FIELD STUDIES OF PHOTOINHIBITION SHOW NON-CORRELATIONS BETWEEN OXYGEN AND FLUORESCENCE MEASUREMENTS IN THE ARCTIC RED-ALGA PALMARIA-PALMATA

Photoinhibition of photosynthesis of the red alga Palmaria palmata was studied on the arctic coast in northern Norway and in laboratory experiments. Studies were carried out with the new portable pulsed modulation fluorometer (PAM 2000, Walt, Effeltrich, Germany) and a home-made oxygen measuring device. Fv/Fm as measure of photoinhibition and Delta F/Fm' as measure of the photosynthetic yield were used. The oxygen production rate caused by daylight or by a constant fluence rate of a red control light was investigated in the course of the day. Fluorescence measurements showed the typical course of photoinhibition during the day. Fv/Fm reached its maximum between noon and the early afternoon, and photosynthesis recovered from afternoon until midnight. Full recovery of photosynthesis required a longer period than in some tropic and antarctic algae investigated recently. Midnight sun did not induce photoinhibition of photosynthesis. Photosynthetic capacity was determined by means of oxygen production caused by a saturating photon fluence rate. It showed no significant change during the course of the day. Thus, in contrast to the fluorescence measurements, an acclimation of the photosynthetic capacity to the course of daylight was not observed. These differences between the results obtained with oxygen and fluorescence measurements were re-investigated under controlled laboratory conditions. The saturated oxygen production rate decreased only after the Fv/Fm level had decreased to 60% of the non-inhibited control and the Delta F/Fm' level to 35% of the control, respectively. A comparison between the action spectra of photoinhibition measured by means of fluorescence and oxygen evolution also showed differences. The fluorescence spectrum indicates that all photosynthetic pigments are involved in photoinhibition. The spectrum measured by oxygen evolution shows that wavelengths absorbed by phycocyanin do not induce photoinhibition. The different results obtained with oxygen and fluorescence measurements are discussed.