PHYTOALEXIN ASSAY IN JUVENILE SORGHUM - AN AID TO BREEDING FOR ANTHRACNOSE RESISTANCE

The sorghum [Sorghum bicolor (L.) Moench] plant produces antimicrobial phytoalexins that are essential components of its defense mechanism a gainst attempted infection by Colletotrichum graminicola (Ces.) G.W. W ils., the causal agent or anthracnose. Sorghum phytoalexins are 3-deox yanthocyanidin flavonoids shown to be luteolinidin, apigeninidin, and a caffeic acid ester of arabinosyl 5-O-apigeninidin. Crosses were made between resistant and susceptible sorghum cultivars to examine the ge netic x isolate relationships of the phytoalexin response. Plants were grown in a growth chamber and in the greenhouse and inoculated with t wo virulent isolates and one avirulent isolate of C. graminicola 7, 21 , 35, 49, and 63 d after emergence. Luteolnidin concentration was grea ter than that of the other phytoalexins in juvenile tissues; however, the ratios of the three compounds changed as the tissues aged and extr actable amounts in tissues older than 21 d were lower than in juvenile tissues. All three phytoalexins accumulated to greater concentrations in resistant inbreds and resistant x susceptible F1 progenies than in susceptible inbreds. These relationships were unaffected by the devel opmental stage of the host. Thus, analysis of phytoalexins in juvenile plants appears to be a potential tool for identification and incorpor ation of host resistance to C. graminicola in adult plants.