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CROP ECOLOGY, MANAGEMENT & QUALITY

Quantitative Description of the Phytomers of Big Bluestem

Plant Science Dep., South Dakota State Univ., Brookings, SD 57007-2141 USA

arvid_boe{at}sdstate.edu

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Distribution of dry matter in reproductive culms of grasses is related to plant morphology and can be described based on phytomers. Our objective was to describe the morphology of the phytomers of reproductive culms of big bluestem (Andropogon gerardii Vitman) and to quantify patterns of distribution of dry matter among their components. Reproductive culms in early anthesis from a seed production field, a mixed-grass planting, and a native prairie near Brookings, SD were separated into phytomers and fractionated into leaf blade, leaf sheath, internode, and inflorescence components for the culm axis and axillary branches. Blade length and weight and sheath weight decreased markedly among phytomers of the main shoot in acropetal fashion, whereas sheath length was relatively constant. Blade and sheath weight were similar for the basal phytomer (180 mg), but sheath weight was 10 times greater than blade weight (17 mg) for the apical leaf-bearing phytomer because of significant contributions from sheaths of axillary branches. The longest internodes (220 mm) were in the central phytomers, but internode weight was 10 times greater for the basal (760 mg) than the apical phytomer. Inflorescence weight varied among populations and among phytomers, but accounted for 60% of the dry matter of axillary branches. The axillary branches accounted for 50% of the weight of the four uppermost leaf-bearing phytomers. Our data provide a framework for developing selection criteria based on plant morphology and studying genetic and environmental influences on the developmental morphology of big bluestem.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
THE QUANTITY AND QUALITY of the forage of perennial grasses are directly related to plant morphology, which can be described based on a hierarchical arrangement of structural subunits. The basic subunits of a grass tiller are called phytomers (Moore and Moser, 1995). Clark and Fisher (1986) described three phytomer models, but the one generally used to elucidate the rhythm of grass growth (Hyder, 1972; Stubbendieck and Burzlaff, 1971) was first proposed by Gray (1849), elaborated on by Weatherwax (1923), and recently discussed by Moore and Moser (1995). A phytomer, as it is usually defined, consists of a leaf and its subjacent internode, node, and lateral bud (Moore and Moser, 1995).

Kalmbacher (1983) studied distribution of chemical constituents at anthesis in leaf blades, leaf sheaths, nodes, internodes, and inflorescences of four warm-season grasses native to the southeastern USA. He found that crude protein, in vitro organic matter digestibility, and mineral concentration increased acropetally on the shoot. He also reported that most of the dry matter was in the nodes and internodes, with leaf blades contributing the smallest proportion to dry matter production. However, he did not report the contribution from each component of each phytomer to dry matter production .

Krueger et al. (1969) fractionated headed tillers of smooth bromegrass (Bromus inermis Leyss.) and timothy (Phleum pratense L.) into an inflorescence component and by internode into leaf blade, leaf sheath, and internode fractions. They found that leaf blades from the second and third internodes from the top of the stem contributed more to total dry matter than the flag leaf and leaves from lower internodes. Leaf blades contributed from 30 to 38% of the total dry matter of the tiller compared with 24 % for leaf sheaths, and from 24 to 33% for internode fractions. The inflorescences contributed from 12 to 15%.

Smith (1973) studied the distribution of dry matter at anthesis among the vegetative and reproductive shoots of six introduced cool-season grasses. As was reported by Krueger et al. (1969), he found that the blade fractions from the second through fourth internodes from the top contributed more to total dry matter than those above or below them. Sheath weights generally decreased from top to bottom of the shoot for all species other than reed canarygrass (Phalaris arundinacea L.), which showed little variability among sheaths along the shoot axis. The top internode was markedly lighter than those below it, and the general trend was a basipetal increase in internode contribution to total dry matter.

Krueger et al. (1969) and Smith (1973) provided general descriptions of distribution of dry matter in leaf and stem fractions along the axes of reproductive culms of cool-season forage grasses based on vascular and physiological associations (Clark and Fisher, 1986). However, little is known about the contribution of individual phytomers (Weatherwax, 1923) to dry matter production in grasses, especially warm-season grasses native to North America.

Each phytomer is a constructional subunit in a phenological sequence. Knowledge of phytomer morphology provides a basis for understanding inherent morphogenetic constraints to plant design and genetic and environmental influences on phenotypic plasticity and patterns of biomass distribution. Big bluestem differs from most other important forage grasses by producing floriferous branches from axillary buds of the upper phytomers. These branches are a source of morphological plasticity in components of forage and seed production. Therefore, our objectives were (i) to quantitatively describe the morphology of the phytomers and (ii) to determine the distribution of dry matter among components of phytomers of reproductive culms of big bluestem.

