Inbreeding usually reduces offspring fitness (‘inbreeding depression’, ID), and may affect the plasticity of functional traits involved in the response to stress. ID is often found to increase under stress, but there are also reports of no effects or even a reduction of ID under stress. One reason for this variation that has received little attention may be related to different concepts of stress. In particular, the magnitude of ID may be unrelated to the effect of an environment on fitness (evolutionary stress concept), but increase particularly during the ‘alarm phase’ after a stress has been initiated (physiological stress concept). We clonally replicated inbred and outbred Mimulus guttatus plants, for which ID was known to increase under flooding. We exposed the clonal replicates to control and flooding conditions and harvested replicates of each genotype after two, six and 11 weeks of growth. As functional traits related to stress response we measured chlorophyll fluorescence, root mass and the production of stolons and adventitious roots. As fitness estimates we measured biomass and flower number, and we pollinated a subset of plants and grew a second generation of plants under control and flooding conditions to calculate multiplicative fitness. Overall, M. guttatus proved to be very flooding-tolerant. Chlorophyll fluorescence (F_v/F_m) was not influenced by flooding, but decreased with leaf age and increased after fertilization. At the end of the experiment, biomass and flower number (F₁ generation) as well as multiplicative fitness (including performance in the F₂ generation) were even higher under flooding than under control conditions. Flooding reduced the root mass in the pots, but increased the production of stolons and floating roots. Plasticity in these traits can be regarded as beneficial, although selection gradient analysis failed to identify plasticity in stolon number as adaptive. Only two functional traits were influenced by an interaction between flooding and inbreeding, early stolon length (suggesting a reduced flooding escape response of inbred plants) and root tissue density of floating adventitious roots (suggesting a reduced aeration of the roots of inbred offspring). ID in fitness-related traits was higher under flooding, but its magnitude changed strongly over the course of the experiment. ID under flooding was particularly high after two weeks (δ = 0.42 vs. 0.05 in the control), suggesting sensitivity of inbred plants to the initiation of flooding (‘alarm phase’ of stress response). This effect had disappeared after 6 weeks when plants had acclimated to ongoing flooding. However, under flooding ID increased again after 11 weeks, this time because outbred plants grew much better under flooded than control conditions, and the same pattern was found for the multiplicative fitness function (δ = 0.68 under flooding vs. 0.36 in the control). Our results suggest that ID was higher under flooding, but not because this environment was generally more stressful. Instead, at an early stage ID increased because inbred offspring was more sensitive to the physiological stress after the initiation of flooding (‘alarm phase’), whereas at a late stage ID increased because outbred offspring was more capable of exploiting the favourable flooding conditions. In general, our results show that phenotypic plasticity may often be robust against the effects of inbreeding. Moreover, ID may increase in particular under conditions of physiological stress, during which many stress-specific genes are expressed, whereas ID may not necessarily increase under constant poor conditions that reduce fitness, even if the stress is very strong in the evolutionary sense.

近亲繁殖通常会降低后代的适应性（“近交抑郁症”，ID），并可能影响与压力反应有关的功能性状的可塑性。经常发现 ID 在压力下会增加，但也有报告称在压力下 ID 没有影响甚至减少。这种变化很少受到关注的一个原因可能与不同的压力概念有关。特别是，ID 的大小可能与环境对健康的影响无关（进化压力概念），但在压力开始后的“警报阶段”期间尤其增加（生理压力概念）。我们克隆复制了近交系和远交系Mimulus guttatus植物，已知其 ID 在洪水下会增加。我们将克隆复制品暴露在控制和泛滥条件下，并在生长 2、6 和 11 周后收获每种基因型的复制品。作为与应激反应相关的功能性状，我们测量了叶绿素荧光、根质量以及匍匐茎和不定根的产生。作为适应度估计，我们测量了生物量和花数，我们对一部分植物授粉并在控制和洪水条件下种植第二代植物，以计算乘法适应度。总的来说，M. guttatus被证明是非常耐洪的。叶绿素荧光 (F _v /F _m) 不受淹水影响，但随叶龄降低，施肥后升高。在实验结束时，生物量和花数（F ₁代）以及乘法适应度（包括 F _{2中的表现）}代）在洪水下甚至高于控制条件下。洪水减少了盆中的根部质量，但增加了匍匐茎和浮根的产生。这些性状的可塑性被认为是有益的，尽管选择梯度分析未能确定匍匐茎数的可塑性是适应性的。只有两个功能性状受到洪水和近交之间相互作用的影响，早期匍匐茎长度（表明近交植物的洪水逃逸反应降低）和浮动不定根的根组织密度（表明近交后代根系通气减少）。在洪水下，健身相关性状的 ID 较高，但其幅度在实验过程中发生了强烈变化。两周后洪水下的 ID 特别高（δ = 0.42 vs. 对照中的 0.05），表明近交植物对洪水开始的敏感性（应激反应的“警报阶段”）。当植物适应持续的洪水时，这种影响在 6 周后消失了。然而，在淹水 11 周后，ID 再次增加，这一次是因为远交植物在淹水条件下的生长比对照条件好得多，并且在乘法适应度函数中发现了相同的模式（淹水条件下 δ = 0.68，对照条件下为 0.36）。我们的结果表明，洪水下 ID 较高，但这并不是因为这种环境通常压力更大。相反，在早期阶段 ID 增加是因为近交后代对洪水开始后的生理压力更敏感（“警报阶段”），而在后期，ID增加，因为远交后代更有能力利用有利的洪水条件。总的来说，我们的研究结果表明，表型可塑性通常可能对近亲繁殖的影响具有很强的抵抗力。此外，ID 可能特别在生理压力条件下增加，在此期间表达许多压力特异性基因，而在降低适应性的持续恶劣条件下，ID 不一定会增加，即使压力在进化意义上非常强。