<p>The aim of this study was to determine the heat stress threshold of grafted cucumber seedlings and examine the effects of potassium sulfate and proline on their stress responses. Temperatures during the experiment were maintained as follows: (ⅰ) optimal (OT) (24/19 ℃); (ⅱ) medium high (MH) (32/27 ℃); and (ⅲ) extremely high (EH) (40/35 ℃). One day after the heat treatment, the adaxial and abaxial surfaces of the leaves were sprayed with 2.5 mL of various treatments per plant, i.e., (i) no treatment (control), (ii) 2% potassium sulfate (K₂SO₄), and (iii) 2.5 mM L-proline (proline), to enhance heat tolerance. The seedling quality under OT conditions was the best, and applying K₂SO₄ and proline delayed shoot development. The roots became thin and yellow under MH, leading to a decline in seedling quality, whereas applying K₂SO₄ significantly increased root growth and improved seedling quality. Heat injury occurred in the EH control group, leading to the death of some seedlings. However, applying exogenous K₂SO₄ and proline to EH effectively enhanced heat tolerance by significantly increasing the concentration of photosynthetic pigments and alleviating negative effects on photosystem II function, with proline being particularly effective. In conclusion, physiological disorders in grafted cucumber seedlings were observed at temperatures exceeding 40 ℃, resulting in abnormal phenotypes. Foliar application of exogenous K₂SO₄ and proline under high-temperature conditions effectively enhanced tolerance to heat stress. Specifically, it is highly recommended to spray foliar K₂SO₄ under mild heat stress and proline under extreme heat stress.</p>