Field-acclimated Gossypium hirsutum cultivars exhibit genotypic and seasonal differences in photosystem II thermostability

Previous investigations have demonstrated that photosystem II (PSII) thermostability acclimates to prior exposure to heat and drought, but contrasting results have been reported for cotton (Gossypium hirsutum). We hypothesized that PSII thermotolerance in G. hirsutum would acclimate to environmental conditions during the growing season and that there would be differences in PSII thermotolerance between commercially-available U.S. cultivars. To this end, three cotton cultivars were grown under dryland conditions in Tifton Georgia, and two under irrigated conditions in Marianna Arkansas. At Tifton, measurements included PSII thermotolerance (T₁₅, the temperature causing a 15% decline in maximum quantum yield), leaf temperatures, air temperatures, midday (1200 to 1400 h) leaf water potentials (Ψ_MD), leaf-air vapor pressure deficit (VPD), actual quantum yield (Φ_PSII) and electron transport rate through PSII (ETR) on three sample dates. At Marianna, T₁₅ was measured on two sample dates. Optimal air and leaf temperatures were observed on all sample dates in Tifton, but PSII thermotolerance increased with water deficit conditions (Ψ_MD = −3.1 MPa), and ETR was either unaffected or increased under water-stress. Additionally, T₁₅ for PHY 499 was ∼5 °C higher than for the other cultivars examined (DP 0912 and DP 1050). The Marianna site experienced more extreme high temperature conditions (20–30 days T_max ≥ 35 °C), and showed an increase in T₁₅ with higher average T_max. When average T₁₅ values for each location and sample date were plotted versus average daily T_max, strong, positive relationships (r² from .954 to .714) were observed between T_max and T₁₅. For all locations T₁₅ was substantially higher than actual field temperature conditions. We conclude that PSII thermostability in G. hirsutum acclimates to pre-existing environmental conditions; PSII is extremely tolerant to high temperature and water-deficit stress; and differences in PSII thermotolerance exist between commercially-available cultivars.     

Effects of polyamines on the functionality of photosynthetic membrane in vivo and in vitro

The three major polyamines are normally found in chloroplasts of higher plants and are implicated in plant growth and stress response. We have recently shown that putrescine can increase light energy utilization through stimulation of photophosphorylation [Ioannidis et al., (2006) BBA-Bioenergetics, 1757, 821-828]. We are now to compare the role of the three major polyamines in terms of chloroplast bioenergetics. There is a different mode of action between the diamine putrescine and the higher polyamines (spermidine and spermine). Putrescine is an efficient stimulator of ATP synthesis, better than spermidine and spermine in terms of maximal % stimulation. On the other hand, spermidine and spermine are efficient stimulators of non-photochemical quenching. Spermidine and spermine at high concentrations are efficient uncouplers of photophosphorylation. In addition, the higher the polycationic character of the amine being used, the higher was the effectiveness in PSII efficiency restoration, as well as stacking of low salt thylakoids. Spermine with 50 μM increase F_V as efficiently as 100 μM of spermidine or 1000 μM of putrescine or 1000 μM of Mg²⁺. It is also demonstrated that the increase in F_V derives mainly from the contribution of PSIIα centers. These results underline the importance of chloroplastic polyamines in the functionality of the photosynthetic membrane.     

Assessing the regulation of leaf redox status under water stress conditions in Arabidopsis thaliana: Col-0 ecotype (wild-type and vtc-2), expressing mitochondrial and cytosolic roGFP1

Using Arabidopsis plants Col-0 and vtc2 transformed with a redox sensitive green fluorescent protein, (c-roGFP) and (m-roGFP), we investigated the effects of a progressive water stress and re-watering on the redox status of the cytosol and the mitochondria. Our results establish that water stress affects redox status differently in these two compartments, depending on phenotype and leaf age, furthermore we conclude that ascorbate plays a pivotal role in mediating redox status homeostasis and that Col-0 Arabidopsis subjected to water stress increase the synthesis of ascorbate suggesting that ascorbate may play a role in buffering changes in redox status in the mitochondria and the cytosol, with the presumed buffering capacity of ascorbate being more noticeable in young compared with mature leaves. Re-watering of water-stressed plants was paralleled by a return of both the redox status and ascorbate to the levels of well-watered plants. In contrast to the effects of water stress on ascorbate levels, there were no significant changes in the levels of glutathione, thereby suggesting that the regeneration and increase in ascorbate in water-stressed plants may occur by other processes in addition to the regeneration of ascorbate via the glutathione. Under water stress in vtc2 lines it was observed stronger differences in redox status in relation to leaf age, than due to water stress conditions compared with Col-0 plants. In the vtc2 an increase in DHA was observed in water-stressed plants. Furthermore, this work confirms the accuracy and sensitivity of the roGFP1 biosensor as a reporter for variations in water stress-associated changes in redox potentials.     

