Quantum Yield Of Photosystem II Research Articles

Influence of the magnesium content in apple leaves photosynthetic capacity

Fruit trees photosynthesis is determined by abiotic factors affecting crop productivity. The aim of this study was to evaluate the influence of magnesium nutritional status and exposure to solar radiation in the photosynthetic capacity of leaves of Malus domestica 'Cripp's Pink'. A two-factor experiment was conducted: high (+Mg) and low (-Mg) foliar Mg content (0.26 and 0.19%, respectively) in leaves were exposed to high (HR) and low (LR) solar radiation (1600 and 600 μmol m-2 s-1, respectively). Chlorophyll concentration, maximum quantum yield of photosystem-II (Fv/Fm) and net photosynthesis rate was determined in leaves. HR and LR -Mg leaves had 11.5 and 1.8% less chlorophyll concentration respectively than HR and LR +Mg leaves; no effects were observed in chlorophyll concentration by exposure. Fv/Fm was affected by sun exposition but not by Mg content. In HR leaves Fv/Fm was 5-6% lower than in LR leaves. Photosynthesis was affected by magnesium content when leaves where exposed to high solar radiation. Maximum net photosynthesis in HR +Mg leaves was 20.0 μmol m-2 s-1, while in HR -Mg was 11.1 μmol m-2 s-1. Net photosynthesis in LR leaves was not affected by Mg content. These results show that apple leaves photosynthetic capacity could be affected by lower Mg content when exposed to high solar radiation.

Effects of the heavy metal cadmium on photosynthetic activity and the xanthophyll cycle in Phaeodactylum tricornutum

Cadmium (Cd) is one of the most common and widespread heavy metals in the environment. Cd has adverse effects on photosynthesis that are countered by photosystem I (PSI) and photosystem II (PSII); however, the protective responses of these photosystems to heavy metal stress remain unclear. Using the model diatom Phaeodactylum tricornutum, a biological indicator that is widely used to assess the impact of environmental toxins, we simultaneously measured the effects of Cd on PSI and PSII and examined the levels of pigments in response to high light treatments before and after Cd exposure. Cd significantly reduced the quantum yield and electron transport rates of PSI and PSII. The quantum yield of non-photochemical energy dissipation in PSI due to donor side limitation increased faster than the quantum yield due to acceptor side limitation. The Cd treatment activated the P. tricornutum xanthophyll cycle under artificial light conditions, as indicated by an increased diatoxanthin content. Xanthophyll is important for photoprotection; therefore, the accumulation of diatoxanthin may down-regulate PSII activities to reduce oxidative damage. Together, our results suggest that the rapid reduction in PSII activities following Cd exposure is an adaptive response to heavy metal stress that reflects the variable exposure to external stressors in the native P. tricornutum environment.

Photoinactivation, repair and the motility-physiology trade-off in microphytobenthos

