Variable Chlorophyll Fluorescence Research Articles

Using Chlorophyll Fluorescence to Determine the Fate of Photons Absorbed by Phytoplankton in the World's Oceans.

Approximately 45% of the photosynthetically fixed carbon on Earth occurs in the oceans in phytoplankton, which account for less than 1% of the world's photosynthetic biomass. This amazing empirical observation implies a very high photosynthetic energy conversion efficiency, but how efficiently is the solar energy actually used? The photon energy budget of photosynthesis can be divided into three terms: the quantum yields of photochemistry, fluorescence, and heat. Measuring two of these three processes closes the energy budget. The development of ultrasensitive, seagoing chlorophyll variable fluorescence and picosecond fluorescence lifetime instruments has allowed independent closure on the first two terms. With this closure, we can understand how phytoplankton respond to nutrient supplies on timescales of hours to months and, over longer timescales, to changes in climate.

Leaf morphology, photosynthesis and pigments change with age and light regime in savin juniper

• Savin juniper is an excellent species for desertification control in arid and semi-arid areas, where it typically establishes under the protection of nurse plants. Ultimately, established plants emerge into full sunlight as they grow, and this transition is accompanied by an increase in the preponderance of scale-like vs. needle-like leaf forms. • To test how age and variable sunlight environments affect shade tolerance in savin juniper, we established a pot study under field conditions with two age cohorts (1- and 4-year-old rooted scions) and three sunlight regimes (10%, 50% and 100% light transmittance). We measured growth, leaf parameters, photosynthesis, chlorophyll fluorescence and foliar pigments on a monthly basis (seven growing months per year, from 2015 to 2017). • Overall, there was little interaction for all variables, and both cohort and light regime had significant effects. Leaf form and spacing varied continuously, tending towards shorter, more closely spaced and more appressed scale leaves with higher dry leaf mass per area in older plants or under higher sunlight. There were no clear age-related patterns in carotenoids but both cohort and light had significant effects on gas exchange and chlorophyll fluorescence variables. • Savin juniper shows an intermediate tolerance to shade that changes with growth in that younger plants were less tolerant of full sun than older plants, consistent with its reliance on nurse plants for ultimate establishment in the open.

Single-Turnover Variable Chlorophyll Fluorescence as a Tool for Assessing Phytoplankton Photosynthesis and Primary Productivity: Opportunities, Caveats and Recommendations

Phytoplankton photosynthetic physiology can be investigated through single-turnover variable chlorophyll fluorescence (ST-ChlF) approaches, which carry unique potential to autonomously collect data at high spatial and temporal resolution. Over the past decades, significant progress has been made in the development and application of ST-ChlF methods in aquatic ecosystems, and in the interpretation of the resulting observations. At the same time, however, an increasing number of sensor types, sampling protocols, and data processing algorithms have created confusion and uncertainty among potential users, with a growing divergence of practice among different research groups. In this review, we assist the existing and upcoming user community by providing an overview of current approaches and consensus recommendations for the use of ST-ChlF measurements to examine in-situ phytoplankton productivity and photo-physiology. We argue that a consistency of practice and adherence to basic operational and quality control standards is critical to ensuring data inter-comparability. Large datasets of inter-comparable and globally coherent ST-ChlF observations hold the potential to reveal large-scale patterns and trends in phytoplankton photo-physiology, photosynthetic rates and bottom-up controls on primary productivity. As such, they hold great potential to provide invaluable physiological observations on the scales relevant for the development and validation of ecosystem models and remote sensing algorithms.

Phosphorus Increases K+ in the Shoot and Improves Salinity Tolerance in Sweetsop Seedlings

