Alamo Switchgrass Research Articles

Sodium hydroxide pretreatment of genetically modified switchgrass for improved enzymatic release of sugars

Overcoming biomass recalcitrance to bioconversion is crucial for cellulosic biofuels commercialization. In this study, Alamo switchgrass (Panicum virgatum L.) was genetically transformed to suppress the expression of 4-coumarate-CoA ligase (4CL). The transgenic plants were determined to have lignin content reductions of up to 5.8%. The ratios of acid soluble lignin (ASL) to acid insoluble lignin (AIL) and syringyl/guaiacyl (S/G) in transgenic plants were 21.4–64.3% and 11.8–164.5%, respectively, higher than those of conventional biomass. Both conventional and transgenic plants were pretreated with 0.5%, 1%, and 2% (w/v) NaOH for 15, 30, and 60min at 121°C, followed by enzymatic hydrolysis with commercial cellulases and xylanases. At the optimal conditions, the glucan and xylan conversion efficiency in the best transgenic plants were 16% and 18% higher than the conventional plant, respectively. The results show that down-regulation of 4CL gene promoted enzymatic hydrolysis of plant cell walls following a mild alkali pretreatment.

Dilute sulfuric acid pretreatment of transgenic switchgrass for sugar production

Conventional Alamo switchgrass and its transgenic counterparts with reduced/modified lignin were subjected to dilute sulfuric acid pretreatment for improved sugar production. At 150°C, the effects of acid concentration (0.75%, 1%, 1.25%) and residence time (5, 10, 20, 30min) on sugar productions in pretreatment and enzymatic hydrolysis were investigated, with the optimal pretreatment conditions determined for each switchgrass genotype based on total sugar yield and the amounts of sugar degradation products generated during the pretreatment. The results show that genetic engineering, although did not cause an appreciable lignin reduction, resulted in a substantial increase in the ratio of acid soluble lignin:acid insoluble lignin, which led to considerably increased sugar productions in both pretreatment and enzymatic hydrolysis. At an elevated threshold concentration of combined 5-hydroxyfuranmethal and furfural (2.0g/L), the overall carbohydrate conversions of conventional switchgrass and its transgenic counterparts, 10/9-40 and 11/5-47, reached 75.9%, 82.6%, and 82.2%, respectively.

Biomass Characterization of Morphological Portions of Alamo Switchgrass

Comparative studies between the leaf and internode portions of switchgrass, Panicum virgatum L., were performed by compositional analysis and structural determination. GC-MS, ICP, and HPAEC-PAD were employed to analyze the chemical compositions of the fractionated switchgrass samples. Quantitative (13)C NMR and CP/MAS (13)C NMR techniques were employed to determine the structures of lignin and cellulose, respectively. These results indicated that the leaves and internodes differed chemically in the amounts of inorganic elements, hot-water extractives, benzene/ethanol extractives, carbohydrates, and lignin content. However, the ultrastructure of isolated cellulose was comparable between leaves and internodes. Ball-milled lignins isolated from leaves and internodes were found to have H/G/S ratios of 12.4/53.9/33.7 and 8.6/54.8/36.6, respectively.

Oxidative Lime Pretreatment of Alamo Switchgrass

Previous studies have shown that oxidative lime pretreatment is an effective delignification method that improves the enzymatic digestibility of many biomass feedstocks. The purpose of this work is to determine the recommended oxidative lime pretreatment conditions (reaction temperature, time, pressure, and lime loading) for Alamo switchgrass (Panicum virgatum). Enzymatic hydrolysis of glucan and xylan was used to determine the performance of the 52 studied pretreatment conditions. The recommended condition (110°C, 6.89 bar O(2), 240 min, 0.248 g Ca(OH)(2)/g biomass) achieved glucan and xylan overall yields (grams of sugar hydrolyzed/100 g sugar in raw biomass, 15 filter paper units (FPU)/g raw glucan) of 85.9 and 52.2, respectively. In addition, some glucan oligomers (2.6 g glucan recovered/100 g glucan in raw biomass) and significant levels of xylan oligomers (26.0 g xylan recovered/100 g xylan in raw biomass) were recovered from the pretreatment liquor. Combining a decrystallization technique (ball milling) with oxidative lime pretreatment further improved the overall glucan yield to 90.0 (7 FPU/g raw glucan).

