Effect of cultivar differences in seed size on early season growth and physiology of cotton under low and high‐temperature extremes

Abstract Extreme temperatures and cultivar variation in seed characteristics affect stand establishment, early‐season growth, and photosynthetic processes. However, studies addressing temperature responses of similarly adapted cultivars of contrasting seed traits are needed. This study assessed the effect of temperature and cultivar on growth, physiological responses, and photosynthetic thermotolerance in cotton ( Gossypium hirsutum L.) seedlings and identified important plant traits contributing to seedling vigor. Two similarly adapted cotton cultivars with contrasting seed sizes were grown in a controlled environment under 4 day/night temperature regimes, 20/15, 30/20, 35/25, and 40/30°C, for 4 weeks. Growth analysis, chlorophyll fluorescence, and gas exchange data were collected. Rapid induction fluorescence × incubation temperature experiments were used to define temperature thresholds, causing a 15% decline in photosynthetic efficiencies ( T 15 ). The large‐seeded cotton cultivar exhibited higher values for most growth traits. The 20/15°C temperature had the lowest values for all growth traits, and 30/20 and 35/25°C were optimal for shoot growth. Leaf area was the most important driver of seedling vigor, and whole‐canopy photosynthesis could be a more accurate predictor of biomass accumulation than net carbon assimilation per unit leaf area. Plants grown at 20/15°C had the lowest net carbon assimilation rates, primarily due to metabolic impairment. Photosystem I (PSI) and photosystem II (PSII) had greater heat tolerance than inter‐photosystem electron transport. The thermotolerance of all thylakoid responses increased as the early‐season growth temperature increased, and there were cultivar differences in acclimation potential for PSI.