Nickel hyperaccumulation by Thlaspi montanum var. montanum (Brassicaceae): a constitutive trait.

Adaptations to particular stresses may occur only in populations experiencing those stresses or may be widespread within a species. Nickel hyperaccumulation is viewed as an adaptation to high-Ni (serpentine) soils, but few studies have determined if hyperaccumulation ability is restricted to populations from high-Ni soils or if it is a constitutive trait found in populations on both high- and low-Ni soils. We compared mineral element concentrations of Thlaspi montanum var. montanum plants grown on normal and high-Ni greenhouse soils to address this question. Seed sources were from four populations (two serpentine, two non-serpentine) in Oregon and northern California, USA. Plants from all populations were able to hyperaccumulate Ni, showing Ni hyperaccumulation to be a constitutive trait in this species. Populations differed in their ability to extract some elements (e.g., Ca, Mg, P) from greenhouse soils. We noted a negative correlation between tissue concentrations of Ni and Zn. We suggest that the ability to hyperaccumulate Ni has adaptive value to populations growing on nonserpentine soil. This adaptive value may be a consequence of metal-based plant defense against herbivores/pathogens, metalbased interference against neighboring plant species, or an efficient nutrient scavenging system. We suggest that the Ni hyperaccumulation ability of T. montanum var. montanum may be an inadvertent consequence of an efficient nutrient (possibly Zn or Ca) uptake system. tanum. Many traits of organisms have been interpreted as adaptations to specific environmental conditions (selective pressures). It is not surprising to find such adaptations in populations that continually or periodically experience a given selective pressure. However, it is more intriguing to find populations that are far removed in time and/or space from a given selective pressure that possess the adaptation to it. If the adaptation is generally possessed by all members of the species it may be termed a con