Absence of Dominance of the Cu Gene in Influencing o-Hydroxycinnamic Acid Content in Melilotus alba

The average content of o-hydroxycinnamic acid in sweetclover plants of the Cucu genotype was 36% of the level found in CuCu plants. Thus, the level of o-hydroxycinnamic acid in the Fl was more like the content of the cucu parent than that of the CuCu parent, indicating that cu was partially dominant over Cu. The calculated value for the degree of dominance of cu over Cu was +0.299. The partial dominance of the cu gene provides an explanation for the difficulty experienced in classifying plants in segregating populations. Additional index words: Sweetc1over, Coumarin. I N sweetclover (Melilotus alba Desr.), the allelic pair designated Cujcu influences content of o-coumari.c and coumarinic acid glucosides (the trans and CIS isomers of p-D-glucosyl-o-hydroxycinnamic acid, respectively) apparently by controlling the o-hydroxylation of trans-cinnamic acid (1, 8). The cu allele was 1 Contribution from the Crops Research Division, Agricultural Research Service, u. S. Department of Agriculture, and the Nebraska Agricultural Experiment Station, Lincoln, Nebr. Supported III part by the National Science Foundation (Grant No. GB-1l48). Published with the approval of the Director as Paper no. 2387, Journal Series, Nebraska Agr. Exp. Sta. Received July 24, 1968. 2 Research Geneticist, Crops Research Division, ARS, USDA, and Bert Rodgers Professor of Agronomy, University of Nebraska, Lincoln 68503, respectively. The assistance of Dr. C. o. Gardner and J. J. Hammond in analyzing the data and the technical assistance of J. F. Hermanussen and A. F. Welty are gratefully acknowledged. derived from M. dentata (Waldst. & Kit.) Pers.; the transfer was effected by interspecific hybridization with M. alba, followed by grafting of the albinistic Fl hybrids to plants of M. officinalis (L.) Lam. (12). Sweetclover plants of the CuCu genotype contain substantial amounts of both o-coumaric acid glucoside and coumarinic acid glucoside (also known as bound coumarin); cucu plants are low in content of both glucosides (6). In tissues that are disrupted, coumarinic acid glucoside is rapidly hydrolyzed by the action of endogenous p-glucosidase to yield coumarinic acid which lactonizes spontaneously to form coumarin (6). p-Glucosidase activity is under the control of the Bjb alleles (7, 11) which are independent of the Cujcu locus. The term "coumarin" has been used extensively in literature pertaining to sweetclover and has a variety of meanings in today's terminology. For example, in some cases the term refers to an unknown portion of the coumarinic acid glucoside or to all of it; but in other cases it designates the sum total of both glucosides. Nevertheless, the term will be retained in some sections below when referring to previous work. The inheritance of coumarin content in M. alba has been studied by several workers but only those investigations dealing with low-coumarin plants derived from the interspecific cross with M. dentata will be considered in this review. Smith (13) reported the transfer from M. dentata to M. alba of two genes influencing coumarin content, one of which displayed 80 CROP SCIENCE. VOL. 9. JANUARY-FEBRUARY 1969 no dominance. In the light of present-day knowledge. the two allelic pairs referred to by Smith undoubtedly were the Gujcu and Bjb genes. Goplen et al. (3) reported that coumarin production in M. alba was governed by a single partially dominant gene. for which the symbol Gu was proposed. They also stated that the average content of F 1 plants was approximately one-half that of the selfs from the high-coumarin parents. Rudorf and Schwarze (10) proposed a two-gene hypothesis in which the homozygous recessive CUI CUI was thought to reduce coumarin content to IjlO of the normal level, while CU2CU2 reduced the coumarin content from 1/10 to 1/100 of the normal level. Micke (9) obtained FI contents varying from 1/10 to 1/2 of the levels found in parental plants high in coumarinic acid glucoside and observed monofactorial, semidominant segregations in the F2 and Fs generations. Although several of the above authors have recognized the intermediate nature of the heterozygote, variability associated with environmental effects and the heterogeneous nature of the plant material used have obscured the precise nature of the dominance inherent in the Gu allele. Accordingly, the present experiments were designed to furnish information on the degree of dominance of the Gu gene with respect to total content of both coumarinic acid and o-coumaric