Acknowledgements:
We thank Chris Cole, Jack Schaefer, Allyson Richins and the Walnut street greenhouse staff for their help with setting up the experiments, Mark Zierden for assistance with the chemical analyses and Rick Jellen for cytotype identification. Funding was provided by the Swiss National Science Foundation (P2BEP3_175254), by the USDA National Institute of Food and Agriculture (grant awards: 2016-67013-25088 and WIS01651). The involvement of coauthor KM in this project was supported by the Utah Agricultural Experiment Station. The authors declare that there is no conflict of interest.