Comparative studies on hydrothermal pretreatment and enzymatic saccharification of leaves and internodes of alamo switchgrass

Hydrothermal pretreatment was performed on the leaves and internodes portions of Alamo switchgrass, Panicum virgatum L., to enhance the digestibility of cellulose towards cellulase. It was observed that extractives free leaves portion provided 18.1% lower pretreatment gravimetrical yield and 33.8% greater cellulose-to-glucose yield than internodes portion. The degree of polymerization (DP) and ultrastructure of cellulose were determined by gel-permeation chromatography and solid-state cross polarization/magic angle spinning 13C NMR experiments. The results suggested that hydrothermal pretreatment hydrolyzed amorphous cellulose and yielded a product enriched in paracrystalline cellulose. Furthermore, the DP of cellulose was reduced to one third of the origin value after hydrothermal pretreatment. The resulting biomass after pretreatment for leaves and internodes has similar cellulose ultrastructure and chemical profiles. The results of the enzymatic hydrolysis studies of cellulose suggest that the reduced DP of cellulose of pretreated switchgrass was an important factor influencing the enhanced digestibility of pretreated switchgrass.

Clash of the Titans: Comparing Productivity Via Radiation Use Efficiency for Two Grass Giants of the Biofuel Field

The comparative productivity of switchgrass (Panicum virgatum L.) and Miscanthus (Miscanthus × giganteus) is of critical importance to the biofuel industry. The radiation use efficiency (RUE), when derived in an environment with non-limiting soil water and soil nutrients, provides one metric of relative productivity. The objective of this study was to compare giant Miscanthus to available switchgrass cultivars, using established methods to calculate RUE of the two species at two disparate sites. Measurements of fraction intercepted photosynthetically active radiation (PAR) and dry matter were taken on plots at Elsberry, MO (Miscanthus and the switchgrass cultivars Alamo, Kanlow, and Cave-in-Rock) and at Gustine, TX (Miscanthus and Alamo switchgrass, irrigated with dairy wastewater and a non-irrigated control). In MO, Miscanthus mean RUE (3.71) was less than Alamo switchgrass mean RUE (4.30). In TX under irrigation, Miscanthus mean RUE was 2.24 and Alamo switchgrass mean RUE was 3.20. In MO, the more northern lowland switchgrass cultivar, Kanlow, showed similar mean RUE (3.70) as Miscanthus. In MO, the northern upland cultivar Cave-in-Rock had a mean RUE (3.17) that was only 85% of that for Miscanthus at MO. Stress (water and nutrients) had a greater effect on Miscanthus RUE than on switchgrass RUE in TX. These results provide realistic RUE values for simulating these important biofuel grasses in diverse environmental conditions.

First Report of Bipolaris oryzae Causing Leaf Spot of Switchgrass in Mississippi.

