Department of Applied Ecology, North Carolina State University, Raleigh, NC, USA.
Rocky Mountain Biological Laboratory, Gothic, CO, USA.
Am J Bot. 2019 May;106(5):643-655. doi: 10.1002/ajb2.1283. Epub 2019 May 2.
Optimal defense theory predicts that selection should drive plants to disproportionally allocate resources for herbivore defense to tissues with high fitness values. Because pollen's primary role is the transport of gametes, plants may be expected to defend it from herbivory. However, for many animal-pollinated plants, pollen serves a secondary role as a pollinator reward. These dual roles may present a conflict between selection to defend pollen from herbivores and selection to reward pollinators. Here, we investigate whether pollen secondary chemistry in three pollen-rewarding Lupinus species better reflects the need to defend pollen or reward pollinators.
Lupinus (Fabaceae) species are nectarless, pollen-rewarding, and produce defensive quinolizidine and/or piperidine alkaloids throughout their tissues. We used gas chromatography to identify and quantitate the alkaloids in four aboveground tissues (pollen, flower, leaf, stem) of three western North American lupines, L. argenteus, L. bakeri, and L. sulphureus, and compared alkaloid concentrations and composition among tissues within individuals.
In L. argenteus and L. sulphureus, pollen alkaloid concentrations were 11-35% of those found in other tissues. We detected no alkaloids in L. bakeri pollen, though they were present in other tissues. Alkaloid concentrations were not strongly correlated among tissues within individuals. We detected fewer alkaloids in pollen compared to other tissues, and pollen contained no unique alkaloids.
Our results are consistent with the hypothesis that, in these pollen-rewarding species, pollen secondary chemistry may reflect the need to attract and reward pollinators more than the need to defend pollen from herbivory.
最优防御理论预测，选择应促使植物将资源不成比例地分配给具有高适应值的组织，以进行食草动物防御。因为花粉的主要作用是配子的运输，所以可以预期植物会保护花粉免受食草动物的侵害。然而，对于许多动物传粉的植物来说，花粉具有作为传粉者奖励的次要作用。这两个角色可能会导致选择保护花粉免受食草动物侵害与选择奖励传粉者之间产生冲突。在这里，我们调查了三个花粉奖励的羽扇豆属物种中的花粉次生化学物质是否更好地反映了保护花粉免受食草动物侵害或奖励传粉者的需求。
羽扇豆属（豆科）物种是无花蜜的，花粉奖励的，并且在其整个组织中产生防御性的喹啉和/或哌啶生物碱。我们使用气相色谱法鉴定和定量三种北美西部羽扇豆属植物（Lupinus argenteus、L. bakeri 和 L. sulphureus）的四个地上组织（花粉、花、叶、茎）中的生物碱，并比较个体内组织间的生物碱浓度和组成。
在 L. argenteus 和 L. sulphureus 中，花粉中的生物碱浓度是其他组织中发现的生物碱浓度的 11-35%。我们在 L. bakeri 花粉中没有检测到生物碱，尽管它们存在于其他组织中。个体内组织间的生物碱浓度没有很强的相关性。我们在花粉中检测到的生物碱比其他组织少，花粉中也没有独特的生物碱。
我们的结果与以下假设一致，即在这些花粉奖励的物种中，花粉次生化学物质可能更反映吸引和奖励传粉者的需求，而不是保护花粉免受食草动物侵害的需求。
