Physiological responses to flooding stress in the federally threatened wetland plant Helonias bullata L (swamp pink)

Helonias bullata is a habitat specializing wetland monocot which has been identified as a candidate for population restoration via reintroduction to extirpated sites. Its habitat has been described as forested wetlands with a stable water table at or near the surface of the soil. This experiment was conducted to investigate physiological factors involved in H. bullata’s adaptation to wetland habitats. Plants were exposed to three soil water regimes: control (moist), waterlogged, and submerged. The production rate of ethylene increased as a result of waterlogged and submerged treatments in both a short (72 hours of stress exposure) and longer term experiment (11 days of stress exposure). There were no significant differences between ethylene production in waterlogged and submerged treatments. Confocal microscopy allowed visualization of aerenchyma in root cross sections of all treatments. The amount of aerenchyma in control plant roots was observably less than in waterlogged and submerged plants; however there were no noticeable differences in the amount of aerenchyma formation between the waterlogged and submerged plants. Aerenchyma was not observed in cross sections of rhizomes or stem tissues. Our results indicated that aerenchyma formation in roots of H. bullata may confer waterlogging or submerging tolerance via increased oxygen supply through roots rather than rhizomes and stems. After 12 days of flooding stress plants exhibited visual leaf senescence and after 20 days plants showed stunted growth of roots and shoots. Our findings suggest that although H. bullata may survive in wet soils by increasing ethylene production and root aerenchyma formation, growth is reduced when plants are waterlogged or submerged. Restoration efforts should therefore focus on sites with a stable water table at or below the bottom of the root zone and not at or near the surface of the soil.