Estimating uptake and internal transport dynamics of irrigation water in apple trees using deuterium-enriched water

Climate change will likely increase crop water demand and reduce the availability of water for irrigation. A deeper knowledge on water uptake dynamics and its translocation inside plants would help optimize irrigation management. Stable isotopes are widely used in ecohydrological studies to track the origin of plant water and to determine the contribution of different water sources to the plants’ requirements. Despite the extensive literature dealing with water relations in fruit orchards, very little is known about the time interval between the irrigation water supply and the presence of irrigation water inside the tree. The present study addressed the following questions: 1. What is the time interval between irrigation and the arrival of irrigation water at different tree heights? 2. To which extent can irrigation water uptake and transport be accelerated by increasing the portion of soil volume receiving drip irrigation water?

To address our goals, we set up a field experiment in summer 2021 in an apple orchard and an ancillary pot experiment in the lab. In the field experiment, we tested the effect of different drip irrigation layouts on the extent and rapidity of water uptake by mature apple trees. Trees were irrigated using deuterium-enriched water (δ²H = 12050 ‰) using 1, 2 or 4 drippers per tree (each dripper delivered 3 L in one hour). Shoot and fruit samples were collected in the bottom (1.5 m) and top (3 m) part of the canopy at regular intervals following the irrigation event. Soil was sampled at different depths and distances from the dripper after the irrigation. In the pot experiment, the soil was saturated with labelled water (δ²H = 1779 ‰) and xylem samples were collected at different time intervals and heights along the apple tree trunk. Water was extracted by cryogenic vacuum distillation and analyzed by IRMS. A two end-member mixing model allowed to quantify the fraction of labelled irrigation water in soil and trees. Irrigation flow velocity within the tree was estimated by the first sampling time at which the shoot δ²H value at a given height was significantly different from the values before the irrigation. Irrigation water could be detected in potted trees at 0.5 m after 1 h and at 1 m after 2 h; in field-grown trees, labelled water first appeared in the shoots in the bottom and top part of the canopy after 2 h and 4-6 h, respectively. By increasing the number of drippers per tree, the fraction of irrigation water in the shoots increased accordingly (ranging from 1% to 3% of total water after 32 h). However, uptake and transport velocity were unaffected by the number of drippers, ranging from 0.6 to 0.8 m h^-1. No irrigation water was detected in the fruits.