'Alamo' switchgrass (Panicum virgatum L.) seedlings growing in a soilless mix exhibiting dark brown, irregular-shaped foliar lesions with black borders were submitted to the Mississippi State University Plant Disease Diagnostic Laboratory in the summer of 2009 from a local forest products company. Symptomatic tissues were plated onto water agar (WA) and incubated for 21 days on a laboratory bench top with a 12-h photoperiod at 22°C. An asexual, sporulating, dematiaceous hyphomycete identified as Bipolaris oryzae (Breda de Haan) Shoemaker was observed. Conidiophores were single, mostly straight, multiseptate, brown, and ranging from 138 to 306 × 7.7 to 15.3 μm and averaged 221.6 × 10.7 μm. Conidia were golden brown, multiseptate, ranging from 3 to 10 septa, straight to slightly curved to fusoid, wider midway, and tapering toward the terminal cells. Conidia measured 40.8 to 109.7 × 10.2 to 20.4 μm and averaged 75.8 × 13.8 μm. Morphological characteristics of B. oryzae were similar to those described by Drechsler (1) and Sivanesan (3). The internal transcribed spacer (ITS) region of ribosomal DNA from four pure culture colonies derived from single conidia was amplified by PCR using ITS1 and ITS4 primers. The resultant 572 bp was sequenced for isolates 86 through 89 (GenBank Accession Nos. GU222690-93). The sequences were 99% similar to the sequence of B. oryzae strain ATCC-MYA 3330 (GenBank No. FJ746665) isolated from P. virgatum. Pathogenicity of isolates 86 and 88 was confirmed by inoculating sterile potting mix with a fungal slurry. Sterile Alamo switchgrass seeds were sown into the infested soil in Magenta boxes and incubated for 6 weeks in a growth chamber with a 14-h photoperiod at 30°C. Leaf lesions and leaf blight were observed in seedling stands growing in B. oryzae-infested soil. Lesions were excised and plated onto WA. Sporulation of B. oryzae was observed on symptomatic tissue. In the interim, 300 nonsterilized Alamo switchgrass seeds of the same seed lot as the original symptomatic seedlings and originating from Oklahoma were plated onto WA (10 seed per plate). The seeds were incubated on the bench top as previously described. The experiment was repeated and B. oryzae colonized 1.4% of the total switchgrass seeds evaluated, indicating seed transmission and subsequent seedling infection as previously observed in the original seedlings. Leaf spot, caused by B. oryzae, was first reported as a new disease of switchgrass in North Dakota (2). In the summer of 2009, the authors observed leaf spot in four cultivars of switchgrass, including Alamo, growing in research plots in Webster County, MS. Twenty-two isolates of B. oryzae were recovered from diseased leaves of these switchgrass cultivars. To our knowledge, this is the first report of B. oryzae causing leaf spot of switchgrass in Mississippi, which broadens the natural distribution of this disease.

Structural Characterization and Comparison of Switchgrass Ball-milled Lignin Before and After Dilute Acid Pretreatment

To reduce the recalcitrance and enhance enzymatic activity, dilute H(2)SO(4) pretreatment was carried out on Alamo switchgrass (Panicum virgatum). Ball-milled lignin was isolated from switchgrass before and after pretreatment. Its structure was characterized by (13)C, HSQC, and (31)P NMR spectroscopy. It was confirmed that ball-milled switchgrass lignin is of HGS type with a considerable amount of p-coumarate and felurate esters of lignin. The major ball-milled lignin interunit was the beta-O-4 linkage, and a minor amount of phenylcoumarin, resinol, and spirodienone units were also present. As a result of the acid pretreatment, there was 36% decrease of beta-O-4 linkage observed. In addition to these changes, the S/G ratio decreases from 0.80 to 0.53.

