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.     

High photosynthetic efficiency of a rice (Oryza sativa L.) xantha mutant

Comparative analysis revealed that a xantha rice mutant (cv. Huangyu B) had higher ratios of chlorophyll (Chl) a/b and carotenoids/Chl, and higher photosynthetic efficiency than its wild type parent (cv. II32 B). Unexpectedly, the mutant had higher net photosynthetic rate (P _N) than II32 B. This might have resulted from its lower non-photochemical quenching (q_N) but higher maximal photochemical efficiency (F_V/F_M), higher excitation energy capture efficiency of photosystem 2 (PS2) reaction centres (F_V′/F_M′), higher photochemical quenching (q_P), higher effective PS2 quantum yield (Φ_PS2), and higher non-cyclic electron transport rate (ETR). This is the first report of a chlorophyll mutant that has higher photosynthetic efficiency and main Chl fluorescence parameters than its wild type. This mutant could become a unique material both for the basic research on photosynthesis and for the development of high yielding rice cultivars.     

Inhibition of photosynthetic processes in foliose lichens induced by temperature and osmotic stress

Negative effects of osmotically-induced dehydration of two foliose lichen species, Lasallia pustulata and Umbilicaria hirsuta, was studied at physiological (22 °C), low (5 °C) and freezing temperature (−10 °C), using chlorophyll (Chl) fluorescence. In both species, exposure to increasing sucrose concentrations led to a pronounced decrease in potential (F_V/F_M), and actual (Φ₂) quantum yields of photochemical processes in photosystem 2. L. pustulata was more sensitive to osmotic stress, because comparable osmotic dehydration inhibited F_V/F_M and Φ₂ more than in U. hirsuta. Critical concentration of sucrose that fully inhibited photochemical processes of photosynthesis was 2.5 M, which represented water potential (Ψ_w) of −18.8 MPa. Decrease in background Chl fluorescence (F₀) and increase in non-photochemical quenching (qN) revealed two phases of osmotic stress in lichens: phase I with no change (Ψ_w 0 to −6.6 MPa) and phase II (Ψ_w −11.3 to −18.8 MPa) typical by substantial change in Chl fluorescence parameters. Effects of thallus anatomy on species-specific response to osmotic dehydration is discussed and attributed to the results obtained by optical microscopy and Chl fluorescence imaging technique.     

Photosynthetic light-response curves and photoinhibition of the deep-water laminaria abyssalis and the intertidal laminaria digitata (phaeophyceae)

Photosynthetic light‐response curves of the deep‐water Laminaria abyssalis Oliveira and of the intertidal L. digitata Lamoroux were determined and related to photoinhibition phenomena as monitored by oxygen evolution and photosystem II efficiency (F_V/F_M). L. abyssalis has half the pigment content, number of cells and plastids, and photosynthetic capacity per unit area compared with L. digitata. L. abyssalis showed a higher in vivo Chl a absorption coefficient and higher photosynthetic efficiency on a Chl a basis, although the two algae showed somewhat similar light‐response curves on a Chl a basis. Both species showed similar Chl a/Chl c and Chl a/fucoxanthin ratios, and similar dark respiration rates and light compensation points. In addition, they also showed similar convexities in their light‐response curves and no differences in their light saturation of F_V/F_M. Room temperature chlorophyll fluorescence induction measurements of fronds incubated in 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU) suggest that both species may have a similar PSII absorption cross section. Thus, L. abyssalis appears to optimize its light absorption at very low light intensities, not by increasing the pigment content, but by absorbing light more efficiently. However, L. abyssalis was more sensitive to photoinhibition than L. digitata and showed no recovery of F_V/F_M and O₂ evolution after a photoinhibitory treatment, even with a subsequent exposure to 24 h of dim light. L. digitata, on the other hand, recovered its photosynthetic capacity within 6 h under dim light. These results suggest that photosynthetic light‐induction curves based on Chl a are not a good indicator of either the photosynthetic capacity or the sensitivity to photoinhibition when macroalgae of different species are being compared. Based on their light‐response and photoinhibition characteristics, we suggest that L. abyssalis, a deep‐water oceanic macroalgae, is an atypical shade alga whereas L. digitata has the properties of a sun alga.     

