Gynodioecy in Lycopus maackianus Makino (Lamiaceae) in Korea: floral dimorphism and nutlet production

Abstract

The sexual system of Lycopus maackianus Makino (Lamiaceae), a perennial herb, has never been investigated in depth before. The present study is focusing on the careful investigation of L. maackianus flowers collected from a natural population in Korea to clarify their precise sexual system. The results confirm that L. maackianus is clearly a gynodioecious plant. It appears that the hermaphrodite individuals of L. maackianus are protandrous. On average, females produced more than 37 times as many nutlets as did hermaphrodites. In L. maackianus, all investigated floral characters (e.g., corolla, calyx, filament, anther, style, stigma, nutlet) are significantly different between the two morphs. Hermaphroditic plants are larger than the female ones in corolla length (P < 0.0001), calyx width (P = 0.006), filament length (P < 0.0001), anther length (P < 0.0001) and style length (P < 0.0001). Female plants have longer stigmas (P < 0.0001) and nutlets (P < 0.0001) than the hermaphrodites. The additional observation of some micromorphological characters (e.g., petal and sepal cell structure, leaf surface, nutlet morphology) on the two morphs using SEM did not reveal significant differences between female and hermaphrodite plants.