Illumination of a suspension of thylakoids with light at high intensity causes inhibition of the photosystem II electron transport activity and loss from the membrane of the D1 protein of the photosystem II reaction center. Impairment of the electron transport activity and depletion of D1 protein from the thylakoid membrane of pea were investigated with reference to the presence or absence of oxygen in the suspension. The breakdown products of the D1 protein were identified by immunoblotting with anti-D1 polyclonal antibodies which were proven to recognize mainly the C-terminal region of the protein. The results obtained show that (i) the light-induced inactivation of the photosystem II electron transport activity under anaerobic conditions is faster than in the presence of oxygen; (ii) depletion of D1 protein is observed on a longer time scale with respect to loss of electron transport activity and is faster when photoinhibition is performed in the presence of oxygen; (iii) C-terminal fragments of D1 are only observed when photoinhibition is carried out anaerobically and are mainly localized in the stroma-exposed regions; and (iv) the fragments observed after anaerobic photoinhibition are quickly degraded on further illumination of the thylakoid suspension in the presence of oxygen.

were studied in vivo and in vitro. Treatments of pea (Pisum sativum) and broad bean (Vicia faba) plants with 0·05-5 mM cadmium (CdCl 2 ) modified PSII activity with a resulting increase in electron transfer followed by an inhibition and damage to the oxygen-evolving complex. Pulsechase experiments with [35 S]methionine in vivo followed by the separation of the radiolabelled thylakoids into grana and stroma exposed regions indicated that the synthesis, degradation and assembly of the D1 protein were greatly affected by cadmium. Initially D1 synthesis increased, later slowing down when the stress became advanced; at the same time the D1 degradation was increased. Binding studies with radiolabelled [ 14 C]herbicide revealed that the Q B pocket activity was also altered. However, the primary consequence of cadmium stress was the disassembly of the stacked regions. The measurements indicated differential tolerance to cadmium stress between the two plant species, which was not caused by either differential metal uptake or binding to the PSII complex. This suggests that the resulting changes in D1 turnover are a consequence of an unknown primary effect of cadmium on the PSII apparatus. However, we show that the higher tolerance to heavy metal stress found in broad bean plants relative to pea is accompanied by stimulation of D1 turnover. These experiments supported by previous data suggest that modulation of D1 turnover under stress is a commonly occurring process.Key-words: cadmium stress; D1 protein turnover; photosystem II; tolerance. INTRODUCTIONPhotosynthesis represents a central anabolic pathway in plants that results in the production of energy-rich organic compounds necessary for growth. The interaction between the production and consumption of assimilates requires a strict regulation of light energy conversion and of photosynthetic electron transport to satisfy the demand for energy and reduction equivalents used in the Calvin cycle. Photosystem II (PSII) is essential to the regulation of photosynthesis because it catalyses the oxidation of water into oxygen and supports electron transport. PSII consists of a core, a light-harvesting antenna and an oxygen-evolving system. The core is comprised of the reaction centre proteins, D1 and D2, cytochrome b 559 , the internal antennae chlorophyll proteins CP43 and CP47, the 33 kDa manganese stabilizing protein and several minor proteins PsbI, PsbW, PsbT . All pigments and prosthetic groups necessary for charge separation and stabilization are bound to the D1 and D2 proteins (Nanba & Satoh 1987). Under optimal physiological conditions, the D1 protein exhibits a light-dependent turnover several times higher than that of other chloroplast proteins (Mattoo, Marder & Edelman 1989). Its synthesis occurs at the stroma exposed membranes and D1 is probably immediately incorporated into the PSII complex because only minor amounts of free protein have been detected so far (Mattoo et al. 1989;Elich, Edelman & Mattoo 1992). PSII is inactivated by a variety of stresses -high irradia...

Effects of abiotic stresses on the turnover of the Di reaction centre II protein. -Physiol. Plant. 101: 635-642.In the 199O's, evidence has accumulated that various unfavourable environmental conditions substantially affect the turnover of the D| protein of reaction centre II, the psbA gene product. Biochemical, molecular and physiological studies in higher plants indicate that alterations of Di protein turnover occur under drought, nutrition deficiency, heat, chemical stress, ozone fumigation as well as UV-B and visible photo-stresses. The behaviour of photosystem II under these various stress conditions indicates that the response of Di protein turnover can be interpreted as a general adaptive response to environmental extremes.

We have developed a biosensor for the detection of residual triazine-, urea-and phenolic-type herbicides, using isolated photosystem II (PSII) particles from the thermophilic cyanobacterium, Synechococcus elongatus, as biosensing elements. The herbicide detection was based on the fact that, in the presence of artificial electron acceptors, the light-induced electron transfer through isolated PSII particles is accompanied by the release of oxygen, which is inhibited by the herbicide in a concentration-dependent manner. The PSII particles were immobilized between dialysis membrane and the Teflon membrane of the Clark oxygen electrode mounted in a flow cell that was illuminated. Inclusion of the antibiotic chloramphenicol in the reaction mixtures prolonged, by 50%, the lifetime of the biosensor. The use of highly active PSII particles in combination with the flow system resulted in a reusable herbicide biosensor with good stability (50% of initial activity was still remaining after 35-h use at 25°C) and high sensitivity (detection limit for diuron was 5 × 10 −10 M).

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