Effects of root phenotypic changes on the deep rooting of Populus euphratica seedlings under drought stresses

Deep roots are critical for the survival of Populus euphratica seedlings on the floodplains of arid regions where they easily suffer drought stress. Drought typically suppresses root growth, but P. euphratica seedlings can adjust phenotypically in terms of root-shoot allocation and root architecture and morphology, thus promoting deep rooting. However, the root phenotypic changes undertaken by P. euphratica seedlings as a deep rooting strategy under drought conditions remain unknown.We quantified deep rooting capacity by the relative root depth (RRD), which represents the ratio of taproot length to plant biomass and is controlled by root mass fraction (RMF), taproot mass fraction (TRMF), and specific taproot length (STRL). We recorded phenotypic changes in one-year-old P. euphratica seedlings under control, moderate and severe drought stress treatments and assessed the effects of RMF, TRMF, and STRL on RRD.Drought significantly decreased absolute root depth but substantially increased RRD via exerting positive effects on TRMF, RMF, and STRL. Under moderate drought, TRMF contributed 55%, RMF 27%, and STRL 18% to RRD variation. Under severe drought, the contribution of RMF to RRD variation increased to 37%, which was similar to the 41% for TRMF. The contribution of STRL slightly increased to 22%.These results suggest that the adjustments in root architecture and root-shoot allocation were predominantly responsible for deep rooting in P. euphratica seedlings under drought conditions, while morphological changes played a minor role. Moreover, P. euphratica seedlings rely mostly on adjusting their root architecture to maintain root depth under moderate drought conditions, whereas root-shoot allocation responds more strongly under severe drought conditions, to the point where it plays a role as important as root architecture does on deep rooting.