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

Dep. of Agronomy and USDA-ARS, Univ. of Missouri-Columbia, Columbia, MO 65211

^* Corresponding author (Email: agrofesq{at}mizzoul.missouri.edu).

Information is needed on the relationship between anatomical structure and the rate and extent of cell wall degradation in warm-season forage grasses. The objective was to study the pattern, rate, and extent of tissue degradation of three warm-season grasses in a prepared cellulase solution, using light microscopy. Most recent fully collared leaf blades from field-grown big bluestem (Andropogon gerardii Vitman), indiangrass [Sorghastrum nutans (L.) Nash] and switchgrass (Panicum virgatum L.) were cut into 5-cm sections and digested in 10 mL prepared cellulase solution with an acid-pepsin pretreatment. All material was in a vegetative state. Leaf segments were retrieved after 3, 9, 24, 48, and 72 h incubation and prepared for light microscopy by fixation, dehydration and embedding in Spurr's low-viscosity resin. Sections 5 µm thick were made using an ultramicrotome, and undigested tissues in cross section were measured with a computer-based image analysis system. The proportion of undigested tissue in cross section after 72 h incubation ranged from 25.4% in big bluestem to 35.0% in switchgrass. Switchgrass contained a higher percentage of parenchyma bundle sheath tissue than either big bluestem or indiangrass. Digestion of all tissues except mesophyll and phloem was delayed until after 24 h incubation. Differences among species tended to be greater in extent of digestion than in rate. Degradation of the parenchyma bundle sheath was significantly less in switchgrass than in either big bluestem or indiangrass after 72 h incubation. Digestion of the abaxial epidermis was nearly complete in big bluestem after 72 h incubation. The adaxial epidermis was more readily degraded than the abaxial epidermis. These results imply that differences in digestibility among the three warm-season grasses could be attributed to differences in the degradation of their individual tissue types.

Received for publication January 4, 1994.