Submergence, seed germination, and seedling development of the Amazonian floodplain tree Pseudobombax munguba: evidence for root oxytropism

Abstract

Key message: Primary root of seeds germinating while submerged grew upwards towards oxygen-rich surface layers. Height of water column influenced germination and root growth. Seedlings removed from water attached to substrate and grow vertically. Abstract: Oxygen and light are potentially limiting resources in floodplain forests where plants are subjected to long periods of flooding, particularly in early stages of the life cycle. We experimentally evaluated the effect of flooding and availability of oxygen and light on germination and initial growth of Pseudobombax munguba (Malvaceae), a tree characteristic of the lower portions of the flood-level gradient in Central Amazonian floodplains. Neither flooding nor darkness affected germination (≥93%); however, only seeds that germinated in light developed into seedlings. Germinated seeds floating in water showed positive gravitropic curvature of the primary root and presence of starch-dense amyloplasts (statoliths) in the root cap. Seed germination decreased under 5–7 cm of non-aerated water and the primary root curved upward, extending towards the water surface where oxygen concentration would be higher. Statoliths were not present in the cap cells of these upwardly growing roots suggesting an absence of gravity-directed growth and the involvement, instead, of re-orientation along an oxygen gradient. Although about 50% of seeds germinated under 10 cm of non-aerated water, their primary root did not elongate further after emergence. Seedlings removed from water and positioned horizontally on the surface of a moist, well-aerated substrate attached themselves as roots curved downward, penetrated into the substrate and anchored the plant. The stem bent upright and resumed vertical growth. These features contribute to reducing the time required for establishment in this type of environment where successful colonization is constrained by the short terrestrial phase.