PHOTOINDUCED DAMAGE IN LEAF SEGMENTS OF WHEAT (TRITICUM-AESTIVUM L) AND LETTUCE (LACTUCA-SATIVA L) TREATED WITH 5-AMINOLEVULINIC ACID .1. EFFECT ON STRUCTURAL COMPONENTS OF THE PHOTOSYNTHETIC APPARATUS

Two different plant species, wheat (Triticum aestivum L.) and lettuce (Lactuca sativa L.) were investigated for the sensitivity of thylakoid membrane structures to photoinduced damage caused by treatment with 5 mM 5-aminolevulinic acid (ALA) in the dark. At the end of a 16-h period in darkness lettuce leaves accumulated significantly lower amounts of protochlorophyllide (PChlide) per unit leaf area, however, when calculated on a protein basis, considerably higher amounts of PChlide compared to wheat. Additionally small amounts of Magnesium-protoporphyrin-IX (monomethylester) were found. Lettuce leaf segments were characterized, furtheron, by a higher ALA uptake and a higher endogenous pool of non-metabolized ALA in comparison to wheat. PChlide and non-metabolized ALA that accumulated in darkness were strongly reduced after 1 h of illumination and disappeared nearly completely after a 5-h period of illumination with 120 Wm-2. Light exposure to ALA-pretreated leaves revealed typical symptoms for the involvement of activated oxygen species in photodynamic action, to a greater extent in lettuce than in wheat. Photobleaching of pigments was accompanied by degradation of proteins and peroxidation of thylakoid lipids detected by the increase in the formation of thiobarbituric acid-reactive substances. The fluidity of the thylakoid membranes isolated from treated leaves decreased (as measured with 5-doxylstearic acid as spin label).