Simultaneously measuring pulse-amplitude-modulated (PAM) chlorophyll fluorescence of leaves at wavelengths shorter and longer than 700 nm

PAM fluorescence of leaves of cherry laurel (Prunus laurocerasus L.) was measured simultaneously in the spectral range below 700 nm (sw) and above 700 nm (lw). A high-sensitivity photodiode was employed to measure the low intensities of sw fluorescence. Photosystem II (PSII) performance was analyzed by the saturation pulse method during a light response curve with subsequent dark phase. The sw fluorescence was more variable, resulting in higher PSII photochemical yields compared to lw fluorescence. The variations between sw and lw data were explained by different levels of photosystem I (PSI) fluorescence: the contribution of PSI fluorescence to minimum fluorescence (F₀) was calculated to be 14% at sw wavelengths and 45% at lw wavelengths. With the results obtained, the validity of an earlier method for the quantification of PSI fluorescence (Genty et al. in Photosynth Res 26:133–139, 1990, https://doi.org/10.1007/BF00047085) was reconsidered. After subtracting PSI fluorescence from all fluorescence levels, the maximum PSII photochemical yield (F_V/F_M) in the sw range was 0.862 and it was 0.883 in the lw range. The lower F_V/F_M at sw wavelengths was suggested to arise from inactive PSII reaction centers in the outermost leaf layers. Polyphasic fluorescence transients (OJIP or OI₁I₂P kinetics) were recorded simultaneously at sw and lw wavelengths: the slowest phase of the kinetics (IP or I₂P) corresponded to 11% and 13% of total variable sw and lw fluorescence, respectively. The idea that this difference is due to variable PSI fluorescence is critically discussed. Potential future applications of simultaneously recording fluorescence in two spectral windows include studies of PSI non-photochemical quenching and state I–state II transitions, as well as measuring the fluorescence from pH-sensitive dyes simultaneously with chlorophyll fluorescence.

     

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of <Emphasis Type="Italic">F</Emphasis><Subscript>V</Subscript>/<Emphasis Type="Italic">F</Emphasis><Subscript>M</Subscript> on excitation wavelength

The F ₀ and F _M level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F _V/F _M (=(F _M − F ₀)/F _M) have been derived: within each of the three spectra of F _V/F _M, statistically meaningful variations were detected. Also, at distinct wavelength regions, the F _V/F _M differed significantly between leaf types. From spectra of F _V/F _M, excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F _V/F _M between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F _V/F _M at these wavelengths. All three leaves exhibited low values of F _V/F _M around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550–650 nm region, the F _V/F _M in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C₄ species, than in barley, a C₃ species. Finally, low values of F _V/F _M at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F _V/F _M ratio to derive information on spectral absorption properties of PS I and PS II is discussed.     

Long‐term measurements of chlorophyll a fluorescence using the JIP‐test show that combined abiotic stresses influence the photosynthetic performance of the perennial ryegrass (Lolium perenne) in a managed temperate grassland

Several experiments have highlighted the complexity of stress interactions involved in plant response. The impact in field conditions of combined environmental constraints on the mechanisms involved in plant photosynthetic response, however, remains understudied. In a long‐term field study performed in a managed grassland, we investigated the photosynthetic apparatus response of the perennial ryegrass (Lolium perenne L.) to environmental constraints and its ability to recover and acclimatize. Frequent field measurements of chlorophyll a fluorescence (ChlF) were made in order to determine the photosynthetic performance response of a population of L. perenne. Strong midday declines in the maximum quantum yield of primary photochemistry (F_VF_M) were observed in summer, when a combination of heat and high light intensity increased photosynthetic inhibition. During this period, increase in photosystem I (PSI) activity efficiency was also recorded, suggesting an increase in the photochemical pathway for de‐excitation in summer. Strong climatic events (e.g. heat waves) were shown to reduce electron transport between photosystem II (PSII) and PSI. This reduction might have preserved the PSI from photo‐oxidation. Periods of low soil moisture and high levels of sun irradiance increased PSII sensitivity to heat stress, suggesting increased susceptibility to combined environmental constraints. Despite the multiple inhibitions of photosynthetic functionality in summer, the L. perenne population showed increased PSII tolerance to environmental stresses in August. This might have been a response to earlier environmental constraints. It could also be linked to the selection and/or emergence of well‐adapted individuals.     