Pigment composition and functional state of the thylakoid membranes during preparation of samples for pigment-protein complexes separation by nondenaturing gel electrophoresis

The present study was conducted to examine changes in photosynthetic pigment composition and functional state of the thylakoid membranes during the individual steps of preparation of samples that are intended for a separation of pigmentprotein complexes by nondenaturing polyacrylamide gel electrophoresis. The thylakoid membranes were isolated from barley leaves (Hordeum vulgare L.) grown under low irradiance (50 μmol m⁻² s⁻¹). Functional state of the thylakoid membrane preparations was evaluated by determination of the maximal photochemical efficiency of photosystem (PS) II (F_V/F_M) and by analysis of excitation and emission spectra of chlorophyll a (Chl a) fluorescence at 77 K. All measurements were done at three phases of preparation of the samples: (1) in the suspensions of osmotically-shocked broken chloroplasts, (2) thylakoid membranes in extraction buffer containing Tris, glycine, and glycerol and (3) thylakoid membranes solubilized with a detergent decyl-β-D-maltosid. F_V/F_M was reduced from 0.815 in the first step to 0.723 in the second step and to values close to zero in solubilized membranes. Pigment composition was not pronouncedly changed during preparation of the thylakoid membrane samples. Isolation of thylakoid membranes affected the efficiency of excitation energy transfer within PSII complexes only slightly. Emission and excitation fluorescence spectra of the solubilized membranes resemble spectra of trimers of PSII light-harvesting complexes (LHCII). Despite a disrupted excitation energy transfer from LHCII to PSII antenna core in solubilized membranes, energy transfer from Chl b and carotenoids to emission forms of Chl a within LHCII trimers remained effective.     

Photosynthesis performance in sweet almond [<Emphasis Type="Italic">Prunus dulcis</Emphasis> (Mill) D. Webb] exposed to supplemental UV-B radiation

Due to anthropogenic influences, solar UV-B irradiance at the earth’s surface is increasing. To determine the effects of enhanced UV-B radiation on photosynthetic characteristics of Prunus dulcis, two-year-old seedlings of the species were submitted to four levels of UV-B stress, namely 0 (UV-B_c), 4.42 (UV-B₁), 7.32 (UV-B₂) and 9.36 (UV-B₃) kJ m⁻² d⁻¹. Effects of UV-B stress on a range of chlorophyll (Chl) fluorescence parameters (FPs), Chl contents and photosynthetic gas-exchange parameters were investigated. UV-B stress promoted an increase in minimal fluorescence of dark-adapted state (F₀) and F₀/F_m, and a decrease in variable fluorescence (F_v, F_v/F_m, F_v/F₀ and F₀/F_m) due to its adverse effects on photosystem II (PSII) activity. No significant change was observed for maximal fluorescence of dark-adapted state (F_m). Enhanced UV-B radiation caused a significant inhibition of net photosynthetic rate (P _N) at UV-B₂ and UV-B₃ levels and this was accompanied by a reduction in stomatal conductance (g _s) and transpiration rate (E). The contents of Chl a, b, and total Chl content (a+b) were also significantly reduced at increased UV-B stress. In general, adverse UV-B effects became significant at the highest tested radiation dose 9.36 kJ m⁻² d⁻¹. The most sensitive indicators for UV-B stress were F_v/F₀, Chl a content and P _N. Significant P<0.05 alteration in these parameters was found indicating the drastic effect of UV-B radiation on P. dulcis.     

