Changes of leaf water potential and endogenous cytokinins in young apple trees treated with or without paclobutrazol under drought conditions

Leaf water potential and endogenous cytokinins in xylem sap of young apple trees treated with or without paclobutrazol (PBZ) under drought stress conditions were investigated. Three drought and rewatering cycles were used in the experiments and leaf water potential was measured every 2-4 days during the experimental period (39 days). Zeatin (Z) and zeatin riboside (ZR) from the xylem sap in the trunks were measured during the drought periods. The results showed that leaf water potential of the plants decreased significantly when drought was applied, while for the PBZ treatment the reduction of leaf water potential was significantly less in all the three drought cycles, particularly in the first and second cycles. The concentrations of Z and ZR in xylem sap of the drought treated plants decreased significantly in the first drought cycle. In the second cycle, Z continued to decrease but ZR did not change significantly by day 4 but increased at day 8 for both treatments compared with the control. In the third cycle, there was no difference in Z concentration between the drought treatment alone and the control, but Z increased significantly for the PBZ treatment. ZR increased significantly for the drought and drought plus PBZ treatments compared with the control in the third cycle. (C) 2003 Elsevier B.V. All rights reserved.