PTOX-DEPENDENT ELECTRON FLOW IS INVOLVED, IN PHOTOSYNTHETIC ELECTRON TRANSPORT PROTECTION AGAINST HEAVY METAL TOXICITY IN SUNFLOWER LEAVES

Changes in photosynthetic electron transport were studied in sensitive to heavy metals sunflower plants grown in contaminated soils collected in the region of Factory for Non-ferrous Metals near Plovdiv, Bulgaria. Elevated contents of lead (Pb), cadmium (Cd) cooper (Cu) and zinc (Zn) were found in the contaminated soils. The most contaminated soil was collected in the closest green belt (highly contaminated soil, HCS) while soils at a greater distance from the factory ("The Fountain") were less contaminated (moderately contaminated soil, MCS). Accelerated rate of the slower phase of variable fluorescence relaxation after light excitation with is saturate pulse was found in leaves of sunflower plants grown in HCS. Retarded rate of actinic light-induced P700 oxidation was established in leaves of sunflower plants grown in both HCS and MCS determined by P700 absorbance. Heavy metal-induced changes were paralleled with higher potential for oxidation of PQ pool by photosynthetic terminal oxidase (PTOX) and/or retarded oxidation in PS1 acceptor side. Such responses can be considered as protective mechanisms against heavy metal stress helping to avoid production of toxic oxygen byproducts in the PS1 acceptor side.