongate shape could also serve other purposes such as enhancement of short-distance transport of sugars and other metabolites. The more random tissue arrangement of the underlying spongy mesophyll may enhance light scattering such that unabsorbed light that passes through the palisade is redirected back into the interior of the leaf.
The contrast- ing light scattering properties of the palisade and spongy mesophyll, along with various epicuticular structures e. Chloroplast to Leaf 25 observed phenomenon of leaf bicoloration where the lower abaxial surface of leaves often appears less green than the upper surface. Absorption Profiles Within Leaves Determine the Energy Input to Photosynthesis In order to ascertain how much each mesophyll layer contributes to whole-leaf photosynthesis, a first step is to know how much light is absorbed within each layer.
In combination with photo synthetic capacity, this sets an upper boundary on the amount of carbon that can be fixed within each tissue layer Evans , Evans and Vogelmann and ultimately the whole leaf. Owing to optical complications introduced by light scattering and the sieve effect, it has been dif- ficult to estimate light absorption profiles. Complex mathematical models have been constructed using radiation transport Richter and Eukshansky a,b, and ray tracing Ustin et al. Until re- cently, experimental verification of these models has not been possible owing to a lack of an experimental approach to measure absorption profiles.
A new method measures chlorophyll fluorescence profiles from a cross sectional view of a leaf irradiated on its adaxial or abaxial surface Koizumi et al. Assuming con- stant quantum efficiency for chlorophyll fluorescence throughout the leaf, ab- sorption profiles can be determined from the chlorophyll fluorescence profile. In spinach, chlorophyll fluorescence profiles are determined by wavelength and leaf orientation.
When leaves were irradiated on their upper adaxial sur- face , the amount of fluorescence typically increased to a maximum beneath the epidermis and then it declined Fig. The rate of decline was related to wave- length, being greatest for the nm blue light and less for nm red and nm green see Fig.
For inverted leaves, the maximum fluorescence occurred closer to the leaf surface and it declined more rapidly with depth see Fig 2. Infiltrating spinach leaves with water greatly reduces light scattering between the air and cell wall interfaces see Fig. Under these conditions, light ab- sorption profiles decline more gradually with depth compared with native leaves and the effect is larger at wavelengths where there is weaker absorption by chloro- phyll e.
By com- paring the profiles of control and water-infiltrated leaves, it is possible to calcu- late pathlength within the tissues of a leaf Fig. For a spinach leaf irradiated on the adaxial surface, initially pathlength was 1. Going deeper into the leaf, the value for red light was constant across the leaf whereas it increased to around 2.
For abaxial illumina- tion, initially the values for red and blue light were similar to the adaxial surface, but they declined with depth suggesting that for these wavelengths, spongy and 26 Sunlight Capture Depth pm Figure 2. Chlorophyll fluorescence in spinach leaves irradiated with monochromatic light. Both native A,B and water-infiltrated leaves C,D were measured. Symbols identify the lines; actual data points are spaced 6. Erom Vogelmann and Evans Green light had a pathlengthen- ing value around 2 in the spongy tissue, which declined steadily toward the adax- ial surface, suggesting greater scattering in spongy than palisade tissue.
In view of the contrasting optical properties of the palisade and spongy tissues, it is curi- ous that pathlength was relatively uniform in these tissues. Despite the fundamental relationships between leaf optics, light absorption pro- files in leaves, and photosynthetic performance, we have a way to go in eluci- dating the important relationships. Some of the details have been worked out for spinach but it remains to be demonstrated whether leaves from other species have similar profiles of light absorption and photosynthetic capacity.
The same can be said for sun and shade leaves of the same plant. Current data present an average 2. Chloroplast to Leaf 27 2. Optical pathlengthening by intercellular air spaces was calculated from the data shown in Fig. Symbols as in Fig. From Vogelmann and Evans Vascular tissues of most C 3 leaves are curiously devoid of photosynthetic pigmentation, and the transparent nature of bundle sheath extensions has been proposed to help light travel deeper in the leaf than is possible through mesophyll tissues Karabourniotis et al.
