Abstract: The sexual system of the perennial shrub Capparis spinosa L. (Capparaceae), which is distributed in arid deserts of northern Xinjiang, was investigated. The main results can be summarized as follows. (1) The species is andromonoecious, i.e. individuals possess both male and perfect flowers. Stamens of two floral morphs are normal and can be divided into long and short ones in each flower. The perfect flowers have well developed pistils, but male flowers have aborted pistils and only function as males. (2) There were very significant differ-ences among the populations in daily ratio of the two floral morphs, number of long and short stamens of perfect flowers and length of filaments and anthers of short stamens in male flowers (P<0.01), but no significant differ-ences occurred in biomass of floral organ (P>0.05). (3) Anthesis was nocturnal and lasted 15–16 h. Both male and perfect flowers opened about 18:00 at dusk. The number of two floral morphs produced on each individual was indeterminate every day during flowering, which would make the individual temporally androdioecious. However, the total number of male flowers was more than that of perfect flowers within the population every day. (4) The P/O ratios of perfect flowers in three populations were 1.57×104, 1.65×104 and 1.71×104. There was no significant difference in pollen numbers (P>0.05) within population between male and perfect flowers or between long and short stamens, and also no significant difference among the populations in pollen numbers of male and perfect flowers, and in ovule numbers and P/O ratios of perfect flowers (P>0.05). (5) Dynamic curves of pollen viability of long and short stamens in the two floral morphs were similar in three populations. Pollen longevity of both long and short stamens was about 18–20 h, and duration of stigmatic receptivity was about 16–18 h. (6) Floral visitors were hymenopterous and lepidopterous insects. There was a total of seven species of floral visitors in the three populations. The activities of them were greatly affected by the climate of the desert environment. (7) Perfect flowers were not apomictic and could produce fruits after self-pollination (autogamy and geitonogamy) and cross-pollination; thus they had a mixed mating system. These results suggest that the sexual system of andro-monoecy may reflect an environment-heredity interaction in C. spinosa. The male flowers increase the number of pollen grains, the P/O ratio and enhance male fitness of individuals. They also increase the floral display and consequently effect cross pollination by attracting more pollinators that bring cross-pollen to the stigmas of per-fect flowers, thus increase outcrossing rate and female fitness of individuals, which insure reproductive success of C. spinosa in the extreme desert environment.

Key words: adaptation, andromonoecy, Capparis spinosa L., desert environment, mixed mating system, reproductive success