Photosynthesis, photorespiration, and productivity in Lemna minor L.1

Abstract

The roles of aqueous vs. atmospheric CO2 fixation, photorespiration, dark respiration, and organic release in the primary productivity of Lemna minor L. were investigated experimentally in field populations and in the laboratory. The mean rate of net photosynthesis through the growth season was 2.48 mg C g−1 h−1 during which an average of 86% of carbon fixed was from aqueous inorganic carbon. Seasonal variations in photosynthesis correlated principally with light intensity and temperature. Presence of photorespiration was suggested by early afternoon depressions of photosynthesis, by experimental enhancement and inhibition of photosynthesis with low and high oxygen, respectively, and by enhancement of light:dark respiration ratios with high oxygen. However, under natural conditions L:D ratios were below unity most of the time, and CO2 loss in the light was never >4% of photosynthate fixed per hour. Experimental enhancement of photorespiration with oxygen was never as great as the inhibition of photosynthesis at comparable oxygen concentrations. Although plants exhibited a low CO2 compensation point and high light and temperature optima, photosynthesis enzymology and fixation products indicated that C4 photosynthesis was not a significant factor in maintenance of the low photorespiration rates.