Relationship between Leachate Pollution Index and growth response of two willow and poplar hybrids: Implications for phyto-treatment applications.

Phytoremediation is a potentially suitable technique for the reclamation of toxic landfill leachate (LL) by decreasing its volume through water uptake and improving its composition by uptake, accumulation and amelioration of pollutants. We investigated the use of two parameters, the LL concentration and the Leachate Pollution Index (LPI), a method used to determine the phytotoxicity potential of a leachate source based on a weighted sum of its components, to set the best LL dilution to apply when poplar clone 'Orion' and willow clone 'Levante' are selected for phytoremediation. Cuttings were watered with five LL concentrations ranging from 0 to 100%. The poplar clone showed significantly higher values than the willow clone for lowest effective concentration index (LOEC) for leaf (i.e. 11.3% vs 10.5%; p = 0.0284) and total biomass (i.e. 10.9% vs 10.6%; p = 0.0402) and for lowest effective LPI for leaf (i.e. 12.3 vs 12.1; p = 0.0359) and total biomass (i.e. 12.8 versus 12.2; p = 0.0365), respectively, with effectiveness demonstrating the LOEC or LPI value at which the parameter is negatively affected. Photosynthetic rates were higher in poplar than willow in both control and the lowest LL dilution, but rapidly declined in both at higher LL dilutions. Although a direct translation of data from bench trials to field conditions should be investigated, we concluded that in the establishment phase, the poplar hybrid is more tolerant than the willow hybrid to LL. We also provide evidence for LPI as a potential predictor for setting LL irrigation levels in the initial phase of a phyto-treatment approach.

Willow and poplar for the phyto-treatment of landfill leachate in Mediterranean climate.

Phytotechnological approaches using living plants are currently being proposed to address a wide range of environmental purposes including the treatment of landfill leachate (LL). Despite their popularity, few studies have investigated this possibility under actual Mediterranean conditions using fast-growing trees. This research reports the results of a two-year project where poplar and willow grown in mesocosm were tested for their ability to withstand and remove specific pollutants from different [Low: 7% (1st year) and 15% (2nd year); High: 15% (1st year) and 30% (2nd year)] amounts of LL. Results indicate that both species were able to treat 340 (Low) and 680 (High) m3 ha-1 in the establishment year (70 days) and 2470 (Low) and 4950 (High) m3 ha-1 in the second year (150 days). Both species yielded the same aboveground biomass, but under high LL treatment, poplar performed better than willow. Poplar showed on average significantly higher extraction rates for Cd, Cu, P, and N than willow. Moreover, under high LL treatment, poplar also seemed more efficient than willow in decreasing the concentration of specific pollutants (BOD5, COD and As) in output effluent. However, with low LL loads both species were able to significantly reduce other compounds (i.e. NH4-N, Cu and Ni). By contrast, Cl, surfactants, and NO3-N, had a tendency to accumulate over time in the effluent and could still represent an actual constraint to large-scale application of the technique. The fate of such pollutants should be investigated with further research to better inform strategies used to manage low amounts of high-concentrated effluent.

Smelling the metal: Volatile organic compound emission under Zn excess in the mint Tetradenia riparia.

This work investigated the effect of Zn excess on growth, metal accumulation and photosynthetic changes in Tetradenia riparia, in relation to possible variations in the composition of the plant volatilome. Experiments were carried out in hydroponics exposing plants to a range of Zn concentrations. Zinc excess negatively affected plant growth in a dose-dependent manner. The metal was accumulated proportionally to its concentration in the medium and preferentially allocated to roots. All the photosynthetic parameters and the concentration of some photosynthetic pigments were negatively affected by Zn, whereas the level of leaf total soluble sugars remained unchanged. Twenty-three different VOCs were identified in the plant volatilome. Each compound was emitted at a different level and intensity of emission was manifold increased by the presence of Zn in the growth medium. The Zn-induced compounds could represent both an adaptive response (f.i. methanol, acetylene, C6-aldehydes, isoprene, terpenes) and a damage by-product (f.i. propanal, acetaldehyde, alkyl fragments) of the metal presence in the culture medium. Given that the Zn-mediated induction of those VOCs, considered protective, occurred even under a Zn-limited photosynthetic capacity, our work supports the hypothesis of an active role of such molecules in an adaptive plant response to trace metal stress.

Meta-transcriptomics indicates biotic cross-tolerance in willow trees cultivated on petroleum hydrocarbon contaminated soil.

BACKGROUND: High concentrations of petroleum hydrocarbon (PHC) pollution can be hazardous to human health and leave soils incapable of supporting agricultural crops. A cheap solution, which can help restore biodiversity and bring land back to productivity, is cultivation of high biomass yielding willow trees. However, the genetic mechanisms which allow these fast-growing trees to tolerate PHCs are as yet unclear. METHODS: Salix purpurea 'Fish Creek' trees were pot-grown in soil from a former petroleum refinery, either lacking or enriched with C10-C50 PHCs. De novo assembled transcriptomes were compared between tree organs and impartially annotated without a priori constraint to any organism. RESULTS: Over 45% of differentially expressed genes originated from foreign organisms, the majority from the two-spotted spidermite, Tetranychus urticae. Over 99% of T. urticae transcripts were differentially expressed with greater abundance in non-contaminated trees. Plant transcripts involved in the polypropanoid pathway, including phenylalanine ammonia-lyase (PAL), had greater expression in contaminated trees whereas most resistance genes showed higher expression in non-contaminated trees. CONCLUSIONS: The impartial approach to annotation of the de novo transcriptomes, allowing for the possibility for multiple species identification, was essential for interpretation of the crop's response treatment. The meta-transcriptomic pattern of expression suggests a cross-tolerance mechanism whereby abiotic stress resistance systems provide improved biotic resistance. These findings highlight a valuable but complex biotic and abiotic stress response to real-world, multidimensional contamination which could, in part, help explain why crops such as willow can produce uniquely high biomass yields on challenging marginal land.