A large subset of plant stress-signaling pathways, including those related with chemical defense production, exhibit diurnal or circadian oscillations. However the extent to which diurnal or circadian time influences the stress mediated accumulation of plant specialized metabolites remains largely unknown. Because plant responses to physical stress (e.g., wounding) is considered a common component of mounting a response against a broad range of environmental stresses, including herbivory, we have utilized mechanical wounding as the stress stimulus to determine the direct contribution of time of day on the induced defenses of Brassica crops. We analyzed glucosinolates (GSLs) from leaves of broccoli (Brassica oleracea) and turnip greens (Brassica rapa) following exposure to mechanical wounding at dawn (ZT0), mid-day (ZT4), and dusk (ZT8). Several GSLs differentially accumulated and their changes depended upon the time of day at wounding was performed. This response varied considerably between species. In a parallel experiment, we investigated whether diurnal activation of Brassica phytochemicals in response to wounding might prime plants against herbivore attack. Results showed that maximal response of plant chemical defense against larvae of the generalist pest Mamestra brassicae occurred at ZT0 in broccoli and ZT8 in turnip greens. Metabolome analysis for global trends of time dependent compounds showed that sulfur-containing phytochemicals, GSL hydrolysis products, auxin-signaling components, and other metabolites activators of plant disease resistance (nicotinamide and pipecolate) had important contributions to the responses of M. brassicae feeding behavior in broccoli at morning. Overall, the findings in this study highlight a significant role for time of day in the wound stress responsive metabolome, which can in turn affect plant-herbivore interactions.

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.