Plants produce an array of defensive compounds with toxic or deterrent effects on insect herbivores. Pollen can contain relatively high concentrations of such defense compounds, but the causes and consequences of this enigmatic phenomenon remain mostly unknown. These compounds could potentially protect pollen against antagonists but could also reduce flower attractiveness to pollinators. We combined field observations of the pollen‐rewarding Lupinus argenteus with chemical analysis and laboratory assays to test three hypotheses for the presence of pollen defense compounds: (1) these compounds are the result of spillover from adjacent tissues, (2) they protect against pollen thieves, and (3) they act as antimicrobial compounds. We also tested whether pollen defense compounds affect pollinator behavior. We found a positive relationship between alkaloid concentrations in pollen and petals, supporting the idea that pollen defense compounds partly originate from spillover. However, pollen and petals exhibited quantitatively (but not qualitatively) distinct alkaloid profiles, suggesting that plants can adjust pollen alkaloid composition independently from that of adjacent tissues. We found no relationship between pollen alkaloid concentration and the abundance of pollen thieves in Lupinus flowers. However, pollen alkaloids were negatively associated with bacterial abundance. Finally, plants with more alkaloids in their pollen received more pollinator visits, but these visits were shorter, resulting in no change in the overall number of flowers visited. We propose that pollen defense compounds are partly the result of spillover from other tissues, while they also play an antimicrobial role. The absence of negative effects of these compounds on pollinator visitation likely allows their maintenance in pollen at relatively high concentrations. Taken together, our results suggest that pollen alkaloids affect and are mediated by the interplay of multiple interactions.

中文翻译：

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花粉防御化合物对花粉奖励植物中传粉媒介和拮抗剂的影响

植物产生一系列防御性化合物，对食草昆虫具有毒性或威慑作用。花粉中可能含有相对较高浓度的此类防御化合物，但这种神秘现象的原因和后果仍然未知。这些化合物可能会保护花粉免受拮抗剂的侵害，但也可能会降低花朵对传粉者的吸引力。我们结合了花粉奖励的实地观察银羽扇豆通过化学分析和实验室测定来测试花粉防御化合物存在的三个假设：（1）这些化合物是邻近组织溢出的结果，（2）它们可以防止花粉窃贼，（3）它们充当抗菌化合物。我们还测试了花粉防御化合物是否影响传粉昆虫的行为。我们发现花粉和花瓣中的生物碱浓度之间存在正相关关系，支持花粉防御化合物部分源自溢出的观点。然而，花粉和花瓣表现出数量上（但不是质量上）不同的生物碱谱，表明植物可以独立于邻近组织调整花粉生物碱组成。我们发现花粉生物碱浓度与花粉窃贼的丰度之间没有关系。羽扇豆属花朵。然而，花粉生物碱与细菌丰度呈负相关。最后，花粉中含有更多生物碱的植物接受更多的传粉者访问，但这些访问时间较短，导致访问的花朵总数没有变化。我们认为花粉防御化合物部分是其他组织溢出的结果，同时它们也发挥抗菌作用。这些化合物对传粉媒介的访问没有负面影响，可能允许它们在花粉中维持相对较高的浓度。综上所述，我们的结果表明花粉生物碱影响并由多种相互作用的相互作用介导。