Thermal tolerance,net CO2 exchange and growth of a tropical tree species, Ficus insipida, cultivated at elevated daytime and nighttime temperatures

Global warming and associated increases in the frequency and amplitude of extreme weather events, such as heat waves, may adversely affect tropical rainforest plants via significantly increased tissue temperatures. In this study, the response to two temperature regimes was assessed in seedlings of the neotropical pioneer tree species, Ficus insipida. Plants were cultivated in growth chambers at strongly elevated daytime temperature (39 °C), combined with either close to natural (22 °C) or elevated (32 °C) nighttime temperatures. Under both growth regimes, the critical temperature for irreversible leaf damage, determined by changes in chlorophyll a fluorescence, was approximately 51 °C. This is comparable to values found in F. insipida growing under natural ambient conditions and indicates a limited potential for heat tolerance acclimation of this tropical forest tree species. Yet, under high nighttime temperature, growth was strongly enhanced, accompanied by increased rates of net photosynthetic CO₂ uptake and diminished temperature dependence of leaf-level dark respiration, consistent with thermal acclimation of these key physiological parameters.     

Luminostat operation: a tool to maximize microalgae photosynthetic efficiency in photobioreactors during the daily light cycle?

The luminostat regime has been proposed as a way to maximize light absorption and thus to increase the microalgae photosynthetic efficiency within photobioreactors. In this study, simulated outdoor light conditions were applied to a lab-scale photobioreactor in order to evaluate the luminostat control under varying light conditions. The photon flux density leaving the reactor (PFD_out) was varied from 4 to 20 μmol photons m⁻² s⁻¹and the productivity and photosynthetic efficiency of Chlorella sorokiniana were assessed.Maximal volumetric productivity (1.22 g kg⁻¹ d⁻¹) and biomass yield on PAR photons (400-700 nm) absorbed (1.27 g mol⁻¹) were found when PFD_out was maintained between 4 and 6 μmol photons m⁻² s⁻¹. The resultant photosynthetic efficiency was comparable to that already reported in a chemostat-controlled reactor. A strict luminostat regime could not be maintained under varying light conditions. Further modifications to the luminostat control are required before application under outdoor conditions.     

ATP binding and hydrolysis steps of the uni-site catalysis by the mitochondrial F1-ATPase are affected by inorganic phosphate

The effect of inorganic phosphate (P_i) on uni-site ATP binding and hydrolysis by the nucleotide-depleted F₁-ATPase from beef heart mitochondria (ndMF₁) has been investigated. It is shown for the first time that P_i decreases the apparent rate constant of uni-site ATP binding by ndMF₁ 3-fold with the K_d of 0.38 ± 0.14 mM. During uni-site ATP hydrolysis, P_i also shifts equilibrium between bound ATP and ADP + P_i in the direction of ATP synthesis with the K_d of 0.17 ± 0.03 mM. However, 10 mM P_i does not significantly affect ATP binding during multi-site catalysis.     

Characterization of Arabidopsis sterol glycosyltransferase TTG15/UGT80B1 role during freeze and heat stress

Sterol glycosyltransferases regulate the properties of sterols by catalyzing the transfer of carbohydrate molecules to the sterol moiety for the synthesis of steryl glycosides and acyl steryl glycosides. We have analyzed the functional role of TTG15/UGT80B1 gene of Arabidopsis thaliana in freeze/thaw and heat shock stress using T-DNA insertional sgt knockout mutants. Quantitative study of spatial as well as temporal gene expression showed tissue-specific and dynamic expression patterns throughout the growth stages. Comparative responses of Col-0, TTG15/UGT80B1 knockout mutant and p35S:TTG15/UGT80B1 restored lines were analyzed under heat and freeze stress conditions. Heat tolerance was determined by survival of plants at 42°C for 3 h, MDA analysis and chlorophyll fluorescence image (CFI) analysis. Freezing tolerance was determined by survival of the plants at -1°C temperature in non-acclimatized (NA) and cold acclimatized (CA) conditions and also by CFI analysis, which revealed that, p35S:TTG15/UGT80B1 restored plants were more adapted to freeze stress than TTG15/UGT80B1 knockout mutant under CA condition. HPLC analysis of the plants showed reduced sterol glycoside in mutant seedlings as compared to other genotypes. Following CA condition, both β-sitosterol and sitosterol glycoside quantity was more in Col-0 and p35S:TTG15/UGT80B1 restored lines, whereas it was significantly less in TTG15/UGT80B1 knockout mutants. From these results, it may be concluded that due to low content of free sterols and sterol glycosides, the physiology of mutant plants was more affected during both, the chilling and heat stress.     

