Erhard Pfündel

Regulation and possible function of the violaxanthin cycle

This paper discusses biochemical and regulatory aspects of the violaxanthin cycle as well as its possible role in photoprotection. The violaxanthin cycle responds to environmental conditions in the short-term and long-term by adjusting rates of pigment conversions and pool sizes of cycle pigments, respectively. Experimental evidence indicating a relationship between zeaxanthin formation and non-photochemical energy dissipation is reviewed. Zeaxanthin-associated energy dissipation appears to be dependent on transthylakoid ΔpH. The involvement of light-harvesting complex II in this quenching process is indicated by several studies. The current hypotheses on the underlying mechanism of zeaxanthin-dependent quenching are alterations of membrane properties, including conformational changes of the light-harvesting complex II, and singlet-singlet energy transfer from chlorophyll to zeaxanthin

Linking chlorophyll a fluorescence to photosynthesis for remote sensing applications: mechanisms and challenges

Chlorophyll a fluorescence (ChlF) has been used for decades to study the organization, functioning, and physiology of photosynthesis at the leaf and subcellular levels. ChlF is now measurable from remote sensing platforms. This provides a new optical means to track photosynthesis and gross primary productivity of terrestrial ecosystems. Importantly, the spatiotemporal and methodological context of the new applications is dramatically different compared with most of the available ChlF literature, which raises a number of important considerations. Although we have a good mechanistic understanding of the processes that control the ChlF signal over the short term, the seasonal link between ChlF and photosynthesis remains obscure. Additionally, while the current understanding of in vivo ChlF is based on pulse amplitude-modulated (PAM) measurements, remote sensing applications are based on the measurement of the passive solar-induced chlorophyll fluorescence (SIF), which entails important differences and new challenges that remain to be solved. In this review we introduce and revisit the physical, physiological, and methodological factors that control the leaf-level ChlF signal in the context of the new remote sensing applications. Specifically, we present the basis of photosynthetic acclimation and its optical signals, we introduce the physical and physiological basis of ChlF from the molecular to the leaf level and beyond, and we introduce and compare PAM and SIF methodology. Finally, we evaluate and identify the challenges that still remain to be answered in order to consolidate our mechanistic understanding of the remotely sensed SIF signal.

Estimating the contribution of Photosystem I to total leaf chlorophyll fluorescence

Chlorophyll a fluorescence characteristics were investigated in 12 species and 2 hybrids from the genus Flaveria exhibiting C3, C3–C4 intermediate, or C4 photosynthesis, and in the C4 species Zea mays. At room temperature, the variable fluorescence divided by the maximum fluorescence (FV/FM) of dark-adapted leaves decreased from C3 to C4 plants. This trend was qualitatively paralleled by an increase of the 735 nm peak relative to the 685 nm peak (F735/F685) of fluorescence emission spectra measured at low temperature (77 K). The variations were analysed using a quantitative model that takes into account higher PS I fluorescence in C4 plants than in C3 plants. The model predicts a linear correlation between 1/(FV/FM) and F735/F685, and was experimentally confirmed. From linear regression analysis, the FV/FM of PS II was calculated to be 0.88. By comparing the FV/FM of PS II with the FV/FM from leaves, the PS I contribution to total F0 fluorescence at wavelengths greater than 700 nm was determined to be about 30% and 50% in C3 and C4 plants, respectively. The corresponding values for the FM fluorescence were 6% and 12%. It is concluded that the effects of PS I fluorescence are significant and should be taken into account when analysing fluorescence data.

A new monitoring PAM fluorometer (MONI-PAM) to study the short- and long-term acclimation of photosystem II in field conditions

We present and evaluate the performance of a new field monitoring PAM fluorometer (MONI-PAM) which is intended for short- and long-term monitoring of the acclimation of photosystem II (PSII). The instrument measures chlorophyll fluorescence, photosynthetic photon flux density (PPFD), and temperature in the field, and monitors exactly the same leaf area over prolonged periods of time, facilitating the estimation of both rapidly reversible and sustained non-photochemical quenching (NPQ). The MONI-PAM performance is evaluated in the lab and under natural conditions in a Scots pine canopy during spring recovery of photosynthesis. The instrument provides a new tool to study in detail the acclimation of PSII to the environment under natural field conditions.

