Robert W. Pearcy

THE IMPORTANCE OF SUNFLECKS FOR FOREST UNDERSTORY PLANTS : PHOTOSYNTHETIC MACHINERY APPEARS ADAPTED TO BRIEF, UNPREDICTABLE PERIODS OF RADIATION

n many forests with closed canopies, only a small fraction (0.55%) of the solar radiation incident above the canopy reaches the understory. Understory plants of these forests experience a highly dynamic light environment, with brief, often unpredictable periods of direct solar irradiance (sunflecks) punctuating the dim, diffuse background irradiance. The contribution of sunflecks to the daily and seasonal light environment of forest understory plants has long been appreciated (Evans 1956, Lundegarth 1921), but only within the last ten years have detailed studies focused on the extent to which photosynthesis and growth of understory plants are influenced by sunfleck activity (Chazdon 1986, Chazdon and Pearcy 1986a,b, Pearcy 1983, 1987, Pearcy et al. 1985, Pfitsch and Pearcy 1989a,b). These studies have revealed that sunflecks are a vital resource for light-limited understory plants (Chazdon 1988, Pearcy 1988, 1990).

Photosynthetic Responses of Tropical Forest Plants to Contrasting Light Environments

Across the complex matrix of microsites that compose tropical forests, light availability varies more dramatically than any other single plant resource. On a sunny day, instantaneous measurements of photosynthetically active radiation range over 3 orders of magnitude, from less than 10 µmol m-2 s-1 in closed-canopy understory of mature forests to well over 1000 µmol m-2 s-1 in exposed microsites of gaps and large clearings, or at the top of the forest canopy (Chazdon & Fetcher, 1984b; Figure 1.1). Among the environmental factors that influence plant growth and survival in tropical forests, light availability is likely to be the resource most frequently limiting growth, survival, and reproduction (Chazdon, 1988; Fetcher, Oberbauer & Chazdon, 1994). Photosynthetic utilization of light is therefore a major component of the regeneration responses of forest species within the larger context of forest dynamics and succession.

Plant Physiological Ecology

Lecture Course Objective: A survey of physiological approaches to understanding plantenvironment interactions from the functional perspective. Lectures cover physiological adaptation; limiting factors; resources acquisition/allocation; photosynthesis, carbon, energy balance; water use and relations; nutrient relations; linking physiology, stable isotope applications ecophysiology; stress physiology; life history, physiology; evolution of physiological performance; physiology population, community, ecosystem levels. Laboratory Course Objectives: The laboratory is focused on instructing you on observational and experimental approaches and methods used in plant physiological ecology. Students are introduced to a wide range of techniques and will make measurements on different plant species growing in the field or greenhouse (weeks 1-7). A group research project is required (weeks 912).

Carbon gain by plants in natural environments

P hysiological ecologists have long been concerned with photosynthesis, which incorporates carbon to provide plants with all their energy and structural building blocks. This carbon gain is an important aspect of plant performance in natural environments. The acquisition and use of other resources, such as nitrogen and water, are tightly linked to photosynthetic performance. For example, energy from photosynthesis is employed for nitrogen acquisition and reduction, and photosynthetic capacity is strongly coupled to leaf nitrogen content (Chapin et al., p. 49, this issue). Photosynthesis is also closely related to water movement in a plant. To facilitate absorption of carbon dioxide for use in photosynthesis, a leaf must have wet cell surfaces. As a consequence, transpiration uses nearly all of the water taken up

Regulation of Photosynthetic Induction State by the Magnitude and Duration of Low Light Exposure

This study was undertaken to examine the dependence of the regulatory enzymes of photosynthetic induction on photon flux density (PFD) exposure in soybean (Glycine max L.). The induction state varies as a function of both the magnitude and duration of the PFD levels experienced prior to an increase in PFD. The photosynthetic induction state results from the combined activity of separate processes that each in turn depend on prior PFD environment in different ways. Direct measurement of enzyme activities coupled with determination of in situ metabolite pool sizes indicated that the fast-induction component was associated with the activation state of stromal fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) and showed rapid deactivation in the dark and at low PFD. The fast-induction component was activated at low PFD levels, around 70 [mu]mol photons m-2 s-1. Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 2.7.1.19) deactivated very slowly in the dark and required higher PFD for activation. Both enzymes saturated at lower PFD than did photosynthesis, around 400 [mu]mol photons m-2 s-1. Ribulose-5-phosphate kinase (EC 2.7.1.19) appeared never to be limiting to photosynthesis, and saturated at much lower PFD than either FBPase or Rubisco. Determination of photosynthetic metabolite pool sizes from leaves at different positions within a soybean canopy showed a limitation to carbon uptake at the stromal FBPase and possibly the sedoheptulose-1,7-bisphosphatase (EC 3.1.3.37) in shade leaves upon initial illumination at saturating PFD levels.

