Within-season adjustment of sex expression in females and hermaphrodites of the clonal gynodioecious herb <Emphasis Type="Italic">Glechoma longituba</Emphasis> (Lamiaceae)

The within-season modification or adjustment of sex expression was studied in the gynodioecious herb Glechoma longituba. The results from seven natural populations indicated that the seed sets of hermaphrodite ramets were relatively stable compared to those of female ramets. However, a negative correlation was detected between the seed set of female ramets and their frequency of occurrence in a population. Moreover, a negative correlation was also detected between the degree of gender modification in the late flowering period and the seed set in female ramets. Controlled experiments on both wild and cultivated plants indicated that the resource adjustment, presumably caused by flower removal or the hand pollination of flowers, in the early-middle flowering season could induce a greater degree of gender modification in later flowers from female to hermaphrodite and vice versa. However, the ranges of degree of gender modification were different between the two sex morphs, and the variation in females was greater than in hermaphrodites. This phenomenon could not be adequately explained based on the sequential adjustment of investment hypothesis which has been invoked in previous studies. We propose that the results of this study may provide a variation to the model of sequential adjustment of investment in serial flowering annual gynodioecious plants, including Glechoma longituba. We postulate that the variation of sex expression might be beneficial for the female in order to avoid pollen limitation and, further, to obtain a selective advantage.