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(American Journal of Botany. 2007;94:1061-1066.)
© 2007 Botanical Society of America, Inc.

Anatomy and Morphology

Do epidermal lens cells facilitate the absorptance of diffuse light?1

Craig R. Brodersen2 and Thomas C. Vogelmann

University of Vermont, Department of Plant Biology, 120 Marsh Life Science, Burlington, Vermont 05405 USA

Received for publication October 25, 2006. Accepted for publication May 10, 2007.

ABSTRACT

Many understory plants rely on diffuse light for photosynthesis because direct light is usually scattered by upper canopy layers before it strikes the forest floor. There is a considerable gap in the literature concerning the interaction of direct and diffuse light with leaves. Some understory plants have well-developed lens-shaped epidermal cells, which have long been thought to increase the absorption of diffuse light. To assess the role of epidermal cell shape in capturing direct vs. diffuse light, we measured leaf reflectance and transmittance with an integrating sphere system using leaves with flat (Begonia erythrophylla, Citrus reticulata, and Ficus benjamina) and lens-shaped epidermal cells (B. bowerae, Colocasia esculenta, and Impatiens velvetea). In all species examined, more light was absorbed when leaves were irradiated with direct as opposed to diffuse light. When leaves were irradiated with diffuse light, more light was transmitted and more was reflected in both leaf types, resulting in absorptance values 2–3% lower than in leaves irradiated with direct light. These data suggest that lens-shaped epidermal cells do not aid the capture of diffuse light. Palisade and mesophyll cell anatomy and leaf thickness appear to have more influence in the capture and absorption of light than does epidermal cell shape.

Key Words: absorptance • diffuse light • epidermal focusing • lens cells • optics • papillose cells • reflectance • transmittance

The forest floor beneath a dense canopy is a unique environment for understory plants. The light regime is typically diffuse and light intensities are much lower compared to the primarily intense direct light received at the top of the canopy. Direct light can penetrate to the understory through gaps in the canopy, appearing as sun flecks (Smith et al., 1989 ; Pearcy, 1990 ). Aside from these infrequent, short, intense bursts of direct light, plants in the understory rely on diffuse light for photosynthesis. Crop physiologists observed many years ago that photosynthesis within canopies increases under diffuse light (Norman and Miller, 1971 ), and more recently remote sensing research has shown that community level productivity of forests also increases under diffuse light conditions (Roderick et al., 2001 ; Farquhar et al., 2003; Gu et al., 2003 ). Climate change scenarios suggest an increase in diffuse light coupled with more moisture in the atmosphere (Geider and Delucia, 2001 ; Pounds and Puschendorf, 2004 ). Therefore, it has become increasingly important to understand how direct and diffuse light penetrates leaves and how the directional quality of light affects photosynthesis.

Leaf epidermal cells constitute an important boundary between the mesophyll and external environments, and they have evolved to serve many purposes such as retaining water, controlling transpiration and CO2 uptake, repelling water, and discouraging predation by insects (Bone et al., 1985 ). Epidermal cells of most leaves provide a clear window for light to reach the mesophyll where it is absorbed for photosynthesis. Although usually transparent and free of chloroplasts, the epidermis is often pigmented by anthocyanins, which are synthesized in response to environmental stress (Bone et al., 1985 ) or as part of normal plant growth and development in special habitats (Lee et al., 1979 ; Lee and Graham, 1986 ). Epidermal cells also contain UV-absorbing compounds, which protect mesophyll cells against harmful short wave radiation (Smith et al., 1997 ; Turunen et al., 1999 ; Mazza et al., 2000 ).

In addition to pigments, which affect the spectral quality of the transmitted light, epidermal cell shape influences the amount of light that enters a leaf, primarily through lens action. Most epidermal cells have a convex shape that focuses light as it passes into a leaf (Vogelmann, 1993 ). These cells are fairly widespread throughout the plant kingdom, though they are typically associated with tropical understory herbs and plants that grow in areas with high moisture (Bone et al., 1985 ). In the most striking examples where epidermal cells are conical or even papillose, leaves have a velvet appearance when viewed from above and a satin sheen when viewed from one side. The focal properties of these cells have been described (Bone et al., 1985 ; Gorton and Vogelmann, 1996 ; Vogelmann et al., 1996 ), but their functional significance remains to be determined.

