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Title: A comparative study of alfalfa and Medicago truncatula stem traits: morphology, chemical composition, and ruminal digestibility

item Jung, Hans Joachim
item Samac, Deborah - Debby

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2007
Publication Date: 7/30/2007
Citation: Schnurr, J.A., Jung, H.G., Samac, D.A. 2007. A comparative study of alfalfa and Medicago truncatula stem traits: morphology, chemical composition, and ruminal digestibility. Crop Science. 47:1672-1680.

Interpretive Summary: Alfalfa is a forage crop rich in dietary fiber that is fed to cattle, sheep, and horses. It is the third most important crop (tied with wheat) grown in the U.S.A., behind corn and soybeans, in terms of number of acres planted and economic value. Alfalfa can also be used for biomass energy production because of its high yield potential. It provides nitrogen fertilizer from the air to both itself and crops that follow it in rotation. Alfalfa would be of even greater value if it were genetically modified using biotechnology to make its dietary fiber more digestible by livestock and fermentable to ethanol. Unfortunately, alfalfa is genetically very complex and difficult to analyze using modern biotechnology tools. However, alfalfa has a closely related cousin, barrel medic, that has simpler genetics and is a model plant for biotechnology. A study was conducted to determine if alfalfa and barrel medic are similar in the development and composition of their stems, the site of most dietary fiber accumulation. For all the stem traits examined (amount, composition, and digestibility of dietary fiber), significant variation was found within alfalfa and barrel medic lines. But the alfalfa and barrel medic lines overlapped with each other such that overall these two species of plants were not found to be different from each other. Because these two species are known to be very similar at the level of individual genes, the results of the current study provide confidence that genetic information on dietary fiber accumulation in barrel medic stems can be used for biotechnological improvement of alfalfa for livestock and bioenergy.

Technical Abstract: Alfalfa (Medicago sativa L.) is an agronomically important forage, but digestibility of stem cell wall material is low. Because the tetraploid genome of alfalfa complicates genetic dissection of complex pathways, Medicago truncatula (Gaertn.) could serve as a model for stem cell wall development in alfalfa. We compared stem morphology, chemical composition (protein, soluble carbohydrates, cell wall polysaccharides, and lignin), and in vitro ruminal cell wall polysaccharide digestibility of two alfalfa clones (Regen-SY27 and 718) and four M. truncatula inbred lines (A17, A20, DZA315.16, and R108) in a replicated growth chamber experiment. Stem tissue development and cell wall lignification observed by light microscopy were similar between the species. While differences in stem morphology, composition, and digestibility were observed among the germplasms, there was overlap between the alfalfa and M. truncatula germplasms for all traits except protein concentration, which was greater for the two alfalfa clones. Younger stem internodes (top third of the stem) of both species had a higher protein concentration and greater cell wall polysaccharide digestibility, and lower cell wall concentration than older internodes (bottom third of stem). Based on the data presented here, it appears that M. truncatula is an excellent model for stem development, composition, and digestibility of alfalfa.