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) were measured in sections of rye leaves (Secale cereale L. cv. Halo) that were exposed for 24 h to moderately high irradiance under osmotic or chemical stress conditions (paraquat, DCMU, mannitol, NaCl, CdCl2 , CuSO4 , Pb(NO3 )2 , KNO2 , or K2 SO3 ). Changes of the chlorophyll content and of enzyme activities related to peroxide metabolism, such as glycolate oxidase, glutathione reductase, and peroxidase, were assayed for comparison. In the presence of the herbicides paraquat and low DCMU concentrations that exert only partial inhibition of photosynthesis, as well as after most treatments with osmotic or chemical stress factors, catalase markedly declined due to a preferential photoinactivation. At higher DCMU levels catalase did not decline. At low KNO2 concentrations catalase activity was preferentially increased. In general, photoinactivation of catalase was accompanied by a decline of the F/Fm ratio, indicating photoinhibition of photosystem II, while other parameters were much more stable. Inasmuch as both catalase and the D1 reaction center protein of photosystem II have a rapid turnover in light, their 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. Photoinactivation of catalase and of photosystem II represent specific and widely occurring early symptoms of incipient photodamage indicating stress conditions where the repair capacity is not sufficient. During prolonged exposures, e. g. to NaCl and CuSO4 , chlorophyll was bleached in light and the rate of its photodegradation increased in proportion as the catalase level had declined. The results suggest that the enhanced susceptibility of leaf tissues to photooxidative damage which is widely observed in stressed plants is related to the early loss of catalase.