Elevated CO2 Enhances Heat Tolerance in Celery by Restoring Photosynthesis and Strengthening Antioxidant Defense

Heat stress resulting from global warming severely constrains the growth and quality of cool-season vegetables. While CO2 enrichment is known to improve crop stress tolerance, the mechanisms by which it mitigates heat stress remain unclear. In this study, celery plants were subjected to four treatments: ambient temperature with ambient CO2 (CK), ambient temperature with elevated CO2 (EC), high temperature with ambient CO2 (CKH), and high temperature with elevated CO2 (ECH). Growth, photosynthetic efficiency, and antioxidant systems were systematically analyzed, with key genes involved in photosynthesis and light-harvesting antenna protein pathways identified by transcriptomic and qRT-PCR analysis. Heat stress significantly inhibited celery growth, reducing the net photosynthetic rate by 37.3% and increasing leaf H2O2 and MDA contents by 104.6% and 87.4%, respectively, after 24 h. Heat stress also down-regulated 4275 genes in CKH compared with CK. In contrast, CO2 enrichment promoted biomass accumulation (total dry weight increasing nearly four-fold in EC vs. CK comparison group) and restored the net photosynthetic rate under heat stress (43.7% higher in ECH than in CKH). Furthermore, it comprehensively activated the antioxidant defense system, with SOD and POD activities remaining consistently higher in the ECH group than in CKH, while O2− and H2O2 contents were reduced by 30.9% and 11.6%, respectively, following 24 h of heat exposure. Transcriptomic analysis identified that the photosynthesis and light-harvesting antenna protein pathways are pivotal in mediating the response to CO2 enrichment under heat stress, with 62 out of 63 differentially expressed genes (DEGs) in these pathways being up-regulated, particularly AgFNR, AgFd3, and AgLhca3. Additionally, DEGs were significantly enriched in pathways related to redox metabolism and plant hormone signal transduction. These findings demonstrate that CO2 enrichment alleviates heat stress damage in celery by repairing photosynthetic impairment, activating antioxidant defenses, and up-regulating heat-responsive genes, thereby offering a theoretical foundation for cultivating stress-resistant vegetables in protected environments.