Photosystem II Yield Research Articles

Photoprotection and recovery of photosystem II in the Southern Ocean phytoplankton

The future shoaling of surface mixed layer depth due to global warming will expose natural assemblages of phytoplankton to increased mean light. Under these conditions, photoprotective acclimation against high light can determine ecological success. We investigated photoprotective responses to sunlight and recovery from photodamage of photosystem II (PSII) in natural assemblages north and south of the Polar Front (PF). The decrease in the maximum quantum yield (Fv/Fm) of PSII during direct sunlight exposure for 2 h was moderated progressively by the enhancement of diatoxanthin synthesis. When the light-exposed cells were incubated under three reduced light conditions, Fv/Fm recovered to more than the initial values north of the PF but did not reach initial values south of the PF. Temperatures higher in the north than the south of the PF could have induced the faster recovery from photodamage of PSII in assemblages north of the PF. These northern assemblages may be able to acclimate to fast-changing light conditions.

Photosystem II Functionality in Barley Responds Dynamically to Changes in Leaf Manganese Status.

A catalytic manganese (Mn) cluster is required for the oxidation of water in the oxygen-evolving complex (OEC) of photosystem II (PSII) in plants. Despite this essential role of Mn in generating the electrons driving photosynthesis, limited information is available on how Mn deficiency affects PSII functionality. We have here used parameters derived from measurements of fluorescence induction kinetics (OJIP transients), non-photochemical quenching (NPQ) and PSII subunit composition to investigate how latent Mn deficiency changes the photochemistry in two barley genotypes differing in Mn efficiency. Mn deficiency caused dramatic reductions in the quantum yield of PSII and led to the appearance of two new inflection points, the K step and the D dip, in the OJIP fluorescence transients, indicating severe damage to the OEC. In addition, Mn deficiency decreased the ability to induce NPQ in the light, rendering the plants incapable of dissipating excess energy in a controlled way. Thus, the Mn deficient plants became severely affected in their ability to recover from high light-induced photoinhibition, especially under strong Mn deficiency. Interestingly, the Mn-efficient genotype was able to maintain a higher NPQ than the Mn-inefficient genotype when exposed to mild Mn deficiency. However, during severe Mn deficiency, there were no differences between the two genotypes, suggesting a general loss of the ability to disassemble and repair PSII. The pronounced defects of PSII activity were supported by a dramatic decrease in the abundance of the OEC protein subunits, PsbP and PsbQ in response to Mn deficiency for both genotypes. We conclude that regulation of photosynthetic performance by means of maintaining and inducing NPQ mechanisms contribute to genotypic differences in the Mn efficiency of barley genotypes growing under conditions with mild Mn deficiency.

Relative sensitivity of DNA and photosystem II in Ulva intestinalis (Chlorophyta) under natural solar irradiation

High intensities of sunlight can result in DNA and photosystem II (PSII) damage. However, the relative sensitivity of both these targets under natural sunlight and especially over a long period has not been studied in algae so far. Although DNA damage is highly specifically induced by ultraviolet-B radiation (UVB, 280-315 nm), PSII is inactivated by a broad spectrum. The green macroalga Ulva intestinalis is an appropriate and interesting study organism with which to investigate the relative importance of the 2 different targets of sunlight because this alga contains no UV-screening protective pigments, although it is exposed to strong solar irradiation in its natural habitat. This entails a high risk of DNA damage. Therefore, diel time courses and long-term development of DNA damage and the optimal quantum yield of PSII (F-v/F-m) were studied in situ. F-v/F-m was extremely reduced at noon, but a fast recovery was observed in the afternoon. As dark-adapted basal fluorescence (F-o) of PSII was substantially decreased during the day, non-photochemical quenching is suggested to be a key photoprotective strategy in U. intestinalis. In contrast, even in samples with strongly reduced F-v/F-m, only very low DNA damage was found, irrespective of the accumulated UVB dose. We propose that efficient photoreactivation driven by natural sunlight balances the induction of dimers. This leads to a higher UVB tolerance of DNA than that observed in algae under experimental UVB irradition. In this field study, U. intestinalis suffered more from photoinhibition than from DNA damage.