Comparative excitation‐emission dependence of the FV/FM ratio in model green algae and cyanobacterial strains

The emission spectra collected under conditions of open (F₀) and closed (F_M) photosystem II (PSII) reaction centres are close‐to‐independent from the excitation wavelength in Chlamydomonas reinhardtii and Chlorella sorokiniana, whereas a pronounced dependence is observed in Synechocystis sp. PCC6803 and Synechococcus PCC7942, instead. The differences in band‐shape between the F₀ and F_M emission are limited in green algae, giving rise only to a minor trough in the F_V/F_M spectrum in the 705–720 nm range, irrespectively of the excitation. More substantial variations are observed in cyanobacteria, resulting in marked dependencies of the measured F_V/F_M ratios on both the excitation and the detection wavelengths. In cyanobacteria, the maximal F_V/F_M values (0.5–0.7), observed monitoring at approximately 684 nm and exciting Chl a preferentially, are comparable to those of green algae; however, F_V/F_M decreases sharply below approximately 660 nm. Furthermore, in the red emission tail, the trough in the F_V/F_M spectrum is more pronounced in cyanobacteria with respect to green algae, corresponding to F_V/F_M values of 0.25–0.4 in this spectral region. Upon direct phycobilisomes excitation (i.e. >520 nm), the F_V/F_M value detected at 684 nm decreases to 0.3–0.5 and is close‐to‐negligible (approximately 0.1) below 660 nm. At the same time, the F_V spectra are, in all species investigated, almost independent on the excitation wavelength. It is concluded that the excitation/emission dependencies of the F_V/F_M ratio arise from overlapped contributions from the three independent emissions of PSI, PSII and a fraction of energetically uncoupled external antenna, excited in different proportions depending on the respective optical cross‐section and fluorescence yield.     

The use of parameters of chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence induction to evaluate the state of plants under anthropogenic load

The physiological state of the leaves of the small-leaved linden (Tilia cordata), silver birch (Betula pendula), and northern white cedar (Thuja occidentalis) under urban conditions was assessed via recording the kinetics of chlorophyll under fluorescence induction. Different sensitivities of the plants to adverse growing conditions were revealed. The most sensitive parameters of the fluorescence JIP test, viz., PI _ABS , F _V/F ₀, and F _V/F _M, were identified as indicators of the physiological state of the urban phytocoenosis. Recommendations for the application of the method for monitoring studies are presented.     

Phosphorus alleviates aluminum-induced inhibition of growth and photosynthesis in Citrus grandis seedlings

Limited data are available on the effects of phosphorus (P) and aluminum (Al) interactions on Citrus spp. growth and photosynthesis. Sour pummelo (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 50, 100, 250 and 500 μM KH₂PO₄× 0 and 1.2 mM AlCl₃· 6H₂O. Thereafter, P and Al in roots, stems and leaves, and leaf chlorophyll (Chl), CO₂ assimilation, ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) and Chl a fluorescence (OJIP) transients were measured. Under Al stress, P increased root Al, but decreased stem and leaf Al. Shoot growth is more sensitive to Al than root growth, CO₂ assimilation and OJIP transients. Al decreased CO₂ assimilation, Rubisco activity and Chl content, whereas it increased or did not affect intercellular CO₂ concentration. Al affected CO₂ assimilation more than Rubisco and Chl under 250 and 500 μM P. Al decreased root, stem and leaf P, leaf maximum quantum yield of primary photochemistry (F_v/F_m) and total performance index (PI_tot,abs), but increased leaf minimum fluorescence (F_o), relative variable fluorescence at K‐ and I‐steps. P could alleviate Al‐induced increase or decrease for all these parameters. We conclude that P alleviated Al‐induced inhibition of growth and impairment of the whole photosynthetic electron transport chain from photosystem II (PSII) donor side up to the reduction of end acceptors of photosystem I (PSI), thus preventing photosynthesis inhibition through increasing Al immobilization in roots and P level in roots and shoots. Al‐induced impairment of the whole photosynthetic electron transport chain may be associated with growth inhibition.     

