RAPID INHIBITION OF HCO-3 USE BY HIGH-CONCENTRATION OF FREE CO2 IN ELODEA-CANADENSIS

The effect of exposure at a high concentration of free CO2 (denoted henceforth as CO2) in simple bicarbonate media on alkalization (increase of pH) of the media was investigated in the submerged macrophyte, Elodea canadensis Michx., in short-term experiments. Experimental plants were exposed at a high CO2 concentration of 0.45-1.35 mM in bicarbonate media of total alkalinity (TA) 0.3-9.8 mmol l-1 directly at the beginning of pH-drift experiments. In other cases, the plants were pretreated with 1 MM CO2 for a given period ( 1-60 min) and exposed afterwards. Plants having been exposed in the media with 1 mM CO2 for 30-60 min alkalized these media to a lesser extent thereafter, i.e. their photosynthetic HCO3- affinity was strongly reduced. The HCO3- compensation point of photosynthesis (carbon-based rates) was about 0.22 mM at pH 10.47 in the untreated controls, but about 0.69 mM at pH 9.60 in CO2-pretreated plants. The pretreated plants showed a substantial reduction in HCO3- affinity lasting up to 12 h, although a very slow recovery of HCO3- affinity took place during this time. In Ceratophyllum demersum L., however, no marked reduction in HCO3- affinity was found after the CO2 pretreatment. In all measurements, the initial TA of media did not stay constant and dropped by 0.02-0.20 mmol l-1. This finding shows that the employment of the pH-drift method is problematic.