Photosynthetic Performance of Ocimum sanctum Morphotypes in a Semiarid Region

ABSTRACTComparative ecophysiological studies by measurements of photosystem II (PSII) chlorophyll a fluorescence, pigment composition, and proline content were used in the synecological assessments of two Ocimum sanctum (holy basil) morphotypes, viz., Sri Tulsi and Krishna Tulsi, to compare their overall “adaptiveness.” The effective quantum yield of PSII (ΔF/Fm’sat) and maximum apparent electron transport rate (ETRmax) was 30% lower in Krishna Tulsi than in Sri Tulsi plants. Chlorophyll and carotenoid content in Sri Tulsi were twofold higher than in Krishna Tulsi. Results indicated that differences in photosynthetic efficiency observed in Sri Tulsi may determine ecological dominance.

Physiological and biochemical responses of photomorphogenic tomato mutants (cv. Micro-Tom) under water withholding

In addition to mediating photomorphogenesis, phytochromes are responsible for many abiotic stress responses, acting upon biochemical and molecular mechanisms of cell signaling. In this work, we measured the physiological and biochemical responses of phytochrome-mutant plants under water stress. In tomato (Solanum lycopersicum L.), the aurea mutant (au) is phytochrome-deficient and the high-pigment-1 mutant (hp1) has exaggerated light responses. We examined the effects of water withholding on water potential, leaf gas exchange, chlorophyll fluorescence, chloroplast pigment content and antioxidant enzyme activity in au and hp1 and their wild-type cultivar Micro-Tom (MT). Initial fluorescence and potential quantum efficiency of photosystem II (PSII) photochemistry were not affected by the treatment, but effective quantum yield of PSII, electron transport rate decreased and non-photochemical quenching increased significantly in MT. Under water withholding conditions, MT had higher malondialdehyde concentration than the mutants, but au had higher activities of catalase and ascorbate peroxidase compared to the other genotypes. The tolerance of mutants to the effects of water withholding may be explained by the higher activity of antioxidant enzymes in au and by a higher concentration of antioxidant compounds, such as carotenoids, in hp1.

Differential responses of photosystems I and II to seasonal drought in two Ficus species

Hemiepiphytic Ficus species exhibit more conservative water use strategy and are more drought-tolerant compared with their non-hemiepiphytic congeners, but a difference in the response of photosystem I (PSI) and photosystem II (PSII) to drought stress has not been documented to date. The enhancement of non-photochemical quenching (NPQ) and cyclic electron flow (CEF) have been identified as important mechanisms that protect the photosystems under drought conditions. Using the hemiepiphytic Ficus tinctoria and the non-hemiepiphytic Ficus racemosa, we studied the water status and the electron fluxes through PSI and PSII under seasonal water stress. Our results clearly indicated that the decline in the leaf predawn water potential (ψpd), the maximum photosynthetic rate (Amax) and the predawn maximum quantum yield of PSII (Fv/Fm) were more pronounced in F. racemosa than in F. tinctoria at peak drought. The Fv/Fm of F. racemosa was reduced to 0.69, indicating net photoinhibition of PSII. Concomitantly, the maximal photo-oxidizable P700 (Pm) decreased significantly in F. racemosa but remained stable in F. tinctoria. The fraction of non-photochemical quenching [Y(NPQ)] and the ratio of effective quantum yield of PSI to PSII [Y(I)/Y(II)] increased for both Ficus species at peak drought, with a stronger increase in F. racemosa. These results indicated that the enhancement of NPQ and the activation of CEF contributed to the photoprotection of PSI and PSII for both Ficus species under seasonal drought, particularly for F. racemosa.

Photosynthetic response of soybean to L-DOPA and aqueous extracts of velvet bean

L-3,4-Dihydroxyphenylalanine (L-DOPA) is an allelochemical released by roots of velvet bean (Mucuna pruriens) that affects the growth of several plant species. However, its mechanism of action is inconclusive. In this work, we compared the effects of L-DOPA (0.01–1.0 mmol L−1) and of aqueous extracts (300, 1200, and 3000 mg L−1) of velvet bean on growth and photosynthesis (gas exchange and chlorophyll a fluorescence) of soybean (Glycine max). Overall, the results showed that both L-DOPA and aqueous extracts of velvet bean reduced the growth, leaf area, photosynthetic rate (A), stomatal conductance (gs), transpiration (E), and quantum yield of electron flow through photosystem II (PSII) in vivo (ΦF). In addition, L-DOPA and aqueous extracts increased the internal CO2 concentration (Ci) and the leaf wax and trichome density on the leaf surface, while the maximum quantum yield of PSII (Fv/Fm) was not changed. These results suggest that the reduction of A should not be related exclusively to the stomatal closure, but also to limitations of the carbon metabolism, as indicated by the increase of Ci and decrease of ΦF. Briefly, we concluded that soybean growth inhibition by L-DOPA and aqueous extracts of velvet bean is due to the combination of damage in the root meristem and reduction in A.

