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Published online 27 October 2005
Published in Crop Sci 45:2465-2472 (2005)
© 2005 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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CROP BREEDING, GENETICS & CYTOLOGY

Hierarchical Analysis of Switchgrass Morphology

Arvid Boea,* and Michael D. Caslerb

a Plant Science Dep., South Dakota State Univ., Brookings, SD 57007
b U.S. Dairy Forage Research Center, Madison, WI 53706

* Corresponding author (arvid.boe{at}sdstate.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Switchgrass (Panicum virgatum L.) has potential as a biomass crop in North America. Our objective was to determine effects of cultivar and location on morphological traits that influence biomass in switchgrass. Six cultivars with origins from 37° N, 88° W (Cave-In-Rock and Shawnee) to 46° N, 100° W (Dacotah) were evaluated in 1-yr-old swards at Bristol and South Shore, SD; in 3-yr-old swards at Brookings, SD, and Arlington, WI; and in 15-yr-old swards at Pierre, SD, for biomass; tillers m–2; reproductive tiller proportions by count and weight; weight tiller–1; phytomers tiller–1; leaf, stem, and inflorescence components of tiller weight; and sheath and stem components of phytomer weight. Biomass production was related to region of cultivar origin [e.g., Shawnee produced two times more than Dacotah (6.2 Mg ha–1)]. Tiller density was highest for Dacotah (1090 tillers m–2) and lowest for Cave-In-Rock (520 tillers m–2). Reproductive tiller fractions by count were plastic and higher at Arlington (0.81) than Brookings (0.08). Weights per reproductive tiller ranged from 0.7 g (Dacotah) to 3.4 g (Cave-In-Rock). Phytomers per tiller was not plastic (5.2 for Dacotah to 7.4 for Cave-In-Rock). Internode weight exhibited a basipetal increase and was highly plastic. Cultivars responded similarly to location effects on tillers m–2, weight tiller–1, and biomass production. Cultivar differences for biomass production were attributed to variation at tiller (phytomers tiller–1) and phytomer (weight phytomer–1) levels.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
SWITCHGRASS (Panicum virgatum L.), a perennial warm-season grass indigenous to the eastern two-thirds of the contiguous USA, has shown promise for sustainable herbaceous biomass production. Evaluations of cultivars and improved populations in the USA and Canada have identified inherently productive and well-adapted populations for individual regions and harvest schedules and soil nutrient management schemes for maximizing sustainable biomass production (e.g., Sanderson et al., 1996; Madakadze et al., 1999; Sanderson et al., 1999; Vogel et al., 2002; Casler and Boe, 2003; Casler et al., 2004). Delaying harvest for biomass until early fall was beneficial for sustainable biomass production and sward longevity in the Northern Plains and Great Lakes regions. Even swards of cultivars that reached maturity in August exhibited long-term benefits from delaying harvest until at least mid September (Casler and Boe, 2003).

Several studies have described morphological development in response to environmental variation (e.g., Sanderson and Wolf, 1995; Mitchell et al., 1997) and the relationship between canopy architecture and biomass production and quality (Redfearn et al., 1997) of switchgrass. Genetic variation among cultivars has been reported for tiller density (Madakadze et al., 1998) in eastern Canada. Redfearn et al. (1997) found significant location, population, and location x population interaction effects for tiller densities of six populations grown at Ames, IA, and Mead, NE. Patterns of dry matter partitioning among leaf and stem components were shown to be similar for ‘Alamo’ and ‘Cave-In-Rock’ in Virginia; however, in Texas, leaf and stem partitioning were slightly different for the two cultivars (Sanderson and Wolf, 1995).

Switchgrass plants are clonal modular organisms. Each module (i.e., tiller) grows by reiteration of a basic structural subunit (i.e., metamer) produced by a single apical meristem (White, 1979; Room et al., 1994). For the Poaceae, this metameric subunit is the phytomer (Moore and Moser, 1995; Boe et al., 2000). The morphology of a tiller reflects the number, size, and spatial orientation of its phytomers (Briske, 1991; Boe et al., 2000).

