Low atmospheric CO2 induces nocturnal carbon accumulation in the lycophyte genus Isoetes

Background and AimsCrassulacean Acid Metabolism (CAM) is an adaptation to increase water use efficiency in dry environments. Oddly, similar biochemical patterns occur in the submerged aquatic lycophyte genus Isoetes. It has long been assumed that CAM-like nocturnal carbon accumulation in aquatic plants is an adaptation to low daytime carbon levels in aquatic ecosystems, but this has never been directly tested. MethodsTo test this hypothesis, populations of terrestrial Isoetes engelmannii and I. tuckermanii were grown in climate-controlled chambers and starved of atmospheric CO2 during the day while pH was measured for 24-hours. Key resultsWe demonstrate that terrestrial plants exposed to low atmospheric CO2 display diel acidity cycles similar to those in both xerophytic CAM plants and submerged Isoetes. ConclusionsCO2 starvation induces CAM-like nocturnal carbon accumulation in terrestrial Isoetes, substantiating the hypothesis that carbon starvation is a selective pressure for nocturnal carbon accumulation in Isoetes. Furthermore, while aquatic carbon levels undoubtedly promote nocturnal carbon accumulation in extant Isoetes, the induction of this behavior in terrestrial plants suggests a possible earlier terrestrial evolution of this metabolism in Isoetalean ancestors in response to low atmospheric CO2 levels. We both provide support for a long-standing assumption about nocturnal carbon accumulation Isoetes as well as suggest an earlier evolution of this behavior.