How to invest carbon under drought, different strategies in early root system development among barley cultivars

Plant roots exposed to water scarcity respond by modulating root functional traits, such as deep and prolific root growth, to maximize resource acquisition. Such adjustments, which include the alterations of root morphology and anatomy to optimize water uptake and/or maximize root survival, may also increase the carbon demand for soil exploration. But this altered carbon allocation to foster belowground resource acquisition limits aboveground plant development. In the present study we investigated the relationship between root physiology, root trait plasticity and whole plant growth under drought stress. We quantified shoot and root traits of nine contrasting spring barley cultivars grown in soil-filled rhizoboxes under well-watered (4 weeks, 55% of field capacity) or drought conditions (2 weeks, 55% of field capacity + 2 weeks without water). Time-lapse imaging was applied to quantify root and shoot growth rates, and combined with measurements of root distribution, morphology, anatomy as well as mycorrhizal colonization. Aboveground traits had a strong and uniform response to drought compared to belowground traits. Root traits’ plasticity were variable and differed among cultivars. The differences between cultivars were particularly pronounced for the proportion of root length and root biomass in deep soil layers as well as changes in root morphological and anatomical traits. We suggest that cultivars characterized by an increase in root conduits, a greater hierarchical structure and a reduction of specific root length and area may be good candidates to promote hydraulic lift while lowering carbon cost for root growth. Increased root length and depth, root density and specific root length in drought condition are different cultivars’ strategies that may promote soil exploration and optimize water uptake. This is directly linked to the interplay of above and belowground carbon investment, with some cultivars yielding both a high shoot biomass and enhanced resource acquisition. Based on our findings, testing new agronomic strategies to mobilize the diversity of cultivars could be key to enhance drought resistance and resilience of barley cropping systems.