Variations in anatomy and ultraviolet microspectrometry between normal and brown midrib mutant maizes possessing different rumen degradabilities

Normal and brown midrib mutant (bmr) maize (Zea mays L) were examined for variations in their morphological composition. The degradability of the leaf blade, leaf sheath and stem, proportional area of specific tissues in leaf blade, and the ultraviolet (UV) absorption spectra of cell walls were measured and related to variations in cell wall degradability by rumen microorganisms. The UV and infrared (IR) absorption spectra of the lignins isolated from leaf blades of both types, before and after reduction with sodium borohydride, were recorded. The bmr3 maize had higher dry matter (DM) and neutral detergent fibre (NDF) degradabilities for leaf blade, leaf sheath, and stem than the normal counterpart. Approximately 35% and 26% of the observed difference in DM degradability was attributed to the difference in DM degradability of stem and leaf blade. respectively, and 39% to the difference in DM composition of stem. Distinct differences in tissue degradation of the leaf blades were observed for mesophyll cell walls in the midrib portion, which were thinner and of greater number in the bmr3 maize. Sclerenchyma cells were present only in the vascular bundles in the bmr3 leaf blade, while in the normal type those cells were underneath the epidermis tissue. The bmr3 plant also had large epidermal cells. UV microspectrometry of mesophyll cell walls of the bmr3 maize showed their lower UV absorbance around 320 nm compared to that of the normal, but not at 280 nm. Considerable increase in the UV absorbance at 280 nm was observed for the isolated lignins after reduction, suggesting a lesser degree of lignification in the bmr3 maize tissues. Lowered UV absorbance of the isolated lignin around 320 nm after reduction was associated with the removal of the IR bands at 1730, 1660, 1600, and 1250 cm-1.