The use of brackish waters is an alternative for the cultivation of fruit-bearing plants in semi-arid regions; nonetheless, sweetsop trees (Annona squamosa L.) are sensitive to these types of waters. However, phosphate fertilization can be a technique used to mitigate the harmful effects of salinity. Thus, our objective was to evaluate the application of phosphorus to mitigate the salinity in irrigation water. The experiment was conducted in a randomized block design, with six treatments S1 (2.5 dS m−1) S2 = (4 dS m−1), SP1 (2.5 dS m−1 + 1.35 g dm−3 of P2O5), SP2 = (4 dS m−1 + 1.35 g dm−3 of P2O5), T (control), and P (1.35 g dm−3 of P2O5). After 78 days of transplantation, the seedlings were evaluated for the total dry weight (TDW), leaf area (LA), salt tolerance index (STI), water use efficiency (WUE), Na+ and K+ levels in the shoot and root, chlorophylls a and b, maximum (Fm) and variable (Fv) chlorophyll fluorescence, quantum efficiency of photosystem II (Fv/Fm), total soluble sugar (TSS), total free amino acids (TFAA), proline, and glycine betaine (GB). It was observed that plants treated with SP1 and SP2 show higher results of TDW, LA, STI, WUE, and K in the shoot, Chl a, Chl b, Fm, Fv, Fv/Fm, TFAA, proline, GB, and TSS in relation to S1 and S2. The presence of phosphorus under salinity conditions increased the K+ concentration in the shoot, which was responsible for the improvement of pigments as well as increasing the quantum efficiency of photosystem II and carbohydrates, maintaining the growth and, consequently, the salinity tolerance of the sweetsop seedlings.

Photosynthetic monitoring techniques indicate maximum glycogen accumulation in nitrogen-limited Synechocystis sp. PCC 6803 culture

Chlorophyll fluorescence and oxygen evolution are rapid and non-invasive monitoring techniques to obtain information about the photosynthesis performance of microalgae cultures. These methods may be employed for optimizing the biomass productivity as well as indicate increased production of some valuable compounds.In this work photosynthesis monitoring techniques – saturation pulse analysis of fluorescence quenching, fast fluorescence induction kinetics and photosynthetic oxygen production/respiration – were employed for monitoring of the physiological state of the nitrogen-limited culture of Synechocystis sp. PCC 6803. The correlation between changes of photosynthetic activities, growth and glycogen accumulation was examined in these cultures. The aim was to determine proper point for biomass harvest comparing the changes of photosynthetic activities and the kinetics of glycogen accumulation. In a 4-day trial the highest glycogen accumulation in biomass was found on day 2 showing an inflection point which was accompanied by the reversal of the chlorophyll fluorescence variables, namely maximal photochemical efficiency of PSII Fv/Fm, relative electron transport rate rETR, photochemical efficiency α and photosynthesis saturating irradiance Ik. The rapid increase of cell respiration after day 2 also indicated the relation of these changes with onset of glycogen catabolism.Our laboratory experiments revealed that the three photosynthetic monitoring techniques provided good indication of the physiological changes leading to high glycogen accumulation in biomass of the nitrogen-limited Synechocystis culture. In this way the glycogen accumulation can be manipulated. The photosynthesis measurements can indicate the right time to harvest the culture rich in glycogen in biotechnological applications.

Physiology and production of yellow passion fruit with hydroabsorbent polymer and different irrigation depths

ABSTRACT The yellow passion fruit is grown under different edaphoclimatic conditions. Irrigation standardizes and increases the development and productivity of the plants. The objective of this study was to evaluate the physiological and productive aspects of yellow passion fruit with application of hydroabsorbent polymer at different irrigation depths. The experimental design was in randomized blocks in a 2 × 5 factorial arrangement. The treatments were soil with and without polymer and five dephts of irrigation (60 to 100% of crop evapotranspiration). Initial, maximum, and variable chlorophyll fluorescence, physiology (photosynthetic rate, internal carbon concentration, transpiration, stomatal conductance, water use efficiency, and instantaneous carboxylation efficiency) and production of yellow passion fruit were the variables analyzed. The hydrogel probably retained water in its structure, reducing the ability to dissipate excess energy. Setting the irrigation depth at an estimated 82% of crop evapotranspiration increased photosynthetic rate, stomatal conductance, and water use efficiency of yellow passion fruit plants. Maximum yield was 17.2 t ha-1 in irrigated plants at 100% evapotranspiration (ETc) in soil without hydrogel.

A simple method to produce Synechocystis PCC6803 biofilm under laboratory conditions for electron microscopic and functional studies.

