Decomposer biomass in the rhizosphere to assess rhizodeposition

Quantification of the organic carbon released from plant roots is a challenge. These compounds of rhizodeposition are quickly transformed into CO₂ and eventually bacterial biomass to be consumed by bacterivores (protozoa and nematodes). Microbes stimulate rhizodeposition several-fold so assays under sterile conditions give an unrealistic value. Quantifying bacterial production from ³H-thymidine incorporation falls short in the rhizosphere and the use of isotopes does not allow clear distinction between labeled CO₂ released from roots or microbes. We reduced rhizodeposition in 3–5 week old barley with a 2 week leaf aphid attack and found that biomass of bacterivores but not bacteria in the rhizosphere correlated with plant–induced respiration activity belowground. This indicated top-down control of the bacteria. Moreover, at increasing density of aphids, bacterivore biomass in the rhizosphere decreased to the level in soil unaffected by roots. This suggests that difference in bacterivore biomass directly reflects variations in rhizodeposition. Rhizodeposition is estimated from plant-induced increases in bacterial and bacterivore biomass, and yield factors, maintenance requirements, and turnover rates from the literature. We use literature values that maximize requirements for organic carbon and still estimate the total organic rhizodeposition to be as little as 4–6% of the plant-induced respiration belowground.     

Heterogeneous mechanics of the mouse pulmonary arterial network

Individualized modeling and simulation of blood flow mechanics find applications in both animal research and patient care. Individual animal or patient models for blood vessel mechanics are based on combining measured vascular geometry with a fluid structure model coupling formulations describing dynamics of the fluid and mechanics of the wall. For example, one-dimensional fluid flow modeling requires a constitutive law relating vessel cross-sectional deformation to pressure in the lumen. To investigate means of identifying appropriate constitutive relationships, an automated segmentation algorithm was applied to micro-computerized tomography images from a mouse lung obtained at four different static pressures to identify the static pressure–radius relationship for four generations of vessels in the pulmonary arterial network. A shape-fitting function was parameterized for each vessel in the network to characterize the nonlinear and heterogeneous nature of vessel distensibility in the pulmonary arteries. These data on morphometric and mechanical properties were used to simulate pressure and flow velocity propagation in the network using one-dimensional representations of fluid and vessel wall mechanics. Moreover, wave intensity analysis was used to study effects of wall mechanics on generation and propagation of pressure wave reflections. Simulations were conducted to investigate the role of linear versus nonlinear formulations of wall elasticity and homogeneous versus heterogeneous treatments of vessel wall properties. Accounting for heterogeneity, by parameterizing the pressure/distention equation of state individually for each vessel segment, was found to have little effect on the predicted pressure profiles and wave propagation compared to a homogeneous parameterization based on average behavior. However, substantially different results were obtained using a linear elastic thin-shell model than were obtained using a nonlinear model that has a more physiologically realistic pressure versus radius relationship.     

Le cancer à cellules germinales du testicule, élément constitutif du Syndrome de Dysgénésie Testiculaire: rôle des facteurs environnementaux et de la susceptibilité génétique

The incidence of testicular germ cell cancer has been increasing over recent decades in many countries of the world. Many studies over recent years have reported adverse trends in other aspects of male reproductive health, such as high and possibly increasing frequencies of undescended testis and hypospadias, declining semen quality, and an apparently growing demand for assisted reproduction due to male infertility. This article summarises the available evidence supporting a new concept that these male reproductive abnormalities may be signs of a single underlying entity: testicular dysgenesis syndrome (TDS). This syndrome, caused by nonspecific delays and aberrations of early testicular development, may be increasingly common because of deteriorating environmental and life-style factors that impair gonadal development. Geographical and ethnic differences in the incidence of various forms of TDS could be explained either by differences in exposure to adverse factors or by differences in genetic susceptibility to these factors.