Abstract. Long-term drought stress on photosystem II (PSII) was studied in pea (Pisum sativum L.) seedlings. Drought stress (reduction of water content by 35-80%) led to a considerable depletion of the PSII core, and the remaining PSII complex appeared to be functional and reorganized, with a unit size (LHCP/PSII core) twofold greater than that of well-irrigated plants. By immunoblotting analysis of the PSII proteins from grana and stroma lamellae, the enhanced degradation of CP43 and D1 proteins was observed in water-stressed plants. Also, water stress caused increased phosphorylation of the PSII core and increased D1 protein synthesis. Water-stress-mediated increase in D1 synthesis did not occur when plants were exposed to photoinhibitory light. The depletion of the PSII core was essentially reversed when water-stressed plants grown at low visible irradiance were watered. We suggest that the syndrome caused by the effect of long-term water stress on photosynthesis is a combination of at least two events: a reduction in the number of active PSII centres caused by a physical destabilization of the PSII core and a PSII reorganization with enhanced D1 turnover to counteract the core depletion. Key words: D1 protein (turnover, modification) - DroughtHigh irradiance -Photosystem II (core phosphorylation) -Pisum sativum (drought stress) -Stress syndrome

Photoinhibition was analyzed in O2-evolving and in Tris-treated PS II membrane fragments by measuring flash-induced absorption changes at 830 nm reflecting the transient P680(+) formation and oxygen evolution. Irradiation by visible light affects the PS II electron transfer at two different sites: a) photoinhibition of site I eliminates the capability to perform a 'stable' charge separation between P680(+) and QA (-) within the reaction center (RC) and b) photoinhibition of site II blocks the electron transfer from YZ to P680(+). The quantum yield of site I photoinhibition (2-3×10(-7) inhibited RC/quantum) is independent of the functional integrity of the water oxidizing system. In contrast, the quantum yield of photoinhibition at site II depends strongly on the oxygen evolution capacity. In O2-evolving samples, the quantum yield of site II photoinhibition is about 10(-7) inhibited RC/quantum. After selective elimination of the O2-evolving capacity by Tris-treatment, the quantum yield of photoinhibition at site II depends on the light intensity. At low intensity (<3 W/m(2)), the quantum yield is 10(-4) inhibited RC/quantum (about 1000 times higher than in oxygen evolving samples). Based on these results it is inferred that the dominating deleterious effect of photoinhibition cannot be ascribed to an unique target site or a single mechanism because it depends on different experimental conditions (e.g., light intensity) and the functional status of the PS II complex.

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...

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