Abstract

Plant acclimation to varying light environments is a critical process for optimizing photosynthetic efficiency and ensuring survival. This study investigates the differential effects of acclimation on the photosynthetic electron transport chain (ETC) in chloroplast membranes of two contrasting species from the Cucumis genus: the shade-tolerant cucumber (*Cucumis sativus*) and the light-loving melon (*Cucumis melo*). The research objective was to elucidate the species-specific molecular and biophysical adjustments in the thylakoid membrane ETC in response to high and low light conditions. Using chlorophyll fluorescence analysis and spectrophotometric measurements of isolated chloroplasts, we assessed key parameters such as Photosystem II (PSII) efficiency, the redox state of the plastoquinone pool, and the maximum electron transport rate. Our key findings indicate that *C. melo* exhibits a greater capacity for upregulating its maximum electron transport rate under high-light acclimation, primarily through enhanced Photosystem I activity and non-photochemical quenching mechanisms. Conversely, *C. sativus* maintains a more stable, albeit lower, basal electron transport rate, suggesting a strategy focused on photoprotection and resource conservation. These results highlight distinct photosynthetic acclimation strategies between the two species, providing valuable insights into the genetic and physiological basis of light-use efficiency in economically important crops and contributing to the broader understanding of plant adaptation to environmental stress.