Periwinkle, a member of the Apocynaceae family, has long-lasting flowering periods under warm climates, with many flower color variations. Periwinkle has become an important bedding plant or potted flowering plant in the world owing to its tolerance to heat and drought, and well adaptation (Curry, 2012). Periwinkle is also known for its use of terpenoid indole alkaloids for treating cardiac diseases and certain tumors in mammals (Zhou et al., 2009).
Most periwinkle cultivars have an upright growth habit, although trailing cultivars are desirable as they have increased their utility as ornamental plants for uses in hanging baskets and mixed containers, and as groundcover in the landscape. Trailing habits can be related to less or gradual loss of fiber tissue within stems (Evert, 2006). Fibers provide much of the mechanical support; therefore, subshrubs are expected to have a higher portion of fibers than climbers (Crivellaro et al., 2012). In non-self-supporting Clematis vitalba L. and Clematis maritima L., primary phloem fiber and collenchyma are displaced outward by plant development, which led to disrupting the mechanical integrity, but not in semiself-supporting Clematis recta L. (Rowe et al., 2004).
Modulus of elasticity measures the resistance of an object or substance to being deformed elastically when a mechanical force is applied to it. Biomechanical properties, such as stem stiffness or elasticity can be quantitatively compared with MOE (Niklas, 1995). MOE decreased significantly after loss of fibers in C. vitalba and C. maritima, but not in C. recta, which remained higher MOE (Rowe et al., 2004). Stems of upright periwinkle contained phloem fiber (Kartmazova, 1972), but no information is available on stem anatomy or MOE of trailing cultivars.
Genetic control of growth habit varies among plant species. For example, growth habit of cowpea [Vigna unguiculata (L.) Walp.] is governed by two allelic pairs, AA and BB. The genotype A_B_ has the prostrate growth habit; the genotype aaB_ has the climbing growth habit, and aabb has the bushy habit (Uguru and Uzo, 1991). In red clover (Trifolium pretense L.), a dominance × dominance epistasis controls prostrateness, with prostrate being partially dominant to erect (Mirzaie-Nodoushan et al., 1999). Egyptian clover (Trifolium alexandrinum L.) is erect, whereas Trifolium apertum Bobrov is prostrate, and all their F1 plants were erect, indicating thereby dominance of erect habit over the prostrate (Malaviya et al., 2004). Growth habit of coleus [Solenostemon scutellarioides (L.) Codd] is controlled by a single gene (U) with additive effects, and upright is dominant to trailing. Upright growth habit is designated with a UUUU genotype and trailing with a uuuu genotype (Nguyen et al., 2008). Information on the genetics for the trailing growth habit of periwinkle is presently limited.
Flower doubleness generally increases the ornamental value of floriculture crops. We have released double-flowered periwinkle cultivars (Chen and Yeh, 2012) and proposed that a nuclear recessive gene controls the double-flowering phenotype, and the dominant allele conditions single flowers in either the homozygous or heterozygous state (Chen et al., 2012). Presently, there appears no double-flowered periwinkle cultivar with a trailing growth habit.
The objectives of this study were to 1) compare the stem anatomy and MOE between upright and trailing cultivars, 2) determine the genetics and inheritance of trailing habit and flower form, and 3) develop new periwinkle progeny with trailing habit and double flowers.
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