Crassulacean acid metabolism in the seasonally submerged aquatic Isoetes howellii

Evidence to date is consistent with the hypothesis that the submerged aquatic Isoetes howellii Engelmann possesses crassulacean acid metabolism. Quantitative ¹⁴C uptake studies indicate that CO₂ assimilation in both the light and dark are functions of pH and total inorganic carbon level. In both the light and dark, maximum uptake rates in 0.6 mM NaHCO₃ were double the rates in 0.3 mM NaHCO₃. At both carbon levels there was a large drop in carbon assimilation rate between pH 6 and 8. In nature water pH and inorganic carbon level fluctuated diurnally thus complicating the determination of the contribution of light vs dark CO₂ uptake to the total carbon gain. On a sunny day between 0600 and 1200 h water chemistry changed markedly with ∼40% reduction in total carbon, ∼2 pH unit rise resulting from ∼100% depletion of free CO₂. Under such conditions daytime deacidification in Isoetes leaves was 88% complete by noon. In contrast, on an overcast day, reduction of carbon in the water was much slower, deacidification was only 46% complete by noon and substantial malic acid levels remained in the leaves at the end of the day. Upon emergence crassulacean acid metabolism was largely lost in Isoetes leaves. Preliminary estimates suggest that under natural submerged conditions, early morning photosynthetic rates may be substantially higher than dark CO₂ uptake rates, though uptake rates throughout much of the day could be substantially lower than nightime CO₂ assimilation.