Electron Transport-Dependent Chlorophyll-a Fluorescence Quenching by O(2) in Various Algae and Higher Plants

A comparison of chlorophyll-a fluorescence in brown algae (Macrocystis integrifolia, Fucus vesiculosis), green algae (Scenedesmus obliquus, Ulva sp.) and higher plants (bean, corn) show differences in the relative fluorescence intensities and induction time courses which characterize each type of plant. These differences are not reflected in either the maximum fluorescence emission in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (F_max) or the nonvariable fluorescence (F_o). Constancy of F_o and F_max suggests functional similarities of photosystem II and associated antennae pigments in the various classes of plants. The time course differences are observed only in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and appear, therefore, to be electron transport dependent. During induction, the peak in fluorescence (F_p) is much lower in all of the algae studied than in the higher plants. Exogenous O₂ strongly quenches F_p in all plants studied and our data indicate that the low F_p in the algae can be partially accounted for by endogenous O₂ quenching.     

Light Energy Distribution in the Brown Alga Macrocystis pyrifera (Giant Kelp)

The brown alga Macrocystis pyrifera (giant kelp) was studied by a combination of fluorescence spectroscopy at 77 kelvin, room temperature modulated fluorimetry, and photoacoustic techniques to determine how light energy is partitioned between photosystems I and II in states 1 and 2. Preillumination with farred light induced the high fluorescence state (state 1) as determined by fluorescence emission spectra measured at 77K and preillumination with green light produced a low fluorescence state (state 2). Upon transition from state 1 to state 2, there was an almost parallel decrease of all of the fluorescence bands at 693, 705, and 750 nanometers and not the expected decrease of fluorescence of photosystem II and increase of fluorescence in photosystem I. The momentary level of room temperature fluorescence (fluorescence in the steady state, F_s), as well as the fluorescence levels corresponding to all closed (F_m) or all open (F_o) reaction-center states were measured following the kinetics of the transition between states 1 and 2. Calculation of the distribution of light 2 (540 nanometers) between the two photosystems was done assuming both the `separate package' and `spill-over' models. Unlike green plants, red algae, and cyanobacteria, the changes here of the light distribution were rather small in Macrocystis so that there was approximately an even distribution of the photosystem II light at 540 nanometers to photosystem I and photosystem II in both states 1 and 2. Photoacoustic measurements confirmed the conclusions reached as a result of fluorescence measurements, i.e. an almost equal distribution of light-2 quanta to both photosystems in each state. This conclusion was reached by analyzing the enhancement phenomenon by light 2 of the energy storage measured in far red light. The effect of light 1 in decreasing the energy storage measured in light 2 is also consistent with this conclusion. The photoacoustic experiments showed that there was a significant energy storage in light 1 which could be explained by cyclic electron transport around photosystem I. From a quantitative analysis of the enhancement effect of background light 2 (maximum enhancement of 1.4-1.5) it was shown that around 70% of light 1 was distributed to this cyclic photosystem I transport.     

Quantitative analysis of State 1–State 2 transitions in intact leaves using modulated fluorimetry — evidence for changes in the absorption cross-section of the two photosystems during state transitions

Using a combination of modulated and non-modulated light with synchronized detection it has been possible to monitor State 1–State 2 transitions in intact leaves as changes in the yield of modulated chlorophyll fluorescence. In the presence of excess far-red non-modulated light (713 nm) absorbed mainly by Photosystem I (PS I), the modulated fluorescence intensity was taken to represent F_o — the emission yield which occurs when the reaction centres of Photosystem II (PS II) are all open. On the other hand, superimposing saturating non-modulated wide-band, blue-green light resulted in a transitory maximum yield of modulated chlorophyll fluorescence, F_m, due to the total closure of the PS II reaction centres. In the absence of these additional lights the fluorescence level assumed a steady-state value, F_s, between F_o and F_m. All these parameters changed as the leaf slowly adapted to light of a given spectral composition. It was found that both F_o and F_m increased reversibly (by about 15–20%) during the transition from State 2 to State 1 such that the ratio of F_m to F_o remained constant, indicative of changes in absorption cross-section of PS II and PS I rather than alterations in ‘spillover’ which would cause preferential changes in F_m. It was also possible to estimate the fractions of light, β and α, channeled to PS II and PS I, respectively, from the values of F_o, F_m and F_s. In one approach, β was estimated in State 1, using the assumption that α + β = 1, and its variation during the subsequent state transition was assumed to follow proportional changes in F_o (or F_m). It was found that in State 2 there is a small loss (about 4%) of the total utilization of light in both photosystems. However, if such loss is neglected, assuming α + β is always unity, the calculated β was found to vary in the same direction and almost with the same magnitude as F_o (or F_m), indicating independently that a change in absorption cross-section in PS II (and PS I) had occurred. Consistent with these data were the light-saturation curves for the non-modulated far-red light-quenching effect in bringing the fluorescence from F_s to F_o in States 1 and 2. The ratio of the initial slopes of these curves indicates quantitatively both redistribution of light between PS I and PS II during the State 1–State 2 transitions and a partial loss of excitation energy in State 2.     

