Breeding the hyperaccumulator Noccaea caerulescens for trace metal phytoextraction: First results of a pure-line selection
Abstract
Phytoextraction still appears to bethe most relevant solution for decontaminating vast areas of cultivated soils contaminated with Cd, one of the most preoccupying contaminants of the food chain. Among those species which could be used for Cd phytoextraction, the hyperaccumulator Noccaea caerulescenshas the highest potential in a temperate climate. However, until now, little has been done in relation to breeding N. caerulescens. The initial seeds were sampled from 60 populations, mostly located in the Western distribution zone of N. caerulescens. Among them were 20 calamine populations, 36 non-metallicolous populations and 4 serpentine populations. The cultivation device was composed of 30 polyethylene trays each filled with 140 kg of homogenised sandy-loamy soil taken from the ploughed horizon of an agricultural soil. The top layer was contaminated with Cd, Ni and Zn salts, mainly, with concentrations of nearly 9, 200 and 1000 mg kg-1. A bit of Cu and Pb was added, to make the selection pressure a bit closer to that of multicontaminated soils. In order to initiate the creation of phytoextraction cultivars, the best individuals were selected for their high shoot biomass or Cd, Ni and Zn concentrations. They were self-pollinated and the selection and fixation was continued for three generations in greenhouse conditions. Selected plants showed a potential to produce 5 to 10 t dry matter ha-1, which is required to decontaminate soils moderately contaminated with Cd. However, the high biomass genotypes could not be fixed, probably because of their complexity and to the sensitivity of this trait to the environmental conditions, particularly to plant density. Indeed, giving more space to the plants enabled them to lengthen their development stages, to produce more aerial dry matter and to increase their reproductive potential. The selection led to an improvement of the Cd and Zn accumulation capacities of the plants but caused a decrease of their Ni accumulation. This is more likely due to a decline in Ni availability in soil than to a deleterious effect of inbreeding. Metal accumulation appeared more heritable than biomass production and fixation for the former trait should be quicker than for the latter. The accumulation capacities of the selected plants enabled offtakes representing around 25% of the soil Cd in one cropping. However, this potential expressed in greenhouse conditions has to be confirmed in the field, in cooler and more contrasted climates, with more biotic and abiotic stresses