Ultrastructural Techniques to Investigate Cell Wall Degradation and Antiquality Factors in Two Bermudagrass Cultivars

Abstract

Two bermudagrass (Cynodon dactylon L. Pers.) hybrids, ‘Tifton 78’ (Tilt‐78) and ‘Coastal’ (CBG), differ in dry matter digestibility. The objective of this study was to distinguish factors at the ultra‐structural level which may contribute to the differences in digestibility. Percent tissue types and histological reactions for lignin were similar for both cultivars. Scanning electron microscopy (SEM) leaf blades incubated in rumen fluid or cellulase revealed that non‐lignified tissues were digested to a greater extent in Tift‐78 than in CBG. Pretreatment with 0.1 M HCI 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 CI was significantly higher in untreated tissues of CBG than Tift‐78 but higher in Tift‐78 after cellulase incubation. The higher concentration of CI 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. Responses 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.

Degradation of Three Warm‐Season Grasses in a Prepared Cellulase Solution

“…For example, delignification of Coastal bermudagrass with potassium permanganate resulted in increased in vitro digestion (32% after 72 h incubation) of sclerenchyma tissue (Barton and Akin, 1977); however, much of this was from sclerenchyma adjacent to minor vascular bundles. Woodward et al (1989) subjected two bermudagrass hybrids to digestion in rumen fluid and a prepared cellulase solution. They found that sclerenchyma, xylem, and parenchyma bundle sheath cells of large vascular bundles were most resistant to digestion, but concluded that pretreatment with acid pepsin facilitated degradation of sclerenchyma by prepared cellulase solution.…”

“…The cellulase procedure is both faster and more cost effective than the rumen fluid; therefore, future work focusing on the anatomy of these grasses could better utilize the cellulase procedure. Woodward et al (1989) compared cell wall degradability oftwo bermudagrass cultivars using rumen fluid or a commercial cellulase solution. They found that 48 h pretreatment with acid pepsin plus 72 h incubation with the cellulase solution compared favorably with digestion with rumen microbes and also gave the most extensive digestion.…”