On the other hand, matching light absorption between chloroplasts within the bun- dle sheath and mesophyll tissues of C 4 plants is critical for efficient photosyn- thetic performance. Finally, little is known about how light direction interacts with leaf anatomy to set up internal absorption profiles. Current experimental data have been collected when light strikes the leaf perpendicularly, yet this is rarely the case in nature. Indeed, it is well known that photosynthesis within canopies is enhanced when the light is diffuse Farquhar and Roderick , Roderick et al.
Given recent advances in experimental techniques, essential information about light and photosynthesis within leaves no longer comprises a missing link in scaling photosynthetic processes from the chloroplast to the canopy level. Chloroplast Movements Chloroplasts are usually closely appressed to the cell membranes of mesophyll cells, facilitating gas exchange. However, their position within cells is dynamic and they may shift along the cell membrane to assume positions adjacent to per- 28 Sunlight Capture iclinal parallel to the leaf surface or anticlinal perpendicular to the leaf sur- face walls under different circumstances.
These movements change the optical properties of the leaf and have the potential to alter photosynthetic performance and perhaps whole-plant productivity. Light is the dominant signal controlling chloroplast movements in higher plants and light-driven chloroplast movements have attracted attention for more than a century, largely because of the optical changes they engender. There are several reviews that describe the phenomenon Britz , Haupt, , Haupt and Scheuerlein , Wada and Kagawa , Wada et al.
Patterns of chloroplast movement differ among various algae, mosses, ferns, and higher plants. We focus here on higher plants. If light is dim, chloroplasts are generally found along the periclinal walls, perpendicular to the incident light, where they intercept and absorb light maximally. In contrast, when light is bright, chloroplasts move to anticlinal walls, forming an optically dense cylinder in the peripheral cytoplasm but leaving a chloroplast-free path through which light can penetrate to deeper cell layers. This movement causes whole-leaf transmittance and reflectance to increase, so leaves in high light appear paler than in low light, a change that is fully reversible as light fades and chloroplasts return to pericli- nal walls Fig.
Chloroplast movements have generally been studied either directly, using mi- croscopy, or indirectly, using optical techniques exploiting changes in transmit- tance that accompany chloroplast movement.
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Using these techniques, light-driven chloroplast movements have been observed in most groups of photosynthetic or- ganisms, including algae, bryophytes, ferns, and higher plants Table 2. They occur in hydrophytes e. The extent of light-mediated chloroplast movement varies with species; it may be hampered by chloroplast crowding McCain as well as by large chloroplast size Jeong et al.
There are reports of plants that do not show chloroplast movements, but some Lechowski are based on transmittance changes alone, which may not al- ways detect chloroplast movement. Thick leaves with multiple cell layers would show negligible change in overall leaf transmittance. If chloroplasts move to an- ticlinal walls, light might penetrate one cell layer only to be absorbed by the next. For example, chloroplast movements are evident in micrographs of pea leaves, but corresponding changes in leaf transmittance are small Park et al.
A few plants, such as the moss Fontinalis antipyretica, do not show significant chloroplast movement detectable using either technique Zurzycki , but such reports are rare. Light-driven chloroplast movements, while perhaps not ubiqui- tous, are extremely widespread in photosynthetic organisms. Mechanism Studies on the mechanism of chloroplast movements have focused on two areas, light perception and force generation. In higher plants, chloroplast movement is 2. Chloroplast to Leaf 29 Figure 2. Striking changes in leaf appearance accompany chloroplast movements. The sunburst pattern visible here was shaded while this Alocasia leaf was irradiated, so chloro- plasts in shaded cells remained in their low-light position along periclinal walls; ab- sorbance was therefore relatively high, so the shaded area appears dark.
In the irradiated area, chloroplasts moved to the periclinal walls, absorbance was lower, and the tissue ap- pears a lighter shade of green. Diagrams represent chloroplast position in cells in the two areas of the leaf. Movement to anticlinal walls in dim light is me- diated by both phototropin 1 photl and phototropin 2 phot2 , while movement to periclinal walls in high light appears to be mediated by phot2 alone Briggs et al. In some cases, however, microtubules may be in- volved, as in red-light mediated chloroplast movement in the moss Physcomitrella Sato et al.
Widespread occurrence of light-driven chloroplast movement in algae and the pant kingdom. Chloroplast movement was demonstrated either directly by microscopy M , indirectly by monitoring changes in the optical properties of the tissues O , or, in one case, by nuclear magnetic resonance spectroscopy NMR. However, available instrumentation has restricted kinetic measurements of chloroplast movements to the laboratory.
