Data from: Endure and call for help: strategies of black mustard plants to deal with a specialised caterpillar

Plants have evolved inducible resistance and tolerance mechanisms against insect herbivores. Resistance mechanisms that affect herbivorous insects directly can be effective against generalist herbivores, but will not deter specialist herbivores from attacking the plant. Tolerance mechanisms and indirect plant resistance are more likely effective strategies used by plants when dealing with specialist herbivores. However, inducible indirect resistance and tolerance mechanisms have rarely been investigated within the same study system.
We studied multiple tolerance mechanisms and the role of natural enemies in reducing fitness loss of Brassica nigra plants incurred by feeding by the specialist herbivore Pieris brassicae. For this, we measured the changes in carbon and nitrogen triggered by herbivore attack and quantified plant biomass and seed production under field conditions, in the presence or absence of natural enemies of the herbivores. We also assessed whether B. nigra increased selfing rates when exposed to herbivore infestation, and investigated whether infestation by P. brassicae rendered B. nigra plants more attractive to night-active pollinators than control plants.
We found that B. nigra flowers are rarely visited by insects during the night, and exposure to herbivores did not influence selfing rates. Brassica nigra plants compensated for herbivory in terms of vegetative biomass. Seed set was negatively affected by herbivory in the absence of natural enemies, but not in the presence of natural enemies. Plants responded to herbivory with drastic changes in nitrogen contents of leaves and flowers, whereas no changes in carbon concentrations were detected.
The investment in reproduction or re-growth of vegetative tissues is not sufficient to sustain plant fitness. Reproductive output of flowering mustard plants is only sustained when interactions with the natural enemies of the herbivores are preserved. We conclude that natural enemies of herbivorous insects play an important role as component of the plant's defence strategy. This study reveals that both tolerance and indirect resistance are strategies of this plant species to sustain fitness.

植物已演化出针对昆虫植食者（insect herbivores）的诱导抗性（inducible resistance）与耐害性（tolerance）两类防御机制。直接作用于植食性昆虫的抗性机制，可对广食性植食者（generalist herbivores）产生防控效果，但无法阻止专食性植食者（specialist herbivores）对植物的侵染。耐害性机制与植物间接防御（indirect plant resistance），在植物应对专食性植食者时更易成为有效的防御策略。然而，目前鲜有研究在同一研究系统（study system）中同时探究诱导型间接防御与耐害性机制。

本研究以专食性植食者大菜粉蝶（Pieris brassicae）取食导致黑芥（Brassica nigra）产生的适合度损失（fitness loss）为研究对象，探讨了黑芥的多重耐害性机制，以及天敌（natural enemies）在缓解该损失中的作用。为此，我们测定了植食者侵染（herbivore infestation）引发的植株碳氮含量变化，并在田间条件（field conditions）下，分别在植食者天敌存在与缺失的情境中，量化了植物生物量（plant biomass）与种子产量（seed production）。此外，我们评估了黑芥在遭遇植食者侵染时是否会提升自交率（selfing rates），并探究了大菜粉蝶侵染是否会使黑芥植株较对照植株（control plants）更能吸引夜行性传粉者（night-active pollinators）。

研究结果显示，黑芥的花朵在夜间极少有昆虫造访，且植食者侵染并未对其自交率产生显著影响。黑芥植株可通过营养生物量（vegetative biomass）的调整补偿植食为害造成的损失。在天敌缺失的情况下，植食为害会显著降低结籽率（seed set），但当天敌存在时则无此负面效应。植株在遭遇植食为害时，叶片与花朵的氮含量会发生剧烈变化，但未检测到碳浓度（carbon concentrations）的显著改变。

仅依靠营养组织的繁殖投入或再生生长，不足以维持植物的适合度。开花芸苔属植物的繁殖产出，仅在保留与植食者天敌的互作关系时才能得以维持。我们认为，植食性昆虫的天敌作为植物防御策略的重要组成部分，发挥着关键作用。本研究表明，耐害性与间接防御均为该植物物种维持适合度的有效策略。