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 601:41-57 (2018) - DOI: https://doi.org/10.3354/meps12670 Photoinactivation, repair and the motility-physiology trade-off in microphytobenthos Silja Frankenbach*, William Schmidt, Jörg C. Frommlet, João Serôdio Departamento de Biologia and CESAM - Centro de Estudos do Ambiente e do Mar, Universidade de Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal *Corresponding author: s.frankenbach@ua.pt ABSTRACT: Microphytobenthos (MPB) inhabiting intertidal flats of estuaries form highly productive diatom-dominated biofilms. The capacity to sustain such high photosynthetic activity under conditions prone to cause photoinhibition is thought to be enabled by efficient photoprotective mechanisms, the main ones being the xanthophyll cycle (XC) and vertical migration (VM). This study compared the photoprotective capacity of 2 MPB communities inhabiting contrasting sedimentary habitats and relying on distinct light responses: epipelic communities, colonizing muddy sediments and using motility (VM) to regulate light exposure; and epipsammic communities, inhabiting sandier sediments and relying solely on physiological photoprotection (XC). The efficiency of physiological photoprotection of the 2 communities was compared regarding Photosystem II (PSII) photoinactivation caused by light stress. Lincomycin was used to distinguish photoinactivation from counteracting repair. Rate constants of PSII photoinactivation (kPI) and repair (kREC) were determined on cell suspensions, based on the light and time dependence of maximum quantum yield of PSII, Fv/Fm, as measured using multi-actinic imaging fluorometry. The results show that motile species, in comparison to epipsammic ones, are inherently more susceptible to photoinactivation (higher kPI), less dependent on the XC for preventing photodamage (smaller increase of kPI induced by nigericin) and more efficient regarding repair capacity (higher kREC). The distinct strategies exhibited by epipelic and epipsammic communities to cope with light stress support the hypothesized trade-off between photoprotective motility and photophysiology. Motile forms have a diminished physiological capacity for preventing photodamage and compensate using VM and a better repair capacity. Non-motile epipsammic forms rely mostly on physiological mechanisms to optimize photoprotection capacity. KEY WORDS: Microphytobenthos · Diatoms · Photoinactivation · Photoprotection · Repair · Vertical migration · Photoinhibition Full text in pdf format PreviousNextCite this article as: Frankenbach S, Schmidt W, Frommlet JC, Serôdio J (2018) Photoinactivation, repair and the motility-physiology trade-off in microphytobenthos. Mar Ecol Prog Ser 601:41-57. https://doi.org/10.3354/meps12670 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 601. Online publication date: August 09, 2018 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2018 Inter-Research.

Nutrient allocation and photochemical responses of Populus × canadensis ‘Neva’ to nitrogen fertilization and exogenous Rhizophagus irregularis inoculation

Arbuscular mycorrhizal fungi (AMF) can promote plant growth performance, but their effectiveness varies depending on soil nitrogen (N) availability. To clarify the effectiveness of exogenous AMF along an N-fertilization gradient (0, 2, 10, 20, and 30 mM), the impacts of exogenous Rhizophagus irregularis and N on the growth, photochemical activity, and nutritional status of Populus × canadensis ‘Neva’ in natural soil were evaluated in a pot experiment. The results showed that the 10 mM N level was the optimal fertilization regime with the highest promotion effect on plant growth and the maximum quantum yield of photosystem II (PSII) (Fv/Fm). Excess N (20 and 30 mM) fertilization reduced the actual quantum yield of PSII (ФPSII) and the Fv/Fm of the plants. Regardless of the N availability, inoculated plants exhibited greater Fv/Fm values than did non-inoculated plants. The biomass of inoculated plants was significantly higher compared with the control under low N levels (0 and 2 mM). Under high N levels, inoculated plants showed significant increases in ФPSII. Moreover, the nutrient imbalance of plants inoculated with exogenous R. irregularis was eased by increasing P, Fe, Mn and Cu uptake in roots and higher P, Ca, Mg, Fe, Mn and Zn concentrations in leaves. Moreover, the Fv/Fm and ФPSII exhibited positive correlations with P, Ca, Mg and Zn concentrations in leaves. In conclusion, inoculation with exogenous R. irregularis can benefit plant fitness by improving the photochemical capacity and nutrient composition of poplar under different N levels.

Effects of 1-butyl-3-methylimidazolium chloride on the photosynthetic system and metabolism of maize (Zea mays L.) seedlings

Ionic liquids (ILs) are widely used in various chemical processes. However, a growing number of studies have found that ILs are potentially toxic to different types of living organisms, including crops. The present study analysed the effects of 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) on the photosynthetic system and metabolism of maize seedlings. Results showed that [C4mim]Cl could significantly reduce maize leaf chlorophyll level and cause extensive leaf bleaching. The activity of photosystem II (PSII) was significantly inhibited when seedlings exposed to higher concentration of [C4mim]Cl. The maximum quantum yield of PSII and the potential efficiency of PSII were reduced by 63% and 88% under 800 mg/L [C4mim]Cl treatment in comparison with the control treatment. The RNA sequencing analysis performed to examine gene expression profiles of maize leaves under [C4mim]Cl treatment revealed 639 differentially expressed genes (DEGs), 115 of which were categorized into different metabolic pathways. Among these DEGs, the seven genes involved in the photosynthetic Calvin cycle were down-regulated by [C4mim]Cl exposure. For carbohydrates and amino acids metabolism, the genes for starch synthesis were down-regulated, while the genes for amino acids and protein degradation were up-regulated. The changes observed in these major metabolic pathways might be an important reason for [C4mim]Cl toxicity.

