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Todd C. Einhorn, Horst W. Caspari, and Steve Green

Approach-grafted 1-year-old `Gala'/M7 apple trees were grown with both tops for the remainder of the 2003 season in a greenhouse. Trees were supplied with >100% (control, PRD100) or 50% (PRD50, DI50) of daily ETc either applied to one root compartment only (PRD100, PRD50) or divided evenly across both root compartments (control and DI50). ETc was estimated from gravimetric measurements, and irrigation was switched between wet and dry root compartments several times throughout the experiment. Soil moisture was measured both gravimetrically (tripod) and volumetrically (time-domain reflectometry). Predawn leaf water potential (υpd) and single leaf gas exchange (photosynthesis, stomatal conductance, and transpiration) were recorded daily, and sap flow in stems and roots was monitored continuously using the heat-pulse technique. Leaves were collected for abscisic acid (ABA) determination following gas exchange measurements. Regardless of irrigation placement (i.e., PRD or DI), both 50% ETc treatments experienced similar declines in υpd and single leaf gas exchange relative to control levels. In addition, ABA concentrations were similar for PRD50 and DI50, and were significantly higher than the control and PRD100 treatments. PRD100 trees had similar υpd as control trees; however, gas exchange was reduced >25% compared to the control. Bulk leaf ABA concentration did not differ significantly from control levels and does not by itself explain the down regulation of stomata with PRD100.

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Todd C. Einhorn, Horst W. Caspari, Steve Green, and Greg Litus

One-year-old `Gala'/M7 apple trees were potted into 30-L containers and approach-grafted about 45 cm above the graft union in late Spring 2003. Trees were grown with both tops for the remainder of the 2003 season in a greenhouse. In Apr. 2004, one of the tops was removed. Trees were fully watered by an overhead irrigation system until July 2004, when trees were subjected to one of four irrigation regimes: control received >100% of ETc applied evenly to the two pots; PRD100 received >100% ETc applied to one pot only; and two regimes received 50% ETc applied to either one (PRD50) or both pots (DI50). Both gravimetric (tripod) and volumetric (time-domain reflectometry) soil moisture measurements were taken daily prior to and after irrigations. In addition, heavy isotope H2O (18O) was applied to one of the two root compartments and analyzed in the leaves to further determine the validity of the model. Sap flow was monitored in six split-rooted trees using miniaturized heat-pulse probes inserted into the stem above the graft union and into each of the two root systems below the graft union. Under fully irrigated conditions, root sap flow was proportional to root trunk cross-sectional area, and was not a function of root system origin (i.e., roots of mother plant with original top remaining or roots of daughter plant with original top detached). Water uptake from a previously dried root zone was rapid when the irrigated side was switched, but much more gradual when the other side was maintained wet. Interactions between soil moisture and sap flow in relation to factors governing canopy demand will be presented.