Influence of phytohormones on morphology and chlorophyll a fluorescence parameters in embryos of Fucus vesiculosus L. (Phaeophyceae)

While a variety of plant hormones from brown algae were described, there were few studies that examined the combined effects of these hormones on morphogenesis and photosynthetic physiology in developing fucoid embryos. We evaluated the effects of phytohormones to determine the extent, to which responses were similar to those of terrestrial plants. Kinetin, IAA, ABA, GA₃, and kinetin + IAA were added to seawater at a physiological concentration (1 mg/L), and embryos of Fucus vesiculosus L. were grown for 10 days. Photosynthetic activity of single embryos or embryo cells were characterized using the following fluorescence parameters: minimum fluorescence yield (F ₀), maximum quantum yield (F _v/F _m), relative maximum rate of electron transfer to photosystem II under saturation irradiances (rETR_max), photosynthetic efficiency under non-saturating irradiances (αETR) and saturation irradiance (E _k). In addition, embryo length and diameter and apical hair length and number were determined. Morphological changes associated with hormone treatments included an increase in the embryo length in the presence of IAA, an increase in the embryo diameter in the presence of IAA, kinetin, and kinetin + IAA, an increase in the maximum hair length and number in the presence of kinetin + IAA, and a decrease in the hair length and number in the presence of ABA. With respect to fluorescence parameters, significant effects of phytohormones included an increase in the F ₀ and F _v/F _m at kinetin treatment, a synergistic effect of kinetin + IAA on F _v/F _m, rETR_max, and αETR, a promotion of F _v/F _m by GA, and a decrease of the parameters by ABA. These results are consistent with the data on responses of land plants to the same hormones and suggest that brown algae have evolved regulatory mechanisms for morphogenesis and photosynthetic regulation similar to plants.     

Dynamics of photosystem II heterogeneity in Dunaliella salina (green algae)

Based on the electron-transport properties on the reducing side of the reaction center, photosystem II (PS II) in green plants and algae occurs in two distinct forms. Centers with efficient electron-transport from Q_A to plastoquinone (Q_B-reducing) account for 75% of the total PS II in the thylakoid membrane. Centers that are photochemically competent but unable to transfer electrons from Q_A to Q_B (Q_B-nonreducing) account for the remaining 25% of total PS II and do not participate in plastoquinone reduction. In Dunaliella salina, the pool size of Q_B-nonreducing centers changes transiently when the light regime is perturbed during cell growth. In cells grown under moderate illumination intensity (500 E m^-2s^-1), dark incubation induces an increase (half-time 45 min) in the Q_B-nonreducing pool size from 25% to 35% of the total PS II. Subsequent illumination of these cells restores the steady-state concentration of Q_B-nonreducing centers to 25%. In cells grown under low illumination intensity (30 µE m^–2s^–1), dark incubation elicits no change in the relative concentration of Q_B-nonreducing centers. However, a transfer of low-light grown cells to moderate light induces a rapid (half-time 10 min) decrease in the Q_B-nonreducing pool size and a concomitant increase in the Q_B-reducing pool size. These and other results are explained in terms of a pool of Q_B-nonreducing centers existing in a steady-state relationship with Q_B-reducing centers and with a photochemically silent form of PS II in the thylakoid membrane of D. salina. It is proposed that Q_B-nonreducing centers are an intermediate stage in the process of damage and repair of PS II. It is further proposed that cells regulate the inflow and outflow of centers from the Q_B-nonreducing pool to maintain a constant pool size of Q_B-nonreducing centers in the thylakoid membrane.Abbreviations Chl chlorophyll - PS photosystem - Q_A primary quinone electron acceptor of PS II - Q_B secondary quinone electron acceptor of PS II - LHC light harvesting complex - F_o non-variable fluorescence yield - F_pl intermediate fluorescence yield plateau level - F_max maximum fluorescence yield - F_i mitial fluorescence yield increase from F_o to F_pl(F_pl-F_o) - F_v total variable fluorescence yield (F_max-F_o) - DCMU dichlorophenyl-dimethylurea     

