Changes In Photosynthetic Activity Research Articles

Photosynthesizing on metal excess: Copper differently induced changes in various photosynthetic parameters in copper tolerant and sensitive Silene paradoxa L. populations

This work investigated Cu-induced changes in photosynthetic activity in contrasting populations of Silene paradoxa L. A metallicolous Cu-tolerant population and a non-metallicolous sensitive population were grown in hydroponics and exposed to different CuSO4 treatments for different times. Copper accumulation, MDA concentrations, and several photosynthetic parameters were measured to assess different effects of Cu exposure on plants from the two populations.A more efficient ability to photosynthesize in the presence of Cu excess was showed by the Cu-tolerant population with respect to the sensitive one. Interestingly, Cu-imposed limitations were present not only at a different degree, but also of different nature in the two populations. In the tolerant population, the most limiting factor to photosynthesis seemed to be Cu-imposed stomatal closure, whereas Cu-mediated biochemical limitation was scarce and Cu-mediated reduction in mesophyll conductance almost non-existent. In the sensitive population, Cu largely affected all the measured parameters, so that its photosynthetic activity experienced any kind of limitation, diffusional and especially biochemical. The lower Cu concentrations accumulated in the tolerant plant could be one of the factors concurring to the reported differences in photosynthetic activity, but also a higher capacity of internal detoxification and compartmentalization of the metal could not be excluded.

Selecting microalgae with high lipid productivity and photosynthetic activity under nitrogen starvation

An economically feasible microalgal lipid industry heavily relies on the selection of suitable strains. Because microalgae lipid content increases under a range of adverse conditions (e.g. nutrient deprivation, high light intensity), photosynthetic activity is usually strongly reduced. As a consequence, lipid productivity rapidly declines overtime, after reaching a maximum within the first days of cultivation. The microalgae Chlorella vulgaris, Chlorococcum littorale, Nannochloropsis oculata, Nannochloropsis sp., Neochloris oleoabundans, Stichococcus bacillaris and Tetraselmis suecica were compared on fatty acid content and productivity, and also on photosynthetic activity under nitrogen (N) starvation. Cultures in N-replete conditions were used as reference. Photosystem II (PSII) maximum efficiency was followed during the experiment, as proxy for the change in photosynthetic activity of the cells. Strains with a high capacity for both lipid accumulation as well as high photosynthetic activity under N starvation exhibited a high lipid productivity over time. Among the tested strains, Nannochloropsis sp. showed highest fatty acid content (45 % w/w) and productivity (238 mg L−1 day−1) as well as PSII maximum efficiency, demonstrating to be the most suitable strain, of those tested, for lipid production. This study highlights that for microalgae, maintaining a high photosynthetic efficiency during stress is the key to maintain high fatty acid productivities overtime and should be considered when selecting strains for microalgal lipid production.

Retrieval of the photochemical reflectance index for assessing xanthophyll cycle activity: a comparison of near-surface optical sensors

Abstract. Unattended optical sensors are increasingly being deployed on eddy covariance flux towers and are often used to complement existing vegetation and micrometeorological measurements to enable assessment of biophysical states and biogeochemical processes over a range of spatial scales. Of particular interest are sensors that can measure the photochemical reflectance index (PRI), which can provide information pertaining to leaf pigments and photosynthetic activity. This interest has facilitated the production of a new range of lower-cost multispectral sensors specifically designed to measure temporal changes in the PRI signal. However, little is known about the characteristics (spectral, radiometric and temporal) of many of these PRI sensors, making it difficult to compare data obtained from these sensors across time, geographical locations and instruments. Furthermore, direct testing of the capability of these sensors to actually detect the conversion of the xanthophyll cycle, which is the original biological basis of the PRI diurnal signal, is largely absent, often resulting in an unclear interpretation of the signal, particularly given the wide range of factors now known to influence PRI. Through a series of experiments, we assess the sensitivity of one of the leading brands of PRI sensor (Skye SKR 1800) to changes in vegetation photosynthetic activity in response to changing irradiance. We compare the results with those obtained using a more expensive industry-standard visible to near-infrared hyperspectral spectrometer (PP Systems UniSpec) and determine the radiometric compatibility of measurements made by the different instruments. Results suggest that the SKR 1800 instrument is able to track rapid (seconds to minutes) and more gradual diurnal changes in photosynthetic activity associated with xanthophyll cycle pigment conversion. Measurements obtained from both the high and lower cost instrument were significantly linearly correlated but were subject to a large systematic bias, illustrating that differences in instrument configuration (e.g. spectral response functions and band positions) can have a large impact on the PRI measurement values obtained. Despite differences in absolute PRI values, significant correlations were observed between the canopy PRI derived from both the SKR 1800 and the UniSpec instruments, and the epoxidation state of the xanthophyll cycle (r2 = 0.46 p < 0.05 and r2 = 0.76 p < 0.01, respectively). However, the dynamic range of the SKR 1800 PRI signal was often lower than more expensive instruments and thus the lower cost multispectral instrument may be less sensitive to pigment dynamics related to photosynthetic activity. Based on our findings, we make a series of recommendations for the effective use of such sensors under field conditions and advocate that sensors should be fully characterized prior to their field deployment.

