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 approximate to 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.
