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Influence of Leaf Anatomy on the Dry Matter Digestibility of C₃, C₄, and C₃/C₄ Intermediate Types of Panicum Species¹

Twenty-eight Panicum species were examined to evaluate the influence of anatomy on leaf digestibility. Eighteen species had Kranztype leaf anatomy (C₄ photosynthetic type), seven were non-Kranz (C₃ type) and three were intermediate (designated C₃/C₄) in their anatomical characteristics. Plants were grown in a greenhouse in summer and were sampled on two occasions for measurement of leaf dry matter digestibility (DMD), cell wall content (CWC) and the percentage of mesophyll, vascular, bundle sheath, epidermis and sclerenchyma tissues in leaf cross section. In rank order the C₃ Panicums were high in DMD and low in CWC, the C₃/C₄ species were intermediate and the C₄ species were low in DMD and high in CWC. Average values for the C₃, C₃/C₄ and C₄ species were 76, 70 and 69%, respectively, for DMD, and 33, 42 and 50% for CWC. The C₃ species, P. rivulare Trin., was an exception with low DMD and high CWC. The average leaf tissue proportions for the C₃, C₃/C₄ and C₄ groups were, respectively, 66, 48 and 43% mesophyll, 10, 18 and 20% bundle sheath, 3, 6 and 8% vascular tissue, 22, 26 and 27% epidermis, and 0.5, 1.7 and 1.7% sclerenchyma. Correlations between DMD and proportions of bundle sheath, vascular or mesophyll tissue were highly significant (P < 0.001) for all species or within the C₃ group with coefficients ranging from –0.53 to –0.88 for relationships with bundle sheath or vascular tissue and 0.62 to 0.63 for mesophyll. Within the C₄ group, variation in gross tissue proportions was not closely correlated with DMD. The results indicate that the gross difference in tissue proportions between Kranz and non-Kranz anatomy is probably an important factor contributing to the general difference in digestibility between tropical and temperate grasses. Leaf bulk density (dry weight/leaf volume) also appeared to be an important factor the dry matter digestibility difference between groups and within the C₄ group.

Key Words: Photosynthetic types • Cell wall content • Microscopy • Tissue proportions • Tropical grasses • Panicum spp.

² Plant physiologist, CSIRO, Div. of Trop. Crops and Pastures, Chancellors' Place, St. Lucia, Qld, Australia, 4067; professor, Agronomy Dep., Univ. of Georgia, Athens, GA 30602; animal physiologist, Richard B. Russell Res. Ctr., USDA-ARS, Athens GA 30604.

Received for publication May 24, 1982.