Abstract

Marginally digested tissues in the brown midrib-12 (bmr) sorghum (Sorghum bicolor (L.) Moench.) mutant are more readily degraded than similar tissues in the normal (N) line. Reduced lignin concentrations have been reported to be a factor affecting digestibility of bmr genotypes, but lignin type could also influence digestibility. The objectives of this work were to (i) examine N-12 and bmr-12 plants for variations in chemical composition, including phenolic acids and phenolic structure, using solid-state nuclear magnetic resonance (NMR) spectroscopy; and (ii) evaluate by microscopy the effect of permanganate delignification. Leaf blades of bmr plants were significantly (Ρ ≤ 0.05) lower in neutral detergent fiber (NDF) and permanganate lignin (PML), and higher in in vitro dry matter digestibility; leaf sheaths contained significantly less PML. Solid-state NMR spectroscopy suggested that bmr leaf blades had lower amounts of bound syringyl moieties. The predominant alkali-soluble (1.0 M N_aOH) phenolic acids in both plant types were ρ-coumaric and ferulic acids, and ρ-coumaric acid was significantly lower (Ρ ≤ 0.05) in concentration in bmr leaves. Treatment of intact blade sections with permanganate to oxidize lignin compounds resulted in a reduced staining intensity for lignin identified by both the acid phloroglucinol and chlorine-sulfite tests in all but midrib sclerenchyma; lignin staining intensity did not differ between plant types. Degradation of delignified midribs by rumen microbes indicated that the tissues were more degradable in bmr than in N plant types. Results suggest that reduced concentrations of phenolic acids (especially ρ-coumaric acid) and reduced levels of etherated syringyl moieties in conjunction with reduced concentrations of lignin are factors in the improved quality of bmr-12 sorghum.

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