Stress response and tolerance of the submerged macrophyte Elodea nuttallii (Planch) St. John to heat stress: a comparative study of shock heat stress and gradual heat stress

Sudden and gradual increases of temperature in aquatic environments play important roles in determining growth and physiological dynamics of aquatic macrophytes. However, a lesser attention has been paid to identify the effects of different temperature regimes on aquatic macrophytes. Therefore, the present study is focused on comparing the effects of shock and gradual heat stresses (SHS and GHS) on growth, photosynthetic attributes, and oxidative damage on Elodea nuttallii as a model plant. Laboratory-oriented two experimental setups were maintained to induce the SHS and GHS. A significant decline in shoot elongation coupled with a decline in endogenous indoleacetic acid (IAA) and an increase in hydrogen peroxide (H₂O₂) was observed in both temperature treatments. These effects were further accompanied by oxidative damage to photosynthetic pigments and cell membrane structures in E. nuttallii. Temperature-mediated oxidative stress was significantly pronounced under SHS, which induced the activation of different defensive mechanisms against reactive oxygen species, including antioxidant enzymes, secondary metabolites, and osmoprotectants. The present study revealed that temperature-induced oxidative damage was more severe when the temperature increased suddenly. Further, heat acclimation was observed when the plant was exposed to 30 °C under GHS, although this treatment induces significant oxidative stress under 35 °C.