Effects of ambient versus reduced UV-B radiation on high arctic Salix arctica assessed by measurements and calculations of chlorophyll a fluorescence parameters from fluorescence transients

A UV-B exclusion-experiment was conducted in the high Arctic Zackenberg, NE Greenland, in which Salix arctica leaves during most of the growing season were fixed perpendicular to the solar zenith angle, thereby receiving maximal solar radiation. Covered with Teflon and Mylar foil, the leaves received approximately 90 and 40% of the ambient UV-B irradiance, respectively. The effects were examined through recordings of chlorophyll a fluorescence transients, determination of biomass and analysis of total carbon and nitrogen content and amount of soluble flavonoids in the leaves. The processing of light was analysed by means of the chlorophyll a fluorescence transient, using the so-called JIP test, as evolved by Reto J. Strasser and his coworkers. Reduction of the UV-B irradiance caused a rise in many of the fluorescence parameters during July, but not in August (late season). Thus increases in the efficiency that an absorbed photon will be trapped by the PSII reaction centre with the resultant reduction of Q_A to Q_A^– (ET₀/ABS = F_V/F_M) and the efficiency that an electron residing on Q_A^– will enter the intersystem electron transport chain (ET₀/TR₀) were observed in reduced UV-B. Moreover, estimated per cross-section of leaf sample, the number of active PSII reaction centres (RC/CS_M) and electron transport rate (ET_M/CS_M) and all performance indexes (PI_ABS, PI_CSo and PI_CSm) were increased in reduced UV-B. The total soluble flavonoid content was highest in ambient UV-B. The treatment effects on fluorescence parameters that were directly measured (e.g. F₀ and F_M) and those that were derived (e.g. quantum efficiencies, parameters per PSII reaction centres and per cross-section of leaf sample) are discussed in relation to one another, in relation to daily and seasonal variation, and from the perspective of evaluating the relative importance of UV-B of donor and acceptor side capacity in Photosystem II. In conclusion, the experimental set-up and non-invasive measurements proved to be a sensitive method to screen for effects of UV-B stress.     

In search of a reversible stage of photoinhibition in a higher plant: No changes in the amount of functional Photosystem II accompany relaxation of variable fluorescence after exposure of lincomycin-treated Cucurbita pepo leaves to high light

Pumpkin (Cucurbita pepo L.) leaves in which chloroplast protein synthesis was inhibited with lincomycin were exposed to strong photoinhibitory light, and changes in F_O, F_M, F_V/F_M and in the amount of functional Photosystem II (O₂ evolution induced by saturating single-turnover flashes) were monitored during the high-light exposure and subsequent dark or low-light incubation. In the course of the photoinhibitory illumination, F_M, F_V/F_M and the amount of functional PS II declined continuously whereas F_O dropped rapidly to some extent and then slowly increased. If the experiments were done at room temperature, termination of the photoinhibitory illumination resulted in partial relaxation of the F_V/F_M ratio and in an increase in F_O and F_M. The relaxation was completed in 10–15 min after short-term (15 min) photoinhibitory treatment but continued 30–40 min if the exposure to high light was longer than 1 h. No changes in the amount of functional PS II accompanied the relaxation of F_V/F_M in darkness or in low light, in the presence of lincomycin. Transferring the leaves to low temperature (+4°C) after the room-temperature illumination (2 h) completely inhibited the relaxation of F_V/F_M. Low temperature did not suppress the relaxation if the photoinhibitory illumination had also been done at low temperature. The results indicate that illumination of lincomycin-poisoned pumpkin leaves at room temperature does not lead to accumulation of a reversibly photoinactivated intermediate.Abbreviations F_O, F_M chlorophyll fluorescence with all reaction centres open or closed, respectively - F_V variable fluorescence (F_V=F_M–F_O) - LHC Light-harvesting complex - PS II Photosystem II - Q_A, Q_B primary and secondary quinone electron acceptors of PS II, respectively - qN_E, qN_T, qN_I non-photochemical quenching due to high-energy state, state transition or photoinhibition, respectively     

Action of UV and visible radiation on chlorophyll fluorescence from dark-adapted grape leaves (Vitis vinifera L.)

