Light scattering, chlorophyll fluorescence and state of the adenylate system in illuminated spinach leaves

A tribute to Ulrich Heber (1930-2016) for his contribution to photosynthesis research: understanding the interplay between photosynthetic primary reactions, metabolism and the environment

The dynamic and efficient coordination of primary photosynthetic reactions with leaf energization and metabolism under a wide range of environmental conditions is a fundamental property of plants involving processes at all functional levels. The present historical perspective covers 60 years of research aiming to understand the underlying mechanisms, linking major breakthroughs to current progress. It centers on the contributions of Ulrich Heber who had pioneered novel concepts, fundamental methods, and mechanistic understanding of photosynthesis. An important first step was the development of non-aqueous preparation of chloroplasts allowing the investigation of chloroplast metabolites ex vivo (meaning that the obtained results reflect the in vivo situation). Later on, intact chloroplasts, retaining their functional envelope membranes, were isolated in aqueous media to investigate compartmentation and exchange of metabolites between chloroplasts and external medium. These studies elucidated metabolic interaction between chloroplasts and cytoplasm during photosynthesis. Experiments with isolated intact chloroplasts clarified that oxygenation of ribulose-1.5-bisphosphate generates glycolate in photorespiration. The development of non-invasive optical methods enabled researchers identifying mechanisms that balance electron flow in the photosynthetic electron transport system avoiding its over-reduction. Recording chlorophyll a (Chl a) fluorescence allowed one to monitor, among other parameters, thermal energy dissipation by means of 'nonphotochemical quenching' of the excited state of Chl a. Furthermore, studies both in vivo and in vitro led to basic understanding of the biochemical mechanisms of freezing damage and frost tolerance of plant leaves, to SO₂ tolerance of tree leaves and dehydrating lichens and mosses.

Storage of light-driven transthylakoid proton motive force as an electric field (Deltapsi) under steady-state conditions in intact cells of Chlamydomonas reinhardtii

Proton motive force (pmf) is physiologically stored as either a DeltapH or a membrane potential (Deltapsi) across bacterial and mitochondrial energetic membranes. In the case of chloroplasts, previous work (Cruz et al. 2001, Biochemistry 40: 1226-1237) indicates that Deltapsi is a significant fraction of pmf, in vivo, and in vitro as long as the activities of counterions are relatively low. Kinetic analysis of light-induced changes in the electrochromic shift (ECS) in intact leaves was consistent with these observations. In this work, we took advantage of the spectroscopic properties of the green alga, Chlamydomonas reinhardtii, to demonstrate that light-driven Deltapsi was stored in vivo over the hours time scale. Analysis of the light-induced ECS kinetics suggested that the steady-state Deltapsi in 400 micromol photons m(-2) s(-1) red light was between 20 and 90 mV and that this represented about 60% of the light-induced increase in pmf. By extrapolation, it was surmised that about half of total (basal and light-induced) pmf is held as Deltapsi. It is hypothesized that Deltapsi is stabilized either by maintaining low chloroplast ionic strength or by active membrane ion transporters. In addition to the strong implications for regulation of photosynthesis by the xanthophyll cycle, these results imply that pmf partitioning is important across a wide range of species.

Chloroplast energization and oxidation of P700/plastocyanin in illuminated leaves at reduced levels of CO2 or oxygen

Chlorophyll fluorescence, light scattering, the electrochromic shift P515 and levels of some photosynthetic intermediates were measured in illuminated leaves. Oxygen and CO2 concentrations in the gas phase were varied in order to obtain information on control of Photosystem II activity under conditions such as produced by water stress, when stomatal closure restricts access of CO2 to the photosynthetic apparatus. Light scattering and energy-dependent fluorescence quenching indicated a high level of chloroplast energization under high intensity illumination even when linear electron transport was curtailed in CO2-free air or in 1% oxygen with 35 μll(-1) CO2. Calculations of the phosphorylation potential based on measurements of phosphoglycerate, dihydroxyacetone phosphate and NADP revealed ratios of intrathylakoid to extrathylakoid proton concentrations, which were only somewhat higher in air containing 35 μl l(-1) CO2 than in CO2-free air or 1% oxygen/35 μl l(-1) CO2. Anaerobic conditions prevented appreciable chloroplast energization. Acceptor-limitation of electron flow resulted in a high reduction level of the electron transport chain, which is characterized by decreased oxidation of P700, not only under anaerobic conditions, but also in air, when CO2 was absent, and in 1% oxygen, when the CO2 concentration was reduced to 35 μll(-1). Efficient control of electron transport was indicated by the photoaccumulation of P700 (+) at or close to the CO2 compensation point in air. It is proposed to require the interplay between photorespiratory and photosynthetic electron flows, electron flow to oxygen and cyclic electron flow. The field-indicating electrochromic shift (P515) measured as a rapid absorption decrease on switching the light off followed closely the extent of photoaccumulation of P700 (+) in the light.