Photosynthesis monitoring to optimize growth of microalgal mass cultures: application of chlorophyll fluorescence techniques

Since the mid-1990s, chlorophyll a (chl a) fluorescence measurement has become widespread to monitor photosynthetic performance of microalgal mass cultures. This paper aims to provide practical instructions for microalgal physiologists and biotechnologists on how fluores- cence monitoring can be used to explain changes in photosynthetic activity of microalgal mass cultures. Emphasis is placed on the 2 most common fluorescence techniques—pulse-amplitude- modulation and fluorescence induction kinetics—and the interpretation of important variables that reflect changes of photosynthesis and physiological status of microalgal cultures. In particu- lar, consideration is given to problems associated with the estimation of the photochemical yield of photosystem II and its relationship to linear electron transport rate and overall photosynthesis. Emphasis is also given to the applications and limitations of these techniques through several case studies. In model experiments with microalgal cultures, typical records and their interpretation under various culture conditions are illustrated. Changes of photosynthetic activity and selected variables monitored by chl a fluorescence techniques can thus be related to changes of cultivation conditions, physiological status and growth of microalgal cultures for a given microalgal strain and cultivation system. In this way, chl a fluorescence may be used as a rapid screening technique to monitor photosynthetic activity and subsequently to estimate growth rate in both indoor and outdoor studies. This text and results formed the basis of a key lecture at the 9 th International GAP workshop held at the University of Malaga in September 2012.

Diurnal changes in photosynthetic activity of the biological soil crust and lichen: Effects of abiotic factors (Petuniabukta, Svalbard)

In polar ecosystems, primary producers have to cope with a very harsh climate that limits the time available for growth and biomass production. In this study, diurnal measurement of photosynthetic processes in biological soil crust and a lichen were carried out in Petuniabukta, Spitsbergen. For field measurements, a method of induced fluorescence of chlorophyll was used. Measurements of photosynthetic activity were taken as repetitive measurements of effective quantum yield of photosystem II (ΦPSII). The short-term field measurements were carried out for 10 days in summer 2014. ΦPSII was recorded each 5 minutes as well as microclimatic data (air temperature, air humidi-ty, photosynthetically active radiation - PAR). The microclimatic parameters were recorded by a datalogger. In general, physiological activity of both biological soil crust and a lichen showed daily courses. Tested lichen was Cladonia rangiferina and the most dominant species in biological soil crust was Nostoc sp. Typically, most of ΦPSII values ranged 0.6 – 0.7 in both model organisms. The results have shown that photosynthetic activity was strongly correlated with all observed abiotic factors in both study objects. Particularly important was the relation found between PAR and ΦPSII in biological soil crust. When the biological soil crust was exposed to high PAR doses of irradiation (about 2300 µmol m-2 s-1) photoinhibition of primary processes of photosynthesis was observed as ΦPSII decrease, while photosynthetic activity of lichen remained at same level. Furthermore, it has been demonstrated increasing that in situ photosynthetic activity increased in both biological soil crust and lichen with a decrease in temperature.