Grapevine plants (Vitis vinifera L. cv. Silvaner) were cultivated under shaded conditions in the absence of UV radiation in a greenhouse, and subsequently placed outdoors under filters transmitting natural radiation, or screening out the UV-B (280 to 315 nm), or screening out the UV-A (315 to 400 nm) and the UV-B spectral range. All conditions decreased maximum chlorophyll fluorescence (F_M) and increased minimum chlorophyll fluorescence (F₀) from dark-adapted leaves; however, with increasing UV, F_M quenching was stimulated but increases in F₀ were reduced. The F_V/F_M ratio (where F_V=F_M-F₀) was clearly reduced by visible radiation (VIS): UV-B caused a moderate extra-reduction in F_V/F_M. Exposure of leaves (V. vinifera L. cv. Bacchus) to UV or VIS lamps quenched the F_M to similar extents; further, UV-B doses comparable to the field, quenched F₀. A model was developed to describe how natural radiation intensities affect PS II and thereby change leaf fluorescence. Fitting theory to experiment was successful when the same F_M yield for UV- and VIS-inactivated PS II was assumed, and for lower F₀ yields of UV- than for VIS-inactivated PS II. It is deduced, that natural UV can produce inactivated PS II exhibiting relatively high F_V/F_M. The presence of UV-inactivated PS II is difficult to detect by measuring F_V/F_M in leaves. Hence, relative concentrations of intact PS II during outdoor exposure were derived from F_M. These concentrations, but not F_V/F_M, correlated reasonably well with CO₂ gas exchange measurements. Consequently, PS II inhibition by natural UV could be a main factor for UV inhibition of photosynthesis.This revised version was published online in October 2005 with corrections to the Cover Date.     

Excitation and emission wavelength dependence of fluorescence spectra in whole cells of the cyanobacterium Synechocystis sp. PPC6803: Influence on the estimation of Photosystem II maximal quantum efficiency

The fluorescence emission spectrum of Synechocystis sp. PPC6803 cells, at room temperature, displays: i) significant bandshape variations when collected under open (F₀) and closed (F_M) Photosystem II reaction centre conditions; ii) a marked dependence on the excitation wavelength both under F₀ and F_M conditions, due to the enhancement of phycobilisomes (PBS) emission upon their direct excitation. As a consequence: iii) the ratio of the variable and maximal fluorescence (F_V/F_M), that is a commonly employed indicator of the maximal photochemical quantum efficiency of PSII (Φ_{pc, PSII}), displays a significant dependency on both the excitation and the emission (detection) wavelength; iv) the F_V/F_M excitation/emission wavelength dependency is due, primarily, to the overlap of PSII emission with that of supercomplexes showing negligible changes in quantum yield upon trap closure, i.e. PSI and a PBS fraction which is incapable to transfer the excitation energy efficiently to core complexes. v) The contribution to the cellular emission and the relative absorption-cross section of PSII, PSI and uncoupled PBS are extracted using a spectral decomposition strategy. It is concluded that vi) Φ_{pc, PSII} is generally underestimated from the F_V/F_M measurements in this organism and, the degree of the estimation bias, which can exceed 50%, depends on the measurement conditions. Spectral modelling based on the decomposed emission/cross-section profiles were extended to other processes typically monitored from steady-state fluorescence measurements, in the presence of an actinic illumination, in particular non-photochemical quenching. It is suggested that vii) the quenching extent is generally underestimated in analogy to F_V/F_M but that viii) the location of quenching sites can be discriminated based on the combined excitation/emission spectral analysis.     

The ratio of variable to maximum chlorophyll fluorescence from photosystem II,measured in leaves at ambient temperature and at 77K,as an indicator of the photon yield of photosynthesis

The response of a number of species to high light levels was examined to determine whether chlorophyll fluorescence from photosystem (PS) II measured at ambient temperature could be used quantitatively to estimate the photon yield of O₂ evolution. In many species, the ratio of the yield of the variable (F_V) and the maximum chlorophyll fluorescence (F_M) determined from leaves at ambient temperature matched that from leaves frozen to 77K when reductions in F_V/F_M and the photon yield resulted from exposure of leaves to high light levels under favorable temperatures and water status. Under conditions which were less favorable for photosynthesis, F_V/F_M at ambient temperature often matched the photon yield more closely than F_V/F_M measured at 77K. Exposure of leaves to high light levels in combination with water stress or chilling stress resulted in much greater reductions in the photon yield than in F_V/F_M (at both ambient temperature and 77K) measured in darkness, which would be expected if the site of inhibition was beyond PSII. Following chilling stress, F_V/F_M determined during measurement of the photon yield in the light was depressed to a degree more similar to that of the depression of photon yield, presumably as a result of regulation of PSII in response to greatly reduced electron flow.Abbreviations and Symbols F_o yield of instantaneous fluorescence - F_M yield of maximum fluorescence - F_V yield of variable fluorescence - PFD photon flux density (400–700 nm) - PSI (II) photosystem I (II) This work was supported by the Deutsche Forschungsgemeinchaft. W.W.A. gratefully acknowledges the support of Fellowships from the North Atlantic Treaty Organization and the Alexander von Humboldt-Stiftung. We also thank Maria Lesch for plant maintenance.     