Photoinhibition and photoinhibition-like damage to the photosynthetic apparatus in tobacco leaves induced by pseudomonas syringae pv. Tabaci under light and dark conditions.

BackgroundPseudomonas syringae pv. tabaci (Pst), which is the pathogen responsible for tobacco wildfire disease, has received considerable attention in recent years. The objective of this study was to clarify the responses of photosystem I (PSI) and photosystem II (PSII) to Pst infection in tobacco leaves.ResultsThe net photosynthetic rate (Pn) and carboxylation efficiency (CE) were inhibited by Pst infection. The normalized relative variable fluorescence at the K step (Wk) and the relative variable fluorescence at the J step (VJ) increased while the maximal quantum yield of PSII (Fv/Fm) and the density of QA-reducing PSII reaction centers per cross section (RC/CSm) decreased, indicating that the reaction centers, and the donor and acceptor sides of PSII were all severely damaged after Pst infection. The PSI activity decreased as the infection progressed. Furthermore, we observed a considerable overall degradation of PsbO, D1, PsaA proteins and an over-accumulation of reactive oxygen species (ROS).ConclusionsPhotoinhibition and photoinhibition-like damage were observed under light and dark conditions, respectively, after Pst infection of tobacco leaves. The damage was greater in the dark. ROS over-accumulation was not the primary cause of the photoinhibition and photoinhibition-like damage. The PsbO, D1 and PsaA proteins appear to be the targets during Pst infection under light and dark conditions.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0723-6) contains supplementary material, which is available to authorized users.

Differences in the Responses of Photosystems I and II in Cymbidium sinense and C. tracyanum to Long-Term Chilling Stress.

The susceptibility of photosystem I (PSI) and photosystem II (PSII) to chilling stress depends on plant species, and cyclic electron flow (CEF) plays an important role in photoprotection for some species under short stress periods. However, little is known about the responses of PSI and PSII to long-term chilling stress. We studied two orchid species—Cymbidium sinense and C. tracyanum— that differ in their capacity to adapt to low temperature, and exposed plants for 19 d to stress conditions that included 4°C and a light intensity of 250 to 350 μmol photons m-2 s-1. Meanwhile, we investigated their dynamic variations in Chl fluorescence and P700 parameters. After exposure to 4°C and 250 μmol photons m-2 s-1 for 6 h, PSI activity was maintained stable in both species, but stronger PSII photoinhibition was observed in C. sinense. During the long-term treatment, the maximum quantum yield of PSII was significantly reduced, with that decrease being greater in C. sinense. After 19 d of chilling treatment, the maximum photo-oxidizable P700 declined only slightly in C. tracyanum but dropped significantly in C. sinense. Linear electron flow was largely depressed during the long-term chilling treatment, especially in C. sinense. Meanwhile, C. tracyanum showed higher CEF activity than C. sinense. These results indicate that PSII is more sensitive to chilling-light stress than PSI in both species. The rate of PSII photodamage at chilling-light stress is higher in C. sinense than C. tracyanum, and CEF contributes to photoprotection for PSI and PSII under long-term chilling stress in C. tracyanum.

Physiological characterization of grapevine rootstocks grown in soil with increasing zinc doses

ABSTRACTThis study aimed to evaluate the performance of grapevine rootstocks under increasing levels of Zn in the soil and to identify physiological variables that can be used as indicators of excess of Zn in the soil. The rootstocks SO4, Paulsen1103, IAC572, IAC313 and 420A were grown in pots containing soil, which received Zn doses of 0, 20, 40, 80 or 160 mg kg-1 of soil. Dry matter (DM), Zn content in shoots and roots, chlorophyll index, initial fluorescence (Fo), maximum fluorescence (Fm), maximum quantum yield of photosystem II (Fv/Fm), effective quantum yield of photosystem II (Y-II) and non-photochemical quenching (NPQ) were evaluated. The increase of Zn levels in the soil decreased DM in all rootstocks, and IAC572 was superior to the others. The variation in the indices of chlorophyll a and b had little expression in relation the soil Zn levels, but allowed identifying that the rootstocks Paulsen 1103, 420A and SO4 are sensitive to Zn toxicity and that IAC572 and IAC313 were not sensitive to the tested levels. Fluorescence analysis showed a negative effect of Zn contents on the variables Fo, Fm, Y-II and NPQ in all rootstocks, which proved to be good indicators of Zn phytotoxicity.