Cyanobacteria can form biofilms in nature, which have ecological roles and high potential for practical applications. In order to study them we need biofilm models that contain healthy cells and can withstand physical manipulations needed for structural studies. At present, combined studies on the structural and physiological features of axenic cyanobacterial biofilms are limited, mostly due to the shortage of suitable model systems. Here, we present a simple method to establish biofilms using the cyanobacterium Synechocystis PCC6803 under standard laboratory conditions to be directly used for photosynthetic activity measurements and scanning electron microscopy (SEM). We found that glass microfiber filters (GMF) with somewhat coarse surface features provided a suitable skeleton to form Synechocystis PCC6803 biofilms. Being very fragile, untreated GMFs were unable to withstand the processing steps needed for SEM. Therefore, we used polyhydroxybutyrate coating to stabilize the filters. We found that up to five coats resulted in GMF stabilization and made possible to obtain high resolution SEM images of the structure of the surface-attached cells and the extensive exopolysaccharide and pili network, which are essential features of biofilm formation. By using pulse-amplitude modulated variable chlorophyll fluorescence imaging, it was also demonstrated that the biofilms contain photosynthetically active cells. Therefore, the Synechocystis PCC6803 biofilms formed on coated GMFs can be used for both structural and functional investigations. The model presented here is easy to replicate and has a potential for high-throughput studies.

Changes in Photo-Protective Energy Dissipation of Photosystem II in Response to Beneficial Bacteria Consortium in Durum Wheat under Drought and Salinity Stresses

The present research aimed at evaluating the harmless dissipation of excess excitation energy by durum wheat (Triticum durum Desf.) leaves in response to the application of a bacterial consortium consisting of four plant growth-promoting bacteria (PGPB). Three pot experiments were carried out under non-stress, drought (at 40% field capacity), and salinity (150 mM NaCl) conditions. The results showed that drought and salinity affected photo-protective energy dissipation of photosystem II (PSII) increasing the rate of non-photochemical chlorophyll fluorescence quenching (NPQ (non-photochemical quenching) and qCN (complete non-photochemical quenching)), as well as decreasing the total quenching of chlorophyll fluorescence (qTQ), total quenching of variable chlorophyll fluorescence (qTV) and the ratio of the quantum yield of actual PSII photochemistry, in light-adapted state to the quantum yield of the constitutive non-regulatory NPQ (PQ rate). Our results also indicated that the PGPB inoculants can mitigate the adverse impacts of stresses on leaves, especially the saline one, in comparison with the non-fertilized (control) treatment, by increasing the fraction of light absorbed by the PSII antenna, PQ ratio, qTQ, and qTV. In the light of findings, our beneficial bacterial strains showed the potential in reducing reliance on traditional chemical fertilizers, in particular in saline soil, by improving the grain yield and regulating the amount of excitation energy.

Photosynthetic energy conversion efficiency in the West Antarctic Peninsula.

The West Antarctic Peninsula (WAP) is a highly productive polar ecosystem where phytoplankton dynamics are regulated by intense bottom‐up control from light and iron availability. Rapid climate change along the WAP is driving shifts in the mixed layer depth and iron availability. Elucidating the relative role of each of these controls and their interactions is crucial for understanding of how primary productivity will change in coming decades. Using a combination of ultra‐high‐resolution variable chlorophyll fluorescence together with fluorescence lifetime analyses on the 2017 Palmer Long Term Ecological Research cruise, we mapped the temporal and spatial variability in phytoplankton photophysiology across the WAP. Highest photosynthetic energy conversion efficiencies and lowest fluorescence quantum yields were observed in iron replete coastal regions. Photosynthetic energy conversion efficiencies decreased by ~ 60% with a proportional increase in quantum yields of thermal dissipation and fluorescence on the outer continental shelf and slope. The combined analysis of variable fluorescence and lifetimes revealed that, in addition to the decrease in the fraction of inactive reaction centers, up to 20% of light harvesting chlorophyll‐protein antenna complexes were energetically uncoupled from photosystem II reaction centers in iron‐limited phytoplankton. These biophysical signatures strongly suggest severe iron limitation of photosynthesis in the surface waters along the continental slope of the WAP.

