Physiological and transcriptomic analysis of cranberry (Vaccinium macrocarpon) in response to drought stress

Cranberry is a small fruit crop with great development potential. Due to its shallow root system and dominance of fibrous roots, it is highly susceptible to drought stress, but the response mechanism is still unclear. In this study, "Bain 11" cranberry was used as the material, and four pot treatments were set up: normal water supply (CK), mild (D1), moderate (D2), and severe (D3) drought. After reaching the set stress levels, functional leaves were collected. The response mechanism to different degrees of drought was clarified by combining paraffin sectioning, chlorophyll fluorescence, spectrophotometry, and transcriptome sequencing technology. The results showed that there was no significant change in the rate of photosynthetic oxygen release under D1 conditions; under D2 conditions, the function of PSII was impaired, but the heat dissipation capacity was enhanced, and the protective mechanism was activated; under D3 conditions, the performance of both PSII and PSI was significantly inhibited, the damage to the oxygen-evolving complex was the most severe, and the photosynthetic oxygen production decreased significantly. Leaf anatomical structure showed that with the intensification of drought, the morphology, area, and density of stomata changed, and the thickness of palisade tissue, epidermis, and spongy tissue decreased; under D2 conditions, organelle membranes were deformed but intact, and the thylakoid lamellar spacing increased; under D3 conditions, organelle membranes ruptured, nuclear material leaked, chloroplasts vacuolated, and thylakoid lamellae disintegrated. The oxidative stress indicators MDA and H₂O₂ increased with the intensity of the stress, and the activities of SOD and CAT significantly increased under D2 and D3 conditions. Transcriptome sequencing identified 58,020 single genes, and KEGG enrichment analysis showed that 55 continuously differentially expressed genes were involved in 54 metabolic pathways (19 upregulated and 36 downregulated), among which photosynthesis-related genes accounted for the highest proportion (13%) and played a key role in the drought response, providing important insights into the drought stress response mechanism of cranberry.