The use of fluorescence induction measurements in leaves Infiltrated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea has been evaluated as a routine method for estimation of the concentration of the reaction centers of photosystem II relative to total chlorophyll in a wide variety of plant species. The procedure is based on a simple theory that takes into account the attenuation of light in passing through the leaf and the linear dependence of the fluorescence induction time from different parts of the leaf on the inverse of the local ight intensity. A formula to calculate the reaction center concentration of photosystem II was obtained. The effect of the light attenuation is accounted for by a correction factor which could become practicafly insignificant by an optimal choice of the excitation and emission wavelengths and the geometry of the photodetector with respect to the sample. Estimation of quantum yields for pimary photochemistry and influence of light scattering were considered. The results demonstrate the effect of the above factors under various circumstances and are in agreement, to a first approximation, with the theory.The utility of the method is demonstrated by a detailed study of four desert plant species: estimation of reaction center concentrations of both photosystem I (by estimation of P700) and photosystem II (by the fluorescence induction method) were made and were compared to the rates of CO2 fixation. There was a good quantitative correlation between the photosynthetic rates and the concentration of photosystem II reaction centers (expressed as per chlorophyll or per unit area of the leaf), but no such correlation was found with photosystem I reaction centers.The ratio of total chlorophyll per reaction centers II varied in the range of about 200 to 800 in different species, but there was no variation of this parameter in any single species.It is well established that the primary charge transfer processes of photosynthesis take place at reaction centers served by a large number of light-collecting antennae pigments (13,15,17,32,33,40) plexes serving both PSII and PSI (1,2,26,27,38). These numbers are probably quite variable in different species of the plant kingdom and in different environments. Since the over-all rate of photosynthesis on a Chl basis may be related to these numbers (39), it follows that they are important in dealing with questions such as the efficiency of photosynthesis (24,28,30), including adaptation to the environment [r.g. ambient light intensity level (5, 6, 31) or other conditions (3, 7, 23, 37)].Fluorescence induction in chloroplasts was previously used to obtain the concentration of the PSII electron-acceptor pool (19,40), as well as the concentration of the reaction centers (10,11,40). The principle of the method lies in the equivalence between the number of quanta, which bring about the fluorescence change, and the magnitude ofthe electron acceptor pool. In the case where DCMU was added, the electron-acceptor pool is limited to the primary acceptor of th...

Energy storage by cyclic electron flow through photosystem I (PSI) was measured in vivo using the photoacoustic technique. A wide variety of photosynthetic organisms were considered and all showed measurable energy storage by PSI-cyclic electron flow except for higher plants using the C-3 carbon fixation pathway. The capacity for energy storage by PSI-cyclic electron flow alone was found to be small in comparison to that of linear and cyclic electron flows combined but may be significant, nonetheless, under conditions when photosystem 11 is damaged, particularly in cyanobacteria. Light-induced dynamics of energy storage by PSI-cyclic electron flow were evident, demonstrating regulation under changing environmental conditions.The oxygen-evolving photosynthetic apparatus contains two reaction center complexes, designated PSI electron flow through PSI in C3 plants has been proposed to generate ATP over and above that produced by linear electron flow, adjusting the ratio of ATP to NADPH generated by the light reactions of photosynthesis in accordance with the needs of the plant (1). As an example, cyclic electron flow through PSI has been proposed as a source of ATP for repair of PSII units damaged by environmental stress, since PSI is typically much less susceptible to stress than PSII (8, 9). Much of the ambiguity concerning the function and significance of cyclic electron flow through PSI is due to the difficulty of measuring it in whole cells and tissues. Most previous studies ofPSI-cyclic electron flow and accompanying phosphorylation have necessarily used either in vitro measurements of thylakoid fragments or somewhat ambiguous light-induced absorbance changes in whole cells or tissues (12,17). The photoacoustic method for measurement of photosynthetic energy storage is well suited for study of PSI-cyclic electron flow, however, because it is capable of simple, direct, and quantitative measurement of energy storage by cyclic electron flow in intact leaves and algae as well as in thylakoid preparations (9,10,16,18). Presumably, the bulk of such energy storage by PSI-cyclic electron flow represents photophosphorylation of ADP.In the present report, the occurrence, capacity, and regulation of energy storage by PSI-cyclic electron flow in whole tissues of a variety of photosynthetic organisms are characterized using the photoacoustic method. MATERIALS AND METHODS Plant Material

Time courses of state I‐state II transitions were measured in the thermophilic blue‐green alga (Cyanobacterium), Synechococcus lividus, that was grown at 55°C. The rate of the state I–II transition using light II illumination was the same as that in the dark, and the dark state was identified to be state II. Therefore, light regulation attained by state transitions is produced by the state II–I transition induced by system I light. The redox level of plastoquinone did not affect this dark state II. Arrhenius plots of the state transitions showed a break point around 43°C that corresponded to the phase transition temperature of this alga. Since both the state I–II and II–I transitions were very much temperature‐independent, we could keep the alga in either state for a long time at a “low” temperature such as room temperature. Activities of both photosystems I and II in states I and II were also measured. After a state II–I transition, the system II activity increased about 16% and at the same time, svstem I activity decreased about 30%.