FIRe glider: Mapping in situ chlorophyll variable fluorescence with autonomous underwater gliders

AbstractNutrient and light availability regulate phytoplankton physiology and photosynthesis in the ocean. These physiological processes are difficult to sample in time and space over physiologically and ecologically relevant scales using traditional shipboard techniques. Gliders are changing the nature of data collection, by allowing a sustained presence at sea over regional scales, collecting data at resolution not possible using traditional techniques. The integration of a fluorescence induction and relaxation (FIRe) sensor in a Slocum glider allows autonomous high‐resolution and vertically‐resolved measurements of photosynthetic physiological variables together with oceanographic data. In situ measurements of variable fluorescence under ambient light allows a better understanding of the physical controls of primary production (PP). We demonstrate this capability in a laboratory setting and with several glider deployments in the Southern Ocean. Development of these approaches will allow for the in situ evaluation of phytoplankton light stress and photoacclimation mechanisms, as well as the role of vertical mixing in phytoplankton dynamics and the underlying physiology, especially in remote locations and for prolonged duration.

Measuring photosynthesis of both oxygenic and anoxygenic photosynthetic organisms using pulse amplitude modulation (PAM) fluorometry in wastewater ponds

Oxygenic photosynthesis can be measured easily using O2 or CO2 gas exchange, oxygen electrodes, Winkler titration, 14CO2-fixation and by PAM (pulse amplitude modulation) fluorometry. PAM estimates the photosynthetic electron transport rate (ETR) by measuring the variable fluorescence of chlorophyll (Chl) a (> 695 nm) induced by absorption of blue or red light. Anoxygenic photosynthetic bacteria (APB) do not use water as an electron source and are typically photoheterotrophic rather than photoautotrophic and so 14CO2 fixation is a misleading estimate of photosynthetic electron transport in APB photosynthesis. In vivo bacteriochlorophyll a (BChl a) absorbs blue light similar to Chl a but its characteristic longer-wavelength absorption is in the infrared and fluorescence is at > 800 nm. Blue light-induced PAM fluorescence can be used to measure the ETR in purple non-sulphur anoxygenic photobacteria and purple sulphur photobacteria because their RC-2 type BChl a complexes fluoresce similarly to PSII but at longer wavelengths than Chl a. Conventional PAM fluorometers using blue light cannot readily distinguish between oxygenic and RC-2 type anoxygenic photosynthesis because they use a simple > 700 nm highpass filter in front of the detector diode. We modified one fluorometer to use a 695–750-nm bandpass filter to measure Chl a fluorescence from PS-II, representing oxygenic photosynthesis. Similarly, we modified another fluorometer to use a highpass filter (> 830 nm) to measure BChl a fluorescence, representing anoxygenic photosynthesis. However, the fluorescence bands of Chl a and BChl a were found to be too wide to unambiguously distinguish between oxygenic and anoxygenic photosynthesis purely by fluorometry. Treatment with the specific PS-II inhibitor DCMU (Diuron) did enable discrimination of the two types of photosynthesis in a mixture of oxygenic and anoxygenic organisms. Ecological niches made up of both oxygenic and anoxygenic organisms such as microbial mats and hypereutrophic environments such as sewage ponds, wastewater ponds and prawn farm ponds are much more common than often realized. Anoxygenic photosynthesis in such systems is significant yet largely unquantified.

Fluorescence of chlorophyll a in transgenic maize with herbicide application and attacked by Spodoptera frugiperda (Lepidoptera: Noctuidae)

Physiological changes in maize plants, submitted to herbicides and pests, should be studied for integrated management in this crop. The objective was to evaluate the chlorophyll a fluorescence in maize genotypes damaged or not by Spodoptera frugiperda, and sprayed with herbicides. The herbicides atrazine and nicosulfuron were applied to the plants at doses of 3 kg ha− 1 and 0.06 kg ha− 1, respectively. A randomized block design in a factorial scheme with an additional treatment 3 (2 × 2 + 1) was used, being the factor A: maize leaves exposed or not to the attack of S. frugiperda; factor B: Herculex® (transgenic maize encoding the Cry1F protein) and Isohybrid (non-transgenic maize) genotypes and an additional treatment represented by the PowerCore® genotype (stacked transgenic maize encoding the Cry1F, Cry1A.105 and Cry2Ab2 proteins). The initial fluorescence of chlorophyll a (F0 - electrons quantum− 1), maximum fluorescence (Fm - electron quantum− 1), ratio between variable fluorescence and maximum fluorescence of chlorophyll a (Fv/Fm) and electron transport rate ETR-µmols electrons m− 2 s− 1) were analyzed. The initial fluorescence of chlorophyll a increased and Fm, Fv/Fm and ETR decreased in plants attacked by the pest. The PowerCore® chlorophyll fluorescence remained in characteristic patterns. Chlorophyll a fluorescence was similar between genotypes with or without herbicides application. Damage by S. frugiperda increased the initial fluorescence of chlorophyll a and reduced the variable fluorescence/maximum fluorescence ratio and electron transport rate, indicating physiological stress in maize plants. The herbicides do not cause physiological stress in plants of maize genotypes with characteristic values of chlorophyll a fluorescence.

