Increase in the Quantum Yield of Photoinhibition Contributes to Copper Toxicity in Vivo1

Abstract

The effect of copper on photoinhibition of photosystem II in vivo was studied in bean (Phaseolus vulgaris L. cv Dufrix). The plants were grown hydroponically in the presence of various concentrations of Cu 2؉ ranging from the optimum 0.3 M (control) to 15 M. The copper concentration of leaves varied according to the nutrient medium from a control value of 13 mg kg ؊1 dry weight to 76 mg kg ؊1 dry weight. Leaf samples were illuminated in the presence and absence of lincomycin at different light intensities (500-1500 mol photons m ؊2 s ؊1 ). Lincomycin prevents the concurrent repair of photoinhibitory damage by blocking chloroplast protein synthesis. The photoinhibitory decrease in the light-saturated rate of O 2 evolution measured from thylakoids isolated from treated leaves correlated well with the decrease in the ratio of variable to maximum fluorescence measured from the leaf discs; therefore, the fluorescence ratio was used as a routine measurement of photoinhibition in vivo. Excess copper was found to affect the equilibrium between photoinhibition and repair, resulting in a decrease in the steadystate concentration of active photosystem II centers of illuminated leaves. This shift in equilibrium apparently resulted from an increase in the quantum yield of photoinhibition (⌽ PI ) induced by excess copper. The kinetic pattern of photoinhibition and the independence of ⌽ PI on photon flux density were not affected by excess copper. An increase in ⌽ PI may contribute substantially to Cu 2؉ toxicity in certain plant species. Cu2ϩ is an essential micronutrient but in excess is toxic for plants. It is a redox-active metal that functions as an enzyme activator and is an important part of prosthetic groups of many enzymes (for review, see Sandmann and Bö ger, 1983). Copper concentrations in healthy plant tissues range from 5 to 20 mg kg Ϫ1 dry weight. In Cu 2ϩ -rich environments, accumulation of Cu 2ϩ in plant tissues depends on the species and cultivar. Cu 2ϩ seems to have several sites of action, which vary among plant species. Toxic concentrations of Cu 2ϩ inhibit metabolic activity, which leads to suppressed growth and slow development. Most Cu 2ϩ ions are immobilized to the cell walls of roots or of mycorrhizal fungi (Kahle, 1993).When the tolerance mechanisms in the root zone become overloaded, Cu 2ϩ is translocated by both the xylem and phloem up to the leaves. Excess Cu 2ϩ may replace other metals in metalloproteins or may interact directly with SH groups of proteins (Uribe and Stark, 1982). Cu 2ϩ -induced free-radical formation may also cause protein damage (for review, see Fernandes and Henriques, 1991; Weckx and Clijsters, 1996). High concentrations of Cu 2ϩ may catalyze the formation of the hydroxyl radical from O 2 and H 2 O 2 . This Cu 2ϩ -catalyzed Fenton-type reaction takes place mainly in chloroplasts (Sandmann and Bö ger, 1980). The hydroxyl radical may start the peroxidation of unsaturated membrane lipids and chlorophyll (Sandmann and Bö ger, 1980), and these inhibitory mechanisms might contribu...

Impact of copper on reactive oxygen species, lipid peroxidation and antioxidants in Lemna minor

“…it is severely toxic to cells (Marschner 1995). Reduction in biomass, symptoms of chlorosis, decline in pigment content and alterations in the chloroplast and thylakoid ultrastructure have been reported (Baszynski et al 1988, Patsikka et al 1998 in plants under Cu stress. Toxic concentration of Cu results in generation of reactive oxygen species (ROS) like hydrogen peroxide, superoxide radical, and hydroxyl radical that exerts oxidative stress in cells (Panda et al 2003).…”

Utilization of inorganic carbon in the edible cyanobacterium Ge‐Xian‐Mi (Nostoc) and its role in alleviating photo‐inhibition

“…The extent of photo-inhibition (Y, %) was calculated according to Pätsikkä, Aro & Tyystjärvi (1998).…”

GC-TOF-MS based metabolomics and ICP-MS based metallomics of cucumber (Cucumis sativus) fruits reveal alteration of metabolites profile and biological pathway disruption induced by nano copper

“…Previous studies showed that nano copper depleted PSII action centers, leading to photoinhibition 25 and disruption of the repair cycle. 33,34 In addition, stomatal conductance and transpiration rates tended to increase in nCu treatments compared to the control. The actual mechanism for Cu to increase stomatal conductance and transpiration rate is still unknown.…”