	Materials and methods

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
At early anthesis during 1997, reproductive culms from each of three populations of big bluestem near Brookings, SD were randomly selected and clipped near ground level. The populations were (i) a 5-yr-old seed production field of `Sunnyview' (Boe and Ross, 1999) planted in 75-cm rows, (ii) a 5-yr-old mixed planting of `Bonilla' (Barker et al., 1990) big bluestem and `Sunburst' (Boe and Ross, 1998) switchgrass (Panicum virgatum L.), and (iii) a native prairie owned and managed by the Nature Conservancy. Hereafter, the populations will be referred to as Sunnyview, Bonilla, and Prairie, respectively. To insure genetic variation within samples, we collected culms separated by a distance of at least 10 m. Freshly harvested culms were separated into phytomers according to the traditional concept of Weatherwax (1923). Most of the culms of Sunnyview had at least seven complete phytomers above ground level, whereas the other populations had a preponderance of culms with six phytomers. Therefore, we conducted the study on 25 culms with seven phytomers for Sunnyview and 25 culms with six phytomers for the other populations. Each phytomer was separated into internode, blade, sheath, and inflorescence (if present). In addition, axillary branches were divided into prophyll (the first leaf of a lateral branch), internode, sheath, and inflorescence. The level of branching ranged from primary to tertiary. Lengths of each component were determined on fresh material that had air dried for 2 to 3 d. Components were dried at 35°C until constant weight. For dry matter determinations, the internodes of the axillary branches were included with the main shoot internode, and the prophylls and sheaths of the branches were included with the main shoot sheath of the associated phytomer. Inflorescence weights for each phytomer were determined by combining all of the rames associated with the main shoot phytomer. A rame is defined as an inflorescence branch that bears repeating pairs of sessile and pedicellate spikelets (Allred, 1982). The number of inflorescences per phytomer ranged from one to three depending on the level of axillary branching. We realized that each axillary branch has its own phytomeric subunits. However, our rationale for including the axillary branch components with main shoot components for dry matter determinations was that floral branches develop from the lateral bud component of the phytomer as defined by Weatherwax (1923).

At early anthesis during 1998, we resampled the Sunnyview population to obtain estimates of the contributions by each component of axillary branches to total dry matter mass of individual phytomers and additional information on the distribution of inflorescence mass among phytomers. The population exhibited greater variability for number of phytomers per culm in 1998 than 1997. Consequently, the 25-culm sample randomly selected for this part of the study was comprised of culms with seven to 11 phytomers. Axillary branches were separated into leaf, internode, and inflorescence fractions and weighed separately from the associated main shoot phytomer components. Prophylls were included in the leaf fraction for mass determination. Length and mass measurements for the other components were done as in 1997.

Length and dry matter mass data were analyzed by two-way analyses of variance for each population. Culms were considered random and phytomers were considered fixed. Experimental error (i.e., culm x phytomer mean square) was used to test culm and phytomer effects. Because of large differences among phytomer means for most of the parameters, Bartlett's procedure was used to test homogeneity of phytomer variances. Heterogeneity of error was only significant (P < 0.05) for internode weight. Internode weights for Phytomers 1 and 2 were considerably lighter and less variable than those for Phytomers 3 to 7; consequently, separate analyses of variance for internode weight were conducted for Phytomers 1 and 2 and Phytomers 3 through 7. Means for Phytomers 1 through 7 for all parameters other than internode weight were separated by Fisher's protected least significant difference at the 0.05 level of probability. Least significant differences for separation of phytomer means for internode weight were calculated for Phytomers 3 through 7. An F test was used to test the difference between the mean internode weights for Phytomers 1 and 2. We designated phytomers in increasing order from the apex to the base of the culm (e.g., the apical phytomer was Phytomer 1).

	Results and discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
1997 Collections
Differences among culms and among phytomers for leaf blade length and weight, leaf sheath length and weight, and internode length and weight were significant (P < 0.01) for each population. As examples of the breadth of variation among culms within a population, mean blade weights for individual culms of Sunnyview ranged from 35 to 147 mg, mean sheath weights ranged from 106 to 286 mg, and mean internode weights ranged from 248 to 599 mg. Similar magnitudes of variation also occurred for the five other leaf and internode traits.