Tansley Review No. 36 Excited leaves

Photosynthesis is largely to do with energy transduction; the conversion of light energy into electrical energy into chemical energy. Precisely how much light energy is needed to bring about the reduction of one molecule of carbon dioxide and the release of one molecule of oxygen (the quantum requirement) is a matter of fundamental importance and one which has attracted much past controversy. This article concludes that a minimum quantum requirement of eight, as demanded by the Z-scheme, is obviously consistent with much contemporary work which puts the measured value for C₃ leaves close to nine. Moreover, while values of less than eight (obtained in some circumstances with micro-organisms), are a reminder that nothing is beyond challenge they are not, in the absence of confirmation and extension, sufficiently compelling to demand rejection of either the Z-scheme or current measuring procedures. This article also shows why, even if the underlying minimum requirement was now accepted beyond all reasonable doubt, there would still be very good reasons for continuing, indefinitely, to measure actual photosynthetic efficiency in the natural environment. It discusses some of the implications of the fact that all plants, if not stressed, appear to photosynthesize at the same rate in low light. It explains the role of fluorescence in its relation to quantum yield, the possibility that the rate of photosynthesis might be determined from fluorescence measurements alone, and that a combination of fluorescence and gas exchange measurements could provide new information about the manner in which 'dark respiration' is affected by light. It indicates how contemporary interest in all of these matters has focused attention on the necessity for safe dissipation of excitation energy by leaves and on the manner by which this might be achieved. CONTENTS Summary 325 I. Excitation 325 II. Quantum requirement 326 III. Learning from fluorescence 335 IV. Safely dissipated 340 Acknowledgements 342 References 342.

Chloroplast movements in leaves: Influence on chlorophyll fluorescence and measurements of light-induced absorbance changes related to ΔpH and zeaxanthin formation

Light-induced chloroplast movements were found to cause changes in chlorophyll fluorescence emission, closely matching those in leaf absorptance, both in terms of the kinetics and the maximum extent of the changes observed in different species. The results demonstrate that chloroplast movements can have a significant effect on the efficiency of light utilization in photosynthesis. They further show that chloroplast movements need to be taken into account in measurements of fluorescence quenching and especially in measurements of light-induced optical changes used to monitor zeaxanthin formation and ΔpH associated light scattering in leaves. Means of minimizing and of adjusting for the influences of chloroplast movements in such measurements are discussed.

Light-dependent pH changes in leaves of C3 plants : I. Recording pH changes in various cellular compartments by fluorescent probes

Chloroplasts, mesophyll protoplasts, cytoplasts, vacuoplasts, vacuoles and leaves were stained with pH-indicating fluorescent dyes of differing pK values. Fluorescence microscopy was used to obtain information on the intracellular and intercellular distribution of the probes. The kinetics of blue or green fluorescence emitted from chloroplasts, protoplasts, cytoplasts and leaves was measured during illumination with red light. The intensity of light used for fluorescence excitation was so low that it had little effect on photosynthesis. In leaves, fluorescence signals emitted from chloroplasts were small and usually insignificant compared to signals originating from the cytosol. Both indicated light-dependent alkalization and reversal of alkalization on darkening. Vacuolar signals were opposite in sign to cytosolic signals. They indicated acidification of the vacuole in the light-dark transient and reversal of this effect on darkening.