Biomass production in swards of switchgrass is a function of tillers m–2, phytomers tiller–1, and weight phytomer–1. A few studies have addressed certain aspects of sward and tiller morphology of switchgrass, but none have evaluated several cultivars in multiple environments to determine effects of genetic and environmental sources of variation on expression of morphological traits at sward, tiller, and phytomer levels. Therefore, our objective was to determine the importance of environment and genetic variation at the cultivar level on expression of morphological traits that influence biomass production in swards, tillers, and phytomers of switchgrass.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experiment 1 was composed of six cultivars (Cave-In-Rock, Dacotah, Forestburg, Shawnee, Sunburst, and Trailblazer) planted in a split plot in randomized complete block designs with four replications during May 1997 at Arlington, WI (43° N, 89° W), and Brookings, SD (44° N, 97° W). Areas of origin ranged from the northern Great Plains for Dacotah (46° N, 101° W), Forestburg (44° N, 98° W), and Sunburst (42° N, 97° W), to southeastern Illinois for Cave-In-Rock and Shawnee (37° N, 88° W) and Nebraska and Kansas for Trailblazer (synthetic cultivar from Nebraska and Kansas collections). Harvest dates (mid-August, mid-September, and mid-October) were whole plots and cultivars were subplots. Subplot sizes were 1.6 by 1.8 m at Arlington and 1.6 by 3.0 m at Brookings. Soil types were Plano silt loam (fine-silty, mixed, superactive, mesic Typic Argiudoll) at Arlington, and Vienna silt loam (fine-loamy, mixed, superactive, frigid, Calcic Hapludoll) at Brookings. Planting was done with drills with beveled packer-wheel openers and press wheels at a rate of 400 pure live seeds m–2 in 15-cm row spacings. Plots were fertilized each spring of 1997–2000 at 112 kg N ha–1 with ammonium nitrate. Weeds were controlled by clipping and 0.45 kg a.i. ha–1 2,4-D amine [(2,4-dichlorophenoxy) acetic acid] during the establishment year. Samples were collected during mid-September 2000 from cultivar subplots within the September harvest whole plots. Sub-subplot sizes were 0.19 m2 at Arlington and 0.15 m2 at Brookings. Tillers within sub-subplots were excised at ground level with rice knives and dried at 60°C to determine total dry matter yield. After drying, tillers were separated into reproductive and vegetative fractions and counted. Growing season precipitation was about 6 cm below the 30-yr average at Brookings and about 23 cm above the 30-yr average at Arlington.

Experiment 2 was composed of the same cultivars in Exp. 1 planted in randomized complete block designs at Bristol (45° N, 98° W) and South Shore (45° N, 97° W), SD, in May 1999. Planting rate was 450 pure live seeds m–2 in 1.2- by 6.0-m plots with 15-cm row spacings, and seedbeds were disked and packed before planting as in Exp. 1. Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) was applied preemergence at 2.2 kg a.i. ha–1. Plots were fertilized with urea at 85 kg N ha–1 at Bristol. No fertilizer was applied at South Shore. Soil types were Nutley (fine, smectitic, frigid Chromic Hapludert) and Sinai (fine, smectitic, frigid Typic Hapludert) silt loam at Bristol, and Brookings silt loam (fine-silty, mixed, superactive, frigid Aquic Hapludoll) at South Shore. Randomly selected 0.15-m2 subplots were harvested in late September 2000 at each location. Collection and handling of tillers obtained from subplots were as in Exp. 1. Growing season precipitation was about 6 cm below the 30-yr average at Bristol and about 5 cm below the 30-yr average at South Shore.

Experiment 3 was composed of the six cultivars used in Exp. 1 and 2 minus Cave-In-Rock, but with the addition of ‘Summer’ (origin 41° N, 96° W), ‘Blackwell’ (origin 37° N, 97° W), and ‘Nebraska 28’ (origin 42° N, 99° W) in an experiment containing more than 30 cultivars of native warm-season grasses planted during May 1986 at Pierre, SD (44° N, 100° W) by personnel from the USDA-NRCS Plant Materials Center at Bismarck, ND. Soil type was a Promise clay (very-fine, montmorillonitic, mesic, Udic Chromstert). Plot size was 4.3 x 13.7 m. Experimental design was a randomized complete block with three replications. Planting was in a conventional seedbed with a grass drill with double-disk furrow openers with 2-cm depth bands and press wheels in 20-cm spacing. Seeding rate was about 275 pure live seeds m–2. Atrazine was applied at 2.2 kg a.i. ha–1 each spring through 1992. In addition, plots were burned in the spring seasons of 1987–1992, with the exception of 1990. No fertilizer was applied. Data on biomass production were collected from 0.6- by 3.0-m subplots during late September of each year through 1992. After 1992, plots were not harvested but were burned occasionally in the spring to remove accumulated biomass. For our study, randomly selected 0.4-m2 subplots were harvested from each plot during late September 2000 and 2001. Biomass harvested in the field was handled as in Exp. 1 and 2 for determination of dry matter production, reproductive and vegetative tiller fractions, and stand densities. Total precipitation was about 15 cm above the 30-yr average during 1999. Growing season precipitation was about 17 cm below the 30-yr average during 2000, and about 1 cm above the 30-yr average during 2001.