Analysis of changes in minimal and maximal fluorescence yields with irradiance and o(2) level in tobacco leaf tissue

The responses of minimal and maximal fluorescence yields of chlorophyll a to irradiance of actinic white light were determined by pulse modulated fluorimetry in leaf discs from tobacco, Nicotiana tabacum, at 1.6, 20.5, and 42.0% (v/v) O₂. Steady-state maximal fluorescence yield (F_m′, measured during a saturating light pulse) declined with increasing irradiance at all O₂ levels. In contrast, the steady-state minimal fluorescence yield (F_o′, measured during a brief dark interval) increased with irradiance relative to that recorded for the fully dark-adapted leaf (F_o) or that observed after 5 minutes of darkness (F_o^*). The relative magnitude of this increase was somewhat greater and extended to higher irradiances at the elevated O₂ levels compared with 1.6% O₂. Suppression of F_o′ was only observed consistently at saturating irradiance. The results are interpreted in terms of the occurrence of photosystem II units possessing exceedingly slow turnover times (i.e. “inactive” units). Inactive units play an important role, along with thermal deactivation of excited chlorophyll, in determining the response of in vivo fluorescence yield to changes in irradiance. Also, a significant interactive effect of O₂ concentration and the presence or absence of far red light on oxidation of photosystem II acceptors in the dark was noted.     

Analysis of Light-Induced Depressions of Photosynthesis in Leaves of a Wheat Crop during the Winter

The photosynthetic performances of individual leaves of a wheat (Triticum aestivum cv Bezostaya) crop were assessed daily and throughout individual days during the winter when temperature and light levels were fluctuating. Measurements of chlorophyll fluorescence induction and the maximum quantum yield of O₂ evolution were made on individual leaves. Depressions in the ratio of variable to maximal fluorescence (F_v/F_m) were correlated with low temperatures and high light levels throughout the winter and during the course of individual days. Depressions in F_v/F_m observed in the field during the day were not accompanied by any significant change in the ability of photosystem II complexes to bind 3-(3,4-dichlorophenyl)-1-dimethyl urea, indicating that the depressions in F_v/F_m were not attributable to photodamage to the D1 protein of the photosystem II reaction center. Decreases in F_v/F_m were associated with increases in the rate of dissipation of excitation energy by radiationless decay processes and decreases in the quantum efficiency of CO₂ assimilation, indicative of a rapidly reversible light-induced “downregulation” of photosynthesis. No major changes were observed in the maximum quantum efficiency of O₂ evolution of leaves throughout periods of fluctuating temperature and light, because light-induced depressions in photosynthetic efficiency recovered within the time required to make these measurements.     