Role of the xanthophyll cycle in photoprotection elucidated by measurements of light-induced absorbance changes, fluorescence and photosynthesis in leaves of Hedera canariensis

The role of the xanthophyll cycle in regulating the energy flow to the PS II reaction centers and therefore in photoprotection was studied by measurements of light-induced absorbance changes, Chl fluorescence, and photosynthetic O2 evolution in sun and shade leaves of Hedera canariensis. The light-induced absorbance change at 510 nm (ΔA510) was used for continuous monitoring of zeaxanthin formation by de-epoxidation of violaxanthin. Non-radiative energy dissipation (NRD) was estimated from non-photochemical fluorescence quenching (NPQ).High capacity for zeaxanthin formation in sun leaves was accompanied by large NRD in the pigment bed at high PFDs as indicated by a very strong NPQ both when all PS II centers are closed (F'm) and when all centers are open (F'o). Such Fo quenching, although present, was less pronounced in shade leaves which have a much smaller xanthophyll cycle pool.Dithiothreitol (DTT) provided through the cut petiole completely blocked zeaxanthin formation. DTT had no detectable effect on photosynthetic O2 evolution or the photochemical yield of PS II in the short term but fully inhibited the quenching of Fo and 75% of the quenching of Fm, indicating that NRD in the antenna was largely blocked. This inhibition of quenching was accompanied by an increased closure of the PS II reaction centers.In the presence of DTT a photoinhibitory treatment at a PFD of 200 μmol m(-2) s(-1), followed by a 45 min recovery period at a low PFD, caused a 35% decrease in the photon yield of O2 evolution, compared to a decrease of less than 5% in the absence of DTT. The Fv/Fm ratio, measured in darkness showed a much greater decrease in the presence than in the absence of DTT. In the presence of DTT Fo rose by 15-20% whereas no change was detected in control leaves.The results support the conclusion that the xanthophyll cycle has a central role in regulating the energy flow to the PS II reaction centers and also provide direct evidence that zeaxanthin protects against photoinhibitory injury to the photosynthetic system.

Light-dependent pH changes in leaves of C3 plants : II. Effect of CO2 and O 2 on the cytosolic and the vacuolar pH

Illumination of leaves of C3 plants caused cytosolic alkalization and vacuolar acidification in the mesophyll cells. Both phenomena were particularly pronounced when CO2 was absent, were suppressed by CO2, and were related to the activation state of the photosynthetic apparatus. The cytosolic alkalization reaction has at least two major components. Trivalent cytosolic phosphoglycerate must be protonated before it can be transferred into the chloroplasts for reduction. Pumping of protons from the cytosol into the vacuole also contributes to cytosolic alkalization. The dependence of light scattering by chloroplast thylakoids on the energy fluence rate was closely related to that of vacuolar acidification under different conditions for chloroplast energization. This indicates (i) transport of energy from the chloroplasts to the cytosol in the light and (ii) use of this energy for the transport of protons into the vacuoles. The light-dependent vacuolar acidification is interpreted to be caused by the increase in the activity of a proton-translocating enzyme of the tonoplast. The decrease of vacuolar acidification during photosynthetic carbon reduction or photorespiration is indicative of decreased cytosolic energization. In low light, the light-dependent vacuolar acidification was stimulated in the absence of CO2 when photorespiration was inhibited. The data do not support the view that photorespiration is capable of increasing the cytosolic energy state in the light.

Diurnal changes in adenylates and nicotinamide nucleotides in sugar beet leaves

Sugar beets (Beta vulgaris L. cv. F58-554H1) were cultured hydroponically in growth chambers at 25°C, with a photon flux density of 500 μmol m(-2)s(-1). Measurements were made of net CO2 exchange, leaf adenylates (ATP, ADP and AMP), and leaf nicotinamide nucleotides (NAD(+), NADP(+), NADH, NADPH), over the diurnal period (16h light/8 h dark) and during photosynthetic induction. All the measurements were carried out on recently expanded leaves from 5-week-old plants. When the lights were switched on in the growth chamber, the rate of photosynthetic CO2 uptake, and the levels of leaf ATP and NADPH increased to a maximum in 30 min and remained there throughout the light period. The increase in ATP over the first few minutes of illumination was associated with the phosphorylation of ADP to ATP and the increase in NADPH with the reduction of NADP(+); subsequently, the increase in ATP was associated with an increase in total adenylates while the increase in NADPH was associated with an accumulation of NADP(+) and NADPH due to the light-driven phosphorylation of NAD(+) to NADP(+). On return to darkness, ATP and NADPH values decreased much more slowly, requiring 2 to 4 hours to reach minimum values. From these results we suggest that (i) the total adenylate and NADPH and NADP(+) (but not NAD(+) and NADH) pools increase following exposure to light; (ii) the increase in pool size is not accompanied by any large change in the energy or redox states of the system; and (iii) the measured ratios of ATP/ADP and NADPH/NADP(+) for intact leaves are low and constant during steady-state illumination.