Morphological Data
Sward Morphology
Traits measured for describing swards in Exp. 1, 2, and 3 were: (i) biomass, (ii) reproductive and vegetative tiller fractions by count and by weight, and (iii) tiller density.

Tiller Morphology
Traits measured for describing tillers in Exp. 1, 2, and 3 were (i) weight per reproductive tiller; (ii) weight per vegetative tiller; (iii) blade, sheath, internode, and inflorescence fractions of reproductive tiller biomass; and (iv) phytomers per reproductive tiller. Mean weight per reproductive tiller and weight per vegetative tiller were determined by dividing weight of each tiller-type fraction by number of tillers in the sample. Blade, sheath, internode, and inflorescence fractions of reproductive-tiller biomass, as well as phytomers per tiller, were determined from a sample of three randomly selected tillers. These three tillers were also used to determine morphological characteristics of phytomers for Cave-In-Rock and Sunburst from Exp. 1 and 2.

Phytomer Morphology
Traits measured to describe phytomers of reproductive tillers of Cave-In-Rock and Sunburst from Exp. 1 and 2 were sheath and internode lengths and weights for each complete aerial phytomer. Blades were not included, since they were frequently incomplete. Cave-In-Rock and Sunburst were chosen for the phytomer-level evaluation because (i) of their widely separated geographic origins and different phytomers per reproductive tiller; and (ii) Sunburst (Boe and Ross, 1998) has shown excellent persistence and biomass production, whereas Cave-In-Rock has shown high biomass yield potential (Casler and Boe, 2003) but poor persistence in eastern South Dakota (Boe, 2005, unpublished data) and central North Dakota (J. Berdahl, personal communication, 2004). All weights for tiller and phytomer description were determined on a laboratory balance with milligram accuracy.

Statistical Analysis
For morphological traits descriptive of swards, subplot raw data were subjected to separate ANOVAs for each of Exp. 1, 2, and 3. For traits descriptive of tillers, subplot means were subjected to ANOVA for each of the three experiments. Cultivars, locations, and years were considered fixed, and replications within locations were considered random. For traits descriptive of phytomers, raw data from three randomly selected reproductive tillers from each subplot were subjected to across-locations ANOVA. The four locations from Exp. 1 and 2, replications within locations, and tillers within replications within locations were considered random. Phytomers were considered fixed. Analyses of variance were conducted using the Linear Models procedure in Statistix 7 (Analytical Software, 2000). Fisher's Protected LSD was used to compare means at P = 0.05.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Sward Morphology
Experiment 1
Differences (P < 0.01) were found between Arlington and Brookings for biomass, tiller density, and reproductive-tiller fractions by count and weight and among cultivars for biomass and tiller density. Differences between location means ranged from about twofold for Dacotah to more than fourfold for Shawnee (Table 1). Averaged across locations, Cave-In-Rock, Shawnee, and Trailblazer were the highest-yielding entries, and Forestburg and Dacotah were the lowest-yielding entries (Table 2).


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  		Table 1. Location means for sward morphology traits averaged across six switchgrass cultivars in two experiments in south central Wisconsin and eastern South Dakota during 2000.

 

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  		Table 2. Biomass and tiller density means for six cultivars averaged across two environments in each of two experiments composed of 1-yr-old stands in northeastern South Dakota (Bristol and South Shore, Exp. 2) or 3-yr-old stands at Arlington, WI and Brookings, SD (Exp. 1) during September 2000.

 
Tiller density, averaged across cultivars, was 25% greater at Brookings than at Arlington (Table 1), but ranking of cultivars was similar. Dacotah produced twice as many tillers m–2 as Cave-In-Rock and at least 40% more tillers m–2 than all of the other cultivars (Table 2).

