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Plants with multiple flowers could be prone to autonomous self‐pollination and insect‐mediated geitonogamy, but physiological and ecological features have evolved preventing costs related to autogamy. We studied the rare perennial herb Dictamnus albus as a model plant, with the aim of describing the plant–pollinator system from both plant and pollinator perspectives and analysing features that promote outcrossing in an entomophilous species. The breeding system and reproductive success of D. albus were investigated in experimental and natural conditions, showing that it is potentially self‐compatible, but only intra‐inflorescence insect‐mediated selfing is possible. Nectar analysis showed gender‐biased production towards the female phase, which follows the male phase, and during flowering, full blooming is found in flowers at the bottom of the raceme. Among a wide spectrum of insect visitors, three genera (Bombus, Apis, Megachile) were found to be principal pollinators. A study of insect behaviour showed a tendency towards bottom‐to‐top flights for the most important pollinators Bombus spp. and Apis mellifera: upward movements on the racemes could be explained by foraging behaviour, from more to less rewarding flowers. In accordance with the ‘declining reward hypothesis’, bumblebees and honeybees leave the plant when gain of reward is low, after which few flowers are visited, reducing the chance of self‐pollen transfer among flowers. Intra‐flower self‐pollination is prevented in D. albus by protandry and herkogamy, while the nectar‐induced sequential pattern of pollinator visits avoids geitonogamy and tends to maximise pollen export, promoting outcrossing. All these features for preventing selfing benefit plant fitness and population genetic structure.

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Most plant species are visited by a diverse array of visitors that may contribute differently to reproductive success. In particular, the size of visitors, relative to flower size, may have severe implications for plant fitness, as it can affect the mechanics of the pollination process. Ononis masquillierii, a rare endemic legume in northern Italy, is visited by numerous bee species of different families and sizes. To evaluate the effect of bee guilds on seed set, we compared the relative efficiency of four size classes of bees visiting flowers in eight plant populations occurring in the Apennines. Size classes were assorted by body weight. Reproductive success was positively related to plant population size and affected by the visitation frequency of the bee size guilds: higher visitation rates by larger bees (medium-large Andrenidae and large Apidae) increased seed set, whereas high visitation rates by smaller bees (small Halictidae and small-medium Megachilidae) reduced seed set. We conclude that the influence of bee size on seed set is related to the pollen foraging techniques of the different bee guilds. Larger bees paying short visits to flowers remove one 'batch' of pollen and transfer it to another flower. In contrast, smaller bees enter the flower entirely and activate the pump-like pollen dispensing mechanism several times, resulting in a higher degree of selfing. These behaviours may result in differential seed sets as the flowers are self-incompatible, i.e. seed set may be reduced after visitation by smaller bees. Such variations in pollinator quality among populations coupled with the ephemeral nature of the habitat of O. masquillierii may represent a risk for its persistence. Our results show that such a risk can be confronted only through the conservation of large plant populations and of large bees capable of flying long-distances.

No abstract