Stem-Water Potential Reading Variability in Olive (Olea europaea)

In the face of growing water insecurity, California farmers have embraced irrigation management technologies which help ensure a greater "crop per drop."  Stem-water potential (ψ_stem) is a plant based irrigation management tool that measures water stress.  Previous research has established numerous technician measurement protocols to minimize variability in almond, walnut and prune water stress readings; these protocols have been adopted in olive with only anecdotal evidence of their validity. The objective of this study is to explore variability in ψ_stem readings in olive (Olea europaea cv. ‘Arbequina’ and ‘Manzanillo’).  Potential variability arising from crop load, the presence of olive knot, the location of the sampled shoot on the tree, shoot samples containing fruit, a two minute post excision interval before placement in the pressure chamber, the length of the stem protuberant from the pressure chamber, re-pressurizing a bagged shoot, and examining differences between technicians are all investigated.  Readings are principally taken from several replicates on four to five trees in a single fully irrigated row.  Data sets are analyzed using ANOVA as a single factor randomized block design (RBD - 1 factor), blocking by tree.  In the overall analysis of all investigations, the variability tested was insignificant ( P > 0.05).  An exception to the overall findings occurred in two of three investigations regarding the location of the sampled shoot on the tree, as well as the first of nine trials comparing readings between two operators.  These results suggest a robustness of ψ_stem readings, despite variance in tree physiology and operator technique.  The exceptions noted are also consistent with literature finding bag placement and operator as potentially significant sources of variation.