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CROP BREEDING & GENETICS

Variation between Two Switchgrass Cultivars for Components of Vegetative and Seed Biomass

Plant Science Dep., South Dakota State Univ., Brookings, SD 57007. This research was supported by the South Dakota Agricultural Experiment Station, the U.S. Department of Energy through contract DE-FC36-02G012028, A000 with Great Plains Institute for Sustainable Development, Minneapolis, MN, and U.S. Department of Energy's Biomass program through contract DE-A105-900R2194 with Oak Ridge National Lab. (ORNL). ORNL is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. South Dakota State Agric. Expt. Stn. Journal Series No. 3445

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

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Biomass production potential in switchgrass (Panicum virgatum L.) populations is inversely related to latitude of origin. However, phenological adaptation limits the latitudinal range from which to select populations for breeding for biomass in the northern Great Plains. Objectives of this study were to compare patterns of biomass partitioning, determine importance of tiller density and size, and identify morphological traits as potential selection criteria for two cultivars, Summer (origin 40° 42' N, 95° 52' W) and Sunburst (origin 42° 42' N, 96° 41' W), of comparable phenology. Summer (12.6 Mg ha^–1) produced 20% more vegetative biomass than Sunburst and had higher percent reproductive tillers (62% vs. 40%) and more phytomers tiller^–1 (7.9 vs. 6.4). Sunburst had more tillers m^–2 (677 vs. 530). Cultivars did not differ for seed biomass (311 kg ha^–1), but seeds of Sunburst (1.8 mg seed^–1) were 90% heavier than seeds of Summer. Vegetative biomass decreased acropetally among phytomers. Reproductive tillers per m² and seed mass per panicle were accurate predictors of vegetative and seed biomass, respectively. Frequency of reproductive tillers, number of phytomers per tiller, and rate of phytomer development morphologically differentiated Summer and Sunburst and were potential selection criteria for improving biomass yield within a maturity class of switchgrass adapted to the northern Great Plains.

Abbreviations: MSC, mean stage by count • MSW, mean stage by weight.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
VARIATION AMONG switchgrass (Panicum virgatum L.) cultivars for biomass production is strongly associated with variation in phenology (e.g., days to heading). Cultivars from lower latitudes (e.g., Cave-In-Rock and Shawnee, origin 37° N lat) have more disease resistance (Gustafson et al., 2003), larger tillers with more phytomers tiller^–1 (Berdahl et al., 2005; Boe and Casler, 2005), and higher biomass yield potential (Casler and Boe, 2003; Boe and Casler, 2005; Lee and Boe, 2005) than cultivars with origins in the northern Great Plains (e.g., Sunburst [origin 43° N lat] and Dacotah [origin 46° N lat]). Casler et al. (2004) attributed higher biomass yield potential of southern-origin ecotypes to their later maturity and more rapid stem elongation rate. However, in the northern Great Plains, expression of superior biomass yield potential of southern-origin cultivars is dependent on winter survival (Berdahl et al., 2005) and adequate soil moisture for phytomer development and biomass accumulation during late summer (Lee and Boe, 2005).

In the northern Great Plains, differences for biomass yield have also occurred among cultivars with similar maturities (Berdahl et al., 2005; Boe and Casler, 2005). However, little is known about the morphological basis for differences in biomass yield between cultivars that emerge at the same time in the spring and reach reproductive development at the same time during the summer. Summer (Alderson and Sharp, 1994), an upland tetraploid cultivar (Vogel, 2004), and Sunburst (Boe and Ross, 1998), an upland octaploid cultivar (Vogel, 2004), exsert inflorescences and begin anthesis within a day or two of each other during early to mid-August in northeastern South Dakota (unpublished data, 2002 through 2006). Summer produced two more phytomers tiller^–1, more mass tiller^–1, and more biomass than Sunburst in 15-yr-old swards in central South Dakota (Boe and Casler, 2005) and has shown promise for biomass production in eastern South Dakota (unpublished data, 2002 through 2004) and southern Canada (R. Samson, personal communication, 2005). However, in North Dakota, Sunburst established better and consequently produced more biomass than Summer (Berdahl et al., 2005).

