Elevated CO2 alleviates negative effects of salinity on broccoli (Brassica oleracea L. var Italica) plants by modulating water balance through aquaporins abundance

•Elevated CO2 improved water relations in broccoli plants under salt stress.•A tighyly control of aquaporins PIPs in the roots and leaves was observed.•A reduction of transpiration rate combined to the increased hydraulic conductance maintained plant water balance.•Improved water balance under enhanced CO2 contributed to an increased plant biomass under salinity.

In the global change scenario, increased CO2 may favour water use efficiency (WUE) by plants. By contrast, in arid and semiarid areas, salinity may reduce water uptake from soils. However, an elevated WUE does not ensure a reduced water uptake and upon salinity this fact may constitute an advantage for plant tolerance. In this work, we aimed to determine the combined effects of enhanced [CO2] and salinity on the plant water status, in relation to the regulation of PIP aquaporins, in the root and leaf tissues of broccoli plants (Brassica oleracea L. var Italica), under these two environmental factors. Thus, different salinity concentrations (0, 60 and 90 mM NaCl) were applied under ambient (380 ppm) and elevated (800 ppm) [CO2]. Under non-salinised conditions, stomatal conductance (Gs) and transpiration rate (E) decreased with rising [CO2] whereas water potential (Ψω) was maintained stable, which caused a reduction in the root hydraulic conductance (L0). In addition, PIP1 and PIP2 abundance in the roots was decreased compared to ambient [CO2]. Under salinity, the greater stomatal closure observed at elevated [CO2] – compared to that at ambient [CO2] – caused a greater reduction in Gs and E and allowed plants to maintain their water balance. In addition, a lower decrease in L0 under salt stress was observed at elevated [CO2], when comparing with the decrease at ambient [CO2]. Modifications in PIP1 and PIP2 abundance or their functionality in the roots is discussed. In fact, an improved water status of the broccoli plants treated with 90 mM NaCl and elevated [CO2], evidenced by a higher Ψω, was observed together with higher photosynthetic rate and water use efficiency. These factors conferred on the salinised broccoli plants greater leaf area and biomass at elevated [CO2], in comparison with ambient [CO2]. We can conclude that, under elevated [CO2] and salt stress, the water flow is influenced by the tight control of the aquaporins in the roots and leaves of broccoli plants and that increased PIP1 and PIP2 abundance in these organs provides a mechanism of tolerance that maintains the plant water status.