Résumé
•Roots matter for drought adaptation but root depth and density are not enough.•Crop tolerance to water stress relies on water availability at key time, which comes from plant traits controlling water use.•Some of these traits depend on plant hydraulics.•Several root anatomical and biochemical traits could influence the root hydraulic conductivity and influence traits responsible for water budgeting.
Roots have long been proposed as a major avenue of research to improve crop adaptation to water limitations. The simple assumption is that deeper and more profuse root systems could tap extra water from the soil profile and alleviate drought effects. However, after decades of research, success in breeding cultivars with improved root systems is lagging behind. Here, we attempt to analyze the possible reasons for this, and re-focus on what root traits might provide the most promising avenues for drought adaptation. We approach the root system from the angle of water extraction, using data from a lysimetric system that allows monitoring and comparing plant water use over the entire crop life cycle and yield, and analyze whether and how differences in water extraction lead to improved yield across different crops. The main message from that analysis is that water extraction during reproduction and grain filling is critical and comes from a number of traits that influence the rate at which plant use the available water before and during stress. Roots may have an effect on this, not from the traditionally thought density or depth, but rather from their hydraulic characteristics. Plants can indeed control water use by controlling leaf area development and this is a “long term” control. Plants also control water losses by controlling stomata opening under high vapor pressure deficit (VPD) conditions, in a transient manner. Both processes (leaf development and stomata opening) are mostly controlled by hydraulic processes. The role of roots in drought adaptation could be there, along with the soil, in setting an hydraulic environment that allow plants to use water in a way that allow maximizing water use for these critical stages.