What are the consequences of water stress on the development of late postzygotic reproductive isolation in an orchid hybrid zone?

Abstract Studies on reproductive isolation in angiosperms often focus on early postzygotic barriers, whereas research on established hybrid generations and their ecological interactions remains limited. This study aimed to compare the morphophysiological performance of Epidendrum × purpureum and its parental species, Epidendrum orchidiflorum and Epidendrum denticulatum , under different water‐deficit conditions. To investigate the effects of water deficit on the parental species and the hybrids, we conducted an experiment comprising one control and two water‐deficit treatments of different severity over time. We measured several ecophysiological and anatomical traits, including leaf water potential, leaf succulence, titratable acidity, and membrane resistance. We also compared growth and survival between parental species and hybrids to understand their performance under contrasting water conditions. The results showed hybrid plants had lower survival rates than parental species, with all plants dying by the end of the experiment. Membrane resistance was also lower in the hybrid plants, which led to increased electrolyte leakage and decreased the water storage potential. Cuticle width was significantly different between parental species and their hybrids. Significant changes in titratable acidity were observed, indicating alterations in the crassulacean acid metabolism. The greater vulnerability of the hybrids to water stress can be interpreted as a strong late postzygotic barrier that limits its distribution and survival. Such barriers may increase in climate change scenarios predicting the aridification in some regions, reducing the potential for heterospecific gene exchange between co‐occurring species and influencing the occurrence and persistence of natural hybrid zones.