THE RESPONSE OF SAP FLOW IN APPLE ROOTS TO LOCALIZED IRRIGATION

?The oft-touted reason for the efficiency of drip irrigation is that r oots can preferentially take up water from localised zones of water av ailability. Here we provide definitive evidence of this phenomenon. Th e heat-pulse technique was used to monitor rates of sap flow in the st em and in two large surface roots of a 14 year old apple tree (Malus d omestica Borkh. cv. Braeburn). The aim was to determine the ability of an apple tree to modify its pattern of root water uptake in response to local changes in soil water content. We monitored the water status of the soil close to the instrumented roots by using time domain refle ctometry (TDR) to measure the soil's volumetric water content, theta, and by using ceramic-tipped tensiometers to measure the soil's matric pressure head, h. A variation in soil water content surrounding the tw o roots was achieved by supplying a single localised irrigation to jus t one root, while the other root remained unwatered. Sap flow in the w etted root increased straight away by 50% following this drip irrigati on which wetted the soil over a zone of approximately 0.6 m in diamete r and 0.25 m in depth. Sap flow in the wetted root remained elevated f or a period of about 10 days, that is until most of the irrigation wat er had been consumed. A comparative study of localised and uniform irr igation was then made. Following irrigation over the full root zone no further change in sap flow in the previously wetted root was observed when referenced to the corresponding sap flow measured in the stem of the apple tree. However sap flow in the previously dry root responded to subsequent irrigations by increasing its flow rate by almost 50%. These results show that apple roots have the capacity to transfer wate r from local wet areas at much higher rates than normally occurs when the entire root zone is supplied with water. They are also able to shi ft rapidly their pattern of uptake and begin to extract water preferen tially from those regions where it is more freely available. Such an a bility supports the use of drip irrigation for the efficient use of sc arce water resources. We conclude that the soil-to-root pathway repres ents a major resistance to water uptake by apple, even at the relative ly high soil water pressure heads developed during parts of this exper iment, during which the tree was not even under any stress. (C) 1997 E lsevier Science B.V.