Physiological responses of the hybrid larch (Larix × eurolepis Henry) to cadmium exposure and distribution of cadmium in plantlets

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Abstract
Phytoextraction of Cd is a growing biotechnology although we currently know few Cd hyperaccumulators, i.e., plant species able to accumulate at least 0.1 mg Cd g−1 dry weight in aerial organs. Owing their deep root system and high biomass, trees are more and more preferred to herbaceous species for phytoextraction. Assuming that conifers could be relevant models under cold climates, we investigated cadmium tolerance of the hybrid larch Larix × eurolepis Henry (Larix decidua × Larix kaempferi) and the efficiency of this species to store this metal. In vitro grown larches were chosen in order to reduce time of exposure and to more rapidly evaluate their potential efficiency to accumulate Cd. One-month-old plantlets were exposed for 2 and 4 weeks to 250 and 500 μM Cd. Results showed that they tolerated a 4-week exposure to 250 μM Cd, whereas the content of photosynthetic pigment strongly dropped in plantlets growing in the presence of 500 μM Cd. In the presence of 250 μM Cd, shoot growth slightly decreased but photosynthetic pigment and total soluble carbohydrate contents were not modified and no lipid peroxidation was detected. In addition, these plantlets accumulated proline, particularly in shoots (two to three times more than control). In roots, Cd concentration in the intracellular fraction was always higher than in the cell wall fraction contrary to shoots where Cd concentration in the cell wall fraction increased with time and Cd concentration in the medium. In shoots, Cd concentration was lower than in roots with a ratio of 0.2 after 4 weeks of exposure but stayed around 0.2 mg g−1 dry weight, thus a value higher than the threshold requested for Cd hyperaccumulators. Hybrid larch would thus be a relevant candidate for field test of Cd phytoextraction.

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