Aerenchyma development and elevated alcohol dehydrogenase activity as alternative responses to hypoxic soils in the Piriqueta caroliniana complex

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Abstract
The ability of plants to make morphological or physiological adjustments in response to environmental cues allows them to survive and reproduce under a wide range of conditions. One stress that plants are often exposed to is soil oxygen depletion due to flooding. Plants can respond to hypoxic soils by producing oxygen‐conducting aerenchymous tissue or through induction of enzymes in the ethanolic fermentation pathway. Here we use greenhouse experiments to examine flood responses in plants of the Piriqueta caroliniana (Turneraceae) complex, which occupy a range of moisture regimes. Morphotypes and hybrids in this complex exhibited contrasting responses to hypoxic conditions. Genotypes from flooded habitats developed aerenchyma and did not substantially elevate levels of alcohol dehydrogenase (ADH) activity, an enzyme associated with anaerobic respiration. Plants from drier sites, on the other hand, did not develop aerenchyma but had much higher levels of ADH activity. Plants with aerenchymous tissue had substantially higher rates of growth under sustained flooding. Results are consistent with the hypothesis that aerenchyma development is an effective strategy in habitats subject to persistent flooding, while elevating activity of enzymes for ethanolic fermentation is effective only under ephemeral flooding. The range of phenotypic responses observed illustrates contrasting adaptive strategies that can lead to habitat isolation and evolutionary divergence.