Spectroscopic studies of the chlorophyll d containing photosystem I from the cyanobacterium, Acaryochloris marina

Absorbance difference spectroscopy and redox titrations have been applied to investigate the properties of photosystem I from the chlorophyll d containing cyanobacterium Acaryochloris marina. At room temperature, the (P740⁺ − P740) and (F_A/B⁻ − F_A/B) absorbance difference spectra were recorded in the range between 300 and 1000 nm while at cryogenic temperatures, (P740⁺A₁⁻ − P740A₁) and (³P740 − P740) absorbance difference spectra have been measured. Spectroscopic and kinetic evidence is presented that the cofactors involved in the electron transfer from the reduced secondary electron acceptor, phylloquinone (A₁⁻), to the terminal electron acceptor and their structural arrangement are virtually identical to those of chlorophyll a containing photosystem I. The oxidation potential of the primary electron donor P740 of photosystem I has been reinvestigated. We find a midpoint potential of 450 ± 10 mV in photosystem I-enriched membrane fractions as well as in thylakoids which is very similar to that found for P700 in chlorophyll a dominated organisms. In addition, the extinction difference coefficient for the oxidation of the primary donor has been determined and a value of 45,000 ± 4000 M^− 1 cm^− 1 at 740 nm was obtained. Based on this value the ratio of P740 to chlorophyll is calculated to be 1:~ 200 chlorophyll d in thylakoid membranes. The consequences of our findings for the energetics in photosystem I of A. marina are discussed as well as the pigment stoichiometry and spectral characteristics of P740.     

MIZ1-regulated hydrotropism functions in the growth and survival of Arabidopsis thaliana under natural conditions

Background and Aims

Root hydrotropism is a response to water-potential gradients that makes roots bend towards areas of higher water potential. The gene MIZU-KUSSEI1 (MIZ1) that is essential for hydrotropism in Arabidopsis roots has previously been identified. However, the role of root hydrotropism in plant growth and survival under natural conditions has not yet been proven. This study assessed how hydrotropic response contributes to drought avoidance in nature.

Methods

An experimental system was established for the study of Arabidopsis hydrotropism in soil. Characteristics of hydrotropism were analysed by comparing the responses of the miz1 mutant, transgenic plants overexpressing MIZ1 (MIZ1OE) and wild-type plants.

Key Results

Wild-type plants developed root systems in regions with higher water potential, whereas the roots of miz1 mutant plants did not show a similar response. This pattern of root distribution induced by hydrotropism was more pronounced in MIZ1OE plants than in wild-type plants. In addition, shoot biomass and the number of plants that survived under drought conditions were much greater in MIZ1OE plants.

Conclusions

These results show that hydrotropism plays an important role in root system development in soil and contributes to drought avoidance, which results in a greater yield and plant survival under water-limited conditions. The results also show that MIZ1 overexpression can be used for improving plant productivity in arid areas.     

Effects of light environment on the induction of chlorophyll fluorescence in leaves: a comparative study of Tradescantia species of different ecotypes

In this work, using a PAM-fluorimetry technique, we have compared effects of plant adaptation to the light or dark conditions on the kinetics of chlorophyll a fluorescence yield in Tradecantia leaves of several species (Tradescantia albiflora, Tradescantia fluminensis, Tradescantia navicularis, and Tradescantia sillamontana), which represent plants of different ecotypes. Two fluorescence parameters were used to assess photosynthetic performance in vivo: non-photochemical quenching (NPQ) of chlorophyll fluorescence (q_NPQ) determined by energy losses in the light-harvesting antenna of photosystem 2 (PS2), and PS2 operating efficiency (Φ_PSII). Comparative study of light-induced changes in q_NPQ and Φ_PSII has demonstrated that shade-tolerant Tradecantia species (T. albiflora Kunth, T. fluminensis Vell.) reveal higher capacities for NPQ and demonstrate slower transitions between the ‘light-adapted’ and ‘dark-adapted’ states than succulent species T. navicularis and T. sillamontana, which are typical habitats of semi-deserts. We analyze the photosynthetic performance of Tradescantia species in the context of their adaptabilities to variable environment conditions. The ability of shade-tolerant plants to retain a relatively long-term (∼40-60 min) ‘memory’ for illumination history may be associated with the regulatory mechanisms that provide the flexibility of photosynthetic apparatus in response to fluctuations of light intensity.     

