Physiological Changes and Transcriptomics of Elodea nuttallii in Response to High-Temperature Stress

<i>Elodea nuttallii</i> is a significant submerged macrophyte utilized in shrimp and crab aquaculture, yet it exhibits low thermotolerance. This study investigated the physiological responses and transcriptomic characteristics of <i>E. nuttallii</i> under high-temperature stress (HTS). The results indicated that HTS significantly reduced the absolute growth rate (AGR) and photosynthetic efficiency of <i>E. nuttallii</i> while concurrently elevating antioxidant enzyme activities, malondialdehyde (MDA) content, and concentrations of osmotic adjustment compounds. Furthermore, the apical segments of <i>E. nuttallii</i> demonstrated greater sensitivity to HTS compared to the middle segments. Under exposure to 35 °C and 40 °C, antioxidant enzyme activities, MDA content, and osmotic adjustment compound levels were significantly higher in the apical segments than in the middle segments. Transcriptomic analysis revealed 7526 differentially expressed genes (DEGs) in the apical segments at 35 °C, a number substantially exceeding that observed in the middle segments. Enrichment analysis of DEGs revealed significant upregulation of key metabolic regulators under HTS, including carbohydrate metabolism genes (<i>HXK</i>, <i>FRK</i>) and phenylpropanoid biosynthesis enzymes (<i>4CL</i>, <i>COMT</i>). This transcriptional reprogramming demonstrates <i>E. nuttallii</i>'s adaptive strategy of modulating carbon allocation and phenolic compound synthesis to mitigate thermal damage. Our findings not only elucidate novel thermotolerance mechanisms in aquatic plants but also provide candidate genetic targets (<i>HXK</i>, <i>4CL</i>) for molecular breeding of heat-resilient cultivars through transcriptomic screening.