One hypothesis is that focusing light in the mesophyll might create a more favorable light environment for photosynthesis for at least some of the chloroplasts. But it is difficult to envision how this would be advantageous because adding light to some chloroplasts means that light is taken away from others (Bone et al., 1985 ). Another idea is that conical epidermal cells make the leaf surface more hydrophobic (Wagner et al., 2003 ; Bhushan, 2006), thereby reducing the ability of pathogens to colonize the leaf surface and also keeping the stomata clear for gas exchange. These hypotheses are not mutually exclusive, and a third possibility is that these cells might facilitate the capture of the diffuse light prevalent under forest canopies. When light strikes a flat surface, some of it is reflected by specular (mirror-like) reflection. The more oblique the light, the more is reflected, and light that barely glances a flat surface will be almost completely reflected. Adding conical cells increases surface roughness, which could aid the capture of low angle light and increase the amount of usable light in the understory for photosynthesis.

The purpose of this study was to test this idea by measuring the differences in reflectance and transmittance of both direct and diffuse light in leaves with two different types of epidermal cell shape—flat and conical. If leaves with conical epidermal cells reflect less diffuse light than flat leaves, then this would support the hypothesis that conical cells aid in the capture of light. To the best of our knowledge, no studies have addressed whether direct and diffuse light is captured similarly by leaves. Here we report experimental results using newly developed instrumentation that makes it possible to measure reflectance from leaves irradiated with diffuse light.

MATERIALS AND METHODS

Plant species
Plants chosen for study with convexly shaped leaf epidermal cells were Begonia bowerae Ziesenh., Colocasia esculenta (L.) Schott., and Impatiens velvetea. Species with topographically flat epidermal cells were Begonia erythrophylla Neum., Citrus reticulata Blanco, and Ficus benjamina L. Plants were grown in a glasshouse under 500-1400 µmol·m–2·s–1 and a 21°/18.5°C day/night temperature. Mature leaf samples were collected and stored in a moist, sealed plastic bag until measurements were conducted (less than 30 min). Leaf disks were taken for optical measurements (described later) and adjacent leaf tissue was sampled for anatomical measurements of epidermal, palisade, and mesophyll cell dimensions as well as total leaf thickness and a measure of the curvature of epidermal cells. Twenty measurements were made for each species for each anatomical attribute (Table 1, Fig. 1). Cell dimensions and tissue layer thickness were measured using an ocular micrometer calibrated against a stage micrometer. Epidermal cell surface angle was assessed by measuring the angle bisecting the apex of the cell using the "Measure" tool in Adobe Photoshop CS (Adobe Systems Incorporated, San Jose, California, USA) (Fig. 2). Twenty epidermal cells were assessed for this measurement per species.


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  		Table 1. Physical properties and dimensions of epidermal cells by species

 

Figure 1
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  		Fig. 1. Cross sections of leaves showing varying epidermal cell characteristics and leaf anatomy. Scale bar = 100 µm. (A) Begonia bowerae (B) B. erythrophylla (C) Colocasia esculenta (D) Ficus benjamina (E) Impatiens velvetea (F) Citrus reticulata

 

Figure 2
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  		Fig. 2. Method for quantifying epidermal cell curvature ({Theta}), where {Theta} was determined by analyzing a digital image of a cross section of each species using the "measure" tool of Adobe Photoshop CS to calculate the angle of cell curvature from parallel to the leaf surface

 
Optics measurements under direct and diffuse light
Reflectance and transmittance spectra were measured from leaf samples using an integrating sphere (Spectralon interior, 15.25 cm diameter, Labsphere, North Sutton, New Hampshire, USA). White light from the xenon arc lamp (150 W, Photon Technology International, Monmouth Junction, New Jersey, USA) passed through an entrance port and was directed to an exit port on the opposite side of the sphere. For measuring transmittance, a leaf disk, 2.38 cm in diameter was cut with a cork borer and affixed to the entrance port. For measuring reflectance, the leaf sample was attached to the exit port. Measurements were calibrated against a 99% reflectance standard (Spectralon SRS-99–010, Labsphere).

