ABA signaling and stomatal control in tomato plants exposure to progressive soil drying under ambient and elevated atmospheric CO2 concentration

The objective of this study was to investigate the relative significance of hydraulic and chemical signals in controlling stomatal conductance (gs) of drought-stressed tomato plants grown under ambient (a[CO2], 400 ppm) and elevated atmospheric CO2 concentration (e[CO2], 800 ppm). The fraction of transpirable soil water (FTSW) was lower and decreased much faster in plants grown at e[CO2] than those grown at a[CO2]. During soil drying, midday leaf water potential (Ψl), osmotic potential (Ψπ), and turgor pressure (Ψp) were kept constant until FTSW reached ca. 0.2; while the plants grown under e[CO2] had significantly lower Ψπ but significantly higher Ψp than those grown under a[CO2]. The An was significantly greater in plants grown under e[CO2] than those grown under a[CO2] during soil drying. The gs was significant higher for plants grown under a[CO2] at onset of drought but decreased linearly with decreasing FTSW, whereas it was kept almost unchanged from onset of drought until FTSW reached ca. 0.2 in plants grown under e[CO2]. The intrinsic water use efficiency (WUEi) increased linearly with decreasing FTSW to ca. 0.2 in plants grown under a[CO2], while a decline of WUEi was observed for plants grown under e[CO2]. The xylem sap ABA concentration ([ABA]xylem) increased exponentially with decreasing FTSW, and the increase was more pronounced in plants grown under e[CO2]. When FTSW > 0.3, gs decreased linearly with increasing [ABA]xylem for plants grown under a[CO2]; while for plants grown under e[CO2], gs decreased linearly with decreasing Ψp, indicating that at moderate drought stress the gs of tomato leaves was mainly regulated by the xylem-borne ABA signaling when grown at a[CO2]; whereas under e[CO2] the gs was insensitive to [ABA]xylem and controlled predominantly by Ψp. The results provide some novel insights into the mechanisms of plant response to drought stress in future CO2 enriched environment.