Genetics and Biochemistry of Flower Color in Stokes Aster [Stokesia laevis (J. Hill) Greene]

Abstract

The flowers of 9 cultivars, 4 F₁ hybrid plants, and one F₂ hybrid plant of stokes aster [Stokesia laevis (J. Hill) Greene] were analyzed using high performance liquid chromatography (HPLC) to determine pigment composition. Flowers that were pale blue ('Omega Skyrocket', 'Blue Danube'), lavender ('Peaches'), or violet ('Honeysong Purple', 'Purple Parasols') each contained a single anthocyanidin, petunidin. Pale magenta flowers ('Maroon', 'Colorwheel') contained a single anthocyanidin, cyanidin. Albescent flowers ('Alba') contained pigment, but the amount was substantially smaller than the amount that was isolated from the other cultivars. We were not able to identify which anthocyanidin(s) was (were) present because the quantity was too small. Flowers of F¹ hybrid progeny from crosses of 'Maroon' (cyanidin) x 'Honeysong' (petunidin), 'Maroon' (cyanidin) x ;Peaches' (petunidin), and 'Peaches' (petunidin) x 'Colorwheel' (cyanidin) contained only petunidin demonstrating that petunidin synthesis is dominant to the synthesis of cyanidin. These biochemical results support a previous genetic study that determined that pale blue/lavender flower color (petunidin) is completely dominant to pale magenta flower color (cyanidin) in the F₁ generation. Pale yellow flowers ('Mary Gregory') did not contain anthocyanidins or carotenoids, but did contain the copigment, luteolin. Thus, it was proposed that yellow flower color in this individual is attributable to the presence of luteolin. Analysis of other cultivars, revealed that all flowers of stokes aster contain luteolin.

Inheritance of flower color was investigated in F₁, F₂, and BC₁ families derived from various combinations of different cultivars.The results of these studies suggest that two genes control pale blue/lavender, albescent, and pale yellow flower color in stokes aster. It was proposed that albescent flower color is controlled by a single gene, designated alb. This gene has two alleles and a system of complete dominance. Based on this classification, plants with the AlbAlb or Albalb genotype have pale blue/lavender flowers that contain wild type amounts of petunidin and luteolin. Plants with the albalb genotype have albescent flowers practically devoid of anthocyanidins and contain reduced amounts of luteolin. The second gene, designated y, controls pale yellow flower color. This gene has two alleles and a system of complete dominance where plants with the YY or Yy genotype have blue/lavender flowers that contain wild type amounts of petunidin and luteolin. Plants with the yy genotype are pale yellow and do not contain anthocyanidins, but do contain luteolin. It was proposed that Y is the structural gene that encodes for the enzyme, flavanone 3-hydroxylase (F3H). Further analysis suggested that the homozygous recessive form of Y is epistatic to Alb. As of yet, it is unclear whether the double recessive genotype albalb yy is lethal. A third gene, designated Mag, was proposed to encode for flavonoid 3',5'-hydroxylase (F3'5'H). A hypothesis was presented to explain how Mag controls the synthesis of petunidin versus cyanidin. A theoretical biochemical pathway for flavonoid biosynthesis in stokes aster and suggestions for future research are proposed.