Photosynthetic adaptation to high temperatures: a field study in death valley, california.

The photosynthesis of Tidestromia oblongifolia (Amranthaceae) is remarkably well adapted to operate at the very high summer temperatures of the native habitat on the floor of Death Valley. The photosynthetic rate was very high and reached its daily maximum when the light intensity reached its noon maximum at the high leaf temperatures of 460� to 50�C which occurred at this time. At the intensity of noon sunlight the rate decreased markedly when the leaf temperature was experimentally reduced to below 44�C. The optimum rate occurred at 47�C. At this temperature the photosynthetic rate was essentially directly proportional to light intensity up to full sunlight.

The taxonomic distribution of C4 photosynthesis in Amaranthaceae sensu stricto.

C(4) photosynthesis evolved multiple times in the Amaranthaceae s.s., but the C(4) evolutionary lineages are unclear because the photosynthetic pathway is unknown for most species of the family. To clarify the distribution of C(4) photosynthesis in the Amaranthaceae, we determined carbon isotope ratios of 607 species and mapped these onto a phylogeny determined from matK/trnK sequences. Approximately 28% of the Amaranthaceae species use the C(4) pathway. C(4) species occur in 10 genera-Aerva, Amaranthus, Blutaparon, Alternanthera, Froelichia, Lithophila, Guilleminea, Gomphrena, Gossypianthus, and Tidestromia. Aerva, Alternanthera, and Gomphrena contain both C(3) and C(4) species. In Aerva, 25% of the sampled species are C(4). In Alternanthera, 19.5% are C(4), while 89% of the Gomphrena species are C(4). Integration of isotope and matK/trnK data indicated C(4) photosynthesis evolved five times in the Amaranthaceae, specifically in Aerva, Alternanthera, Amaranthus, Tidestromia, and a lineage containing Froelichia, Blutaparon, Guilleminea, Gomphrena pro parte, and Lithophila. Aerva and Gomphrena are both polyphyletic with C(3) and C(4) species belonging to distinct clades. Alternanthera appears to be monophyletic with C(4) photosynthesis originating in a terminal sublineage of procumbent herbs. Alpine C(4) species were also identified in Alternanthera, Amaranthus, and Gomphrena, including one species (Gomphrena meyeniana) from 4600 m a.s.l.

Plant Physiological Ecology TodayRecent advances are helping to determine the biochemistry and physiology behind plant performance under natural conditions

D uring the past few decades plant physiological ecology has expanded tremendously. This growth has come partly from substantial technological advances that now make it possible to quantify precisely, under natural conditions, the microenvironment of plants and plant tissues as well as their metabolic responses. In addition, accompanying theoretical developments have provided a conceptual framework for relating environmental factors to plant mass and energy exchanges. Such information has been incorporated into simulation and optimization models of both morphological characteristics (e.g., leaf color, size, angle) and physiological properties (e.g., photosynthesis, transpiration, stomatal conductance). Plant physiological ecology is thus becoming increasingly predictive and is providing management tools in a number of areas, including forestry and pollution control. It is also providing a new understanding of community function and evolutionary development. To summarize past progress and set priorities for future research in this field, the National Science Foundation sponsored a symposium at Asilomar, California, in December 1985.

Concurrent Measurements of Oxygen and Carbon Dioxide Exchange during Lightflecks in Maize (Zea mays L.)

Leaves of maize (Zea mays L.) were enclosed in a temperature-controlled cuvette under 35 Pa (350 [mu]bars) CO2 and 0.2 kPa (0.2%)O2 and exposed to short periods (1–30 s) of illumination (light-flecks). The rate and total amount of CO2 assimilated and O2 evolved were measured. The O2 evolution rate was taken as an indicator of the rate of photosynthetic noncyclic electron transport (NCET). In this C4 species, the response of electron transport during the lightflecks qualitatively mimicked that of C3 species previously tested, whereas the response of CO2 assimilation differed. Under short-duration lightflecks at high photon flux density (PFD), the mean rate of O2 evolution was greater than the steady-state rate of O2 evolution under the same PFD due to a burst of O2 evolution at the beginning of the lightfleck. This O2 burst was taken as indicating a high level of NCET involved in the buildup of assimilatory charge via ATP, NADPH, and reduced or phosphorylated metabolites. However, as lightfleck duration decreased, the amount of CO2 assimilated per unit time of the lightfleck (the mean rate of CO2 assimilation) decreased. There was also a burst of CO2 from the leaf at the beginning of low-PFD lightflecks that further reduced the assimilation during these lightflecks. The results are discussed in terms of the buildup of assimilatory charge through the synthesis of high-energy metabolites specific to C4 metabolism. It is speculated that the inefficiency of carbon uptake during brief light transients in the C4 species, relative to C3 species, is due to the futile synthesis of C4 cycle intermediates.