Root anatomical plasticity contributes to the different adaptive responses of two Phragmites species to water-deficit and low-oxygen conditions

The runner reed (Phragmites japonica) is the dominant species on riverbanks, whereas the common reed (Phragmites australis) thrives in continuously flooded areas. Here, we aimed to identify the key root anatomical traits that determine the different adaptative responses of the two Phragmites species to water-deficit and low-oxygen conditions. Growth measurements revealed that P. japonica tolerated high osmotic conditions, whereas P. australis preferred low-oxygen conditions. Root anatomical analysis revealed that the ratios of the cortex to stele area and aerenchyma (gas space) to cortex area in both species increased under low-oxygen conditions. However, a higher ratio of cortex to stele area in P. australis resulted in a higher ratio of aerenchyma to stele, which includes xylem vessels that are essential for water and nutrient uptakes. In contrast, a lower ratio of cortex to stele area in P. japonica could be advantageous for efficient water uptake under high-osmotic conditions. In addition to the ratio of root tissue areas, rigid outer apoplastic barriers composed of a suberised exodermis may contribute to the adaptation of P. japonica and P. australis to water-deficit and low-oxygen conditions, respectively. Our results suggested that root anatomical plasticity is essential for plants to adapt and respond to different soil moisture levels.