Fitness consequences of natural variation in flooding-induced shoot elongation in rumex palustris

• Plants can respond to their environment by morphological plasticity. Generally, the potential benefits of adaptive plastic responses are beyond doubt under predictable environmental changes. However, the net benefits may be less straightforward when plants encounter temporal stresses, such as flooding in river flood plains. • Here, we tested whether the balance of costs and benefits associated with flooding-induced shoot elongation depends on the flooding regime, by subjecting Rumex palustris plants with different elongation capacity to submergence of different frequency and duration. • Our results showed that reaching the surface by shoot elongation is associated with fitness benefits, as under less frequent, but longer, flooding episodes plants emerging above the floodwater had greater biomass production than plants that were kept below the surface. As we predicted, slow-elongating plants had clear advantages over fast-elongating ones if submergence was frequent but of short duration, indicating that elongation also incurs costs. • Our data suggest that high costs select for weak plasticity under frequent environmental change. In contrast to our predictions, however, fast-elongating plants did not have an overall advantage over slow-elongating plants when floods lasted longer. This indicates that the delicate balance between benefits and costs of flooding-induced elongation depends on the specific characteristics of the flooding regime.