PS 43-176 - Light, CO2 and temperature responses and long-term capacity of underwater photosynthesis of Taxodium distichum (L.) Rich.

Abstract: Background

Global climate change has increased the frequency and intensity of floods, and more terrestrial plants would be in the risk of submergence. Actually, scope floodwaters are variable in dissolved O₂, CO₂ and light which could be used by aquatic plants for underwater photosynthesis (Pn) to growth. However, rare relevant references were found on terrestrial plants, especially Taxodium distichum which had been observed to survive after submerged in a water-fluctuation zone.

Aim and method

To characterize underwater Pn performance of T. distichum, the underwater Pn at various light intensities, CO₂ concentrations and temperatures were determined with a rotating wheel incubator using microelectrodes. Then, a 30-d complete submergence experiment was conducted to investigate the long-term capacity of underwater P<sub>N. Meanwhile, leaf gas films that formed on the superhydrophobic leaves were left intact, or experimentally removed with Triton X, to elucidate their effect on underwater P<sub>N.

Results

Underwater P<sub>N of T. distichum increased rapidly with the increasing light intensities and CO₂ concentrations until they were at 1000 µmol photons m^-2 s^-1 PAR and 500 mmol m^-3 CO₂ concentration, respectively. And then, the underwater P<sub>N reached a stable value. Both standard Gaussian function and standard exponential function indicated the appropriate temperature for underwater P<sub>N was 24-25℃. T. distichum leaves have a hydrophobic surface and thus form complete leaf gas films which could significantly increase the underwater P<sub>N when compared with those without gas films. At early submerged stage, leaf gas films made stomata open, which could promote the entry of O₂ and alleviate the restriction of submergence on underwater P<sub>N. After one week, the underwater P<sub>N decreased below 2 µmol O₂ m^-2 s^-1 and remained stable until the end of the 30-day submergence experiment. Moreover, partly degraded leaves palisade tissue and spongy tissue and the decreased chlorophyll contents were observed at the end.

Conclusions

The survival strategy of underwater Pn could be employed by T. distichum during complete submergence. Leaf characteristics and the external environments commonly affect the underwater Pn. The optimal conditions for T. distichum were 1000 µmol photons m^-2 s^-1 PAR, 500 mmol m^-3 CO₂ and 24-25℃. The presence of leaf gas films was conducive to the diffusion of O₂ to leaves, and thus enhancing underwater Pn. In summary, underwater Pn could provide carbohydrates and O₂ to T. distichum, extending its underwater survival time.