Light in the sphere was transmitted through a fiber optic cable, which was attached to a port 90 degrees from the entry port, then directed to a spectrometer (S2000, Ocean Optics, Dunedin, Florida, USA). Reflectance (R), transmittance (T), and absorptance (A) were calculated as described previously (Gorton et al., 2001 ) according to the relationship:


Formula 1

(1)
where 1 is the total fractional quantity of light that strikes a leaf.

Optical properties of leaves irradiated with diffuse light
Measurements of leaf reflectance under diffuse light required special instrumentation that will be described in detail elsewhere. Briefly, a dual-beam integrating sphere spectrometer was constructed in which monochromatic light was split into two beams, each of which was directed into an integrating sphere, one for sample and one for reference (Model CA-06050–000, Labsphere). The light was chopped such that it was alternately directed into the sample and reference spheres, and light was detected in each sphere by a bifurcated optical cable attached to a photomultiplier. The signal from the photomultiplier was sent to a lock-in amplifier. Lock-in detection allowed measurement of small leaf reflectance signals against the large amount of background light within the sphere.

Reflectance was measured by placing a leaf disk on a port, located 90° from the entrance port, such that the disk was irradiated with diffuse light emanating from the interior of the sphere. With the leaf sample in place, measurements were made at each wavelength as the monochromator advanced from 400–700 nm. Similar measurements were made with the port left open (baseline, Bl) and in the presence of a 99% reflectance standard (Sl). Total reflectance (Rtl) was calculated as:


Formula 2

(2)
where Kl = spectral calibration constant at each wavelength for the reflectance standard.

For measuring the amount of light that was transmitted through leaves when they were irradiated with diffuse light, diffuse incident light was created by directing white light into an integrating sphere as described earlier. A leaf sample was placed on an exit port of the sphere where it was irradiated with diffuse light, and then a second detector integrating sphere moved in place such that it captured the light that was transmitted through the leaf (Ts). Light was measured in the detector sphere through a fiber optic cable and spectrometer as described earlier. A reference baseline was measured with no sample in place (Tr). Transmittance (Td) was calculated as


Formula 3

(3)
Representative values of the reflectance, transmittance, and absorptance of direct and diffuse light have been provided.

RESULTS

Leaf anatomy of study species
Clear differences in epidermal cell shape were evident between the two study groups. Leaves with perfectly flat epidermal cells would theoretically have a curvature value of 180°, parallel with the leaf surface, while leaves with curvature values of 130° would have more lens-like cells. Species with lens cells had lower curvature angles, with an average of 30.5° less curvature than leaves with a flat surface. Leaf anatomical characteristics varied within the study group, and no clear trends were evident between the two study groups regarding palisade, mesophyll, or total leaf thickness (Table 1, Fig. 1).

The two Begonia species, which had similar leaf morphology, offered the opportunity to measure the effect of the epidermis on leaf reflectance, the primary difference being the presence or absence of lens-shaped epidermal cells. For these two species, the general trends observed in this study apply. Begonia erythrophylla responds like the other glossy plants, while B. bowerae responds like the other plants with lens cells.

