Using Sap Flow, Stem Diameter Micro-variations to Assess Water Use in Grapefruit Trees under Water-saving Irrigation Strategies

The objectives of this study were: 1) to study the response of grapefruit trees to regulated deficit irrigation (RDI) and partial rootzone drying (PRD); and 2) to evaluate the use of different methods, including stem water potential, sap flow, and tree trunk micro-variations, to estimate tree water use. A greenhouse experiment was conducted at the Texas A&M University-Kingsville Citrus Center during 8 weeks. Root systems of two-year-old grapefruit trees were split into half and allowed to become established in adjacent pots. Four irrigation strategies were applied: 1) Control: irrigated with 100% of plant evapotranspirative needs (ET); 2) RDI60: 60% ET; 3) RDI30: 30% ET; and 4) PRD: 100% ET applied to only one-half of root zone, changing the side every four weeks. Stem water potential (SWP), stomatal conductance, leaf abscisic acid concentration, chlorophyll fluorescence, and plant growth (total plant dry weight and shoot length) were measured. The RDI strategies increased tree water use efficiency; trees with RDI60 and RDI30 used 30% and 52% less water than control, respectively, without showing differences in total plant dry weight or shoot length. RDI30 trees had a lower SWP than RDI60 at the end of the experiment, although the value (–1.48 MPa) was very mild and RDI30 trees were not considered to be drought stressed. PRD did not show any differences in water savings (probably because of the short duration of this experiment), SWP or plant growth. Trees under RDI60, RDI30, and PRD treatments showed higher leaf ABA concentration at the end of the experiment as compared to control trees; however, no differences were found in stomatal conductance, which may indicate that hydraulic mechanisms played a role in water movement in the three irrigation strategies evaluated. Regulated deficit irrigation treatments (30 and 60 RDI) had smaller sap flow rates than control and PRD treatments. RDI60 trees had a smaller maximum daily shrinkage than control trees but no differences were observed between the other treatments. This study showed that sap flow sensors and stem diameter variations can be good indicators to detect water stress in grapefruit trees.