Loss Of Variable Fluorescence Research Articles

Photosynthetic response to different light exposures and associated environmental conditions of the subaerial, epilithic green alga Trentepohlia umbrina (Chlorophyta, Ulvophyceae)

ABSTRACT The potentially biodeteriogenic, subaerial green alga Trentepohlia umbrina is widely distributed on shaded substrates with northern aspects. This preference for low light was confirmed by a recent survey in Lazio, Italy. However, the same survey revealed an isolated occurrence of extensive growths of T. umbrina-dominated biofilms on surfaces with diverse aspects that ranged from highly exposed to shaded. Prior studies of Trentepohlia photosynthesis have been restricted to single sites, so this multi-aspect site was particularly interesting to understand the photoacclimation potential of Trentepohlia. Photosynthetic pigment concentrations and chlorophyll fluorescence (in situ and ex situ) were used to assess the photosynthetic response to changes in light exposure and humidity. Pigment contents and photoinhibition decreased, while the maximum photosynthetic rate and the level of light that saturated photosynthesis increased with an increasing light exposure duration (ranging from c. 5 to 300 min d–1 of exposure to sunlight). Laboratory tests of relative humidity (RH) on biofilm photosynthesis showed no recorded activity up to 32% RH, only 26% of the maximum activity at 77% RH, and full activity at 92% RH. When T. umbrina was in the desiccated state a lowering of basal of chlorophyll a fluorescence emission and a loss of variable fluorescence occurred. This suppression of fluorescence when cells are desiccated is known to result in photoprotection of reaction centres from continuing high irradiances, as the enhanced thermal dissipation replaces slower and potentially damaging energy transfer to the functional reaction centres. It was concluded that T. umbrina has a much wider environmental range than previously thought, but its growth is very slow and may be overlooked during cultural heritage surveillance, leading to inconspicuous chronic degradation of outdoor stone substrata.

Photosynthetic performance of freshwater green microalgae Chlorella vulgaris to air-drying treatment

Photosynthetic performance in air-dried green microalgae Chlorella vulgaris was investigated. We attempted to identify how air-drying treatment affects chlorophyll a fluorescence OJIP transient and its derived fluorescence parameters. The recorded OJIP fluorescence transients showed a decrease of the variable fluorescence yield at all fluorescence transitions mainly after 4 h treatment compared to measurement at 0 h. The amplitude of the I–P phase from OJIP transient induced by a down-regulation at the PSII level. We suggest that the photochemical state responsible on the variable fluorescence during the first hours (up to 4 h) was maintained by turgid cells. However, the loss of variable fluorescence at sever air-drying treatment was caused by reducing the conversion of light energy in active reaction centers. Furthermore, the performance index (PIABS) parameter used here to quantify the PSII behavior decreased from 0.15 to 0.016 respectively at 0 h and 24 h treatment.

Desiccation-induced changes of photosynthetic transport in Parmelina tiliacea (Hoffm.) Ach. analysed by simultaneous measurements of the kinetics of prompt fluorescence, delayed fluorescence and modulated 820 nm reflection

Simultaneous in vivo measurements of prompt fluorescence (PF), delayed fluorescence (DF) and modulated reflection (MR) at 820-nm were used to assess effect of short desiccation period (4.5 h) on Parmelina tiliacea lichen. The two performance indexes (PIABS and PItotal) as a measure of an overall photosynthetic thalli performance showed a negative effect of desiccation treatment on photosynthetic activity. The maximal intensity of PF and DF recorded during desiccation treatment decreased and at 4.5 h desiccation time Parmelina tiliacea thalli loss their variable fluorescence and DF amplitude. This loss of variable fluorescence was due to an increase in inactive reaction centers and a limitation of electron donation on the donor side of photosystem II (PSII) that caused a down-regulation of electron transport chain at the PSII level. However, the efficiency with which an electron can move from the reduced intersystem electron acceptors to the PSI end electron acceptors was less affect by desiccation treatment. In respect to MR change, re-reduction kinetics of the primary electron donors of photosystems I (P700) and plastocyanin (PC) in desiccated thalli seem to be faster, their amplitudes gradually decrease and a disconnection between the two photosystem (PSII and PSI) were observed. These responses allow to Parmelina tiliacea lichen a photoprotection mechanism from the excess light excitation.

Ultraviolet B Exposure of Whole Leaves of Barley Affects Structure and Functional Organization of Photosystem II

This study examines the effects of ecologically important levels of ultraviolet B radiation on protein D1 turnover and stability and lateral redistribution of photosystem II. It is shown that ultraviolet B light supported only limited synthesis of protein D1, one of the most important components of photosystem II, whereas it promoted significant degradation of proteins D1 and D2. Furthermore, dephosphorylation of photosystem II subunits was specifically elicited upon exposure to ultraviolet B light. Structural modifications of photosystem II and changes in its lateral distribution between granum membranes and stroma-exposed lamellae were found to be different from those observed after photoinhibition by strong visible light. In particular, more complete dismantling of photosystem II cores was observed. Altogether, the data reported here suggest that ultraviolet B radiation alone fails to activate the photosystem II repair cycle, as hypothesized for visible light. This failure may contribute to the toxic effect of ultraviolet B radiation, which is increasing as a consequence of depletion of stratospheric ozone.

