Gert Schansker

Drought stress effects on photosystem I content and photosystem II thermotolerance analyzed using Chl a fluorescence kinetics in barley varieties differing in their drought tolerance

Drought stress has multiple effects on the photosynthetic system. Here, we show that a decrease of the relative contribution of the I-P phase, DeltaV(IP) = -V(I) = (F(M)-F(I))/(F(M)- F(o)), to the fluorescence transient OJIP is observed in 10 drought-stressed barley and 9 chickpea varieties. The extent of the I-P loss in the barley varieties depended on their drought tolerance. The relative loss of the I-P phase seems to be related to a loss of photosystem (PS) I reaction centers as determined by 820-nm transmission measurements. In the second part of this study, the interaction of drought and heat stress in two barley varieties (the drought tolerant variety Aït Baha and the drought sensitive variety Lannaceur) was studied using a new approach. Heat stress was induced by exposing the plant leaves to temperatures of 25-45 degrees C and the inactivation of the O(2)-evolving complex (OEC) was followed measuring chlorophyll a (Chl a) fluorescence using a protocol consisting of two 5-ms pulses spaced 2.3 ms apart. In active reaction centers, the dark interval is long enough to allow the OEC to recover from the first pulse; whereas in heat-inactivated reaction centers it is not. In the latter category of reaction centers, no further fluorescence rise is induced by the second pulse. Lannaceur, under well-watered conditions, was more heat tolerant than Aït Baha. However, this difference was lost following drought stress. Drought stress considerably increased the thermostability of PS II of both varieties.

The IP amplitude of the fluorescence rise OJIP is sensitive to changes in the photosystem I content of leaves: a study on plants exposed to magnesium and sulfate deficiencies, drought stress and salt stress

The hypothesis that changes in the IP amplitude of the fluorescence transient OJIP reflect changes in leaf photosystem I (PSI) content was tested using mineral-deficient sugar beet plants. Young sugar beet plants (Beta vulgaris) were grown hydroponically on nutrient solutions containing either 1 mM or no Mg(2+) and 2.1 µM to 1.88 mM SO(4)(2-) for 4 weeks. During this period two leaf pairs were followed: the already developed second leaf pair and the third leaf pair that was budding at the start of the treatment. The IP amplitude [ΔF(IP) (fluorescence amplitude of the I-to-P-rise) and its relative contribution to the fluorescence rise: ΔV(IP) (amplitude of the relative variable fluorescence of the I-to-P-rise = relative contribution of the I-to-P-rise to the OJIP-rise)] and the amplitude of the transmission change at 820 nm (difference between all plastocyanin and the primary electron donor of photosystems I oxidized and reduced, respectively) relative to the total transmission signal (ΔI(max) /I(tot)) were determined as a function of the treatment time. Correlating the transmission and the two fluorescence parameters yielded approximately linear relationships in both cases. For the least severely affected leaves the parameter ΔV(IP) correlated considerably better with ΔI(max) /I(tot) than ΔF(IP) indicating that it is the ratio PSII:PSI that counts. To show that the relationship also holds for other plants and treatments, data from salt- and drought-stressed plants of barley, chickpea and pea are shown. The relationship between ΔV(IP) and PSI content was confirmed by western blot analysis using an antibody against psaD. The good correlations between ΔI(max) /I(tot) and ΔF(IP) and ΔV(IP) , respectively, suggest that changes in the IP amplitude can be used as semi-quantitative indicators for (relative) changes in the PSI content of the leaf.

Models of Chlorophyll a Fluorescence Transients

86 I. Fluorescence Induction 86 A. Relation Between Photosynthesis and Fluorescence 86 B. Summary of Used Fluorescence Techniques 87 C. Summary of Processes Reflected in the Fluorescence Rise 90 II. Approaches and Assumptions in the Modeling of the Fluorescence Rise 91 A. Why Model? 91 B. Why Measure and Simulate the Fluorescence Rise? 92 C. Relationship Between the Origin of Fluorescence and the Model Structure 92 D. Basic Types of Photosystem II Models 93 E. Kinetics and Rate Constants 95 F. Fluorescence and “Closed” Reaction Centers 97 G.Model Formulation and Simplification 99 III. Particular Models for the Fluorescence Rise 100 A. Modeling Fluorescence Rise in DCMU Inhibited Samples 100 1. Using Analytical Functions 100 2. The Application of Reversible Radical Pair Models 101 B. Modeling of the O–(J–)I–P Phases in Fluorescence Rise 102 1. Using Analytical Functions 102 2. Two-electron Gate Models 103 3. Extended Two-electron Gate Models 104 4. Combined Reversible Radical Pair and Two-electron Gate Models 104 5. Complex Models 107 IV. Modeling the Whole Fluorescence Induction 111 A. Experimental Observations and Models 111 B. Oscillations in the Fluorescence Intensity 114 V. Conclusions and Future Perspectives 115 Acknowledgments 115 References 115