Fusion of sensor data for the detection and differentiation of plant diseases in cucumber

The development of plant diseases is associated with biophysical and biochemical changes in host plants. Various sensor methods have been used and assessed as alternative diagnostic tools under greenhouse conditions. Changes in photosynthetic activity, spectral reflectance and transpiration rate of diseased leaves, inoculated with Cucumber mosaic virus (CMV), Cucumber green mottle mosaic virus (CGMMV), and the powdery mildew fungus Sphaerotheca fuliginea were assessed by the use of non‐invasive sensors during disease development. Spatiotemporal changes in leaf temperature related to transpiration were visualized by digital infrared thermography. The maximum temperature difference within a leaf was an appropriate parameter to differentiate between healthy and diseased plants. The photosynthetic activity of healthy and diseased cucumber plants varied as measured by chlorophyll fluorescence and compared to the actual chlorophyll content. Hyperspectral imaging data were analysed using spectral vegetation indices. The results from this study confirm that each pathogen has a characteristic influence on the physiology and vitality of cucumber plants, which can be measured by a combination of non‐invasive sensors. Whereas thermography and chlorophyll fluorescence are unspecific indicators for plant diseases, hyperspectral imaging offers the potential for an identification of plant diseases. In a sensor data fusion approach, an early detection of each pathogen was possible by discriminant analysis. Although it still needs to be validated under real conditions, the combination of information from different sensors seems to be a promising tool.

A potential physiological role for cyanotoxins in cyanobacteria of arid environments

The ecological or physiological function of most cyanotoxins has remained largely unknown despite much speculation and the development of several hypotheses. The possible role of two of these toxins, microcystin and β-N-methylamino-l-alanine (BMAA) within producing organisms is discussed within the context of survival in arid environments and the consequent potential for the presence of such toxins in these environments. Cellular microcystin and BMAA concentrations are modulated by cellular carbon:nitrogen balance which in turn reflects environmental nitrogen availability and light intensity or duration. Both toxins result in decreased photosynthesis. However, available nitrogen relative to light and/or carbon fixation has opposite effects on the concentration of these toxins. We present data to explain this apparent contradiction and to support the role of these toxins in inhibition of photosynthesis. Additionally, we discuss how their different modes of achieving inhibition of photosynthesis may offer a survival advantage to cyanobacteria exposed to rapid changes in photosynthetic activity in arid environments.

Long-distance signal transmission and regulation of photosynthesis in characean cells.

Photosynthetic electron transport in an intact cell is finely regulated by the structural flexibility of thylakoid membranes, existence of alternative electron-transport pathways, generation of electrochemical proton gradient, and continuous exchange of ions and metabolites between cell organelles and the cytoplasm. Long-distance interactions underlying reversible transitions of photosynthetic activity between uniform and spatially heterogeneous distributions are of particular interest. Microfluorometric studies of characean cells with the use of saturating light pulses and in combination with electrode micromethods revealed three mechanisms of distant regulation ensuring functional coordination of cell domains and signal transmission over long distances. These include: (1) circulation of electric currents between functionally distinct cell domains, (2) propagation of action potential along the cell length, and (3) continuous cyclical cytoplasmic streaming. This review considers how photosynthetic activity depends on membrane transport of protons and cytoplasmic pH, on ion fluxes associated with the electrical excitation of the plasmalemma, and on the transmission of photoinduced signals with streaming cytoplasm. Because of signal transmission with cytoplasmic flow, dynamic changes in photosynthetic activity can develop far from the point of photostimulus application and with a long delay (up to 100 s) after a light pulse stimulus is extinguished.

Development of oogonia of Sargassum horneri (Fucales, Heterokontophyta) and concomitant variations in PSII photosynthetic activities

:Sargassum horneri is one of the most important seaweeds used to restore degraded coastal environments near Nanji Island, China. However, its reproductive characteristics, especially those of oogenesis and the concomitant changes in photosynthetic activities, remained uncertain. Herein we documented the processes of conceptacle and female gamete formation and the photosystem II (PSII) changes that accompanied them. Thalli of S. horneri were dioecious, the oogonial conceptacles maturing acropetally on the axillary receptacles. Conceptacles were originated from the epidermal cells, the cells lining the inner surface of the cavity ultimately differentiating into either sterile unbranched paraphyses or oogonia. The PSII photosynthetic activities of oogonia continuously diminished with increasing maturation. The value of Fv/Fm decreased from 0.55 ± 0.01 for recently initiated oogonia to 0.27 ± 0.05 for mature oogonia. The diameters of the oogonia increased approximately 20 times from initiation to maturity. The possible nutrition sources of oogonia during their development are discussed.

