PREFERENTIAL PHOTOINACTIVATION OF CATALASE AND PHOTOINHIBITION OF PHOTOSYSTEM-II ARE COMMON EARLY SYMPTOMS UNDER VARIOUS OSMOTIC AND CHEMICAL STRESS CONDITIONS

Activity of catalase (EC 1.11.1.6) and variable fluorescence (F(v)) we re measured in sections of rye leaves (Secale cereale L. cv. Halo) tha t were exposed for 24 h to moderately high irradiance under osmotic or chemical stress conditions (paraquat, DCMU, mannitol, NaCl, CdCl2, Cu SO4, Pb(NO3)2, KNO2, or K2SO3. Changes of the chlorophyll content and of enzyme activities related to peroxide metabolism, such as glycolate oxidase, glutathione reductase, and peroxidase, were assayed for comp arison. In the presence of the herbicides paraquat and low DCMU concen trations that exert only partial inhibition of photosynthesis, as well as after most treatments with osmotic or chemical stress factors, cat alase markedly declined due to a preferential photoinactivation. At hi gher DCMU levels catalase did not decline. At low KNO2 concentrations catalase activity was preferentially increased. In general, photoinact ivation of catalase was accompanied by a decline of the F(v)/F(m) rati o, indicating photoinhibition of photosystem II, while other parameter s were much more stable. Inasmuch as both catalase and the D1 reaction center protein of photosystem II have a rapid turnover in light, thei r steady state levels appear to decline whenever stress effects either excessively enhance deleterious oxidative conditions and degradation (e. g. paraquat, low DCMU), or inhibit repair synthesis. Photoinactiva tion of catalase and of photosystem II represent specific and widely o ccurring early symptoms of incipient photodamage indicating stress con ditions where the repair capacity is not sufficient. During prolonged exposures, e.g. to NaCl and CuSO4, chlorophyll was bleached in light a nd the rate of its photodegradation increased in proportion as the cat alase level had declined. The results suggest that the enhanced suscep tibility of leaf tissues to photooxidative damage which is widely obse rved in stressed plants is related to the early loss of catalase.