Differential tolerance to summer stress conditions in two olive cultivars using the dendro-isotopic approach

In this study the interannual and seasonal dynamics of carbon and oxygen stable isotope composition (δ¹³C, δ¹⁸O), and the resulting intrinsic water use efficiency (iWUE) in xylem rings of Olea europaea L. were investigated. The study was conducted on two high-quality Italian olive cultivars (cv Moraiolo and cv Maurino), grown in central Italy, during the seasons 2020–2022. Variations in both C and O isotope compositions revealed seasonal patterns characterised by the lowest values within the transition from late to early wood rings and the highest values within the transition from early to late wood. The wider seasonal range of δ¹³C, δ¹⁸O and iWUE observed in cv Moraiolo highlighted its ability to adapt to changing environmental conditions. During periods of summer stress, Moraiolo trees close their stomata to reduce transpiration rates, prioritising water conservation to sustain growth. In contrast, Maurino displayed less flexibility in vary its iWUE based on water availability, exhibiting limited responsiveness to environmental fluctuations. The relationship between ecophysiological traits and above-ground development of each caultivar was also discussed.