Determination of critical low light limit and adaptive physiological and biochemical traits regulating growth and yield of mustard (Brassica juncea Coss.).

Growth and physio-biochemical traits under different incident solar light intensities (100, 67, 50 and 25%) were studied in mustard in a semi-arid agroclimate region of Central India. Our comprehensive studies revealed that incident solar light intensities below about 67% were highly detrimental in mustard for its growth and grain yield. Major factors that contributed to the differential responses under varying light intensities were identified which holds importance for better understanding of low light adaptability in an important oilseed crop like mustard. Biomass index (ratio of dry biomass to height) has been established and evaluated for the differential growth performance of the crop under different light intensities. Biomass index progressively declined from 0.48 (open sunlight) to 0.11 (25% sunlight). Physio-biochemical factors were identified that were playing major role in manifestation of the differential growth and grain yield. Mustard exhibited its low light adaptive trends through differential down-regulation in the rates of net CO2 assimilation (PN), stomatal conductance, transpiration, thylakoid electron transport rate (ETR) and leaf wax level. For example, PN decreased from 35.88 (open light) to 11.64µmolm-2s-1 (25% sunlight). Photochemical events showed critical impact as evidenced by decreased PSII quantum yield, photochemical quenching (qP) and higher non-photochemical quenching (qN) that were clearly associated with physiological efficiency of the plants under varying light intensities. Leaf wax level decreased from 1.69mgg-1 fresh weight (open light) to 0.96mgg-1 fresh weight (25% sunlight). Our results indicated that limited ETR supply across photosystem II (PSII) decreased the photochemical efficiency and carbon gain under low light which resulted in reduction of biomass index and grain yield. Besides, it was found that overexpression of protein band around ~ 26kDa in low light could be another adaptive feature for mustard related to light harvesting complex. Our findings would augment selection of traits for optimizing growth and grain yield of mustard for low light or light limiting agro-ecosystem.

Moderate Photoinhibition of Photosystem II Significantly Affects Linear Electron Flow in the Shade-Demanding Plant Panax notoginseng.

Although photoinhibition of photosystem II (PSII) frequently occurs under natural growing conditions, knowledge about the effect of moderate photoinhibition on linear electron flow (LEF) remains controversial. Furthermore, mechanisms underlying the decrease in LEF upon PSII photoinhibition are not well clarified. We examined how selective PSII photoinhibition influenced LEF in the attached leaves of shade-demanding plant Panax notoginseng. After leaves were exposed to a high level of light (2258 μmol photons m-2 s-1) for 30 and 60 min, the maximum quantum yield of PSII (Fv/Fm) decreased by 17 and 23%, respectively, whereas the maximum photo-oxidizable P700 (Pm) remained stable. Therefore, this species displayed selective PSII photodamage under strong illumination. After these treatments, LEF was significantly decreased under all light levels but acidification of the thylakoid lumen changed only slightly. Furthermore, the decrease in LEF under low light was positively correlated with the extent of PSII photoinhibition. Thus, the decline in LEF was not caused by the enhancement of lumenal acidification, but was induced by a decrease in PSII activity. These results indicate that residual PSII activity is an important determinant of LEF in this shade-adapted species, and they provide new insight into how strong illumination affects the growth of shade-demanding plants.

Effects of cadmium on photosynthesis of Schima superba young plant detected by chlorophyll fluorescence.