Chlorophyll fluorescence tracks seasonal variations of photosynthesis from leaf to canopy in a temperate forest

Accurate estimation of terrestrial gross primary productivity (GPP) remains a challenge despite its importance in the global carbon cycle. Chlorophyll fluorescence (ChlF) has been recently adopted to understand photosynthesis and its response to the environment, particularly with remote sensing data. However, it remains unclear how ChlF and photosynthesis are linked at different spatial scales across the growing season. We examined seasonal relationships between ChlF and photosynthesis at the leaf, canopy, and ecosystem scales and explored how leaf‐level ChlF was linked with canopy‐scale solar‐induced chlorophyll fluorescence (SIF) in a temperate deciduous forest at Harvard Forest, Massachusetts, USA. Our results show that ChlF captured the seasonal variations of photosynthesis with significant linear relationships between ChlF and photosynthesis across the growing season over different spatial scales (R² = 0.73, 0.77, and 0.86 at leaf, canopy, and satellite scales, respectively; P < 0.0001). We developed a model to estimate GPP from the tower‐based measurement of SIF and leaf‐level ChlF parameters. The estimation of GPP from this model agreed well with flux tower observations of GPP (R² = 0.68; P < 0.0001), demonstrating the potential of SIF for modeling GPP. At the leaf scale, we found that leaf F_q’/F_m’, the fraction of absorbed photons that are used for photochemistry for a light‐adapted measurement from a pulse amplitude modulation fluorometer, was the best leaf fluorescence parameter to correlate with canopy SIF yield (SIF/APAR, R² = 0.79; P < 0.0001). We also found that canopy SIF and SIF‐derived GPP (GPP_SIF) were strongly correlated to leaf‐level biochemistry and canopy structure, including chlorophyll content (R² = 0.65 for canopy GPP_SIF and chlorophyll content; P < 0.0001), leaf area index (LAI) (R² = 0.35 for canopy GPP_SIF and LAI; P < 0.0001), and normalized difference vegetation index (NDVI) (R² = 0.36 for canopy GPP_SIF and NDVI; P < 0.0001). Our results suggest that ChlF can be a powerful tool to track photosynthetic rates at leaf, canopy, and ecosystem scales.     

Impacts of leaf age and heat stress duration on photosynthetic gas exchange and foliar nonstructural carbohydrates in Coffea arabica

Given future climate predictions of increased temperature, and frequency and intensity of heat waves in the tropics, suitable habitat to grow ecologically, economically, and socially valuable Coffea arabica is severely threatened. We investigated how leaf age and heat stress duration impact recovery from heat stress in C. arabica. Treated plants were heated in a growth chamber at 49°C for 45 or 90 min. Physiological recovery was monitored in situ using gas exchange, chlorophyll fluorescence (the ratio of variable to maximum fluorescence, F_V/F_M), and leaf nonstructural carbohydrate (NSC) on mature and expanding leaves before and 2, 15, 25, and 50 days after treatment. Regardless of leaf age, the 90‐min treatment resulted in greater F_V/F_M reduction 2 days after treatment and slower recovery than the 45‐min treatment. In both treatments, photosynthesis of expanding leaves recovered more slowly than in mature leaves. Stomatal conductance (g_s) decreased in expanding leaves but did not change in mature leaves. These responses led to reduced intrinsic water‐use efficiency with increasing heat stress duration in both age classes. Based on a leaf energy balance model, aftereffects of heat stress would be exacerbated by increases in leaf temperature at low g_s under full sunlight where C. arabica is often grown, but also under partial sunlight. Starch and total NSC content of the 45‐min group significantly decreased 2 days after treatment and then accumulated 15 and 25 days after treatment coinciding with recovery of photosynthesis and F_V/F_M. In contrast, sucrose of the 90‐min group accumulated at day 2 suggesting that phloem transport was inhibited. Both treatment group responses contrasted with control plant total NSC and starch, which declined with time associated with subsequent flower and fruit production. No treated plants produced flowers or fruits, suggesting that short duration heat stress can lead to crop failure.     

