Natural variation in cadmium tolerance and its relationship to metal hyperaccumulation for seven populations of Thlaspi caerulescens from western Europe

Abstract

Thlaspi caerulescens J. & C. Presl is a distinctive metallophyte of central and western Europe that almost invariablyhyperaccumulates Zn to > 1.0% of shoot dry biomass in its natural habitats, and can hyperaccumulate Ni to > 0.1% when growing on serpentine soils. Populations from the Ganges region of southern France also have a remarkable ability to accumulate Cd in their shoots to concentrations well in excess of 0.01% without apparent toxicity symptoms. Because hyperaccumulation of Cd appears to be highly variable in this species, the relationship between Cd tolerance and metal accumulation was investigated for seven contrasting populations of T. caerulescens grown under controlled conditions in solution culture. The populations varied considerably in average plant biomass (3.1-fold), shoot : root ratio (2.2-fold), Cd hyperaccumulation (3.5-fold), shoot : root Cd-concentration ratio (3.1-fold), and shoot Cd : Zn ratio (2.6-fold), but the degree of hyperaccumulation of Cd and Zn were strongly correlated. Two populations from the Ganges region were distinct in exhibiting high degrees of both Cd tolerance and hyperaccumulation (one requiring 3 m m m m M Cd for optimal growth), whereas across the other five populations there was an inverse relationship between Cd tolerance and hyperaccumulation, as has been noted previously for Zn. Metal hyperaccumulation was negatively correlated with shoot : root ratio, which could account quantitatively for the differences between populations in shoot Zn (but not Cd) concentrations. On exposure to 30 m m m m M Cd, the two Ganges populations showed marked reductions in shoot Zn and Fe concentrations, although Cd accumulation was not inhibited by elevated Zn; in the other five populations, 30 m m m m M Cd had little or no effect on Zn or Fe accumulation but markedly reduced shoot Ca concentration. These results support a proposal that Cd is taken up predominantly via a high-affinity uptake system for Fe in the Ganges populations, but via a loweraffinity pathway for Ca in other populations. Total shoot Cd accumulated per plant was much more closely related to population Cd tolerance than Cd hyperaccumulation, indicating that metal tolerance may be the more important selection criterion in developing lines with greatest phytoremediation potential.

Metal concentration and metal mass of metallicolous, non metallicolous and serpentine Noccaea caerulescens populations, cultivated in different growth media

“…Roosens et al (2003) further showed that the addition of Zn (10 or 100 μM) in hydroponic solutions containing Cd (30 μM) did not decrease leaf Cd concentration in SF population (also called "Ganges ecotype"), confirming the idea that Cd is accumulated independently of Zn in these populations. We tested this hypothesis for the AV population and two others by comparing the Cd concentration between the two salt mixtures (100Zn250Cd and 250Zn250Cd) and the monometallic salt 250 CdSO 4 at the same Cd concentration (250 mg Cd/kg).…”

“…This is in agreement with Reeves et al (2001), who showed that field-collected N. caerulescens of PW accumulated up to 6000 mg/kg, despite the fact that the soil Zn concentration was below 70 mg/kg (Table 1). In hydroponic experiments, SERP populations had higher (Roosens et al 2003) or similar (Assunção et al 2003a; values of foliar Zn concentration compared to MET populations.…”

“…Despite the high number of papers concerning N. caerulescens, most experimental studies investigating metal concentration patterns have compared a very low number (usually 1 or 2) of MET populations (with the exception of Peer et al 2003 andRoosens et al 2003). Most experiments were performed with nutrient solutions (e.g.…”

“…Meerts and Van Isacker 1997;Escarré et al 2000;Lombi et al 2001a). In addition, experimental studies comparing MET with NMET populations simultaneously are scarce and were mostly conducted by Belgian and French teams (Meerts and Van Isacker 1997;Escarré et al 2000;Meerts et al 2003;Noret et al 2005Dechamps et al 2007Dechamps et al , 2008b, and even less common are studies that compared populations from metal-contaminated and serpentine soils (Peer et al 2003;Roosens et al 2003). Only two studies (Assunção et al 2003a; compared one population of each of the three origins.…”

Evolutionary aspects of elemental hyperaccumulation

“…This may be true for both essential and non-essential elements. For instance, the hyperaccumulator Noccaea caerulescens had a 75 % increase in biomass when supplied with Cd after 31 days (Roosens et al 2003). The element Se is not known to be essential for plants, but is beneficial for the growth of many plant species, especially Se hyperaccumulators (virupaksha and Shrift 1965;el Mehdawi et al 2012).…”