Regulation of photosynthesis during heterocyst differentiation in Anabaena sp. strain PCC 7120 investigated in vivo at single-cell level by chlorophyll fluorescence kinetic microscopy

Changes of photosynthetic activity in vivo of individual heterocysts and vegetative cells in the diazotrophic cyanobacterium Anabaena sp. strain PCC 7120 during the course of diazotrophic acclimation were determined using fluorescence kinetic microscopy (FKM). Distinct phases of stress and acclimation following nitrogen step-down were observed. The first was a period of perception, in which the cells used their internally stored nitrogen without detectable loss of PSII activity or pigments. In the second, the stress phase of nitrogen limitation, the cell differentiation occurred and an abrupt decline of fluorescence yield was observed. This decline in fluorescence was not paralleled by a corresponding decline in photosynthetic pigment content and PSII activity. Both maximal quantum yield and sustained electron flow were not altered in vegetative cells, only in the forming heterocysts. The third, acclimation phase started first in the differentiating heterocysts with a recovery of PSII photochemical yields [Formula: see text] Afterwards, the onset of nitrogenase activity was observed, followed by the restoration of antenna pigments in the vegetative cells, but not in the heterocysts. Surprisingly, mature heterocysts were found to have an intact PSII as judged by photochemical yields, but a strongly reduced PSII-associated antenna as judged by decreased F 0. The possible importance of the functional PSII in heterocysts is discussed. Also, the FKM approach allowed to follow in vivo and evaluate the heterogeneity in photosynthetic performance among individual vegetative cells as well as heterocysts in the course of diazotrophic acclimation. Some cells along the filament (so-called "superbright cells") were observed to display transiently increased fluorescence yield, which apparently proceeded by apoptosis.

Seasonal changes in photosynthetic activity and carbohydrate content in leaves and fruit of three fig cultivars (Ficus carica L.)

This study was carried out to investigate the seasonal photosynthetic capacity of ‘Kalamon’, ‘Fracasana’ and ‘Mission’ fig cultivars and the effect of chlorophyll (Chl), nitrogen, specific leaf weight (SLW) and leaf anatomy on net photosynthesis rate (Pn). The seasonal changes in carbohydrate concentrations in leaves and fruit were also studied. Pn and stomatal conductance (gs) in leaves were measured in the field using an infra-red gas analyser. Soluble sugars were analyzed in leaves and fruit using a high-performance liquid chromatographic system, and starch with an enzymatic method.The concentration of Chl (a+b) increased rapidly up to early June, when the leaves reached their maximum area, then started decreasing slowly. The main soluble sugars found in both fig leaves and fruits were sucrose, glucose and fructose. Leaves of all cultivars contained much more sucrose than other sugars. Soluble sugar and starch concentrations increased until leaves reached their maximum area, decreasing thereafter. This period (late May) coincided with the emergence of inflorescences. Leaves of ‘Kalamon’ had higher concentrations of sucrose, fructose, total soluble sugars and starch than did the other cultivars.In contrast to leaves, in fruits the sucrose concentration was much lower than that of other sugars throughout the periods of fruit development and maturation. No significant differences in sugars were found between the cultivars during the period of fruit development, but mature fruit of ‘Kalamon’ and ‘Mission’ had higher concentrations of all sugars than did those of ‘Fracasana’.Pn increased rapidly from leaf emergence, reaching maximum values for all cultivars at full leaf expansion (FLE), then decreased and increased again during the period of fruit maturation. ‘Kalamon’ had higher Pn than the other cultivars. This is probably due to the higher SLW, nitrogen and Chl concentrations, the greater palisade parenchyma and leaf thickness, as well as to the higher gs found in ‘Kalamon’ compared to the other cultivars.

