Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant Biology

Abstract

The light environment of plants is extremely complex and questions relating to how direct, diffuse, or low-angle light affect plants at the leaf-level have remained largely unanswered. Global-change scenarios suggest a trend of increasing diffuse light due to expected increases in cloud cover and atmospheric water vapor concentrations. Here we present three different examples where changes in the directional quality of light affect leaf-level processes. First, some understory plants have well-developed lens-shaped epidermal cells, which have been shown to focus collimated light, but their optical function under diffuse light has been largely speculative. 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 and lens-shaped epidermal cells. Regardless of epidermal cell shape, direct light was absorbed more than diffuse light in all species studied by approximately 2–3%. These data suggest that lensshaped epidermal cells do not aid the capture of diffuse light, and 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. Second, community-level productivity has been shown to increase under diffuse light conditions and has been attributed to more uniform distribution of light within the forest canopy. Leaf-level responses to the directional quality of light, however, are unknown. Here we show that leaf-level photosynthesis in sun leaves of both C3 and C4 plants can be 10–15% higher under direct light compared to equivalent absorbed irradiances of diffuse light, while shade-adapted leaves showed no preference for direct or diffuse light at any irradiance. Sun leaves with multiple palisade layers may be adapted to better utilize direct than diffuse light, while shade leaf structure does not appear to discriminate light based on its directionality. Thus, it appears that leaf-level and canopy-level photosynthetic processes react differently to the directionality of light, and previously observed increases in canopy-level photosynthesis occur even though leaf-level photosynthesis decreases under diffuse light. Third, we tested how changes in the directional quality of light affect the penetration of light at the leaf-level. Using chlorophyll fluorescence imaging we were able to determine that low-angle and diffuse light do not penetrate as deeply into leaves as direct light. Upon entering the leaf, diffuse light appears to scatter and remain in the upper tissue layers, while direct light penetrates through more leaf tissue. Absorption of diffuse light is reduced compared to direct light, with the greatest differences in absorption occurring near the interface of the palisade and spongy mesophyll tissue. Changes in the directional quality of light can therefore alter the absorption of light at the leaf-level, and a shift in the absorption profile could potentially decrease light utilization, potentially contributing to the leaf-level photosynthetic differences observed. Overall, it is now clear that plants are much more sensitive to the directional quality of light than we once believed. Also, the directional quality of light has different effects when scaling from the leaf to the landscape, and models of both leaf-level and community-level photosynthesis should be revised to account for these new findings.

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