Photosynthetic pathway diversity in a seasonal pool community

1. Photosynthetic pathway diversity was evaluated for the dominant species in a seasonally aquatic community in the south-western USA using C-14 pulse-chase techniques. 2. Under submerged conditions, only about half of the species were clearly C-3 three of the 15 dominants were CAM, one species was C-4 and three were potentially assimilating carbon with both C-3 and C-4 fixation. 3. During the brief terrestrial stage in the Life history of these: amphibious plants, both the CAM and the C-3 + C-4 species switched to C-3 whereas the C-4 species did not switch. 4, Numerous variations were apparent; for example, the C-4 species, while exhibiting a biochemical pathway indistinguishable from terrestrial C-4 plants, lacked Kranz anatomy in the aquatic foliage. Also, despite well-developed CAM in several species, others exhibited low-level diel changes in acidity, apparently not indicative of CAM. 5. Species with C-4 or CAM CO2 concentrating mechanisms lacked the capacity for bicarbonate uptake, an alternative CO2 concentrating mechanism found in certain C-3 species in this community. 6. Rubisco/PEPC in aquatic foliage was higher in C-3 species than in C-4 CAM or putative C-3 + C-4 species. In the terrestrial phase, as expected, the switch from CAM or C-3 + C-4 to strictly C-3 assimilation was associated with a substantial increase in Rubisco/PEPC. Quite unexpected, however, was the substantial increase in this ratio in terrestrial C-3 foliage. It is hypothesized that submerged C-3 plants utilize PEPC for recycling of respiratory CO2 and/or C-2 phototrophism under field conditions of Limited CO2 and O-2 saturation, and this is lost in the terrestrial foliage.