Contamination by heavy metals has become a serious environmental pollution issue today due to its potential threat to plant, wildlife, and human health. Photosynthesis, a process in which light energy is used to produce sugar and other organic compounds, is sensitive to heavy metals. In the present study, the response of photosynthetic process and carbon assimilation of Schima superba was investigated under cadmium (Cd) stress. Three Cd concentrations (0, 300, and 600mgkg-1) were used designated as control (CK), low Cd (L1), and high Cd treatment (L2) of plants. Results showed that photosystem II (PSII) acceptor and donor side electron transport were more easily blocked in treatment compared to control, and L2 have more significant changes than L1. A substantial decrease of 820 nm reflection curve absorption was observed both in L1 and L2 treatments. Special energy fluxes showed significant difference between the control group and the treated group, which indicated that low concentration Cd stress can cause decrease in quantum yield of PSII in plants studied. Non-stomatal factors resulted in a decrease in net photosynthetic rate and a decrease in photosystem activity. Our results suggested that Cd can damage structure and function of the photosynthesis of S. superba young plants.

Dynamic interplay between photodamage and photoprotection in photosystem II.

Photoinhibition is the light-induced reduction in photosynthetic efficiency and is usually associated with damage to the D1 photosystem II (PSII) reaction centre protein. This damage must either be repaired, through the PSII repair cycle, or prevented in the first place by nonphotochemical quenching (NPQ). Both NPQ and D1 repair contribute to light tolerance because they ensure the long-term maintenance of the highest quantum yield of PSII. However, the relative contribution of each of these processes is yet to be elucidated. The application of a pulse amplitude modulation fluorescence methodology, called protective NPQ, enabled us to evaluate of the protective effectiveness of the processes. Within this study, the contribution of NPQ and D1 repair to the photoprotective capacity of Arabidopsis thaliana was elucidated by using inhibitors and mutants known to affect each process. We conclude that NPQ contributes a greater amount to the maintenance of a high PSII yield than D1 repair under short periods of illumination. This research further supports the role of protective components of NPQ during light fluctuations and the value of protective NPQ and qPd as unambiguous fluorescence parameters, as opposed to qI and Fv /Fm , for quantifying photoinactivation of reaction centre II and light tolerance of photosynthetic organisms.

Photochemical Efficiencies of Diatom Symbionts in Hospite in Amphistegina Gibbosa (foraminifera) Across Seasons in the Florida Keys, Usa

Abstract Benthic foraminiferal taxa that host algal endosymbionts can serve as model organisms in understanding photo-oxidative stress in environments where algal symbioses are prevalent. This study examined photochemical performance of the diatom symbionts within the foraminifer Amphistegina gibbosa. Data on bleaching prevalence and photochemical efficiency for photosystem II (PSII) were obtained from specimens collected at Tennessee Reef, Florida reef tract, USA, as close as logistically feasible to each equinox and solstice in 2012 and 2013. Specimens were collected at 6 m and 18 m depths, isolated, visually characterized, and assessed for maximum quantum yield of PSII using a pulse-amplitude modulated (PAM) fluorometer. Rapid-light curves (RLCs) were performed in the laboratory on specimens within a few hours of collection. Trials examined the effects of depth, sampling date, and degree of host bleaching. Consistent with field studies in the 1990s, proportions of A. gibbosa populations exhibiting visible loss of symbionts (i.e., “bleaching”) were lowest in December, with between-season and between-year differences in maxima. Maximum quantum yields ranged from 0.66–0.76. Significant differences between sampling dates were found at the 6 m site, with highest yields in winter and lowest in summer. At the 18 m site, maximum quantum yields differed both by sampling date and by degree of bleaching. Rapid light curves revealed that relative electron transport rates were highest in specimens exhibiting no visible symbiont loss; derived-photosynthetic parameters of α (initial slope of the RLC) and Ek (minimum saturating irradiance) did not differ significantly across different degrees of symbiont loss. Overall results indicate that even individuals exhibiting substantial symbiont loss retain some fully functional symbionts.