Investigating UV screening in leaves by two different types of portable UV fluorimeters reveals in vivo screening by anthocyanins and carotenoids

Two portable instruments, designed to evaluate epidermal UV screening in leaves, were compared: the Dualex and the UV-A-PAM fluorimeter. Both instruments excite chlorophyll fluorescence at the same UV wavelengths but reference excitation is in the red and the blue spectral range in the former and the latter fluorimeter, respectively. When analyzing green leaves, general agreement of the data is obtained with the two instruments. In the presence of anthocyanins, the UV-A-PAM fluorimeter provided higher estimates for epidermal UV transmittance than the Dualex fluorimeter, which was attributed to absorption of blue excitation light by anthocyanins. By comparing data from the instruments, anthocyanin-dependent transmittance of 50% was determined in abaxial sides of some autumn leaves, and also in abaxial sides of tropical shade plants. Further, with leaves of chlorophyll b-less mutants of H. vulgare, unusually high epidermal UV transmittance was detected but this was attributed to the lack of chlorophyll b absorption and, in addition, to absorption of blue radiation by xanthophylls which are not functionally connected to photosystems.

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 (FM) and increased minimum chlorophyll fluorescence (F0) from dark-adapted leaves; however, with increasing UV, FM quenching was stimulated but increases in F0 were reduced. The FV/FM ratio (where FV=FM-F0) was clearly reduced by visible radiation (VIS): UV-B caused a moderate extra-reduction in FV/FM. Exposure of leaves (V. vinifera L. cv. Bacchus) to UV or VIS lamps quenched the FM 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 FM 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 FV/FM. The presence of UV-inactivated PS II is difficult to detect by measuring FV/FM in leaves. Hence, relative concentrations of intact PS II during outdoor exposure were derived from FM. These concentrations, but not FV/FM, 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.

Violaxanthin de-epoxidase in etiolated leaves.

In etiolated leaves the occurrence of the enzymatic violaxanthin de-epoxidation to zeaxanthin is shown. The carotenoid transformation is provoked by the infiltration of whole leaves with ascorbate at pH 5 and is susceptible to DTT. Identification of the de-epoxidase activity is achieved by in vivo spectroscopy and pigment analysis (TLC).

A quantitative description of fluorescence excitation spectra in intact bean leaves greened under intermittent light

We present a simple approach for the calculation of in vivo fluorescence excitation spectra from measured absorbance spectra of the isolated pigments involved. Taking into account shading of the pigments by each other, energy transfer from carotene to chlorophyll a, and light scattering by the leaf tissue, we arrive at a model function with 6 free parameters. Fitting them to the measured fluorescence excitation spectrum yields good correspondence between theory and experiment, and parameter estimates which agree with independent measurements. The results are discussed with respect to the origin and the interpretation of in vivo excitation spectra in general.

Chlorophyll a Fluorescence (77 K) and Zeaxanthin Formation in Leaf Disks (Nicotiana tabacum) and Isolated Thylakoids (Lactuca sativa)

In higher plants and in most of the eukaryotic algal classes light-dependent reversible interconversions of 5,6-epoxycarotenoids (xanthophyll cycles) were found (see [1,2,3] for a review). Two types of cycle are known. In higher plants as well as in Chlorophyceae and in Phaeophyceae the violaxanthin cycle is known to be present. It starts with the enzymatic de-epoxidation of violaxanthin (5,6,5′,6′-diepoxyzeaxanthin) via antheraxanthin (5,6-monoepoxyzeaxanthin) to zeaxanthin induced by strong light; the back reaction occurring on a different pathway is light-independent. According to the transmembrane model of the violaxanthin cycle, the de-epoxidase which has its optimum at pH 5 and requires ascorbate for its activity in vitro, is located at the loculus side of the thylakoid membrane, while epoxidation takes place at the stroma side of the membrane (optimum at pH 7.5, cosubstrates: NADPH, O2). The de-epoxidation system is present in both grana and stroma thylakoids [2]. The ability to de-epoxi-date violaxanthin has been established in etiolated leaves [4].

Linking chloroplast relocation to different responses of photosynthesis to blue and red radiation in low and high light-acclimated leaves of Arabidopsis thaliana (L.)

Low light (LL) and high light (HL)-acclimated plants of A. thaliana were exposed to blue (BB) or red (RR) light or to a mixture of blue and red light (BR) of incrementally increasing intensities. The light response of photosystem II was measured by pulse amplitude-modulated chlorophyll fluorescence and that of photosystem I by near infrared difference spectroscopy. The LL but not HL leaves exhibited blue light-specific responses which were assigned to relocation of chloroplasts from the dark to the light-avoidance arrangement. Blue light (BB and BR) decreased the minimum fluorescence (