Partial Root Zone Drying Increases Peppermint Essential Oil Yield and Water Productivity

The effects of varying irrigation volume and placement (regulated deficit irrigation that irrigated the entire root zone versus partial root zone drying that irrigated only half of the root zone at a time), on drip-irrigated peppermint (Mentha piperita L.) were studied. Treatments included full irrigation (FI) and regulated deficit irrigation receiving 85%, 70%, 55% and 40% of the irrigation volume of the full irrigation treatment (RDI85, RDI70, RDI55 and RDI40 respectively), and partial root zone drying receiving 70%, 55% and 40% of the irrigation volume of the full irrigation treatment(PRD70, PRD55 and PRD40 respectively). Plants were harvested at mid flowering stage over two consecutive seasons. The steam distillation method used to extract essential oil. At the first harvest, biological yield (dried leaf and stem weight) generally did not significantly differ between the full irrigation, partial root zone drying, and regulated deficit irrigation treatments, but yield of RDI40 decreased by 33% compared to FI plants. However, deficit irrigation strategies significantly increased essential oil yield and ratio (Percentage of essential oil in 80 g of leaves), with the highest biological yield in FI and highest essential oil yield in PRD70, at rates of 1942 kg/ha and 23.7 kg/ha, respectively. Irrigation water productivity based on oil yield was significantly higher in partial root zone drying treatments than full irrigation and regulated deficit irrigation treatments, with the highest value achieved in PRD55 at a rate of 0.0191 kg/m(3). At the second harvest, plant biological yield components were not affected by the irrigation treatments, but again the partial root zone drying treatments produced higher essential oil ratio and yield. Since the major goal of peppermint production is to extract its oil, it is recommended to apply a moderate water deficit with the PRD70 and PRD55 techniques, to allow significant (30-45%) water-saving and also reasonable yield.