Spatiotemporal Variations in Nectar Robbing and Its Effects on Reproduction in Salvia castanea Diels (Lamiaceae)

Abstract

Nectar robbing typically reduces nectar availability to pollinators, damages flower structure, and/or induces secondary robbing. Consequently, it may reduce pollen deposition and seed set, increase pollination efficiency and outcrossing, and/or not affect reproduction in some species. However, spatiotemporal variations in nectar robbing and their effects on plant reproduction have received little attention. In this study, we assessed the effects of nectar robbing on floral visits, seed set, nectar volume and concentration, and flower longevity in two populations of Salvia castanea Diels (Lamiaceae) in the Himalayan region of Southwestern China in 2014-2020. We also examined whether one or a few visits by pollinators can result in the stigma receiving sufficient pollen to fertilize all ovules of S. castanea. We found that significant differences in the nectar robbing rate did not affect seed set in any of the years for either population of S. castanea. In the robbed and unrobbed flowers, nectar was consistently replenished every night at higher concentrations. Bagging, nectar robbing, and sufficient pollination did not affect flower longevity. Salvia castanea required only 5-10 pollen grains to achieve the maximum seed set. However, pollinators depositing more than 10 pollen grains after a single visit ensured a high seed set of >80%. Our results suggest that nectar availability, floral longevity maintenance, and sufficient pollen deposition mitigate the effects of nectar robbing on the reproductive success of S. castanea. These results are expected to further our understanding of plant-animal interactions and the ecological consequences of nectar robbing.

Keywords: Salvia; floral longevity; nectar availability; nectar robbing; pollen deposition; reproductive success.

Figure 1

Visitation frequency of pollinators and robbers in both populations of Salvia castanea from 2014 to 2020. YL, Yulong Mountain; MY, Mingyin Town; NR, nectar robber; P, pollinator.

Figure 2

Nectar robbing rate (purple) and seed set (pink) in different years in both populations of Salvia castanea. Different lowercase letters indicate significant differences in nectar robbing rates. The boxes in the figure indicate the 25–75% data distribution range; the black horizontal line inside the box indicates the median; the hollow square inside the box indicates the mean; the upper and lower whisker lines indicate the maximum and minimum values, respectively; and the black solid rhombus outside the upper and lower whisker lines indicates the outliers (All of the following box plots are annotated to match here). YL, Yulong Mountain; MY, Mingyin Town.

Figure 3

Temporal variation in nectar volume (a) and concentration (b) in robbed and unrobbed flowers. Different capital letters indicate significant differences in nectar volume and concentration between robbed and unrobbed flowers at different times of the day. Different lowercase letters indicate significant differences in nectar volume and concentration between robbed and unrobbed flowers at the same time.

Figure 4

Effect of different treatments on flower longevity in Salvia castanea. There was no significant difference in flower lifespan among different treatments.

Figure 5

The seed set of different pollen amounts for hand pollination (a), the number of pollen deposited (purple) and seed set (pink) by pollinators in a single visit in two populations (b). Different lowercase letters indicate significant differences in the seed set of different pollen amounts for hand pollination.