357
nitens. The more rounded juvenile leaves of this species have larger air spaces than
the adult leaves, suggesting they may be adapted to more rapid growth but have less
tolerance to drought (Gras et al. 2005). This type of adult leaf anatomy increases
light interception at lower solar zenith angles closer to sunrise and sunset and
enhances potential for photosynthesis.
The asymmetric leaf structure of typical dicots enhances capture of energy for
photosynthesis by orienting the leaf upright, facilitated by branching angle and petiole orientation. It makes a difference whether light enters the leaf from the upper or
lower surfaces because the anatomical structures and biochemistry across the leaf
are different. The effect of the asymmetric distribution on reflectance and absorptance patterns is shown in Fig. 14.4 for photosynthetic light at two solar zenith
angles entering the leaf from the upper or lower side of the leaf.
The downward flux constitutes most of the net flux when the solar zenith angle
is higher (left side); when the solar zenith angle is lower (right side), the upward flux
is much larger. The panel (14.4. #1 and 14.4. #2) with the typical dorsiventral orientation has higher net flux through the epidermis than panels (14.4. #3 and 14.4. #4)
with the abaxial side receiving the incident flux. About 20% of the net flux enters
the palisade parenchyma in 14.4. #1 and 14.4. #2, while 14.4.#3 and 14.4. #4 show
very low net flux. Because the chlorophyll concentration is highest in the palisade
parenchyma, comparatively little photosynthesis occurs in the spongy mesophyll,
with higher net flux of 675 nm light (suitable for chlorophyll a absorption in photosystem II).
Panel 14.4. #1, with the typical dorsiventral orientation (adaxial side up) at the
higher solar zenith angle, has higher net flux downward (about 20%) through the
epidermis than Panel 14.4. #2 with the lower solar zenith angle, where the net flux
is much lower in both palisade and spongy parenchyma and the upward flux is much
larger. Panel 14.4. #3, with the abaxial side up, shows little difference in upward
flux, but most of the net flux continues to be located in the palisade parenchyma.
Panel 14.4. #4, with the abaxial side receiving the light at the lower zenith angle, has
virtually no net flux into the palisade parenchyma and high upward flux in the abaxial epidermis. Because the chlorophyll concentration is highest in the palisade
parenchyma, there is little photosynthesis elsewhere, even with high net flux of
675 nm light (red wavelength region, suitable for chlorophyll a absorption).
14.7 The Epidermis
The epidermis is the outermost layer of leaf cells and is generally one cell layer
thick, but some species have several cell layers. The epidermis lacks pigments and
is generally transparent to light. The outer cuticle surface is covered by wax to limit
uptake and loss of gases, except at the stomatal complexes, which are generally
located on the abaxial (lower) side of the leaf. Stomata are composed of two kidneyshaped stomatal cells and two to four guard cells at the ends. The stomata are generally located above an open space in the mesophyll where gases can collect, termed
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…
nitens. The more rounded juvenile leaves of this species have larger air spaces than
the adult leaves, suggesting they may be adapted to more rapid growth but have less
tolerance to drought (Gras et al. 2005). This type of adult leaf anatomy increases
light interception at lower solar zenith angles closer to sunrise and sunset and
enhances potential for photosynthesis.
The asymmetric leaf structure of typical dicots enhances capture of energy for
photosynthesis by orienting the leaf upright, facilitated by branching angle and petiole orientation. It makes a difference whether light enters the leaf from the upper or
lower surfaces because the anatomical structures and biochemistry across the leaf
are different. The effect of the asymmetric distribution on reflectance and absorptance patterns is shown in Fig. 14.4 for photosynthetic light at two solar zenith
angles entering the leaf from the upper or lower side of the leaf.
The downward flux constitutes most of the net flux when the solar zenith angle
is higher (left side); when the solar zenith angle is lower (right side), the upward flux
is much larger. The panel (14.4. #1 and 14.4. #2) with the typical dorsiventral orientation has higher net flux through the epidermis than panels (14.4. #3 and 14.4. #4)
with the abaxial side receiving the incident flux. About 20% of the net flux enters
the palisade parenchyma in 14.4. #1 and 14.4. #2, while 14.4.#3 and 14.4. #4 show
very low net flux. Because the chlorophyll concentration is highest in the palisade
parenchyma, comparatively little photosynthesis occurs in the spongy mesophyll,
with higher net flux of 675 nm light (suitable for chlorophyll a absorption in photosystem II).
Panel 14.4. #1, with the typical dorsiventral orientation (adaxial side up) at the
higher solar zenith angle, has higher net flux downward (about 20%) through the
epidermis than Panel 14.4. #2 with the lower solar zenith angle, where the net flux
is much lower in both palisade and spongy parenchyma and the upward flux is much
larger. Panel 14.4. #3, with the abaxial side up, shows little difference in upward
flux, but most of the net flux continues to be located in the palisade parenchyma.
Panel 14.4. #4, with the abaxial side receiving the light at the lower zenith angle, has
virtually no net flux into the palisade parenchyma and high upward flux in the abaxial epidermis. Because the chlorophyll concentration is highest in the palisade
parenchyma, there is little photosynthesis elsewhere, even with high net flux of
675 nm light (red wavelength region, suitable for chlorophyll a absorption).
14.7 The Epidermis
The epidermis is the outermost layer of leaf cells and is generally one cell layer
thick, but some species have several cell layers. The epidermis lacks pigments and
is generally transparent to light. The outer cuticle surface is covered by wax to limit
uptake and loss of gases, except at the stomatal complexes, which are generally
located on the abaxial (lower) side of the leaf. Stomata are composed of two kidneyshaped stomatal cells and two to four guard cells at the ends. The stomata are generally located above an open space in the mesophyll where gases can collect, termed
14 How the Optical Properties of Leaves Modify the Absorption and Scattering…