PREFERENTIAL PHOTOINACTIVATION OF CATALASE AND PHOTOINHIBITION OF PHOTOSYSTEM-II ARE COMMON EARLY SYMPTOMS UNDER VARIOUS OSMOTIC AND CHEMICAL STRESS CONDITIONS
P. Streb et al., PREFERENTIAL PHOTOINACTIVATION OF CATALASE AND PHOTOINHIBITION OF PHOTOSYSTEM-II ARE COMMON EARLY SYMPTOMS UNDER VARIOUS OSMOTIC AND CHEMICAL STRESS CONDITIONS, Physiologia Plantarum, 88(4), 1993, pp. 590-598
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.