It has been difficult to assess, for example, how laboratory-derived light-response curves might relate to actual chloroplast movements with changing solar irradiance throughout the day. Now, with a field-portable device that monitors leaf trans- mittance using a pulsed measuring beam and lock-in detection, it is possible to follow chloroplast movements in the field Fig. Data are shown for Aquilegia flavescens columbine , a plant that grows at high alti- tude, in a high-PAR, high-UV environment.
The transmittance signal obtained in the field is not directly comparable with that obtained in the laboratory be- cause of changes in parameters such as angle and collimation of irradiation and because of differences in instrument design. However, observed transmittance changes are consistent with light-mediated chloroplast movement.
In this exam- ple, leaf transmittance tracks PAR, decreasing during the late afternoon as PAR drops and chloroplasts move to periclinal walls, remaining low overnight, and increasing again the next day as chloroplasts move to the anticlinal walls. Functionality Chloroplast movements are widespread yet require energy; they occur in the field as well as under controlled laboratory conditions.
These observations lead one to consider the possible adaptive advantage they may have for plants. Hypothe- ses about the possible benefits of chloroplast movement generally have been based on the changes they cause in optical properties of the leaves. Chloroplast move- ments under dim light and under bright light are distinct responses, cause leaf absorptance changes in different directions, and likely serve different adaptive roles.
One adaptive advantage of chloroplast movement to periclinal walls under dim light seems clear: How- ever, changes in the rate of photosynthesis may be greater than expected based on changes in whole-leaf absorptance alone Zurzycki Several hypothe- ses, perhaps complementary, have been suggested to explain chloroplast move- ment to anticlinal walls in bright light. Chloroplast movement in high light protects chloroplasts from photodamage.
This hypothesis is based on the idea that fewer chloroplasts would be exposed to potentially damaging rays; mutual shading would protect most of 2. Chloroplast to Leaf 33 Figure 2. Measure- ments were made in the field in the Snowy Mountains of southwestern Wyoming at about m eleva- tion. It is an early hypothesis Zurzycki , and there is recent evidence to support it. The greater tolerance of light stress in Tradescantia than in Piswn has been attributed to light-induced chloroplast move- ment in Tradescantia Park et al.
However, although absorptance changes were greater in the thinner-leafed Tradescantia, both species showed marked chloroplast movement, so there may be other factors involved. Another study employed transgenic tobacco plants deficient in plastid division with only a few large chloroplasts per cell rather than many small ones Jeong et al. Al- though the large chloroplasts did move in response to light, transmittance changes were smaller than for wild-type leaves, and the transgenic plants were more sus- ceptible to photodamage.
Both of these studies depend on correlation between transmittance changes and susceptibility to photodamage, and for both, factors other than chloroplast movement could cause the observed differences in photo- sensitivity. Experiments using other ways to block chloroplast movement, such as with the phot mutants that do not show chloroplast movement, will be im- portant to confirm these conclusions.
In addition, little is known about how dif- 34 Sunlight Capture ferences in sensitivity to photodamage found in the laboratory translate to dif- ferences for plants growing in the field, where irradiation regimes are more vari- able and where small differences accumulate over time. Chloroplast movement to anticlinal walls under high light opens channels of light penetration to deeper cell layers within the leaf and relieves light limitation of photosynthesis in those deeper layers Brugnoli and Bjorkman , Terashima and Hikosaka One prediction from this hypothesis is supported: However, there is no evidence that the observed redistribution of light absorption leads to any in- crease in photosynthesis, at least in the short term Lechowski , Zurzycki Fluorescence was induced by blue light normal to the adaxial surface arrows.
Chloroplast to Leaf 35 Hypothesis 3: Chloroplast movement in bright light reduces Cdiffusion dis- tance and enhances CO 2 uptake. Under this hypothesis, chloroplasts would move to their low-light position to maximize light absorption when light was limiting, and to their high-light position to maximize the rate of Cdiffusion when CO 2 was limiting. Both distance in the liquid phase and surface area of chloroplasts adjacent to intracellular air spaces affect the rate of CO 2 diffusion from the in- tercellular air spaces to the chloroplasts Evans , Evans and Vellen Thus, chloroplast movement in high light could increase CO 2 diffusion by mov- ing the chloroplasts more tightly against the plasma membrane, or if it flattened them so that more chloroplast surface was against the membrane.
