Physiological and growth responses of pomegranate tree (Punica granatum (L.) cv. Rabab) under partial root zone drying and deficit irrigation regimes

• Physiological and growth responses of pomegranate to water stress were studied.
• Mild water stress maintains the physiological parameters at an acceptable level.
• High tolerance to water stress occurs in pomegranate subjected to water stress.
• In arid areas, the pomegranate stomatal conductance is controlled by hydraulic signaling.

This study evaluated the effects of full irrigation (FI, 100% ETC), partial root zone drying and deficit irrigation with mild (75%ETC) and severe (50% ETC) water stress on the physiological and growth behavior of pomegranate trees in southern Iran in 2011 and 2012. The experiment was set up as a split plot design in completely randomized blocks design with three replications. Results showed that the stem water potential (ψstem) was directly related to the soil water deficit (SWD), and it varied from −2.07 to −3.27 MPa in 2011 and −1.67 to −3.23 MPa in 2012. In all times of measurements, higher values of stomatal conductance (gs), net photosynthesis (An) and leaf scale transpiration (Tr) were obtained in FI and mild water stress treatments (An: 3.15–12.64 μmol m−2 s−1, gs: 0.032–0.129 mol m2 s−1 and Tr: 2.72–9.48 mmol m2 s−1) in comparion with those obtained in the severe water stress treatments (An: 2.11–11.81 μmol m−2 s−1, gs: 0.021–0.11 mol m2 s−1 and Tr: 1.89–7.58 mmol m2 s−1). Results generally showed low values of gs in the Rabab cultivar in the studied arid area. Furthermore, the hydraulic signaling (ψstem) in response to the water stress and weather conditions controlled the seasonal variation of gs. Transpiration efficiency (TE) increased with increasing the degree of water stress and PRD showed higher TE (1.17–1.18 μmol mmol−1) than that in DI strategies (1.06–1.07 μmol mmol−1). Furthermore, the value of intrinsic water use efficiency (IWUE) in FI (85 μmol mol−1) was statistically lower than that obtained in mild (PRD75, DI75) and severe (PRD50, DI50) water stress treatments (87.8–89.9 μmol mol−1). Different irrigation strategies did not significantly affect the fruit growth rate, and its maximum values occurred at about 170 DAB (0.27–0.35 mm d−1) in all irrigation strategies. The LAI values in FI and mild water stress strategies were significantly different compared with those in the severe water stress treatments. For all irrigation strategies, the maximum LAI values occurred at flowering stage (68 DAB), and was 6.09 and 6.15 (FI), 5.97 and 6.04 (DI75), 6.33 and 6.64 (PRD75), 4.60 and 4.82 (DI50) and 4.91 and 5.03 m2 m−2 (PRD50) in 2011 and 2012, respectively. In conclusion, PRD75 is the recommended, applicable water-saving strategy and good alternative with respect to FI among other irrigation managements when water resources are limited in order to increase TE and IWUE while other physiological and growth parameters are maintained at an acceptable level.