Background and aims
The root barrier to radial O2 loss is a trait induced during soil flooding restricting oxygen loss from the roots to the anoxic soil. It can also restrict radial water loss, potentially providing tolerance towards drought during conditions of water deficit. Several root traits (aerenchyma and xylem vessels area) respond in a similar way to soil flooding and low soil water potentials. Therefore, we hypothesised that root acclimations to soil flooding prime plants to withstand conditions of water deficit.

Methods
We raised plants in hydroponics mimicking contrasting soil water conditions (aerated controls for well-watered soils; stagnant, deoxygenated solutions for flooded soils, and aerated solutions with different PEG6000 concentrations to mimic conditions of water deficit). We used O2 microsensors and gravimetric measurements to characterize the formation of a barrier to radial O2 loss during conditions of water deficit, and measured key anatomical root traits using light microscopy.

Results
Several root traits were induced in stagnant conditions as well as in conditions of water deficit, including the barrier to radial O2 loss. The tightness of the barrier to water loss was similar in both stagnant and PEG6000 treatments. Moreover, plants growing in stagnant conditions tolerated a following severe condition of water deficit, whereas those growing in mimicked well-watered conditions did not.

Conclusions
We demonstrated that plants growing in stagnant conditions can withstand following severe conditions of water deficit. We propose that key root traits, such as the barrier to radial O2 loss, which are induced in stagnant conditions as well as mild conditions of water deficit, prime the plants for a following severe condition of water deficit.