Short-term dynamics of physico-chemical and biological features in a shallow, evaporative antarctic lake

Lakes are among the most productive and biodiverse ecosystems in Antarctica, and they behave as important indicators of local climatic and environmental changes. However, few studies have focused on the local drivers of short-term temporal variability in lacustrine biogeochemical variables. In the present study, measurements of physical, chemical, biological and optical characteristics of the shallow endorheic Lake 14 at Edmonson Point (74.33° S, 165.13° E) were made over the ice-free period in December 2006. A significant variation in most variables was observed. Possible drivers for these changes were the loss of the ice cover, an increase in solar irradiance, a change in photosynthetic activity and the evaporative loss of water. By removing relative changes due to evaporative losses, new insights were gained into the driving factors controlling the biogeochemistry and primary productivity in the shallow Antarctic lake. In particular, a decrease in phytoplankton biomass was observed and was probably linked to photoinhibition as revealed by an increase in photoprotective pigments. The absorbance by dissolved organic matter, when weighted with respect to evaporative loss, shows an overall reduction in humic-like absorption, most likely linked to photodegradation.

Diurnal changes of photosynthetic quantum yield in the intertidal macroalga Sargassum thunbergii under simulated tidal emersion conditions

In this study, a three-way factorial experimental design was used to investigate the diurnal changes of photosynthetic activity of the intertidal macroalga Sargassum thunbergii in response to temperature, tidal pattern and desiccation during a simulated diurnal light cycle. The maximum (Fv/Fm) and effective (ΦPSII) quantum yields of photosystem II (PSII) were estimated by chlorophyll fluorescence using a pulse amplitude modulated fluorometer. Results showed that this species exhibited sun-adapted characteristics, as evidenced by the daily variation of Fv/Fm and ΦPSII. Both yield values decreased with increasing irradiance towards noon and recovered rapidly in the afternoon suggesting a dynamic photoinhibition. The photosynthetic quantum yield of S. thunbergii thalli varied significantly with temperature, tidal pattern and desiccation. Thalli were more susceptible to light-induced damage at high temperature of 25°C and showed complete recovery of photosynthetic activity only when exposed to 8°C. In contrast with the mid-morning low tide period, although there was an initial increase in photosynthetic yield during emersion, thalli showed a greater degree of decline at the end of emersion and remained less able to recover when low tide occurred at mid-afternoon. Short-term air exposure of 2h did not significantly influence the photosynthesis. However, when exposed to moderate conditions (4h desiccation at 15°C or 6h desiccation at 8°C), a significant inhibition of photosynthesis was followed by partial or complete recovery upon re-immersion in late afternoon. Only extreme conditions (4h desiccation at 25°C or 6h desiccation at 15°C or 25°C) resulted in the complete inhibition, with little indication of recovery until the following morning, implying the occurrence of chronic PSII damage. Based on the magnitude of effect, desiccation was the predominant negative factor affecting the photosynthesis under the simulated daytime irradiance period. These results may explain the distribution pattern of this species in natural habitats, where it is generally restricted to tide pools in the intertidal zone of wave-swept rocky shores which could provide shelter from desiccation stress during low tide.

Changes in photosynthetic activity in five common hybrids of oil palm (Elaeis guineensis Jacq.) seedlings under water deficit

The leaf water potential, gas-exchange parameters and chlorophyll fluorescence were evaluated in five common oil palm (Elaeis guineensis Jacq.) tenera hybrids 913X1988, 1425X2277, 748X1988, 7418X1988, and 690X1988 under water deficit with an aim to identify hybrids which can cope up better under such conditions and understand possible differences among hybrids in relation to the physiological mechanisms triggered by water deficit. Our findings indicate oil palm hybrids 913X1988, 1425X2277, and 7418X1988 maintained higher leaf water potentials than the other hybrids. Hybrids 7418X1988 and 1425X2277 recorded lower stomatal conductance after water deficit, which resulted in higher intrinsic water-use efficiency. The excess light energy produced due to decreased photosynthesis in 7418X1988 and 690X1988 hybrids under water deficit was dissipated as heat by higher nonphotochemical quenching. The maximum efficiency of photosystem II was not affected, even after withholding water for 24 days, suggesting an increased efficiency of photoprotection mechanisms in all these oil palm hybrids.

