Variation in flooding tolerance in <i>Populus deltoides</i> ‘D-124’ and <i>P. trichocarpa</i>  ×  <i>P. deltoides</i> hybrid ‘52-225’

Flooding poses a substantial challenge to plant survival and productivity, particularly in riparian genera like Populus. This study examines the physiological, morphological, metabolic and molecular responses of Populus deltoides Bartram ex Marshall 'D-124' and P. trichocarpa Torr. & Gray × P. deltoides hybrid clone '52-225' under control and inundated conditions to identify differences in flooding tolerance. Under flooding conditions, physiological and cellular stress was more pronounced in P. deltoides 'D-124' than in the hybrid clone '52-225,' as evidenced by lower transpiration (E), photosynthesis (A) and chlorophyll content. In contrast, '52-225' showed reduced reactive oxygen species accumulation, suggesting better cellular function under stress. Morphologically, '52-225' produced more shoot-born roots, which likely enhance oxygen transport and metabolic activity during flooding. Metabolite profiling revealed both overlapping and distinct patterns of sugar and amino acid accumulation between genotypes. Gene expression analysis revealed that flooding-responsive genes, including ALCOHOL DEHYDROGENASE 1 and HYPOXIA RESPONSIVE ERF 2, were activated in both genotypes, with a more pronounced response noted in '52-225'. These findings extend our understanding of flooding tolerance mechanisms in Populus by connecting physiological traits, stress responses and genetic regulation. This research contributes to the development of more flooding-resilient poplar varieties, with potential applications in breeding and restoration programs for flooding-prone environments.