Shearing force of alfalfa stems: effects of cultivar and shearing site

Abstract

Smith, J. G. 1999. Shearing force of alfalfa stems: effects of cultivar and shearing site. Can. J. Plant Sci. 79: 49-55. A 3-yr study was conducted to investigate the effect of shearing position on shearing force characteristics in alfalfa (Medicago sativa L.) stems and to evaluate the relationship between cell wall (CW) chemical constituents and stem shearing force. Stems from plots of 24 alfalfa cultivars were harvested from the second, third and fourth production years. Each stem was divided into three 16-cm segments: top, middle and bottom, and the force required to shear each segment was measured using a modified Ottawa Texture Measuring System. Cultivar by stem segment interactions were significant (P < 0.05) for stem diameter, weight and shearing force. The middle and bottom stem segments were the best sampling site to detect cultivar differences for shearing force. Stem diameter and weight were generally the variables most associated with shearing force for all three stem segments. However, shearing force characteristics for certain cultivars could not be entirely explained by stem diameter or weight and may be affected by CW chemical constituents and/or structural tissues. The ADL concentrations were not consistently correlated to shearing force at the top and bottom segments over the 3 yr. For the bottom stem segment a negative relationship (r = -0.42, P < 0.001) resulted between shearing force and hemicellulose, while a positive relationship (r = 0.40, P < 0.001) resulted between shearing force and cellulose. Nous avons examiné pendant trois ans les effets de l'emplacement choisi sur la tige de la luzerne (Medicago sativa L.) sur la mesure de la force de cisaillement, ainsi que le rapport entre cette mesure et les constituants membranaires chimiques de la tige. Les tiges de 24 cultivars de luzerne cultivés en parcelles d'essais étaient récoltées dans les 2 e , 3 e et 4 e années d'exploitation. Chaque tige était divisée en trois segments de 16 cm : haut, moyen, bas, sur lesquels on a mesuré la force de cisaillement au moyen d'une modification du système de mesure rhéologique Ottawa (Ottawa Texture Measuring system). On constatait des interactions cultivars x segments significatives quant au diamètre, au poids et à la force de cisaillement de la tige. Les segments du milieu et du bas de la tige étaient ceux qui permettaient le mieux de déceler les différences variétales pour la force de cisaillement. Le diamètre et le poids de la tige étaient généralement, quel que soit le segment, les variables les plus étroitement associées avec la force de cisaillement. Cependant, la force de cisaillement de certains cultivars ne pouvait pas s'expliquer entièrement par le diamètre ou par le poids de la tige et serait éventuellement liée à la nature des constituants membranaires chimiques et/ou des tissus structuraux. Il n'y avait pas de corrélation stable entre les concentrations en LDA (lignine au détergent acide) et la force de cisaillement dans les segments du haut et du bas de la tige. Dans les segments du bas, ...

Lignification and Tannin Localization During the Development of Birdsfoot Trefoil Stems

“…All stems sampled for this study began growth at approximately the same time and therefore added a similar number of internodes to their tips over the 15-wk regrowth period. Shear force was also found to be greater in lower than upper stem segments of alfalfa and was significantly correlated with lignin concentration (Ames et al, 1995;Iwaasa et al, 1995Iwaasa et al, , 1999. This progression of growth and lignification of a basal stem internode has already been assessed for alfalfa (Engels and , which forms a dark blueblack precipitate with tannins.…”

“…They reported a decrease in both internode diameter and secondary xylem thickness that progressed linearly from the base to the tip of stems and found that internodes located close to the base of the stem, such as Internode 6, had more lignified xylem tissue than internodes closer to the stem tip. Shear force was also found to be greater in lower than upper stem segments of alfalfa and was significantly correlated with lignin concentration (Ames et al, 1995;Iwaasa et al, 1995Iwaasa et al, , 1999. The elevated level of shear strength that prevented sheep from harvesting mature BFT stems, as observed by Douglas et al (1999), was likely due to thickening and lignification of the xylary ring of BFT that had progressed to stem internodes above a desirable grazing height.…”

“…Jung (1998, 2005) and Jung andEngels (2001, 2002) thoroughly described the deposition of lignin in cell walls of the seventh internode from the base of alfalfa stems and the implications for rumen digestibility as plants matured during the growing season but no similar histological examination of BFT stem development can be found in the literature. The objective of this study, therefore, was to document the growth of BFT Internode 6 and the accumulation of lignin in stem internode secondary xylem cell walls because lignification is not only negatively associated with fiber digestion ( Jung, 1989) but positively associated with an increase in shear strength of alfalfa stems (Iwaasa et al, 1995(Iwaasa et al, , 1999.…”