{"id":40456,"date":"2021-03-22T21:48:15","date_gmt":"2021-03-22T21:48:15","guid":{"rendered":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=40456"},"modified":"2021-03-22T21:48:19","modified_gmt":"2021-03-22T21:48:19","slug":"leaf-optical-properties","status":"publish","type":"post","link":"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=40456","title":{"rendered":"Leaf Optical Properties"},"content":{"rendered":"<p id=\"top\" \/>\n<p><em>by St\u00e9phane Jacquemoud and Susan Ustin<\/em><\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Leaf Biophysics<\/h3>\n\n\n\n<p>The epidermis, also called the dermal tissue, is the specialized outermost cell layer of the leaf, which is typically one cell thick. &#8211; p.14<\/p>\n\n\n\n<p>The cuticle is a non-cellular protective layer that is produced by the epidermal cells and that covers the entire leaf surface (Figure 2.4). It may be covered with wax that can have many forms from smooth to complex three-dimensional multicellular structures. Therefore it reflects the cell pattern of the epidermis. &#8211; p.14<\/p>\n\n\n\n<p>The micro- and nanostructures of the cuticle confer defensive properties that prevent penetration by dust, atmospheric pollutants, and microorganisms including virus particles, bacterial cells, and the spores or growing filaments of fungi &#8211; p.15<\/p>\n\n\n\n<p>The waxes and other cuticular materials differ in chemical composition between species, causing differences in the luster (glossiness vs. glaucousness or bloom) of leaves that affects the light-scattering properties. &#8211; p.16<\/p>\n\n\n\n<p>The epidermis in both angiosperms and gymnosperms consists of thick-walled epidermal cells without chloroplasts, which are coated on the outer side with a waxy cuticle of thickness and structure vary depending on the species (Figure 2.5). &#8211; p.16<\/p>\n\n\n\n<p>The epidermal surfaces of plant organs, such as stems and leaves, are often ornamented unicellular to complex multi-cellular outgrowth structures called trichomas, the most common typeof which is hairs, but which also include other structures like scales (Figure 2.6). &#8211; p.16<\/p>\n\n\n\n<p>Leaves and hairs occur in many forms. They can be hirsute (covered with coarse hairs), hispid (covered with stiff or rough hairs), downy (covered with fine soft hairs), or strigose (having stiff, straight, closely appressed hairs), etc. as shown in Figure 2.7. &#8211; p.16<\/p>\n\n\n\n<p>Finally in some species, the palisade parenchyma is present on both faces of the leaf blade. This bilateral symmetry is often found in species where a combination of wind and canopy architecture results in similar illumination on both leaf surfaces. &#8211; p.19<\/p>\n\n\n\n<p>A light that is too intense causes photoinhibition, which results in a significant reduction of photosynthetic activity. This photoinhibition performs at several scales; at the cell scale the chloroplasts that are under optimal lighting conditions preferentially move towards the periclinal cell wall to maximize their exposure to incident light. When illumination becomes too strong, they move to rearrange the chloroplasts along the anticlinal cell walls to be oriented sideways to the incident light, which reduces possible damage of the photosynthetic apparatus. &#8211; p.46<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Spectroscopy of leaf molecules<\/h3>\n\n\n\n<p><br>With plant leaves we face two kinds of difficulties: first, as pointed out earlier, the absorption properties of the biochemical constituents which, except for water, are poorly known. This problem is most acute for chemicals that are embedded in membrane bound complexes, such as the photosynthetic pigments. Second, leaves are not homogeneous media. &#8211; p.48<\/p>\n\n\n\n<p>Carotenoids are a large family of water-repelling red, orange, or yellow accessory pigments, which primarily absorb in the blue-green wavelengths and extend the wavelength range for energy absorption by photosynthesis. &#8211; p.57<\/p>\n\n\n\n<p>Anthocyanins are flavonoid pigments not associated with photosynthesis. The spectral features of anthocyanins, and hence their colors, are very dependent on pH, temperature, solvent, and the presence of other molecules that may interact with them. &#8211; p.58<\/p>\n\n\n\n<p>Flavonoids that are ubiquitous fulfill many plant functions. They accumulate primarily in epidermal cells where they protect photosynthetic activity from high sunlight, particularly UV-B radiation. p.60<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Leaf optical properties in different wavelength domains<\/h3>\n\n\n\n<p>In opposition to coherent scattering that can produce the phenomenon of iridescence, incoherent Rayleigh scattering can be caused by the presence of an intact crystalline epicuticular wax layer at the surface of glaucous leaves: it typically occurs when particles are much smaller than the wavelength of incident light. In that