Photosystem II efficiency of the palisade and spongy mesophyll in <Emphasis Type="Italic">Quercus coccifera</Emphasis> using adaxial/abaxial illumination and excitation light sources with wavelengths varying in penetration into the leaf tissue

The existence of major vertical gradients within the leaf is often overlooked in studies of photosynthesis. These gradients, which involve light heterogeneity, cell composition, and CO₂ concentration across the mesophyll, can generate differences in the maximum potential PSII efficiency (F _V/F _M or F _V/F _P) of the different cell layers. Evidence is presented for a step gradient of F _V/F _P ratios across the mesophyll, from the adaxial (palisade parenchyma, optimal efficiencies) to the abaxial (spongy parenchyma, sub-optimal efficiencies) side of Quercus coccifera leaves. For this purpose, light sources with different wavelengths that penetrate more or less deep within the leaf were employed, and measurements from the adaxial and abaxial sides were performed. To our knowledge, this is the first report where a low photosynthetic performance in the abaxial side of leaves is accompanied by impaired F _V/F _P ratios. This low photosynthetic efficiency of the abaxial side could be related to the occurrence of bundle sheath extensions, which facilitates the penetration of high light intensities deep within the mesophyll. Also, leaf morphology (twisted in shape) and orientation (with a marked angle from the horizontal plane) imply direct sunlight illumination of the abaxial side. The existence of cell layers within leaves with different photosynthetic efficiencies makes appropriate the evaluation of how light penetrates within the mesophyll when using Chl fluorescence or gas exchange techniques that use different wavelengths for excitation and/or for driving photosynthesis.     

Contributions of the free oxidized and QB-bound plastoquinone molecules to the thermal phase of chlorophyll-a fluorescence

Variable chlorophyll a (Chl a) fluorescence is composed of a photochemical and a thermal phases of similar amplitudes. The photochemical phase can be induced by a saturating single turnover flash (STF) and reflects the reduction of the Photosystem II (PS II) Q_A primary electron acceptor. The thermal phase requires multiple turnover flash (MTF) and is somehow related to the reduction of the plastoquinone (PQ) molecules. This article aimed to determine the relative contributions of the Q_B-bound and the free oxidized PQ molecules to the thermal phase of Chl a fluorescence. We thus measured the interactive effects of exogenous PQ (PQex), of an inhibitor (DCMU) acting at the Q_B site of PS II and of an artificial quencher, 2-methyl-1,4-naphtoquinone, on Chl a fluorescence levels induced by STF (F_F) and MTF (F_M) in spinach thylakoids. We observed that: (1) the incorporation of PQex in thylakoids stimulated photosynthetic electron transport but barely affected F_F and F_M in the absence of DCMU; (2) DCMU significantly increased the amplitude of F_F but slightly quenched F_M; (3) 2-methyl-1,4-naphtoquinone quenched F_M to a larger-extent than F_F; (4) DCMU increased the quenching effects of PQex on F_F and F_M and also, of methyl-1,4-naphtoquinone on F_F. These results indicate that: (1) the Q_B-bound and the free PQ molecules contribute to about 56% and 25%, respectively, to the thermal phase Chl a fluorescence in dark-adapted thylakoids; and (2) the thermal phase of Chl a fluorescence is more susceptible than the photochemical phase to the non-photochemical quenching effect of oxidized quinones. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of different excitation and detection spectral regions on room temperature chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence parameters

The effects of different spectral region of excitation and detection of chlorophyll (Chl) a fluorescence at room temperature on the estimation of excitation energy utilization within photosystem (PS) 2 were studied in wild-type barley (Hordeum vulgare L. cv. Bonus) and its Chl b-less mutant chlorina f2 grown under low and high irradiances [100 and 1 000 μmol(photon) m⁻² s⁻¹]. Three measuring spectral regimes were applied using a PAM 101 fluorometer: (1) excitation in the red region (maximum at the wavelength of 649 nm) and detection in the far-red region beyond 710 nm, (2) excitation in the blue region (maximum at the wavelength of 461 nm) and detection beyond 710 nm, and (3) excitation in the blue region and detection in the red region (660– 710 nm). Non-photochemical quenching of maximal (NPQ) and minimal fluorescence (SV₀), determined by detecting Chl a fluorescence beyond 710 nm, were significantly higher for blue excitation as compared to red excitation. We suggest that this results from higher non-radiative dissipation of absorbed excitation energy within light-harvesting complexes of PS2 (LHC2) due to preferential excitation of LHC2 by blue radiation and from the lower contribution of PS1 emission to the detected fluorescence in the case of blue excitation. Detection of Chl a fluorescence originating preferentially from PS2 (i.e. in the range of 660–710 nm) led to pronounced increase of NPQ, SV₀, and the PS2 photochemical efficiencies (F_V/F_M and F_V′/F_M′), indicating considerable underestimation of these parameters using the standard set-up of PAM 101. Hence PS1 contribution to the minimal fluorescence level in the irradiance-adapted state may reach up to about 80 %.     