Effect of arbuscular mycorrhizal inoculation on water status and photosynthesis of Populus cathayana males and females under water stress.

Drought is one of the most serious environmental limitations for poplar growth. Although the ways in which plants deal with water stress and the effects of arbuscular mycorrhizal (AM) formation have been well documented, little is known about how the male and female plants of Populus cathayana respond to drought and AM formation. We also aimed to investigate the potential role of AM fungi in maintaining gender balance. We tested the impact of drought and AM formation on water status and photosynthesis. The results suggested that both sexes showed similar responses to water stress: drought decreased the growth of stem length (GSL), growth of ground diameter (GGD), relative water content (RWC), increased the relative electrolyte leakage (REL), and limited the photosynthesis and chlorophyll fluorescence indexes. However, the responses of the two sexes to drought and AM formation differed to some extent. AM formation had positive effects on RWC, photosynthesis and the intrinsic water use efficiency (WUEi) but negative effects on the REL of males and females, especially under drought. AM formation enhanced the maximum quantum yield of photosystem II (PSII) (Fv/Fm), the actual quantum yield of PSII (ΦPSII), non-photochemical quenching (qN) and photochemical quenching (qP) under drought conditions, and had no significant effects under well-watered conditions except on the qP of males. Principal component analysis showed that males were significantly more drought tolerant than females, and AM formation enhanced drought tolerance, particularly among males, which suggested that AM fungi are beneficial for ecological stability and for P. cathayana survival under drought conditions.

Growth and photosynthetic responses of Lemna minor L. exposed to cadmium in combination with zinc or copper.

Metals have a variety of negative outcomes on plants, essential components of any ecosystem. The effects of CdCl2 (5 μmol L-1), ZnCl2 (25 or 50 μmol L-1), and CuCl2 (2.5 or 5 μmol L-1) and combinations of CdCl2 with either ZnCl2 or CuCl2 on the growth, photosynthetic pigments, and photosystem II (PSII) efficiency of duckweed (Lemna minor L.) were investigated. All of the treatments caused growth inhibition and remarkable metal accumulation in plant tissue after 4 and 7 days. In the combined treatments, the accumulation of each metal applied was lesser in comparison to treatments with single metals. After 4 days, all of the treatments generally diminished chlorophyll a content and decreased the maximum quantum yield (Fv/Fm) and effective quantum yield (ΔF/F'm) of PSII. However, after 7 days of exposure to a combination of Cd and Zn, pigment content and PSII activity recovered to control levels. A higher concentration of Cu (5 μmol L-1) as well as Cd in combination with Cu had a prolonged inhibitory effect on photosynthetic features. Our results suggest that growth inhibition was due to the toxic effect of absolute metal quantity in plant tissue. Zn counteracted Cd uptake, as seen from the recovery of pigment content and PSII efficiency in plants exposed for 7 days to the Cd and Zn combination. Cu-induced oxidative stress led to a prolonged inhibitory effect in plants treated both with a higher concentration of Cu (5 μmol L-1) and simultaneously with Cd and Cu. Our findings could contribute to general knowledge on anthropogenic and environmental contaminants that endanger plant communities and significantly disrupt the sensitive balance of an ecosystem by influencing photosynthetic mechanisms.

