Physiological Responses of Switchgrass (Panicum virgatum L.) to Organic and Inorganic Amended Heavy-Metal Contaminated Chat Tailings
A. L. Youngman
Department of Biological Sciences, Wichita State University, Wichita, KS 67260-0026,Phone: (316) 978-3111, FAX: (316) 978-3772
Study plots established at the Galena subsite of the Cherokee County Superfund Site in Southeastern Kansas by the U.S. Bureau of Mines in 1990 were examined during the summer of 1996 to determine whether physiological criteria could be used to determine suitability of switchgrass for remediation of heavy-metal contaminated substrates. Switchgrass was chosen because it was the most frequently encountered species on these plots. Treatment plots included a treatment control, an organic residue treatment of 89.6 Mg Ha-1 composted cattle manure, and two inorganic fertilizer treatments recommended for either native grass or grass/legume mixtures. Plant response variables were photosynthetic rate, leaf conductance to water vapor, internal concentration of carbon dioxide in leaves, foliar transpiration rate, leaf water-use-efficiency, predawn leaf xylem water potential, and midday leaf xylem water potential. Predawn and midday xylem water potentials were higher for grass/legume inorganic treatment than for the other inorganic treatments. Leaf conductances were lower for organically treated plots than those plots not organically amended and both photosynthesis and transpiration were lower for organically treated plots. Leaf conductances and transpiration were higher for grass/legume treated plots than for plots lacking inorganic treatment. Water-use-efficiency was higher for native grass inorganically treated plots than for other inorganic treatments.
Keywords: photosynthesis, transpiration, xylem water potential
Solid waste produced by the mineral industry accounts for 80% of non-agricultural land-disposed solid waste in the U.S., potentially producing widespread human health and environmental risks (Veith, et al. 1988). Although a number of options have been proposed for restoration of areas impacted by these wastes (Palmer, 1990), phytoremediation may present the most cost-effective means of restoring these areas. A study of mineral lands in Cherokee County in Southeastern Kansas provides an opportunity to examine the establishment of vegetation on chat tailings produced by lead and zinc smelting operations. Comparing chat tailings at this site with a typical soil for the region, Norland, et al . (1990) found that chat tailings had 54% less organic matter and 26% less available phosphorus than did the native soil, suggesting that the addition of organic and inorganic amendment might aid the establishment of vegetation on chat tailings. Norland (1994) determined plant density and coverage data on vegetation he established on plots amended with a variety of organic and inorganic treatments. Among the treatments he found that composted cattle manure applied at the rate of 89.6 Mg ha-1 produced the highest total cover of warm-season and cool-season grasses and legumes, and an inorganic fertilizer treatment recommended for grass legume mixtures produced a higher cover value than a fertilizer recommended for native grass establishment, but not higher than unamended plots.
The purpose of the present study is to provide plant water potential and gas exchange data in addition to the vegetation data provided by Norland. It is anticipated that this study would provide useful information for the selection of candidate species for the establishment of vegetation at these sites.
The study plots were established at Galena in Cherokee County, Kansas, in 1990 on an unremediated portion of a 285 km2 National Priorities List site by Norland (1991). Results presented in this study are based on physiological measurement of plants sampled from a subset of plots included in a larger factorial experiment designed by Norland to investigate organic and inorganic amendment to chat tailings at this site. The subset used in this study consisted of six 2.5 X4.0 m plots comprising all combinations of two levels of organic amendment (none and composted cattle manure at the rate of 89.6 Mg ha-1); and three levels of inorganic amendment: none, a fertilizer recommended for establishing native grasses (22.4 kg ha-1 nitrogen, 67.2 kg ha-1 phosphorus, and 89.6 kg ha-1 potassium), and a fertilizer treatment recommended for the establishment of grass and legume mixtures (44.8 kg ha-1 nitrogen, 112.9 kg ha-1 phosphorus, and 156.8 kg ha-1 potassium) were selected. The plots had been seeded with a mixture of two cool season grasses, four warm season grasses and four leguminous forbs. One of the warm season grasses, Panicum virgatum, was present in all plots and was used in this study. Soil moisture was monitored on selected plots by means of soil psychrometers (Wescor, Inc., Model PCT-55-15) and physiological data were collected after soil moisture had declined over a three-week period with 0.58 cm precipitation in midsummer.
Gas Exchange Measurements
Photosynthesis, transpiration, stomatal conductance, and internal leaf CO2 concentrations were determined by a Portable Photosynthesis System (LI-COR, Inc, Model 6200) operated in closed mode with a one-quarter liter chamber. In each of the six plots, four plants were randomly chosen. For each plant, three replicate sets of gas exchange data were collected for a pair of intact leaves (with a combined mean leaf area of 4.72 cm2) to give a sample size of 12 for each plot. The measurement endpoint was reached for each replicate when a 5 ppm reduction in chamber CO2 concentration had occurred. Water-use-efficiency was determined by dividing the rate of photosynthesis (µmol CO2 m-2 s-1) by the rate of transpiration (mmol H2O m-2 s-1).
