Corolla Abscission Triggered by Nectar Robbers Positively Affects Reproduction by Enhancing Self-Pollination in Symphytum officinale (Boraginaceae)

Abstract

Nectar robbers, which affect plant fitness (directly or indirectly) in different degrees and in different ways, potentially constitute a significant part of mutualistic relationships. While the negative effects of nectar robbing on plant reproductive success have been widely reported, the positive effects remain unknown. The target of our study was to evaluate the effects of nectar robbers on the reproductive success of Symphytum officinale (Boraginaceae). We observed the behavior, species and times of visitors in the field, and we assessed the effect of nectar robbers on corolla abscission rate and time. To test the fitness of corolla abscission, we detected the changes in stigma receptivity, pollen viability, pollen amount and appendage opening size along with the time of flower blossom. The flowering dynamics and floral structure were observed to reveal the mechanism of self-pollination. Finally, pollen deposition seed set rate and fruit set rate were determined to estimate the effect of nectar robbers on reproduction success. We observed 14 species of visitors and 2539 visits in 50 h of observation; 91.7% of them were nectar robbers. The pressure and nectar removal of nectar robbers significantly promoted corolla abscission during a period when pollen grains are viable and the stigma is receptive. In addition, corolla abscission significantly increased the pollen deposition and seed setting rate. Our results demonstrate that nectar robbing contributes to enhancing seed production and positively and indirectly impacts the reproductive success of S. officinale. This mechanism involved the movement of anthers and indirect participation by nectar robbers, which was rarely investigated. Considering the multiple consequences of nectar robbing, understanding the impact of nectar robbers on plant reproduction is essential to comprehend the evolutionary importance of relationships between plants and their visitors.

Nectar robbing by bees affects the reproductive fitness of the distylous plant Tirpitzia sinensis (Linaceae)

“…Neutral consequences have been inferred when nectar robbing had no significant effects on plant reproductive success because pollinator behavior was not modified by nectar robbing (Elena & Jaime, 2019; Rojas‐Nossa et al., 2016a; Souza et al., 2019; Ye et al., 2017). Positive effects on plant reproductive success have been related to the pollination mediated by nectar robbers in Primula (Zhu et al., 2010) and enhanced self‐pollination in Symphytum (Hou et al., 2021).…”

A Decrease in the Staminode-Mediated Visitor Screening Mechanism in Response to Nectar Robbers Positively Affects Reproduction in Delphinium caeruleum Jacq. ex Camb. (Ranunculaceae)

“…However, nectar robbers can also indirectly contribute to the reproductive success of plants in some cases. For example, corolla abscission in Symphytum officinale triggered by nectar robbers positively affects reproduction by enhancing self-pollination [ 16 ]. In addition, legitimate pollinators are forced to fly longer distances to visit flowers of other populations of the plant species due to reduced nectar volume availability as a result of nectar robbing, hence increasing genetic variability through increasing pollen flow and outcrossing, effects that are beneficial to the reproduction (and survival) of plant populations in the long run [ 17 ].…”