This hypothe- sis is consistent with the light requirements for chloroplast movement to anticli- nal walls; saturation of the response generally requires at least as much light as saturation of photosynthesis and can require much more Gorton et al. However, photoacoustic experiments designed to assess the diffusion rate of CO 2 into chloroplasts do not show that chloroplast movement under bright light en- hances gas diffusion Gorton et al.
Although much of its specific function remains to be elucidated by future ex- periments, it seems clear that chloroplast movement is an important part of the armamentarium that includes leaf shape, display angle, active movement, and wilting, that plants have evolved for keeping them precisely adapted to a vary- ing light environment. Summary and Conclusions Despite tremendous diversity in leaf anatomy and light environments, there are many features that allow generalization.
Leaves exist to increase the efficiency with which a plant can capture light and gain CO 2. The diversity of solutions reflect the differ- ent environmental pressures, and diversity appears to increase as one scales up from the photosystem reaction center to the chloroplast, cell, leaf, and plant. There is little difference between species in the pigment-protein complexes of chloroplast thylakoids. Eor shade chloroplasts either those deep within a leaf, or from a leaf growing under low irradiance , there is an increased proportion of chlorophyll associated with light-harvesting complexes compared to sun or high- light chloroplasts.
This is associated with an increased number of thylakoids per granum, but curiously, does not seem to alter the ratio of stroma exposed to ap- pressed thylakoid membrane surface. We still lack experimental techniques to probe the fate of light as it is absorbed in the thylakoids to definitely know how much is absorbed by each photosystem. The complexity in the distribution and function of the various types of each photosystem, whilst known, is not properly understood.
A key question that awaits definitive explanation is the wavelength dependence of quantum yield. Perhaps, rather than waiting for the invention of new techniques, progress can be made by using mutant and transgenic plants. While multiple interpretations 36 Sunlight Capture of such experiments can be made, having independent ways of asking the same question improves our chances of correct understanding.
Unfortunately, what is often revealed is the flexibility a plant has in bypassing the problem. For exam- ple, removing the two major LHCII proteins was compensated by increased ex- pression of another gene. Lhcb5 is usually a minor component restricted to a place close to the reaction center complex of photosystem II. However, in trans- genic plants lacking Lhcbl and Lhcb2, the expression of Lhcb5 increased suffi- ciently for it to be able to assemble into trimeric complexes, a role previously thought to be specific for Lhcbl and Lhcb2 Ruban et al.
Chloroplast movement is a widespread phenomenon and yet it has been diffi- cult to demonstrate any substantial benefit to performance in terms of protection from damage in bright light, enabling light to penetrate more deeply into a leaf, or reducing the resistance to CO 2 diffusion into the chloroplast. Here too, mutants and transgenic plants may assist in revealing the function of chloroplast movement. By far the greatest diversity for light capture by the leaf exists among plants in their anatomy.
It is now possible to infer the profile of light absorption through the leaf by measuring chlorophyll fluorescence Vogelmann and Evans This revealed the dominant role that chlorophyll distribution played, as well as the impact of light scattering by the intercellular air spaces. Considerable work remains to be done in examining a broad range of foliage, from thick needles to thin leaves. The problem of dealing with diffuse light and light striking the leaf at different angles also requires effort. It is known that the photosynthetic ca- pacity of chloroplasts can vary and adapt.
Usually it is thought that chloroplast photosynthetic capacity will relate to the amount of light absorbed by the chloro- plast. However, are there limits to the flexibility of a given chloroplast? What are the lower and upper bounds in photosynthetic capacity per unit chlorophyll? It is not known whether different chloroplasts within a given cell have to share the same capacity per unit chlorophyll. It is also only possible to match one pro- file in light absorption, which means that it is not perfectly matched to other profiles that may occur due to changing angle of incidence of the light, altered proportion of diffuse light, or light falling onto the other surface.
