Thomas C. Vogelmann

Leaf form and photosynthesis

M orphological and anatomical features of plant leaves are commonly associated with metabolic type (e.g., Kranz anatomy of C4 species), amount of sun exposure (e.g., sun and shade leaves), or water stress (e.g., xeromorphism). However, although the primary function of the leaf is to absorb and process sunlight and carbon dioxide for photosynthesis, few structural features of leaves have been related mechanistically to these tasks. For example, it has been known for over a century that the internal anatomy of leaves is characterized by different cell layers (e.g., the palisade and spongy mesophyll) and that stomatal pores can be located on one or both sides of a leaf. Yet, only recently has any functional relationship between leaf form and photosynthetic performance been suggested. A variety of ecological studies have correlated numerous leaf structural parameters with photosynthetic performance (e.g., Abrams and Kubiske 1990, 1994, Hinckley et al. 1989,

Are some plant life forms more effective than others in screening out ultraviolet-B radiation?

SummaryThe unprecedented rate of depletion of the stratospheric ozone layer will likely lead to appreciable increases in the amount of ultraviolet-B radiation (UV-B, 280–320 nm) reaching the earths surface. In plants, photosynthetic reactions and nucleic acids in the mesophyll of leaves are deleteriously affected by UV-B. We used a fiber-optic microprobe to make direct measurements of the amount of UV-B reaching these potential targets in the mesophyll of intact foliage. A comparison of foliage from a diverse group of Rocky Mountain plants enabled us to assess whether the foliage of some plant life forms appeared more effective at screening UV-B radiation. The leaf epidermis of herbaceous dicots was particularly ineffective at attenuating UV-B; epidermal transmittance ranged from 18–41% and UV-B reached 40–145 μm into the mesophyll or photosynthetic tissue. In contrast to herbaceous dicots, the epidermis of 1-year old conifer needles attenuated essentially all incident UV-B and virtually none of this radiation reached the mesophyll. Although the epidermal layer was appreciably thinner in older needles (7 y) at high elevations (Krumholtz), essentially all incident UV-B was attenuated by the epidermis in these needles. The same epidermal screening effectiveness was observed after removal of epicuticular waxes with chloroform. Leaves of woody dicots and grasses appeared intermediate between herbaceous dicots and conifers in their UV-B screening abilities with 3–12% of the incident UV-B reaching the mesophyll. These large differences in UV-B screening effectiveness suggest that certain plant life forms may be more predisposed than others to meet the challenge of higher UV-B levels resulting from stratospheric ozone depletion.

Carbon Fixation Gradients across Spinach Leaves Do Not Follow Internal Light Gradients.

In situ measurements of 14C-CO2 incorporation into 40-[mu]m paradermal leaf sections of sun- and shade-grown spinach leaves were determined. Chlorophyll, carotenoid, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) content in similar 40-[mu]m paradermal leaf sections was also measured. The carbon fixation gradient did not follow the leaf internal light gradient, which decreases exponentially across the leaf. Instead, the 14C-CO2 fixation was higher in the middle of the leaf. Contrary to expectations, the distribution of carbon fixation across the leaf showed that the spongy mesophyll contributes significantly to the total carbon reduced. Approximately 60% of the carboxylation occurred in the palisade mesophyll and 40% occurred in the spongy mesophyll. Carbon reduction correlated well with Rubisco content, and no correlation between chlorophyll and carotenoid content and Rubisco was observed in sun plants. The correlation among chlorophyll, carotenoids, Rubisco, and carbon fixation was higher in shade leaves than in sun leaves. The results are discussed in relation to leaf photosynthetic and biochemical measurements that generally consider the leaf as a single homogeneous unit.

PENETRATION OF LIGHT INTO PLANTS

Plant growth, development, and photosynthetic performance is tightly coupled to the light environment, but surprisingly little attention has been paid to how physiological processes are related to the light microenvironment present within plant cells and tissues. As light enters and moves through a plant it is attenuated both by absorption and scattering, which creates a light gradient. Depending upon the optical properties of the tissues, these gradients can be relatively steep or gradual, exponential or linear in shape. Multiple internal reflections trap light so that the space irradiance, or the internal fluence rate, near the irradiated surface of plant tissues can exceed by several-fold the amount of light incident upon the surface. Although this may seem counterintuitive, light trapping has been described in mathematical terms (Seyfried and Fukshansky, 1983; Kaufmann and Hartmann, 1988) and is a major feature of light gradients in plants. A previous lack of attention to the relationship between light gradients and physiological processes probably stems from the fact that light gradients and other characteristics of the light microenvironment within plants are difficult to calculate and measure. However, within the last 5 years, new experimental methods and novel approaches have been introduced that now make it possible to characterize the light regime within plants. Although several experimental and theoretical milestones have been passed, much remains to be done. The optical properties of plants and features of the internal light microenvironment have been discussed previously (Fukshansky, 1981; Osborne and Raven, 1986; Vogelmann, 1986). This review will summarize some of the developments that have occurred in the field largely over the last 5 years.