Salt pretreatment alleviated salt-induced photoinhibition in sweet sorghum

Sweet sorghum is an important energy crop. This study aimed to investigate the effects of salt pretreatment on the interaction between photosystem II (PSII) and photosystem I (PSI) upon salt stress. In this study, sweet sorghum was pretreated with 150 mM NaCl for 10 days, and subsequently, the pretreated plants were subjected to severe salt stress at 300 mM NaCl. PSII and PSI photoinhibition occurred in non-pretreated plants after 4 days of salt stress, as the maximum quantum yield of PSII (Fv/Fm) and the maximal photochemical capacity of PSI (Delta MR/MR0) significantly decreased, and their normal coordination was destroyed. The significant positive correlation between Fv/Fm and Delta MR/MR0 under salt stress indicated that PSII photoinhibition was in relation to PSI photoinhibition, and PSI photoinhibition might lead to PSII photoinhibition through inhibiting electron transport at the acceptor side of PSII. Salt stress did not induce PSII photoinhibition in salt-pretreated plants, and thus, salt pretreatment protected PSI against photoinhibition not by aggravating PSII photoinhibition. Salt pretreatment mitigated the decrease in CO2 assimilation, reduced the feedback inhibition on photosynthetic electron transport and then contributed to suppressing PSI and PSII photoinhibition in sweet sorghum under salt stress. Therefore, the normal coordination between PSII and PSI was maintained in salt-pretreated plants. In conclusion, salt pretreatment ensured normal PSII and PSI coordination by preventing photoinhibition in sweet sorghum under salt stress.

Assessing temporal and spatial variation in sensitivity of communities of periphyton sampled from agroecosystem to, and ability to recover from, atrazine exposure

Lotic systems in agriculturally intensive watersheds can experience short-term pulsed exposures of pesticides as a result of runoff associated with rainfall events following field applications. Of special interest are herbicides that could potentially impair communities of primary producers, such as those associated with periphyton. Therefore, this study examined agroecosystem-derived lotic periphyton to assess (1) variation in community sensitivity to, and ability to recover from, acute (48h) exposure to the photosystem II (PSII)-inhibiting herbicide atrazine across sites and time, and (2) attempt to determine the variables (e.g., community structure, hydrology, water quality measures) that were predictive for observed differences in sensitivity and recovery. Periphyton were sampled from six streams in the Midwestern U.S. on four different dates in 2012 (April to August). Field-derived periphyton were exposed in the laboratory to concentrations of atrazine ranging from 10 to 320μg/L for 48h, followed by untreated media for evaluation of recovery for 48h. Effective quantum yield of PSII was measured after 24h and 48h exposure and 24h and 48h after replacement of media. Inhibition of PSII EC50 values ranged from 53 to >320µg/L. The majority of periphyton samples (16 out of 22) exposed to atrazine up to 320µg/L recovered completely by 48h after replacement of media. Percent inhibition of effective quantum yield of PSII in periphyton (6 of 22 samples) exposed to 320µg/L atrazine that were significantly lower than controls after 48h ranged from 2% to 24%. No distinct spatial or temporal trends in sensitivity and recovery potential were observed over the course of the study. Conditional inference forest analysis and variation partitioning were used to investigate potential associations between periphyton sensitivity to and ability to recover from exposure to atrazine. Although certain environmental variables (i.e., proximity of high flow/velocity events and dissolved solutes) were significantly associated with sensitivity to atrazine, recovery was not significantly associated with any variables, which is predicted by the rapid reversible binding at PSII. Consistent and rapid recovery of effective quantum yield of PSII across sites and sampling dates indicates that acute exposure to atrazine is unlikely to adversely affect function of these communities in their current state in intensive agroecosystems.

Contrasting photosynthesis and photoinhibition in tetraploid and its autodiploid honeysuckle (Lonicera japonica Thunb.) under salt stress.

Honeysuckle (Lonicera japonica Thunb.) is a popular landscape plant. This study was to explore leaf photosynthetic characterization with emphasis on the coordination between photosystem II (PSII) and photosystem I (PSI) in tetraploid and its autodiploid honeysuckle (TH and DH) upon salt stress (300 mM NaCl). Leaf photosynthetic rate and carboxylation efficiency in DH and TH were significantly decreased under salt stress, and the decrease was greater in DH. PSII photoinhibition was induced in DH under salt stress, as the maximum quantum yield of PSII (Fv/Fm) was significantly decreased. PSII photoinhibition declined electron flow to PSI, but did not prevent PSI photoinhibition, as the maximal photochemical capacity of PSI (MR/MR0) was significantly decreased by salt stress. According to the significant decrease in PSI oxidation amplitude in the first 1 s red illumination, PSI photoinhibition was more severe than PSII photoinhibition. As a result, PSII and PSI coordination was destroyed. Comparatively, salt-induced photoinhibition did not occur in TH, as no significant change was observed in Fv/Fm and MR/MR0. Consequently, PSII and PSI coordination was not significantly affected by salt stress. In conclusion, TH maintained normal coordination between PSII and PSI by preventing photoinhibition and exhibited higher leaf photosynthetic activity than DH under salt stress. Compared with DH, lower leaf ionic toxicity due to greater root Na+ extrusion and restriction of Na+ transport to leaf might be responsible for maintaining higher leaf photosynthetic capacity in TH under salt stress.

