© 2007 Heron Publishing—Victoria, Canada
Leaf responsiveness of Populus tremula and Salix viminalis to soil contaminated with heavy metals and acidic rainwater
Sandra Hermle (1), Pierre Vollenweider (1), Madeleine S. Günthardt-Goerg (1, 2), Carolyn J. McQuattie (3) and Rainer Matyssek (4)
1. Swiss Federal Institute for Forest, Snow and Landscape Research, WSL, Zürcherstrasse 111, CH-8903, Birmensdorf, Switzerland / 2. Corresponding author (madeleine.goerg@wsl.ch) / 3. USDA Forest Service Laboratory, 359 Main Road, Delaware, OH 43015, USA / 4. Ecophysiology of Plants, Technische Universität München, Am Hochanger 13, D-85354 Freising-Weihenstephan, Germany / Received July 21, 2006; accepted November 15, 2006; published online August 1, 2007
Summary
Fast-growing trees such as Salix viminalis L. and Populus tremula L. are well suited to phytoremediate heavy metal contaminated soils. However, information on tree performance, particularly
leaf function, under conditions of heavy metal contamination is scarce. We used yearly coppiced saplings of S. viminalis and P. tremula growing in model ecosytems to test four hypotheses: (1) heavy metal contamination impairs photosynthesis by injuring leaf
structure; (2) the effects of heavy metal contamination are enhanced by acidified rainwater and low soil pH; (3) heavy metal
contamination increases dark respiration and, thus, repair processes; and (4) heavy metal contamination is tolerated and remediated
better by S. viminalis than by P. tremula.
We investigated heavy metal accumulation, tissue injury and gas exchange in leaves of plants subjected to controlled soil
contamination with heavy metal dust. Additional treatments included acidic and calcareous natural forest subsoils in combination
with irrigation with rainwater at pH 5.5 or 3.5. In both provenances of P. tremula that were studied, but not in S. viminalis, heavy metal treatment reduced photosynthesis and transpiration by varying amounts, except in the hot and dry summer of 2003,
but had no effect on dark respiration. At light saturation, net CO2 uptake and water-use efficiency were reduced by heavy metal contamination, whereas the CO2 concentration in the leaf intercellular air space was increased. Rainwater pH and subsoil pH only slightly modified the effects
of the heavy metal treatment on P. tremula. Gas exchange responses of P. tremula to heavy metals were attributed to leaf structural and ultrastructural changes resulting from hypersensitive-response-like
processes and accelerated mesophyll cell senescence and necroses in the lower epidermis, especially along the transport pathways
of heavy metals in the leaf lamina. Overall, the effects of heavy metals on P. tremula corroborated Hypothesis 1, but refuted Hypotheses 2 and 3, and were inconclusive for Hypothesis 4. Both P. tremula and S. viminalis showed appreciable potential for storing heavy metals in aging foliage.
Keywords:
accelerated cell senescence, cadmium, chloroplasts, CO2 assimilation rate, dark respiration rate, histological changes, hypersensitive
response-like reaction, stomatal conductance, visible injury, water-use efficiency, zinc
.
