Holly L. Gorton

Ultraviolet Radiation and the Snow Alga Chlamydomonas nivalis (Bauer) Wille

Aplanospores of Chlamydomonas nivalis are frequently found in high‐altitude, persistent snowfields where they are photosynthetically active despite cold temperatures and high levels of visible and ultraviolet (UV) radiation. The goals of this work were to characterize the UV environment of the cells in the snow and to investigate the existence and localization of screening compounds that might prevent UV damage. UV irradiance decreased precipitously in snow, with UV radiation of wavelengths 280–315 nm and UV radiation of wavelengths 315–400 nm dropping to 50% of incident levels in the top 1 and 2 cm, respectively. Isolated cell walls exhibited UV absorbance, possibly by sporopollenin, but this absorbance was weak in images of broken or plasmolyzed cells observed through a UV microscope. The cells also contained UV‐absorbing cytoplasmic compounds, with the extrachloroplastic carotenoid astaxanthin providing most of the screening. Additional screening compound(s) soluble in aqueous methanol with an absorption maximum at 335 nm played a minor role. Thus, cells are protected against potentially high levels of UV radiation by the snow itself when they live several centimeters beneath the surface, and they rely on cellular screening compounds, chiefly astaxanthin, when located near the surface where UV fluxes are high.

The light environment and cellular optics of the snow alga Chlamydomonas nivalis (Bauer) Wille.

The alga Chlamydomonas nivalis lives in a high‐light, cold environment: persistent alpine snowfields. Since the algae in snow receive light from all angles, the photon fluence rate is the critical parameter for photosynthesis, but it is rarely measured. We measured photon irradiance and photon fluence rate in the snow that contained blooms of C. nivalis. On a cloudless day the photon fluence rate at the snow surface was nearly twice the photon irradiance, and it can be many times greater than the photon irradiance when the solar angle is low or the light is diffuse. Beneath the surface the photon fluence rate can be five times the photon irradiance. Photon irradiance and photon fluence rate declined exponentially with depth, approximating the Bouguer–Lambert relationship. We used an integrating sphere to measure the spectral characteristics of a monolayer of cells and microscopic techniques to examine the spectral characteristics of individual cells. Astaxanthin blocked blue light and unknown absorbers blocked UV radiation; the penetration of these wavelengths through whole cells was negligible. We extracted astaxanthin, measured absorbance on a per‐cell basis and estimated that the layer of astaxanthin within cells would allow only a small percentage of the blue light to reach the chloroplast, potentially protecting the chloroplast from excessive light.

Measurement of the Optical Properties of Leaves Under Diffuse Light

Measuring leaf light absorptance is central to many areas of plant biology including photosynthesis and energy balance. Absorptance is calculated from measured values of transmittance and reflectance, and most such measurements have used direct beam light. However, photosynthesis and other processes can differ under direct and diffuse light. Optical properties under diffuse light may be different, but there have been technical difficulties involved in measuring total reflectance of diffuse light. We developed instrumentation to measure this reflectance using a chopped measuring beam delivered alternately to sample and reference integrating spheres, and lock‐in detection. We also built instrumentation for measuring transmittance of diffuse light. We developed standards to calibrate our instruments and correct for substitution error, a known systematic error with integrating sphere‐based measurements. Helianthus annuus leaves measured under diffuse light reflected 5–10% more and transmitted a few percent less 400–700 nm light than under direct light. Overall absorptance was only a few percent higher under direct light, but leaves may utilize absorbed direct and diffuse light differently. For example, of the light entering the leaf, significantly more direct light than diffuse light is transmitted through the leaf, suggesting differences in localization of absorption within the leaf.

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.