Mechanism of interaction of Al3+ with the proteins composition of photosystem II.

The inhibitory effect of Al3+on photosystem II (PSII) electron transport was investigated using several biophysical and biochemical techniques such as oxygen evolution, chlorophyll fluorescence induction and emission, SDS-polyacrylamide and native green gel electrophoresis, and FTIR spectroscopy. In order to understand the mechanism of its inhibitory action, we have analyzed the interaction of this toxic cation with proteins subunits of PSII submembrane fractions isolated from spinach. Our results show that Al 3+, especially above 3 mM, strongly inhibits oxygen evolution and affects the advancement of the S states of the Mn4O5Ca cluster. This inhibition was due to the release of the extrinsic polypeptides and the disorganization of the Mn4O5Ca cluster associated with the oxygen evolving complex (OEC) of PSII. This fact was accompanied by a significant decline of maximum quantum yield of PSII (Fv/Fm) together with a strong damping of the chlorophyll a fluorescence induction. The energy transfer from light harvesting antenna to reaction centers of PSII was impaired following the alteration of the light harvesting complex of photosystem II (LHCII). The latter result was revealed by the drop of chlorophyll fluorescence emission spectra at low temperature (77 K), increase of F0 and confirmed by the native green gel electrophoresis. FTIR measurements indicated that the interaction of Al 3+ with the intrinsic and extrinsic polypeptides of PSII induces major alterations of the protein secondary structure leading to conformational changes. This was reflected by a major reduction of α-helix with an increase of β-sheet and random coil structures in Al 3+-PSII complexes. These structural changes are closely related with the functional alteration of PSII activity revealed by the inhibition of the electron transport chain of PSII.

Introduction of the Arabidopsis PHYB gene increases resistance of photosynthetic apparatus in transgenic Solanum tuberosum plants to UV-B radiation

The influence of phytochrome B overproduction in 50- to 60-day-old transgenic potato plants (Solanum tuberosum L., lines Dara 5 and Dara 12 with moderate and intense expression of PHYB, respectively) on the resistance of photosynthetic apparatus to UV-B irradiation was investigated. In plants unexposed to UV-B, there was no significant difference in photosynthetic rates (P n) and fluorescence parameters (F v/F m, qN, qP) between the nontransformed (NT) line and Dara-5 and Dara-12 lines, whereas the content of photosynthetic pigments per 1 cm2 leaf area was higher in the transgenic plants. Irradiation with UV-B resulted in the decrease in photosynthetic rate in NT plants by 35–45%, whereas in Dara-12 line this rate was lowered by only 20–25%. Exposure to UV-B reduced the amplitudes of both fast and slow components of delayed fluorescence (DF), which indicated the diminished efficiency of photosystem II (PSII). The decrease in the maximum amplitude of the slow DF component was markedly lower in Dara-12 (19%) than in NT line (33%). The maximal photochemical quantum yield of PSII (F v/F m ratio) in plants exposed to UV-B was also suppressed stronger in NT line than in Dara-12. The line Dara-5 had intermediate position among other lines in terms of UV-B resistance of photosynthesis and PSII activity, but it was closer to NT than to Dara-12. Thus, the potato plants actively expressing the gene of Arabidopsis phytochrome B apoprotein (PHYB) demonstrated a higher resistance of photosynthetic apparatus to UV-B radiation compared to nontransformed plants. The elevated UV-B tolerance is most likely related to the increased leaf content of chlorophyll, carotenoids, and flavonoids.

A miniature bioassay for testing the acute phytotoxicity of photosystem II herbicides on seagrass.

