The role of elevation and soil chemistry in the distribution and ion accumulation of floral morphs of Streptanthus polygaloides Gray (Brassicaceae), a Californian nickel hyperaccumulator

Abstract

Background: The flora of serpentine/ultramafic soils provides an excellent model system for the study of natural selection in plant populations. Streptanthus polygaloides is a nickel hyperaccumulator that is endemic to serpentine soils in the Sierra Nevada of California, and has four floral morphs (yellow, purple, yellow-to-purple and undulate).

Aims: We investigate three hypotheses: (1) the purple morph occurs in colder, wetter climates than the yellow morph; (2) tissue–soil ionic relationships differ among morphs; and (3) morphs occur on soils with differing elemental concentrations.

Methods: We queried herbarium records to investigate patterns of occurrence among the yellow and purple floral morphs, and analysed soil and tissue samples from wild populations of all four morphs.

Results: The purple morph inhabited serpentine outcrops with colder temperatures and greater precipitation levels than the yellow morph. Concentrations of elements in leaf tissue and rhizosphere soil differed little among populations of the morphs, but showed substantial within-site variation.

Conclusions: Our results suggest that a climatic gradient may be responsible for divergence in floral colour among populations of S. polygaloides. Because of the large within-site variation in soil and tissue elemental concentrations, plants appear to have a varied physiological response to edaphic factors, regardless of morph membership.

Keywords:

• floral divergence
• local adaptation
• plant endemism
• serpentine ecology
• Streptanthus polygaloides

Acknowledgements

We are grateful to K. Kersh and D. Moe (JEPS, UC), J. England (RSA), D. Kelch (CDA), M. Williams (CHSC), J. Shepard (UCD), A. Sanders (UCR), S. Junak (SBBG) and M. Alvear (CAS) for providing calyx colour for herbarium vouchers accessible via the California Consortium of Herbaria Database (ucjeps.berkeley.edu/consortium/); to S. Woolhouse and J. Vieto for assistance in the field; and to four anonymous reviewers for their comments and suggestions. Additionally, we are grateful to Auburn University, San José State University and the College of the Atlantic for providing the funding to make this project possible.

Additional information

Notes on contributors

Nathaniel Pope

Nathaniel Pope is a graduate student. His research interests include pollinator-plant interactions and population ecology.

Michael Fong

Mike Fong is a botanist. His research focuses on the geoecology of Streptanthus polygaloides.

Robert S. Boyd

Robert Boyd is a professor. His research interests include ecology of metal hyperaccumulator plants, as well as management and ecology of rare plants.

Nishanta Rajakaruna

Nishanta Rajakaruna is a professor of botany. His research focuses on the factors and mechanisms driving plant speciation and community assembly on serpentine and other edaphically harsh environments.