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

This work aims to: (1) correlate photochemical activity and productivity, (2) characterize the flow pattern of culture layers and (3) determine a range of biomass densities for high productivity of the freshwater microalga Chlorella spp., grown outdoors in thin-layer cascade units. Biomass density, irradiance inside culture, pigment content and productivity were measured in the microalgae cultures. Chlorophyll-fluorescence quenching was monitored in situ (using saturation-pulse method) to estimate photochemical activities. Photobiochemical activities and growth parameters were studied in cultures of biomass density between 1 and 47 g L(-1). Fluorescence measurements showed that diluted cultures (1-2 g DW L(-1)) experienced significant photostress due to inhibition of electron transport in the PSII complex. The highest photochemical activities were achieved in cultures of 6.5-12.5 g DW L(-1), which gave a maximum daylight productivity of up to 55 g dry biomass m(-2) day(-1). A midday depression of maximum PSII photochemical yield (F (v)/F (m)) of 20-30% compared with morning values in these cultures proved to be compatible with well-performing cultures. Lower or higher depression of F (v)/F (m) indicated low-light acclimated or photo-inhibited cultures, respectively. A hydrodynamic model of the culture demonstrated highly turbulent flow allowing rapid light/dark cycles (with frequency of 0.5 s(-1)) which possibly match the turnover of the photosynthetic apparatus. These results are important from a biotechnological point of view for optimisation of growth of outdoor microalgae mass cultures under various climatic conditions.

A biosensor for the detection of triazine- and phenylurea-type herbicides was constructed using isolated Photosystem II (PS II) complexes as a biosensing element. PSII isolated from the thermophilic cyanobacterium Synechococcus elongatus was immobilized on the surface of a screen-printed sensor composed of a graphite working electrode and Ag/AgCl reference electrode deposited on a polymeric substrate. The biosensor was mounted in a flow microcell with illumination. The principle of the detection was based on the fact that herbicides selectively block PSII electron transport activity in a concentration-dependent manner. Changes of the activity were registered amperometrically as the rate of photoreduction of an artificial electron acceptor. The setup resulted in a reusable herbicide biosensor with a good stability (half-life of 24 h) and limit of detection of approximately 10(-9) M for diuron, atrazine and simazine.

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The saturating pulse fluorescence technique was applied to study photoinhibition of photosynthesis in outdoor cultures of the cyanobacterium Spirulina platensis (Nordstedt) Geitler strain M2 grown under high oxygen and low temperature stress. Diurnal changes in maximum photochemical yield (Fv/Fm), photon yield of PSII (ΔF/F′m), and nonphotochemical quenching (qN) were measured using a portable, pulse‐amplitude–modulated fluorometer. When solar irradiance reached the maximum value, the Fv/Fm and ΔF/F′m ratios of theSpirulina cultures grown under high oxygen stress decreased by 35% and 60%, respectively, as compared with morning values. The depression of the Fv/Fm and ΔF/F′m ratios reached 55% and 84%, respectively, when high oxygen stress was combined with low temperature (i.e. 10° C below the optimal value for growth). Photoinhibition reduced the daily productivity of the culture grown under high oxygen stress by 33% and that of the culture grown under high oxygen–low temperature stress by 60%. Changes in the biomass yield of the cultures correlated well with changes in the daily integrated value of the estimated electron transport rate through the PSII (ΔF/F′m × photon flux density). The results indicate that on‐line chlorophyll fluorescence measurement is a powerful tool for assessing the photosynthetic performance of outdoorSpirulina cultures.

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The role of the xanthophyll cycle in the adaptation of two chlorococcal algae Scenedesmus quadricauda and Chlorella sorokiniana to high irradiance was studied under laboratory and outdoor conditions. We wished to elucidate whether the xanthophyll cycle plays a key role in dissipating the excesses of absorbed light, as in higher plants, and to characterise the relationship between chlorophyll fluorescence parameters and the content of xanthophyll-cycle pigments. The xanthophyll cycle was found to be operative in both species; however, its contribution to overall non-photochemical quenching (NPQ) could only be distinguished in Scenedesmus (15-20% of total NPQ). The Scenedesmus cultures showed a larger pool of xanthophyll-cycle pigments than Chlorella, and lower sensitivity to photoinhibition as judged from the reduction of maximum quantum yield of photosystem II. In general, both algae had a larger xanthophyll-cycle pool when grown outdoors than in laboratory cultures. Comparing the two species, Scenedesmus exhibited a higher capacity to adapt to high irradiance, due to an effective quenching mechanism and high photosynthetic capacity; in contrast, Chlorella represents a species with a larger antennae system, less-efficient quenching and lower photosynthetic performance. Non-photochemical quenching (NPQ) induced through the xanthophyll cycle can, to a limited extent, represent a regulatory factor in diluted algal cultures grown in outdoor solar photobioreactors, as well as in natural algal phytoplankton populations exposed transiently to high irradiance. However, it does not play an appreciable role in dense, well-mixed microalgal suspensions.