Responses of woody plant root and xylem sap ATP to soil drying

The ATP in roots and xylem sap of two woody plant species, Platycladus orientalis and Acacia auriculiformis, subjected to rapid and slow soil drying has been determined employing firefly luciferase ATP assay method (sensitivity is at 10^-12 mol ATP L^-1). The ATP levels in the two species were 1.6 nmol·g^-1_DW and 0.6 nmol·g^-1_DW in roots, and 5.6 μmol·m^-3 and 8 μmol ·m^-3 in xylem sap, respectively. When plants of P. orientalis and A. auriculiformis were subjected to rapid soil drying, respectively, as soil water content (SWC) decreased from the normal level (0.25 g·g^-1_DW) to 0.02 and 0.06 g·g^-1_DW, separately, plant water potential (ψ) dropped to -4 and -3.2 MPa, differently, the ATP in roots decreased 99.7% and 42%, respectively. When the rapidly dried soil was watered for up to 6 d, SWC and ψ were found to recover to their normal levels, but ATP content in roots of P. orientalis and A. auriculiformis recovered by 10% and 23%, respectively. When plants of P. orientalis and A. auriculiformis were subjected to slow soil drying, respectively, as SWC was found to decrease from the normal level to 0.1 and 0.13 g·g^-1_DW, separately, ψ dropped to -2.5 and -2 MPa, differently, the ATP content in roots decreased 98% and 38%, respectively. When the slowly dried soil was watered for up to 8 d, ATP levels in roots of P. orientalis and A. auriculiformis recovered by 30% and 24%, respectively. However, ATP concentrations in xylem sap were not directly influenced by either SWC or ψ. The plot dots of xylem sap ATP concentration versus root ATP content were found to be distributed diagonally. It is concluded that ATP in roots is more sensitive in response to SWC changes than that to ψ and root-sourced ATP is a source of that in xylem sap. When plants are subjected to slow soil drying, the ATP levels in both root and xylem sap are higher than that to rapid soil drying.