Relationships between yield and tillers m^–2 and mass tiller^–1 have been described for alfalfa (Medicago sativa L.) (Berg et al., 2005) and several cool-season forage grasses (e.g., Thomson et al., 1973). Redfearn et al. (1997) determined that stands of switchgrass managed for biomass production should contain predominantly reproductive tillers with high mass tiller^–1, which may not coincide with high tiller density. They utilized multiple linear regression to develop prediction equations for the effects of blade, sheath, and stem weight, tiller density, and mean stage by count (MSC) and weight (MSW) effects on yield in ‘Trailblazer,’ ‘Pathfinder,’ Cave-In-Rock, and three selected populations of switchgrass in Iowa and Nebraska. Blade and sheath dry weights and MSC and MSW were most useful for predicting yield in those populations, with a single independent variable model accounting for the significant variation in most cases.

Phenological adaptation is crucial for sustainable biomass production from switchgrass in large areas of the northern Great Plains (unpublished data, 2000 through 2006; Berdahl et al., 2005). Berdahl et al. (2005) concluded that cultivars that did not head until after mid-August in western North Dakota did not acquire adequate dormancy to avoid injury due to winter stresses. Therefore, development of new higher-yielding cultivars intended for long-term (i.e., more than 5 yr) biomass production in regions of the northern Great Plains where moisture and winter stresses can be severe should be approached by selection for increased biomass yield within cultivars or populations with acceptable biomass yield potential and demonstrated winter hardiness and persistence in long-term yield trials or other evaluations.

Therefore, Summer and Sunburst were chosen for this study, and the objectives were to (i) compare mature swards of Summer and Sunburst for partitioning of the vegetative biomass among tiller types (reproductive and vegetative), among phytomers, and among sheath and internode components of phytomers, (ii) compare tiller density and tiller size for predicting vegetative and seed biomass yields of Summer and Sunburst, and (iii) identify morphological traits in swards of Summer and Sunburst that have potential as selection criteria for increasing biomass yield within a maturity class (i.e., heading during early August in North Dakota and South Dakota) of switchgrass adapted for sustainable biomass production in the northern Great Plains.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Summer and Sunburst were planted on the Northeast Research Station near South Shore, SD (45° N, 97° W) during early June 1999 into a conventional seedbed with a cultipacker seeder with 15-cm row spacing. Experimental design was a randomized complete block with three replications. Individual plot size was 1.8 m x 6.1 m. Planting rate was 450 pure live seeds m^–2. Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) was applied preemergence at 2.2 kg a.i. ha^–1. Soil type was a Brookings silty clay loam (fine-silty, mixed, superactive, frigid, Aquic Hapludolls). Plots were burned in early April 2000 to remove standing crop and harvested with a sickle-bar mower at a stubble height of 10 cm, and the biomass removed during late September 2000 and 2001.

Data collection for this study began during 2002. On 5 Aug. 2002, 0.19-m^–2 plots from each cultivar in two replications were excavated below the crowns to provide complete tillers with intact proaxes suitable for accurate developmental staging. During early October 2002 and 2003, three randomly selected 0.19 m² subplots were harvested at ground level from each plot for vegetative and seed biomass and morphological descriptions. The entire experiment was burned during early April 2003 to remove standing crop from the plot areas not harvested during October 2002. No fertilizer was applied during the conduct of the experiment. Previous vegetation was a spaced-plant nursery of red clover (Trifolium pratense L.) for 2 yr before 1999.

