Diurnal Acid Metabolism in Isoetes Howellii From a Temporary Pool and a Permanent Lake

Abstract

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 ~ 300 μequivalents 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 lake had a diurnal change of ~100 μeq g−1 fresh wt, daytime deacidification was only 45% complete by noon but nighttime acidification was 80% complete by midnight. Water characteristics were distinctly different between these two systems. In the seasonal pool there were marked diurnal changes in temperature, pH, oxygen and carbon dioxide. Free‐CO2 levels were an order of magnitude greater in the early morning than in the late afternoon. In contrast, the conditions in the oligotrophic lake showed no marked diurnal fluctuation, though total inorganic carbon levels were extremely low relative to other aquatic systems.

CAM-cycling in the cycad Dioon edule Lindl. in its natural tropical deciduous forest habitat in central Veracruz, Mexico

“…CAM has also been observed in aquatic vascular plants that grow in environments where there is high carbon dioxide and/or bicarbonate competition during daylight hours, especially in moderately fertile shallow temporary pools that experience extreme diel fluctuations in inorganic carbon availability. Isoëtes is an example of an aquatic CAM plant reported by Keeley (1981Keeley ( , 1983 and Keeley et al (1983). Although at first this met with scepticism and was considered an oxymoron by some, it has since been shown that some aquatic species do indeed possess CAM photosynthesis (Keeley, 1998).…”

Diurnal Acid Metabolism in the Submerged Aquatic Plant, Isoetes tuberculata

“…An absence of pH changes may reflect a well-buffered cytosol, rather than the absence of CAM. Titration (Keeley, 1983) is a more accurate method for elucidating CAM. Further investigations in this regard are being undertaken in this laboratory.…”

“…plant, Isoetes howellii Engelm ime the diurnal acid metabolism kn 1985, 1987Keeley and Bowes, 1982;Keeley et al, 1983). In India, the genus Isoetes is represented by several species (Gena and Bhardwaja, 1984;Panigrahi, 1981;Pant and Srivastava, 1962;Sharma et al, 1985), and they occur as terrestrial, amphibious, or submerged aquatic plants.…”

In situ O2 dynamics in submerged Isoetes australis: varied leaf gas permeability influences underwater photosynthesis and internal O2

“…Most, if not all, Isoetes species tested so far exhibit indicators of CAM photosynthesis such as dark fixation of CO 2 , overnight accumulation of malate followed by daytime decarboxylation, and large diurnal amplitudes in tissue acidity ( Keeley, 1998 b ). However, CAM is often lost in specimens that develop functional stomata upon air exposure ( Keeley et al , 1983 ). The CAM photosynthetic mechanism is of great advantage in the shallow pools as CO 2 can be fixed during the night-time when it is readily available and subsequently released internally and used in daytime photosynthesis at a time when CO 2 becomes depleted in the water ( Keeley, 1998 a , b , c ).…”