No abstract

Hydrophobic exudate from roots of sorghum [Sorghum bicolor(L.) Moench. #3SORVU ‘IS 8768’] contain fourp-benzoquinones which in the dihydroquinone form are active as germination stimulants of witchweed [Striga asiatica(L.) Kuntz. # STRLU]. The three minorp-benzoquinones were partially characterized and found to be structurally similar to sorgoleone, the majorp-benzoquinone of this exudate. Herbicidal activity of the hydrophobic exudate was due to concentration- and pH-dependent inhibition of root elongation in some but not all weeds tested. Witchweed has apparently adapted these “defense” compounds of sorghum as host-specific germination stimulants.

Roots of Sorghum are known to exude materials that exhibit allelochemical activity, but the compounds identified do not completely account for the observed species‐specific allelochemical activities. The purpose of this investigation was to characterize both water‐soluble and water‐insoluble exudates from roots of sorghum [Sorghum bicolor (L.) Moench] seedlings grown in petri dishes. Hydrophilic exudates included phenols, protein, and 3‐deoxyanthocyanidin derivatives. Hydrophobic droplets, exuded from root hairs, tested positively for phenols and lipids. These hydrophobic exudates strongly inhibit (85%) root elongation in lettuce (Lactuca sativa cv. Great Lakes) seedlings, but do not affect that of corn (Zea mays L. cv. B73Ht), nor the germination of either of these plants. Associated with these hydrophobic droplets are novel quinones, yet unidentified. The results indicate that these hydrophobic exudate droplets contain components that may have species‐specific biological activities.

Selected fungi were grown on agar plates in the presence of naringenin, apiforol, apiforol 7-O-rhamnoglucoside, or apigeninidin. Of the four compounds tested, only apigeninidin inhibited the growth ofFusarium oxysporum, Gibberella zeae, Gliocladium roseum, Altemaria solani, and Phytophthora infestons. In contrast, the growth ofRhizoctonia solani, Sclerotium rolfsii, and Rhizopus stolonifer (- and +) was not effected by any compound. Since apigeninidin is present in seeds ofSorghum sp., we hypothesize that apigeninidin may play a role in mold resistance and that apiforol accumulates as a biosynthetic precursor of apigeninidin, not as a fungal defense compound.

Apigeninidin, a 3-deoxyanthocyanidin present in sorghum, was chemically synthesized and effectively purified by extracting impurities from aqueous 2 N HC1 into ethyl acetate. Apigeninidin (50μmol) applied to filter paper disks inhibited the growth on agar plates of selected gram-positive bacteria (Bacillus cereus, Staphylococcus aureus, Staphylococcus epidermidis, andStreptococcus faecalis) and to a lesser extent gram-negative bacteria (Escherichia coli, Serratia marcens, andShigella flexneri). Apigeninidin was not mutagenic based on the Ames assay. The ability of apigeninidin to precipitate proteins from plant extracts could be physiologically important if it can be shown to bind to specific plant proteins.