Changes in Photosynthetic Characteristics and Antioxidative Protection in Male and Female Ginkgo during Natural Senescence

Ginkgo (Ginkgo biloba), a dioecious tree species, is widely distributed throughout the world, yet little is known about sex-related responses to autumnal senescence in ginkgo. The aim of this study was to investigate changes in photosynthetic activities, concentration of oxidative stress parameters [malondialdehyde (MDA) and H2O2] and antioxidant systems, and ultrastructure of chloroplasts in the naturally senescing leaves of two ginkgo sexes and to examine whether progression of senescence is sex-specific in ginkgo. Photosynthesis in ginkgo leaves of both sexes was not limited by stomatal factors, but rather non-stomatal factors such as decreased photosynthetic pigments and photochemical activities that became more important during autumnal senescence. The responses of antioxidative enzymes were different from those of antioxidants to leaf senescence. Correlation analysis revealed that autumnal leaf senescence was significantly correlated to antioxidative enzymes changes but not to antioxidants such as ascorbate (ASA) and glutathione (GSH). Guaiacol peroxidase (POD) became more important in senescing leaves and played a major protective role, especially at the late stage of senescence. The shape of chloroplasts of both sexes changed from oblong to round, and there was an increase in the number and size of osmiophilic granules during senescence; swollen thylakoid membranes in the stroma and grana with a significant increase in MDA content were also observed. During autumnal senescence, female ginkgo plants showed smaller decreases in net photosynthetic rates, photosynthetic pigments, photochemical activities, superoxide dismutase, ascorbate peroxidase and catalase activities, higher POD activity, ASA and GSH contents, and smaller increases in H2O2 and MDA contents than did males. In addition, female plants had a later senescence of chloroplasts, a smaller accumulation of osmiophilic granules, and a slower rate of membrane damage. These results show that female ginkgo exhibit slower leaf senescence, which may be related to increased reproductive costs.

Molecular Crosstalk Between PAMP-Triggered Immunity and Photosynthesis

The innate immune system allows plants to respond to potential pathogens in an appropriate manner while minimizing damage and energy costs. Photosynthesis provides a sustained energy supply and, therefore, has to be integrated into the defense against pathogens. Although changes in photosynthetic activity during infection have been described, a detailed and conclusive characterization is lacking. Here, we addressed whether activation of early defense responses by pathogen-associated molecular patterns (PAMPs) triggers changes in photosynthesis. Using proteomics and chlorophyll fluorescence measurements, we show that activation of defense by PAMPs leads to a rapid decrease in nonphotochemical quenching (NPQ). Conversely, NPQ also influences several responses of PAMP-triggered immunity. In a mutant impaired in NPQ, apoplastic reactive oxygen species production is enhanced and defense gene expression is differentially affected. Although induction of the early defense markers WRKY22 and WRKY29 is enhanced, induction of the late markers PR1 and PR5 is completely abolished. We propose that regulation of NPQ is an intrinsic component of the plant's defense program.

Seasonal changes in photosynthetic activity and photochemical efficiency of the Mediterranean shrub Phillyrea angustifolia L

This research aimed to investigate the physiological strategies that allow Phillyrea angustifolia, a sclerophyllous shrub widespread in the Mediterranean maquis, to cope successfully with the most stressful conditions of the Mediterranean climate i.e. summer drought and winter cold. Gas exchange and chlorophyll a fluorescence measurements as well as pigment content determinations were performed periodically throughout one whole year (spring 2004–spring 2005) on plants growing outdoor in the Botanical Garden of the Naples University. In June and July 2004 and in February 2005 the light-saturated net photosynthetic rate (P N) and stomatal conductance (gH2O) were lower compared to May and October 2004. Carboxylation rate (v c) was negatively affected by summer drought and low temperature. The quantum yield (ΦPSII) and maximum photochemical efficiency of PSII (F v/F m) significantly declined in February. All monitored parameters increased after each stressful period. The highest carotenoid content was detected in July whereas total pigment content was lower both in July and in February as compared to May and October, respectively. The photosynthetic apparatus of P. angustifolia is able to cope with summer drought and winter cold by different strategies: in summer, efficient stomatal control reduces water loss whereas in winter, PSII efficiency is down-regulated to avoid irreversible damage.

Salt-induced changes in photosynthetic activity and oxidative defense system of three cultivars of mustard (Brassica juncea L.)