Effect of zinc and cadmium on physiological and production characteristics in <Emphasis Type="Italic">Matricaria recutita</Emphasis>

Effects of zinc (12–180 μM) alone and in mixtures with 12 μM Cd on metal accumulation, dry masses of roots and shoots, root respiration rate, variable to maximum fluorescence ratio (F_V/F_M), and content of photosynthetic pigments were studied in hydroponically cultivated chamomile (Matricaria recutita) plants. The content of Zn in roots and shoots increased with the increasing external Zn concentration and its accumulation in the roots was higher than that in the shoots. While at lower Zn concentrations (12 and 60 μM) the presence of 12 μM Cd decreased Zn accumulation in the roots, treatment with 120 and 180 μM Zn together with 12 μM Cd caused enhancement of Zn content in the root. Presence of Zn (12–120 μM) decreased Cd accumulation in roots. On the other hand, Cd content in the shoots of plants treated with Zn + Cd exceeded that in the plants treated only with 12 μM Cd. Only higher Zn concentrations (120 and 180 μM) and Zn + Cd mixtures negatively influenced dry mass, chlorophyll (Chl) and carotenoid content, F_V/F_M and root respiration rate. Chl b was reduced to a higher extent than Chl a.     

In vivo analysis of chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence induction

Quantitative characteristics of photosynthetic electron transport were evaluated in vivo on the basis of the multi-exponential analysis of OJIP fluorescence transients induced by saturating actinic light. The OJIP fluorescence curve F(t), measured in Chlamydomonas reinhardtii cells, was transformed into the (1 − F _O/F(t)) × (F _V /F _M)⁻¹ transient, which is shown to relate to PS 2 closure. We assumed that kinetics of PS 2 closure during OJIP rise reflects time-separated processes related to the establishment of redox equilibrium at the PS 2 acceptor side (OJ), PQ pool (JI), and beyond Cyt b/f (IP). Three-exponential fitting was applied to (1 − F _O/F(t)) × (F _V /F _M)⁻¹ transient to obtain lifetimes and amplitudes of the OJ, JI, and IP components of PS 2 closure, which were used to calculate overall rates of reduction and re-oxidation of the PS 2 acceptor side, PQ pool, and intermediates beyond Cyt b/f complex. The results, obtained in the presence of inhibitors, oxidative reagents, and under different stress conditions prove the suggested model and characterize the introduced parameters as useful indicators of photosynthetic function.     

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.     

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.     

Sensitivity of Photosystem 2 of Antarctic Lichens to High Irradiance Stress: Fluorometric Study of Fruticose (Usnea antarctica) and Foliose (Umbilicaria decussata) Species

