Do roots etiolate in response to drought? An experimental test with very young seedlings of three Mediterranean shrub species

Seedlings are very sensitive to dehydration and recruitment depends on their ability to cope with dryness, particularly in dry habitats where root morphology plays an important role in their responses. Here we explore how drought affected root growth at the very early stages of plant development. We carried out a greenhouse experiment exposing two-week-old seedlings of three Mediterranean shrub species, Genista umbellata, Lycium intricatum, and Retama sphaerocarpa, to different watering regimes and monitored root growth for five weeks in translucent glass cases. We found that reduced watering enhanced root growth in all three species. Seedlings responded to lower water availability with faster root elongation rate, greater root area, and longer roots in a response analogous to the etiolation of shoots in shaded conditions. Plants did not shift allocation patterns (i.e., root: shoot ratio) in response to watering reduction. Our data evidence critical effects of water availability on early development of plants. In seedlings, drought-induced root elongation is a strategy to deal with decreasing soil moisture, as fast root extension allows to tap water from unexplored soil areas and contributes to a successful establishment.