International Society for Plant Anaerobiosis

International Society for Plant Anaerobiosis - flooded Rumex crispus (c) Ole Pedersen

   

CONFERENCES - International Society for Plant Anaerobiosis (ISPA)

 

Underwater CAM photosynthesis: benefits for carbon gain in some aquatic plants

 

Pedersen O1 & Colmer TD2

 

1) Freshwater Biological Laboratory, University of Copenhagen, Denmark; 2) School of Plant Biology, The University of Western Australia, Australia

 

CAM photosynthesis in terrestrial plant is thought to have evolved as a trait to conserve water and is thus particularly frequent among desert plants. CAM is also present in some aquatic plants inhabiting freshwater lakes and temporary pools. Here, CAM is unimportant as a water-conserving feature and instead it is considered of adaptive importance in carbon limited environments where CO2 is stored as malate during the night and subsequently released during the day and fixed in the normal Calvin cycle. As a consequence of extensive underwater photosynthesis, CO2 drops to very low concentration during the afternoon in vegetation-rich temporary pools. In contrast, CO2 builds up to several times air-equilibrium during the night when respiration processes dominates. Under these environmental conditions, plants with CAM photosynthesis may maintain positive underwater net photosynthesis also during times when the external CO2 concentration drops to sub air-equilibrium values. However, CAM photosynthesis may also restrict photorespiration which is often high in aquatic plants as a consequence of the slow gas diffusion in water compared to in air; CO2 is only slowly replenished at the site of Rubisco. In addition, O2 tends to build up in the tissue because of the lower solubility of O2 compared to CO2. CAM activity in the present study was indicated by 9.7-fold higher leaf malate at dawn, compared with at dusk, and CAM activity was confirmed also as changes in titratable acidity (μmol H+ equivalents) of leaves. Leaves high in malate not only showed higher underwater net photosynthesis at low external CO2 but also less apparent photorespiration. Suppression by CAM of apparent photorespiration was evident at a range of O2 concentrations, including values below air equilibrium. At high O2 of 2.2-fold atmospheric equilibrium, net photosynthesis was reduced substantially and although it remained positive in leaves containing high malate, became negative in those low in malate. In situ measurements of internal leaf O2 concentrations in IsoŽtes australis, an Australian aquatic CAM plant inhabiting granite rock pools, showed that late afternoon O2 increased to 32 kPa in the leaf lacunae while dropping to below 1 kPa during the night. It is thus suggested that CAM in aquatic plants enables higher rates of underwater net photosynthesis over large O2 and CO2 concentration ranges in floodwaters, via increased CO2 fixation and suppression of photorespiration.