Depth of Light Penetration into Leaf Tissues

Light attenuation in pecan leaves at four wavelengths including UV-B, UV-A, blue light, and red light into leaf tissues is illustrated in Fig. 18.5. The depths of the light penetration into the leaf tissues are presented in Fig. 18.6. Pecan leaf epidermis strongly attenuated 310 nm UV-B, 98% of which was absorbed within the first 10 ^m of the 15 ^m-thick upper epidermal tissue (Figs. 18.5 and 18.6). High UV-B attenuation by the epidermal layer means low epidermal transmittance to UV-B.

The Penetration Depth Light

Figure 18.4 Leaf reflectance, transmittance, and absorbance to 300 nm UV-B radiation on a whole leaf basis in the selected broadleaf tree species. The measurements were made on the mature leaves collected in August in Baton Rouge, LA. The species were ranked based on the reflectance from the highest to the lowest

Figure 18.4 Leaf reflectance, transmittance, and absorbance to 300 nm UV-B radiation on a whole leaf basis in the selected broadleaf tree species. The measurements were made on the mature leaves collected in August in Baton Rouge, LA. The species were ranked based on the reflectance from the highest to the lowest

Since the epidermis of the pecan leaves allowed very little UV-B (< 2%) transmittance into the mesophyll tissues, the photosynthetic apparatus were essentially protected from the UV-B damage. Thus, such an effective epidermal function of the UV-B screening characterizes the UV-B tolerance mechanism in pecan. In addition to the UV-B attenuation, pecan leaf epidermis attenuated 96% of the UV-A (360 nm), 83% of the blue light (430 nm), and 58% of the red light (680 nm). However, the blue and the red light penetrated much deeper into the mesophyll tissues. The 430 nm light was mainly attenuated within the first 100 ^m thickness of the leaf, including the upper epidermis and the palisade mesophyll tissue. The 680 nm penetrated even deeper into the sponge mesophyll and was mainly attenuated within the first 160 |im thickness of the leaf. Overall, mesophyll tissues alone attenuated 17% of the blue light and 42% of the red light that were available for photosynthesis (Figs. 18.5 and 18.6).

Comparisons among the species are presented in Fig. 18.7, which contains 31 species, excluding dogwood, red mulberry, pin oak, and post oak, for which the data were not available. The 31 species were ranked from the lowest to the highest based on their epidermal UV-B transmittance. The differences in the depth of the UV-B penetration, epidermal transmittance to UV-B, and upper epidermal thickness are clearly considerable amongst the species (Fig. 18.7). The first 23 species in Fig. 18.7, including Arizona ash, chestnut oak, mocker nut hickory, pecan, American sycamore, bitternut hickory, green ash, sawtooth oak, American elm, blue Japanese oak, cherrybark oak, cottonwood, southern live oak, southern magnolia, shumard oak,

360nm Wavelength

Figure 18.5 Light penetration into leaf tissues from the adaxial surfaces in pecan by four different wavelengths, the error bar represents + 1SD. The measurements were made on 15 mature leaves sampled in August

100 120 140 Leaf thickness (Jim)

Figure 18.5 Light penetration into leaf tissues from the adaxial surfaces in pecan by four different wavelengths, the error bar represents + 1SD. The measurements were made on 15 mature leaves sampled in August

UVB UVA Blue Red

310nm 360nm 430nm 6 8 On m

UVB UVA Blue Red

310nm 360nm 430nm 6 8 On m

The Penetration Depth Light

Figure 18.6 Visual illustration of the light penetration into leaf tissues in pecan. (a) Shows a pecan leaf cross-section. UE = upper epidermis, PM = palisade mesophyll, SM = sponge mesophyll, LE = lower epidermis, VS = vascular system. The downward arrows show the relative positions of depths of light penetration at different wavelengths; (b) shows the upper leaf surface and trichome; and (c) shows the lower leaf surface and trichome

Figure 18.6 Visual illustration of the light penetration into leaf tissues in pecan. (a) Shows a pecan leaf cross-section. UE = upper epidermis, PM = palisade mesophyll, SM = sponge mesophyll, LE = lower epidermis, VS = vascular system. The downward arrows show the relative positions of depths of light penetration at different wavelengths; (b) shows the upper leaf surface and trichome; and (c) shows the lower leaf surface and trichome sweetgum, American beech, white oak, Chinese tallow, water oak, yellow poplar, Bradford pear, and red maple, had relatively low epidermal transmittance, ranging from 1% - 8%. This means the leaf epidermises of these species were capable of attenuating 92% - 99% of the UV-B absorbed. Such a strong epidermal function of the UV-B screening underlies the fundamental UV-B protection mechanism in these species. This finding also concurs with the research by Caldwell et al.

(1983) and Day (1993), asserting that the epidermal attenuation appears to be the dominant UV-B screening mechanism in the majority of plants. On the other hand, the last five species in Fig. 18.7, including river birch, American chestnut, red bud, Chinese elm, and sugarberry, exhibited high epidermal transmittance (ranging from 27% - 49%). These species may be less UV-B tolerant. They may also experience up to 50% UV-B penetration into their mesophyll tissues, which could result in damage to their photosynthetic apparatus, unless there are additional UV-B absorbing compounds present in their mesophyll tissues. In addition, the three species in Fig. 18.7, including sweet bay magnolia, red oak, and willow oak, are intermediate regarding their epidermal transmittance (13% -17%), which falls between the low values (under 8%) of the first 23 species and the high values (27% - 49%) shown by the last 5 species.

Light Penetration Depth

Figure 18.7 Comparisons of upper epidermal transmittance to 310 nm UV-B radiation, depth of 310 nm penetration into leaves, and epidermal thickness among the selected broadleaf tree species. Note: The species were ranked based on the epidermal transmittance from the lowest to the highest. In all cases, mature leaves were used and light was illuminated toward the upper leaf surfaces in the light penetration study

Figure 18.7 Comparisons of upper epidermal transmittance to 310 nm UV-B radiation, depth of 310 nm penetration into leaves, and epidermal thickness among the selected broadleaf tree species. Note: The species were ranked based on the epidermal transmittance from the lowest to the highest. In all cases, mature leaves were used and light was illuminated toward the upper leaf surfaces in the light penetration study

Was this article helpful?

0 0

Post a comment