Floral scent dynamics of Schiedea kaalae and Schiedea hookeri

Floral scent often intensifies during periods of pollinator activity, but
the degree of this synchrony may vary among scent compounds depending on
their function. Related plant species with the same pollinator may exhibit
similar timing and composition of floral scent. We compared timing and
composition of floral volatiles for two endemic Hawaiian plant species,
Schiedea kaalae and S. hookeri (Caryophyllaceae). For S. kaalae, we also
compared the daily timing of emission of floral volatiles to evening
visits of their shared pollinator, an endemic Hawaiian moth
(Pseudoschrankia brevipalpis; Erebidae). The identity and amount of floral
volatiles were measured in the greenhouse during day and evening periods
with dynamic headspace sampling and GC-MS (gas chromatography – mass
spectrometry). The timing of emissions (daily rise, peak, and fall) was
measured by sampling continuously for multiple days in a growth chamber
with PTR-MS (proton transfer reaction mass spectrometry). Nearly all
volatiles detected underwent strong daily cycles in emission. Timings of
floral volatile emissions were similar for S. kaalae and S. hookeri, as
expected for two species sharing the same pollinator. For S. kaalae, many
volatiles known to attract moths, including several linalool oxides and
2-phenylacetaldehyde, peaked within 2 h of the peak visitation time of the
moth which pollinates both species. Floral volatiles of both species that
peaked in the evening were also emitted several hours before and after the
brief window of pollinator activity. Few volatiles followed a daytime
emission pattern, consistent with increased apparency to visitors only at
night. The scent blends of the two species differed in their major
components and were most distinct from each other in the evening. The
qualitative difference in evening scent composition between the two
Schiedea species may reflect their distinct evolutionary history and may
indicate that the moth species uses several different floral cues to
locate rewards.

花香通常会在传粉者活动时段增强，但这类同步性的强弱会因香气成分的功能差异而有所不同。共享同一种传粉者的近缘植物类群，其花香的释放时序与成分组成往往趋于相似。本研究以两种夏威夷特有植物——Kaala希氏草（Schiedea kaalae）与Hooker希氏草（S. hookeri，石竹科Caryophyllaceae）为研究对象，比较了二者的花香挥发物释放时序与成分组成。针对Kaala希氏草，本研究还将其花香挥发物的每日释放节律，与二者共享的传粉者——夏威夷特有短须拟狭须夜蛾（Pseudoschrankia brevipalpis，夜蛾科Erebidae）的晚间访花行为进行了关联对比。研究人员于日间与晚间在温室环境中，采用动态顶空采样法结合气相色谱-质谱联用仪（gas chromatography – mass spectrometry, GC-MS），测定了花香挥发物的成分与含量；通过在生长箱中连续多日采样，利用质子转移反应质谱法（proton transfer reaction mass spectrometry, PTR-MS），监测了挥发物释放的日周期变化（包括每日的上升、峰值与回落阶段）。检测到的几乎所有挥发物均呈现显著的每日释放节律。正如共享传粉者的两个物种所预期的那样，Kaala希氏草与Hooker希氏草的花香挥发物释放时序较为相似。针对Kaala希氏草，多种已知可吸引蛾类的挥发物（包括多种芳樟醇氧化物与苯乙醛（2-phenylacetaldehyde））的释放峰值，均出现在该蛾类（同时为两种植物传粉）的峰值访花时段前后2小时内。两种植物的晚间峰值释放挥发物，均在传粉者活动的短暂窗口前后数小时内出现释放。仅有极少数挥发物呈现日间释放模式，这与植物仅在夜间提升对访花者的吸引可见性的结论相符。两种植物的花香混合组分在主要成分上存在差异，且在晚间时段的组分差异最为显著。两个希氏草属物种间晚间花香组成的定性差异，可能反映了二者独特的演化历史，同时也表明该蛾类可利用多种不同的花香线索来定位花蜜奖励。