All tillers in subplots were excised at ground level with a rice knife. Spikelets and inflorescence branches were removed from reproductive tillers by using a rub board. Reproductive and vegetative tiller fractions were separated, dried at 60°C for 72 h, counted, and weighed. Vegetative biomass was determined from the combined dry weights of reproductive (with seed removed) and vegetative tiller fractions from the subplots. Mass tiller^–1 for subplots was determined from vegetative biomass and total number of tillers. Phytomers per reproductive tiller was determined from five-tiller and six-tiller samples randomly collected from each subplot during 2002 and 2003, respectively. Mass phytomer^–1 (internode + leaf sheath) was determined from total weight of the five- and six-tiller samples and total number of phytomers in those samples.

Fertile florets were isolated by screening to remove panicle branches, followed by use of a rub board to remove glumes from florets and a South Dakota–style seed blower to remove inert material (i.e., glumes and empty florets). Total weights of the fertile florets (hereafter referred to as seeds) obtained from the subplots were used to determine seed biomass. Seed mass per panicle for subplots was determined from seed biomass and number of reproductive tillers. Two 100-seed samples from each plot were weighed on an analytical balance to determine mass per 100 seeds.

Tillers m^–2 and reproductive tiller fractions by count and weight were determined to describe sward morphology. Five each of reproductive and vegetative tillers in 2002 and six reproductive tillers in 2003 were randomly selected from each subplot to determine phytomers tiller^–1 and to describe biomass partitioning among phytomers of reproductive tillers. Phytomers tiller^–1 were determined by counting all complete and incomplete phytomers. Excision at ground level frequently resulted in the basal phytomer being incomplete as a result of severance within the internode.

Sheath length and weight and internode weight and length were determined for each phytomer from the reproductive tillers. Weights were determined on a scale with milligram accuracy. Leaf blades were not used for this analysis because they were often incomplete due to the advanced stage of development of the tillers at harvest. The numbering system for phytomers of mature reproductive tillers (S5 stage in Moore et al., 1991) used throughout this article designates the apical phytomer as phytomer 1 and subtending phytomers in sequence (Boe et al., 2000). For the analysis of internode and sheath characteristics, the balanced data set was confined to phytomers 1 through 5 and five tillers subplot^–1 yr^–1. Only the top five phytomers were included in the analyses because 10% of the tillers of Sunburst did not have complete sixth phytomers.

Data were analyzed using the Linear Models General AOV procedure in Statistix 7 (Analytical Software, 2000). Cultivars, phytomers, and years were fixed. Replications were random. Main effects and interactions were considered significant when P < 0.05 for the appropriate mean square ratio.

Simple linear regression was used to determine the usefulness of linear equations for describing the nature of the relationships between vegetative biomass and tillers m^–2, mass tiller^–1, and reproductive tillers per m². Linear regression analyses were also used to determine the relationships between seed biomass and panicles m^–2, seed mass per panicle, and mass per 100 seeds. For individual cultivars, across-years regression analyses were conducted on data sets of 18 pairs (nine subplots in each of 2002 and 2003) of observed values; whereas, the across-cultivars and years analyses were conducted on data sets of 36 pairs of observed values.

Growing season (April through October) precipitation was 383 mm in 2002 and 304 mm in 2003. These amounts were 82% of the 30-yr average in 2002 and 65% of the 30-yr average in 2003.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Examination of excavated tillers during early August corroborated previous descriptions of the phenological development of Summer and Sunburst (unpublished data, 2002 through 2005; Berdahl et al., 2005). On 5 Aug. 2002, the most developmentally advanced tillers for both cultivars were in the R2 (spikelets fully emerged, peduncle not emerged) substage within the reproductive primary growth stage (Moore et al., 1991). Dissection of these tillers revealed that the peduncle (phytomer 1) and internodes of the top two leaf-bearing phytomers (phytomers 2 and 3) had active intercalary meristems. As expected, the extent of elongation and differentiation of those internodes decreased acropetally. Proaxis development was also similar for the two cultivars, with proaxes of tillers in the R2 stage usually having at least one rhizome greater than 1 cm in length (data not shown).

