Induced water stress affects seed germination response and root anatomy in Robinia pseudoacacia (Fabaceae)

Key message: The different germination behaviours of the seeds under induced water limitations may be related to the different adaptive capacities acquired at the diverse collection sites, as a response to the different environmental parameters. The island of Pianosa resulted the most performant in term of germination responses and the co-occurrence of xeromorphic anatomical evidences at root level confirmed this trend. Abstract: Regeneration from seeds is an important co-determinant in the invasion ecology of black locust. In the attempt of providing new information on its invasion potential in Mediterranean Europe under the future scenario of global warming, we investigated the effects of induced water-deficit regimes on: (1) seed germination performance and (2) root growth and anatomy. Ripe seeds were collected from four populations established in Tuscany (Central Italy): mechanically scarified seeds were incubated in a range (− 0.2/− 0.8 MPa) of constant water stress conditions at 21 °C. The final germination rate drastically declined with increasing induced water-deficit conditions, with the highest value at control and at − 0.2 MPa (ca 50–97%), and the lowest at − 0.6 MPa (ca 10–33%). The mean germination time decreased with increasing water stress. At root level, xeromorphism relies on the combination of different anatomical traits which co-optimize water uptake/loss: thinner roots, higher number of xylem vessels, vessels with small-sized lumen and thinner cell walls. Seeds collected in sites characterized by different environmental parameters display a noteworthy difference in the germination dynamics: as far as the beginning and ongoing of germination, as well as the germination response in time is concerned, the seeds from the island of Pianosa showed the highest performance; the major arid conditions in Pianosa could have caused a “stress imprint” able to facilitate a fast and protective response to future drought events. As a whole, our results confirmed the great phenotypic plasticity of black locust as a response to variable water availability and provided evidence for the potential high germination capacity in drier environments, as seems to be the future Mediterranean Europe.