The effect of inorganic (IC) depletion on fluorescence quenching was studied under laboratory conditions using the chlorococcal alga Scenedesmus quadricauda strain Greifswald/15. The absence of IC caused a decrease in photochemical quenching (^p), fluorescence yield and the photosystem 2 photochemical yield (dF/F^), and an increase of non-photochemical quenching {q^). High extemal pH (about 11), which accompanies the IC-depletion, did not háve any remarkable effect on the algae. Fluorescence parameters were reversed by a resupply of CO2. The IC-defícient algae when exposed to high irradiance appeared to be less sensitive to the photoinhibition than the algae supplied with CO2. Increased thermal dissipation of the excitation energy (non-photochemical quenching) in the absence of IC is one of the probable protective mechanisms against photoinhibitory damage.
A decrease of F,„ followed with a ceitain delay by an increase of was detected in the cells of Synechococcus elongatus in the first 120 min of the photoinhibitory treatment at 56 <>0 (growth temperature). Then F,n started to rise in parallel with F,, and this process proceeded widi the same rate both in the light and in the dark at 56 °C after light treatment. On tíie contrary, an increase of F^ observed during the light treatment at 15 °C was largely reversed after subsequent transfer of the ceUs to the dark at 56 <>€ but F,q remained nearly unchanged during the dark incubation.
The effects of drought oř salinity (NaCl) stress in combination wifh high irradiance (HI) were studied under the laboratory conditions, using young plants of a drought- resistant cultivar of Sorghum bicolor in order to understand leaf observed effects at the chloroplast level. Water potential and net photosynthetic rate (P^j) in leaves were analyzed in parallel with photosynthetic electron transport, electron spin resonance and chlorophyll-protein (CP) composition in chloroplasts isolated from these stressed leaves. The H1 slightly increased and electron transport activities of the irrigated plants, whereas drougbt and salinity stresses (leaf water potential lower than -1 MPa) decreased these activities. Under the combined effects of drought or salinity stress (200 mM NaCl) with HI the leaf Pjj was severely inhibited, as were photosystem (PS) 2 chloroplast electron transport activities (but not PS 1 activity). NaCl concentrations lower than 100 mM stimulated the electron transport activities in non-photoinhibited plants. A degradation of the PS 2 CFs and a decrease of ESR signál n were seen in the drought and salinity stressed plants afler HI.
High (HI, 200 W m'^) and low (LI, 30 W m"^) irradiance treatments on the cells of cyanobactenum Synechococcus elongatus Nfig., var. thermalis Geitl. strain KOVROV 1972/8 were perfonned in the presence of chloramphenicol (CAP), and with addition of DCMU and hydroxylamine (HA), respectively, at growth (56 °C) and low (20 °C) temperatures, to distínguish allegedly different mechanisms of PS 2 photoinactivation (PS 2 PI). At both temperatures HI caused a decline of Fy and Hill reaction activity (HRA) followed by degradation of Dl and to a lesser extent also D2 protein. Fq increased slowly during irradiance at 56 while at 20 ®C it quickly rose to constant level. Degradation of proteins was slowed at a lower temperature. The presence of DCMU during photoinhibition significantly blocked the Fq rise and also prevented PS 2 protein degradation at both temperatures. The course of PS 2 PI under LI resembled tlmt, observed at HI, but changes were much slower. During irradiation of the cells, in which oxygen evolving complex (OEC) was impaired by HA, we observed: (/) at least a ten-fold faster decline of PS 2 electron transport activity than in the cells vňth fimctional OEC under the same conditions; (2) an extensive degradation not only of Dl and D2, but also of the apoprotein of chlorophyll-protein CP43 {ACP43y, (3) almost complete inhibition of PS 2 protein degradation in the presence of DCMU. Thus under all conditions tested in vivo which do not affect the fimction of OEC, the fimction of OEC, PS 2 PI proceeds via the acceptor side and a fimctional impairment of OEC is necessary for induction of the donor side mechanism. When OEC is impaired (e.g. by HA) this mechanism can come in action.