Photochemical efficiency of basil cultivars fertigated with salinized nutrient solutions

ABSTRACT Reduction in plant growth under salt stress is due, among other factors, to changes in photochemical efficiency and, consequently, in photosynthesis. The objective of this study was to evaluate the effect of salt stress on the chlorophyll fluorescence variables in cultivars of basil (Ocimum basilicum L.). The experiment was conducted in a greenhouse, using a randomized block design in a 5 x 3 factorial scheme, corresponding to five basil cultivars (‘Grecco a Palla’, ‘Alfavaca Basilicão’, ‘Alfavaca Verde’, ‘Lemoncino’ and ‘Roxo’) and three salinity of the nutrient solution (2.0, 3.5 and 5.0 dS m-1), with three repetitions, and the experimental plot was represented by three pots containing 3.0 dm3 of coconut fiber. At the full flowering stage, plants were evaluated for chlorophyll fluorescence, when adapted to the dark, as well as under saturating light conditions. In general, there were changes in fluorescence variables only at salinity above 3.5 dS m-1. Fertigation using high-salinity water promotes strong changes in the chlorophyll a fluorescence of sensitive basil cultivars. The cultivars ‘Grecco a Palla’ and ‘Alfavaca Verde’ were little affected by the increase of salinity, being the most tolerant to salt stress. In the cultivars ‘Alfavaca Basilicão’, ‘Lemoncino’ and ‘Roxo’, chlorophyll fluorescence was strongly affected by salinity, being the most sensitive. The tolerance ranking based on chlorophyll fluorescence is: ‘Grecco a Palla’ = ‘Alfavaca Verde’ > ‘Alfavaca Basilicão’ > ‘Lemoncino’ > ‘Roxo’.

Morphophysiological responses of young oil palm plants to salinity stress

Abstract: The objective of this work was to assess the morphophysiological characteristics and the ionic imbalance in the substrate, roots, and leaves of young oil palm (Elaeis guineensis) plants under different levels of saline stress, following a substrate salinization protocol in which the level of salt was known. Bifid oil palm plants were subjected to different NaCl rates (0.0, 0.5, 1.0, 1.5, and 2.0 g NaCl per 100 g substrate on a dry basis), and their morphophysiological responses were evaluated for a period of 12-14 days. This protocol generated different levels of stress due to the gradients of electrical conductivity and water potential in the saturation extract of the substrate, according to the added NaCl. Based on the rates of real evapotranspiration and leaf gas exchange, the osmotic effect of the salt reflected negatively on leaf temperature, on the chlorophyll content index, and on the chlorophyll fluorescence variables. The increase in Na and Cl levels in the saturation extract culminated with the increasing availability of Ca, K, and Mg in the solution and their accumulation in the leaves. However, the plants poorly absorbed Na and Cl. The obtained results are indicative that, for a better characterization of the osmotic and ionic phases of salinity stress, it is necessary to reduce the applied level of salinity stress and to increase the evaluation period.

Cortical photosynthesis as a physiological marker for grape breeding: methods and approaches

Photosynthesis occurring in chlorenchymal tissues of lignified branches of perennial plants (cortical photosynthesis) has a significant impact on their productivity and resistance to adverse environmental conditions, such as water deficiency and low temperatures. Cortical photosynthesis occurring under the outer bark of a lignified grape vine can become a convenient marker for breeding freeze-tolerant varieties. The following approaches can be undertaken to assess the functional state of the cortical photosynthetic apparatus: (1) analysis of the variable chlorophyll fluorescence parameters and (2) biochemical analysis of photosynthetic membrane preparations. To evaluate these approaches, in this work we have carried out the comparative analysis of characteristics of the cortical photosynthetic apparatus in grape varieties differing in freeze tolerance. This work was supported by grant №18-04-00079 from the Russian Foundation for Basic Research.