case, shorter wavelengths (the blue end of the visible spectrum) are preferentially scattered, causing the characteristic bluish gray-green color found in the leaves (or needles) of a number of plant species, including blue spruce (picea pungens), Atlas cedar {Cedrus atlantica), chalk dudleya {Dudleya brittonii), and blue gum (Eucalyptus globulusy). &#8211; p.127<\/p>\n\n\n\n<p>In windy locations, hairs break up the flow of air across the plant surface and increase the depth of the leaf boundary layer, reducing evapotranspiration and CO2 flux. Leaf pubescence also tends to increase along environmental gradients of decreasing precipitation: therefore it is presented as an adaptative feature of plants growing in hot or arid climates because pubescence greatly reduces the heat load of leaves by increasing the reflectance from their surface, which consequently reduces the amount of absorbed radiation. &#8211; p.128<\/p>\n\n\n\n<p>The convex cells of the upper epidermis, which are sensitive to the direction of light, seem to act as photoreceptors and also they can focus it on the epidermis itself somewhere within the mesophyll at the level of the chloroplasts, based on the curvature of their outer wall. Besides the control of leaf orientation, light-focusing properties of epidermal cells may increase the capture of diffuse light, and then the photosynthesis rate of leaves growing in the extreme slide which is prevalent in tropical rain forests. &#8211; p.134<\/p>\n\n\n\n<p>Recent years have seen substantial advances understanding of the sensitivity of higher plants to UV-B radiation: it may change their structural (e.g., leaf thickness), physiological, and biochemical properties; it may damage the photosynthetic apparatus by altering the chloroplast structure; and different lesions of DNA that controls the synthesis of proteins may also be observed. Plant mechanisms of resistance to UV-B radiation fall into three categories: epidermal screening, high antioxidative status, and efficient repair of damage. Changes in the optical properties of outer tissues of the leaf are considered to play a primary role, and much attention has been paid to the cuticle and the epidermis that function like a skin to the plant, providing a barrier between the outside world and the leaf interior &#8211; p.139<\/p>\n\n\n\n<p>However, if glaucous leaves are very effective reflectors of both UV and visible radiation, it seems that pubescent leaves are more effective in reflecting visible wavelengths than ultraviolet radiation. &#8211; p.141<\/p>\n\n\n\n<p>Depending on the amount of incident radiation and the physiological state of the plant, variable proportions of the energy absorbed by photosynthetic pigments may be converted into a stable chemical form through photosynthesis. Excess energy may be dissipated and lost as heat, and some may be emitted as fluorescence (Figure 5.24). &#8211; p.148<\/p>\n\n\n\n<p>The leaf epidermal layer is known to absorb harmful UV-A (~ 360 nm) and UV-B (~ 310 nm) radiation, acting as an effective filter that protects photosynthetically sensitive tissues from damage (see Section 5.2.2). Such an effect may be reinforced by the presence of trichomes or epicuticular waxes, which develop on the adaxial and abaxial side of certain leaf species. &#8211; p.165<\/p>\n\n\n\n<p>Most of the UV light (98% at 310 nm and 96% at 360 nm) is attenuated within the first 5 micro meters of the upper epidermis. &#8211; p.166<\/p>\n\n\n\n<p>Some authors noted that the palisade mesophyll might propagate light better than the sponge mesophyll. The allocation of light between the two mesophyll tissues is mainly due to differences in cell shape and in chlorophyll content. The particular shape of these cells seems to facilitate the penetration of light because of their central vacuole that presses the chloroplasts closely against the cell wall and serves as a transparent channel. Under diffuse light conditions, such as the sky on a cloudy day, or in the shade of plant canopies, internal light gradients are steeper in the palisade than when the leaves are irradiated with collimated light. It also shows that the transmission of light at 0\u00b0 is much larger than at the other sampling orientations &#8211; p.166<\/p>\n\n\n\n<p>Shortly after the first light profiles were published, a surprising optical phenomenon, predicted by models, was reported &#8211; the ability of plant tissues to trap light. Optical boundaries, which are created when the refractive index between two media changes very rapidly control the penetration and distribution of light within leaves. These boundaries act as mirrors that can bounce photons back and forth between the different tissue layers, which can result in more light inside the leaf than outside. For instance, the amount of light immediately beneath the irradiated surface can exceed by several times that of incident light. The degree of light trapping depends upon the amount of absorption by the leaf. It is higher at 730 nm, where absorption is low, than at 660 nm, where absorption by chlorophyll is high. Until recently the measurement of absorption profiles within plant leaves was more relevant to plant physiology than to remote sensing studies. The introduction of within-leaf chlorophyll profile gradients into leaf optical properties models rna\/ provide an improved estimation of this photosynthetic pigment by remote sensing techniques. &#8211; p.167<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Variation due to leaf structural, chemical and physiological traits.