Differences (P < 0.01) were found between locations for reproductive tiller fractions by count and by weight (Table 1). At Arlington, 81% of the tillers were reproductive compared with 8% at Brookings. Correspondingly, reproductive tillers accounted for 95% of the biomass at Arlington and 28% at Brookings (Table 1).

Experiment 2
Significant differences occurred among cultivars but not between locations for biomass and tiller density. Sunburst, Trailblazer, and Shawnee produced more biomass than Dacotah, but Dacotah had the densest stands (Table 2). Reproductive tiller fractions by count and by weight also varied significantly between locations, but the differences were smaller than those that occurred between Arlington and Brookings for those traits (Table 1).

Experiment 3
Differences (P < 0.01) were found between years and among cultivars in 15-yr-old swards in central South Dakota for sward morphology traits. As in Exp. 1 and 2, Dacotah had the thickest stands and the lowest biomass. Summer, which was not included in Exp. 1 and 2, had the highest biomass and thinnest stands (Table 3). Only 13% of the tillers of Summer were reproductive, yet they accounted for 44% of the biomass. Reproductive tiller fractions by count and by weight were highest for Dacotah and lowest for Pathfinder and Trailblazer.


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  		Table 3. Experiment 3 means for sward morphology traits of eight switchgrass cultivars averaged across two years in a 15-yr-old stand in central South Dakota during September 2000 and 2001.

 
Tiller Morphology
Experiment 1
Location x cultivar interactions were significant for weight per reproductive tiller, weight per vegetative tiller, and sheath, internode, and inflorescence fractions of reproductive tillers (Table 4). Large differences (P < 0.05) were found between locations for weight per reproductive tiller and weight per vegetative tiller for Cave-In-Rock and Shawnee, but not for Dacotah, Forestburg, and Sunburst (Table 5). This greater plasticity in tiller weight in cultivars with origins east and south of Arlington and Brookings reflects the superior biomass potential of southern cultivars when grown north of their origins, if conditions are favorable (Casler et al., 2004).


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  		Table 4. Experiment 1 mean squares for tiller morphology traits of six cultivars of switchgrass evaluated at Arlington, WI, and Brookings, SD, during September 2000.

 

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  		Table 5. Experiment 1 means for tiller morphology traits of six switchgrass cultivars evaluated at Arlington, WI, and Brookings, SD, during September 2000.

 
The difference (P < 0.01) between Arlington and Brookings means for weight per reproductive tiller for Shawnee may have been due, in part, to differences for phytomers tiller–1. However, differences between location means for Cave-In-Rock and Trailblazer were due to plasticity of individual phytomers (Table 5). Variability within cultivars for length of the first internode above the proaxis influenced the number of phytomers in excised tillers. Consequently, we considered cultivars with location means that differed by less than one phytomer to be phenotypically stable for that trait. Shawnee was the only cultivar that had a difference in location means that exceeded one phytomer (Table 5).

The blade fraction of reproductive tillers was greater for Cave-In-Rock, Shawnee, and Trailblazer than for cultivars of more northern origin. Since harvest was during mid-September, this may be a reflection of more weathering of leaf blades of the earlier-maturing northern types (Table 6). Sheaths comprised a larger fraction of tiller weight at Brookings compared with Arlington for all cultivars, but relative differences between location means differed significantly among cultivars (Table 6). In contrast, contribution of internodes to tiller weight was significantly greater at Arlington than Brookings, but relative differences between location means were also not consistent across cultivars (Table 6). Inflorescence comprised a larger fraction of tiller weight at Arlington than at Brookings for all cultivars, other than Dacotah and Sunburst (Table 6).


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  		Table 6. Experiment 1 mean proportions of biomass distributed among leaf, stem, and inflorescence components of tillers of six switchgrass cultivars evaluated at Arlington, WI, and Brookings, SD, during September 2000.

