Photosynthetic pathway diversity in a seasonal pool community

Abstract

1. Photosynthetic pathway diversity was evaluated for the dominant species in a seasonally aquatic community in the south-western USA using 14 C 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 CO 2 concentrating mechanisms lacked the capacity for bicarbonate uptake, an alternative CO 2 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 CO 2 and/or C 4 phototrophism under field conditions of limited CO 2 and O 2 saturation, and this is lost in the terrestrial foliage.