Changes in the salinity tolerance of sweet pepper plants as affected by nitrogen form and high CO2 concentration

The assimilation and availability of nitrogen in its different forms can significantly affect the response of primary productivity under the current atmospheric alteration and soil degradation. An elevated CO2 concentration (e[CO2]) triggers changes in the efficiency and efficacy of photosynthetic processes, water use and product yield, the plant response to stress being altered with respect to ambient CO2 conditions (a[CO2]). Additionally, NH4+ has been related to improved plant responses to stress, considering both energy efficiency in N-assimilation and the overcoming of the inhibition of photorespiration at e[CO2]. Therefore, the aim of this work was to determine the response of sweet pepper plants (Capsicum annuum L.) receiving an additional supply of NH4+ (90/10 NO3−/NH4+) to salinity stress (60 mM NaCl) under a[CO2] (400 μmol mol−1) or e[CO2] (800 μmol mol−1). Salt-stressed plants grown at e[CO2] showed DW accumulation similar to that of the non-stressed plants at a[CO2]. The supply of NH4+ reduced growth at e[CO2] when salinity was imposed. Moreover, NH4+ differentially affected the stomatal conductance and water use efficiency and the leaf Cl−, K+, and Na+ concentrations, but the extent of the effects was influenced by the [CO2]. An antioxidant-related response was prompted by salinity, the total phenolics and proline concentrations being reduced by NH4+ at e[CO2]. Our results show that the effect of NH4+ on plant salinity tolerance should be globally re-evaluated as e[CO2] can significantly alter the response, when compared with previous studies at a[CO2].