Leaf blade length and mass exhibited marked acropetal decreases (Table 1) . The apical phytomer, as defined by Weatherwax (1923), is leafless. The blades of the second phytomers were very small and delicate and often partially or totally missing. Consequently, we did not collect blade data for the second phytomer. Blades from the basal phytomer (Phytomers 6 and 7) were up to five times longer and 10 times heavier than those from Phytomer 3. Weights of blades from Phytomer 6 were about equal to the sum of the weights of those from Phytomers 3 through 5. Our results differ from those of Krueger et al. (1969) and Smith (1973), who found that blades from the central part of the stem of introduced cool-season grasses contributed more to dry matter production than those above or below them. Evans (1927) found that lengths of the blades of the third, second, and next to upper leaves were comparable (145 mm), whereas the blade of the upper leaf was much shorter (86 mm) for reproductive culms of timothy. Green and Goetz (1973), on the other hand, found that blade length increased basipetally across seven internodes for western wheatgrass [Pascopyrum smithii (Rydb.) Love] and blue grama [Bouteloua gracilis (H. B. K.) Lag. ex Steud].

The percentage of phytomers with inflorescences and the contribution of inflorescence to phytomer weight decreased basipetally (Table 4) . For example, inflorescence comprised about 80% of the weight of Phytomer 1, but only 30% of the weight of Phytomer 3 (Table 4). Although large differences occurred among populations for percentage of phytomers with reproductive development and inflorescence dry matter at each phytomer, all three exhibited a general decrease in reproductive development basipetally from Phytomer 1 to Phytomer 6 (Table 4).

All branches were subtended by prophylls. Significant (P < 0.01) variation occurred among culms within populations for prophyll length. For example, the range in mean length of the prophyll of the primary branch from Phytomers 2 through 4 of Sunnyview was from 40 to 78 mm. On the other hand, there was little variation among phytomers within a culm for length of prophylls from the same branch level. However, prophylls from the secondary branch were about 25% shorter than those from the primary branch of the same phytomer.

Branches with exserted inflorescences had two (rarely three) internodes, the first subtended by a prophyll and the second surrounded by a sheath. The length of the first internode of the primary branch increased significantly (P < 0.01) from 125 to 145 mm basipetally for Phytomers 2 through 4 of Sunnyview, for example. The second internode of primary and secondary branches was about 25% longer than the first. Sheaths attached at the second node were about 10% shorter than those of the associated main shoot phytomer. As was reported by Evans (1927) for timothy, the blade of the first leaf above the prophyll was absent or rudimentary.

Although the average culm weight for Sunnyview was nearly twice (5.4 g) that of the Prairie (2.9 g) and Bonilla (2.8 g) populations, the distribution of dry matter among leaf, internode, and inflorescence fractions were similar for all three. Leaf blades comprised from 7 (Bonilla) to 9% (Prairie) of the total dry matter of a culm, sheaths comprised from 18 (Prairie and Bonilla) to 19% (Sunnyview), internodes comprised from 56 (Sunnyview) to 62% (Prairie and Bonilla), and inflorescence comprised from 11 (Prairie) to 17% (Sunnyview). These proportions were similar to that found by Smith (1973) for patterns of dry matter distribution in shoots of six cool-season grasses. In his study ranges for the six species were from 10 to 17% for leaf blades, 15 to 21% for leaf sheaths, 35 to 48% for internodes, 12 to 22% for inflorescence, and 7 to 13% for stem bases (unelongated internodes in the lower 7 cm of the shoot).

1998 Collection
All of the culms had axillary branch development at Phytomers 2 to 4, 65% had axillary branches at Phytomer 5, and 23% had branches at Phytomer 6. In 1997, 30% of the culms had branches at Phytomer 5 and only 8% had them at Phytomer 6. No axillary branches occurred basipetal to Phytomer 6 on any of the culms in either year. Axillary branches comprised 53% of the mass of Phytomer 4, 63% of Phytomer 3, and 72% of Phytomer 2 (Table 5) . Inflorescence comprised about 60% of the weight of the axillary branches, with leaf and internode comprising about equal fractions of the remaining 40% (Table 5).

These results revealed the potential importance of axillary branches to forage production in big bluestem, since they comprised about 60% of the total weight of Phytomers 2 through 4 at early anthesis. In addition inflorescence contributed more to axillary branch weight than leaf and internode components combined. However, since the culms fractionated for this part of the study were from Sunnyview managed for seed production, they may reflect a relatively high level of biomass allocated to reproductive effort that may not occur in solid stands or under more stressful conditions (Table 4).