Symbiosis extended: exchange of photosynthetic O2 and fungal-respired CO2 mutually power metabolism of lichen symbionts

Lichens are a symbiosis between a fungus and one or more photosynthetic microorganisms that enables the symbionts to thrive in places and conditions they could not compete independently. Exchanges of water and sugars between the symbionts are the established mechanisms that support lichen symbiosis. Herein, we present a new linkage between algal photosynthesis and fungal respiration in lichen Flavoparmelia caperata that extends the physiological nature of symbiotic co-dependent metabolisms, mutually boosting energy conversion rates in both symbionts. Measurements of electron transport by oximetry show that photosynthetic O2 is consumed internally by fungal respiration. At low light intensity, very low levels of O2 are released, while photosynthetic electron transport from water oxidation is normal as shown by intrinsic chlorophyll variable fluorescence yield (period-4 oscillations in flash-induced Fv/Fm). The rate of algal O2 production increases following consecutive series of illumination periods, at low and with limited saturation at high light intensities, in contrast to light saturation in free-living algae. We attribute this effect to arise from the availability of more CO2 produced by fungal respiration of photosynthetically generated sugars. We conclude that the lichen symbionts are metabolically coupled by energy conversion through exchange of terminal electron donors and acceptors used in both photosynthesis and fungal respiration. Algal sugars and O2 are consumed by the fungal symbiont, while fungal delivered CO2 is consumed by the alga.

The negative effects of short-term extreme thermal events on the seagrass Posidonia oceanica are exacerbated by ammonium additions.

Global warming is increasingly affecting our biosphere. However, in addition to global warming, a panoply of local stressors caused by human activities is having a profound impact on our environment. The risk that these local stressors could modify the response of organisms to global warming has attracted interest and fostered research on their combined effect, especially with a view to identifying potential synergies. In coastal areas, where human activities are heavily concentrated, this scenario is particularly worrying, especially for foundation species such as seagrasses. In this study we explore these potential interactions in the seagrass Posidonia oceanica. This species is endemic to the Mediterranean Sea. It is well known that the Mediterranean is already experiencing the effects of global warming, especially in the form of heat waves, whose frequency and intensity are expected to increase in the coming decades. Moreover, this species is especially sensitive to stress and plays a key role as a foundation species. The aim of this work is thus to evaluate plant responses (in terms of photosynthetic efficiency and growth) to the combined effects of short-term temperature increases and ammonium additions.To achieve this, we conducted a mesocosm experiment in which plants were exposed to three thermal treatments (20°C, 30°C and 35°C) and three ammonium concentrations (ambient, 30 μM and 120 μM) in a full factorial experiment. We assessed plant performance by measuring chlorophyll fluorescence variables (maximum quantum yield (Fv/Fm), effective quantum yield of photosystem II (ΔF/Fm’), maximum electron transport rate (ETRmax) and non-photochemical quenching (NPQ)), shoot growth rate and leaf necrosis incidence. At ambient ammonium concentrations, P. oceanica tolerates short-term temperature increases up to 30°C. However, at 35°C, the plant loses functionality as indicated by a decrease in photosynthetic performance, an inhibition of plant growth and an increase of the necrosis incidence in leaves. On the other hand, ammonium additions at control temperatures showed only a minor effect on seagrass performance. However, the combined effects of warming and ammonium were much worse than those of each stressor in isolation, given that photosynthetic parameters and, above all, leaf growth were affected. This serves as a warning that the impact of global warming could be even worse than expected (based on temperature-only approaches) in environments that are already subject to eutrophication, especially in persistent seagrass species living in oligotrophic environments.