Nutrient Uptake by ‘Alamo’ Switchgrass Used as an Energy Crop

Field studies were conducted in 2000 and 2001 to examine yields and nutrient removal by Alamo switchgrass (Panicum virgatum L.) grown at eight locations within five states in the upper southeastern USA. Plots, which had been established for >5 years as part of a larger study, were cut either once (late fall) or twice (midsummer and late fall). Plots cut once received 50 kg N per hectare per year, while twice-cut plots received 100 kg N per hectare per year. Nutrient concentrations of and nutrient removal by harvested biomass were determined. Partitioning of nutrients into leaf and stem fractions was determined at the time of the midsummer harvest in 2000. Biomass production during 2000 and 2001 averaged 15.9 Mg/ha per year across all sites and was as high as 21.7 Mg/ha per year at one site. Two cuttings plus the additional 50 kg N per hectare did not generally increase seasonal yields; and, in one quite productive location, that management caused a yield reduction. Nitrogen removal with two cuts was much higher than with a single cut due largely to the higher N content in the midsummer harvest. Over the 2 years, twice as much N was removed with the two annual cuts as with one cut. Nitrogen removal exceeded the amounts of N applied in both managements, suggesting N was being supplied via mineralization or other processes. Phosphorus removal also increased significantly with the two-cut management. Seasonal K and Ca removals were more similar between the two managements. Nitrogen and P concentrations generally declined basipetally in tillers, with older leaves and internodes having lower concentrations of both nutrients. Potassium was more uniformly distributed than N throughout the tiller components (leaf and stem). Calcium was higher in older leaf blades. Levels of soil P, K, and Ca at most locations appeared not to be limiting biomass production and were adequate for long-term productivity.

Afternoon harvest increases readily fermentable carbohydrate concentration and voluntary intake of gamagrass and switchgrass baleage by beef steers12

Our objective was to determine if harvest in the morning (AM, 0600) vs. the afternoon (PM, 1800) affects composition and voluntary DMI of gamagrass (GG) or switchgrass (SG) stored as baleage. Iuka GG (Tripsacum dactyloides L.) and Alamo SG (Panicum virgatum L.) were cut with a mower-conditioner, immediately round-baled, wrapped in plastic, and stored as baleage. Beef steers (255 +/- 7 kg of BW) were assigned (5 steers/treatment) to GG/AM, GG/PM, SG/AM, or SG/PM. Ad libitum intake was measured for 21 d (7-d adjustment and 14-d intake estimate) followed by 7-d adjustment and 5-d digestion and N balance study. Chewing behavior was recorded during the balance study. Compared with AM, PM had more (P < 0.01) starch (9.3 vs. 4.7 g/kg of DM), total nonstructural carbohydrate (30.4 vs. 19.0 g/kg of DM), and monosaccharides (17.1 vs. 11.2 g/kg of DM). Compared with AM, PM had less (P = 0.05) acetate (13.0 vs. 18.6 g/kg of DM) and propionate (0.29 vs. 0.82 g/kg of DM) and tended (P < 0.13) to have less lactate (2.9 vs. 3.5 g/kg of DM) and butyrate (3.9 vs. 5.1 g/kg of DM). Compared with SG, GG had more (P = 0.01) DM (324 vs. 242 g/kg of baleage), CP (114 vs. 97 g/kg of DM), lactate (4.8 vs. 1.6 g/kg of DM), starch (9.4 vs. 4.7 g/kg of DM), total nonstructural carbohydrate (34.2 vs. 15.2 g/kg of DM), and monosaccharides (20.8 vs. 7.4 g/kg of DM). However, GG had a lower (P = 0.01) pH (5.32 vs.5.79) and less (P < 0.01) ethanol (18.7 vs. 27.3 g/kg of DM), acetate (12.3 vs. 19.2 g/kg of DM), propionate (0.00 vs. 1.11 g/kg of DM), and butyrate (0.6 vs. 8.4 g/kg of DM). Daily DMI (2.16 vs. 1.83% of BW) and digestible DMI (1.15 vs. 0.95% of BW) were greater (P = 0.03) for PM than AM. Plasma urea N concentrations at the end of the ad libitum intake phase were greater (P = 0.01) for AM (3.91 mM) than for PM (2.31 mM) and greater (P = 0.07) for GG (3.51 mM) than for SG (2.71 mM). Steers fed PM spent more time eating (P = 0.04) and less time resting (P = 0.01) during meals than steers fed AM. Apparent digestibility of DM and fiber components was not affected (P < 0.18) by treatment. Apparent digestibility and retention of N decreased from PM to AM for SG, but increased for GG (P = 0.05). Retention of N as a percentage of N intake or N digested decreased more from PM to AM for SG than for GG (P < 0.05). We conclude that increased nonstructural carbohydrate content of the PM harvest of these grasses stored as baleage caused increased voluntary intake and improved use of dietary N by beef steers.