The different effects of chilling stress under moderate light intensity on photosystem II compared with photosystem I and subsequent recovery in tropical tree species

Tropical plants are sensitive to chilling temperatures above zero but it is still unclear whether photosystem I (PSI) or photosystem II (PSII) of tropical plants is mainly affected by chilling temperatures. In this study, the effect of 4°C associated with various light densities on PSII and PSI was studied in the potted seedlings of four tropical evergreen tree species grown in an open field, Khaya ivorensis, Pometia tomentosa, Dalbergia odorifera, and Erythrophleum guineense. After 8 h chilling exposure at the different photosynthetic flux densities of 20, 50, 100, 150 μmol m⁻² s⁻¹, the maximum quantum yield of PSII (F _v /F _m) in all of the four species decreased little, while the quantity of efficient PSI complex (P _m) remained stable in all species except E. guineense. However, after chilling exposure under 250 μmol m⁻² s⁻¹ for 24 h, F _v /F _m was severely photoinhibited in all species whereas P _m was relative stable in all plants except E. guineense. At the chilling temperature of 4°C, electron transport from PSII to PSI was blocked because of excessive reduction of primary electron acceptor of PSII. F _v /F _m in these species except E. guineense recovered to ~90% after 8 h recovery in low light, suggesting the dependence of the recovery of PSII on moderate PSI and/or PSII activity. These results suggest that PSII is more sensitive to chilling temperature under the moderate light than PSI in tropical trees, and the photoinhibition of PSII and closure of PSII reaction centers can serve to protect PSI.     

Photosystem 2 is more tolerant to high temperature in apple (<Emphasis Type="Italic">Malus domestica</Emphasis> Borkh.) leaves than in fruit peel

Tolerance of photosystem 2 (PS2) to high temperature in apple (Malus domestica Borkh. cv. Cortland) leaves and peel was investigated by chlorophyll a fluorescence (OJIP) transient after exposure to 25 (control), 40, 42, 44, and 46 °C in the dark for 30 min. The positive L-step was more pronounced in a peel than in leaves when exposed to 44 °C. Heat-induced K-step became less pronounced in leaves than in peel when exposed to 42 °C or higher temperature. Leaves had negative L-and K-steps relative to the peel. The decrease of oxygen-evolving complex (OEC) by heat stress was higher in the peel than in the leaves. OJIP transient from the 46 °C treated peel could not reach the maximum fluorescence (F_m). The striking thermoeffect was the big decrease in the relative variable fluorescence at 30 ms (V_I), especially in the leaves. Compared with the peel, the leaves had less decreased maximum PS2 quantum efficiency (F_v/F_m), photochemical rate constant (K_P), F_m and performance index (PI) on absorption basis (PI_abs) and less increased minimum fluorescence (F₀) and non-photochemical rate constant (K_N), but more increased reduction of end acceptors at PS1 electron acceptor side per cross section (RE₀/CS₀) and per reaction center (RE₀/RC₀), quantum yield of electron transport from Q_A ⁻ to the end acceptors (ϕ _R0) and total PI (PI_abs,total) when exposed to 44 °C. In conclusion, PS2 is more thermally labile than PS1. The reduction of PS2 activity by heat stress primarily results from an inactivation of OEC. PS2 was more tolerant to high temperature in the leaves than in the peel.     

Kinetics of prolonged photoinhibition revisited: photoinhibited Photosystem II centres do not protect the active ones against loss of oxygen evolution

