Responses of Ottelia alismoides, an aquatic plant with three CCMs, to variable CO(2) and light

Ottelia alismoides is a constitutive C(4) plant and bicarbonate user, and has facultative crassulacean acid metabolism (CAM) at low CO(2). Acclimation to a factorial combination of light and CO(2) showed that the ratio of phosphoenolpyruvate carboxylase (PEPC) to ribulose-bisphosphate carboxylase/oxygenase (Rubisco) (>5) is in the range of that of C(4) plants. This and short-term response experiments showed that the activity of PEPC and pyruvate phosphate dikinase (PPDK) was high even at the end of the night, consistent with night-time acid accumulation and daytime carbon fixation. The diel acidity change was maximal at high light and low CO(2) at 17–25 µequiv g(−1) FW. Decarboxylation proceeded at ~2–3 µequiv g(−1) FW h(−1), starting at the beginning of the photoperiod, but did not occur at high CO(2); the rate was greater at high, compared with low light. There was an inverse relationship between starch formation and acidity loss. Acidity changes account for up to 21% of starch production and stimulate early morning photosynthesis, but night-time accumulation of acid traps <6% of respiratory carbon release. Ottelia alismoides is the only known species to operate CAM and C(4) in the same tissue, and one of only two known aquatic species to operate CAM and bicarbonate use.