Switchgrass simulation by the ALMANAC model at diverse sites in the southern US

Simulation models for plant species important for biofuel such as switchgrass (Panicum virgatum L.) can be used to make management decisions related to biomass productivity and related to environmental impacts such as soil erosion and changes in surface and groundwater quality. The present study was designed to evaluate the accuracy of simulation of switchgrass biomass production by the ALMANAC (Agricultural Land Management Alternatives with Numerical Assessment Criteria) model at sites in Texas, Arkansas, and Louisiana. We used multi-year field data for Alamo switchgrass yields at each of five sites to evaluate ALMANAC. The model realistically simulated mean switchgrass yields at each of the locations and the total variability of all the data pooled, but did not perform as well in accounting for the year-to-year variability within some locations. Sensitivity analysis showed that changes in runoff curve number (CN) and changes in maximum stomatal conductance (GSI) had variable impacts on simulated values among the sites. A 15% change in CN changed mean annual biomass yield from 0% to 16% depending on location. Changing GSI from 4 to 8 mm s−1 changed mean annual biomass from 1% to 31% depending on location. ALMANAC shows promise as a tool to realistically simulate mean biomass yields and variability around the mean for multi-year runs of switchgrass at these diverse sites. Further research, with more extensive measurements of soil parameters including soil nutrients is needed to determine why the model reasonably simulated individual years’ yields at Stephenville, TX and Dallas, TX, but had difficulty at other sites.

Genetic variation and heritability of phosphorus uptake in Alamo switchgrass grown in high phosphorus soils

Developing crop cultivars with high P uptake may prevent the accumulation of phosphorus in the soil and its escape to streams and water reservoirs. The purpose of this investigation was to assess the potential for P uptake in switchgrass ( Panicum virgatum L.), an important bioenergy and forage crop, and to determine the nature of genetic variation and heritability of this trait. Thirty genotypes from a population of ‘Alamo’ switchgrass were evaluated in the greenhouse [Tifton loamy sand (fine, loamy, siliceous thermic Plinthic Paludults)] and the field [Wedowee coarse sandy loam (fine, kaolinitic, thermic family of the Typic Kanhapludults)] at a rate of 450 mg P kg −1 and 200 mg N kg −1 soil. Significant genetic variation was observed among the parental genotypes for P concentration, biomass production, and P uptake. Genotype × location interaction was significant for all three traits. Nevertheless, rank correlation of the genotypes between the two locations was high for P uptake ( r = 0.83, P < 0.01) indicating that little of the genotype × location interaction is associated with changes in ranking. Narrow sense heritability based on individual plant and family means were 2 and 13% for P concentration, 60 and 69 for biomass production, and 67 and 73 for P uptake, respectively. Expected genetic gain based on individual plant selection were <1, 51, and 65% of the parental means for P concentration, biomass production, and P uptake, respectively. Expected gain from selection based on half-sib progeny testing was slightly higher (2, 55, and 68% for P concentration, biomass, and P uptake). Although little gain from selection is predicted for P concentration, the magnitude of heritability values for biomass production and P uptake indicate that substantial genetic progress in increasing P uptake can be achieved through breeding for higher biomass production.