<\/h3>\n\n\n\n<p><br>Light scattering is more closely related to internal structure (proportions of parenchyma) rather than to thickness per se. &#8211; p.172<\/p>\n\n\n\n<p>In an intensely scattering environment, the primary effect of visible and shortwave infra-red (SWIR) light scattering is to increase the light optical path length which, in turn, increases the probability of light absorption within leaf tissues. This lengthening of the optical path length, which explains the difference in optical properties between leaves and chloroplast suspensions, is referred to as the detour effect. The detour effect leads both to increased absorption and to substantial flattening of the absorption spectrum, in comparison with the hypothetical spectrum that would be recorded in the absence of such an effect. &#8211; p.172<\/p>\n\n\n\n<p>Absorbers in a chloroplast, a vacuole, or the cytoplasm are not distributed homogeneously throughout the leaf but are rather packaged. Therefore when light travels through a leaf, it may be intercepted and absorbed by foliar constituents or propagated without encountering them. This phenomenon, known as the sieve effect (also called package effect or flattening effect), reduces the probability of light absorption within leaf tissues. &#8211; p.172<\/p>\n\n\n\n<p>This simulation shows that the increase of transmittance (loss of absorptance) by the sieve effect is large at wavelengths with strong absorption (low transmittance). In Figure 6.5, the absorption coefficient of the medium sigmaA = C kA is assumed to be constant along the whole depth, which of course is not true in a real leaf. Pigments in tissues are usually nonhomogeneously distributed due to the layered structure, to cell type variety, and to the absence of pigments in some cells. &#8211; p.174 [Fukshansky 1978]<\/p>\n\n\n\n<p>It has been well documented that detour and sieve effects not only have opposite influences in the absorption profile of turbid materials, but they are also more pronounced in different regions of the light spectrum. The absorption increase caused by the detour effect favors the wavelengths where absorption is minimum (green, red-edge), while the absorption decrease caused by the sieve effect is more noticeable at the wavelengths where it is maximum (blue, red). In conclusion, for the same amount of pigments with the same specific absorption coefficient and a fixed thickness, the transmittance of the leaf can be different depending on the spatial distribution of pigments. Since chloroplasts can move within minutes in response to how much light they receive, this movement can influence leaf optical properties. &#8211; p.174<\/p>\n\n\n\n<p>Some leaves display areas of transparent epidermis that is underlain by translucent water storage parenchyma. These structures allow light to directly penetrate into the leaf&#8217;s interior where it can be absorbed by chlorenchyma. This can occur naturally in &#8220;window-leaved&#8221; succulent desert plants or can be caused by insects, for instance, nocturnal caterpillars. The clear areas can take the form of scattered dots, lines, wide streaks, or even complete large portions of a leaf. Although strictly speaking such leaves do not cause a sieve effect as defined above, their transmittance could be calculated using the same formalism. &#8211; p.175<\/p>\n\n\n\n<p>In Angiosperms, the vascular system of the leaf is composed of a network of primary, secondary, and tertiary veins that may represent up to 25% of the leaf area (Figure 6.7). &#8211; p.176<\/p>\n\n\n\n<p>Monocotyledons are distinguished from dicotyledons in that most leaves have a parallel (or striate) venation system, with equally sized secondary veins that run lengthwise along the lamina between the midrib and leaf edge. On the contrary, leaves of most dicots have netted (reticulate) venation that follows a branching pattern originating from the midrib, into successively smaller, secondary, and tertiary veins in a recursive fashion. In some leaves, even the very small veinlets are surrounded by bundle-sheath cells arranged into tightly packed sheaths, which separate the mesophyll into many small compartments. These extensions may guide light deeper into the mesophyll of thick sun-adapted leaves. As a consequence, the reflectance may vary from one spot to another. &#8211; p.177<\/p>\n\n\n\n<p>Under low light