Effects of cages, plant age and mechanical clipping on plantain chemistry

In plant-insect herbivore field studies, effects of cages, plant age, and mechanical clipping on host plant chemistry are often postulated but not well documented. We examined the effects of cages (for the purpose of restraining insects on experimental plots), plant age over the course of the experiment and mechanical clipping on plantain (Plantago lanceolata) chemistry. Leaf age affected the concentrations of nitrogen and iridoid glycosides (IGs; specifically aucubin and catalpol), with higher levels in newer leaves. Caged plants had higher levels of IGs and lower concentrations of nitrogen than uncaged plants. The IG concentrations were greater in new leaves of caged plants than uncaged plants, whereas the concentrations in mature leaves were unaffected by caging. Plants that were 5 weeks older had higher levels of IGs and lower nitrogen than plants harvested 5 weeks earlier. Comparison of three studies suggested that over the summer IG concentrations increase during dry years but decrease during wet years. Plants with above-ground parts clipped and then allowed to regrow for five weeks had similar concentrations of IGs and nitrogen compared to control plants; but the regrowth plants had a lower catalpol to total IG ratio. We conclude that cages and time can have significant positive effects on iridoid glycoside concentrations and significant negative effects on leaf nitrogen concentration. But our results also indicate that the direction and magnitude of the effects of cages, time and mechanical damage are not easily predicted. Therefore, it is advisable to determine and/or control for such effects in field experiments on plant-insect interactions.

在植物 - 植食性昆虫的田间研究中，笼子、植物年龄以及机械修剪对寄主植物化学成分的影响常常被假定，但缺乏充分的文献记载。我们研究了笼子（用于在试验田限制昆虫）、实验过程中的植物年龄以及机械修剪对车前草（Plantago lanceolata）化学成分的影响。叶片年龄影响氮和环烯醚萜苷（IGs；特别是桃叶珊瑚苷和梓醇）的浓度，新叶中的含量较高。与未罩笼的植物相比，罩笼植物的环烯醚萜苷含量较高，氮浓度较低。罩笼植物新叶中的环烯醚萜苷浓度高于未罩笼植物，而成熟叶片中的浓度不受罩笼影响。比提前5周收获的植物年长5周的植物，其环烯醚萜苷含量较高，氮含量较低。对三项研究的比较表明，在夏季，干旱年份环烯醚萜苷浓度增加，而湿润年份则降低。地上部分被修剪后再生长5周的植物，与对照植物相比，其环烯醚萜苷和氮的浓度相似；但再生植物的梓醇与总环烯醚萜苷的比率较低。我们得出结论，笼子和时间对环烯醚萜苷浓度有显著的正向影响，对叶片氮浓度有显著的负向影响。但我们的研究结果也表明，笼子、时间和机械损伤的影响方向和程度不易预测。因此，在植物 - 昆虫相互作用的田间实验中，确定和/或控制这些影响是明智的。