The mismatch between light absorption and photosynthetic capacity reduces the efficiency of the leaf because less photosynthesis is achieved for a given investment in re- sources. Within the leaf, this is the area where the greatest potential exists to im- prove photosynthesis. The huge diversity in leaf anatomy shows that there are many solutions to the problems faced by a plant. This also provides us with the ability to tease apart the different contributions that tissue, cell, and chloroplast morphology make toward light capture by a leaf.
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Crystal structure of photosystem II from Synechococcus elongatus at 3. Chloroplasts arrangement as a factor in photosynthesis. The destructive effect of intense light on the photosynthetic apparatus. The influence of chloroplast displacements on the optical properties of leaves. Action dichroism in the chloroplasts rearrangements in various plant species. Leaf light interception scales linearly with incident irradiance, but plant photosynthesis and photomorphogen- esis typically exhibit a saturating response to light.
Because of the inherent non- linearity in light responses, estimates of the photosynthetic rate at canopy scale cannot be obtained from mean irradiance values, but require a full description of the radiative field. This means that scaling of light harvesting from leaf to land- scape is a central issue for the prediction and understanding of plant canopy processes Asner and Wessman Because of the strong linkage between photosynthesis and plant water use, canopy radiative field is not only relevant for primary plant productivity, but also for the partitioning of ecosystem energy fluxes between sensible and latent heat.
Thus, architecture of plant stands and resulting light environment exert a major control over the meteorology of plant communities Baldocchi and Harley , Lai et al. Detailed upscaling methodologies are required both in ecological modeling and remote sensing to deal with the spatio-temporal distribution of the irradiance on leaf surface Myneni and Ross The increasing awareness of the role of terrestrial ecosystem in global bio-geochemical cycles enhances the interest in the application of soil-vegetation-atmosphere transfer SVAT models. This class of process-oriented models has typically a hierarchical structure, scaling physical and physiological processes from a single leaf to higher hierarchical or- ders such as single crown, vegetation canopies, and landscape mosaics e.
The predictions of SVAT models are strongly affected by the scaling strategies adopted for the integration of the leaf-level photosyn- thetic response with the light climate Law et al. There is a plethora of architectural strategies for light harvesting, and the ef- ficiency of these strategies is evident in numerous physiological characteristics, for example, compensation and saturation points in the light response curve.
This wealth of biological details implies that modes of light environment have to tackle 42 3. Leaf to Landscape 43 with extremely complex plant geometries. With development of computer tech- nology, very detailed models can be developed to described light environment of single leaves and shoots Valladares and Pearcy , or even clonal plant stands Casella and Sinoquet with very high degree of predictability.
How- ever, extensive parameterization requirements limit application of fine-detailed schemes to larger scales. Thus, architectural drivers of light harvesting must be simplified to scale up plant physiological processes in larger scale models. Such a model simplification requires a full understanding of the relative impact of dif- ferent architectural levels on light capture as well as the interactions between the structural and temporal scales of light harvesting and plant functioning.
For in- stance, most SVAT models rely on the assumption of homogeneous canopy to simplify the scaling of light interception, that is, leaves are considered to be ran- domly dispersed in a series of horizontal layers Baldocchi and Wilson However, only a few sensitivity analyses have been conducted to test for the ap- propriateness of such important assumptions. In parallel, scaling canopy processes at larger scales, from canopy to the land- scape, region, and biome levels, is often based on remote sensing techniques to retrieve structural and physiological information of the canopies from the spec- tra of reflected radiation.
To predict intensity and angular distribution of reflected radiation, these procedures are based on direct or inverse application of detailed radiative transfer models that link leaf-level optics with canopy-scale architec- ture Pinty et al. In the last decades, the increasing attention on these is- sues and the development of instrumentation, theories, and models have signif- icantly improved the capability to upscale light harvesting through architectural, spatial, and temporal scales. However, recent evidence of major differences in direct and diffuse irradiance use-efficiency at the ecosystem level Roderick et al.