Photoinhibition of Photosystem II (PSII) in lincomycin-treated leaves begins as a first-order reaction, but fluorescence measurements have suggested that after prolonged illumination, the number of active PSII centres stabilizes to 15–20% of control. The stabilization has been interpreted to indicate that photoinhibited PSII centres protect the remaining active centres against photoinhibition (Lee, Hong and Chow, Planta 212:332–342, 2001). In an attempt to study the mechanism of this protection, we measured the reaction kinetics of photoinhibition in lincomycin-treated pumpkin (Cucurbita pepo L.) and pepper (Capsicum annuum L.) leaves in vivo. The light-saturated rate of PSII oxygen evolution, assayed from thylakoids and isolated from the treated leaves, was used as a direct measure of the number of remaining active PSII centres, and the fluorescence parameters F _V/F _M and (F _V/F _M)/F ₀ (=1/F ₀ − 1/F _M) were measured for comparison. To our surprise, no stabilization of PSII activity was observed and photoinhibition followed first-order kinetics until PSII activity had virtually declined to zero. A series of in vitro experiments was carried out to see whether stabilization of PSII activity occurs if a particular combination of light intensity and wavelength range is applied, or if a specific PSII preparation is used as experimental material. The results of the in vitro experiments confirmed the in vivo result about persistent first-order kinetics. We conclude that photoinhibited PSII centres offer no measurable protection against photoinhibition.     

Effect of 24-Epibrassinolide on Chlorophyll Fluorescence and Photosynthetic CO<Subscript>2</Subscript> Assimilation in <Emphasis Type="Italic">Vicia faba</Emphasis> Plants Treated with the Photosynthesis-Inhibiting Herbicide Terbutryn

Simultaneous measurements of chlorophyll (Chl) fluorescence and CO₂ assimilation (A) in Vicia faba leaves were taken during the first weeks of growth to evaluate the protective effect of 24-epibrassinolide (EBR) against damage caused by the application of the herbicide terbutryn (Terb) at pre-emergence. V. faba seeds were incubated for 24 h in EBR solutions (2 × 10⁻⁶ or 2 × 10⁻⁵ mM) and immediately sown. Terb was applied at recommended doses (1.47 or 1.96 kg ha⁻¹) at pre-emergence. The highest dose of Terb strongly decreased CO₂ assimilation, the maximum quantum yield of PSII photochemistry in the dark-adapted state (F _V/F _M), the nonphotochemical quenching (NPQ), and the effective quantum yield (ΔF/F′_M) during the first 3–4 weeks after plant emergence. Moreover, Terb increased the basal quantum yield of nonphotochemical processes (F ₀/F _M)_, the degree of reaction center closure (1 − q _p), and the fraction of light absorbed in PSII antennae that was dissipated via thermal energy dissipation in the antennae (1 − F′_V/F′_M). The herbicide also significantly reduced plant growth at the end of the experiment as well as plant length, dry weight, and number of leaves. The application of EBR to V. faba seeds before sowing strongly diminished the effect of Terb on fluorescence parameters and CO₂ assimilation, which recovered 13 days after plant emergence and showed values similar to those of control plants. The protective effect of EBR on CO₂ assimilation was detected at a photosynthetic photon flux density (PFD) of 650 μmol m⁻² s⁻¹ and the effect on ΔF/F′_M and photosynthetic electron transport (J) was detected under actinic lightings up to 1750 μmol m⁻² s⁻¹. The highest dose of EBR also counteracted the decrease in plant growth caused by Terb, and plants registered the same growth values as controls.     

Photosynthetic Gas Exchange and Chlorophyll a Fluorescence in Salicornia brachiata (Roxb.) Under Osmotic Stress

Osmotic stress negatively affects the photosynthetic efficiency and cause a significant loss of crop productivity. Salicornia brachiata (Roxb.) is a eu-halophyte. We hereby report on photosynthetic gas exchange and chlorophyll fluorescence in S. brachiata under sodium chloride (NaCl), seawater and polyethylene glycol (PEG) induced osmotic stress. It grows luxuriantly and exhibited a higher tolerance index and better accumulation of organic solutes under 100% strength of seawater (32.5 ppt) and 0.5 M NaCl salinity. It exhibited comparatively better gas exchange, stomatal conductance, PSII photochemistry and electron transfer under 100% strength of seawater salinity. Higher chlorophyll a/b ratio under stress conditions indicated a lower ratio of PSII to PSI and balanced excitation of PSI and PSII in S. brachiata resulting in efficient photosynthetic processes. The lower total chlorophyll/carotenoids ratio and higher non-photochemical quenching indicated the photo-protection and safer dissipation of heat energy in S. brachiata under stress. The 100% strength of seawater and 0.5 M NaCl salinity in S. brachiata did not cause significant changes in antenna size, connectivity between PSII reaction centres (RCs) and reduction of electrons on PSII donor side. The 20% PEG induced the inactivation of RCs and cause damage to PSII RCs in S. brachiata thus reduced the electron transfer from QA^– to QB pool-sized and activity of water-splitting complex. Higher φ(P₀) and F_V/F_M in S. brachiata under seawater salinity indicated a comparatively better quantum yield of primary photochemistry. The higher PI_Total in S. brachiata under 100% strength of seawater and 0.5 M NaCl stress indicated a better energy flux reaching to PSII RCs, electron transport and performance of RCs. The higher strengths of osmotic stress cause reduction in the quantum yield of PSII electron transport and capturing efficiency of excitation energy by open PSII RCs in S. brachiata.