Protection of Photosystem II by Light in Heat-Stressed Pea Leaves

Modulated 685-nm chlorophyll fluorescence was measured under steady-state conditions in pea leaves adapted to moderate actinic light. When the measurements were performed after a short exposure to heat (42 °C) in darkness, the maximal (Fm) and steady-state (Fs) fluorescence levels were dramatically and irreversibly quenched whereas the basic fluorescence Fo remained unchanged. Concomitantly , photo synthetic O2 evolution was irreversibly inhibited. Analysis of the fluorescence data suggested that the heat treatment affected primarily the PS II reaction center, with the main effect being presumably an increased thermal dissipation of the excita­tion energy transferred to the reaction centers. In contrast, when heat stress was imposed in the presence of light, the loss of variable fluorescence (Fm-Fo) was much less marked and was fully reversible. In addition , no inhibition of in vivo O2 : evolution was observed when the samples heated in the light were recooled at 25 °C. The results indicate that light acted as an efficient protector of PS II against heat injury.

Photoinactivation of Photosystem II and Degradation of the D1 Protein are Reduced in a Cytochrome b6/f -Less Mutant of Chlamydomonas reinhardtii

The effect of unoccupancy of the QB site by plastoquinone on the photoinactivation of reaction center II in a Cyt b6/f-less mutant of Chlamydomonas reinhardtii, B6, was investigated. In these cells the oxidation of plastoquinol generated by electron flow via RC II to plastoquinone and thus the turnover of PQH2/PQ via the QB site are drastically reduced. Reaction center II of the mutant cells was resistant to photoinactivation relative to the control cells as demonstrated by measurements of light-induced destabilization of S2-QB- charge recombination, rise in intrinsic fluorescence and loss of variable fluorescence. These parameters relate to functions involving the reaction center II D 1 protein. The light-induced degradation of D 1 in the mutant cells was also considerably reduced, with a t1/2 value of 7 h as compared, under similar conditions, to about 1.5 h for the control cells. These results indicate that the photoinactivation of RC II and turnover of the D 1 protein are related and require occupancy of the QB site by PQ and its light-driven reduction.

A Comparison of the Effects of Chilling on Leaf Gas Exchange in Pea (Pisum sativum L.) and Cucumber (Cucumis sativus L.)

The effects of chilling on the photosynthesis of a chilling-resistant species, pea (Pisum sativum L. cv Alaska) and a chilling-sensitive species, cucumber (Cucumis sativus L. cv Ashley) were compared in order to determine the differences in the photosynthetic chilling sensitivity of these two species. For these experiments, plants were chilled (5 degrees C) for different lengths of time in the dark or light. Following a 1 hour recovery period at 25 degrees C, photosynthetic activity was measured by gas exchange (CO(2) uptake and H(2)O release), quantum yield, and induced chlorophyll fluorescence. The results show that pea photosynthesis was largely unaffected by two consecutive nights of chilling in the dark, or by chilling during a complete light and dark cycle (15 hours/9 hours). Cucumber gas exchange was reduced by one night of chilling, but its quantum yield and variable fluorescence were unaffected by dark chilling. However, chilling cucumber in the light led to reduced CO(2) fixation, increased internal leaf CO(2) concentration, decreased quantum yield, and loss of variable fluorescence. These results indicate that chilling temperatures in conjunction with light damaged the light reactions of photosynthesis, while chilling in the dark did not.

Regulation of photosynthesis in isolated spinach chloroplasts during orthophosphate limitation

