Disparate Style Lengths Contribute to Isolation of Species in Rhododendron

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Abstract
For a group of closely related Rhododendron species in section Vireya, it is shown that success of interspecific pollinations is significantly dependent on male/female style length ratio (SLR), and apparently independent of whether both species belong to the same taxonomic subsection. Crosses with SLR < 0.2 or SLR > 6 were unsuccessful, with the probability of success increasing as SLR approached 1.0. When SLR was < 0.2 pollen tubes of the shorter-styled species were unable to reach the ovary. When SLR was > 5-6, pollen tubes of the longer-styled species penetrated deeply into the ovary but were rarely observed to enter ovules. Disparate style lengths are therefore a major incongruity factor among Vireya rhododendrons, and may contribute to species isolation.