Graphic Abstract

     

Impacts of forest management type and season on soil carbon fluxes in Eastern Mau Forest,Kenya

The value of ecosystems functions performed by forests in the climate change era has prompted increasing attention towards assessment of carbon stocks and fluxes in tropical forests. The aim of this study was to understand how forest management approaches and environmental controls impacted on soil CO₂ efflux in a tropical Eastern Mau forest which is one of the blocks of the greater Mau complex in Kenya. Nested experimental design approach was employed where 32 plots were nested into four blocks (disturbed natural, undisturbed natural, plantation and glades). In 10 m² plots, data were collected on soil CO₂ efflux, soil temperature and soil moisture using soda lime methods, direct measurement and proxy techniques, respectively. There was significant forest management type effect (F_3,127 = 3.01, p = 0.033) and seasonality effect (t test = 3.31, df = 1, p < 0.05) on mean soil CO₂ efflux. The recorded mean soil CO₂ efflux levels were as follows: plantation forest (9.219 ± 3.067 g C M^?2 day^?1), undisturbed natural forest (8.665 ± 4.818 g C M^?2 day^?1), glades (8.592 ± 3.253 g C M^?2 day^?1) and disturbed natural forest (7.198 ± 3.457 g C M^?2 day^?1). The study concludes that managing a forest in plantation form is primarily responsible for forest soil CO₂ efflux levels due to aspects such as increased microbial activity and root respiration. However, further studies are required to understand the role and impact of soil CO₂ efflux on the greater forest carbon budget.     

Estimation of the effect of photoinhibition on the carbon gain in leaves of a willow canopy

The occurrence of photoinhibition of photosynthesis in leaves of a willow canopy was examined by measuring the chlorophyll-a fluorescence ratio of F _V/F _M (F_M is the maximum fluorescence level of the induction curve, and F_V is the variable fluorescence, F _V=F _M–F ₀, where F₀ is the minimal fluorescence). The majority of the leaves situated on the upper parts of peripheral shoots showed an afternoon inhibition of this ratio on clear days. This was the consequence of both a decrease in F _M and a rise in F _O. In the same leaves the diurnal variation in intercepted photosynthetic photon flux density (PPFD) was monitored using leaf-mounted sensors. Using the multivariate method, partial least squares in latent variables, it is shown that the dose of PPFD, integrated and linearly weighted over the last 6-h period, best predicts photoinhibition. Photoinhibition occurred even among leaves that did not intercept PPFDs above 1000 mol·m^–2·s^–1. Exposure of leaves to a standard photoinhibitory treatment demonstrated that the depression in the F _V/F _M ratio was paralleled by an equal depression in the maximal quantum yield of CO₂ uptake and a nearly equal depression in the rate of bending (convexity) of the light-response curve of CO₂ uptake. As a result, the rate of net photosynthesis is depressed over the whole natural range of PPFD. By simulating the daily course in the rate of net photosynthesis, it is estimated that in the order of one-tenth of the potential carbon gain of peripheral willow shoots is lost on clear days as a result of photoinhibition. This applies to conditions of optimal temperatures. Photoinhibition is even more pronounced at air temperatures below 23° C, as judged from measurements of the F_V/F_M ratio on clear days: the afternoon inhibition of this ratio increased in a curvilinear manner from 15% to 25% with a temperature decrease from 23° to 14° C.Abbreviations and Symbols F_O minimum fluorescence - F_V variable fluorescence - F_M maximum fluorescence - PLS partial least squares in latent variables - PPFD photosynthetic photon flux density - VPD water vapour-pressure deficit This study was supported by the Swedish Natural Science Research Council. We are indebted to Dr. Jerry Leverenz (Department of Plant Physiology, University of Umeå, Sweden) for guidance with the modelling of the photosynthesis data.