Dynamic response of UV-absorbing compounds, quantum yield and the xanthophyll cycle to diel changes in UV-B and photosynthetic radiations in an aquatic liverwort

We studied the diel responses of the liverwort Jungermannia exsertifolia subsp. cordifolia to radiation changes under laboratory conditions. The samples were exposed to three radiation regimes: P (only PAR), PA (PAR + UV-A), and PAB (PAR + UV-A + UV-B). The day was divided in four periods: darkness, a first low-PAR period, the high-PAR plus UV period, and a second low-PAR period. After 15 days of culture, we measured photosynthetic pigments, chlorophyll fluorescence and UV-absorbing compounds in the four periods of the day on two consecutive days. With respect to UV-absorbing compounds, we analyzed their global amount (as the bulk UV absorbance of methanolic extracts) and the concentration of seven hydroxycinnamic acid derivatives, both in the soluble (mainly vacuolar) and insoluble (cell wall-bound) fractions of the plant extracts. PAB samples increased the bulk UV absorbance of the soluble and insoluble fractions, and the concentrations of p-coumaroylmalic acid in the soluble fraction and p-coumaric acid in the cell wall. Most of these variables showed significant diel changes and responded within a few hours to radiation changes (more strongly to UV-B), increasing at the end of the period of high-PAR plus UV. F_v/F_m, Φ_PSII, NPQ and the components of the xanthophyll cycle showed significant and quick diel changes in response to high PAR, UV-A and UV-B radiation, indicating dynamic photoinhibition and protection of PSII from excess radiation through the xanthophyll cycle. Thus, the liverwort showed a dynamic protection and acclimation capacity to the irradiance level and spectral characteristics of the radiation received.     

Photosynthetic electron transport activity in heat-treated barley leaves: The role of internal alternative electron donors to photosystem II

Electron transport processes were investigated in barley leaves in which the oxygen-evolution was fully inhibited by a heat pulse (48 °C, 40 s). Under these circumstances, the K peak (∼ F_400 μs) appears in the chl a fluorescence (OJIP) transient reflecting partial Q_A reduction, which is due to a stable charge separation resulting from the donation of one electron by tyrozine Z. Following the K peak additional fluorescence increase (indicating Q_A⁻ accumulation) occurs in the 0.2-2 s time range. Using simultaneous chl a fluorescence and 820 nm transmission measurements it is demonstrated that this Q_A⁻ accumulation is due to naturally occurring alternative electron sources that donate electrons to the donor side of photosystem II. Chl a fluorescence data obtained with 5-ms light pulses (double flashes spaced 2.3-500 ms apart, and trains of several hundred flashes spaced by 100 or 200 ms) show that the electron donation occurs from a large pool with t_1/2 ∼ 30 ms. This alternative electron donor is most probably ascorbate.     

Spin-probes designed for measuring the intrathylakoid pH in chloroplasts

Nitroxide radicals are widely used as molecular probes in different fields of chemistry and biology. In this work, we describe pH-sensitive imidazoline- and imidazolidine-based nitroxides with pK values in the range 4.7-7.6 (2,2,3,4,5,5-hexamethylperhydroimidazol-1-oxyl, 4-amino-2,2,5,5-tetramethyl-2,5-dihydro-1H-imidazol-1-oxyl, 4-dimethylamino-2,2-diethyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl, and 2,2-diethyl-5,5-dimethyl-4-pyrrolidyline-1-yl-2,5-dihydro-1H-imidazol-1-oxyl), which allow the pH-monitoring inside chloroplasts. We have demonstrated that EPR spectra of these spin-probes localized in the thylakoid lumen markedly change with the light-induced acidification of the thylakoid lumen in chloroplasts. Comparing EPR spectrum parameters of intrathylakoid spin-probes with relevant calibrating curves, we could estimate steady-state values of lumen pH_in established during illumination of chloroplasts with continuous light. For isolated bean (Vicia faba) chloroplasts suspended in a medium with pH_out = 7.8, we found that pH_in ≈ 5.4-5.7 in the state of photosynthetic control, and pH_in ≈ 5.7-6.0 under photophosphorylation conditions. Thus, ATP synthesis occurs at a moderate acidification of the thylakoid lumen, corresponding to transthylakoid pH difference ΔpH ≈ 1.8-2.1. These values of ΔpH are consistent with a point of view that under steady-state conditions the proton gradient ΔpH is the main contributor to the proton motive force driving the operation of ATP synthesis, provided that stoichiometric ratio H⁺/ATP is n ≥ 4-4.7.     