In this chapter, we first review new developments in the description of plant architecture and radiative field that allow accounting for the effect of canopy het- erogeneity on leaf irradiance Gastellu-Etchegorry et al. The other task of this chapter is to outline a series of challenging objectives, such as simulation of penumbra and clustering of light- harvesting elements at various hierarchical scales, that should be included in the future generation of global-scale simulation models. Methods in Scaling the Light Climate The major sources of strong temporal and spatial dynamics of light in the vege- tation are 1 changes in the radiative field above the canopy, for example, due to modifications in solar elevation, cloudiness, or angular distribution of diffuse radiation; 2 complexity of canopy structure, for example, angular and spatial distribution of the leaves in shoots and crowns, and 3 the dynamic nature of 44 Sunlight Capture the vegetation, for example, seasonal trends in plant phenology and leaf flutter- ing Baldocchi and Collineau The variability of irradiance on leaf surface, resulting from the interplay of radiative field and canopy optical and geometri- cal assets, largely affects the scaling of physiological and biophysical processes from the leaf to the landscape photosynthesis, transpiration, energy balance.
The variation in canopy light climate can be directly investigated with experi- mental measurements of incoming photosynthetic photon flux density PPFD , or can be predicted by radiative transfer models starting from the quantitative description of canopy architecture and optics.
Direct Measurements of the Radiative Regime The experimental description of the light climate within plant canopies is par- ticularly complex because larger variations in the geometry of the radiation field above the canopy are further amplified by canopy structure and optics, implying an inherently large variability in the distribution of irradiance on leaf surface. The structure of the radiative field can experimentally be investigated in time and space, and separating between angular and spectral variation of irradiance Norman and Jarvis , Campbell and Norman , Baldocchi and Collineau , Ross et al.
Because of the complexity of the radiative field within canopies, direct meas- urements of light climate or canopy reflectance generally aim to indirectly esti- mate canopy architecture parameters by the inversion of canopy radiative trans- fer models Welles , Weiss et al. In addition, direct light measurements are used to verify the predictive capacity of radiative transfer models RTMs or to investigate the modification of the radiative field by plant characteristics such as leaf fluttering and movement due to wind as well as penumbra, which are not included in most of the existing radiative transfer models Roden and Pearcy , Palmroth et al.
The high spatio-temporal variability of radiation in plant canopies severely limits the possibilities to scale light climate by direct measurements, and it is essential to clearly define which features of the radia- tive field should be measured and for which aims. Overall, three general goals can be pursued: The number of independent sensors primarily depends on the vari- ability of the light regime in the investigated canopy type and the time scale of interest.
For instance, uniform broadleaf canopies show a lower spatial variabil- ity compared with conifers or sparse canopies Baldocchi and Collineau , while the spatial variability largely decreases with increasing averaging time. When the ultimate goal is upscaling photosynthesis from leaf to canopy, it is es- sential to consider that sensors should be distributed in space and oriented in an- gles to mimic leaf distribution Palva et al. Very often cosine-corrected 3. Leaf to Landscape 45 flat sensors are erroneously assumed to correctly represent the irradiance distri- bution on leaf surface.
Spatial and temporal sampling can be combined using mobile sensors. Language English View all editions Prev Next edition 1 of 2. Check copyright status Cite this Title Photosynthetic adaptation: Other Authors Smith, William K. William Kirby , Vogelmann, Thomas Craig. Series Ecological studies ; v.
Ecological studies ; volume Ecological studies ; volume Summary "The impact of global change on sources, sinks, and sequestration of carbon and, ultimately, on future changes in plant distribution and biodiversity patterns depends upon the capacity of plants for light capture and CO[subscript 2] assimilation. This book provides a detailed analysis of photosynthetic mechanisms across the structural and spatial hierarchy from cells to leaves, crowns, canopies, stands and landscapes.
The authors question whether photosynthetic adaptations are taking place primarily at the metabolic and biochemical level, or through changes in structure and form, or both. In the interest of genetic engineering applications for plant improvement, they consider the relative importance of genes controlling both metabolic and light reactions, as opposed to the development and arrangement of photosynthetic components.
The impact of global change on sources, sinks, and sequestration of carbon and, ultimately, on future changes in plant distribution and biodiversity patterns depends upon the capacity of plants for light capture and CO2 assimilation. Vogelmann, and Christa Critchley Part 2. Williams, and Holly L. Ian Woodward, Dennis D. Baldocchi, and David Ellsworth Part 5.
Standish, and Ichiro Terashima 8. Ellsworth, Ulo Niinemets, and Peter B. Baker, and Donald R. Vogelmann, and Christa Critchley. Notes Includes bibliographical references and index.
Full text of "Photosynthetic adaptation : chloroplast to landscape"
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