Photoinhibition of photosystem II was studied in vivo with bean (Phaseolus vulgaris) plants grown in the presence of 0.3 (control), 4, or 15 m Cu 2ϩ . Although photoinhibition, measured in the presence of lincomycin to block concurrent recovery, is faster in leaves of Cu 2ϩ -treated plants than in control leaves, thylakoids isolated from Cu-treated plants did not show high sensitivity to photoinhibition. Direct effects of excess Cu 2ϩ on chloroplast metabolism are actually unlikely, because the Cu concentration of chloroplasts of Cu-treated plants was lower than that of their leaves. Excess Cu in the growth medium did not cause severe oxidative stress, collapse of antioxidative defenses, or loss of photoprotection. Thus, these hypothetical effects can be eliminated as causes for Cu-enhanced photoinhibition in intact leaves. However, Cu treatment lowered the leaf chlorophyll (Chl) concentration and reduced the thylakoid membrane network. The loss of Chl and sensitivity to photoinhibition could be overcome by adding excess Fe together with excess Cu to the growth medium. The addition of Fe lowered the Cu 2ϩ concentration of the leaves, suggesting that Cu outcompetes Fe in Fe uptake. We suggest that the reduction of leaf Chl concentration, caused by the Cu-induced iron deficiency, causes the high photosensitivity of photosystem II in Cu 2ϩ -treated plants. A causal relationship between the susceptibility to photoinhibition and the leaf optical density was established in several plant species. Plant species adapted to high-light habitats apparently benefit from thick leaves because the rate of photoinhibition is directly proportional to light intensity, but photosynthesis becomes saturated by moderate light.

Photoinhibition is light-induced inactivation of PSII. Hypotheses about the photoreceptor(s) of photoinhibition include the Chl antenna of PSII, manganese of the oxygen-evolving complex (OEC), uncoupled Chl and iron-sulfur centres. We measured the action spectrum of photoinhibition in vivo from wild-type Arabidopsis thaliana L. and from the npq1-2 and npq4-1 mutants defective in non-photochemical quenching (NPQ) of excitations of the PSII antenna. The in vivo action spectrum was found to resemble closely the in vitro action spectra published for photoinhibition. We compared the action spectrum with absorbance spectra of model compounds of the OEC complex and other potential photoreceptors of photoinhibition. The comparison suggests that both manganese and Chl function as photoreceptors in photoinhibition. In accordance with the function of two types of photoreceptors in photoinhibition, NPQ was found to offer only partial protection against photoinhibition at visible wavelengths. The low protective efficiency of NPQ supports the conclusion that the Chl antenna of PSII is not the only photoreceptor of photoinhibition. Comparison of the action spectrum of photoinhibition with the emission spectrum of sunlight shows that the UV part of sunlight is responsible for the major part of photoinhibition under natural conditions.

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...

Effects of ozone on spring wheat (Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons (1992)(1993) at Jokioinen in south-west Finland. The wheat was exposed to filtered air (CF), non-filtered air (NF), non-filtered airj35 nl l −" ozone for 8 h d −" (NF + ) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone treatment (NF + , 45 nl l −" 1000-1800 hours, seasonal mean) the net CO # uptake of wheat flag leaves was decreased by c. 40 % relative to CF and NF treatments, both initial and total activity of Rubisco and the quantity of protein-bound SH groups were decreased significantly. Added ozone also significantly accelerated flag leaf senescence recorded as a decrease in chloroplast size. The effect was significant 2 wk after anthesis, and senescence was complete after 4 wk. In the CF and NF treatments senescence was complete 5 wk after anthesis. The significant effect of ozone on the chloroplasts and net CO # uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filling period was shorter for ozone fumigated plants, kernels were smaller. The decrease in 1000-grain weight explained most of the yield reduction in the plants under NF + treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.

The effect of copper on photoinhibition of photosystem II activity of thylakoids that had been illuminated in the presence of Cu 2 + . Second, already moderate light was enough to cause (PSII) in vitro was studied in bean (Phaseolus 7ulgaris L. cv. Dufrix) and pumpkin (Cucurbita pepo L.) thylakoids. The rapid inhibition of PSII, and the inhibition could be saturated thylakoids were illuminated at 200-2000 mmol photons m − 2 by light. Third, the extrinsic polypeptides of the oxygen-evolvs − 1 in the presence of 70-1830 added Cu 2 + ions per PSII. ing complex were found to become oxidized by the combined effect of Cu 2 + and light. The presence of oxygen was not Three lines of evidence show that the irreversible damage of PSII caused by illumination of thylakoids in the presence of necessary for the copper-induced enhancement of photoinhibi-Cu 2 + was mainly due to donor-side photoinhibition resulting tion of PSII. When the illumination was prolonged, copper from inhibition of the PSII donor side by Cu 2 + . First, caused a gradual collapse of the thylakoid structure by inaddition of an artificial electron donor partially restored PSII creasing degradation of thylakoid proteins.