A greenhouse experiment was conducted to assess salt-induced modulation in gas exchange attributes and some key enzymes of oxidative defense system in three cultivars of mustard (Brassica juncea L.), Varuna, RH-30 and Rohini, under saline conditions. After 21 days of seed germination, three salt treatments (0, 100 and 200 mM NaCl) were applied to the root growing medium. Salt treatments for three weeks significantly decreased the biomass, shoot and root lengths, CO2-assimilation rate (A), transpiration rate (E), stomatal conductance (gs), chlorophyll a and b pigments and relative water contents (RWC), while electrolyte leakage, proline, H2O2,malondialdehyde (MDA) and activities of superoxide dismutase (SOD), peroxidise (POD), catalase (CAT) and glutathione reductase (GR) enzymes increased in all three mustard cultivars. In comparison with the two other cultivars, cv. Varuna had higher shoot fresh and dry weight, proline content and activities of SOD, ascorbate peroxidase (APX), CAT and GR, while RH-30 and Rohini were higher in electrolyte leakage, H2O2 and MDA levels. Overall, the differential growth of all three mustard cultivars under saline conditions was found to be attributable to differential photosynthetic and transpiration rates, stomatal conductance, chlorophyll pigments and relative water contents, hence these could be used as potential selection criteria for screening mustard germplasm for salt tolerance. Key words: Salt stress, Brassica juncea, antioxidant enzymes, proline.

Seasonal changes in photosynthetic activity and chlorophylls in the Scots pine and Siberian spruce with optimal or insufficient moistening

The paper studies the seasonal changes in photosynthetic activity and concentration of chlorophylls in the needles of the Scots pine Pinus sylvestris L. and Siberian spruce Picea obovata Ledeb. depending on the environmental factors. In drought, the daily peaks of photosynthesis are regulated by the level of moisture in the needles and concentration of chlorophylls. The work of chlorophyll is 2–6 times more effective with optimal moistening than in drought.

Assessing structural effects on PRI for stress detection in conifer forests

The retrieval of indicators of vegetation stress from remote sensing imagery is an important issue for the accurate assessment of forest decline. The Photochemical Reflectance Index (PRI) has been demonstrated as a physiological index sensitive to the epoxidation state of the xanthophyll cycle pigments and to photosynthetic efficiency, serving as a proxy for short-term changes in photosynthetic activity, stress condition, and pigment absorption, but highly affected by illumination conditions, viewing geometry and canopy structure. In this study, a diurnal airborne campaign was conducted over Pinus sylvestris and Pinus nigra forest areas with the Airborne Hyperspectral Scanner (AHS) to evaluate the effects of canopy structure on PRI when used as an indicator of stress in a conifer forest. The AHS airborne sensor was flown at two times (8:00 GMT and 12:00 GMT) over forest areas under varying field-measured stress levels, acquiring 2 m spatial resolution imagery in 80 spectral bands in the 0.43–12.5 μm spectral range. Five formulations of PRI (based on R 531 as a xanthophyll-sensitive spectral band) were calculated using different reference wavelengths, such as PRI 570 (reference band R REF = R 570), and the PRI modifications PRI m1 (R REF = R 512), PRI m2 (R REF = R 600), PRI m3 (R REF = R 670), and PRI m4 (R REF = R 570, R 670), along with other structural indices such as NDVI, SR, OSAVI, MSAVI and MTVI2. In addition, thermal bands were used for the retrieval of the land surface temperature. A radiative transfer modeling method was conducted using the LIBERTY and INFORM models to assess the structural effects on the PRI formulations proposed, studying the sensitivity of PRI m indices to detect stress levels while minimizing the effects caused by the conifer architecture. The PRI indices were related to stomatal conductance, xanthophyll epoxidation state (EPS) and crown temperature. The modeling analysis showed that the coefficient of variation (CV) for PRI was 50%, whereas the CV for PRIm1 (band R512 as a reference) was only 20%. Simulation and experimental results demonstrated that PRI m1 (R REF = R 512) was less sensitive than PRI (R REF = R 570) to changes in Leaf Area Index (LAI) and tree densities. PRI 512 was demonstrated to be sensitive to EPS at both leaf (r 2 = 0.59) and canopy level (r 2 = 0.40), yielding superior performance than PRI 570 (r 2 = 0.21) at the canopy level. In addition, PRI 512 was significantly related to water stress indicators such as stomatal conductance (Gs; r 2 = 0.45) and water potential (Ψ; r 2 = 0.48), yielding better results than PRI 570 (Gs, r 2 = 0.21; Ψ, r 2 = 0.21) due to the structural effects found on the PRI 570 index at the canopy level.