Inheritance of Flower Color in Guayule1

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Abstract
Five sexual diploid guayule (Parthenium argentatum Gray) plants, two with purple and three with white flower color were selfed, open‐pollinated, and crossed in all possible combinations. Except for one white parent which produced two plants, the parents failed to produce progenies by selfing. All of the parents produced progenies upon open‐pollination. Due to the existence of a sporophytic system of incompatibility in diploid guayule, only compatible crosses produced F1 plants. Crosses among white parents produced only white progenies. Reciprocal crosses between two purple parents produced fifty purple and one white plants. Crosses between purple and white parents produced either all white or white and purple progenies in 1:1 ratio. These results and the results obtained from the open‐pollination of the parents indicated that purple flower color is a recessive trait which is controlled by one pair (p controlling purple and P controlling white) of genes. The presence of modifiers accounting for the light to dark purple variability in flower color is postulated also. Purple flower color is expected to serve as a valuable genetic marker especially in studies directed toward the determination of the degree of apomixis in guayule.