Interactive Effects of Maximum Daytime and Minimum Nighttime Temperatures on Spinach Growth and Physiological Characteristics

High temperatures restrict spinach growth, and the plant’s growth and physiological responses to heat remain poorly understood. It remains unclear whether high daytime or elevated nighttime temperatures have a more negative impact on spinach growth. In addition, the interaction effect of maximum daytime and minimum nighttime temperatures on spinach growth remains unknown. This study was conducted to address these issues. Spinach was grown in controlled environments under four temperature treatments: 30 and 20 °C (T30/20), 30 and 25 °C (T30/25), 35 and 20 °C (T35/20), and 35 and 25 °C (T35/25). These treatments represent the maximum daytime temperature and minimum nighttime temperature, respectively, and were maintained for 45 days. Plant growth characteristics were monitored, and the physiological responses to temperature regimes were assessed. The results show that compared with T30/20, dry matter production decreased by 15.4% with increased nighttime temperature (T30/25), decreased by 42.3% with increased daytime temperature (T35/20), and decreased by 57.7% when both daytime and nighttime temperatures were increased (T35/25). However, there was no statistically significant interaction effect (P > 0.05) between daytime maximum and nighttime minimum temperatures on plant biomass production variables. In comparison with T30/20, the T35/25 treatment increased significantly plant stomatal conductance, stomatal apertures, transpiration rate, and leaf temperature during heat waves. The T35/25 treatment also decreased the quantum efficiency in light compared with the other treatments. Plant biomass production did not improve with the T35/20 and T35/25 treatments, likely as a result of a decoupling of photosynthesis and stomatal conductance during heat waves. Overall, these results reveal that maximum daytime and minimum nighttime temperatures exert additive effects on spinach growth.