Effects of Regulated Deficit Irrigation on Walnut (Juglans regia) Grafted on Northern California Black (Juglans hindsii) or Paradox Rootstock

Chandler walnut (Juglans regia cv Chandler) on Northern California Black (Juglans hindsii) and Paradox (Juglans hindsii x Juglans regia) rootstocks were irrigated to achieve two levels of regulated deficit irrigation (RDI) in 2002 thru 2005. The RDI strategy was to maintain high midday stem water potential (MSWP) early in the growing season to favor shoot growth and nut size then gradually decrease MSWP toward harvest. For the low water stress control (fully irrigated), MSWP was maintained at –0.3 to –0.6 MPa throughout the growing season. For the mild RDI treatment, trees were maintained at –0.3 MPa to –0.6 MPa early in the season and MSWP was gradually decreased to –0.7 to –0.9 MPa by harvest. Moderate stress RDI treatments were the same as mild except MSWP was gradually decreased to –0.9 to –1.1 MPa by harvest. Stem Water Potential was measured midday (12-4 pm) by placing leaves inside foil bags. Leaves remained bagged for at least ten minutes to achieve equilibrium. Bagged leaves were removed, placed inside a pressure chamber and stem water potential was measured. Experimental design was a randomized complete block with three treatments and four replicates. The experiment was located inside a commercial Chandler orchard planted in 1994 to a 5.5m x 9.1m north south hedgerow configuration at a density of 197 trees/ha. Water application used one Nelson R-5 mini sprinkler per tree with an application rate ranging from 0.76 to 1.27 mm/hr. Applied water was manipulated using timers to open and close valves. Each of the twelve plots consisted of three tree rows with 13 trees per row. Six trees in the center of each plot were used for data collection and the remaining trees were used as guards. The six data trees alternated