Photosystem II (PSII) herbicides have been detected in nearshore tropical waters such as those of the Great Barrier Reef and may add to the pressure posed by runoff containing sediments and nutrients to threatened seagrass habitats. There is a growing number of studies into the potential effects of herbicides on seagrass, generally using large experimental setups with potted plants. Here we describe the successful development of an acute 12-well plate phytotoxicity assay for the PSII herbicide Diuron using isolated Halophila ovalis leaves. Fluorescence images demonstrated Diuron affected the entire leaf surface evenly and responses were not influenced by isolating leaves from the plant. The optimum exposure duration was 24 h, by which time the inhibition of effective quantum yield of PSII (∆F/Fm’) was highest and no deterioration of photosystems was evident in control leaves. The inhibition of ∆F/Fm’ by Diuron in isolated H. ovalis leaves was identical to both potted and hydroponically grown plants (with leaves remaining attached to rhizomes), indicating similar reductions in photosynthetic activity in these acute well-plate assays. The sensitivity of the assay was not influenced by irradiance (range tested 40 to 400 μmol photons m-2 s-1). High irradiance, however, caused photo-oxidative stress in H. ovalis and this generally impacted in an additive or sub-additive way with Diuron to damage PSII. The bioassay using isolated leaves is more rapid, uses far less biological material and does not rely on specialised aquarium facilities in comparison with assays using potted plants. The development and validation of this sensitive bioassay will be useful to reliably screen and monitor the phytotoxicity of existing and emerging PSII herbicides and contribute to risk assessments and water quality guideline development in the future.

Assessment of phytotoxicity of anthracene in soybean (Glycine max) with a quick method of chlorophyll fluorescence.

A decrease in photosynthetic efficiency may indicate the toxic effects of environmental pollutants on higher plants. Measurement of chlorophyll (Chl) a fluorescence to assess the performance of photosystem II (PSII) was used as an bioindicator of toxicity of the polycyclic aromatic hydrocarbon (PAH) anthracene (ANT) in soybean plants. The results revealed that ANT treatment caused a reduction in quantum yield of PSII, damage to the oxygen evolving complex, as well as a significant reduction in performance index of PSII. However, change in performance index was more prominent, and it seems that the performance index is a more sensitive parameter to environmental contaminants. Moreover, a change in heterogeneity of PSII was also observed. The number of active reaction centres decreased with increasing concentration of ANT, as secondary plastoquinone reducing centres were converted into non-reducing centres, and PSIIα centres were converted into PSIIβ and PSIIγ centres. The influence of ANT on PSII heterogeneity could be an important reason for reductions in the PSII performance.

Fluorescence phenotyping in blueberry breeding for genotype selection under drought conditions, with or without heat stress

Lack of water and increase in ambient temperature, caused by climate change, are already affecting agriculture worldwide. These factors will affect the physiology and development of plants in general, including blueberry plants (Vaccinium spp.). With this in mind, six cultivars of highbush blueberry (Vaccinium corymbosum L.) (‘Star’, ‘Bluecrisp’, ‘Jewel’, ‘Bluegold’, ‘Elliott’ and ‘Liberty’) and two rabbiteye cvs. (Vaccinium ashei R.) (‘Bonita’ and ‘Powderblue’) were subjected to two water treatments: continuous irrigation (Full irrigation–FI); and with a water deficit (only one third of the volume of water, water deficit–WD). Both treatments were applied in two greenhouses one of which represented ambient conditions (At) and the other simulated heat stress conditions (At+10°C). Measurements were made of chlorophyll fluorescence, stem water potential (Ψs), chlorophyll content, leaf temperature and SPAD. In At conditions, cultivars showed differences in most parameters of chlorophyll fluorescence, but only the quantum yield of energy conversion of non-photochemical quenching (Y(NPQ)) and Ψs were significant, along with interactions between cultivars and irrigation treatments. In addition, cultivars differed in the maximum rate of electron transport (ETRmax), IK and effective photochemical quantum yield of PSII (Y(II)), indicating differences in the efficiency of photosystem II (PSII). Under At+10°C conditions, there were significant interactions in the minimum fluorescence in the dark-adapted state (F0), ETRmax, IK, Y(II), Ψs and photochemical quenching (qP and qL). Thus indicating that when subjected to the two combined stresses (WD−At+10°C) the cultivars showed different responses in the efficiency and operation of PSII. The results of this study indicate that the fluorescence parameters provide a good tool for phenotyping in blueberry breeding programs and enable the detection and elimination of unwanted genotypes at the beginning of the selection process.