HormonesAfter extraction of the xylem sap, it was immediately injectedinto a solid-phase extraction octadecyl carbon chain C-18 column(Waters Corporation, USA), blow-dried with nitrogen and stored at−80◦C for future measurements of hormone concentrations. Freshleaf samples (1–3 g) were frozen in liquid nitrogen for 30 min andstored at −80◦C for the same purpose. Each frozen leaf samplewas cut into several pieces, and approximately 0.7 g was weighedand mixed with 80% methanol containing 1 mmol/L di-tert-butyl-4-methylphenol. The mixture was ground into a homogenate ina water bath and extracted at 4◦C for 4 h. The samples were cen-trifuged at 7000 rpm for 15 min and precipitated. Subsequently, thesupernatant was collected. The precipitate was then extracted with80% methanol for 1 h, and the supernatant was collected again. Thesupernatants were pooled, injected into a solid-phase extraction C-18 column and blow-dried with nitrogen. The residues of leaf andxylem sap extract samples were dissolved in 0.01 mol/L phosphatebuffer solution (pH 7.4). The concentrations of IAA, GA3, ABA andZR were determined using enzyme- linked immunosorbent assay(ELISA), according to previously described methods