intensity chloroplasts generally accumulate parallel to the leaf surface at the top and bottom of palisade cells (accumulation), while under high light intensity they arrange perpendicular to the leaf surface along the sides of cells (avoidance). The position of the chloroplasts in darkness is usually intermediate. &#8211; p.177<\/p>\n\n\n\n<p>The narrower, more columnar cells found in sunlit leaves restrict the ability of chloroplasts to move while the broader, more spherical cells of shaded leaves allow greater chloroplast rearrangements. In low light conditions this would optimize light capture. &#8211; p. 179<\/p>\n\n\n\n<p>The bottom (or dorsal or abaxial) surfaces of most dorsiventral leaves usually reflect more light than the top (or ventral or adaxial) surfaces. On average, the reflectance difference is about 6.6% but it can increase by up to 20% for some leaves. It indicates that the spongy parenchyma contribute more to light scattering than the palisade parenchyma of the leaf mesophyll. As for transmittance values, they are essentially alike in the visible and shortwave infrared regions, but in the reciprocal configuration, they can deviate by a few percent in the near-infrared region. An increase has been measured when the light falls on the abaxial side of the leaf- p.179<\/p>\n\n\n\n<p>Finally, albino leaves are totally devoid of chlorophylls, carotenoids, and other colored pigments, therefore they represent extreme examples that are very useful to study leaf optical properties. In absence of pigments, leaf reflectance over most of the visible region is the same as the maximum leaf reflectance observed in the near infrared. Then it decreases below 450 nm or so due to absorption by non-pigment biochemical constituents such as phenolic compounds, nucleic acids, and proteins. &#8211; p.183<\/p>\n\n\n\n<p>Water stress impacts plant productivity both in natural and crop plant communities. The mechanisms of defense are highly variable from one species to another. Some strategies reduce photon absorption by heliotropic orientation of the leaves or by wilting or curling, in others the chloroplasts are reoriented in the palisade parenchyma, while others increase the dissipation of excess energy through the xanthophyll cycle, which temporarily dissipates light energy. In the longer term, plant adaptations include a reduction in chlorophyll content, leaf abscission, and leaf thickening. All of these mechanisms reduce photosynthetic efficiency. &#8211; p.184<\/p>\n\n\n\n<p>Although these characteristics may differ between individual plants or species, sunlit leaves, in general, are smaller and thicker with less intercellular air spaces than shaded leaves. Their palisade\/ spongy mesophyll ratio is higher and their specific leaf area (SLA) is lower. They contain less chlorophyll per leaf area, and they display higher chlorophyll alb ratios. As a result, leaves exposed to sunlight reflect (absorb) more (less) visible light than the leaves in the shade, while their transmittance remains unchanged. &#8211; p.185<\/p>\n\n\n\n<p>Visible light travels further into sunlit leaves because of their elongated palisade cells. In the ultraviolet, the opposite is true. &#8211; p.186<\/p>\n\n\n\n<p>The juvenile stage starts with leaf emergence and finishes when it is fully expanded. Maturity represents the most important stage because the photosynthetic rate reaches its peak during this period. The onset of senescence can be defined as the time at which apparent photosynthesis begins to decline irreversibly due to chlorophyll breakdown. As leaves become more mature, both their mesophyll structure and biochemical content change. Juvenile leaves have small immature protoplasmic cells that are arranged in compact mesophyil layers. Their low water content is explained by the little water storage capacity of their vacuoles. Very young leaves exhibit primarily protochlorophyll that is rapidly replaced by chlorophyll. In contrast, the vacuolated cells of mature dicot leaves are loosely packed together, with large air spaces between them. They are consequently less compact than younger ones. The chlorophyll and water contents reach their maximum value. With further aging, the mesophyll structure of the leaf begins to deteriorate: the lateral walls of the palisade cells, followed later by those of the spongy cells, collapse; chlorophyll, starch, and protein decline; the preferential degradation of chlorophyll over carotenoids results in leaf yellowing and eventually the development of a bright yellow color; afterwards watersoluble anthocyanins may be produced giving the leaves their orange to red colors. The leaves drop in moisture concurrent with changes in cell geometry- Depending on the species, significant spectral fluctuations are produced in the visible, near, and shortwave infrared during the three growth stages. &#8211; p.187<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Modeling 3D leaf optical