Preliminary Cold Tolerance Evaluation of Seven Ilex Species Based on Physiological Responses of Detached Leaves to Acute Low-Temperature Stress

The genus Ilex L., the sole member of the family Aquifoliaceae, is valued for its high ornamental value. However, low winter temperatures restrict the distribution of its evergreen species in colder regions. In this study, detached leaves of seven evergreen Ilex cultivars were subjected to acute low-temperature stress, and key physiological parameters (cell membrane permeability, osmoregulatory substances, and chloroplast pigments) were measured. The results showed that under low-temperature stress, relative electrical conductivity (REC) and malondialdehyde (MDA) content increased with decreasing temperature, while soluble protein (SP), soluble sugar, and free proline (Pro) contents first increased and then decreased. A positive association was observed between REC and MDA, as well as between REC and SP, while REC showed a negative association with Pro. Furthermore, random forest analysis indicated that MDA, proline, and chlorophyll a together accounted for 72.6% of the variance in REC. These findings demonstrate the physiological responses of detached leaves of evergreen Ilex species to acute low-temperature stress and offer an initial assessment of their cold tolerance.