Analysis of chlorophyll fluorescence induction curves in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea as a function of magnesium concentration and NADPH-activated light-harvesting chlorophyll ab-protein phosphorylation

The effect of Mg²⁺ concentration and phosphorylation of light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ on various chlorophyll fluorescence induction parameters of isolated pea thylakoids has been studied. (1) Lowering the Mg²⁺ concentration from 3 to 0.4 mM decreases only the variable fluorescence (F_v) and the area above the induction curve while at the same time increasing the slow exponential component of the rise (β_max). (2) A further decrease in Mg²⁺ concentration from 0.4 to 0 mM decreases the initial (F₀) fluorescence level such that the ratio $\text{F}{}_{\mathrm{<mtext>v</mtext>}}\text{F}{}_{\mathrm{<mtext>m</mtext>}}$ increases slightly as does the area above the induction curve and β_max. (3) Thylakoid membranes, phosphorylated at 5 mM Mg²⁺, show an equal decrease in F_v and F₀, no change in the area above the induction curve and an increase in β_max. At 2 mM Mg²⁺, however, phosphorylation induced a more extensive quenching of F_v so that the $\text{F}{}_{\mathrm{<mtext>v</mtext>}}\text{F}{}_{\mathrm{<mtext>m</mtext>}}$ ratio was lowered and the area above the induction curve decreased while β_max increased. (4) When phosphorylated membranes were subsequently suspended in an Mg²⁺-free medium the effect on F₀ due to phosphorylation was found to be additive to that due to the absence of Mg²⁺. The effect of membrane phosphorylation on fluorescence is discussed in relation to the control of excitation energy distribution and shows that different mechanisms operate depending on the background Mg²⁺ levels. At high Mg²⁺ the phosphorylation seems to affect the absorption cross-section of Photosystem II while at lower Mg²⁺ levels there is an additional effect of increased spillover from Photosystem II to I.     

Freeze-fracture ultrastructure of thylakoid membranes in chloroplasts from manganese-deficient plants

Leaves from spinach (Spinacia oleracea L. cv Hybrid 102) plants grown in Mn-deficient nutrient solution were characterized by chlorosis, lowered chlorophyll a/b ratio and reduced electron transport. There were characteristic changes in room temperature fluorescence induction kinetics with increased initial yield (F_o) and decreased variable fluorescence (F_v). The fluorescence yield after the maximum fell rapidly to a level below F_o. The shape of the rise from F_o to the maximum was altered and the size of photosystem II units increased, as measured by half-rise time of F_v in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The Mn-deficient leaves were harvested before necrosis, when thin section electron microscopy revealed no disorganization of the thylakoid system. Thylakoid membranes were examined by freeze-fracture electron microscopy. The effect of Mn-deficiency was the specific loss of three-quarters of the particles from the endoplasmic fracture face of appressed thylakoids (EFs). Mn-deficient leaves were restored to near normal 2 days after application of exogenous Mn to the nutrient solution. It is concluded that the loss of most, but not all, functional photosystem II reaction centers from grana, with no alteration in light-harvesting complex or photosystem I, is responsible for the fluorescence and functional properties observed. The response of thylakoids to Mn deficiency shows that there is a fundamental difference in composition and function of stacked and unstacked endoplasmic fracture particles. The stacked endoplasmic fracture particle probably contains, in close association, the photosystem II reaction center and also the Mn-containing polypeptide, the 3-(3,4-dichlorophenyl)-1,1-dimethylurea-binding protein, and all electron transport components in between.     

Some effects of 4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone (San 9785) on the development of chloroplast thylakoid membranes in Hordeum vulgare L.

Chloroplast ultrastructural and photochemical features were examined in 6-d-old barley (Hordeum vulgare L. cv. Sundance) plants which had developed in the presence of 4-chloro-5-(dimethylamino)-2-phenyl-3(2H)-pyridazinone (San 9785). In spite of a substantial modification of the fatty-acid composition of thylakoid lipids there were no gross abnormalities in chloroplast morphology, and normal amounts of membrane and chlorophyll were present. Fluorescence kinetics at 77K demonstrated considerable energetic interaction of photosystem (PS)I and PSII chlorophylls within the altered lipid environment. An interference with electron transport was indicated from altered room-temperature fluorescence kinetics at 20°C. Subtle changes in the arrangements of chloroplast membranes were consistently evident and the overall effects of these changes was to increase the proportion of appressed to nonappressed membranes. This correlated with a lower chlorophyll a/b ratio, an increase in the amount of light-harvesting chlorophylls as determined by gel electrophoresis and fluorescence emission spectra, and an increase in excitation-energy transfer from PSII to PSI, as predicted from current ideas on the organisation of photosystems in appressed and non-appressed thylakoid membranes.Abbreviations CP1 P700-chlorophyll a protein - F_o, F_m, F_v minimal, maximal and variable fluorescence yield - LHCP light-harvesting chlorophyll-protein complex - PSI, PSII photosystem I, II - San 9785 4-chloro-5(dimethylamino)-2-phenyl-3(2H)-pyridazinone     

