Physiological Response of Citrus reticulata Blanco var. Gonggan Seedlings to High-Temperature Stress

The physiological and biochemical responses of <i>Citrus reticulata</i> Blanco var. Gonggan (Gonggan) to high-temperature stress were explored in the present study, offering valuable insights into the growth of this plant in elevated temperature scenarios. Plants were exposed to daytime temperatures of 22 °C, 40 °C, and 45 °C, with corresponding nighttime temperatures of 17 °C, 35 °C, and 40 °C, respectively. Each treatment was administered for 12 h, with a daytime light intensity of 14,400 lux. Key parameters such as the chlorophyll content, peroxidase activity, malondialdehyde content, cytoplasmic membrane permeability, and photosynthetic metrics were assessed. The results showed that the content of malondialdehyde decreased with the increase in temperature, with the highest content at 22 °C. After high-temperature treatment at 40 °C and 45 °C, there was a significant difference (<i>p</i> < 0.05) compared with the Gonggan plants treated at 22 °C. Peroxidase activity exhibited an increasing trend as the temperature increased, and there was a significant difference (<i>p</i> < 0.05) between the peroxidase activity at 22 °C and 45 °C. Similar trends are displayed for high-temperature stress, stomatal conductance, transpiration rate, and intercellular CO₂, which initially decreased and then significantly increased. The net photosynthetic rate (Pn) showed a trend of first increasing and then decreasing. When plants were subjected to high-temperature stress at 40 °C, the net photosynthetic rate showed a significant increase compared to the control group at 22 °C, but in a 45 °C stress environment, the Pn showed a decreasing trend. In the experimental group, relative conductivity decreased with the increase in temperature. Meanwhile, Gonggan plants exhibited moderate heat tolerance to short-term or moderate high-temperature stress, primarily through antioxidant and repair mechanisms. However, their heat tolerance was limited under prolonged or extremely high-temperature conditions, characterized by significant membrane damage and photosynthetic inhibition. Overall, Gonggan plants demonstrate moderate heat tolerance, making them suitable for intermittent high-temperature environments rather than prolonged extreme heat conditions. These findings provide a foundation for understanding the adaptive strategies of Gonggan plants and their cultivation in high-temperature settings.