Abstract:

Aim: The strategies of reproductive adaptation for alpine plants have long been a hot topic in evolutionary ecology, but there is still a lack of quantitative measures for reproductive traits under natural conditions. Here, we investigated reproductive adaptation of Rorippa elata, an endemic Brassicaceae species in one of the world's temperate biodiversity hotspots, the Hengduan Mountains (HDM).

Methods: We established a field-based common garden experiment at Shangri-La Alpine Botanical Garden in Yunnan. During 2019‒2020, we transplanted a total of 633 R. elata plants for 150 accessions across latitudes and performed manipulative field experiments. First, we collected several life history-related traits of R. elata, including germination rate, flowering time, over-winter survival rate, and pollination-relevant characteristics. We then carried out a set of four experimental treatments to characterize natural variation of the mating system in R. elata, including (1) strict selfing rate by bagging inflorescences before flowering; (2) common outcrossing rate by emasculation and open pollination; (3) strict outcrossing rate by emasculation and artificial pollination among accessions; and (4) natural reproductive rate under open pollination. For each treatment, we counted seed setting rate per fruit per accession and used this information to assign a mating system type for each R. elata accession. Furthermore, we also quantified the extents of pollen limitation and reproductive assurance.

Results: Our results showed that the main visitors of R. elata flowers were insects of Syrphidae, Halictus, Musca domestica, and Calliphoridae. Among these, insects of Syrphidae visited flowers most frequently and insects of Calliphoridae had the longest staying time per visiting. According to life history observations in the common garden, we demonstrated that R. elata plants germinated in early spring followed by a long period of reproductive growth throughout the synchronization of both high temperature and precipitation in HDM. They then entered into dormancy with living sprouts belowground during cold and dry winter. Correlation analysis of latitude with seed setting rate and over-winter survival rate showed that latitude was negatively correlated with the reproductive ability but positively correlated with the over-winter survival rate of R. elata. Comparative analysis of seed setting rates among experimental treatments found that R. elatapossessed a mixed mating system, with selfing dominant and intermediate types accounting for 45.5% and 39.4% of accessions, respectively. Furthermore, most R. elata accessions showed signs of reproductive assurance, but to different extents, with the highest in selfing dominant accessions (0.163) and the lowest in intermediate accessions (0.011).

Conclusions: Our findings reveal the reproductive strategies of R. elataat the population level and underscore a trade-off between reproduction and survival along a latitudinal gradient. The mixed mating system with variable reproductive assurance likely plays an important role in the stability of reproduction. Collectively, our results highlight the important contributions of reproductive strategies for alpine plants during their adaptation to mountain environments.

Key words: adaptation, common garden, Hengduan Mountains, trade-off, mating system, seed setting rate