Quantification of non-QB-reducing centers in leaves using a far-red pre-illumination

An alternative approach to quantification of the contribution of non-QB-reducing centers to Chl a fluorescence induction curve is proposed. The experimental protocol consists of a far-red pre-illumination followed by a strong red pulse to determine the fluorescence rise kinetics. The far-red pre-illumination induces an increase in the initial fluorescence level (F25 μs) that saturates at low light intensities indicating that no light intensity-dependent accumulation of QA− occurs. Far-red light-dose response curves for the F25 μs-increase give no indication of superimposed period-4 oscillations. F25 μs-dark-adaptation kinetics following a far-red pre-pulse, reveal two components: a faster one with a half-time of a few seconds and a slower component with a half-time of around 100 s. The faster phase is due to the non-QB-reducing centers that re-open by recombination between QA− and the S-states on the donor side. The slower phase is due to the recombination between QB− and the donor side in active PS II reaction centers. The pre-illumination-induced increase of the F25 μs-level represents about 4–5% of the variable fluorescence for pea leaves (∼2.5% equilibrium effect and 1.8–3.0% non-QB-reducing centers). For the other plant species tested these values were very similar. The implications of these values will be discussed.

The chl a fluorescence intensity is remarkably insensitive to changes in the chlorophyll content of the leaf as long as the chl a/b ratio remains unaffected.

The effects of changes in the chlorophyll (chl) content on the kinetics of the OJIP fluorescence transient were studied using two different approaches. An extensive chl loss (up to 5-fold decrease) occurs in leaves suffering from either an Mg(2+) or SO(4)(2-) deficiency. The effects of these treatments on the chl a/b ratio, which is related to antenna size, were very limited. This observation was confirmed by the identical light intensity dependencies of the K, J and I-steps of the fluorescence rise for three of the four treatments and by the absence of changes in the F(685 nm)/F(695 nm)-ratio of fluorescence emission spectra measured at 77K. Under these conditions, the F(0) and F(M)-values were essentially insensitive to the chl content. A second experimental approach consisted of the treatment of wheat leaves with specifically designed antisense oligodeoxynucleotides that interfered with the translation of mRNA of the genes coding for chl a/b binding proteins. This way, leaves with a wide range of chl a/b ratios were created. Under these conditions, an inverse proportional relationship between the F(M) values and the chl a/b ratio was observed. A strong effect of the chl a/b ratio on the fluorescence intensity was also observed for barley Chlorina f2 plants that lack chl b. The data suggest that the chl a/b ratio (antenna size) is a more important determinant of the maximum fluorescence intensity than the chl content of the leaf.

Chl a Fluorescence and 820 nm Transmission Changes Occurring During a Dark-to-Light Transition in Pine Needles and Pea Leaves: A Comparison

In dark-adapted leaves and needles, inactive ferredoxin-NADP+-reductase (FNR) forms a transient block of electron transport. We show here that the activation of this enzyme during a dark-to-light transition in Pinus brutia needles (and other gymnosperms) is much faster than in pea leaves (and other angiosperms). At the same time, inactivation of FNR in darkness is a much slower process in P. brutia needles than in pea leaves. The consequences of this difference for the interpretation of saturating pulse experiments is discussed. The special properties of FNR in pine needles are used to confirm our earlier observation that it is not possible to determine Fm (all centers closed) in the presence of an active photosystem I acceptor side. A comparison of the first 900 s of illumination following a dark-to-light transition shows that the slow activation kinetics of FNR in pea leaves provide an additional means to control electron flow during the activation of photosynthesis.