Carbon dioxide diffusion across stomata and mesophyll and photo-biochemical processes as affected by growth CO2 and phosphorus nutrition in cotton

Nutrients such as phosphorus may exert a major control over plant response to rising atmospheric carbon dioxide concentration (CO₂), which is projected to double by the end of the 21st century. Elevated CO₂ may overcome the diffusional limitations to photosynthesis posed by stomata and mesophyll and alter the photo-biochemical limitations resulting from phosphorus deficiency. To evaluate these ideas, cotton (Gossypium hirsutum) was grown in controlled environment growth chambers with three levels of phosphate (Pi) supply (0.2, 0.05 and 0.01 mM) and two levels of CO₂ concentration (ambient 400 and elevated 800 μmol mol⁻¹) under optimum temperature and irrigation. Phosphate deficiency drastically inhibited photosynthetic characteristics and decreased cotton growth for both CO₂ treatments. Under Pi stress, an apparent limitation to the photosynthetic potential was evident by CO₂ diffusion through stomata and mesophyll, impairment of photosystem functioning and inhibition of biochemical process including the carboxylation efficiency of ribulose-1,5-bisphosphate carboxylase/oxyganase and the rate of ribulose-1,5-bisphosphate regeneration. The diffusional limitation posed by mesophyll was up to 58% greater than the limitation due to stomatal conductance (g_s) under Pi stress. As expected, elevated CO₂ reduced these diffusional limitations to photosynthesis across Pi levels; however, it failed to reduce the photo-biochemical limitations to photosynthesis in phosphorus deficient plants. Acclimation/down regulation of photosynthetic capacity was evident under elevated CO₂ across Pi treatments. Despite a decrease in phosphorus, nitrogen and chlorophyll concentrations in leaf tissue and reduced stomatal conductance at elevated CO₂, the rate of photosynthesis per unit leaf area when measured at the growth CO₂ concentration tended to be higher for all except the lowest Pi treatment. Nevertheless, plant biomass increased at elevated CO₂ across Pi nutrition with taller plants, increased leaf number and larger leaf area.     

Natural variation in phosphorylation of photosystem II proteins in Arabidopsis thaliana: is it caused by genetic variation in the STN kinases?

Reversible phosphorylation of photosystem II (PSII) proteins is an important regulatory mechanism that can protect plants from changes in ambient light intensity and quality. We hypothesized that there is natural variation in this process in Arabidopsis (Arabidopsis thaliana), and that this results from genetic variation in the STN7 and STN8 kinase genes. To test this, Arabidopsis accessions of diverse geographical origins were exposed to two light regimes, and the levels of phospho-D1 and phospho-light harvesting complex II (LHCII) proteins were quantified by western blotting with anti-phosphothreonine antibodies. Accessions were classified as having high, moderate or low phosphorylation relative to Col-0. This variation could not be explained by the abundance of the substrates in thylakoid membranes. In genotypes with atrazine-resistant forms of the D1 protein, low D1 and LHCII protein phosphorylation was observed, which may be due to low PSII efficiency, resulting in reduced activation of the STN kinases. In the remaining genotypes, phospho-D1 levels correlated with STN8 protein abundance in high-light conditions. In growth light, D1 and LHCII phosphorylation correlated with longitude and in the case of LHCII phosphorylation also with temperature variability. This suggests a possible role of natural variation in PSII protein phosphorylation in the adaptation of Arabidopsis to diverse environments.     