The effects of chilling-light stress on photosystems I and II in three Paphiopedilum species

BackgroundLow temperatures pose a critical limitation to the physiology and survival of chilling-sensitive plants. One example is the genus Paphiopedilum (Orchidaceae), which is mainly native to tropical and subtropical areas from Asia to the Pacific islands. However, little is known about the physiological mechanism(s) underlying its sensitivity to chilling temperature. We examined how chilling-light stress influences the activities of photosystem I (PSI) and photosystem II (PSII) in three species: P. armeniacum, P. micranthum, and P. purpuratum. All originate from different distribution zones that cover a range of temperatures.ResultsPhotosystem II of three Paphiopedilum species was remarkable sensitivity to chilling stress. After 8 h chilling stress, the maximum quantum yield of PSII of three species of Paphiopedilum was significantly decreased, especially in P. purpuratum. The quantity of efficient PSI complex (Pm) value did not significantly differ after 8 h chilling treatment compared to the original value in three species. The stronger PSII photoinhibition and significantly less capacity for cyclic electron flow (CEF) were observed in P. purpuratum.ConclusionsIn conclusion, the three species of Paphiopedilum showed significant PSII photoinhibition when exposed to 4 °C chilling treatment. However, their PSI activities were not susceptible to chilling-light stress during 8 h. The CEF was important for the photoprotection of PSI and PSII in P. armeniacum and P. micranthum under chilling conditions. Our findings suggested that the photosynthetic characteristics of Paphiopedilum were well adapted to their habitat.

Rooting affects the photosystem II activity: in vitro and ex vitro studies on energy hybrid sorrel

Rumex tianschanicus × Rumex patientia is a high-biomass-yielding plant suitable for fuel and biogas production. The protocol of the hybrid sorrel micropropagation was used to study the changes in the photosystem II (PSII) activity as well as to analyse the ultrastructure of the chloroplasts. The lowest effective PSII quantum yield [Y(II)] and an apparent electron transport rate of PSII [ETR(II)] were observed for adventitious shoots that had been regenerated in vitro, before rooting. These fluorescence parameters were higher and similar for both the leaves of the same adventitious shoots that had been rooted under in vitro conditions and for the shoots that had been acclimated and grown in ex vitro conditions. The analysis indicated that the PSII activity strongly depends on the formation of properly functioning roots and that in vitro or ex vitro culture conditions are, at least to some degree, less important. TEM analysis revealed that chloroplasts from plants rooted in vitro were sufficiently mature and acclimatization processes have less impact on their development. This is the first report concerning the analysis of PSII activity and the ultrastructure of the chloroplasts at all of the stages of micropropagation, i.e. adventitious shoot formation in vitro, rooting in vitro and acclimation to ex vitro conditions. It strongly indicated that rooting under in vitro conditions, rather than the acclimation to ex vitro conditions, plays a key role in the development of a completely functional photosynthetic apparatus in hybrid sorrel.

Sub-high Temperature and High Light Intensity Induced Irreversible Inhibition on Photosynthesis System of Tomato Plant (Solanum lycopersicum L.).

High temperature and high light intensity is a common environment posing a great risk to organisms. This study aimed to elucidate the effects of sub-high temperature and high light intensity stress (HH, 35°C, 1000 μmol⋅m-2⋅s-1) and recovery on the photosynthetic mechanism, photoinhibiton of photosystem II (PSII) and photosystem I (PSI), and reactive oxygen (ROS) metabolism of tomato seedlings. The results showed that with prolonged stress time, net photosynthetic rate (Pn), Rubisco activity, maximal photochemistry efficiency (Fv/Fm), efficient quantum yield and electron transport of PSII [Y(II) and ETR(II)] and PSI [Y(I) and ETR(I)] decreased significantly whereas yield of non-regulated and regulated energy dissipation of PSII [Y(NO) and Y(NPQ)] increased sharply. The donor side limitation of PSI [Y(ND)] increased but the acceptor side limitation of PSI [Y(NA)] decreased. Content of malondialdehyde (MDA) and hydrogen peroxide (H2O2) were increased while activity of superoxide dismutase (SOD) and peroxidase (POD) were significantly inhibited compared with control. HH exposure affected photosynthetic carbon assimilation, multiple sites in PSII and PSI, ROS accumulation and elimination of Solanum lycopersicum L.