The stromal concentration of orthophosphate in intact spinach chloroplasts (prepared in the absence of orthophosphate or pyrophosphate but supplied with both in the reaction medium) fell from a value of approx. 20 mM in the dark to a steady-state concentration of approx. 8 mM in the light. Chloroplasts illuminated in the absence of orthophosphate or pyrophosphate showed a similar trend. However, in this situation the stromal inorganic phosphate (P i) concentration rapidly decreased from approx. 10 mM in the dark to a constant steady-state concentration of between 1.5 and 2.5 mM in the light. This P i concentration was not further diminished (even though CO 2-dependent O 2 evolution had ceased) and was therefore considered to be stromal orthophosphate not freely available to metabolism. In the P i-deficient chloroplasts the rate of photosynthesis declined rapidly after 1–2 min in the light such that CO 2-dependent O 2 evolution ceased with 5 min of the onset of illumination. The decline in O 2 evolution was accompanied by an increase in the transthylakoid ΔpH (as measured by 9-aminoacridine fluorescence quenching) and in the high-energy state, non-photochemical component of chlorophyll fluorescence quenching (q E). Measurements of stromal metabolite concentrations showed that the ATP ADP ratio was decreased in the P i-deficient chloroplasts relative to chloroplasts illuminated in the presence of P i. The stromal concentration of glycerate 3-phosphate was comparable in the P i-deficient chloroplasts and those to which P i had been supplied. Chloroplasts which were illuminated in P i-free media showed a large accumulation of ribulose-1,5-bisphosphate relative to those supplied with P i, suggesting inhibition of ribulose-1,5-bisphosphate carboxylase under these conditions. When P i was added to chloroplasts illuminated in the absence of P i, both non-photochemical quenching (q E), photochemical quenching (q Q) and ΔpH increased. This suggests that electron transport was not limited by inability to discharge transthylakoid ΔpH. These observation are consistent with the hypothesis that P i limitation results in decreased ATP production by the thylakoid ATP synthase. The data presented here show that there are multiple sites of flux control exerted by low stromal P i in the chloroplast. At least three factors contribute to the inhibition of photosynthesis under phosphate limitation: (1) there appears to be a direct effect of P i on the energy-transducing system; (2) there is direct inhibition of the Calvin cycle decreasing the ability of the pathway to act as a sink for ATP and NADPH; and (3) feedback inhibition of primary processes occurs either via ΔpH or the redox state of electron carriers. However, ΔpH does not appear to be a limiting factor, but rather an inability to regenerate NADP as electron acceptor is suggested. The addition of DCMU to chloroplasts during illumination in the absence of P i for periods of up to 10 min showed that there was very little loss of variable fluorescence despite a 60% reduction in the capacity for O 2 evolution. This would suggest that photoinhibitory damage to Photosystem II was not the major cause of the inhibition of photosynthesis observed with low P i.

Hydroxylamine, hydrazine and methylamine donate electrons to the photooxidizing side of Photosystem II in leaves inhibited in oxygen evolution due to water stress

When leaf discs are water stressed, they lose the capacity for photosynthetic oxygen evolution and variable (chlorophyll a) fluorescence. Such a loss of variable fluorescence was previously reported by Govindjee et al. (Plant Sci. Lett. 20 (1981) 191–194). The later activity is not lost if prior to the water-stress treatment the leaf is incubated with typical water analogs known to act as electron donors to Photosystem II, such as hydroxylamine and hydrazine. Methylamine also acts in the same fashion. These results indicate that one of the sites of drought damage is the oxidizing side of Photosystem II, and that electron donors can restore electron transport, at least to the plastoquinone pool, similar to their effect in Tris treatment of isolated chloroplasts.

Turgor maintenance during salt stress prevents loss of variable fluorescence in grapevine leaves

Leaf water relations and chlorophyll a fluorescence from grapevines subjected to 0, 25, 50, 75 and 100 mM chloride (osmotic potentials of 0, −0.1, −0.2, −0.3 and −0.4 MPa, respectively) were studied over a 75-day period. Loss of variable fluorescence occurred soon after the imposition of the 100 mM chloride treatment which reduced turgor pressure to zero. Although the other treatments lowered turgor, relative fluorescence was not affected. Osmotic adjustment led to the rapid restoration of turgor in the 100 mM treatment and the recovery of variable fluorescence to pre-stress values. Variable fluorescence remained constant until day 58 for the 75 mM treatment and until day 75 for the 50 mM treatment at which time leaf chloride concentrations reached approx. 350 mM (tissue water basis) and there was a substantial decline in water potential and turgor pressure. It is concluded that the maintenance of turgor pressure and osmotic adjustment during salt stress prevents immediate damage to photosystem II (loss of variable fluorescence), but that the continued accumulation of chloride eventually causes cellular damage resulting in a loss of variable fluorescence and turgor pressure.

Distribution and Effects of Bentazon in Crop Plants and Weeds

Abstract The inhibition of photosynthetic CO2-assimilation and of the variable chlorophyll fluorescence as well as uptake and transport of 14C-labelled bentazon and the possibilities for a herbicideinduced shade-type modification of the photosynthetic apparatus were investigated in bentazonsensitive weeds (Galium, Sinapis, Raphanus) and in the tolerant crop plants wheat and maize. 1. In weeds the depression of photosynthetic CO2-assimilation is irreversible, whereas tolerant plants recover due to the metabolization of the active herbicide. 2. A lower rate of uptake and transport of bentazon associated with its fast metabolization is the reason for the tolerance of crop plants towards bentazon. 3. The transport of [14C]bentazon proceeds in the tracheary elements of the xylem. Uptake and transport of bentazon in the weeds are light dependent. 4. The loss of variable fluorescence (Kautsky effect) in the leaves after root application o f bentazon proceeds much faster at high-light than at low light conditions and confirms the light-dependency of the bentazon transport. 5. In the sensitive dicot weeds bentazon not only inhibits photosynthetic electron flow and depresses CO2-fixation but also induces the formation of shade-type chloroplasts which are less efficient in photosynthetic quantum conversion. This bentazon-induced modification of the photosynthetic apparatus (e.g. changes in ultrastructure, pigment ratios, and levels of chloro-phyll-proteins) contributes to the effectiveness of bentazon as a herbicide.