Melatonin Mitigates Drought Stress on Radish and Promotes its Recovery After Rehydration

The “global boiling” will aggravate drought events and irregular rainfall, extremely impacting crop production. Therefore, strategies to mitigate drought stress, and to promote rapid plant recovery are paramount. In this context, exogenous melatonin has shown promising results in other species; however, its effects on radish remain unknown. Thus, we aimed to evaluate the effects of melatonin on the mitigation and recovery of drought stress on radish. For this, radish plants were subjected to three watering conditions (well-watering, re-watering, and drought), and leaf sprayed with melatonin (100 µM) or water (control). Drought induced oxidative damages, increasing electrolyte leakage by 59%, decreasing relative water content by 19%, and strongly inhibiting photosynthesis (65% lower), which culminated in impaired root growth (53% lower). Re-watering, in turn, reversed the drought effects on photosynthesis, fully recovering growth of shoots but not of roots. Melatonin increased the photosynthetic capacity of re-watered and non-stressed plants, increasing their overall growth. Melatonin also decreased oxidative damages under all watering conditions, increasing the contents of phenolics and flavonoids. Thus, melatonin had a growth promoting effect on non-stressed plants, also mitigating drought stress and promoting a stronger recovery in re-watered plants by inducing their antioxidant defenses, and increasing radish quality.