Effect of High Cation Concentrations on Photosystem II Activities

The effects of wide concentration ranges of NaCl, KCl, and MgCl₂ on ferricyanide reduction and the fluorescence induction curve of isolated spinach (Spinacia oleracea) chloroplasts were investigated. Concentrations of the monovalent salts above 100 mm and MgCl₂ above 25 mm produced a decrease in the rate of ferricyanide reduction by thylakoids uncoupled with 2.5 mm NH₄Cl which cannot be attributed to changes in the primary photochemical capacity of photosystem II. Salt-induced decreases in the effective concentration of the secondary electron acceptor of photosystem II, plastoquinone, reduce the capacity for secondary photochemistry of photosystem II and this could contribute to the reduction in ferricyanide reduction by uncoupled thylakoids at high salinities. The rate of ferricyanide reduction by coupled thylakoids is little affected by salinity changes, indicating that the rate-limiting phosphorylation mechanism in electron flow from water to ferricyanide in coupled thylakoids is salt-tolerant, whereas the rate-limiting reaction in uncoupled ferricyanide reduction is considerably affected by salinity changes. Salt-induced changes in the fluorescence induction curve are interpreted in terms of changes in the rate constants for excitation decay by radiationless transitions, exciton transfer from photosystem II chlorophylls to other associated chlorophyll species, and photochemistry.     

Differential responses of photosystems I and II to seasonal drought in two Ficus species

Hemiepiphytic Ficus species exhibit more conservative water use strategy and are more drought-tolerant compared with their non-hemiepiphytic congeners, but a difference in the response of photosystem I (PSI) and photosystem II (PSII) to drought stress has not been documented to date. The enhancement of non-photochemical quenching (NPQ) and cyclic electron flow (CEF) have been identified as important mechanisms that protect the photosystems under drought conditions. Using the hemiepiphytic Ficus tinctoria and the non-hemiepiphytic Ficus racemosa, we studied the water status and the electron fluxes through PSI and PSII under seasonal water stress. Our results clearly indicated that the decline in the leaf predawn water potential (ψ_pd), the maximum photosynthetic rate (A_max) and the predawn maximum quantum yield of PSII (F_v/F_m) were more pronounced in F. racemosa than in F. tinctoria at peak drought. The F_v/F_m of F. racemosa was reduced to 0.69, indicating net photoinhibition of PSII. Concomitantly, the maximal photo-oxidizable P700 (P_m) decreased significantly in F. racemosa but remained stable in F. tinctoria. The fraction of non-photochemical quenching [Y(NPQ)] and the ratio of effective quantum yield of PSI to PSII [Y(I)/Y(II)] increased for both Ficus species at peak drought, with a stronger increase in F. racemosa. These results indicated that the enhancement of NPQ and the activation of CEF contributed to the photoprotection of PSI and PSII for both Ficus species under seasonal drought, particularly for F. racemosa.     

A new mechanism for adaptation to changes in light intensity and quality in the red alga,Porphyra perforata. I. Relation to State 1—State 2 transitions

Time courses of chlorophyll fluorescence and fluorescence spectra at 77 K after various light treatments were measured in the red alga, Porphyra perforata. Photosystem (PS) I or II light (light 1 or 2) induced differences in the fluorescence spectra at 77 K. Light 2 decreased the two PS II fluorescence bands (F-685 and F-695) in parallel, while light 1 preferentially increased F-695. Light 1 and 2 also produced different effects on the activities of PS I and II. Preillumination with light 1 increased PS II activity and decreased PS I activity. However, preillumination with light 2 decreased PS II activity with no effect on PS I activity. These results show that there are at least two mechanisms that can alter the transfer of light energy in P. perforata. The dark state in this alga was found to be State 2 and light 1 induced a State 2-State 1 transition which retarded the transfer of light energy from PS II to PS I. Light 2 induced another change (which we have called a State 2-State 3 transition) that was accompanied by a change only in PS II activity.     

