Author
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ENGELS, FERDINAND - WAGENINGEN AGRIC UNIV |
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Jung, Hans Joachim |
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Submitted to: Annals of Botany
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 6/11/1998 Publication Date: N/A Citation: N/A Interpretive Summary: Alfalfa is called the "Queen of Forages" because of its high quality as dairy cow feed. However, as alfalfa plants mature, their stems become woody and the feeding value of alfalfa declines dramatically. The cellular structure of tissues that comprise alfalfa stems are very diverse and complex. Microscopic examination of the structure of alfalfa stem tissues during the process of development and maturation characterized the changes which occur. Some alfalfa stem tissues remain the same throughout growth and development whereas other tissues change greatly in their structure. When combined with information on the relative digestibility of these stem tissues, these results identify which alfalfa tissues become resistant to digestion with maturation and when this decline in quality occurs. This information will aid plant breeders and biotechnologists in identifying targets for modification to further improve alfalfa as a feed for dairy cows. Technical Abstract: Alfalfa stems contain a variety of tissues with different patterns of cell- wall development. Development of alfalfa cell walls was investigated after histochemical staining and with polarized light using a light microscope and by scanning electron microscopy. Samples of the seventh internode, from the base of regrowth stems, were harvested at five defined stages of development from early internode elongation through late maturity. Internode seven was elongating up to the third sample harvest, and internode diameter increased throughout the entire sampling period. Chlorenchyma, cambium, secondary phloem, protoxylem parenchyma, and pith parenchyma stem tissues were all comprised of thin primary cell walls. Pith parenchyma underwent a small amount of cell-wall thickening and lignification during maturation. Collenchyma and protophloem tissues developed partially thickened primary walls. In contrast to a recent report, the formation of a ring shaped, lignified portion of the primary wall of the protophloem was found to precede the deposition of a thick, non-lignified secondary wall which was degradable by rumen microbes. In numerous xylem fibers from the fourth harvest date, an additional highly degradable secondary wall layer was deposited against a previously deposited lignified and undegradable secondary wall. The pattern of lignification observed in alfalfa stem tissues suggests that polymerization of monolignols by peroxidases at the luminal border of the primary cell wall creates an impermeable zone which restricts lignification of the middle lamella region of tissues with thick primary walls. |
