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

Abstract

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.