In dioecious plant species, there are sexual dimorphisms between the sexes. The cause may be the different allocation on reproduction between sexes. Helwingia japonica is a deciduous shrub that develops underground asexual reproductive organs called runners. In these plants, it is important to clarify the differences in cost of sexual and asexual reproductive organs and in the allocation patterns between male and female. In this study, we compared the difference of the sexual dimorphism and reproductive allocation between male and female in different light conditions.

The cost for reproductive organs flower and fruit in female is larger than that of male. Therefore female cannot develop sexual reproductive organs well under forest canopy. Female in open condition could have more flowers than that of under forest canopy. The cause is the high ratio of leaves of female and their sufficient amount of photosynthesis in open condition.

On the other hand, male under forest canopy could have many flowers than female. And male in open condition could increase flowers further more. They also increased the ratio of roots to whole plant and the ratio of runners to underground parts. Male in open condition use excessive resources to make runners.

In results, female tend to invest a lot of cost in sexual reproductive organs while male in asexual reproductive organs in open condition.