The Role of Malate, Oxalate, and Mustard Oil Glucosides in the Evolution of Zinc-Resistance in Herbage Plants

Malate, oxalate and mustard oils were analysed in zinc-resistant and zinc-sensitive ecotypes of Silene cucubalus Wib., Rumex acetosa L., Thlaspi alpestre L. and Agrostis tenuis Sibth. The effect of zinc on the activities of carbonic anhydrase and peroxidase in Siiene cucubalus was tested. Carbonic anhydrase of the zinc-resistant ecotypes was stimulated by addition in vivo of high amounts of zinc. The high activity of peroxidase in the non-zinc-resistant ecotypes after the addition of zinc documented a poisoning of the sensitive plants by zinc. The amount of oxalate differed greatly between ecotypes. There was, however, no direct correlation between zinc-resistance and oxalate. When applying zinc to the nutrient medium, the synthesis of oxalate was inhibited in zinc-sensitive, but stimulated in zinc-resistant ecotypes of Silene cucubalus and Rumex acetosa. In Thlaspi alpestre high concentrations of mustard oil glucosides were found. Zinc-resistant plants produced twice as much glucosides as sensitive ones. A possible role of mustard oils in zinc-resistance is discussed. In the content of malate there were great quantitative differences between zinc-resistant and zinc-sensitive plants. All zinc-resistant ecotypes of all the species contained much higher concentrations in their green organs than the sensitive ones. It is assumed that malate is a major factor in the evolution of zinc-resistance. Malate may act as a complexing agent for zinc within the plasma, whereas oxalate and mustard oils may function as “terminal acceptors” of large amounts of zinc. The role of malate may be extended by a special transport mechanism, by which zinc is eliminated from the plasma into the vacuole.     

Dissolved carbon leaching from soil is a crucial component of the net ecosystem carbon balance

Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH₄), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its δ¹³C signature. Leaching of biogenic DIC was 8.3±4.9 g m^?2 yr^?1 for forests, 24.1±7.2 g m^?2 yr^?1 for grasslands, and 14.6±4.8 g m^?2 yr^?1 for croplands. DOC leaching equalled 3.5±1.3 g m^?2 yr^?1 for forests, 5.3±2.0 g m^?2 yr^?1 for grasslands, and 4.1±1.3 g m^?2 yr^?1 for croplands. The average flux of total biogenic carbon across land use systems was 19.4±4.0 g C m^?2 yr^?1. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO₂ in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO₂. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5–98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24–105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO₂ in acidic forest soil solutions and large NEE. Leaching of CH₄ proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems.     

Microclimatic factors influencing refugium suitability for Rhodnius prolixus

Abstract.  Rehydration in the triatomine bug Rhodnius prolixus (Stål) is dependent on the blood meal, and water balance regulation is crucial for survival of starving bugs. In an experimental arena with zones at different climatic conditions, starved R. prolixus nymphs prefer a cooler and more humid zone, stopping there more often and for longer periods. This is probably a behavioural adaptation to limit water loss and reduce metabolic rate. In the Venezuelan State of Portuguesa, temperature and humidity were monitored in three kinds of potential refugia during the dry season: (i) in a palm roof; (ii) in a crack of the wall of a house; and (iii) in a palm tree crown. Fluctuations in temperature and saturation deficit are not very different inside and outside the palm roof except during a few hours of the day when the sun is at its zenith. In the crack of the wall, the diurnal temperature range is reduced to 6.5 °C compared with 12.4 °C outside, and the saturation deficit varies by only 7.6 hPa compared with 28.6 hPa outside. In the palm tree crown, the daily temperature range is only 4.2 °C compared with 13.8 °C outside, and the saturation deficit is permanently < 5 hPa. The microclimatic conditions in the palm tree crown would appear to be ideal for starving R. prolixus , but this kind of refugium generally harbours low densities of bugs, probably related to a combination of predation, pathogens and lower mean temperature within the crown. Such biotic and abiotic constraints play a lesser role in the less ideal palm roof and wall crack refugia where bugs can proliferate as long as hosts are readily available nearby.