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Purpose To explore the regulatory mechanism of rice to Cd stress and/or elevated CO2 concentration. Methods The rice seedlings (Oryza sativa L.) were exposed to two CO2 concentrations (400 ± 20 µmol mol− 1, AC; 800 ± 20 µmol mol− 1, EC) and CdCl2 concentrations (0 µmol/L, 150 µmol/L) for 10 days. Antioxidant enzymes activities, respiration rate, transcriptomics and metabolomics changes of leaves were studied. Results GR activity, respiration rate, many sugars, polyols, amino acids and organic acids contents increased under Cd stress. DEGs (differentially expressed genes) annotated in photosynthesis-antenna proteins were down-regulated; When CO2 increases, some antioxidant enzymes activities and respiration rate decreased. Genes and metabolites related to photosynthesis were enhanced; Under the composite treatment, ASA-GSH cycle was regulated, some amino acids contents increased, respiration rate decreased. The DEGs mainly enriched in substances transmembrane movement and enzymes activities, etc. Conclusion Under Cd stress, GR played an important antioxidant role. Sugar, polyol and amino acid metabolisms were enhanced to provide energy, improve osmotic adjustments, maintain cell membrane stability, etc. Organic acids contents increased for regulating plant nutrition, TCA cycle and as the secondary metabolites. Photosynthesis was adversely affected; Under high CO2, photosynthesis increased, the decrease of partial O2 pressure resulted in the decrease of some antioxidant enzymes activities and respiration rate; Under the composite treatment, Cd stress played a dominant role, elevated CO2 alleviated the Cd stress damage by regulating ASA-GSH cycle and amino acids metabolism.

JA is a defensive plant hormone, and its derivative MeJA is widely used in scientific research as an exogenous plant regulator to plants under biotic or abiotic stress. In recent years, the comparative study of MeJA and JA has become a new field for JA researchers, but most of them remain in laboratory experiments. Differences between the response of JA compounds to biotic stress in crops under greenhouse conditions, is not studied well. In this study, we studied the difference of two JA compounds in the regulation of physiological, growth and biotic stress resistance of cucumber under greenhouse conditions. We have three key findings: (1) JA compounds can significantly enhance the antioxidant activity, defense secondary metabolite accumulation and stress-resistance of cucumber. (2) MeJA increased the accumulation of lignin in cucumber leaves by elevating the activities of enzymes related to lignin biosynthesis (PAL, POD, LAC), which resulted in lower value of plant stress rating than JA. (3) MeJA also had a greater limiting effect on cucumber biomass accumulation than JA, which may be another reason for the lower plant stress rating. Our research has provided suggestions and experience for relevant agricultural production and follow-up scientific research.

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