<p>This study investigated the impact of exogenous L-glutamic acid (L-Glu) combined with supplemental LED lighting on the growth, photosynthetic efficiency, and yield of paprika plants (<i>Capsicum annuum</i> L.) under the challenging environmental conditions of the summer season, in response to climate change. The optimal concentration of 5 mM L-glutamic acid, which was determined through preliminary experiments, was used in conjunction with three types of LED interlighting spectra: quantum dot-applied blue + wide-red + far-red (QD), blue + red (B + R), white (W), and a control with no supplemental LED lighting. The results demonstrated that the foliar application of L-glutamic acid, particularly when combined with supplemental LED lighting, significantly increased the plant height, leaf spectral reflectance indices, and overall fruit yield compared with those of the controls. Notable improvements were observed in photosynthetic parameters such as stomatal conductance (gs), transpiration rate (E), and photosynthetic rate (A), especially in middle canopy leaves. Principal component analysis (PCA) revealed that chlorophyll content, photosynthetic efficiency, and vegetation indices were significant factors driving variations in growth and yield. K-means clustering further demonstrated that the combination of L-glutamic acid with LED lighting consistently resulted in superior plant performance, whereas the absence of both treatments was associated with the most detrimental effects. The study concluded that integrating L-glutamic acid with supplemental LED lighting effectively mitigates the stress associated with high temperatures and low light conditions, thereby increasing paprika productivity in greenhouse environments.</p>