Analyses of variance (not shown) indicated significant cultivar and year effects for 8 out of 10 traits measured in 2002 and 2003. The cultivar x year mean square was nonsignificant for all 10 traits.

Differences were found between cultivars for tillers m^–2, mass tiller^–1, phytomers per reproductive tiller, mass per phytomer of a reproductive tiller, and vegetative biomass. Summer had 1.5 more phytomers per reproductive tiller, heavier tillers, and 20% greater vegetative biomass than Sunburst. On the other hand, Sunburst had 28% more tillers m^–2 and heavier internodes and leaf sheaths in phytomers 1 through 5 of reproductive tillers than Summer (Table 1). Morphological differences between the two cultivars were consistent across diverse environments, since Boe and Casler (2005) reported more phytomers per reproductive tiller, fewer tillers m^–2, and greater biomass yield for Summer than Sunburst in 15-yr-old swards in central South Dakota. In contrast to reproductive tillers, vegetative tillers of the two cultivars had similar numbers of phytomers (i.e., 5.2 for Sunburst and 5.5 for Summer).

Strong linear relationships occurred between vegetative biomass and reproductive tillers per m² for both cultivars. Weaker, but nevertheless significant, linear relationships were found between vegetative biomass and tillers m^–2 and mass tiller^–1 (Table 3). This indicated, within the range of tiller densities encountered (312 to 785 tillers m^–2), mass tiller^–1 and tillers m^–2 were both useful predictors of biomass production. However, vegetative biomass production was more accurately predicted from reproductive tillers per m². Thomson et al. (1973) found number of tillers was more important than weight per tiller in determining yield, but pointed out that the relative importance of weight per tiller increased during reproductive growth of perennial ryegrass (Lolium perenne L.).

Linear regression across cultivars and years indicated mass tiller^–1 was a better predictor (r² = 0.45, P < 0.001) than tillers m^–2 (r² = 0.08, P = 0.047) of vegetative biomass. Although Sunburst had 28% more tillers m^–2 and 13% more mass phytomer^–1 in the uppermost five phytomers, Summer had 21% more reproductive tillers per m² and its reproductive tillers, due to two more phytomers tiller^–1, were 17% heavier than those of Sunburst. Thus, reproductive tillers per m² was, as was found for individual cultivars, a better predictor (r² = 0.77, P < 0.001) than tillers m^–2 or mass tiller^–1 of vegetative biomass for data pooled across cultivars and years.

Differences were found between cultivars for reproductive tiller fractions by count and weight and mass per 100 seeds, but not for seed mass per panicle or seed biomass. A higher proportion of tillers were reproductive for Summer than for Sunburst. In 15-yr-old swards in central South Dakota, Sunburst had a higher percent reproductive tillers (28%) than Summer (13%), but the reproductive tiller fraction of the total biomass was greater for Summer (44%) than for Sunburst (33%). The mass of 100 seeds of Sunburst was nearly twice the mass of 100 seeds of Summer (Table 4). These 100-seed weights for Summer and Sunburst under sward conditions were similar to means of 30 half-sib families of each of the two cultivars in sod and cultivated spaced-plant nurseries in east central South Dakota (Boe, 2003). However, seed yields of Summer were 60% higher than those of Sunburst in the sod spaced-plant nursery, indicating seed yields of cultivars in swards may not be strongly correlated with spaced-plant yields.

Boe and Casler (2005) reported similar linear acropetal decreases in internode weight for phytomers of Cave-In-Rock and Sunburst from one environment in Wisconsin and three environments in South Dakota. However, in that study, sheaths of Cave-In-Rock exhibited a quadratic pattern in weight across phytomers in only one out of four environments. In the other three environments, the pattern was an acropetal increase across phytomers. Pattern of biomass distribution among internodes appears to be more consistent than the pattern of biomass distribution among sheaths across cultivars, environments, and warm-season grass species (Boe et al., 2000).

