Diurnal courses of net photosynthesis and photosystem II quantum efficiency of submerged Lagarosiphon major under natural light conditions

An optode device for net-photosynthesis measurements, based on oxygen-depending quenching of fluorescence from O(2)-specific sensors, and PAM fluorometry have been used to study diurnal courses of net-photosynthesis and the F(v)/F(m) ratio of the submerged plant Lagarosiphon major. Plants were pre-cultivated and studied in large mesocosm flow-through outdoor tanks under 50% and 80% shade cloth, respectively. Growth under the different shade cloths resulted in similar light compensation points (similar to 20 mu mol photons m(-2) s(-1)), but strongly different light saturation levels, with about 150 mu mol m(-2) s(-1) for plants grown under 80% shade cloth and about 350 mu mol m(-2) s(-1) for plants grown under 50% shade cloth. Plants under both growth conditions showed a transient reduction of the maximum F(v)/F(m) value in the afternoon (down to 70% of the morning control values under 80% shade cloth and down to 85% under 50% shade cloth), which was not accompanied by a reduction of the net photosynthetic rate. This indicated that the fluorescence parameter must not be a reliable indicator of the rate of photosynthesis under all conditions. The new photo-optical device became evidenced as a valuable tool not only for laboratory experiments, but also for field studies of gas exchange of submerged plants. (C) 2011 Elsevier GmbH. All rights reserved.