Leaf: Light Capture in the Photosynthetic Organ
Terrestrial photosynthesis fixes about half the carbon on the planet and most of that photosynthesis occurs in chloroplasts in the leaves of higher plants. Leaves protect the chloroplasts, distribute them for light interception, and provide enough surface area interfacing with the atmosphere to facilitate maximum carbon dioxide uptake. There is a balancing act between light levels and carbon dioxide supply: insufficient quantities of either one limit the amount of photosynthesis that a leaf can conduct. When leaves develop under low light they are usually thin because light harvesting limits the amount of carbon that can be fixed and photosynthesis is concentrated within a few cell layers. When leaves develop under high light they are usually thick and light absorption is distributed over many cell layers, greatly increasing the amount of carbon that can be fixed per unit leaf area. In nature, light is rarely constant and leaves are often exposed to too little or too much light. This chapter describes adaptations at the level of the leaf that control the amount of light that is absorbed by the photosynthetic tissues. Some of these adaptations are anatomical and the epidermis is the first optical boundary that can play a key role in controlling entry of light into leaves. The anatomy of the photosynthetic tissues and the directional quality of the ambient light also interact to determine the light absorption profile within the tissues, which sets an energetic boundary on the amount of photosynthesis that can be conducted within the individual cell layers. Other adaptations provide for screening of excess light by pigments and fine-tuning of light absorption through chloroplast movement. Additional adaptations occur at the biochemical and whole-plant level to balance light absorption with carbon fixation and this chapter concentrates on the intermediate level: the leaf.
KeywordsEpidermal Cell Specular Reflectance Diffuse Light Light Gradient Chloroplast Movement
– – Far red;
– – Angle of incidence;
– – Refractive index;
– – Photosynthetically active region of the spectrum;
– – Ultraviolet
We thank William E. Williams for his insightful comments and suggestions on the manuscript.
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