Network disintegration of photosynthesis-antioxidant coordination underpins thermal vulnerability in Ochroma lagopus Swartz: A 5°C threshold triggers carbon starvation

The present study employed Ochromalagopus Swartz seedlings to examine the effects of low-temperature stress (4 gradients: CK, 7°C, 5°C, 3°C; 12–72 h) on photosynthetic traits, antioxidant enzymes, and osmoregulatory substances. Physiological responses were analysed via cluster, correlation, and PCA.The results demonstrated a decline in chlorophyll a and total chlorophyll a+b with decreasing temperature. However, moderate low temperature (5°C) led to a transient increase in chlorophyll b and carotenoids, thereby enhancing light harvesting and antioxidant activity. Chlorophyll fluorescence parameters (Fv/Fm, Fv/Fo) indicated progressive PSII damage with stress intensity/duration. The activities of antioxidant enzymes (SOD, POD, CAT) and the levels of MDA increased. The proline concentration reached its maximum at 48 h at a temperature of 5°C.The principal component analysis (PCA) revealed that PC1 (48.1 % variance) was associated with photosynthetic pigments and photochemical efficiency, while PC2 (18.7 %) was linked to initial fluorescence and lipid peroxidation. A 5°C threshold was observed to trigger carbon starvation (for a minimum of 48 h), with 3°C inducing photosynthetic collapse. The moderation of cold conditions was found to be effective in mitigating damage, a process that was facilitated by pigment adjustment and antioxidant activation. The findings provide a physiological basis for the evaluation of cold tolerance and the management of cultivation of this tropical species. • For the first time, the critical cold acclimatisation threshold of tropical O. lagopus seedlings was determined to be 5°C/48 hours. Beyond this threshold, irreversible PS II damage occurs (Fv/Fm decline >50%). • Extreme cold (3°C) causes thylakoid peroxidation, antioxidant enzymes (SOD, POD and CAT) lose efficacy at 3°C due to SOD denaturation. • Low temperature induces a decrease in non-structural carbohydrates (NSCs), and starch accumulation reflects 'energy storage prior to growth'.