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Phytoalexin Assay in Juvenile Sorghum: An Aid to Breeding for Anthracnose Resistance

Dep. of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843

Richard A. Frederiksen and Ralph L. Nicholson^*

Dep. of Plant Pathology and Microbiology, Texas A&M University, College Station, TX 77843
Dep. of Botany and Plant Pathology, Purdue Univ., West Lafayette, IN 47907

^* Corresponding author.

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 x 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 x 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.

Received for publication March 26, 1992.

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