Natural variability in Olea europaea L. reveals diverse functional strategies to drought under a CO2-enriched atmosphere.

The wild olive (Olea europaea L.) is one of the most iconic Mediterranean forest tree species and the ancestor of many cultivated olive varieties. In this study, twelve wild olive genotypes from three distinct subspecies (subsp. europaea, subsp. guanchica, and subsp. maroccana) were grown under ambient ([CO2]amb) and enriched CO2 atmospheres ([CO2]enr), subject to two watering regimes (WW - Well-watered/WS - Water-stressed). We measured water use at leaf and whole-plant levels, along with growth traits, leaf gas exchange, and leaf isotopic carbon (δ13C) and nitrogen composition (δ15N), to differentiate functional strategies among genotypes in response to CO2 and soil water availability. Growth under [CO2]enr lessened water stress effects by increasing water use efficiency (WUE). The improved WUE came from reduced water consumption at both leaf and plant levels under [CO2]enr, although sensitivities to soil water content varied among genotypes. Growth-related traits responded more to water stress than to CO2. Patterns of N15 isotopic composition (δ15N) and total leaf nitrogen content on a mass and area basis (Nmass and Narea) varied among genotypes in response to atmospheric CO2 and water stress. A negative relationship between δ13C and δ15N indicates a complex interaction between carbon and nitrogen metabolisms under the influence of water availability and CO2 supply. Our study shows that (1) the response of wild olive to atmospheric CO2 enrichment is strongly influenced by resource availability, especially water, and (2) it is necessary to consider genotype- and subspecies-specific responses when predicting vegetation response in a CO2-enriched world.