Concentrations of abscisic acid and other solutes in xylem sap from root systems of tomato and castor-oil plants are distorted by wounding and variable sap flow rates

We investigated if concentrations of abscisic acid (ABA) andother solutes measured in the first few droplets of xylem sapfrom detopped root systems, are good estimates of those in thetranspiration stream as it enters the shoot-base of whole plants.Xylem sap from root systems of pot-grown tomato plants (Lycopersiconesculentum Mill., cv. Ailsa Craig), at the seven-leaf stage,was obtained by placing root systems in chambers pressurizedto 0.3 MPa with air. The first sample was taken from the cut-surfaceof the hypo-cotyl stump within 30 s of removing the shoot. ABA,sucrose and other osmolytes were more concentrated in the initial100–200 mm³ of xylem sap than in subsequent samples. Thissuggested the sap was contaminated and not unchanged transpirationfluid. The effect was reproduced on the same plant, severaltimes, by recutting the hypocotyl prior to reassembling thesap collecting set-up and repressurizing. Similar results werefound with castor-oil plants (Ricinus communis L., cv. Gibsonii).However, neither release of ABA from the cut surface of thehypocotyl stump, nor the effects of pressure to the roots causedthe contamination. Instead, small radial pressures exerted bya rubber sleeve attached to the hypocotyl stump, for collectingthe sap, were responsible. The effect was reproduced by lightlysqueezing the hypocotyl by hand. The possibility was examined that reliable estimates of ABAconcentrations in transpiration stream fluid may be obtainedfrom sap samples taken immediately after rejecting the initial,contaminated 200 mm³. However, ABA concentrations in these latersamples were also unsatisfactory since they changed with rateof sap flow. The problem may be overcome by analysing sap inducedto flow through detached root systems at rates close to thoseof whole-plant transpiration. Key words: Tomato, Lycopersicon esculentum Mill., Castor-oil plant, Ricinus communis L., roots, root to shoot communication, xylem sap, abscisic acid, sucrose, transpiration stream