Photoinhibition, 77K chlorophyll fluorescence quenching and phosphorylation of the light-harvesting chlorophyll-protein complex of photosystem II in soybean leaves

When the capacity of leaves for orderly dissipation of excitation energy in photosynthesis is exceeded, one mechanism by which the excess energy appears to be dissipated is through a nonradiative decay process. This process is observed as a reversible quenching of chlorophyll fluorescence emission (77K) from both photosystem II and photosystem I which persists in darkness (Demmig and Björkman 1987, Planta 171, 171-184). Fluorescence quenching was induced in soybean (Glycine max (L.) Merr.) leaves by two methods: 1) changing the composition of the gas surrounding the leaf from normal air to 2% O2, 0% CO2 at a low, constant photon flux density (PFD=photon fluence rate), and 2) increasing the PFD in the presence of normal air. In either case the quenching was fully reversible after return to the original condition (low PFD, normal air). The half-time of the relaxation of the quenching was in the order of 30 min. Both treatments resulted in reversible dephosphorylation of the light-harvesting chlorophyll-protein complex of photosystem II (LHC-II). Treatment under photoinhibitory conditions (high PFD plus chloramphenicol) also caused dephosphorylation of LHC-II. Therefore, phosphorylation of LHC-II cannot account for the observed fluorescence quenching. In addition, our results indicate that in vivo a factor other than the redox state of the plastoquinone pool controls LHC-II phosphorylation. This factor may be ΔpH, the pH gradient across the thylakoid membranes.

Regulation of the photosynthetic electron transport during dark-light transitions by activation of the ferredoxin-NADP(+)-oxidoreductase in higher plants

Absorbance changes associated with the oxidation and reduction of cytochrome f belong to the classical observations about the interaction of the two photosystems. A complex induction pattern of cytochrome f oxidation results, if both photosystems are excited simultaneously. This indicates a light-modulated regulation of the photosynthetic electron transport, which we examined for intact biological systems of decreasing complexity. The ferredoxin-NADP(+)-oxidoreductase (FNR) is suggested to be activated by light and inactivated in the dark. This is pointed out by the kinetics of variable fluorescence and by the influence of different artificial electron acceptors on the cytochrome f kinetics. The photoreduction of NADP(+) by carefully prepared thylakoids demonstrates the activation process directly.

The midday depression of CO2 assimilation in leaves of Arbutus unedo L.: diurnal changes in photosynthetic capacity related to changes in temperature and humidity

Parts of attached leaves of the sclerophyllous shrub Arbutus unedo were subjected to simulated mediterranean days. Gas exchange was recorded in order to recognize the causes of the midday depression in CO2 assimilation. Depressions could be induced in part of a leaf: they were local responses. The CO2-saturation curves of photosynthesis, determined during the morning and afternoon maxima of CO2 assimilation and during the minimum at midday, established that depressions in CO2 assimilation were in one-half of the investigated cases totally caused by reversible reductions in the photosynthetic capacity of the leaves, and in the other half almost totally caused by such reductions. An analysis of 37 daily courses showed that morning reductions and afternoon recoveries of stomatal conductance and rate of photosynthesis occurred simultaneously and in proportion to each other, with the result that the partial pressure of CO2 in the intercellular spaces remained more or less constant. Midday depressions occurred also in detached leaves standing in water. The initiation of a midday depression was not caused by a circadian rhythm, nor was high quantum flux or high temperature a requirement. There was no correlation between the rate of water loss from the leaves, or the amount of water lost, with the degree of reduction of the photosynthetic capacity. However, depressions occurred if an apparent threshold in the water-vapor pressure difference between leaf and air was exceeded. This critical value varied between about 20 and 30 mbar, depending on the leaf investigated. The dominating role of humidity in the induction of the midday depression was further demonstrated when leaf temperature was held constant and the vapor-pressure difference was made to follow the pattern of the mediterranean day: depressions occurred. Depressions however were hardly noticeable when the water-vapor pressure difference was held constant and leaf temperature was allowed to vary. In another set of experiments, leaves were subjected to variations in temperature and humidity independent of the time of the day, under otherwise constant conditions. Photosynthetic capacity and stomatal conductance proved to be almost insensitive to changes in temperature (in a range extending from 20 to 37° C) as long as the water vapor-pressure difference was held constant. If it was not, the rate of photosynthesis began to decline with increasing temperature after a threshold water-vapor pressure difference was exceeded. The position of the resulting apparent temperature optimum of photosynthesis depended on the humidity of the air. We suggest that the ability of A. unedo to respond to a dry atmosphere with a reversible reduction of its photosynthetic capacity (by a still unknown mechanism) is the result of a co-evolution with the development of a strong stomatal sensitivity to changes in humidity.

