Variation of Morphology and Elemental Concentrations in the California Nickel Hyperaccumulator Streptanthus polygaloides (Brassicaceae)
The Ni hyperaccumulator Strepthanthus polygaloides (Brassicaceae) is one of a handful of Ni hyperaccumulators known from continental North America. Surveys have revealed four distinctive morphs of this species, relying primarily on floral traits (sepal color and shape): a purple sepal morph (P), a yellow sepal morph (Y), a morph in which sepals start yellow and mature to purple (Y/P), and a morph with light yellow undulate sepals (U). In this study, we raised plants from ten populations (five Y, three P, one Y/P, and one U) under uniform greenhouse conditions to determine if morphs varied in morphology and elemental concentrations when grown on Ni-amended potting soil in a common garden. Morphological data included measurements of leaf form (length, width, and degree of lobing) and plant size (height to first flower as they bolted in summer). Phenology was documented by noting flowering timing of plants. Elemental concentrations of plants were also determined for nine elements (Ca, Cu, Fe, K, Mg, Mn, Ni, P, and Zn). All morphological/phenological traits measured varied significantly between at least some morphs. The U and Y/P morphs were larger than Y and P morphs, with larger leaves as well. Leaves of U morph plants had wide sinuses and shallow lobes, whereas Y/P plants had narrow sinuses and long narrow lobes. P morph plants were shortest in stature, with the smallest leaves. Morphs also varied significantly in concentrations of all elements except Fe. All populations hyperaccumulated Ni, but the P morph contained significantly greater Ni levels than the other three morphs. The P morph also had more Mg, and less Mn and P, than the other morphs. The U morph had more K and Zn, but less Ca, than the other morphs. Principal components analysis revealed all four morphs to be distinctive from one another, and also suggested both morphological/phenological and elemental differences between Y morph populations along a north–south gradient. We conclude that there is considerable genetic divergence between morphs. If additional information shows that morphs are reproductively isolated, then these morphs may require taxonomic subdivision.