Xylem Water Potential Measurements
Predawn and midday xylem water potential were determined by a Plant Water Status Console (Soilmoisture Equipment Corp., Model 3000). In each plot, one leaf for each of six randomly selected plants was excised for the determination of predawn xylem water potential and one leaf from each of six plants was excised for determination of midday xylem water potential. Time between excision and determination was approximately two minutes. Water loss was minimized by enclosing excised leaf blade in a plastic bag containing moist paper towel. Differences between predawn and midday xylem water potential were also determined.
Statistical analysis was conducted using the Statistical Analysis System (SAS Institute, Inc.,1982). All physiological responses were analyzed as a 2 X 3 factorial ANOVA with organic and inorganic treatments as the main factors. In those cases in which a main factor was determined to be significant (probability level of p< or = 0.05), the Fishers LSD test was used for multiple comparisons among treatment levels.
Predawn and midday xylem water potentials (Figure 1a) were not significantly different for manured compared to non-manured treatments. Grass/legume inorganic treatments (Figure 2a) produced higher predawn and midday water potentials than the other inorganic treatments (F=3.96; df=2,30; p=0.0297 and F=9.07; df=2,30; p=0.0008, respectively). Differences between midday and predawn xylem water potentials (MD-PD) were not significant for any of the treatment comparisons.
Leaf conductances were higher for non-manured than for manured treatments (Figure 1b) and the grass/legume inorganic treatment (Figure 2b) had higher conductances than for those that were not amended with inorganic fertilizer (F=6.02; df=1,66; p=0.0168 and F=5.89; df=2,66; p=0.0044, respectively). Rates of photosynthesis (Figure 1c and 2c) were higher for non-manured than manured treatments (F=9.89; df=1,66; p=0.0025) but were not different among inorganic treatments. Internal carbon dioxide concentrations (Figure 1d and 2d) were higher for manured than non-manured treatments (F=6.60; df=1,66; p=0.0124) and higher for grass/legume inorganic treatment than for other inorganic treatments (F=5.35; df=2,66; p=0.0071). Transpiration rates (Figure 1c and 2c) were higher for non-manured than manured treatments (F=4.86; df=1,66; p=0.031) and higher for the grass/legume inorganic treatment than for other inorganic treatments (F=6.13; df=2,66; p=0.0036). There were no differences in water-use-efficiency (Figure 1c) between manured and non-manured treatments, but among inorganic treatments (Figure 2c) the native grass treatment produced a higher water-use-efficiency than the other inorganic treatments (F=6.81; df=2,66; p=0.002).
Discussion and Conclusions
There have been several reviews of the literature on plant responses to heavy metals in plant-soil systems (Turner, 1994; Kabata-Pendias, 1992; Baker, et al, 1990; and Påhlsson, 1989). Most of the work cited in these reviews deals with uptake, transport, and tolerance of plants to heavy-metal contaminated soil with little or no emphasis on water potential or gas exchange responses. Only the review by Påhlsson includes work done on photosynthetic and transpiration responses to specific heavy metals including zinc, lead, and cadmium, which are associated with zinc and lead smelting.
The data provided by this study indicates that the organic amendment does not improve in xylem water potential for switchgrass. Leaf conductances for organically amended plots were lower than for unamended plots, providing a means of reducing water loss and the development of drought stress, but at the cost of reduced rates of photosynthesis as indicated by the data. The higher internal carbon dioxide concentrations reflected the reduced rates of photosynthesis.
Among inorganic amendments, the grass-legume treatment produced higher (less negative) predawn and midday xylem water potentials than other inorganic treatments. This is not consistent with higher total coverage of all species for native grass inorganic treatments reported by Norland (1994). Leaf conductances were higher for grass/legume inorganically treated plots than inorganically untreated plots which translated into higher rates of transpiration but not photosynthesis for grass/legume treated plots. Curiously, internal carbon dioxide concentrations were higher for the grass/legume inorganic treatments though there were no differences in rates of photosynthesis among inorganic treatments. Of all inorganic treatments, fertilizer recommended for native grass establishment provided higher water-use-efficiency than did either of the other inorganic treatments. Higher water-use-efficiency may have accounted for the higher total coverage for native grass inorganically treated plots reported by Norland (1994).
Gas exchange and water potential data in addition to vegetation analysis of test plots can provide additional insight for selection of plant species or varieties suitable for the establishment of vegetation on chat tailings. Choice of soil amendment options can also be facilitated by this information.
The author wishes to acknowledge Mike Norland for permission to use his study plots at Galena, KS. I also wish to acknowledge Karen Brown's assistance with statistical analysis and the interpretation of the data. This study was funded by Kansas EPA EPSCoR: Enhancement of Bioremediation Research in Kansas, R82-1829-010.
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Figure 1 Physiological Responses of Panicum virgatum to organic amendments.
Each bar represents the mean of 36 gas exchange measurements or 18 xylem water
potential determinations. Unlike letters at the top of each group of bars indicate significant
differences (p< or =0.05): a. xylem water potentials; b. leaf conductance; c.
photosynthesis, transpiration, and water use efficiency (WUE); d. internal carbon dioxide
Figure 2 Physiological Responses of Panicum virgatum to inorganic amendments Each bar represents the mean of 24 gas exchange measurements of 12 xylem water potential determinations. Unlike letters at the top of each group of bars indicate significant differences (p< or =0.05): a. xylem water potentials; b. leaf conductance; c. photosynthesis, transpiration, and water use efficiency (WUE); d. internal carbon dioxide concentration.