Abstract
The sorghum [Sorghum bicolor (L.) Moench] plant produces antimicrobial phytoalexins that are essential components of its defense mechanism against attempted infection by Colletotrichum graminicola (Ces.) G.W. Wils., the causal agent of anthracnose. Sorghum phytoalexins are 3‐deoxyanthocyanidin 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 genetic × isolate relationships of the phytoalexin response. Plants were grown in a growth chamber and in the greenhouse and inoculated with two virulent isolates and one avirulent isolate of C. graminicola 7, 21, 35, 49, and 63 d after emergence. Luteolnidin concentration was greater than that of the other phytoalexins in juvenile tissues; however, the ratios of the three compounds changed as the tissues aged and extractable 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 × susceptible F1 progenies than in susceptible inbreds. These relationships were unaffected by the developmental stage of the host. Thus, analysis of phytoalexins in juvenile plants appears to be a potential tool for identification and incorporation of host resistance to C. graminicola in adult plants.
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