A clonal fresh water plants acquires transgenerational stress resistance under recurring copper excess

Although non-genetic inheritance is thought to play an important role in plant ecology and evolution, evidence for adaptive transgenerational plasticity is scarce. Here, we investigated the consequences of copper excess on offspring defences and fitness in the giant duckweed (Spirodela polyrhiza) across multiple asexual generations. We found that exposing large monoclonal populations (>10,000 individuals) for 30 generations to copper excess decreased plant fitness during the first few generations but increased their fitness in consecutive generations under recurring stress when plants were grown for 5 generations under control conditions prior recurring conditions. Similarly, propagating individual plants as single descendants for 5 or 10 generations under copper excess decreased plant fitness when 5 generations and improved plant fitness when 10 generations passed between initial and recurring stress; thus, transgenerational stress responses likely contributed to the observed variations in offspring fitness of long-term copper exposed populations. Fitness benefits under recurring stress were partially associated with avoidance of excessive copper accumulation, which in turn correlated with transgenerationally modified flavonoid concentrations. Taken together, these data demonstrate time-dependent adaptive transgenerational responses under recurring stress, which highlights the importance of non-genetic inheritance for plant ecology and evolution.