Soil Type and Moisture Level Influence on Alamo Switchgrass Emergence and Seedling Growth

Attempts to establish lowland types of switchgrass (Panicum virgatum L.) in the Lower South have frequently been unsuccessful. A greenhouse study was conducted to determine the influence of soil texture and moisture on ‘Alamo’ switchgrass emergence and seedling growth. Soils used were a very fine sandy loam, loamy fine sand, silt loam, and clay that were watered every 3 to 4, 7, 10 to 11, or 14 d. Emergence was recorded daily for the first 28 d and seedlings were harvested at 6 wk to determine survival and seedling traits. The study was initiated on 30 March 2001 and repeated on 29 May and 24 July. Run date caused significant (P ≤ 0.001) interactions for all measurements with soil series and watering interval because of increasing greenhouse temperatures with each succeeding run. Watering interval affected emergence only at the 24 July run with emergence decreasing as watering interval increased. However, watering interval of at least 7 d was necessary for greater than 90% seedling survival in all soils. Only the clay soil consistently had good seedling survival (≥94%) at all watering intervals. Shoot stage and weight and root stage and weight decreased as watering interval increased. These seedling traits were usually higher in the loamy fine sand and very fine sandy loam if watered at least every 10 d. The trend for root/shoot ratios was to increase as moisture became limiting because shoot weight decreased faster than root weight. Rainfall occurring at greater than 10 d intervals is one of the factors for unreliable switchgrass establishment on sandy soils in the Lower South.

Adaptation of Tall-Grass Prairie Cultivars to West Louisiana

Big bluestem (Andropogon gerardii Vitman) indiangrass Sorghastrum nutans (L.) Nash] and switchgrass (Panicum virgatum L.) are widely recognized as valuable forage and conservation species in the Great Plains and the upper South. These species occurred in natural fire-maintained longleaf pine (Pinus palustris Mill.) woodland ecosystems from southeast Texas eastward. Recent interest and even recommendations of cultivars of these species have been developed in the Louisiana longleaf pine area. Genotype origin of these species is known to affect adaptation. Adaptation of the cultivars Kaw big bluestem, Lometa indiangrass, Osage indiangrass, Blackwell switchgrass, and Alamo switchgrass to the Louisiana Coastal Plain was assessed under a natural stand of longleaf pines and in full sun at Rosepine, La. Plots were seeded in 1995, and stands were monitored through the spring of 1998. Only Alamo switchgrass and Lometa indiangrass under the pines produced sufficient stands for harvest. Production potential was greatest for Alamo switchgrass with the highest annual yield of 5,580 kg ha-1 from 2 harvests in 1996. Herbage production of 1,500 kg ha-1 did not differ between Alamo switchgrass and Lometa indiangrass in the second year of harvest when harvested in June and September for 2 consecutive years. The longleaf pine overstory apparently provided a competitive advantage during establishment, since weeds dominated plots in full sun. The 2 cultivars from southern latitudes, Alamo switchgrass and Lometa indiangrass, were superior in establishment, productivity, and stand survival. Aggressive competition from dense-growing subtropical grasses was detrimental during establishment and following defoliations. DOI:10.2458/azu_jrm_v53i1_pitman

Adaptation of tall-grass prairie cultivars to West Louisiana

Big bluestem (Andropogon gerardii Vitman) indiangrass Sorghastrum nutans (L.) Nash] and switchgrass (Panicum virgatum L.) are widely recognized as valuable forage and conservation species in the Great Plains and the upper South. These species occurred in natural fire-maintained longleaf pine (Pinus palustris Mill.) woodland ecosystems from southeast Texas eastward. Recent interest and even recommendations of cultivars of these species have been developed in the Louisiana longleaf pine area. Genotype origin of these species is known to affect adaptation. Adaptation of the cultivars Kaw big bluestem, Lometa indiangrass, Osage indiangrass, Blackwell switchgrass, and Alamo switchgrass to the Louisiana Coastal Plain was assessed under a natural stand of longleaf pines and in full sun at Rosepine, La. Plots were seeded in 1995, and stands were monitored through the spring of 1998. Only Alamo switchgrass and Lometa indiangrass under the pines produced sufficient stands for harvest. Production potential was greatest for Alamo switchgrass with the highest annual yield of 5,580 kg ha-1 from 2 harvests in 1996. Herbage production of 1,500 kg ha-1 did not differ between Alamo switchgrass and Lometa indiangrass in the second year of harvest when harvested in June and September for 2 consecutive years. The longleaf pine overstory apparently provided a competitive advantage during establishment, since weeds dominated plots in full sun. The 2 cultivars from southern latitudes, Alamo switchgrass and Lometa indiangrass, were superior in establishment, productivity, and stand survival. Aggressive competition from dense-growing subtropical grasses was detrimental during establishment and following defoliations.