Stomatal aperture, photosythesis and water fluxes in mesophyll cells as affected by the abscission of leaves. Simultaneous measurements of gas exchange, light scattering and chlorphyll fluorescence

Carbon dioxide exchange, transpiration, chlorophyll fluorescence and light scattering of leaves of Lycopersicom esculentum, Helianthus annuus and Arbutus unedo were measured simultaneously before and after abscission of leaves. Scattering of a weak green measuring beam was used to monitor water fluxes across the thylakoid membranes of the mesophyll. When leaves were cut under water, stomata initially closed partially and then occasionally exhibited distinct regulatory oscillations. As stomata closed, light scattering decreased indicating water influx into the mesophyll. Stomatal oscillations were accompanied, with small but noticeable phase shifts, by oscillations of water fluxes at the thylakoid level. These fluxes could be distinguished from the water fluxes accompanying light-dependent ion pumping across the thylakoids by the concomitant chlorophyll fluorescence signals. The latter record energy-dependent ion fluxes in addition to redox changes of the electron-transport chain. As stomata closed partially after cutting a leaf under water, photosynthesis decreased. In Arbutus unedo and Helianthus annuus leaves, transient stomatal closure was insufficient to account for transient inhibition of photosynthesis which appeared to be brought about by transfer of an inhibitory solute through the petiole into the mesophyll. This solute also stimulated respiration in the dark. When leaves were cut in air, stomata opened transiently (Iwanoff effect) before wilting enforced closure. Photosynthesis followed the stomatal responses, increasing during opening and decreasing during closure.

The relationship between the redox state of Q A and photosynthesis in leaves at various carbon-dioxide, oxygen and light regimes

The response of chlorophyll fluorescence elicited by a low-fluence-rate modulated measuring beam to actinic light and to superimposed 1-s pulses from a high-fluence-rate light source was used to measure the redox state of the primary acceptor Q A of photosystem II in leaves which were photosynthesizing under steady-state conditions. The leaves were exposed to various O2 and CO2 concentrations and to different energy fluence rates of actinic light to assess the relationship between rates of photosynthesis and the redox state of Q A. Both at low and high fluence rates, the redox state of Q A was little altered when the CO2 concentration was reduced from saturation to about 600 μl·l(-1) although photosynthesis was decreased particularly at high fluence rates. Upon further reduction in CO2 content the amount of reduced Q A increased appreciably even at low fluence rates where light limited CO2 reduction. Both in the presence and in the absence of CO2, a more reduced Q A was observed when the O2 concentration was below 2%. Q A was almost fully reduced when leaves were exposed to high fluence rates under nitrogen. Even at low fluence rates, Q A was more reduced in shade leaves of Asarum europaeum and Fagus sylvatica than in leaves of Helianthus annuus and Fagus sylvatica grown under high light. Also, in shade leaves the redox state of Q A changed more during a transition from air containing 350 μl·l(-1) CO2 to CO2-free air than in sun leaves. The results are discussed with respect to the energy status and the CO2-fixation rate of the leaves.

Regulation of photosynthetic CO2-pathway enzymes by light and other factors

The regulatory properties of enzymes of the pathway of CO2 fixation are discussed in relation to changes in regulatory parameters with changing light, CO2 and temperature.

Photoinhibition of photosynthesis under anaerobic conditions studied with leaves and chloroplasts of Spinacia oleracea L

The role of oxygen in the photoinactivation of the photosynthetic apparatus of Spinacia oleracea L. was investigated. Moderate irradiation (1200 μmol photons m(-2)s(-1)) of spinach leaves in an atmosphere of pure nitrogen caused strong inhibition of subsequently measured net CO2 assimilation, whereas considerably less photoinhibition was observed in the presence of low partial pressures (10-20 mbar) of O2. The decrease in activity caused by anaerobiosis in the light was not based on stomatal closure; the decline of assimilation represents a photoinhibition, as activity was not impaired by low irradiation (80 μmol photos m(-2)s(-1)). In contrast, gassing with pure N2 in the dark caused strong inhibition. Electron-transport rates and chlorophyll-fluorescence data of thylakoids isolated from photoinhibited leaves indicated damage to the electron-transport system, in particular to photosystem II reaction centers. In vitro, photoinhibition in isolated thylakoid membranes was also strongly promoted by anaerobiosis. Photoinhibition of electron-transport rates under anaerobic conditions was characterized by a pronounced increase in the initial fluorescence level, F0, of chlorophyll-fluorescence induction, in contrast to photoinhibition under aerobic conditions. The results are discussed in terms of two mechanisms of photoinhibition, one that is suppressed and a second that is promoted by oxygen.

