INCREASED CONTENT OF ENDOGENOUS CYTOKININS DOES RIOT DELAY DEGRADATION OF PHOTOSYNTHETIC APPARATUS IN TOBACCO

The effect of stress (long-term darkening) on the structure and functi oning of the photosynthetic apparatus was studied in leaves of non-tra nsformed as well as two types of ipt-transformed tobacco (Nicotiana ta bacum cv. Petit Havana SR1) plants, The ipt-gene controlling the biosy nthesis of cytokinins (CKs) was coupled to the light-inducible Pssu-pr omoter of Pisum sativum of to the heat-inducible hsp-promoter of Droso phila melanogaster. Pssu-ipt transgenic grafts with high contents of e ndogenous CKs retained their chlorophyll (Chi) content during a 15 d d ark treatment while the SR1- and heat-treated Phsp 70-ipt seedlings, w hich did not differ significantly in CKs content, lost up to 60 % of t heir Chi. The normalised variable fluorescence ratio (F-v/F-m) and oxy gen evolution decreased dramatically in the course of continuous dark treatment, indicating a degradation of photosystem. 2 irrespective of the plant type. Changes in the polypeptide composition of thylakoid me mbranes, as analysed by SDS-PAGE, confirmed this degradation process. Light and electron microscopic observations of leaf sections, and of t he ultrastructure of plastids showed changes corresponding to a degrad ation of the photosynthetic apparatus.