Increased content of endogenous cytokinins does riot delay degradation of photosynthetic apparatus in tobacco

Abstract

The effect of stress (long-term darkening) on the structure and functioning of the photosynthetic apparatus was studied in leaves of non-transformed as well as two types of ipt-transformed tobacco (Nicotiana tabacum cv. Petit Havana SR1) plants, The ipt-gene controlling the biosynthesis of cytokinins (CKs) was coupled to the light-inducible Pssu-promoter of Pisum sativum of to the heat-inducible hsp-promoter of Drosophila melanogaster. Pssu-ipt transgenic grafts with high contents of endogenous CKs retained their chlorophyll (Chi) content during a 15 d dark treatment while the SR1- and heat-treated Phsp 70-ipt seedlings, which did not differ significantly in CKs content, lost up to 60 % of their Chi. The normalised variable fluorescence ratio (F-v/F-m) and oxygen 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 membranes, as analysed by SDS-PAGE, confirmed this degradation process. Light and electron microscopic observations of leaf sections, and of the ultrastructure of plastids showed changes corresponding to a degradation of the photosynthetic apparatus.