Photosynthesis is improved by exogenous calcium in heat-stressed tobacco plants

Effects of exogenous calcium chloride (CaCl₂) (20 mM) on photosynthetic gas exchange, photosystem II photochemistry, and the activities of antioxidant enzymes in tobacco plants under high temperature stress (43 °C for 2 h) were investigated. Heat stress resulted in a decrease in net photosynthetic rate (P_n), stomatal conductance as well as the apparent quantum yield (AQY) and carboxylation efficiency (CE) of photosynthesis. Heat stress also caused a decrease of the maximal photochemical efficiency of primary photochemistry (F_v/F_m). On the other hand, CaCl₂ application improved P_n, AQY, and CE as well as F_v/F_m under high temperature stress. Heat stress reduced the activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POD), whereas the activities of these enzymes either decreased less or increased in plants pretreated with CaCl₂; glutathione reductase (GR) activity increased under high temperature, and it increased more in plants pretreated with CaCl₂. There was an obvious accumulation of H₂O₂ and O₂⁻ under high temperature, but CaCl₂ application decreased the contents of H₂O₂ and O₂⁻ under heat stress conditions. Heat stress induced the level of heat shock protein 70 (HSP70), while CaCl₂ pretreatment enhanced it. These results suggested that photosynthesis was improved by CaCl₂ application in heat-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the thermostability of oxygen-evolving complex (OEC), which might be due to less accumulation of reactive oxygen species.     

Mutation of a vitelline membrane protein, BmEP80, is responsible for the silkworm “Ming” lethal egg mutant

The egg stage is an important stage in the silkworm (Bombyx mori) life cycle. Normal silkworm eggs are usually short, elliptical, and laterally flattened, with a sometimes hollowed surface on the lateral side. However, the eggs laid by homozygous recessive “Ming” lethal egg mutants (l-e^m) lose water and become concaved around 1 h, ultimately exhibiting a triangular shape on the egg surfaces. We performed positional cloning, and narrowed down the region containing the gene responsible for the l-e^m mutant to 360 kb on chromosome 10 using 2287 F₂ individuals. Using expression analysis and RNA interference, the best l-e^m candidate gene was shown to be BmEP80. The results of the inverse polymerase chain reaction showed that an ~ 1.9 kb region from the 3′ untranslated region of BmVMP23 to the forepart of BmEP80 was replaced by a > 100 kb DNA fragment in the l-e^m mutant. Several eggs laid by the normal moths injected with BmEP80 small interfering RNAs were evidently depressed and exhibited a triangular shape on the surface. The phenotype exhibited was consistent with the eggs laid by the l-e^m mutant. Moreover, two-dimensional gel electrophoresis showed that the BmEP80 protein was expressed in the ovary from the 9th day of the pupa stage to eclosion in the wild-type silkworm, but was absent in the l-e^m mutant. These results indicate that BmEP80 is responsible for the l-e^m mutation.     

Probing the FQR and NDH activities involved in cyclic electron transport around Photosystem I by the ‘afterglow’ luminescence

Far-red illumination of plant leaves for a few seconds induces a delayed luminescence rise, or afterglow, that can be measured with the thermoluminescence technique as a sharp band peaking at around 40-45 °C. The afterglow band is attributable to a heat-induced electron flow from the stroma to the plastoquinone pool and the PSII centers. Using various Arabidopsis and tobacco mutants, we show here that the electron fluxes reflected by the afterglow luminescence follow the pathways of cyclic electron transport around PSI. In tobacco, the afterglow signal relied mainly on the ferredoxin-quinone oxidoreductase (FQR) activity while the predominant pathway responsible for the afterglow in Arabidopsis involved the NAD(P)H dehydrogenase (NDH) complex. The peak temperature T_m of the afterglow band varied markedly with the light conditions prevailing before the TL measurements, from around 30 °C to 45 °C in Arabidopsis. These photoinduced changes in T_m followed the same kinetics and responded to the same light stimuli as the state 1-state 2 transitions. PSII-exciting light (leading to state 2) induced a downward shift while preillumination with far-red light (inducing state 1) caused an upward shift. However, the light-induced downshift was strongly inhibited in NDH-deficient Arabidopsis mutants and the upward shift was cancelled in plants durably acclimated to high light, which can perform normal state transitions. Taken together, our results suggest that the peak temperature of the afterglow band is indicative of regulatory processes affecting electron donation to the PQ pool which could involve phosphorylation of NDH. The afterglow thermoluminescence band provides a new and simple tool to investigate the cyclic electron transfer pathways and to study their regulation in vivo.     

