Reproductive Biology

Japanese Knotweed, Giant Knotweed and its hybrids and backcrosses have two different sexes. However, it is a rather unusual sex system, known as gynodioecy, where you have female and hermaphrodite individuals. The hermaphrodite individuals produce good pollen, and can also produce small quantities of viable seed when cross-pollinated.  This sex system is thought to have originated from a mutation in a hermaphrodite species, which knocks out pollen production in affected individuals to produce female individuals.  This mutation does not affect floral morphology very much, so that the difference between hermaphrodite and female flowers is basically down to anther size.  When female and hermaphrodite plants are together it is easy to distinguish them.  In female plants the anthers are small empty and are do not stick out from the petals.  Fertile anthers are much larger and are exserted from the perianth. (Beerling et al. 1994).  

Although this flower is female, it still contains small empty anthers Male flowers have long exserted anthers containing pollen, they also have female parts with varying degrees of fertility

Since only a female clone of Japanese Knotweed is found in Britain it is unable to reproduce itself by seed, and any seed found on these plants is the result of pollination by a related species.  It has also been found (Bailey 1989) that the hermaphrodite plants of F. sachalinensis and F. x bohemica are self-incompatible, that is there are unable to form seed without an additional source of pollen.

In all these taxa, the female and hermaphrodite plants may be spotted at the flowering stage due the different ways in which the inflorescences are borne.

Here we have a male plant with erect inflorescences next to a female with drooping flower spikes Flowering occurs in the late Summer and Autumn. This is a male plant in Japan

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