Abstract:

The theory of sex allocation can be used to predict the optimal allocation of reproductive resources and considered as individual fitness between male and female function. Variations in resource allocation between sexes and among different positions in the flower inflorescence have great significance for understanding how choice of reproductive strategy affects reproductive success. Changes in floral sex allocation among different positions, pollinator movement and reproductive success within sequentially flowering inflorescences of Ammopiptanthus mongolicus were studied at the Turpan Eremophytes Botanical Garden of the Chinese Academy of Sciences. Our major results are as follows: (1) Floral longevity shortened significantly, stamen mass/(stamen mass + pistil mass), pollen number and pollen/ovule ratio all increased among positions along with the turn of intermediate earlier developing flowers→lower flowers→upper later developing flowers. Corolla diameter, corolla mass and nectar production, on the other hand, all diminished and ovule number showed no differences. Upper flowers showed male-biased sex allocation; (2) Within an inflorescence, Apis mellifera and Lasiglossum sp.1 landed first on the intermediate flowers, moved among different positions in the inflorescence, and finally flew away from the upper flower. Intermediate flowers had the highest first visiting rate but lowest last visiting rate while upper flowers had the lowest last visiting rate but highest first visiting rate; (3) During the two years of our study, the seed set rate and seed weight increased after supplying outcross pollen to the upper flowers; the fruit set rate, seed set rate and seed weight all increased significantly after simultaneously supplying outcross pollen to the upper flowers and removing the intermediate and lower flowers, and these measures did not differ between the two treatments. These results suggested that lower fruit set, seed set and seed weight of upper flowers under natural conditions can be attributed to the pollinators’ directionality within inflorescence and a lack of outcrossing pollen in upper flowers rather than resource limitation in A. mongolicus. Increasing investment in male functions on the upper fowers may be an adaptive strategy to sustain pollination success for the species.

Key words: Ammopiptanthus mongolicus, sex allocation, pollinator, directionality of movement, across pollen, reproductive success