Study of Resistance to Orobanche ramosa in Host (Oilseed Rape and Carrot) and Non-host (Maize) Plants

Orobanche ramosa is a parasitic Angiosperm responsible for severe yield losses in several economical crops. It is a serious threat in oilseed rape in France and Morocco and is appearing in carrot crops in Morocco. In this study, several varieties of oilseed rape and carrot were screened in order to identify resistant cultivars and to characterize the resistance mechanisms involved. All the 15 oilseed rape varieties tested were susceptible. In carrot, the varieties 'Colmar à coeur rouge' and 'Nantaise demi-longue' were susceptible, whereas 'Palaiseau' and 'Buror' were resistant. In the susceptible 'Colmar à coeur rouge' carrot no defence reactions were found and the development of the parasite inhibited carrot tap root formation. In the resistant carrot varieties, the parasite germinated, became attached to the host root but became necrotic before emergence. In 'Buror' carrot, formation of a mechanical barrier was associated with the restriction to the cortex of the parasite. In maize cv. 'Vigni', a non-host of O. ramosa, thickening of xylem vessels, cell divisions in the central cylinder and formation of an encapsulation layer were observed in association with restricted development of Orobanche haustoria.     

Lateritic and redoximorphic features in a faulted landscape near Manaus, Brazil

The soils and sediments of the uplands in the Manaus region are described and analysed along a representative cross‐section. There are two broad types of features, lateritic and redoximorphic. Their formation is linked to two main processes acting under contrasted hydrological regimes. The first process, acting under well‐drained conditions, is lateritization. It has transformed strongly weathered sediment into soil and led to depletion of silica (mainly quartz) as well as to relative accumulation of both kaolinite and iron oxides (haematite and goethite). Crystallographic changes observed in the latter have resulted from alternating dissolution and crystallization cycles without significant transfer of iron and alumina. However, in the uppermost soil, dissolution of kaolinite has prevailed over crystallization, leading to depletion of clay and the formation of tiny crystals of gibbsite disseminated throughout the groundmass. The second process results from the development of reducing conditions in groundwater giving redoximorphic features in lateritic soils and sediments. In the sediments, iron has been depleted by regional aquifers to form a pallid zone. In the soil, large amounts of iron and minor amounts of alumina, mainly from aluminous goethite, have been mobilized at first in small patches, which with further mobilization and vertical transfer of these elements have increased in size and have led to the formation of bleached horizons over thin iron pans. Iron has crystallized predominantly as haematite in the iron pans and alumina as large crystals of gibbsite in soil voids. Formation of impervious iron pans holds up fluctuating perched groundwater in the overlying horizons depending on rainfall events. Neotectonic events (formation of uplifted blocks and small grabens) have markedly altered the hydrological regimes. In the uplifted blocks, the soil has been deeply truncated and iron loss has been checked in the uppermost sediment. By contrast, mobilization of iron has been initiated at various places in the soil of the small grabens. In this way tectonic events have checked mobilization of iron in sediments but activated it in soils, leaving spectacular fingerprints on the landscape.     

Soil and water degradation processes in burned areas: Lessons learned from a nested approach

Forest fires produce a major impact on soil, water and vegetation. Despite the amount of research published on this subject, there are two major problems that hamper the fully understanding of on and off-site impacts of forest fires. They include methodological problems steaming from the uniqueness of burned soil properties, easily erodible, by the fast degradation they undergo over a short period of time immediately after fire and by the meaning of the impacts at different scales.     

Hydrogeochemistry of a Small Catchment in Northeastern Amazonia: a Comparison Between Natural with Deforested Parts of the Catchment (Serra do Navio,Amapá State,Brazil)

To undertake a comprehensive study of the disturbanceeffects due to mining activities (manganese oreexploitation) on the chemical composition of drainagewaters, a 164 ha catchment in North-eastern Amazoniatropical forest was sampled weekly for a year period(June 1993 to June 1994) to determine the contrasts inrainwater, throughfall and stream water chemistrybetween the upper undisturbed area and the lower partaffected by deforestation and mining works. During the3 dry months season only about 15% of the totalannual input-output of chemical species to and fromthe catchment occurs. In the three wettest months,about 30% of the flux occurs, except 60% of thepotassium and nitrate release from the catchmentoccurs in these three months. The rain waters areslightly acidic (pH ≈ 5.2) with lowalkalinity, while the stream water is nearly neutral(pH ≈ 6.9): alkalinity is generated within thesoil and soil water system. The rainfall-throughfallrelationship of the chemical species in the naturalareas exhibits strong internal recycling and littleexport from the catchment. In the area changed bydeforestation and mining works almost all chemicalspecies show a net export in surface waters: these aremainly derived from the deforested area of the basin,where most probably due to enhanced weatheringprocesses because the vegetation cover has tore-establish itself.     

The use of the scintillation technique for monitoring seasonal water consumption of olive orchards in a semi-arid region

To monitor seasonal water consumption of agricultural fields at large scale, spatially averaged surface fluxes of sensible heat (H) and latent heat (L_vE) are required. The scintillation method is shown to be a promising device for obtaining the area-averaged sensible heat fluxes, on a scale of up to 10 km. These fluxes, when combined with a simple available energy model, can be used to derive area-averaged latent heat fluxes. For this purpose, a Large Aperture Scintillometer (LAS) was operated continuously for more than one year over a tall and sparse irrigated oliveyard located in south-central Marrakesh (Morocco). Due to the flood irrigation method used in the site, which induces irregular pattern of soil moisture both in space and time, the comparison between scintillometer-based estimates of daily sensible heat flux (H_LAS) and those measured by the classical eddy covariance (EC) method (H_EC) showed a large scatter during the irrigation events, while a good correspondence was found during homogenous conditions (dry conditions and days following the rain events). We found, that combining a simple available energy model and the LAS measurements, the latent heat can be reliably predicted at large scale in spite of the large scatter (R² = 0.72 and RMSE = 18.25 W m⁻²) that is obtained when comparing the LAS against the EC. This scatter is explained by different factors: the difference in terms of the source areas of the LAS and EC, the closure failure of the energy balance of the EC, and the error in available energy estimates. Additionally, the irrigation efficiency was investigated by comparing measured seasonal evapotranspiration values to those recommended by the FAO. It was found that the visual observation of the physical conditions of the plant is not sufficient to efficiently manage the irrigation, a large quantity of water is lost (≈37% of total irrigation). Consequently, the LAS can be considered as a potentially useful tool to monitor the water consumption in complex conditions.