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Site and Rate of Tissue Digestion in Leaves of C₃, C₄, and C₃/C₄ Intermediate Panicum Species¹

Digestion is lower for grasses adapted to tropical regions than for species from temperate areas. These differences in digestibility have been attributed to anatomical structure and to effects of temperature. This research compared the degradation of tissue types in leaf blades of 29 Panicum species of tropical origin having diverse anatomies and representing C₃, C₄, and intermediate (C₃/C₄) photosynthetic pathways and grown in identical environments. Blades were incubated in rumen fluid, and tissue degradation was studied by light and scanning electron microscopy. Mesophyll was rapidly degraded in most C₄ and C₃ species and estimated to be 86 and 80%, respectively, after 20 hours incubation; a few species of each type (e.g., P. effusum and P. laxum) exhibited slower mesophyll degradation than other grasses. However, the mesophyll digestibility in the C₃/C₄ species was unexpectedly low, averaging only 72% after incubation for 20 hours and only 50% for P. milioides. Epidermal tissues were readily degraded in most species and the average digestibility of the adaxial epidermis tended to be higher (5 to 8 percentage units) than that the abaxial for all photosynthetic types. Parenchyma bundle sheaths were about 90 to 100% digested both in C₃ and C₃/C₄ species, but those of many C₄ species were poorly degraded with an average for all species of only 50% after incubation for 20 hours. The outer portion of the sheath cell wall was particularly low in degradation and remained in many C₄ species after 48 hours incubation. The low degradation of parenchyma bundle sheath appeared to be a major influence on blade digestibility in C₄ species. Vascular tissues were either not degraded or only partially degraded. These results suggest that improved forage quality could result from plant breeding to increase the degradation of limiting tissues, e.g., the parenchyma bundle sheaths of C₄ species, without alterations in the basic leaf anatomical structure. Improvement in quality may also be possible by breeding to increase the proportion of mesophyll content, but such a change may not always improve digestibility since the mesophyll of some species was more slowly degraded than that of others.

Key Words: Light microscopy • Scanning electron microscopy • Mesophyll • Epidermis • Parenchyma bundle sheath • Forage quality • Leaf anatomy • Panicum spp.

² Microbiologist, USDA-ARS, plant physiologist, CSIRO, and animal physiologist, USDA-ARS, respectively.

Received for publication May 24, 1982.