Stem-Centered Drought Tolerance in Mikania micrantha During the Dry Season

Mikania micrantha, commonly known as mile-a-minute weed, is listed among the world's top 10 worst weeds. Although native to humid regions of South America, it has recently been found to colonize arid habitats as well. Despite pronounced seasonal hydroclimatic variations in South China and increasing drought due to global climate change, the mechanisms underlying M. micrantha's drought tolerance remain poorly understood. In this study, we compared the photosynthetic responses of M. micrantha leaves and stems between the dry (June) and wet (December) seasons through field experiments. We measured changes in phenotype, photosynthetic characteristics, and the content of antioxidant and osmotic adjustment substances, using the co-occurring native vine Paederia scandens as a control. The results revealed that during the dry season, M. micrantha leaves exhibited wilting, along with significant reductions in relative water content (RWC), chlorophyll (Chl), soluble sugar (SS), and soluble protein (SP). In contrast, the stems of M. micrantha maintained relatively stable phenotypes and chlorophyll levels compared to those of P. scandens. Notably, M. micrantha stems exhibited significant increases in vessel wall thickness, vessel density, total phenol content, and the activities of peroxidase (POD) and ascorbate peroxidase (APX). Furthermore, compared to P. scandens, M. micrantha stems displayed a greater increase in cortex proportion, flavonoid content, and soluble protein content. Expression analysis of bZIP transcription factors further revealed drought-responsive upregulation of specific genes (bZIP60, ZIP42-1), suggesting their potential involvement in drought response. These results indicate that although the leaves of M. micrantha are susceptible to prolonged drought, the stems exhibit considerable resilience, which may be attributed to a combination of traits including structural modifications in stem anatomy, enhanced antioxidant capacity, and osmotic adjustment. These insights suggest that stem-specific adaptations are key to its drought tolerance, providing a theoretical foundation for understanding the habitat distribution of M. micrantha and informing effective management strategies.