Light within the plant

A detailed knowledge of the optical properties of plants is necessary to understand how plants detect their light environment and how they may perceive light direction, light quantity, and spectral quality. However, a detailed description of what happens to light after it enters plant tissues is complicated by a number of optical phenomena such as lens effects, light scattering, and the sieve effect. Light scattering, in combination with internal reflection, creates a light trap so that fluence rates within plants can exceed by 3–4 times that of incident light. Internal fluence rates change with increasing depth within the tissues as the light is attenuated by absorption and scattering. This creates a light gradient which is necessary for the perception of light direction in phototropism. Other mechanisms for perception of light may depend upon other optical effects.

Ontogenetic differences in mesophyll structure and chlorophyll distribution in Eucalyptus globulus ssp. globulus (Myrtaceae)

Mesophyll structure has been associated with the photosynthetic performance of leaves via the regulation of internal light and CO(2) profiles. Differences in mesophyll structure and chlorophyll distribution within three ontogenetically different leaf types of Eucalyptus globulus ssp. globulus were investigated. Juvenile leaves are blue-grey in color, dorsiventral (adaxial palisade layer only), hypostomatous, and approximately horizontal in orientation. In contrast, adult leaves are dark green in color, isobilateral (adaxial and abaxial palisade), amphistomatous, and nearly vertical in orientation. The transitional leaf type has structural features that appear intermediate between the juvenile and adult leaves. The ratio of mesophyll cell surface area per unit leaf surface area (A(mes)/A) of juvenile leaves was maximum at the base of a single, adaxial palisade layer and declined through the spongy mesophyll. Chlorophyll a + b content showed a coincident pattern, while the chlorophyll a:b ratio declined linearly from the adaxial to abaxial epidermis. In comparison, the mesophyll of adult leaves had a bimodal distribution of A(mes)/A, with maxima occurring beneath both the adaxial and abaxial surfaces within the first layer of multiple palisade layers. The distribution of chlorophyll a + b content had a similar pattern, although the maximum ratio of chlorophyll a:b occurred immediately beneath the adaxial and abaxial epidermis. The matching distributions of A(mes)/A and chlorophyll provide further evidence that mesophyll structure may act to influence photosynthetic performance. These changes in internal leaf structure at different life stages of E. globulus may be an adaptation for increased xeromorphy under increasing light exposure experienced from the seedling to adult tree, similar to the characteristics reported for different species according to sunlight exposure and water availability within their native habitats.

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.

The blue light gradient in unilaterally irradiated maize coleoptiles: measurement with a fiber optic probe

Abstract— The blue light (450 nm) gradient was measured with a fiber optic microsensor in etiolated maize coleoptiles that were irradiated unilaterally. Patterns of transmitted and scattered light across the shoot were related to the morphology of the coleoptile and varied greatly between the coleoptile base, mid‐region and tip. In the coleoptile base, light was scattered equatorially around the coleoptile sheath so that there was more light on the shaded side of the sheath than in the shaded side of the primary leaves. In the hollow mid‐region there were strong reflecting boundaries at the air‐coleoptile interfaces, which resulted in step like transitions in the light gradient. In the coleoptile tip, there was a steep, near‐linear gradient of blue light, with the greatest amount of light on the irradiated side. There was no evidence that the coleoptile tip acts as a lens or that there are anomalies that result in more light on the shaded than irradiated side. Immediately beneath the irradiated surface, the space irradiance was about twice the fluence rate of the light beam. The magnitude of the light gradient was 4:1 between the irradiated and shaded side of the mid‐region and tip and 8:1 for the coleoptile base.

New applications of photoacoustics to the study of photosynthesis

Photoacoustic methods offer unique capabilities for photosynthesis research. Phenomena that are readily observed by photoacoustics include the storage of energy by electron transport, oxygen evolution by leaf tissue at microsecond time resolution, and the conformational changes of photosystems caused by charge separation. Despite these capabilities, photoacoustic methods have not been widely exploited in photosynthesis research. One factor that has contributed to their slow adoption is uncertainty in the interpretation of photoacoustic signals. Careful experimentation is resolving this uncertainty, however, and technical refinements of photoacoustic methods continue to be made. This review provides an overview of the application of photoacoustics to the study of photosynthesis with an emphasis on the resolution of uncertainties in the interpretation of photoacoustic signals. Recent developments in photoacoustic technology are also presented, including a microphotoacoustic spectrometer, gas permeable photoacoustic cells, the use of photoacoustics to monitor phytoplankton populations, and the use of photoacoustics to study protein dynamics.