Reflectance of direct and diffuse light
Our measured values for direct reflectance fall within the range for most leaves under direct light, with an average reflectance of 3.5% and 4.0% at wavelengths of maximum absorption in the blue (450 nm) and red (650 nm) regions, respectively, and 7.8% and 45.8% in the far (700 nm) and infrared (750 nm), respectively, where there is minimal absorption. Reflectance within the green was variable, depending upon the amount of pigmentation and leaf anatomy, and ranged from 4.2% to 17.4% at 550 nm in our leaves. Reflectance at all wavelengths studied in both diffuse and direct light was typically lower in plants with epidermal lens cells than those without epidermal lens cells (Fig. 3) (B. erythrophylla: 5.8% and 6.0% diffuse reflectance, and 4.7% and 4.5% direct reflectance at 450 nm and 500 nm, respectively; B. bowerae: 3.2% and 4.4% diffuse reflectance, and 2.1% and 3.1% direct reflectance at 450 nm and 500 nm, respectively). In leaves of both surface types, diffuse light was typically reflected more than direct light between 400 nm and 650 nm. Beyond 650 nm, direct light was reflected less than diffuse light in all leaves except the two Begonia species. Diffuse light was consistently reflected more than direct light across the entire spectrum.


Figure 3
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  		Fig. 3. Percentage of reflected light from leaves with either (A–C) flat or (D–F) lenticular epidermal cells after irradiation with direct (black lines, black triangles) or diffuse light (black lines, open circles). (A) Begonia bowerae (B) Colocasia esculenta (C) Impatiens velvetea (D) B. erythrophylla (E) Ficus benjamina (F) Citrus reticulata. Graphs show representative data

 
Transmittance of direct and diffuse light
Diffuse light was transmittance through all leaves slightly less than direct light (Fig. 4). Impatiens velvetea had the lowest transmittance values across the spectrum for diffuse light (2.0, 1.9, and 3.5% at 450, 500, and 700 nm, respectively), while C. reticulata had the lowest values for direct light at 450 and 500 nm (0.1% and 0.1%, respectively). The highest transmittance values for direct light at 450 nm and 500 nm were in F. benjamina (4.9% and 4.9%, respectively), and the highest transmittance values for diffuse light at 450 nm, 500 nm, and 700 nm were in B. bowerae (2.5%, 5.3%, and 17.6%, respectively). When the two Begonia species were compared, B. erythrophylla had higher transmittance of diffuse light at 450 nm (5.8% vs. 3.2% for B. bowerae) and 500 nm (6.0% vs. 4.4% for B. bowerae) as well as of direct light at 450 nm (4.7% vs. 2.1% for B. bowerae) and 500 nm (4.5% vs. 3.1% for B. bowerae). Throughout this range, both types of light were typically transmitted the most around 500 nm and above 700 nm.


Figure 4
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  		Fig. 4. Percentage of transmitted light from leaves with either (A–C) flat or (D–F) lenticular epidermal cells after irradiation with direct (black lines, black triangles) or diffuse light (black lines, open circles). (A) Begonia bowerae (B) Colocasia esculenta (C) Impatiens velvetea (D) B. erythrophylla. (E) Ficus benjamina (F) Citrus reticulata. Graphs show representative data

 
Absorptance of direct and diffuse light
Absorptance of direct light was typically slightly higher or equal to diffuse light absorptance throughout the entire spectrum for plants with and without lens cells, while plants with lens cells absorbed slightly less diffuse light (Fig. 5). Absorption was highest at 450 nm under direct and diffuse light for all species. Absorptance was typically lowest in the 525–550 nm range of the visible spectrum for all species. The only remarkable differences (greater than 1.5%) in diffuse or direct light absorptance occurred in C. esculenta, I. velvetea, and C. reticulata at 550 nm (16.5%, 12.5%, and 5.8%, respectively). Most species absorbed more direct light than diffuse light across the photosynthetically active wavelengths (400–700 nm), except for F. benjamina around 550 nm and B. erythrophylla at 625 nm.