Effects of Growth Temperature on the Thermal Stability of the Photosynthetic Apparatus of Atriplex lentiformis (Torr.) Wats

High growth temperatures induced a substantial increase in the thermal stability of the photosynthetic apparatus of Atriplex lentiformis. This was manifested as a much reduced inhibition of light-saturated photosynthesis and the initial slope of the light-dependence curves by exposure to high temperatures in high as compared to moderate temperature-grown plants. Heat treatment at 46 C of leaves from moderate temperature-grown plants resulted in a marked reduction in photosystem II activities of chloroplasts isolated from them. In contrast, heat treatment of leaves from high temperature-grown plants resulted in no reduction of photosystem II activities. In vivo estimates of photosystem II functioning, the 515 nm light-induced absorbance change, and the ratio initial to maximum fluorescence (F₀/F_max) indicated a similar increase in the thermal stability of photosystem II in high temperature-grown plants.     

Use of zinc ions to study thylakoid protein phosphorylation and the state 1-state 2 transition in vitro

At ATP concentrations less than 0.2 millimolar, zinc ions cause a marked stimulation of endogenous protein phosphorylation in thylakoid membranes isolated from tobacco (Nicotiana tabacum L. cv Turkish Samsun), pea (Pisum sativum L. cv Feltham First) and spinach (Spinacia oleracea L. cv Northland). The greatest stimulatory effect was observed at Zn²⁺ concentrations of 1 to 2 millimolar; higher concentrations were inhibitory. The stimulatory effect of Zn²⁺ was independent of Mg²⁺ concentration from 1 to 5 millimolar and thus does not appear to be due to the formation of a Zn²⁺ -ATP complex. Phosphorylation of histones IIA, an exogenous protein substrate, was inhibited by 2 millimolar Zn²⁺. At low levels of ATP, Zn²⁺ not only stimulates general endogenous protein phosphorylation, but also the phosphorylation of the apoproteins of the light-harvesting chlorophyll a/b-protein complex. However, under these conditions Zn²⁺ inhibits the ATP-induced quenching of photosystem II fluorescence and the increase in the ratio of photosystem I to photosystem II fluorescence which are both characteristic of the State 1-State 2 transition. These results suggest that phosphorylation of the light-harvesting chlorophyll a/b-protein complex may not directly bring about the State 1-State 2 transition.     

Effect of Glomus mosseae on chlorophyll content, chlorophyll fluorescence parameters, and chloroplast ultrastructure of beach plum (Prunus maritima) under NaCl stress

The present study was undertaken to investigate the effect of Glomus mosseae on chlorophyll (Chl) content, Chl fluorescence parameters and chloroplast ultrastructure of beach plum seedlings under 2% NaCl stress. The results showed that compared to control, both Chl a and Chl b contents of NaCl + G. mosseae treatment were significantly lower during the salt stress, while Chl a/b ratio increased significantly. The increase of minimal fluorescence of darkadapted state (F₀), and the decrease of maximal fluorescence of dark-adapted state (F_m) and variable fluorescence (F_v) values were inhibited. The maximum quantum yield of PSII photochemistry (F_v/F_m), the maximum energy transformation potential of PSII photochemistry (F_v/F₀) and the effective quantum yield of PSII photochemistry (??_PSII) increased significantly, especially the latter two variables. The values of the photochemical quenching coefficient (q_P) and the nonphotochemical quenching (NPQ) were similar between G. mosseae inoculation and noninoculation. It could be concluded that G. mosseae inoculation could protect the photosystem II (PSII) of beach plum, enhance the efficiency of primary light energy conversion and improve the primitive response of photosynthesis under salinity stress. Meanwhile, G. mosseae inoculation was beneficial to maintain the integrity of thylakoid membrane and to protect the structure and function of chloroplast, which suggested that G. mosseae can alleviate the damage of NaCl stress to chloroplast.     

