Not only magnitude but also frequency of extreme droughts is predicted to increase under global climate warming. Abiotic stress like drought can result in a loss of resilience or resistance in plants, causing more adverse effects in face of a recurrent stressful event. On the other hand, plants are able to acclimate towards stress, revealing phenotypic plasticity as a response towards environmental conditions. Phenotypic plasticity can later on facilitate fast and protective response under subsequent stress. However, if such an effect can last over a whole vegetation period and even after harvest and regrowth has not been reported up until now. In our study conducted during summer 2009 in Bayreuth, Germany, we investigated if grasses remember early summer drought exposure over a whole vegetation period. Therefore, we compared drought response of potted Arrhenatherum elatius plants that were subjected to recurrent versus single extreme drought manipulations. We determined leaf water content, chlorophyll fluorescence, leaf gas exchange and dead and living aboveground biomass.
Results/Conclusions
Surprisingly, percentage of dead biomass after a late drought was significantly reduced in plants that were previously subjected to drought compared to plants receiving a single drought, although plants had been harvested and regrew after the first drought. Maximum quantum efficiency as well as maximum fluorescence was reduced in plants subjected to recurrent drought indicating photoprotection. Before starting the second drought treatment net photosynthesis was larger in plants that had already received an early drought. Relative water content of the leaves did not differ significantly between the two treatments. These effects appeared to be universal in the sense that they occurred independently of genotypic variation represented by different provenances of tested plants.

Our results show that grasses exhibit an adaptive “drought memory” over a whole vegetation period even after harvest and regrowth: Photochemical efficiency is reduced to prevent photodamage which in turn results in a decrease in dead biomass. Possible mechanisms of this phenotypic plasticity facilitating quick and adequate response towards recurrent drought stress include belowground storage of metabolites or signalling molecules. Furthermore, processes modifying gene expression profile after regrowth, rendering the plant more permissive towards recurrent drought could be responsible for the observed phenomenon.