Diurnal acid metabolism in Isoetes howellii from a temporary pool and a permanent lake.

Water chemistry and titratable acidity and malic acid levels in Isoetes howellii leaves were sampled every 6 hr from plants in a seasonal pool and an oligotrophic lake. Plants in the seasonal pool showed a diurnal fluctuation of approx 300 mu equivalents titratable acidity g-1 fresh wt; daytime deacidification was 75% complete by noon and nighttime acidification was 45% complete by midnight. Late in the season after the pool had dried, emergent leaves showed only a very weak tendency to accumulate acid at night. Plants from the oligotrophic lakes had a diurnal change of approx 100 mu eq g-1 fresh wt, daytime deacidification was only 45% complete by noon but nighttime acidification was 80% complete by midnight. In the seasonal pool there were marked diurnal changes in temperature, pH, oxygen and CO2. Free-CO2 levels were an order of magnitude greater in early morning than in late afternoon. Conditions in the oligotrophic lake showed no marked diurnal fluctuation, though total inorganic carbon levels were extremely low relative to other aquatic systems.-from AuthorsBiology Dept., Occidental College, Los Angeles, CA 90041, USA.