 
Experiment 2
In contrast to Exp. 1, location x cultivar interaction variances were nonsignificant for all traits, other than phytomers per reproductive tiller. Shawnee was the only cultivar that had a difference between location means of at least one phytomer per reproductive tiller. Differences (P < 0.01) were found among cultivars for six of the seven traits chosen to describe tiller morphology. As in Exp. 1, weights per reproductive tiller were three to five times heavier than weights per vegetative tiller, and tillers of cultivars of Kansas, Nebraska, and Illinois origin were heavier than those from origins in the Dakotas, with the exception of Sunburst (Tables 5 and 7). Averaged across locations, Sunburst produced biomass comparable to that of the cultivars with origins south of the Dakotas and significantly greater than that of Dacotah and Forestburg (Table 2). Significant differences between locations for components of reproductive tillers were due to higher contributions of blade and sheath fractions to tiller weight at South Shore compared with higher contributions of internode and inflorescence to tiller weight at Bristol. As in Exp. 1, contribution of internodes to tiller weight was greater for Dacotah, whereas contribution of inflorescence to tiller weight was greater for cultivars of more southern origins (Table 7).


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  		Table 7. Experiment 2 means for tiller morphology and proportions of biomass distributed among leaf, stem, and inflorescence components of six switchgrass cultivars evaluated at Bristol and South Shore, SD, during September 2000.

 
Experiment 3
The cultivar x year interaction variance was significant for five of the seven traits chosen to describe tiller morphology (Tables 8 and 9). Effect of year was significant for tiller weights but not for phytomers per reproductive tiller and blade, internode, and inflorescence fractions of reproductive tillers. Summer and Trailblazer produced larger tillers than Nebraska 28 and cultivars from the Dakotas in 2000. However, in 2001, weights of tillers of Sunburst and Forestburg were comparable to those of the more southern cultivars. As in Exp. 1 and 2, tillers of Dacotah were lighter than those of the other cultivars. Summer had more phytomers per reproductive tiller than the other cultivars (Table 8). However, Trailblazer, which was also developed from natural populations in Nebraska, had phytomers per reproductive tiller similar to cultivars with origins in northern Nebraska (i.e., Nebraska 28) and South Dakota, not only here but in Exp. 1 and 2, as well.


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  		Table 8. Experiment 3 means for tiller morphology traits of six switchgrass cultivars evaluated in a 15-yr-old sward in central South Dakota during September 2000 and 2001.

 

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  		Table 9. Experiment 3 mean proportions of biomass distributed among leaf, stem, and inflorescence components of tillers of six switchgrass cultivars in 15-yr-old swards in central South Dakota during September 2000 and 2001.

 
The pattern of distribution of biomass among leaf, stem, and inflorescence components varied slightly among cultivars and was similar to patterns observed for Exp. 1 and 2. For example, the internode component comprised a greater proportion of tiller weight for Dacotah, the earliest maturing cultivar, than for Trailblazer, the latest maturing cultivar. Logically, the opposite was true for the leaf component (Table 9).

Phytomer Morphology
Across-locations (Exp. 1 and 2) ANOVAs indicated among- and within-location influences on morphology of sheath and internode components of phytomers of Cave-In-Rock and Sunburst. Location x phytomer interactions occurred for internode weight and length for Sunburst and sheath weight and length for Cave-In-Rock. Replications within locations and tillers within replications were significant for internode weight for both cultivars. The location effect was significant for internode and sheath lengths for both cultivars and for internode and sheath weights for Cave-In-Rock. However, no effect of location on internode and sheath weights was detected for Sunburst due to large replications within locations mean squares (Table 10).


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  		Table 10. Mean squares for morphological characteristics of sheath and internode fractions of the five uppermost aerial phytomers of reproductive tillers of Sunburst (SBS) and six uppermost aerial phytomers of reproductive tillers of Cave-In-Rock (CIR) switchgrass from one sward in Wisconsin and three swards in eastern South Dakota (Exp. 1 and 2) during September 2000.

 
Internode weight was related to phytomer position (Table 10). A linear (P < 0.01) basipetal increase in internode weight was the general pattern for both cultivars (Fig. 1 and 2) . Although phytomers of Cave-In-Rock varied substantially for internode weight, they had similar plastic responses to environmental variation (Fig. 1). The plasticity patterns for phytomers of Sunburst resembled those of Cave-In-Rock, except at Arlington, where the fourth phytomer inexplicably produced less biomass than the second and third phytomers (Fig. 2).



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  		Fig. 1. Pattern of biomass distribution among internodes of aerial phytomers of reproductive tillers of ‘Cave-In-Rock’ switchgrass in swards at four locations (BST = Bristol, SD; ARL = Arlington, WI; BRK = Brookings, SD; SSH = South Shore, SD) during September 2000. Standard errors are represented by vertical lines centered on means.

