Ultrastructure of Cell Wall Degradation in Panicum Species Differing in Digestibility1

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Abstract
The genus Panicum has been shown to have species exhibiting a range of leaf digestibility. Leaf anatomy as well as differences in degradation of specific tissues have been implicated as factors influencing the digestibility. The objective of the present report was to examine microscopically specific cell wall types in leaves of upper and lower nodes for the pattern of attack and the microorganisms involved in cell wall degradation in Panicum species having different digestibilities. Lignified tissues (i.e., sclerenchyma and vascular tissue) were the most resistant, with xylem cells essentially not degraded and sclerenchyma cells degraded at the periphery. Mesophyll cells displayed a range in digestibility; bacteria did not adhere to the cells. The mesophyll in bottom lamina tended to be more rapidly degraded than in upper lamina of P. milioides Nees ex Trin. and P. laxum Sw. whereas differences were not observed between upper and lower lamina in other species. Epidermal cells were usually degraded after adherence by bacteria, which formed clearly defined zones of erosion in cell walls and separated the intact cuticle from the blades. However, cell walls of P. tricanthum often showed zones of erosion without adhering types of bacteria. Bundle sheath cells of C4 species were degraded mostly by adhering types of bacteria, with a thin electron dense layer often resisting digestion. Sheath cells in P. milioides and P. tricanthum were totally degraded. Two distinct morphotypes of bacteria, i.e., encapsulated cocci and irregularly‐shaped bacteria, were the predominant ones that adhered to and attacked the more slowly degraded tissues. Easily degraded cells were degraded by nonadhering microorganisms. Results indicated that differences existed in the manner of attack on plant cell walls by rumen bacteria and that these differences were reflected in variation in the ease of digestion.