James A. Weber

Changes in photosynthetic capacity, carboxylation efficiency, and CO2 compensation point associated with midday stomatal closure and midday depression of net CO2 exchange of leaves of Quercus suber.

The carbon-dioxide response of photosynthesis of leaves of Quercus suber, a sclerophyllous species of the European Mediterranean region, was studied as a function of time of day at the end of the summer dry season in the natural habitat. To examine the response experimentally, a “standard” time course for temperature and humidity, which resembled natural conditions, was imposed on the leaves, and the CO2 pressure external to the leaves on subsequent days was varied. The particular temperature and humidity conditions chosen were those which elicited a strong stomatal closure at midday and the simultaneous depression of net CO2 uptake. Midday depression of CO2 uptake is the result of i) a decrease in CO2-saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO2 compensation point caused by an increase in leaf temperature and a decrease in humidity. As a consequence of the changes in photosynthesis, the internal leaf CO2 pressure remained essentially constant despite stomatal closure. The effects on capacity, slope, and compensation point were reversed by lowering the temperature and increasing the humidity in the afternoon. Constant internal CO2 may aid in minimizing photoinhibition during stomatal closure at midday. The results are discussed in terms of possible temperature, humidity, and hormonal effects on photosynthesis.

Interactive effects of light, leaf temperature, CO2 and O2 on photosynthesis in soybean

A biochemical model of C3photosynthesis has been developed by G.D. Farquhar et al. (1980, Planta 149, 78–90) based on Michaelis-Menten kinetics of ribulose-1,5-bisphosphate (RuBP) carboxylase-oxygenase, with a potential RuBP limitation imposed via the Calvin cycle and rates of electron transport. The model presented here is slightly modified so that parameters may be estimated from whole-leaf gas-exchange measurements. Carbon-dioxide response curves of net photosynthesis obtained using soybean plants (Glycine max (L.) Merr.) at four partial pressures of oxygen and five leaf temperatures are presented, and a method for estimating the kinetic parameters of RuBP carboxylase-oxygenase, as manifested in vivo, is discussed. The kinetic parameters so obtained compare well with kinetic parameters obtained in vitro, and the model fits to the measured data give r2values ranging from 0.87 to 0.98. In addition, equations developed by J.D. Tenhunen et al. (1976, Oecologia 26, 89–100, 101–109) to describe the light and temperature responses of measured CO2-saturated photosynthetic rates are applied to data collected on soybean. Combining these equations with those describing the kinetics of RuBP carboxylase-oxygenase allows one to model successfully the interactive effects of incident irradiance, leaf temperature, CO2 and O2 on whole-leaf photosynthesis. This analytical model may become a useful tool for plant ecologists interested in comparing photosynthetic responses of different C3 plants or of a single species grown in contrasting environments.

Analysis of gas exchange in seedlings of Acer saccharum: integration of field and laboratory studies

AbstractIn the field, photosynthesis of Acer saccharum seedlings was rarely light saturated, even though light saturation occurs at about 100 μmol quanta m-2 s-1 photosynthetic photon flux density (PPFD). PPFD during more than 75% of the daylight period was 50 μmol m-2 s-1 or less. At these low PPFDs there is a marked interaction of PPFD with the initial slope (CE) of the CO2 response. At PPFD-saturation CE was 0.018 μmol m-2 s-1/(μl/l). The apparent quantum efficiency (incident PPFD) at saturating CO2 was 0.05–0.08 mol/mol.

Effects of temperature at constant air dew point on leaf carboxylation efficiency and CO2 compensation point of different leaf types.

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Development of a photosynthesis model with an emphasis on ecological applications: II. Analysis of a data set describing theP M surface

and PPFD-saturated CO2 exchange was 6–8 μmol m-2 s-1. The ratio of internal CO2 concentration to external (Ci/Ca) was 0.7 to 0.8 except during sunflecks when it decreased to 0.5. The decrease in Ci/Ca during sunflecks was the result of the slow response of stomates to increased PPFD compared to the response of net photosynthesis. An empirical model, which included the above parameters was used to simulate the measured CO2 exchange rate for portions of two days. Parameter values for the model were determined in experiments separate from the daily time courses being sumulated. Analysis of the field data, partly through the use of simulations, indicate that the elimination of sunflecks would reduce net carbon gain by 5–10%.

5 – Environmental and Hormonal Control of Dormancy in Terminal Buds of Plants

The response of CO2 exchange rate (CER) to temperature and light was determined for 14 dominant plant species of a northern deciduous hardwood forest in northern lower Michigan. Leaves at the top of the canopy had temperature optima near 25 C for CER, whereas leaves in the understory had optima near 20 C. There was no change in optimum temperature over the growing season, and overall shapes of response curves were similar among species. The lack of change in temperature optima may be a result of little change in growing conditions rather than a lack of ability to acclimatize. Nine of 11 species in the understory had no significant differences in light-saturated, maximum CERs, whereas at the top of the canopy Populus grandidentata had a higher maximum CER than Quercus rubra and Betula papyrifera. The species in the understory also differed little in light-saturation points for CER. Species at the top of the canopy had higher values for maximum CER, light-saturation point for CER, and maximum conductance than did species in the understory.

Environmental and hormonal control of turion formation in Myriophyllum verticillatum

The effect of temperature on photosynthesis at constant water-vapor pressure in the air was investigated using two sclerophyll species, Arbutus unedo and Quercus suber, and one mesophytic species, Spinacia oleracea. Photosynthesis and transpiration were measured over a range of temperatures, 20–39° C. The external concentration of CO2 was varied from 340 μbar to near CO2 compensation. The initial slope (carboxylation efficiency, CE) of the photosynthetic response to intercellular CO2 concentration, the CO2 compensation point (Γ), and the extrapolated rate of CO2 released into CO2-free air (Ri) were calculated. At an external CO2 concentration of 320–340 μbar CO2, photosynthesis decreased with temperature in all species. The effect of temperature on Γ was similar in all species. While CE in S. oleracea changed little with temperature, CE decreased by 50% in Q. suber as temperature increased from 25 to 34° C. Arbutus unedo also exhibited a decrease in CE at higher temperatures but not as marked as Q. suber. The absolut value of Ri increased with temperature in S. oleracea, while changing little or decreasing in the sclerophylls. Variations in Γ and Ri of the sclerophyll species are not consistent with greater increase of respiration with temperature in the light in these species compared with S. oleracea.

Short-Term Effects of CO2 on Gas Exchange of Leaves of Bigtooth Aspen (Populus grandidentata) in the Field

Photosynthetic photon flux density (PPFD) and CO2 concentration are critical determinants of net CO2 exchange rate (CER) in leaves. The interaction of these environmental variables through their effects on metabolic processes controlling photosynthesis is not completely understood, especially at the leaf level. There are at least two possible methods by which light intensity could affect the photosynthetic response of a leaf to changing CO2 concentration: 1) changes in ribulose-1,5-isphosphate (RuP2) concentration (Collatz, 1978) and 2) changes in the activation of the enzyme, RuP2 carboxylase (Perchorowicz et al., 1981). The interaction of PPFD with CO2 concentration would be most evident when neither CO2 nor PPFD are saturating. The aim of the research reported here was to investigate the effect of PPFD on the initial slope of the photosynthetic response.