Time of day of leaf wounding determines plant biomass and affects the interplay between growth and defence in Brassica crops.

Recent discoveries pointed out the importance of the mutual correlation between timing of environmental stress and plant fitness. However, the internal reshaping of plant growth under daily stress sensing, and their metabolic coordination remain to be investigated. Thus, we studied the connection between time of day, growth and defence to understand how plant fitness is affected by diurnal stress inputs. We examined if simulated herbivory (leaf wounding) in the morning, at midday or the evening differentially influence plant defence state vs growth in three crop species of Brassica: broccoli (Brassica oleraceae), turnip greens (B. rapa) and rapeseed (B. napus). The data revealed that plant's tolerance of wounding stress is diurnally regulated in Brassica crops. Trade-offs between biomass and investment in glucosinolates (GSL) and phenolics were affected by timing of leaf stress. Negative correlations between biomass and induction of defence compounds were found for plants treeated in the morning and evening. However, the correlations were positive for midday treatment. Interestingly, we revealed a new connection between plant growth and changes in aliphatic GSLs and flavonoids in response to wounding. These data suggest that metabolic stress-dependent circadian oscillations in leaf defences could be one mechanism conferring a competitive advantage to plants to anticipate daily environmental variations by synchronizing them with growth. Moreover, this work provides first insights into how secondary metabolites are linked to growth response in a timing-related manner.