Two lichen species collected in maritime Antarctica (King George Island) were exposed under laboratory conditions to excess irradiance to evaluate the response of photosystem 2 (PS2). The response was measured on fully hydrated lichen thalli at 5 °C by means of a modulated fluorometer using chlorophyll (Chl) fluorescence induction curve supplemented with analysis of quenching mechanisms. Chl fluorescence parameters [i.e. ratio of variable to maximum Chl fluorescence (F_V/F_M), quantum yield of PS2 photochemical reactions (₂), quenching coefficients] were evaluated before and several times after exposition to high irradiance in order to characterise the extent of photoinhibition, fast and slow phase of recovery. Strong irradiance (2 000 mol m^–2 s^–1) caused high degree of photoinhibition, particularly higher in fruticose (Usnea antarctica) than in foliose (Umbilicaria decussata) lichen species. Fast phase of recovery from photoinhibition, corresponding to regulatory mechanisms of PS2, was more apparent in U. decussata and ₂ than in U. antarctica and F_V/F_M and ₂ within 40 min after photoinhibitory treatment. It was followed by a slow phase lasting several hours, corresponding to repair and re-synthesis processes. After photoinhibitory treatment, recovery of non-photochemical quenching (NPQ) was faster and more pronounced in U. decussata than in U. antarctica. Significant differences were found between the two species in the rate of recovery in fast-(q_E) and slow-recovering (q_T+I) component of NPQ.     

Photosynthetic responses of radish (<Emphasis Type="Italic">Raphanus sativus</Emphasis> var. <Emphasis Type="Italic">longipinnatus</Emphasis>) plants to infection by turnip mosaic virus

Plant growth, chlorophyll (Chl) content, photosynthetic gas exchange, ribulose-1,5-bisphosphate carboxylase (RuBPCO) enzyme activity, and Chl fluorescence in radish (Raphanus sativus var. longipinnatus) plants were examined after turnip mosaic virus (TuMV) infection. Plant fresh mass, dry mass, Chl content, net photosynthetic rate (P _N), transpiration rate (E), stomatal conductance (g _s), and RuBPCO activity were significantly lower in infected plants after 5 weeks of virus infection as compared to healthy plants. The 5-week virus infection did not induce significant differences in intercellular CO₂ concentration (C _i, photochemical efficiency of photosystem 2, PS2 (F_v/F_m), excitation capture efficiency of open PS2 reaction centres (F_v'/F_m'), effective quantum efficiency of photosystem 2 (ΔF/F_m'), and photochemical quenching (q_P), but non-photochemical quenching (q_N) and alternative electron sink (AES) were significantly enhanced. Thus the decreased plant biomass of TuMV-infected plants might be associated with the decreased photosynthetic activity mainly due to reduced RuBPCO activity.     

Low Temperature Tolerance of Tobacco Plants Transformed to Accumulate Proline,Fructans, or Glycine Betaine. Variable Chlorophyll Fluorescence Evidence

Tobacco (Nicotiana tabacum L.) has been transformed to accumulate different compatible solutes (proline, fructans, or glycine betaine) in order to improve its tolerance to abiotic stress. Photosynthetic activity of wild Type (wt) and transformed tobacco plants before and after freezing stress was studied by measuring chlorophyll (Chl) fluorescence. The JIP test of Chl fluorescence induction was used to analyze in details the functional activity of photosystem 2. No significant differences were found among wild Type and transgenic plants after 12 h of freezing. Both plant Types maintained the same values of the measured parameters [F_V/F_M, PI(CS_M), ABS/RC, TR₀/RC, ET/RC] after recovery of stress. The studied Chl fluorescence parameters decreased only for the wild Type plants, stressed for 24 h at –2 °C. The strong inhibition of photosynthetic reactions in the wt plant after 24 h of freezing could not be restored. The evaluated parameters of transgenic plants did not change significantly after 24 h at –2 °C and successfully survived freezing stress.     