Mean annual leaf (blade + sheath) weights for each of Phytomers 2, 3, 4, 6, and 7 were similar in 1997 (Tables 1 and 2) and 1998 (Table 5). However, Phytomer 5 had a greater mean leaf weight in 1998 as a result of a higher percentage of culms with axillary branches (Table 5). Mean main-shoot sheath lengths for Sunnyview, averaged across Phytomers 2 to 7, were 109 mm in 1998 compared with 119 mm in 1997 (Table 2). Sheath length was slightly longer for Phytomers 6 (115 ± 4 mm) and 7 (122 ± 5 mm) compared with Phytomers 2 through 5 (100 to 109 mm). For culms that had more than seven phytomers above ground level, mean sheath lengths were 127 ± 6, 130 ± 8, and 132 ± 18 mm for Phytomers 8, 9, and 10, respectively. Sheath weight of Phytomer 2 (140 ± 9 mg) was significantly (P < 0.01) less than that of the other phytomers, just as occurred in 1997 (Table 2). The range in sheath weight among the other phytomers was from 180 ± 10 mg for Phytomer 7 to 209 ± 13 mg for Phytomer 4. Annual means, averaged across phytomers, were 175 and 183 mg for 1997 and 1998, respectively. Main shoot sheaths exhibited marked acropetal decreases in sheath weight, similar to what occurred for leaf blades. Weights of main shoot sheaths were 70 ± 4, 91 ± 3, 110 ± 3, 136 ± 6, 162 ± 8, and 180 ± 7 mg for Phytomers 2 through 7, respectively. Thus the canalization of sheath weight along the culm axis was due to an acropetal increase in axillary branch sheath weight in association with a concurrent acropetal decrease in main shoot sheath weight.

Mean internode lengths, averaged across Phytomers 1 through 7, were 177 mm in 1998 compared with 197 mm in 1997 (Table 3). Phytomer 1 showed the largest interannual difference, with means of 203 mm in 1997 and 142 mm in 1998. Annual means of Phytomers 2 through 7 differed by <20 mm.

Patterns of dry matter distribution that occurred for Sunnyview, Bonilla, and Prairie populations in 1997 were evident for Sunnnyview again in 1998, even though the culms collected in 1998 had, on average, more phytomers than those collected in 1997. More than 80% of the culms collected in 1998 had at least eight phytomers, 53% had at least nine phytomers, and 30% had 10 phytomers above ground level. Most of the culms collected in 1997 had seven phytomers. Mean total weights for Phytomers 1 through 7 were similar for culms of Sunnyview in 1997 (5.4 g) and 1998 (5.7 g). However, mean culm weight was 2.3 g heavier in 1998 due to the increased mean number of phytomers per culm (Table 5). Madakadze et al. (1998) and Gillen and Ewing (1992) found differences between years for leaf number per tiller in big bluestem, indicating that environmental conditions may influence phytomer number. Gillen and Ewing (1992) also found leaf numbers to be much greater under garden conditions than in native prairie.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Morphological characteristics of and relationships between components of comparable phytomers of reproductive culms of big bluestem were relatively consistent across genetically different populations from three diverse environments. Patterns of dry matter distribution among and within phytomers of big bluestem were, with a few exceptions, similar to reports for cool-season forage grasses (e.g., Smith, 1973). The major differences were that big bluestem had (i) marked and relatively uniform acropetal decreases in leaf blade length and weight and main shoot sheath weight, (ii) the longest internodes in the centrally located phytomers, (iii) axillary floriferous branches on Phytomers 2 through 6, and (iv) as a result of axillary branches on the upper phytomers, similar total sheath (main shoot sheath + sheaths of axillary branches) weights for all phytomers within a culm.

Quantification of the contributions of leaf, stem, and inflorescence components to phytomer weights revealed several possible architectural constraints to the range of morphological plasticity that can be expressed by reproductive culms of big bluestem. The most evident constraints were the marked acropetal decreases in leaf blade length and weight, main shoot sheath weight, and internode weight. Axillary branches were an obvious source of morphological plasticity among culms, but their contribution to phytomer weights declined basipetally from 73% for Phytomer 2 to only 34% for Phytomer 6. Our results should be useful for (i) designing additional research to examine genetic and environmental influences on the developmental biology and morphology of big bluestem and (ii) developing selection criteria based on phytomer morphology.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
South Dakota Agric. Exp. Stn. Journal Series no. 3114.

Received for publication April 5, 1999.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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• Madakadze I., Coulman B.E., Stewart K., Peterson P., Samson R., Smith D.L. Phenology and tiller characteristics of big bluestem and switchgrass cultivars in a short growing season area. Agron. J. 1998;90:489-495.[Abstract/Free Full Text]
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• Smith, D. 1973. Distribution of dry matter and chemical constituents among the plant parts of six temperate-origin forage grasses at anthesis. Univ. Wisconsin Res. Rep. R 2552.
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