Chlorophyll a fluorescence and light-scattering kinetics displayed by leaves during induction of photosynthesis

Light-scattering, which can be taken as an indicator of the transthylakoid proton-gradient, and chlorophyll a fluorescence, have been followed simultaneously during re-illumination of spinach leaves at different energy fluence rates and carbon dioxide concentrations. The slow fluorescence transient ("M peak"), which has been associated with photosynthetic induction, was observed in air only at the lower fluence rates used. Data are presented that indicate that M peaks in chlorophyll fluorescence kinetics can only be observed if there is also a simultaneous transient in light-scattering and that these transients are observed when the dark period is relatively long, fluence rate relatively low, and CO2 concentration relatively high.The results are discussed in relation to the varying demands on ATP by carbon assimilation during induction of photosynthesis at different carbon dioxide concentrations and the manner in which these variations influence the quenching of chlorophyll a fluorescence.

The relationship between light scattering and chlorophyll a fluorescence during oscillations in photosynthetic carbon assimilation

Light scattering, which can be taken as an indicator of the transthylakoid proton gradient, and the 518-nm rise, which can be regarded as a measure of the transthylakoid membrane potential, have been followed during oscillations in chlorophyll a fluorescence, which are known to be associated with corresponding changes in photosynthetic carbon assimilation. Both components oscillated in a manner which was broadly reciprocal to chlorophyll a fluorescence. However, there was a phase shift such that the light-scattering change usually anticipated fluorescence and often also the 518-nm shift. It is concluded that the proton motive force rises and falls slightly in advance of rises and falls in carbon assimilation. The relationship of these changes to a possible underlying mechanism is discussed.

Simultaneous and independent effects of abscisic acid on stomata and the photosynthetic apparatus in whole leaves

(±)-Abscisic acid (ABA) at 10(-5) M was added to the transpiration stream of leaves of 16 species (C3 and C4, monocotyledons and dicotyledons). Stomatal responses followed one of three patterns: i) stomata that were wide and insensitive to CO2 initially, closed partially and became sensitive to CO2; ii) for stomata that were sensitive to CO2 before the application of ABA, the range of highest sensitivity to CO2 shifted from high to low intercellular partial pressures of CO2, for instance in leaves of Zea mays from 170-350 to 70-140 μbar; iii) when stomata responded strongly to ABA, their conductance was reduced to a small fraction of the initial conductance, and sensitivity to CO2 was lost. The photosynthetic apparatus was affected by applications of ABA to various degrees, from no response at all (in agreement with several previous reports on the absence of effects of ABA on photosynthesis) through a temporary decrease of its activity to a lasting reduction. Saturation curves of photosynthesis with respect to the partial pressure of CO2 in the intercellular spaces indicated that application of ABA could produce three phenomena: i) a reduction of the initial slope of the saturation curve (which indicates a diminished carboxylation efficiency); ii) a reduction of the level of the CO2-saturated rate of assimilation (which indicates a reduction of the ribulose-1,5-bisphosphate regeneration capacity); and iii) an increase of the CO2 compensation point. Photosynthesis of isolated mesophyll cells was not affected by ABA treatments. Responses of the stomatal and photosynthetic apparatus were usually synchronous and often proportional to each other, with the result that the partial pressure of CO2 in the intercellular spaces frequently remained constant in spite of large changes in conductance and assimilation rate. Guard cells and the photosynthetic apparatus were able to recover from effects of ABA applications while the ABA supply continued. Recovery was usually partial, in the case of the photosynthetic apparatus occasionally complete. Abscisic acid did not cause stomatal closure or decreases in the rate of photosynthesis when it was applied during a phase of stomatal opening and induction of photosynthesis that followed a transition from darkness to light.

