Comparisons of physiology, metabolism, and gene expression of distinct Leymus chinensis germplasms under elevated CO2.

As the atmospheric CO2 concentration continues to rise, understanding the response mechanisms of key species in grasslands is crucial for predicting ecosystem feedbacks. Leymus chinensis, as the dominant species in Eurasian grasslands, the response of its germplasm resources to elevated CO2 (eCO2) remains unclear. Using open-top chambers, we integrated physiological measurements (photosynthetic performance, enzyme activities related to carbon-nitrogen metabolism, and growth traits) with transcriptomic and metabolomic analyses to compare three L. chinensis germplasms with different response intensities to eCO2 (strongly responsive LC05, weakly responsive LC22 and LC52). The results showed that only LC05 had a significant increase in biomass under eCO2 (+50.24%), and its photosynthetic rate, quantum yield of photosystem II and glutamate synthase activity were all significantly enhanced. Principal component analysis revealed that the carbon-nitrogen coordinated growth axis (PC2) was the key factor driving the variation in biomass. Multi-omics integrated analysis revealed that LC05 achieved an optimized allocation of defense resources by up-regulating flavonoid synthesis genes (CHS, F3H, FLS) and down-regulating the phenylpropanoid pathway gene PAL along with other lignin pathway genes (C4H, 4CL), while up-regulating photosynthesis-related genes (LcRBCS2, LcPQL3, LcFBA5) and transcription factors (MYB, HSF) to synergistically promote growth. In contrast, the weakly responsive germplasms showed carbon retention, metabolic imbalance and limited activation of defense signals. This study systematically revealed the physiological and molecular basis of the response of L. chinensis germplasms to eCO2, providing theoretical and technical support for the selection of climate-adaptive grassland varieties.