Investigating Thermotolerance of Thylakoid Processes in Two Cotton Species using Rapid Induction Fluorescence

High temperatures can hinder stand establishment, seedling growth, and photosynthetic processes in cotton. Yet, interpretations of thermotolerance in thylakoid processes often depend on whether measurements follow chronic or acute heat exposure, and species-level differences in these responses remain poorly characterized. This study evaluated the effects of chronic high temperatures (40°C/30°C) on key thylakoid processes in four-week-old Upland and Pima cotton seedlings and assessed their acclimation potential using rapid induction chlorophyll fluorescence across a range of incubation temperatures. As these processes were inferred from OJIP parameters rather than direct measurements of photosystem activity, they are interpreted as fluorescence-based indicators of thylakoid function. Chronic heat enhanced photosystem I (PSI)-related parameters in both species, with Upland exhibiting larger increases (44%-46%) compared to Pima (39%-40%), whereas photosystem II (PSII) photochemistry remained largely stable under chronic heat, declining by less than 2% at 40°C and by 6%-8% at 45°C under acute exposure. Acute temperature responses closely mirrored chronic patterns, and Upland showed higher thermal optima for PSI quantum yield and overall performance (40°C) than Pima (35°C), suggesting greater PSI electron-sink capacity. Collectively, these fluorescence-derived results suggest that PSI-related processes distinguish species-level thermotolerance in cotton. Upland's stronger enhancement of PSI acceptor-side capacity under both sustained and transient heat exposure points to a more robust acclimation strategy, whereas Pima shows limited PSI adjustment despite maintaining PSII efficiency. These findings clarify how thylakoid processes respond to heat across timescales and may help guide the development of heat-resilient cotton as growing-season temperatures rise.