Temperature-dependent chlorophyll accumulation and photosystem II assembly during etioplast to chloroplast transition in sunflower cotyledons

Abstract Despite numerous data dealing with the biogenesis of photosynthetic membranes connected with specific functional alterations in higher plants this is still an insufficiently understood topic and is one of the most promising areas of research in plant biochemistry. The main goal of our study was to detect the impact of different temperatures on chlorophyll biosynthesis and the maximum quantum yield of PSII (Fv/Fm). Therefore, we investigated the greening processes in etiolated sunflower cotyledons (Helianthus annuus L.) grown at different temperatures (10, 20 and 30 °C) during 24 h. The dynamics of chlorophyll a and b (Chl a and Chl b) accumulation as well as photosystem II (PSII) effi ciency were observed. We also evaluated combined effects of different temperatures (20 and 30 °C) and short-term application of increased irradiation (800 μmol m-2 s-1) on effective quantum yield of PSII (ΔF/F’m) and non photochemical quenching (NPQ) in cotyledons with fully developed PSII. Our results showed reduced chlorophyll accumulation and the arrest of PSII assembly at 10 °C in comparison with 20 and 30 °C. Further, the increased irradiance induced equal down regulation of effective quantum yield of PSII at 20 and 30 °C, with significantly higher capability of heat dissipation at 30 °C.

Induction of cyclic electron flow around photosystem I during heat stress in grape leaves

Photosystem II (PSII) in plants is susceptible to high temperatures. The cyclic electron flow (CEF) around PSI is thought to protect both PSII and PSI from photodamage. However, the underlying physiological mechanisms of the photosynthetic electron transport process and the role of CEF in grape at high temperatures remain unclear. To investigate this issue, we examined the responses of PSII energy distribution, the P700 redox state and CEF to high temperatures in grape leaves. After exposing ‘Cabernet Sauvignon’ leaves to various temperatures (25, 30, 35, 40 and 45°C) in the light (600μmol photons m−2s−1) for 4h, the maximum quantum yield of PSII (Fv/Fm) significantly decreased at high temperatures (40 and 45°C), while the maximum photo-oxidizable P700 (Pm) was not affected. As the temperature increased, higher initial rates of increase in post-illumination Chl fluorescence were detected, which were accompanied by an increase in high energy state quenching (qE). The chloroplast NAD(P)H dehydrogenase-dependent CEF (NDH-dependent CEF) activities were different among grape cultivators. ‘Gold Finger’ with greater susceptibility to photoinhibition, exhibited lower NDH-dependent CEF activities under acute heat stress than a more heat tolerant ‘Cabernet Sauvignon’. These results suggest that overclosure of PSII reaction centers at high temperature resulted in the photoinhibition of PSII, while the stimulation of CEF in grape played an important role in the photoprotection of PSII and PSI at high temperatures through contributing to the generation of a proton gradient.

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.

Leaf Gas Exchange and Fluorescence of Two Winter Wheat Varieties in Response to Drought Stress and Nitrogen Supply.

Water and nitrogen supply are the two primary factors limiting productivity of wheat (Triticum aestivum L.). In our study, two winter wheat varieties, Xinong 979 and large-spike wheat, were evaluated for their physiological responses to different levels of nitrogen and water status during their seedling stage grown in a phytotron. Our results indicated that drought stress greatly reduced the net photosynthetic rate (Pn), transpiration rate (E), and stomatal conductance (Gs), but with a greater increase in instantaneous water use efficiency (WUE). At the meantime, the nitrogen (N) supply improved photosynthetic efficiency under water deficit. Parameters inferred from chlorophyll a measurements, i.e., photochemical quenching coefficient (qP), the maximum photochemical efficiency (Fv/Fm), the quantum yield of photosystemII(ΦPSII), and the apparent photosynthetic electron transport rate (ETR) decreased under water stress at all nitrogen levels and declined in N-deficient plants. The root–shoot ratio (R/S) increased slightly with water stress at a low N level; the smallest root–shoot ratio was found at a high N level and moderate drought stress treatment. These results suggest that an appropriate nitrogen supply may be necessary to enhance drought resistance in wheat by improving photosynthetic efficiency and relieving photoinhibition under drought stress. However, an excessive N supply had no effect on drought resistance, which even showed an adverse effect on plant growth. Comparing the two cultivars, Xinong 979 has a stronger drought resistance compared with large-spike wheat under N 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.