Masters Thesis

Effects of Regulated Deficit Irrigation on Fruit Yield, Quality, and Physiology of Washington Navel Orange Trees.

The purpose of this study was 1) to measure fruit yield of navel orange trees and the plants’ physiological responses to regulated deficit irrigation (RDI) and 2) to determine if plant-based measures, including stem water potentials (stem), sap flow (SF), and daily trunk diameter fluctuations (TDF) could be easily used to indicate citrus water need using minimal instruments. The study was conducted in 2014/15 at the Citrus Experiment Station at the University of California, Riverside, and consisted of one control and three deficit irrigation treatments. Control trees were irrigated to replace 100% of water lost due to crop evapotranspiration (ETc). During the early fruit growth period (mid-May to mid-July), RDI1 and RDI3 trees were subjected to 25% of ETc. During fruit maturity period (mid-October to mid-December), RDI2 and RDI3 trees were subjected to 75% of ETc. At all other times, irrigation was 100% of ETc. All RDI treatments resulted in lower yield and higher percentage of small oranges than the control, but the yield effects were only significant for RDI1 and the size effect only for RDI1 and RDI3. Total water savings were 19% for RDI1, 2% for RDI2, and 21% for RDI3. Considering the benefits of water savings and the potential loss/gain in gross revenues, RDI3 and RDI2 were better irrigation strategies than RDI1. Of the plant-based parameters, the labor-intensive discontinuous stem had the highest signal intensity for water stress, while the continuous measurements using plant sensors failed to detect water stress consistently.

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