Drip irrigation frequency leads to plasticity in root water uptake by apple trees

Stable isotopes of hydrogen and oxygen are used in agriculture to investigate the water sources used by crops. Yet, isotopic research on irrigated orchards is still scarce. We investigated the isotopic variability in an apple tree plantation in the Eastern Italian Alps (South Tyrol) during the growing seasons 2020 and 2021. The orchard was subject to an irrigation trial, whereby a drip system was triggered at different soil water potential thresholds at two treatment types: full irrigation (FI, −30 kPa) and deficit irrigation (DI, −60 kPa). On a bi-weekly basis, we sampled precipitation, river water, and groundwater used for irrigation. At both FI and DI, we sampled soil at different depths and bark-devoid branches, and cryogenically extracted their water. Isotopic analyses revealed large differences in δ18O values of soil water belonging to the two irrigation treatments, particularly during the irrigation period (up to 8.9‰). In xylem water, the differences were much smaller (up to 1.6‰). Mixing models (EEMMA) estimated a larger groundwater (vs. rainwater) fraction in the shallow soil (5–10 cm) at FI (25–55%) than at DI (0–5%), compatible with a larger presence of irrigation water in the former. DI plants had a deeper root water uptake (32.0 ± 11.9 cm) than FI ones (19.3 ± 14.5 cm) during the irrigation period. This agreed with the results of mixing models (IsoSource) that estimated a larger use of deeper (60–65 cm) soil water (42 ± 18%) and a lower use of shallow soil water (13 ± 6%) for DI than for FI (34 ± 26% and 27 ± 26%) during the same period. This root water uptake plasticity explains the lacking evidence of physiological stress in sap flux records at DI and supports the potential for further improvements of precision irrigation in similar climatic and edaphic settings.