329 Partial Root Drying as Strategy for Water Use Optimization in a ‘Tahiti’ Lime Orchard under the Semi-Arid of Bahia State A.H.R. Sampaio, M.A. Coelho Filho, E.F. Coelho, R. Daniel, V.V. Machado, G.C. Carvalho, J.A.D.V. Santana and E.B. Santana Junior Embrapa Cassava and Tropical Fruits - CNPNF Universidade Federal do Recôncavo da Bahia - UFRB Cruz das Almas-BA Brazil Keywords: irrigation management, deficit irrigation, citrus Abstract The management of irrigation using partial root drying (PRD) was tested in an orchard of ‘Tahiti’ lime in the semi-arid conditions of Iaçu, BA. Six treatments were evaluated: T1, T2, T3 (50% ETc PRD) with plants irrigated by a microsprinklers with its position alternating every 7, 14 or 21 days, respectively, and wetting on opposite sides of the root system; T4 (DI 50% ETc) irrigation was fixed on one side using one emitter per plant, T5 (100% ETc) was the control treatment using two microsprinklers per plant in a fixed position. In T6, at each interval of 21 days, one and two micro- sprinklers were alternatively used per plant. Results demonstrated significant differences in fruit production by the Scott Knott test (p<0, 05). Treatments T5 and T6 had the highest yield with 23.16 and 22.49 t ha -1 , respectively. Treatments with water deficit reduced average production by 30%. Switching sides of irrigation had no benefit over the treatment with the deficit on only one side. Although there were no statistical differences regarding the efficiency of water use, there was a tendency for greater efficiency for T1 and T4 with 9,32 and 9,48 kg/m³, respectively, compared to the control treatment which produced an average of 6,30 kg/m³. Moreover, physio- logical indicators showed that the treatment with water deficit developed mechanisms to reduce plant transpiration. INTRODUCTION The use of efficient irrigation techniques is necessary to meet the rising demand for food for humanity. Improvements of management strategies are necessary, not only in order to get maximum physical yields, but also to increase water use efficiency and to maintain potential productivities during periods of high climatic risks. The use of water deficit as a strategy for irrigation management has been used in many parts of the world with many fruit cops. The irrigation management inducing temporary drying of part of the root system (Partial Rootzone Drying, PRD) is a kind of strategy that has provided examples of good results of water saving compared to the traditional strategy that gives yields close to the potential values. PRD results are mentioned in the literature with vine (Dry and Loveys, 2000a), olives (Wahbi et al., 2005), apples (Leib et al., 2006), mangoes (Spreer et al., 2007), pears (Kang et al., 2002a) and with citrus (Kirda, 2007; Treeby, 2007; Raveh, 2008). The theory of the PRD technique is based on the fact that the side of root system located at the dry zone produces a great amount of abscisic acid (ABA) that is translocated by the vascular axis to the aerial parts of plant causing partial closure of stomata and, as a consequence, reduced losses of water to the atmosphere (Davies and Zhang, 1991; Gowing et al., 1993), and increased water use efficiency. Few studies have been carried out in Brazil such as the one made by Silva (2005) for vine and by Coelho et al. (2007) and Coelho Filho et al. (2009) for mango. An important point to be noted is that irrigation management is scarce in irrigated Brazil where water is often applied in excess. Therefore, the reduction of irrigation levels could provide an increase in irrigation efficiency. Moreover, studies should answer how biochemical responses to PRD influences plant responses. For instance, the levels of soil drying may affect the signal between root and leaf (Dodd, 2007). These levels should be Proc. XXVIII th IHC – IS on Citrus, Bananas and Other Trop. Fruits under Subtrop. Conditions Eds.: J.-N. Wünsche and L.G. Albrigo Acta Hort. 928, ISHS 2012