Selection Response and Developmental Basis for Early and Late Panicle Emergence in Alamo Switchgrass

For many determinate crop species, delayed flowering, associated with the production of more mainstem leaves and/or a reduced rate of leaf appearance, extends the vegetative phase and results in higher biomass yields. Studies were carried out on 'Alamo' switchgrass (Panicum virgatum L.), a potential biofuel crop, to determine the realized heritability and developmental basis for variation in the time to panicle emergence. A single cycle of divergent selection for time of panicle emergence (earliest and latest 3%) was carried out. Time of panicle emergence for the parent and progeny generation was evaluated in the field for 2 yr and 1 yr in a greenhouse trial. For the parental generation, leaf emergence was monitored weekly and final leaf numbers were recorded. Panicle emergence for the selected early and late parents differed by an average of 22 d (10 d earlier and 12 d later than the original population). Realized heritabilities in the year following establishment were 1.0 and 0.92 for early and late panicle emergence, respectively. The early and late parent plants initiated growth at the same time in spring and produced leaves at the same rate; however, late plants produced from 0.7 to 2.1 more mainstem leaves than early plants. Similar leaf appearance rates, combined with higher final leaf numbers on late than on early plants, strongly suggested that late panicle emergence resulted from delayed floral initiation. This study showed that there is considerable genetic variation for flowering time in Alamo switchgrass and that the extended period of vegetative growth in late flowering types was associated with the production of more leaves.

Potential remediation of 137Cs and 90Sr contaminated soil by accumulation in Alamo switchgrass

Cesium-137 (137Cs) and Strontium-90 (90Sr) are radionuclides characteristic of nuclear fallout from nuclear weapons testing and nuclear reactor accidents. Alamo switchgrass (Panicum virginatum L.) is a perennial C4 species native to central North America that produces exceptionally high biomass yields in short periods of time. In three separate experiments, Alamo switchgrass plants were tested for their ability to accumulate 137 Cs and90 Sr from a contaminated growth medium. Plants in experiment 1 were grown in 33 × 20 × 7 cm plastic pans containing 2.5 kg sand. Plants in experiments 2 and 3 were grown in 30 × 3 cm diameter test tubes containing 0.3 kg growth medium. After 3 months of plant growth, either 102 Bq 137Cs or 73 Bq90 Sr g−1 soil were added to the growth medium. Plants in all three experiments were grown within a greenhouse that was maintained at 22 ± 2 °C with a photosynthetic active radiation of 400–700 µmol m−2 s−1 and a 14–16 h photoperiod. Above-ground plant biomass did not differ between plants that were not exposed to these radionuclides (controls) and those that were exposed to growth medium containing 137Cs or90 Sr over the course of the experiment. Plants accumulated 44 and 36% of the total amount of 90Sr and137 Cs added to growth medium after the first 5 harvests. After the first two harvests, the concentration of 137Cs and90 Sr in plant tissue and the amount of 137Cs or90 Sr removed from growth medium declined with each successive harvest. Duration of exposure correlated curvilinearly with accumulation of both 90Sr and 137Cs by plants (r2 = 0.95 and 0.78, respectively). As concentration of both 137Cs and 90Sr in growth medium increased, plant accumulation of both radionuclides increased and correlated curvilinearly in seedlings (r2 = 0.83 and 0.89 respectively).

