Ultrastructural techniques to investigate cell wall degradation and antiquality factors in two bermudagrass cultivars

Two bermudagrass (Cynodon dactylon L. Pers.) hybrids, 'Tifton 78' (Tift-78) and 'Coastal' (CBG), differ in dry matter digestibility. The objective of this study was to distinguish factors at the ultrastructural level which may contribute to the differences in digestibility. Percent tissue types and histological reactions for lignin were similar for both culltivars. Scanning electron microscopy (SEM) of leaf blades incubated in rumen fluid or cellulase revealed that nonlignified tissues were digested to a greater extent in Tift-78 than in CBG. Pretreatment with 0.1 M HCl or acid-pepsin augmented cellulase digestion of the more slowly degraded tissues. Transmission electron microscopy showed that Tift-78 had a higher number of attached bacteria during incubation with rumen fluid. The ratio of attached Bacteroides to Ruminococcus morphotypes also varied between cultivars. After 24 h, Tift-78 supported a population of predominantly Bacteroides (74%), whereas CBG had only Bacteroides. By 72 h, the attached population of Ruminococcus and Bacteroides had shifted to 59% and 17%, respectively, for Tift-78 and 60% and 0%, respectively, for CBG. A SEM-energy dispersive X-ray spectroscopic analysis of the elemental composition of parenchymal bundle sheath and epidermal cell walls revealed that Cl was significantly higher in untreated tissues of CBG than Tift-78 but higher in Tift-78 after cellulase incubation. The higher concentration of Cl in CBG may prevent initial colonization of cell walls by specific bacterial morphotypes and reduce the number of attaching bacteria, thus reducing digestibility. Silicon, which has been described as an antiquality factor, did not differ between cultivars. Response to enzymes indicated that structural carbohydrates in specific cell walls varied in digestibility between bermudagrass cultivars, and differences in cell walls further influenced the adherence of fiber-digesting bacteria.