Assessing the Impact of Glycine on Elodea nuttallii Growth, Photosynthesis, Antioxidant Activity, and Endogenous Hormones in a High-Temperature Environment.

High temperatures (HTs) severely constrain the growth and development of Elodea nuttallii, highlighting the need for effective mitigation strategies to maintain its ecological functions. This study investigated the effects of exogenous glycine (Gly) on E. nuttallii under high-temperature stress (HTS). Plants were assigned to three groups: (i) CK (no stress, 25 °C, no Gly), (ii) HT (HTS, 35 °C, no Gly), and (iii) HT+Gly (HTS, 35 °C, with Gly). HTS significantly reduced the plant height and fresh weight, chlorophyll fluorescence parameters (Fv/Fm, YII, and NPQ), photosynthetic pigments (Chl-a, Chl-b, and Car), CAT activity, and 6-BA content, while increasing the ABA, IAA, and GA levels. Exogenous Gly application substantially alleviated these adverse effects. Compared with the HT group, Gly increased Fv/Fm, YII, Chl-a, and Chl-b by 16.67%, 17.34%, 86.72%, and 72.22%, respectively, on day 4. Gly also strengthened antioxidant defense by increasing APX activity to as much as 2.38-fold that of the HT group and reducing H2O2 content by 29.03% on day 6. In addition, Gly restored a more balanced phytohormone profile. These findings indicate that exogenous Gly can substantially improve the HT tolerance of E. nuttallii and may provide a practical approach for enhancing macrophyte resilience under high-temperature conditions.