Thylakoid protein kinase activity and associated control of excitation energy distribution during chloroplast biogenesis in wheat

The activity of thylakoid protein kinase and the regulation of excitation energy distribution between photosystems I and II was examined during chloroplast biogenesis in light-grown Triticum aestivum (wheat) leaves. The specific activity of the thylakoid protein kinase decreased some six-fold during development from the young plastids at the base of the 7-d-old leaf to the mature chloroplasts at the leaf tip. Appreciable activity was also detected in plastids isolated from etiolated leaves. In mature chloroplasts the majority of phosphate was incorporated into the M_r=26,000 apo-proteins of the light-harvesting chlorophyll a/b-protein complex (LHCP). However, at early stages of chloroplast development and in the etioplast, the phosphate was predominantly incorporated into a polypeptide of M_r=9,000 dalton. Immature thylakoids, isolated from the base of the leaf, had relatively low concentrations of LHCP and could perform a State 1-State 2 transition, as demonstrated by ATP-induced quenching of photosystem II fluorescence. Analyses of photosystem I and photosystem II fluorescence-induction curves from intact leaf tissue demonstrated that this transition occurs in vivo at early stages of leaf development and, therefore, may play an important role in regulating energy transduction during chloroplast biogenesis.     

Detection of rapid induction kinetics with a new type of high-frequency modulated chlorophyll fluorometer

A newly developed modulation fluorometer is described which operates with 1 sec light pulses from a light-emitting diode (LED) at 100 KHz. Special amplification circuits assure a highly selective recording of pulse fluorescence signals against a vast background of non-modulated light. The system tolerates ratios of up to 1:10⁷ between measuring light and actinic light. Thus it is possible to measure the dark fluorescence yield and record the kinetics of light-induced changes. A high time resolution allows the recording of the rapid relaxation kinetic following a saturating single turnover flash. Examples of system performance are given. It is shown that following a flash the reoxidation kinetics of photosystem II acceptors are slowed down not only by the inhibitor DCMU, but by a number of other treatments as well. From a light intensity dependency of the induction kinetics the existence of two saturated intermediate levels (I₁ and I₂) is apparent, which indicates the removal of three distinct types of fluorescence quenching in the overall fluorescence rise from F₀ to F_max.Abbreviations Q_A and Q_B consecutive electron acceptors of photosystem II - PS II photosystem II - P 680 reaction center chlorophyll of photosystem II - F₀ minimum fluorescence yield following dark adaptation - F_max maximum fluorescence yield - DCMU 3-(3, 4-dichlorophenyl)-1, 1-dimethyl-urea - DCCD N,N-dicyclohexylcarbodiimide - PQ plastoquinone - DAD diaminodurene Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement.     

Responses of photosystem I compared with photosystem II to high-light stress in tropical shade and sun leaves

Sun and shade leaves of several plant species from a neotropical forest were exposed to excessive light to evaluate the responses of photosystem I in comparison to those of photosystem II. Potential photosystem I activity was determined by means of the maximum P700 absorbance change around 810 nm (ΔA_810max) in saturating far-red light. Leaf absorbance changes in dependence of increasing far-red light fluence rates were used to calculate a ‘saturation constant’, K_s, representing the far-red irradiance at which half of the maximal absorbance change (ΔA_810max/2) was reached in the steady state. Photosystem II efficiency was assessed by measuring the ratio of variable to maximum chlorophyll fluorescence, F_v/F_m, in dark-adapted leaf samples. Strong illumination caused a high degree of photo-inhibition of photosystem II in all leaves, particularly in shade leaves. Exposure to 1800–2000 μ mol photons m⁻² s⁻¹ for 75 min did not substantially affect the potential activity of photosystem I in all species tested, but caused a more than 40-fold increase of K_s in shade leaves, and a three-fold increase of K_s in sun leaves. The increase in K_s was reversible during recovery under low light, and the recovery process was much faster in sun than in shade leaves. The novel effect of high-light stress on the light saturation of P700 oxidation described here may represent a complex reversible mechanism within photosystem I that regulates light-energy dissipation and thus protects photosystem I from photo-oxidative damage. Moreover, we show that under high-light stress a high proportion of P700 accumulates in the oxidized state, P700⁺. Presumably, conversion of excitation energy to heat by this cation radical may efficiently contribute to photoprotection.