 


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  		Fig. 2. Pattern of biomass distribution among internodes of aerial phytomers of reproductive tillers of ‘Sunburst’ switchgrass in swards at four locations (BST = Bristol, SD; ARL = Arlington, WI; BRK = Brookings, SD; SSH = South Shore, SD) during September 2000. Standard errors represented by vertical lines centered on means.

 
Internode lengths increased acropetally for both cultivars, with the steepest increase between the second and apical phytomers. The apical internode of Sunburst was longer than the four subtending internodes at Arlington and Bristol. However, at Brookings, it was only longer than the basal internode. For Cave-In-Rock, the apical internode (229 mm) was longer than the five subtending internodes, with the basal internode being shortest (147 mm). Mean internode lengths at Arlington and Bristol were about 50% longer than those at Brookings for both cultivars (data not shown).

Sheath weight increased acropetally across phytomers for Cave-In-Rock at Arlington, Bristol, and South Shore, with sheaths of apical and second phytomers heavier than those of the fifth and sixth phytomers. However, at Brookings, sheaths from the fifth phytomer were heavier than those from the second phytomer (Fig. 3) . For Sunburst, large within-location variation prevented detection of location, phytomer, and location x phytomer effects on sheath weight (Table 10).



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  		Fig. 3. Pattern of biomass distribution among sheaths of aerial phytomers of reproductive tillers of ‘Cave-In-Rock’ switchgrass at four locations (BST = Bristol, SD; ARL = Arlington, WI; BRK = Brookings, SD; SSH = South Shore, SD) during September 2000. Standard errors represented by vertical lines centered on means.

 
Location means for sheath length in Cave-In-Rock ranged from 94 mm at Brookings to 164 mm at Arlington, but ranking of phytomers was not consistent across locations (Table 10). Location means for Sunburst ranged from 126 mm at Brookings to 188 mm at Arlington, and sheaths from the second, fourth, and fifth phytomers were longer (159 mm) than those from the third phytomer (147 mm) (data not shown).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Sward Morphology
Tiller densities varied widely among cultivars, but were fairly consistent between locations for swards of the same age and harvest management. Tiller density for Trailblazer in this study was similar to what Mitchell et al. (1998) reported in Nebraska. In this study, tiller density of Cave-In-Rock (677 tillers m–2) was similar to reports from Canada (725 tillers m–2) (Madakadze et al., 1998) but lower than in Iowa and Nebraska (900 tillers m–2) (Redfearn et al., 1997). In contrast to this study, tiller densities of Cave-In-Rock and Sunburst were significantly greater than tiller densities for Dacotah (555 tillers m–2) in Quebec (Madakadze et al., 1998).

High shoot densities in several herbaceous species have have been shown to result in skewed distributions with high frequencies of small shoots and lower biomass than stands with lower shoot densities and larger mean shoot size (e.g., Barbour et al., 1987; Berg et al., 2005). This pattern was also evident in our study. Averaged across cultivars, biomass production was 3.4 times greater at Arlington than at Brookings, but tiller density at Arlington was only 80% of that at Brookings. In an ongoing complementary study, yield component analysis indicated stronger linear relationships between weight tiller–1 and biomass production than between tiller density and biomass production (Boe, 2005, unpublished data).

Cassida et al. (2005) found differences among switchgrass genotyopes for biomass production were primarily a result of differences in weight tiller–1 at Stephenville, TX, where plant density ranged from 4.84 to 6.45 plants m–2. At Dallas, TX, where plant density ranged from 2.61 to 5.84 plants m–2, variation in plant density or tillers plant–1 largely explained differences among genotypes for biomass yield. Smart et al. (2004) concluded selection for high yield tiller–1 would likely be effective for increasing biomass in swards of switchgrass populations harvested in a single cutting at the end of the growing season.

Differences among cultivars for reproductive tiller fractions by count and weight were more evident between years in a 15-yr-old sward than between locations in 1- and 3-yr-old swards, with late-maturing cultivars (e.g., Blackwell) having lower reproductive tiller fractions by count and weight than those from the Dakotas. Large differences between eastern South Dakota and southern Wisconsin suggested young swards of cultivars had similar plasticities for these traits. On the other hand, weight tiller–1 was strongly influenced by genetics.