properties<\/h3>\n\n\n\n<p>A typical cell may be defined as a set of concentric objects, as &#8220;Russian doll&#8221; structures, filled with three different media: cell wall material (cellulose, hemicellulose, and lignin), chlorophyll, and water (Figure 10.12). These media are assumed homogeneous, that is, their physical properties are independent of the direction. Each cell constituent is therefore characterized by a volume, a refractive index n, and a specific absorption coefficient k to describe the partitioning of light among the reflected, transmitted, or absorbed fractions. &#8211; p.302<\/p>\n\n\n\n<p>Eighty percent or more of the light is exponentially absorbed in the palisade parenchyma. &#8211; p.311<\/p>\n\n\n\n<p>The net flux decreases exponentially in the spongy parenchyma, whereas its decline is smoother in the palisade parenchyma. This effect is linked to the cylindrical structure of the palisade; cells, where some rays penetrate the inner part of the cell, which is filled with water, and then are propagated toward the bottom of the cell without being absorbed. Pigments are concentrated in chloroplasts and not uniformly distributed in the cells, which causes a diminution of the absorption at high absorption wavelengths. In the spongy mesophyll, the large number of small cells that are randomly distributed excludes that situation. &#8211; p.314<\/p>\n\n\n\n<p>Because of its numerous spherical cells, thes spongy mesophyll is the most scattering tissue. Light diffusion is thus essential to absorption efficiency. When setting a specific absorption coefficient for a leaf model, one should also take into account this effect. &#8211; p.316<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Application of leaf optics<\/h3>\n\n\n\n<p>Increased rates of photosynthesis have been observed at the canopy level due to increased proportion of diffuse light. The reason is that two leaves receiving moderate light produce more photosynthesis than if one receives bright light while the other is in deep shade (Farquhar and Roderick, 2003). At the leaf level direct and diffuse light may be utilized differently- Leaves measured under diffuse light reflect more and transmit less light than under direct light. Their overall absorptance may be a few percent lower, as well as their photosynthesis. Finally, leaves that develop under direct high-light condi tions use direct light more effectively than diffuse light, while those that develop under diffuse, low light conditions use diffuse and direct light equally well. &#8211; p.366<\/p>\n\n\n\n<h3 class=\"wp-block-heading\">Conclusion<\/h3>\n\n\n\n<p>Plants have evolved diverse mechanisms to control their interactions with the environment, from changing turgor pressure in epidermal cells that allow light to be focused into the leaf&#8217;s interior, and to motile stomata that move to the edge of palisade parenchyma cells or to their interior to enhance or limit light absorption. When present in excess, photosynthetic light is released through fluorescence, protecting the reaction centers. Even the clustering of chloroplasts or their dispersion in the cell extends or reduces the path length for transmitted light, affecting the efficiency of light absorption. Moreover, leaves change structural properties, such as increased cell wall thicknesses providing additional cellular support during periods of limited water availability and increased leaf pubescence and glandular trichomes that reduce light absorption. &#8211; p.404<\/p>\n","protected":false},"excerpt":{"rendered":"<p>by St\u00e9phane Jacquemoud and Susan Ustin Leaf Biophysics The epidermis, also called the dermal tissue, is the specialized outermost cell layer of the leaf, which is typically one cell thick. &#8211; p.14 The cuticle is a non-cellular protective layer that is produced by the epidermal cells and that covers the entire leaf surface (Figure 2.4). &#8230; <a title=\"Leaf Optical Properties\" class=\"read-more\" href=\"https:\/\/www.labri.fr\/perso\/barla\/blog\/?p=40456\" aria-label=\"Read more about Leaf Optical Properties\">Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[621],"tags":[],"class_list":["post-40456","post","type-post","status-publish","format-standard","hentry","category-books"],"_links":{"self":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/40456","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=40456"}],"version-history":[{"count":4,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/40456\/revisions"}],"predecessor-version":[{"id":40460,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=\/wp\/v2\/posts\/40456\/revisions\/40460"}],"wp:attachment":[{"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=40456"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=40456"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.labri.fr\/perso\/barla\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=40456"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}