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 peri od in darkness lettuce leaves accumulated significantly lower amounts of protochlorophyllide (PChlide) per unit leaf area, however, when cal culated on a protein basis, considerably higher amounts of PChlide com pared to wheat. Additionally small amounts of Magnesium-protoporphyrin -IX (monomethylester) were found. Lettuce leaf segments were character ized, furtheron, by a higher ALA uptake and a higher endogenous pool o f non-metabolized ALA in comparison to wheat. PChlide and non-metaboli zed ALA that accumulated in darkness were strongly reduced after 1 h o f illumination and disappeared nearly completely after a 5-h period of illumination with 120 Wm-2. Light exposure to ALA-pretreated leaves r evealed typical symptoms for the involvement of activated oxygen speci es in photodynamic action, to a greater extent in lettuce than in whea t. Photobleaching of pigments was accompanied by degradation of protei ns and peroxidation of thylakoid lipids detected by the increase in th e formation of thiobarbituric acid-reactive substances. The fluidity o f the thylakoid membranes isolated from treated leaves decreased (as m easured with 5-doxylstearic acid as spin label).