Institute of Evolution and Ecology, School of Life Sciences, Central China Normal University, Wuhan, China.
South China Botanical Garden, Chinese Academy of Sciences, Guangzhou, China.
Ann Bot. 2019 Jan 23;123(2):373-380. doi: 10.1093/aob/mcy102.
Gain or loss of floral nectar, an innovation in floral traits, has occurred in diverse lineages of flowering plants, but the causes of reverse transitions (gain of nectar) remain unclear. Phylogenetic studies show multiple gains and losses of floral nectar in the species-rich genus Pedicularis. Here we explore how experimental addition of nectar to a supposedly nectarless species, P. dichotoma, influences pollinator foraging behaviour.
The liquid (nectar) at the base of the corolla tube in P. dichotoma was investigated during anthesis. Sugar components were measured by high-performance liquid chromatography. To understand evolutionary transitions of nectar, artificial nectar was added to corolla tubes and the reactions of bumble-bee pollinators to extra nectar were examined.
A quarter of unmanipulated P. dichotoma plants contained measurable nectar, with 0.01-0.49 μL per flower and sugar concentrations ranging from 4 to 39 %. The liquid surrounding the ovaries in the corolla tubes was sucrose-dominant nectar, as in two sympatric nectariferous Pedicularis species. Bumble-bees collected only pollen from control (unmanipulated) flowers of P. dichotoma, adopting a sternotribic pollination mode, but switched to foraging for nectar in manipulated (nectar-supplemented) flowers, adopting a nototribic pollination mode as in nectariferous species. This altered foraging behaviour did not place pollen on the ventral side of the bees, and sternotribic pollination also decreased.
Our study is the first to quantify variation in nectar production in a supposedly 'nectarless' Pedicularis species. Flower manipulations by adding nectar suggested that gain (or loss) of nectar would quickly result in an adaptive behavioural shift in the pollinator, producing a new location for pollen deposition and stigma contact without a shift to other pollinators. Frequent gains of nectar in Pedicularis species would be beneficial by enhancing pollinator attraction in unpredictable pollination environments.
花的花蜜的产生或丧失，是花部性状的一个创新，这种现象发生在多样化的开花植物谱系中，但花蜜反转（产生花蜜）的原因仍不清楚。系统发育研究表明，在种类丰富的马先蒿属中，花蜜有多次获得和丧失。在这里，我们探讨了向一种假定无蜜的物种，马先蒿二歧种，人工添加花蜜如何影响传粉者的觅食行为。
在马先蒿二歧种开花期间，我们研究了花冠管底部的液体（花蜜）。通过高效液相色谱法测量糖成分。为了理解花蜜进化的转变，我们向花冠管中添加了人工花蜜，并观察了熊蜂传粉者对额外花蜜的反应。
四分之一未经处理的马先蒿二歧种植物含有可测量的花蜜，每朵花含有 0.01-0.49 μL，糖浓度范围为 4-39%。花冠管中围绕子房的液体是蔗糖占主导地位的花蜜，与两种同域的产蜜马先蒿物种相同。熊蜂只从马先蒿二歧种未处理（未处理）的花中采集花粉，采用 sternotribic 传粉模式，但在处理（添加花蜜）的花中转变为觅食花蜜，采用 nototribic 传粉模式，与产蜜物种相同。这种改变的觅食行为并没有将花粉放在蜜蜂的腹侧，sternotribic 传粉也减少了。
我们的研究首次量化了一个假定的“无蜜”马先蒿物种中花蜜产生的变化。通过添加花蜜进行花朵处理，表明花蜜的获得（或丧失）将很快导致传粉者的适应性行为转变，产生一个新的花粉沉积和柱头接触的位置，而不需要转向其他传粉者。马先蒿属物种中花蜜的频繁获得将通过在不可预测的授粉环境中增强对传粉者的吸引力而受益。
