Dye-sensitized photodegradation of the fungicide carbendazim and related benzimidazoles

The present work studies the visible-light-promoted photodegradation of the colorless fungicide carbendazim (methyl 2-benzimidazolecarbamate) and several 2-substituted benzimidazoles (SBZ's), in water or water-methanol solution, in the presence of air and, as a photosensitizer, the synthetic xanthene dye Rose Bengal (RB) or the natural pigment riboflavin (Rf). The results indicate that the degradation of each particular SBZ depends on its chemical structure and on the sensitizer employed. In the presence of RB, the degradation always operates via a singlet molecular oxygen (O(2)((1)Delta(g)))-mediated mechanism, through a highly efficient process, as deduced from the comparison of the rate constants for physical and chemical quenching of O(2)((1)Delta(g)). In the presence of Rf, the visible-light irradiation of any of the studied SBZ's produces a series of competitive processes that depend on the relative concentrations of Rf and SBZ. These processes include the quenching of excited singlet and triplet Rf states by the SBZ and the generation of both O(2)((1)Delta(g)) and superoxide radical anion (O(2)(-)), the latter generated by electron transfer from excited Rf species to the dissolved oxygen. The overall result is the photodegradation of the SBZ and the photoprotection of the sensitizer.     

Photodegradation of the herbicide Norflurazon sensitised by Riboflavin. A kinetic and mechanistic study

The kinetics and mechanism of the Riboflavin (Rf)-promoted photochemical degradation with visible light of the herbicide Norflurazon (NF) has been studied by time-resolved and stationary techniques. Using light of wavelength higher than 400 nm--a region where NF is totally transparent--and with concentrations of Rf and NF of ca. 0.02 and 1 mM, respectively, only the excited triplet state of Rf ((3)Rf*) is quenched by NF, in competition with dissolved ground state triplet oxygen, O(2)((3)Sigma(g)(-)). NF degradation mainly occurs by reaction with superoxide radical anion O(2)(-) formed through two electron transfer steps: from NF to (3)Rf*, yielding Rf radical anion, and from this anion to O(2)((3)Sigma(g)(-)), regenerating ground state Rf. Although singlet molecular oxygen is also produced, NF only quenches this oxidative species in a physical mode. The global result is the photoprotection of the sensitiser and the photodegradation of NF.     

Model of stomatal ammonia compensation point (STAMP) in relation to the plant nitrogen and carbon metabolisms and environmental conditions

Agricultural crops can be either a source or a sink of ammonia (NH₃). Most NH₃ exchange models developed so far do not account for the plants nitrogen (N) metabolism and use prescribed compensation points. We present here a leaf-scale simplified NH₃ stomatal compensation point model related to the plants N and carbon (C) metabolisms, for C₃ plants. Five compartments are considered: xylem, cytoplasm, apoplasm, vacuole and sub-stomatal cavity. The main processes accounted for are the transport of ammonium (NH₄⁺), NH₃ and nitrate (NO₃⁻) between the different compartments, NH₄⁺ production through photorespiration and NO₃⁻ reduction, NH₄⁺ assimilation, chemical and thermodynamic equilibriums in all the compartments, and stomatal transfer of NH₃.The simulated compensation point is sensitive to paramaters related to the apoplastic compartment: pH, volume and active transport rate. Determining factors are leaf temperature, stomatal conductance and NH₄⁺ flux to the leaf. Atmospheric NH₃ concentration seem to have very little effect on the compensation point in conditions of high N fertilization. Comparison of model outputs to experimental results show that the model underestimates the NH₃ compensation point for high N fertilization and that a better parametrisation of sensitive parameters especially active trasport rate of NH₄⁺ may be required.     

Ammonia stomatal compensation point of young oilseed rape leaves during dark/light cycles under various nitrogen nutritions

The plant can be a source or a sink of ammonia (NH₃) depending on its nitrogen (N) supply, metabolism and on the background atmospheric concentrations. Thus plants play a major role in regulating atmospheric NH₃ concentrations. For a better understanding of the factors influencing the NH₃ stomatal compensation point, it is important to analyse the dynamics of leaf NH₃ fluxes. The relationship between the leaf NH₃ fluxes and the leaf apoplast ammonium and nitrate concentrations, N nutrition and the light and dark periods was studied here.We designed an experiment to quantitatively assess leaf-atmosphere NH₃ exchange and the stomatal compensation point and to identify the main factors affecting the variation of NH₃ fluxes in oilseed rape. We tested day and night dynamics as well as the effect of five different N treatments. Two experimental methods were used: a dynamic open flux chamber and extraction of the apoplastic solution.Chamber measurements showed that there was a good correlation between plant NH₃ fluxes and water fluxes. Compensation points were calculated by two different methods and ranged between 0.8 and 12.2 μg m⁻³ NH₃ (at 20 °C) for the different N treatments. Apoplastic solution measurements showed that there was no significant differences in the apoplastic NH₄⁺ concentrations ([NH₄⁺]_apo) extracted in dark and light periods for the same N treatment. Statistical analysis also showed that [NH₄⁺]_apo was correlated with [NH₄⁺] in the nutrient solution and weakly correlated with [NO₃⁻]. Apoplast NH₄⁺ concentrations ranged between 0.1 and 2.1 mM, bulk tissue NH₄⁺ concentrations between 3.9 and 6.6 mM and xylem concentrations between 2.4 and 6.1 mM depending on the N supply.Calculated NH₃ emission potential from the extraction measurements were over-estimated when compared with the value calculated from chamber measurements. Errors related to chamber measurements included separation of the cuticular and stomatal fluxes and the calculation of total resistance to NH₃ exchange. Errors related to the extraction measurements included assessing the amount of cytoplasmic contamination. We do not have another method to assess the NH₃ stomatal compensation point and the choice between these two measurement techniques should depend on the scales to which the measurements apply and the processes to be studied.     

Relationship between ammonia stomatal compensation point and nitrogen metabolism in arable crops: current status of knowledge and potential modelling approaches

The ammonia stomatal compensation point of plants is determined by leaf temperature, ammonium concentration ([NH4+]apo) and pH of the apoplastic solution. The later two depend on the adjacent cells metabolism and on leaf inputs and outputs through the xylem and phloem. Until now only empirical models have been designed to model the ammonia stomatal compensation point, except the model of Riedo et al. (2002. Coupling soil-plant-atmosphere exchange of ammonia with ecosystem functioning in grasslands. Ecological Modelling 158, 83-110), which represents the exchanges between the plant's nitrogen pools. The first step to model the ammonia stomatal compensation point is to adequately model [NH4+]apo. This [NH4+]apo has been studied experimentally, but there are currently no process-based quantitative models describing its relation to plant metabolism and environmental conditions. This study summarizes the processes involved in determining the ammonia stomatal compensation point at the leaf scale and qualitatively evaluates the ability of existing whole plant N and C models to include a model for [NH4+]apo.