Tiller Morphology
Of all the traits measured, phytomers per reproductive tiller was the least plastic. In species that exhibit ecoclinal variation (Casler et al., 2004), phytomers per tiller determines time elapsed between tiller emergence and anthesis and is an indicator of adaptation to the length of the growing season in the region of origin. A common method for increasing biomass production from native warm-season grasses is to plant a cultivar with origin south of the area of intended production. However, a general rule is not to move them more than 500 km north of their area of origin because of potential winter injury (Vogel, 2000).

In Texas, flowering tillers of Cave-In-Rock produced about nine leaves (Van Esbroeck et al., 1997). We found seven to eight aerial phytomers on flowering tillers of Cave-In-Rock in Wisconsin and South Dakota. Since the proaxis of switchgrass generally produces 2 to 3 leaves (Boe and Bortnem, 2003), phytomers per reproductive tiller of Cave-In-Rock appeared to be comparable in Texas and our region.

Similar to results of this study, Redfearn et al. (1997) found significant differences among cultivars and between locations for weights of reproductive and elongating vegetative tillers. Mean weights of reproductive tillers of Cave-In-Rock at Ames, IA (Redfearn et al., 1997), were comparable with weights of reproductive tillers at Brookings, but only about 65% of weights of tillers at Arlington in our study. However, they harvested in August, whereas we harvested in early autumn.

Biomass partitioning was similar to what was reported for reproductive tillers of ‘Alamo’ (Sanderson, 1992) and Cave-In-Rock (Sanderson and Wolf, 1995) harvested in late August in the southern USA. However, small differences among cultivars for partitioning biomass among leaf, stem, and inflorescence suggested genetic differences for biochemical composition of uninterrupted growth in early autumn.

Phytomer Morphology
Phytomeric position had a strong influence on internode weight in both cultivars. Basal phytomers produced up to twice as much stem biomass as apical phytomers for Cave-In-Rock in all environments. Internodes and sheaths of all phytomers were highly plastic sinks for biomass, with up to four-fold differences between location means for comparable phytomers. However, there was no strong indication of variation among phytomers for plasticity in biomass accumulation. Perhaps this is due in part to the fact that, although phytomers develop in sequence (Moore and Moser, 1995), and internode elongation is progressive from the basal to the apical phytomer, periods of internode elongation overlap for several phytomers (Sims et al., 1971).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Extensive genetic variation occurred among cultivars of switchgrass for morphological traits at sward, tiller, and phytomer levels. Not surprisingly, the magnitude of difference between cultivar means for morphological traits was often a reflection of distance between regions of cultivar origin. Morphological traits influencing biomass production were also highly plastic, with large differences between eastern South Dakota and southern Wisconsin environment means for all eleven traits measured, other than phytomers per reproductive tiller.

The highest biomass swards of each cultivar were composed of predominantly reproductive tillers, with the maximum number of phytomers tiller–1 for that cultivar. On the other hand, the lowest biomass swards were not necessarily associated with low tillers m–2, but rather with a high frequency of vegetative tillers with fewer phytomers and lower weight phytomer–1 than reproductive tillers. This suggested progress from within-population selection for biomass will depend on genetic variance for (i) production of tillers with the maximum number of phytomers, and (ii) accumulation of biomass at the phytomer level. Since phenotypic selection in spaced plantings has been relatively unsuccessful for increasing biomass per unit land area in perennial grasses (Casler et al., 1996), evaluations of half-sib families in swards are needed to provide estimates of (i) effects of tillers m–2, phytomers tiller–1, and weight phytomer–1 on biomass production; and (ii) heritabilities, which would be useful for predicting progress from among family selection for biomass yield and its components in swards.


   ACKNOWLEDGMENTS
 
This research was supported by the South Dakota Agricultural Experiment Station and the U.S. Dep. of Energy's Biomass Program through contracts DE-AIO5-90OR21954 with Oak Ridge National Laboratory (ORNL) and contract DE-FC36-02G012028, A000 with the Great Plains Institute for Sustainable Development, Minneapolis, MN. The ORNL is managed by UT-Battelle, LLC, for the U.S. Dep. of Energy under contract DE-AC05-00OR22725.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
South Dakota Agric. Expt. Stn. Journal Series No. 3435.

Received for publication December 4, 2004.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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