Plant secondary metabolites (SMs) play crucial roles in plant-environment interactions and contribute greatly to human health. Global climate changes are expected to dramatically affect plant secondary metabolism, yet a systematic understanding of such influences is still lacking. Here, we employed medicinal and aromatic plants (MAAPs) as model plant taxa and performed a meta-analysis from 360 publications using 1828 paired observations to assess the responses of different SMs levels and the accompanying plant traits to elevated carbon dioxide (eCO₂), elevated temperature (eT), elevated nitrogen deposition (eN), and decreased precipitation (dP). The overall results showed that phenolic and terpenoid levels generally respond positively to eCO₂ but negatively to eN, while the total alkaloid concentration was increased remarkably by eN. By contrast, dP promotes the levels of all SMs, while eT exclusively exerts a positive influence on the levels of phenolic compounds. Further analysis highlighted the dependence of SM responses on different moderators such as plant functional types, climate change levels or exposure durations, mean annual temperature and mean annual precipitation. Moreover, plant phenolic and terpenoid responses to climate changes could be attributed to the variations in C/N ratio and total soluble sugar levels, while the trade-off supposition contributed to SM responses to climate changes other than eCO₂. Taken together, our results predicted the distinctive SM responses to diverse climate changes in MAAPs, and allowed us to define potential moderators responsible for these variations. Further, linking SM responses to C-N metabolism and growth-defence balance provided biological understandings in terms of plant secondary metabolic regulation.

Peer-reviewed journal articles published online from January 1990 to March 2022 were searched using Web of Science (http://www.isiknowledge.com/), with the following terms: (global change OR climate change OR free-air carbon dioxide enrichment OR free-air CO₂ enrichment OR elevated carbon dioxide OR elevated CO₂ OR elevated atmospheric CO₂ OR CO₂ enrichment OR eCO₂ OR atmospheric CO₂  enrichment OR elevated atmospheric carbon dioxide OR carbon dioxide enrichment OR [carbon dioxide] OR nitrogen deposition OR nitrogen addition OR nitrogen application OR nitrogen fertiliz* OR nitrogen nutrition OR N deposition OR N addition OR N application OR N fertiliz* OR N nutrition OR changing precipitation OR increased precipitation OR decreased precipitation OR drought OR water stress OR water addition OR warming OR elevated temperature OR climate warming OR elevated temperature OR increased temperature) AND (medicinal plant OR aromatic plants).