Breeding of Double-flowered and Trailing Catharanthus roseus

Abstract Periwinkle [Catharanthus roseus (G.). Don] is an important summer bedding and potted plant species. Cultivar with both trailing and double-flower habits is presently limited. The study employs double-flowered and upright ‘Taoyuan’ series, and single-flowered and trailing ‘Cora Cascade’ series and ‘Mediterranean’ series to elucidate 1) mechanism of trailing habit, 2) inheritance of trailing and double-flower habits, and flower color, and 3) to breed new cultivars with both trailing and doulbe-flower habits. Free hand cross sections were made for upright and trailing cultivars at the first to the third internode above cotyledons. Results showed that all 5 upright cultivars had phloem fiber, but respective cells in trailing ‘Cora Cascade’ series, ‘Mediterranean’ series, and ‘Boa’ series did not have secondary wall. Internode below the second leaf pairs from top of upright ‘Vitesse Pink’ were also sectioned, and stained by phloroglucinol-HCl and iodine chloride to reveal lignin and cellulose. Results showed that secondary wall of phloem fiber had low lignin, and high cellulose content, and categorized as gelatinous fiber. Plants of ‘Vitesse Pink’ with 2-3 leaf pairs were tilted-cultivated for 28 days and cross sectioned at bending stems. Phloem fiber at tension side showed increased diameter and thickened secondary wall, indicating the phloem fiber is a reaction fiber and functions as upright growth support. Self-pollination, F1, F2, and backcross generation of upright and double-flowered ‘Taoyuan’ series, and trailing and single-flowered ‘Cora Cascade’ series and ‘Mediterranean’ series were made. Results showed that F1 population between upright and trailing cultivars were all upright, and the F2 population showed 3 : 1 segregation ratio among upright and trailing progenies (χ2 = 0 – 2.84). Trailing habit in periwinkle is recessive to upright habit, and is controlled by a single major gene. In addition, the F2 population showed 9 : 3 : 3 : 1 ratio among upright+single-flowered, trailing+single-flowered, upright+double-flowered, and trailing+double-flowered progenies (χ2 = 0.5 – 7.4). Loci responsible for upright/ trailing and single-/ double-flower habits in periwinkle are inherited independently. Modulus of elasticity (MOE) of upright ‘Little Pinkie’, ‘Vitesse Pink’, and trailing ‘Cora Cascade Polka Dot’, ‘Cora Cascade Strawberry’, and ‘Cora Cascade Magenta’ were measured by microtester. Results of correlation coefficient analysis between MOE and tissue structure showed that trailing phenotype is governed by presence of phloem fiber. Plants that lack phloem fiber tend to have lower MOE and more likely to bend during growth and showed trailing phenotype of 25 days after potting. Moreover, xylem proportion of also contributed to MOE and trailing level, especially for basal stem and more mature position. Periwinkle cultivars without phloem fiber and lower xylem proportion had lower MOE, which had higher bending angle and more trailing level Four ‘Cora Cascade’ series cultivars, two ‘Mediterranean’ series cultivars, and three ‘Taoyuan’ series cultivars were used to study inheritance of flower color and pollen-less male steriling. Flower color of progenies from self-pollination, F1 and F2 population segregated and followed one gene model of dominant rules as: violet > cherry > pink > apricot > white. Progenies from self-pollination of cultivars with red receptacle segregated for 3 : 1 ratio among red and yellow receptacle (χ2 = 0.11 – 3.25). Red receptacle in periwinkle is dominant to yellow receptacle, and is controlled by single gene. Progenies of self-pollinated red medium-eyed ‘Cora Cascade Strawberry’ segregated for small- and medium-sized eye in 3 : 1 ratio (χ2 = 0.35). Red medium-sized eye in periwinkle is dominant to red small-sized eye, and is also controlled by single gene. Male steriling in periwinkle is recessive to fertile male, and is controlled by single gene. Lobules margined ‘Angel Tutu’ was pollinated with double-flowerd ‘Taoyuan No.4-Summer Snow’, and F2 population was made to study petal margin and style-less. Results showed that F1 population all had entire petal margin and style. Progenies of F2 population segregated for entire petal margin and lobules margin in 3 : 1 ratio (χ2 = 0.33-3.02). And style-less trait is linked with lobules margin. Double-flowered plants showed lobules margin even at heterozygous state. NAC secondary wall promoting factor 1 of Arabidopsis thaliana (AtNST1) was blsated with gene sequence of transciptome in periwinkle, and similar sequence was observed. However, coding DNA sequence of CrNST1 in upright ‘Vitesse Pink’ and trailing ‘Cora Cascade Polka Dot’ did not showed difference. CrNST1 expression level is more in ‘Vitesse Pink’than in Cora Cascade Polka Dot’stem. Three vigourous and well-branching double-flowered and trailing plants from F2 population were selected and cutting-propagated. DUS test following announced methods was made to breed first double-flowered and trailing periwinkle in the world.