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.