Ecophysiological aspects of biomass production in higher plants

Photosynthesis as a cosmic process of local entropy decrease is the basic phenomenon for life on Earth. Biomass production is energetically understood as a long-term storage of hydrogen. Endogeneously, the hierarchy of energy storage begins with the electric and proton gradient across the thylakoid membrane. At all higher levels of energy-converting system, efficiency is determined by the interactions between the autotrophic and heterotrophic part of the studied biosystem. Specially, the regulatory function of anabolic and catabolic reduction charge is discussed. From the frame of exogeneous factors, the action of calcium in connection with calmodulin is described as a second messenger.Summarizing, the findings are integrated into the energy-overflow concept in which the loss terms act as system stabilizers especially under injuring conditions.

Steady-state room temperature fluorescence and CO2 assimilation rates in intact leaves

Steady-state room temperature variable fluorescence from leaves was measured as a function of CO2 pressure in Xanthium strumarium L. and Phaseolus vulgaris L. Measurements were made in a range of light intensities, at normal and low O2 parital pressure and over a range of temperatures.At low CO2 pressure fluorescence increased with increasing CO2. At higher CO2 pressure fluorescence usually decreased with increasing CO2 but occasionally increased slightly. The transition CO2 pressure between the responses could be changed by changing light, O2 pressure, or temperature. This breakpoint in the fluorescence-CO2 curve was a reliable indicator of the transition between ribulose 1,5-bisphosphate (RuBP) saturated assimilation and RuBP regeneration limited assimilation. The fluorescence signal was not a reliable indicator of O2-insensitive assimilation in these C3 species.

Thylakoid membrane stability in drought-tolerant and drought-sensitive plants

The stress stability of membranes from two drought-tolerant plants (Craterostigma plantagineum andCeterach officinarum) was compared with that of a drought-sensitive plant (Spinacia oleracea) in model experiments. Thylakoids from these plants were exposed to excessive sugar or salt concentrations or to freezing. All stresses caused loss of membrane function as indicated by the loss of cyclic photophosphorylation or the inability of the membranes to maintain a large proton gradient in the light. However, loss of membrane functions caused by osmotic dehydration in the presence of sugars was reversible. Irreversible membrane damage during freezing or exposure to salt was attributed mainly to chaotropic solute effects. The sensitivity to different stresses was comparable in thylakoid membranes from tolerant and sensitive plants indicating that the stress tolerance of a plant can hardly be attributed to specific membrane structures which would increase membrane stability. Levels of membrane-compatible solutes such as sugars or amino acids, among them proline, were much higher in the drought-tolerant plants than in spinach. Isolated thylakoids suspended in solutions containing an excess of sugars remained functional after dehydration by freeze-drying. This indicates that membrane-compatible solutes are important in preventing membrane damage during dehydration of poikilohydric plants.

Carbon dioxide gas exchange and the energy status of leaves of Primula palinuri under water stress

The photosynthetic rate of water stressed leaves of Primula palinuri was reduced drastically by stomatal closure, not by limitations imposed on the capacity of the photosynthetic apparatus, when water loss exceeded 20% of the water content of turgid leaves. The sudden decrease in phtosynthesis was not observed when the lower epidermis of the leaves had been removed. In these 'stripped' leaves, inhibition of photosynthesis increased only gradually during the wilting caused by increasing water stress and was complete when the relative water content was as low as 20% compared with the initial value. This corresponded to a water potential of about-40 bar. The light intensity at which half-maximum rates of photosynthesis were observed decreased as stress increased. In intact leaves photosynthesizing in the presence of CO2, light scattering, which is a measure of thylakoid energization, increased steeply during stomatal closure. The observed increase corresponded to the light scattering level measured in the absence of CO2. When the lower epidermis was removed, no sudden increase in thylakoid energization could be observed during dehydration. Thylakoid energization remained high even at low water potentials. It decreased drastically only below a relative water content of 20%. Irrespective, of the extent of water stress, CO2 fixation of stripped leaves increased when the oxygen content of air was reduced from 21% to 2%. Usually the transition from 21 to 2% O2 was accompanied by increased thylakoid energization. The increase in energization was more pronounced below than above a relative water content of 50%. The data show that energy-dissipating photorespiratory CO2 turnover in the in tercellular space of water-stressed leaves whose stomata are closed decreases only slowly as water stress increases. Respiratory CO2 production by leaves in the dark was even more resistant to water stress than photosynthesis. It was still significant at water potentials as low as-80 bar.