Physiological, morphological and anatomical leaf traits variation across leaf development in 'Corylus avellana'

The study analyzed the variations of physiological, morphological and anatomical leaf traits during its development in Corylus avellana L. saplings. Three different phases were identify during leaf development: the first phase (hereafter IP) considered in the developing leaves, the second phase (IIP) in the mature green leaves and the third phase (IIIP) in the senescent leaves. In particular, variations in parameters estimated from the photosynthetic light response curves, in chlorophyll fluorescence variables and in morphological leaf traits were analyzed during the three phases. The principal component analysis (PCA) carried out using all the considered morphological characters (leaf mass per area - LMA, and leaf tissue density - LTD) and physiological traits (the maximum net photosynthetic rates - ANmax, dark respiration rates - RD, light compensation point - LCP, light saturation point – LSP, maximum quantum yield - ɸmax, fluorescence-based maximum quantum yield of PSII photochemistry - Fv/FM and effective quantum yield of PSII photochemistry - ɸPSII) showed a complete separation among the considered phases. The results showed that the major differences occur between the phases IP and IIP. In particular, a greater variation was found for LMA, ANmax, RD. On the contrary a lower variation was observed for ɸmax which remain quite constant from IP to IIP indicating that chloroplasts present in juvenile leaves are fully functional.

Responses of mature roadside trees to root severance treatments

Conflicts between trees and infrastructure installation or repair occur frequently in the urban environment and may result in damage to or complete removal of tree roots. We monitored the responses of trees to linear root cutting and examined the effects of the presence of a road within tree root zones. Mature Quercus virginiana Mill trees (n = 31) were exposed to three root pruning treatments (plus a control) consisting of a trench on one side of the tree, offset by a distance as a multiple of trunk diameter (3x, 6x and 12x). Roots were severed manually, and their diameters were measured at the cut point to estimate a total cross-sectional area of severed roots. The total severed root cross-sectional area was subsequently expressed as a ratio of whole trunk cross-sectional areas (termed Ar(BH)), as well as trunk conductive sapwood areas (termed As(BH)) at breast height (1.4 m). The shortest horizontal distance between the tree bases and the edge of a nearby road was also recorded and included in statistical analyses as a continuous variable. Linear root cutting reduced shoot extension and leaf area by as much 146.13 mm (p < 0.01) and 2.92 cm2 (p < 0.001) respectively, one growing season after root severance. Trees nearer the road had significantly reduced trunk diameter growth (p < 0.05), independent of root severance effects, i.e. 12x, 6x and 3x treatments. Pre-dawn leaf water potential was negatively affected for all treatments (p < 0.05) and the symptoms of water stress persisted in the 3x treatment for 440 days. Ar(BH) and As(BH) ratios were regressed against physiological (predawn leaf water potential (Ψ), stomatal conductance (gs), leaf temperature (T) and two chlorophyll fluorescence variables (Fv/Fm and Fv/Fo)) and growth (trunk diameter, shoot extension and leaf area) responses. Ar(BH) was a significant predictor of Ψ, Fv/Fm, Fv/Fo, shoot extension and leaf area responses after one growing season, although model strength varied (R2 = 0.75, 0.14, 0.16, 0.18, 0.24 respectively). To avoid sustained water stress symptoms, linear root cutting on Q. virginiana should not be undertaken closer to trees than six times DBH, equating to ≈ 25% root system loss.

Response of short-term heat shock on photosynthetic activity of soil crust cyanobacteria.

Short-term heat exposure in tropical regions can generate severe stress in the photosynthetic activity of soil crust cyanobacteria. We investigated the responses of two filamentous cyanobacteria, Scytonema tolypothrichoides and Tolypothrix bouteillei, to 1hr exposure at 35, 45, and 55°C using variable chlorophyll fluorescence. Protocols for maximum quantum yield (FV/FM) and dark recovery of chlorophyll a fluorescence (OJIP) transient were applied. Heat exposure caused damage to the donor side of PSII, indicated by a decrease in FV/FM and a rapid increase in F0. After heat stress, photochemical energy utilization (φPo, φETo, and φRE1o) declined and energy dissipation (φDIo) increased. At 45°C, the photosynthetic apparatus was reversibly damaged, since full recovery was observed after 7days of relaxation. S. tolypothrichoides was more resistant to heat stress than T. bouteillei, confirming better adaptation to higher temperatures as observed in growth experiments.