Figure 5
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  		Fig. 5. Percentage of absorbed light from leaves with either (A–C) flat or (D–F) lenticular epidermal cells after irradiation with direct (black lines, black triangles) or diffuse light (black lines, open circles). (A) Begonia bowerae (B) Colocasia esculenta (C) Impatiens velvetea (D) B. erythrophylla (E) Ficus benjamina (F) Citrus reticulata. Graphs show representative data

 
DISCUSSION

The optical properties of the plants in this study were consistent with the ranges of transmittance, reflectance, and absorptance values observed for other species (Woolley, 1971 ; Gausman and Allen, 1973 ; Knapp and Carter, 1998 ). The typical transmission spectrum of leaves has a minimum in the blue wavelengths, a transmission peak within the green wavelengths, and maximum transmission in the far red and infrared wavelengths.

These data suggest that because no consistent differences in the reflectance, transmittance, or absorptance of direct and diffuse light were observed between leaves with and without lens cells lens cells do not appear to aid in the absorptance of diffuse light as was originally hypothesized. The presence of lens cells appeared to negatively influence the absorptance of diffuse light in two of the species (C. esculenta and I. velvetea). The velvety sheen that we refer to is probably a visual confirmation that multiple reflections occur in the epidermal cell layer of leaves with well-developed lens cells (Fig. 6), and those multiple reflections could be responsible for the decreased absorptance of diffuse light in the green wavelengths.


Figure 6
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  		Fig. 6. Origins of reflected light from a leaf. (A) Mirror-like or specular reflection comes from the leaf surface, whereas (B and C) diffuse reflectance originates from light scattering within the leaf

 
Overall, I. velvetea had the lowest transmittance and reflectance values for both diffuse and direct light across the entire spectrum. This is most likely due to the dark green pigmentation, which can appear almost black depending on growing conditions. The leaf pigmentation of this species is fairly plastic, with smaller and lighter-colored leaves in full sun, and broad, dark-colored leaves in heavy shade.

The causes of the greatest differences (greater than 1.5%) in diffuse or direct light absorptance occurring at 550 nm in C. esculenta, I. velvetea, and C. reticulata are not clear, although the leaves of C. esculenta and C. reticulata were very thin. The thicker leaves of the other plants may be able to absorb more direct light at wavelengths near 550 nm. It appears as though this phenomenon was a function of leaf thickness and possibly epidermal cell structure because the trend occurred in both study groups, those with and without lens cells.

The most striking result from this study was the unequal reflectance of diffuse and direct light independent of epidermal cell structure. With a greater proportion of diffuse light reflected from the surface of all leaves in this study, less light enters the leaf for photosynthesis. The unequal absorptance of direct and diffuse light and the extent to which a change in the directional quality of light affects photosynthesis at the leaf level is not yet known, but community level productivity in diffuse light has been estimated to be higher than in direct light, presumably because light is distributed more evenly within the canopy (Roderick et al., 2001 ; Farquhar and Roderick, 2003 ; Gu et al., 2003 ).

Pigment distribution, leaf morphology, and cellular arrangement appear to have significantly more effect than epidermal cell shape on the reflectance, transmittance, and absorption of diffuse light. Lens cells may then be more important for the focusing of direct light (Vogelmann et al., 1996 ) or for other reasons such as storing water and improving the hydrophobicity of the leaf surface. The development of these lens-shaped cells in understory tropical species may be primarily related to chance opportunities to exposure to direct light when sun flecks penetrate to the ground level of the forest. In addition, plants with these types of cells typically have an extremely hydrophobic surface, and convexly shaped cells increase water repellency (Wagner et al., 2003 ; Bhushan and Jung, 2006 ). Lens cells are often found on both the abaxial and adaxial sides of the leaf, and freeing either surface of a film of water may be critical for reducing the presence of fungal and bacterial pathogens, as well as for promoting gas exchange. Knowing how diffuse light affects photosynthesis will ultimately help determine the importance of the percentage of diffuse or direct light a plant receives. Technical limitations have kept such measurements from being performed, but this is a promising area for future research.

FOOTNOTES

1 The authors thank N. Poirier for technical assistance and NSF DBI 0454933 for financial support. Back

2 Author for correspondence (cbroders{at}uvm.edu ) Back

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