Abstract:

Aims Isoetes sinensis Palmer, an endangered aquatic plant in China, possesses a CAM (Crassulacean acid metabolism) photosynthetic pathway in its submerged leaves. Our objectives were to investigate diurnal CO₂ exchange characteristics of submerged leaves and to examine if I. sinensis was capable of net CO₂ uptake in the dark, which was considered evidence of the CAM pathway in aquatic plants by Keeley (1998), and determine if diurnal CO₂ exchange rates were influenced by alkalinity. Results may contribute to conservation of this endangered plant by determining if high alkalinity of water sources limited population sizes.
Methods We used the pH-drift technique to analyze C_T (total carbon) and CO₂ uptake at two alkalinities in light and dark conditions. Photosynthetic C_T/CO₂ response curves were measured at 25 °C and 210 μmol·m ^-2·s^-1 and in a solution designed by Smart and Barko (1985) in which concentrations of NaHCO₃and KHCO₃ were varied to supply different alkalinities. Leaves were placed in a small glass chamber, each containing a known volume of the solution which was stirred by a magnetic stirrer at the bottom. The pH/unit time was recorded by a pH electrode that had been calibrated at the beginning and the end of each experiment, the alkalinities of the solution was measured by Gran titration (Talling, 1973) to determine if alkalinity was constant during each experiment. Plant material was weighed after drying overnight at 80 °C. We calculated the concentration of free CO ₂ and total carbon based on alkalinity, pH, conductivity and temperature according to Mackereth et al. (1978). With the calculated concentration of C_T (or CO₂), unit time, chamber volume and amount of plant material present, a photosynthesis or net CO₂ exchange rate was obtained. For each alkalinity, the CO₂ exchange rate was then plotted against the mean concentration of CO₂. A covariance approach was used to check if the CO₂ exchange significantly differed at two alkalinities.
Important findings Submerged leaves were capable of net CO₂ uptake in both light and dark conditions. The CO₂ uptake rate was a function of the concentration of CO₂ in the medium. The CO₂ uptake in dark was significantly influenced by alkalinity of the medium. Alkalinity does not seem to be a significant factor in reducing the distribution and success of this aquatic species.

Key words: Isoetes sinensisPalmer, photosynthesis, crassulacean acid metabolism, CO₂ exchange, alkalinity