Drying half the root-zone of potted avocado (Persea americana Mill., cv. Hass) trees avoids the symptoms of water deficit that occur under complete root-zone drying

We investigated the hypothesis that drying half the root system, compared with drying the whole root system of avocado plants, would maintain the axial hydraulic conductivity of trunks and generate root signals that cause reduced growth and water use. We exposed young, grafted avocado trees, growing in divided containers, to three treatments: water withheld from both soil compartments (DD); from one compartment (WD); or with both compartments well-watered (WW). After 44 d, the treatments were switched, including the W and D compartments of the WD trees, and recovery of the DD trees was observed for up to 84 d. We examined branch and trunk anatomy, leaf stomatal conductance, leaf water potential, elongation of vegetative and. reproductive plant parts, sap flow velocities (in WW and DD trees), and hydraulic conductivities of excised trunk and branch segments. In the lower trunk of DD trees, even 84 d after re-watering, up to 34% of xylem vessels were occluded by tyloses, an outgrowth into the xylem lumen that contributed to a 50% reduction in hydraulic conductivity. Tyloses were not detected in trunks of either WW or WD trees. In contrast to partial root-zone drying, complete root-zone drying (DD trees) led to closed stomata, reduced water uptake, stopped tissue elongation, and caused defoliation, flower abortion and fruit shedding, while inducing the growth of tyloses. Watering half of the root system maintained vegetative and reproductive growth to fruit set, and WD trees used a similar amount of water as WW trees. It is therefore unlikely that partial root-zone drying would improve water-use efficiency in avocado trees grown in the field.