Interacting effects of soil fertility and atmospheric CO 2 on leaf area growth and carbon gain physiology in Populus × euramericana (Dode) Guinier

Two important processes which may limit productivity gains in forest ecosystems with rising atmospheric CO 2 are reduction in photosynthetic capacity following prolonged exposure to high CO 2 and diminution of positive growth responses when soil nutrients, particularly N, are limiting. To examine the interacting effects of soil fertility and CO 2 enrichment on photosynthesis and growth in trees we grew hybrid poplar ( Populus × euramericana ) for 158 d in the field at ambient and twice ambient CO 2 and in soil with low or high N availability. We measured the timing and rate of canopy development, the seasonal dynamics of leaf level photosynthetic capacity, respiration, and N and carbohydrate concentration, and final above- and belowground dry weight. Single leaf net CO 2 assimilation (A) increased at elevated CO 2 over the majority of the growing season in both fertility treatments. At high fertility, the maximum size of individual leaves, total leaf number, and seasonal leaf area duration (LAD) also increased at elevated CO 2 , leading to a 49% increase in total dry weight. In contrast, at low fertility leaf area growth was unaffected by CO 2 treatment. Total dry weight nonetheless increased 25% due to CO 2 effects on A. Photosynthetic capacity (A at constant internal p(CO 2 ), (( C 1 )) was reduced in high CO 2 plants after 100 d growth at low fertility and 135 d growth at high fertility. Analysis of A responses to changing C 1 indicated that this negative adjustment of photosynthesis was due to a reduction in the maximum rate of CO 2 fixation by Rubisco. Maximum rate of electron transport and phosphate regeneration capacity were either unaffected or declined at elevated CO 2 . Carbon dioxide effects on leaf respiration were most pronounced at high fertility, with increased respiration mid-season and no change (area basis) or reduced (mass basis) respiration late-season in elevated compared to ambient CO 2 plants. This temporal variation correlated with changes in leaf N concentration and leaf mass per area. Our results demonstrate the importance of considering both structural and physiological pathways of net C gain in predicting tree responses to rising CO 2 under conditions of suboptimal soil fertility.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65655/1/j.1469-8137.1995.tb04295.x.pd

Derivation of lowland riparian wetland deposit architecture using geophysical image analysis and interface detection

For groundwater-surface water interactions to be understood in complex wetland settings, the architecture of the underlying deposits requires investigation at a spatial resolution sufficient to characterize significant hydraulic pathways. Discrete intrusive sampling using conventional approaches provides insufficient sample density and can be difficult to deploy on soft ground. Here a noninvasive geophysical imaging approach combining three-dimensional electrical resistivity tomography (ERT) and the novel application of gradient and isosurface-based edge detectors is considered as a means of illuminating wetland deposit architecture. The performance of three edge detectors were compared and evaluated against ground truth data, using a lowland riparian wetland demonstration site. Isosurface-based methods correlated well with intrusive data and were useful for defining the geometries of key geological interfaces (i.e., peat/gravels and gravels/Chalk). The use of gradient detectors approach was unsuccessful, indicating that the assumption that the steepest resistivity gradient coincides with the associated geological interface can be incorrect. These findings are relevant to the application of this approach in settings with a broadly layered geology with strata of contrasting resistivities. In addition, ERT revealed substantial structures in the gravels related to the depositional environment (i.e., braided fluvial system) and a complex distribution of low-permeability putty Chalk at the bedrock surface—with implications for preferential flow and variable exchange between river and groundwater systems. These results demonstrate that a combined approach using ERT and edge detectors can provide valuable information to support targeted monitoring and inform hydrological modeling of wetlands

Response of Trifolium repens Clones to Infection by Meloidogyne incognita and Peanut Stunt Virus

The responses of selected clones of white clover (Trifolium repens) to simultaneous infection by the southern root-knot nematode (Meloidogyne incognita) and peanut stunt virus (PSV) were determined. Two white clover clones, which were resistant (NC-R) or sensitive (NC-S) to ozone injury, were evaluated. Plant growth and M. incognita reproduction were measured. Root, stolon, and top growth were reduced by PSV infection, which affected NC-R more than NC-S. Both clones were tolerant of M. incognita, but NC-R had less root galling and less nematode reproduction than NC-S, and thus was less susceptible to M. incognita. Reductions in root growth of plants infected with both M. incognita and PSV were greater than in plants infected by either pathogen alone. Nematode reproduction tended to be lower on PSV-infected plants

The Ribosome - Three-Dimensional Structure and Ligand-Binding Studies

To date, cryo-electron microscopy has become the most successful technique for exploring the structure of the ribosome and for studying binding positions of its various ligands, with the resolution slowly extending toward 10 Å. Obstacles in the attempts to improve resolution are the limited stability and coherence of the electron microscope, the statistics of data collection, and the conformational heterogeneity of the specimen. The last factor in this list proved to be the reason why it has been difficult to go past 18-20 Å with many specimens despite the use of state-of-the-art electron microscopes and inclusion of tens of thousand of projections. A breakthrough has been achieved with a protein synthesis initiation-like complex in which mRNA and fMet-tRNA is bound to the E. coli ribosome. The high occupancy and extraordinary conformational homogeneity of this specimen has enabled us to reach a resolution of 15 Å