Root-shoot communication of field-grown maize drought-stressed at different rates as modified by atmospheric conditions

Maize is often grown in drought-prone environments and, thus, drought resistance is an important trait. In order to minimize production losses, plants need to respond and adapt early and fast to moisture loss in the root zone. From experiments under controlled conditions, constituents of the xylem sap, such as the plant hormone abscisic acid (ABA), or xylem pH have long been recognized to act as signals in root-shoot communication. To investigate early signals of field-grown maize under conditions of progressive drought, a field trial was set up in a field lysimeter for two consecutive years. Although the experimental set-up was very similar in the two years, plant responses to moisture loss were significantly different in both, the cascade of events and the intensity of responses. The main difference between the two years was in atmospheric vapor-pressure deficit (VPD), accelerating the drying rate of the soil in the second year. In contrast to observations during the first year, the sudden increase in VPD in the second year caused a strong, transient peak in xylem sap ABA concentration, but no change in xylem pH or leaf ABA concentration was observed. Whereas the water relations of the maize plants remained stable in the first year, they were severely unbalanced in the second. It is argued that the strong xylem-ABA signal triggered a change from adaptation mechanisms to survival mechanisms. Modulations due to VPD of constituents of the signal cascade induced by drought are discussed with regard to possible resistance strategies, their initiation, and their modification by combining primary environmental signals.