Noccaea is a genus of roughly 80 species of flowering plants in the mustard family (Brassicaceae), order Brassicales. The genus was established by Moench in 1802 and named in honour of Domenico Nocca (1758–1841), an Italian clergyman, botanist, and director of the botanic garden at Pavia.
Members of the genus are biennials or perennials, often somewhat glaucous, with erect or decumbent stems. Leaves are arranged in a basal rosette and along the stem; basal leaves are petiolate with entire to toothed margins, while the stem leaves typically have auriculate to subamplexicaul (stem-clasping) bases. Flowers are borne in corymbose racemes and have white, pink, or purple spatulate petals that exceed the sepals; stamens are slightly unequal in length (tetradynamous). The fruits are characteristic of the genus: flattened sessile silicles (angustiseptate — compressed at right angles to the narrow septum), obcordate to oblong in outline, sometimes winged at the apex, with 4–14 seeds per locule. The chromosome base number is x = 7.
Noccaea has a broad distribution across temperate regions, occurring in western North America (including arctic Alaska), Mexico, Patagonian South America, Europe, Asia, and northern Africa. The genus was previously accommodated within the broadly circumscribed Thlaspi, but molecular and morphological studies — notably Koch & Al-Shehbaz (2004) — demonstrated that the American Thlaspi species belong to Noccaea and are distinct from the core Eurasian clade.
The genus is particularly well known in ecology and phytoremediation research because several species, most notably Noccaea caerulescens (alpine penny-cress), are hyperaccumulators of heavy metals such as zinc, cadmium, and nickel, concentrating these elements in their tissues at levels toxic to most other plants. This trait has made Noccaea a model system for studying the genetics and physiology of metal tolerance and for practical phytoremediation of contaminated soils.
Etymology
Noccaea was named by the German botanist Conrad Moench in his 1802 Supplementum ad Methodum Plantas in honour of Domenico Nocca (1758–1841), an Italian clergyman, botanist, and director of the botanic garden at the University of Pavia.
Distribution
The genus comprises approximately 80 species distributed across temperate and montane regions of western North America (from arctic Alaska south through the mountain and Pacific states to Mexico), Patagonian South America, Europe, Asia, and northern Africa.
Ecology
Several Noccaea species are facultative or obligate hyperaccumulators of heavy metals, especially zinc, cadmium, and nickel. Noccaea caerulescens (formerly Thlaspi caerulescens) is among the best-studied hyperaccumulator plants in the scientific literature and can accumulate zinc in leaf tissue at concentrations exceeding 10,000 ppm — levels lethal to most plants. This adaptation allows the genus to colonise metalliferous soils (calamine grasslands, mine spoils) where competition from other plants is greatly reduced. The trait is exploited in phytoremediation — using living plants to extract or stabilise heavy-metal contaminants from polluted soils.
Taxonomy Notes
Noccaea was segregated from the broadly defined Thlaspi (Brassicaceae) following molecular and morphological reassessment. Koch & Al-Shehbaz (2004, Systematic Botany 29: 375–384) demonstrated that the American species previously assigned to Thlaspi (e.g., T. montanum, T. fendleri, T. alpestre) belong within Noccaea, distinct from the Eurasian core of Thlaspi sensu stricto. The name Noccaea has priority over the segregate Raparia and related synonyms. Chromosome base number x = 7.