Differences were found between cultivars and among phytomers for sheath-to-internode weight ratio (Table 7). The relative contribution of sheath to biomass of phytomers 2 through 5 showed a strong acropetal linear increase (Table 7), with ratios of 0.41, 0.59, 0.65, and 0.84, for phytomers 5, 4, 3, and 2, respectively. Sheath weight was 66% of internode weight for Summer and 59% of internode weight for Sunburst. The relative contributions of internode and sheath to phytomer biomass did not differ between years (Table 7). The cultivar x phytomer x year interaction was significant for sheath-to-internode weight ratio (Table 7). However, phytomers ranked the same in both years and for both cultivars, and cultivar ranks were the same in both years.

As expected, based on the difference between years for biomass yield, a difference was found between years for internode and sheath weights and lengths (Table 7). Internode weight, averaged across phytomers 1 through 5 and cultivars, in 2002 (318 mg) was 28% greater than in 2003 (248 mg). Correspondingly, sheath weight, averaged across phytomers 2 through 5 and cultivars, was 22% greater in 2002 (158 mg) than in 2003 (129 mg). Similarly lengths of internodes and sheaths were 20% and 17% greater, respectively, in 2002 than in 2003.

Significant variation occurred among tillers within year x replication subplots for both internode and sheath traits (Table 7). This indicated high levels of morphological plasticity in response to microenvironmental variation (Boe and Casler, 2005).

All of the reproductive tillers of Summer had at least 6 phytomers, 90% had at least 7, 70% had at least 8, 24% had at least 9, and 4% had 10. In contrast, all of the reproductive tillers of Sunburst had at least 5 phytomers, 90% had at least 6, 42% had at least 7, and 5% had 8 (Table 9). One-way analysis of variance indicated internode and sheath weights of phytomer 6 and the sheath weights of phytomer 7 were similar for the two cultivars. For those tillers of both cultivars that produced at least 7 phytomers, the internode component was 60% heavier for Summer than Sunburst. This was largely due to a higher percentage of the phytomer 7 internodes for Sunburst being incomplete (70% of the tillers of Summer had 8 phytomers compared with 5% for Sunburst) as a result of tiller excision within the phytomer 7 internode. Only Summer produced tillers with contributions to biomass from phytomers 8 and 9 (Table 9).

Frequency of reproductive tillers, number of phytomers per tiller, and rate of phytomer development morphologically differentiated these two phenologically similar cultivars of switchgrass in terms of biomass yield potential in the northern Great Plains. Other studies have suggested increasing frequency of reproductive tillers should increase biomass yield (Redfearn et al., 1997; Boe and Casler, 2005). Increasing tiller size by selecting for more phytomers tiller^–1, as proposed by Van Esbroeck et al. (1998) for improving biomass yield in cultivars adapted to the southern USA, may also be relevant for populations adapted to the northern Great Plains. Number of phytomers per tiller ranged from 6 to 10 (4% of the tiller had 10 phytomers) for Summer and from 5 to 8 (5% of the tillers had 8 phytomers) for Sunburst. However, increasing phytomers tiller without concurrent attention to rate of phytomer development may delay maturity enough to reduce winter hardiness and stand persistence in the northern Great Plains. Increasing mass tiller^–1 could also be accomplished by increasing mass phytomer^–1, which has been shown to be highly plastic with no effect on maturity (Boe and Casler, 2005). For increasing seed production, seed mass per panicle, largely determined by spikelets panicle^–1and seeds spikelet^–1 (i.e., seed set), should be an important selection criterion.

Received for publication April 20, 2006.

	REFERENCES

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This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Boe, A. Search for Related Content PubMed Articles by Boe, A. Agricola Articles by Boe, A. Related Collections Crop Growth and Development Biofuels Crop Genetics