FIELD STUDIES OF PHOTOINHIBITION SHOW NON-CORRELATIONS BETWEEN OXYGENAND 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 expe riments. Studies were carried out with the new portable pulsed modulat ion 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 pr oduction rate caused by daylight or by a constant fluence rate of a re d control light was investigated in the course of the day. Fluorescenc e 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 recov ery of photosynthesis required a longer period than in some tropic and antarctic algae investigated recently. Midnight sun did not induce ph otoinhibition of photosynthesis. Photosynthetic capacity was determine d by means of oxygen production caused by a saturating photon fluence rate. It showed no significant change during the course of the day. Th us, in contrast to the fluorescence measurements, an acclimation of th e photosynthetic capacity to the course of daylight was not observed. These differences between the results obtained with oxygen and fluores cence measurements were re-investigated under controlled laboratory co nditions. The saturated oxygen production rate decreased only after th e Fv/Fm level had decreased to 60% of the non-inhibited control and th e Delta F/Fm' level to 35% of the control, respectively. A comparison between the action spectra of photoinhibition measured by means of flu orescence and oxygen evolution also showed differences. The fluorescen ce 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. Th e different results obtained with oxygen and fluorescence measurements are discussed.