The effects of ultraviolet radiation on the brown midrib mutants of Zea mays and Sorghum bicolor

Abstract

The brown-midrib varieties (bm/bmr) of corn (Zea mays) and sorghum (Sorghum bicolor) have gained attention as both forage material and feedstock for biofuel. These varieties are attractive due to their low-lignin content resulting from alterations in the phenylpropanoid pathway. These bm/bmr varieties have impaired activity of cinnamyl alcohol dehydrogenase (CAD) or caffeate/5 hydroxyferulate O-methyl transferase (COMT) enzymes. We examined the effects of ultraviolet-B radiation (UV-B; 280–320 nm) on bm3 corn and bmr6 sorghum growing in a UV-transparent greenhouse in southern Minnesota. Plants grew under filters that either transmitted 2.8% (mylar) or 90% (aclar) of UV-B for 71 days. We monitored UV-screening effectiveness and chlorophyll fluorescence of leaves with pulse-amplitude modulated fluorometers. In order to explain differences in UV-screening effectiveness, we estimated concentrations of bulk-soluble UV-absorbing compounds (λ = 300 and 370 nm). We also measured final cellulose, hemicellulose and lignin concentrations of plants using a fiber analyzer. Epidermal transmittance of UV was reduced under aclar filters in both species and decreased from 41.6 to 1.9% in corn and 66.4 to 0.98% in sorghum. Increased epidermal screening may be related to the 35–39 and 37–54% higher concentrations of UV-absorbing compounds in plants growing under aclar. Concentrations of lignin were 1.7% lower in corn and 1.1% higher in sorghum in plants under aclar treatments. These contrasting results may be the result of alterations in the phenylpropanoid pool associated with each bm/bmr variety.