Stomatal closure of Pelargonium x hortorum in response to soil water deficit is associated with decreased leaf water potential only under rapid soil drying

Soil water deficits applied at different rates and for different durations can decrease both stomatal conductance (g(s)) and leaf water potential (Psi(leaf)). Understanding the physiological mechanisms regulating these responses is important in sustainable irrigation scheduling. Glasshouse-grown, containerized Pelargonium x hortorum BullsEye plants were irrigated either daily at various fractions of plant evapotranspiration (100, 75 and 50% ET) for 20 days or irrigation was withheld for 4 days. Xylem sap was collected and g(s) and Psi(leaf) were measured on days 15 and 20, and on days 16-19 for the respective treatments. Xylem sap pH and NO3- and Ca2+ concentrations did not differ between irrigation treatments. Xylem abscisic acid (ABA) concentrations ([ABA](xyl)) increased within 24 h of irrigation being withheld whilst gs and Psi(leaf) decreased. Supplying irrigation at a fraction of daily ET produced a similar relationship between [ABA](xyl) and g(s), but did not change Psi(leaf). Treatment differences occurred independently of whether Psi(leaf) was measured in whole leaves with a pressure chamber, or in the lamina with a thermocouple psychrometer. Plants that were irrigated daily showed lower [ABA](xyl) than plants from which irrigation was withheld, even at comparable soil moisture content. This implies that regular re-watering attenuates ABA signaling due to maintenance of soil moisture in the upper soil levels. Crucially, detached leaves supplied with synthetic ABA showed a similar relationship between [ABA](xyl) and gs as intact plants, suggesting that stomatal closure of P. hortorum in response to soil water deficit is primarily an ABA-induced response, independent of changes in Psi(leaf).