The brown-midrib variety (BMR) of sorghum (Sorghum bicolor) has gained attention as both forage material and feedstock for biofuels. BMR mutants of sorghum have lower lignin concentrations due to alterations in the phenylpropanoid pathway. The bmr6 and bmr12 mutants have impaired activity of cinnamyl alcohol dehydrogenase or caffeate/5 hydroxyferulate O-methyl transferase enzymes, respectively. We investigated the effects of ultraviolet-B radiation (UV-B; 280-320 nm) on wild type (WT), bmr6, bmr12, and double mutant (DM) varieties growing in a UV-transparent greenhouse in southern Minnesota. Plants were grown under canopies with filters that transmitted either 2.8% or 90% of ambient UVB for 63 days. We monitored UV-screening effectiveness and the ratio of variable to maximal chlorophyll fluorescence (Fv/Fm) 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 (ℷ=300 and 370 nm). We also measured final cellulose, hemicellulose and lignin concentrations of plants using a fiber analyzer.
Plants growing under near-ambient UV-B were 4-6% shorter than those under reduced-UV-B treatments for the WT, bmr12 and DM varieties. There were no consistent differences in Fv/Fm between treatments over the course of the experiment. Epidermal transmittance was typically lower under near-ambient UV-B treatments for all four varieties and values ranged from 89% to 51% over the course of 63 d. However, there were no differences in concentrations of bulk-soluble UV-absorbing compounds. Constituent analyses of plant material is ongoing. It appears that near-ambient UV-B influences growth and UV-screening properties of sorghum regardless of alterations in phenlypropanoid metabolism.