Photoinhibition at chilling temperature

The effects of moderate light at chilling temperature on the photosynthesis of unhardened (acclimated to +18° C) and hardened (cold-acclimated) spinach (Spinacea oleracea L.) leaves were studied by means of fluorescence-induction measurements at 20° C and 77K and by determination of quantum yield of O₂ evolution. Exposure to 550 mol photons·m^-2·s^-1 at +4° C induced a strong photoinhibition in the unhardened leaves within a few hours. Photoinhibition manifested by a decline in quantum yield was characterized by an increase in initial fluorescence (F _o) and a decrease in variable fluorescence (F _v) and in the ratio of variable to maximum fluorescence (F _V/F _M), both at 77K and 20° C. The decline in quantum yield was more closely related to the decrease in the F _V/F _M ratio measured at 20° C, as compared with F _V/F _M at 77K. Quenching of the variable fluorescence of photosystem II was accompanied by a decline in photosystem-I fluorescence at 77K, indicating increased thermal de-excitation of pigments as the main consequence of the light treatment. All these changes detected in fluorescence parameters as well as in the quantum yield of O₂ evolution were fully reversible within 1–3 h at a higher temperature in low light. The fast recovery led us to the view that this photoinhibition represents a regulatory mechanism protecting the photosynthetic apparatus from the adverse effects of excess light by increasing thermal energy dissipation. Long-term cold acclimation probably enforces other protective mechanisms, as the hardened leaves were insensitive to the same light treatment that induced strong inhibition of photosynthesis in unhardened leaves.Abbreviations F ₀ initial fluorescence - F _M maximum fluorescence - F _V variable fluorescence (F _M-F ₀ - PFD photon flux density - PS photosystem     

Measurement of Differences in Red Chlorophyll Fluorescence and Photosynthetic Activity between Sun and Shade Leaves by Fluorescence Imaging

With a flash-lamp chlorophyll (Chl) fluorescence imaging system (FL-FIS) the photosynthetic activity of several thousand image points of intact shade and sun leaves of beech were screened in a non-destructive way within a few seconds. The photosynthetic activity was determined via imaging the Chl fluorescence at maximum F_p and steady state fluorescence F_s of the induction kinetics (Kautsky effect) and by a subsequent determination of the images of the fluorescence decrease ratio R_Fd and the ratio F_p/F_s. Both fluorescence ratios are linearly correlated to the photosynthetic CO₂ fixation rates. This imaging method permitted to detect the gradients in photosynthetic capacity and the patchiness of photosynthetic quantum conversion across the leaf. Sun leaves of beech showed a higher photosynthetic capacity and differential pigment ratios (Chl a/b and Chls/carotenoids) than shade leaves. Profile analysis and histogram of the Chl fluorescence yield and the Chl fluorescence ratios allow to quantify the differences in photosynthetic activity between different leaf parts and between sun and shade leaves with a high statistical significance.     

Acclimation of winter wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis>, cv. Yangmai 13) to low levels of solar irradiance

Winter wheat is a grass species widely planted in northern and central China, where the increase of aerosols, air pollutants and population density are causing significant reduction in solar irradiance. In order to investigate the adaptation of winter wheat (Triticum aestivum L., cv. Yangmai 13) to low irradiance conditions occurring in the downstream plain of the Yangtze River (China), plants were subjected to four solar irradiance treatments (100%, 60%, 40%, and 20% of environmental incident solar irradiance). Significant increases in chlorophyll (Chl) and xanthophyll (Xan) pigments, and decreases in Chl a/b and Xan/Chl ratios were observed in plants under low light. Light-response curves showed higher net photosynthetic rates (P _N) in fully irradiated plants, that also showed a higher light-compensation point. Shaded plants maintained high values of minimal fluorescence of dark-adapted state (F_o) and maximum quantum efficiency of PSII photochemistry (F_v/F_m) that assess a lower degree of photoinhibition under low light. Reduced irradiance caused decreases in effective quantum yield of PSII photochemistry (Φ_PSII), electron transport rate (ETR), and nonphotochemical quenching coefficient (q_N), and the promotion of excitation pressure of PSII (1 − q_P). The activities of the antioxidant enzymes superoxide dismutase and peroxidase were high under reduced light whereas no light-dependent changes in catalase activity were observed. Thiobarbituric acid reactive species content and electrolyte leakage decreased under shaded plants that showed a lower photooxidative damage. The results suggest that winter wheat cv. Yangmai 13 is able to maintain a high photosynthetic efficiency under reduced solar irradiance and acclimates well to shading tolerance. The photosynthetic and antioxidant responses of winter wheat to low light levels could be important for winter wheat cultivation and productivity.