PHOTOINDUCED DAMAGE IN LEAF SEGMENTS OF WHEAT (TRITICUM-AESTIVUM L) AND LETTUCE (LACTUCA-SATIVA L) TREATED WITH 5-AMINOLEVULINIC ACID .2. CHARACTERIZATION OF PHOTODYNAMIC DAMAGE BY MEANS OF DELAYED CHLOROPHYLLFLUORESCENCE AND P700 PHOTOOXIDATION

In a preceding paper (Hartel et al., this issue, pp. 230-236), light-i nduced damage of structural components of the photosynthetic apparatus has been demonstrated after pretreatment of green leaf segments of wh eat and lettuce with 5-aminolevulinic acid (ALA) in darkness. In the p resent study, the time course of this process has been investigated by means of chlorophyll fluorescence and measurement of the light-induce d electron spin resonance signal I of P700+. After exposure of leaves pretreated with ALA in darkness to continuous light, a decrease in del ayed chlorophyll fluorescence intensity coincided with an increase in P700+ formation. This was observed in both plant species during the fi rst two hours, indicating disturbance of the electron transfer between the two photosystems. Later a complete inhibition of photosystem-II-m ediated electron transfer was found in lettuce. P700+ measurements 're flected a severe structural disintegration of thylakoid membranes whic h increased with duration of light exposure and light intensity, and w hich appeared to be accelerated when photosystem II activity was lost. In contrast, photodynamic damage in wheat leaf segments did not incre ase after 2h of illumination. The electron transfer inhibition was inc omplete, and P700+ measurements revealed no indication of strong struc tural injury.