Simulating Alamo Switchgrass with the ALMANAC Model

AbstractA model for forage yield with adequate details for leaf area, biomass, nutrients, and hydrology would be valuable for making management decisions. The objectives of this study were to develop Alamo switchgrass (Panicum virgatum L.) parameters for the Agricultural Land Management Alternatives with Numerical Assessment Criteria (ALMANAC) model and demonstrate its accuracy across a wide range of environments. Derived plant parameters included potential leaf area index (LAI), potential biomass growth per unit intercepted light, optimum nutrient concentrations, and growth responses to temperature. The model's simulated yields accounted for 79% of the variability in measured yields for one‐cut and two‐cut harvest systems from six diverse sites in Texas in 1993 and 1994. Simulated yields for three locations differed in sensitivity to potential LAI, heat units to maturity, radiation use efficiency (RUE), and soil depth. The ALMANAC model shows promise as a management tool for this important forage and bioenergy crop.

CONTROL OF STREAMBANK EROSION DUE TO BED DEGRADATION WITH VEGETATION AND STRUCTURE1

ABSTRACT: Combinations of vegetation and structure were applied to control streambank erosion along incised stream channels in northwest Mississippi. Eleven sites along seven channels with contributing drainage areas ranging from 12–300 km2 were used for testing. Tested configurations included eroding banks protected by vegetation alone, vegetation with structural toe protection, vegetation planted on re‐graded banks, and vegetation planted on regraded banks with toe protection. Monitoring continued for up to 10 years, and casual observation for up to 18 years. Sixteen woody and 13 nonwoody species were tested. Native woody species, particularly willow, appear to be best adapted to stream‐bank environments. Sericea lespedeza and Alamo switchgrass were the best nonwoody species tested. Vegetation succeeded in reaches where the bed was not degrading, competition from kudzu was absent, and bank slopes were stabilized by grading or toe protection. Natural vegetation invaded planted and unplanted stable banks composed of fertile soils. Designs involving riprap toe protection in the form of a longitudinal dike and woody vegetation appeared to be most cost‐effective. The exotic vine kudzu presents perhaps the greatest long‐term obstacle to restoring stable, functional riparian zones along incised channels in our region. (KEY TERMS: vegetation; streambank protection; bioengineering; stream restoration; channel incision; riparian zone.)

Shoot Growth and Development of Alamo Switchgrass as Influenced by Mowing and Fertilization

The response of shoot development and forage yield of a 2-year old 'Alamo' switchgrass stand to mowing and fertilization was evaluated to provide information needed for effective management of this variety. Mowing to a 20-cm stubble height in mid-spring removed only a few apical me&ems and had little impact on shoot development. Late spring and early summer mowing were done when apical meristems of primary compound shoots were elevated to near the 20-cm cutting height in May and over 20 cm in June. Secondary nonrooted shoot and aerial shoot numbers were increased and plant vigor, measured by spring growth in 1980, was decreased slightly the following spring. Mowing in mid-summer removed apical meristems from essentially all primary compound shoots and many secondary compound shoots. Regrowth was slight during the remainder of the summer, but the number of secondary and tertiary nonroqted shoots and aerial shoots in- creased. The number of proaxis buds decreased, and plant vigor was severely decreased the following spring. Mowing twice includ- ing early fall, removed apical me&ems from secondary com- pound shoots and some primary and secondary nonrooted shoots. Numbers of secondary, tertiary, and quarternary nonrooted shoots increased, but proaxis bud numbers were reduced. Plant vigor was very low the following spring, possibly due to exposure of mowed plants to cold winter temperatures. Fertilization increas- ed the rate of development of compound and nonrooted shoots, the number of secondary and compound shoots in spring, the number of proaxis buds in fall and the weight of primary and secondary compound shoots. Fertilized stands mowed during summer and early fall were more productive than all other mowed stands. Fertilized plants mowed in mid-summer were vigorous and productive the following spring. However, fertilization did not overcome the loss of vigor caused by fall mowing.