Physiological response and tolerance of Sesuvium portulacastrum L. to low temperature stress

The plant Sesuvium portulacastrum L., commonly referred to as sea purslane, is a perennial halophytic species with significant potential for development in marine ecological restoration. However, its growth is limited in high-latitude regions with lower temperatures due to its subtropical nature. Furthermore, literature on its cold tolerance is scarce. This study, therefore, focused on sea purslane plants naturally overwintering in Ningbo (29°77’N), investigating their morphological, histological, rooting, and physiological responses to low temperatures (7 °C, 11 °C, 15 °C, and 19 °C). The findings indicated an escalation in cold damage severity with decreasing temperatures. At 7 °C, the plants failed to root and subsequently perished. In contrast, at 11 °C, root systems developed, while at 15 °C and 19 °C, the plants exhibited robust growth, outperforming the 11 °C group in terms of leaf number and root length significantly (P < 0.05). Histological analyses showed a marked reduction in leaf thickness under cold stress (P < 0.05), with disorganized leaf structure observed in the 7 °C group, whereas it remained stable at higher temperatures. No root primordia were evident in the vascular cambium of the 7 and 11 °C groups, in contrast to the 15 and 19 °C groups. Total chlorophyll content decreased with temperature, following the order: 19 °C > 15 °C > 11 °C > 7 °C. Notably, ascorbic acid levels were significantly higher in the 7 and 11 °C groups than in the 15 and 19 °C groups. Additionally, the proline concentration in the 7 °C group was approximately fourfold higher than in the 19 °C group. Activities of antioxidant enzymes—superoxide dismutase, peroxidase, and catalase—were significantly elevated in the 7 and 11 °C groups compared to the 15 and 19 °C groups. Moreover, the malondialdehyde content in the 7 °C group (36.63 ± 1.75 nmol/g) was significantly higher, about 5.5 and 9.6 times, compared to the 15 °C and 19 °C groups, respectively. In summary, 7 °C is a critical threshold for sea purslane stem segments; below this temperature, cellular homeostasis is disrupted, leading to an excessive accumulation of lipid peroxides and subsequent death due to an inability to neutralize excess reactive oxygen species. At 11 °C, although photosynthesis is impaired, self-protective mechanisms such as enhanced antioxidative systems and osmoregulation are activated. However, root development is compromised, resulting in stunted growth. These results contribute to expanding the geographic distribution of sea purslane and provide a theoretical basis for its ecological restoration in high-latitude mariculture.