Partial root-zone simulated drought induces greater flavonoid accumulation than full root-zone simulated water deficiency in the leaves of Ginkgo biloba

To elucidate the physiological and transcriptomic regulatory mechanisms underlying flavonoid concentrations in ginkgo leaves in response to full and partial root-zone drying, double-rootstock grafted ginkgo saplings that could split roots easily were exposed to one of three water treatments: well-watered (WW); PEG-6000 simulated full root-zone drought (DD); and simulated partial root-zone drought (WD). Both DD and WD resulted in decreased chlorophyll concentrations and increased total flavonoid concentrations in ginkgo leaves and led to increased levels of scavenging OH and DPPH radicals and total reducing power in flavonoid extracts. The concentrations of chlorophyll and total flavonoids and the total reducing availability under WD were significantly greater than those under DD. Consistently, numerous differentially abundant metabolites and differentially expressed genes involved in the flavonoid biosynthetic pathway and hormone metabolism were identified in the leaves of DD- and WD-exposed G. biloba. For instance, the abundances of various precursors for flavonoid biosynthesis and gibberellin (GA) involved in modulating flavonoid accumulation and the transcript levels of several genes involved in GA and flavonoid biosynthesis in WD were greater than those in WW- and DD-treated ginkgo leaves. Through association analysis, three metabolites (sakuranetin, cinnamic acid and naringenin), six structural genes, and two transcription factors (enhancer of GLABRA 3 (EGL3) and ethylene-insensitive 3 (EIN3)) that play key roles in regulating flavonoid accumulation in ginkgo leaves were identified. These results suggest that WD induced greater flavonoid accumulation than DD, and metabolites and structural genes involved in the flavonoid biosynthetic pathway and hormone metabolism as well as transcription factors play crucial roles in regulating flavonoid concentrations in response to full and partial root-zone drought stress. This study also implies that partial root-zone drying is an efficient method to harvest ginkgo leaves with high concentration of flavonoids.