Sorghum (Sorghum bicolor) synthesizes a complex mixture of 3-deoxyanthocyanidin phytoalexins in response to
inoculation with the non-pathogenic fungus Bipolaris maydis. The anthocyanin cyanidin 3-dimalonyl glucoside,
is also synthesized naturally in response to light. To determine the order and time of appearance of these
compounds, etiolated sorghum mesocotyls were inoculated with B. maydis and tissue extracts were analysed by
photodiode array-HPLC every 2 h for the first 24 h and at 48 h post inoculation (hpi). Uninoculated and inoculated
etiolated mesocotyls were also allowed to incorporate L-[U-14C] phenylalanine. Apigeninidin appeared at 10 hpi,
followed by luteolinidin and apigeninidin 5-O-arabinoside at 14 hpi. Luteolinidin 5-methylether was not detected
until 18 hpi and apigeninidin 7-methylether not until 20 hpi. The concentrations of the primary phytoalexins,
apigeninidin, luteolinidin and apigeninidin 5-O-arabinoside, rose steadily between 12 and 24 hpi, and the levels
of apigeninidin and luteolinidin were approximately equivalent by 24 hpi. However, between 24 and 48 hpi
luteolinidin and luteolinidin 5-methylether accumulated rapidly so that by 48 hpi the amounts of luteolinidin and
luteolinidin 5-methylether had increased approximately twofold. Radiolabelling also showed that 14C was
incorporated into the 3-deoxyanthocyanidins and cyanidin 3-dimalonyl glucoside. Several other unidentified
phenolic compounds also accumulated radioactivity.