Changes in photosynthetic capacity, carboxylation efficiency, and CO 2 compensation point associated with midday stomatal closure and midday depression of net CO 2 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 CO 2 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 CO 2 uptake. Midday depression of CO 2 uptake is the result of i) a decrease in CO 2 -saturated photosynthetic capacity after light saturation is reached in the early morning, ii) a decrease in the initial slope of the CO 2 response curve (carboxylation efficiency), and iii) a substantial increase in the CO 2 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 CO 2 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 CO 2 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.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47468/1/425_2004_Article_BF00397440.pd

Disturbance creates stability. Experimental investigations on Mid-European inland sand dunes

Beyschlag W, Friedrich S, Weigelt A, Jentsch A. Disturbance creates stability. Experimental investigations on Mid-European inland sand dunes. In: Gao J, Veste M, Sun B, Beyschlag W, eds. Desertification and Rehabilitation in Arid and Semi-arid Regions. Proceedings International Symposium Beijing 2004. Beijing: Science Press; 2006: 56-64

Plant Competition for Light Analyzed with a Multispecies Canopy Model. III. Influence of Canopy Structure in Mixtures and Monocultures of Wheat and Wild Oat

A multispecies canopy photosynthesis simulation model was used to examine the importance of canopy structure in influencing light interception and carbon gain in mixed and pure stands of wheat (Triticum aestivum L.) and wild oat (Avena fatua L.), a common weedy competitor of wheat. In the mixtures, the fraction of the simulated canopy photosynthesis contributed by wheat was found to decline during the growing season and this decline was closely related to reductions in the amount of leaf area in upper canopy layers. For both species in mixture and in monoculture, simulated photosynthesis was greatest in the middle or upper-middle canopy layers and sensitivity analyses revealed that canopy photosynthesis was most sensitive to changes in leaf area and leaf inclination in these layers. Changes in LAI and leaf inclination affected canopy carbon gain differently for mixtures and monocultures, but the responses were not the same for the two species. Results from simulations where the structural characteristics of the two species were substituted indicated that species differences in leaf inclination, sheath area and the fraction of leaf area alive were of minor consequence compared with the differences in total leaf area in influencing relative canopy carbon gain in mixtures. Competition for light in these species mixtures appears to be influenced most by differences in the positioning of leaf area in upper canopy layers which determines, to a great extent, the amount of light intercepted

Plant Competition for Light Analyzed with a Multispecies Canopy Model. II. Influence of Photosynthetic Characteristics in Mixtures of Wheat and Wild Oat

The importance of photosynthetic characteristics such as quantum efficiency or carboxylation efficiency for carbon gain of plants competing for light in dense stands is dependent on several environmental factors and structural features of the canopy. A quantitative analysis of photosynthesis of competing plants in mixed stands of wheat and wild oat (Avena fatua L.), a common weed of wheat, involved measuring photosynthetic parameters of individual leaves at different heights in the canopy throughout the growing season. This information combined with detailed assessments of canopy structure was used with a multispecies canopy model to evaluate the importance of different photosynthetic characteristics for carbon gain in this canopy environment. Independent photosynthesis data sets were used to validate predictions of the model. Carboxylation efficiency (CE) and CO2-and light-saturated photosynthetic capacity (AML) were highly correlated and decreased with depth in the canopy for both species. Quantum efficiency (agr) did not tend to decrease with depth in the canopy. Sensitivity analyses with the model for whole-plant carbon gain of each species over entire day periods were conducted. These showed that changes in CE and AML had an influence similar to that of changes in agr on carbon gain for both species. This was not necessarily expected from single-leaf photosynthetic behavior in response to changes in CE, AML and agr. The influence of agr is more pronounced in the lower, more shaded portions of the canopy than are changes in CE and AML. Appreciable differences between the species were apparent for carbon gain under different weather conditions. The differences between the species in carbon gain when in competition for light were associated more with structural features rather than with photosynthetic characteristics

Plant Competition for Light Analyzed with a Multispecies Canopy Model. I. Model Development and Influence of Enhanced UV-B Conditions on Photosynthesis in Mixed Wheat and Wild Oat Canopies

Competition for light among species in a mixed canopy can be assessed quantitatively by a simulation model which evaluates the importance of different morphological and photosynthetic characteristics of each species. A model was developed that simulates how the foliage of all species attenuate radiation in the canopy and how much radiation is received by foliage of each species. The model can account for different kinds of foliage (leaf blades, stems, etc.) for each species. The photosynthesis and transpiration for sunlit and shaded foliage of each species is also computed for different layers in the canopy. The model is an extension of previously described single-species canopy photosynthesis simulation models. Model predictions of the fraction of foliage sunlit and interception of light by sunlit and shaded foliage for monoculture and mixed canopies of wheat (Triticum aestivum) and wild oat (Avena fatua) in the field compared very well with measured values. The model was used to calculate light interception and canopy photosynthesis for both species of wheat/wild oat mixtures grown under normal solar and enhanced ultraviolet-B (290–320 nm) radiation (UV-B) in a glasshouse experiment with no root competition. In these experiments, measurements showed that the mixtures receiving enhanced UV-B radiation had a greater proportion of the total foliage area composed of wheat compared to mixtures in the control treatments. The difference in species foliage area and its position in the canopy resulted in a calculated increase in the portion of total canopy radiation interception and photosynthesis by wheat. This, in turn, is consistent with greater canopy biomass of wheat reported in canopies irradiated with supplemental UV-B

Linear relationships between aboveground biomass and plant cover in low open herbaceous vegetation

Rottgermann M, Steinlein T, Beyschlag W, Dietz H. Linear relationships between aboveground biomass and plant cover in low open herbaceous vegetation. JOURNAL OF VEGETATION SCIENCE. 2000;11(1):145-148.Four herbaceous plant species of a sand dune area and several herb species of an open early-successional patch were investigated for the occurrence of a simple relationship between aboveground biomass and plant cover. Without exception linear regressions of aboveground biomass on plant cover were found with slope factors depending on the growth form of the species. These results suggest that (early) growth of herbaceous plants in low and (temporarily) open vegetation is not affected by possible constraints caused by a decreasing ratio of plant cover to aboveground biomass. The obtained linear relationships could be used for rapid non-destructive determination of aboveground biomass by image-analysis of cover data