Melatonin enhances drought resistance by regulating leaf stomatal behaviour, root growth and catalase activity in two contrasting rapeseed (Brassica napus L.) genotypes.

Two contrasting rapeseed (Brassica napus L.) genotypes, Qinyou 8 (drought-sensitive) and Q2 (drought-tolerant), were studied under drought stress with or without pretreatment with melatonin to (i) explore whether melatonin enhances drought resistance by regulating root growth and (ii) determine the relationship between the belowground and aboveground responses to melatonin under drought stress. Results show that the light-saturated rate of photosynthesis (Pn), stomatal conductance (gs), water use efficiency (WUE) and chlorophyll content were decreased by drought for Qinyou 8, whereas drought only decreased Pn and chlorophyll content for Q2. Drought decreased actual photochemical efficiency in saturated light (Fv'/Fm'), actual photochemical efficiency (PhiPSⅡ), quenching of photochemical efficiency (qL) and electron transport rate (ETR) in Qinyou 8. However drought only decreased Fv'/Fm' and qL in Q2. Drought increased malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents in the roots of both genotypes. Melatonin had no significant additional effects on root guaiacol peroxidase (POD) and superoxide dismutase (SOD) activities, but enhanced root catalase (CAT) activity of droughted plants further. Melatonin promoted taproot and lateral root growth under drought stress. Melatonin also promoted stomatal opening resulting in enhanced photosynthesis in the two genotypes. The two mechanisms induced by melatonin synergistically enhance drought resistance of rapeseed as indicated by enhanced gas exchange parameters under melatonin pretreatment. The findings provide evidence for a physiological role of melatonin in improving drought resistance, especially in belowground parts.