Abstract: To investigate the physiological mechanism of β-cyclocitral (βcyc), as a retrograde signaling molecule in alleviating salt stress in watermelon, photosynthetic CO₂ response curves were measured using a Li-6800 portable photosynthesis system. Combined with enzyme activity assays and high-throughput sequencing technology, principal component analysis (PCA) and grey relational analysis (GRA) were employed to construct a systematic evaluation model to analyze the changes in growth, physiological and biochemical indices, and rhizosphere microbial communities. The results showed that NaCl stress alone significantly inhibited the growth and photosynthetic carbon assimilation capacity of watermelon seedlings. Under non-stress conditions, the application of βcyc alone led to an increase in the chlorophyll a/b ratio and the accumulation of malondialdehyde (MDA), inducing a mild oxidative stress in the plants. However, under salt stress, βcyc treatment significantly increased the activities of leaf catalase (CAT) and peroxidase (POD), and decreased the MDA content by 74.9% compared to NaCl stress alone, thereby alleviating cell membrane damage. Photosynthetic physiological analysis revealed that βcyc not only maintained a relatively high stomatal conductance (G_s), but also enhanced the carbon assimilation capacity of plants under high CO₂ concentrations, maintaining the net photosynthetic rate (P_n) at levels comparable to the control. Furthermore, βcyc treatment altered the rhizosphere bacterial community structure, and the enrichment of core beneficial bacterial taxa exhibited a significant positive correlation with the improvement of aboveground physiological indices. βcyc could activate the antioxidant defense system of watermelon seedlings, alleviate salt stress-induced damage to the photosynthetic apparatus, and promote the enrichment of beneficial rhizosphere microorganisms. This demonstrated a synergistic effect of “aboveground physiological defense and belowground microecological improvement”, thereby enhancing the salt tolerance of watermelon seedlings.

Key words: watermelon, salt stress, β-cyclocitral, photosynthesis, antioxidant enzymes, rhizosphere microorganisms