Fluorescence induction in the phycobilisome-containing cyanobacterium Synechococcus sp PCC 7942: Analysis of the slow fluorescence transient

At room temperature, the chlorophyll (Chl) a fluorescence induction (FI) kinetics of plants, algae and cyanobacteria go through two maxima, P at ∼ 0.2-1 and M at ∼ 100-500 s, with a minimum S at ∼ 2-10 s in between. Thus, the whole FI kinetic pattern comprises a fast OPS transient (with O denoting origin) and a slower SMT transient (with T denoting terminal state). Here, we examined the phenomenology and the etiology of the SMT transient of the phycobilisome (PBS)-containing cyanobacterium Synechococcus sp PCC 7942 by modifying PBS → Photosystem (PS) II excitation transfer indirectly, either by blocking or by maximizing the PBS → PS I excitation transfer. Blocking the PBS → PS I excitation transfer route with N-ethyl-maleimide [NEM; A. N. Glazer, Y. Gindt, C. F. Chan, and K.Sauer, Photosynth. Research 40 (1994) 167-173] increases both the PBS excitation share of PS II and Chl a fluorescence. Maximizing it, on the other hand, by suspending cyanobactrial cells in hyper-osmotic media [G. C. Papageorgiou, A. Alygizaki-Zorba, Biochim. Biophys. Acta 1335 (1997) 1-4] diminishes both the PBS excitation share of PS II and Chl a fluorescence. Here, we show for the first time that, in either case, the slow SMT transient of FI disappears and is replaced by continuous P → T fluorescence decay, reminiscent of the typical P → T fluorescence decay of higher plants and algae. A similar P → T decay was also displayed by DCMU-treated Synechococcus cells at 2 °C. To interpret this phenomenology, we assume that after dark adaptation cyanobacteria exist in a low fluorescence state (state 2) and transit to a high fluorescence state (state 1) when, upon light acclimation, PS I is forced to run faster than PS II. In these organisms, a state 2 → 1 fluorescence increase plus electron transport-dependent dequenching processes dominate the SM rise and maximal fluorescence output is at M which lies above the P maximum of the fast FI transient. In contrast, dark-adapted plants and algae exist in state 1 and upon illumination they display an extended P → T decay that sometimes is interrupted by a shallow SMT transient, with M below P. This decay is dominated by a state 1 → 2 fluorescence lowering, as well as by electron transport-dependent quenching processes. When the regulation of the PBS → PS I electronic excitation transfer is eliminated (as for example in hyper-osmotic suspensions, after NEM treatment and at low temperature), the FI pattern of Synechococcus becomes plant-like.     

Early Zn2+-induced effects on membrane potential account for primary heavy metal susceptibility in tolerant and sensitive Arabidopsis species

Background and Aims

Uptake of heavy metals by plant root cells depends on electro-physiological parameters of the plasma membrane. In this study, responses of the plasma membrane in root cells were analysed where early reactions to the metal ion-induced stress are localized. Three different Arabidopsis species with diverse strategies of their adaptation to heavy metals were compared: sensitive Arabidopsis thaliana and tolerant A. halleri and A. arenosa.

Methods

Plants of A. thaliana Col-0 ecotype and plants of A. arenosa and A. halleri originating from natural metallicolous populations were exposed to high concentrations of Zn²⁺. Plants were tested for root growth rate, cellular tolerance, plant morphology and cell death in the root apex. In addition, the membrane potential (E_M) of mature cortical root cells and changes in the pH of the liquid culture media were measured.

Key Results

Primary roots of A. halleri and A. arenosa plants grew significantly better at increased Zn²⁺ concentrations than A. thaliana plants. Elevated Zn²⁺ concentrations in the culture medium induced rapid changes in E_M. The reaction was species-specific and concentration-dependent. Arabidopsis halleri revealed the highest insensitivity of the plasma membrane and the highest survival rate under prolonged treatment with extra-high concentrations. Plants were able to effectively adjust the pH in the control, but much less at Zn²⁺-induced lower pH.

Conclusions

The results indicate a similar mode of early reaction to Zn²⁺, but with different extent in tolerant and sensitive species of Arabidopsis. The sensitivity of A. thaliana and a high tolerance of A. halleri and A. arenosa were demonstrated. Plasma membrane depolarization was lowest in the hyperaccumulator A. halleri and highest in A. thaliana. This indicates that rapid membrane voltage changes are an excellent tool to monitor the effects of heavy metals.