The enzyme superoxide dismutase is ubiquitous in aerobic organisms where it plays a major role in alleviating oxygen-radical toxicity. An insertion mutation introduced into the iron superoxide dismutase locus (designated sodE) of the cyanobacterium Synechococcus sp. PCC 7942 created a mutant strain devoid of detectable iron superoxide dismutase activity. Both wild-type and mutant strains exhibited similar photosynthetic activity and viability when grown with 17 jmolm-2-s'1 illumination in liquid culture supplemented with 3% carbon dioxide. In contrast, the sodB mutant exhibited significantly greater damage to its photosynthetic system than the wild-type strain when grown under increased oxygen tension or with methyl viologen. Although damage occurs at both photosystems I and II, it is primarily localized at photosystem I in the sodB mutant. Growth in 100% molecular oxygen for 24 hr decreased photoacoustically measured energy storage in 3-(3,4-dichlorophenyl)-1,1-dimethylurea and abolished the fluorescence state 2 to state 1 transition in the sodB mutant, indicating interruption of cyclic electron flow around photosystem I. Analysis of the flash-induced absorption transient at 705 nm indicated that the interruption of cyclic electron flow occurred in the return part of the cycle, between the two [4 Fe-4 SI centers of photosystem I, FA and FB, and cytochrome f. Even though the sodB mutant was more sensitive to damage by active oxygen than wild-type cells, both strains were equally sensitive to the photoinhibition of photosystem II caused by exposure to strong light.

Recently, a number of techniques, some of them relatively new and many often used in combination, have given a clearer picture of the dynamic role of electron transport in Photosystem I of photosynthesis and of coupled cyclic photophosphorylation. For example, the photoacoustic technique has detected cyclic electron transport in vivo in all the major algal groups and in leaves of higher plants. Spectroscopic measurements of the Photosystem I reaction center and of the changes in light scattering associated with thylakoid membrane energization also indicate that cyclic photophosphorylation occurs in living plants and cyanobacteria, particularly under stressful conditions.In cyanobacteria, the path of cyclic electron transport has recently been proposed to include an NAD(P)H dehydrogenase, a complex that may also participate in respiratory electron transport. Photosynthesis and respiration may share common electron carriers in eukaryotes also. Chlororespiration, the uptake of O2 in the dark by chloroplasts, is inhibited by excitation of Photosystem I, which diverts electrons away from the chlororespiratory chain into the photosynthetic electron transport chain. Chlororespiration in N-starved Chlamydomonas increases ten fold over that of the control, perhaps because carbohydrates and NAD(P)H are oxidized and ATP produced by this process.The regulation of energy distribution to the photosystems and of cyclic and non-cyclic phosphorylation via state 1 to state 2 transitions may involve the cytochrome b 6-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b 6-f complex, stimulates phosphorylation of the light-harvesting chlorophyll-protein complex of Photosystem II and decreases energy input to Photosystem II upon induction of state 2. The resulting increase in the absorption by Photosystem I favors cyclic electron flow and ATP production over linear electron flow to NADP and 'poises' the system by slowing down the flow of electrons originating in Photosystem II.Cyclic electron transport may function to prevent photoinhibition to the photosynthetic apparatus as well as to provide ATP. Thus, under high light intensities where CO2 can limit photosynthesis, especially when stomates are closed as a result of water stress, the proton gradient established by coupled cyclic electron transport can prevent over-reduction of the electron transport system by increasing thermal de-excitation in Photosystem II (Weis and Berry 1987). Increased cyclic photophosphorylation may also serve to drive ion uptake in nutrient-deprived cells or ion export in salt-stressed cells.There is evidence in some plants for a specialization of Photosystem I. For example, in the red alga Porphyra about one third of the total Photosystem I units are engaged in linear electron transfer from Photosystem II and the remaining two thirds of the Photosystem I units are specialized for cyclic electron flow. Other organisms show evidence of similar specialization.Improved understanding of the biological role of cyclic pho...

The transition of the physical phase of lipids in membrane fragments of a blue-green alga Anacystis nidulans was studied by a spin labeling technique. The maximum hyperfine splitting of the electron spin resonance spectrum of the N-oxyl -4', 4'-dimethyloxazolidine derivative of 5 -ketostearic acid plotted against the reciprocal of the absolute temperature gave a discontinuity point that was characteristic of a transition of the physical phase of the hydrocarbon region of membrane lipids. The phase transition appeared at approximately 13 or 24 C in the organisms grown at 28 or 38 C, respectively.The and state 2 shift is dependent on the physical state of membrane lipids. In the chloroplasts of lettuce and spinach, on the other hand, there was no break in the Arrhenius plot of the electron transport reactions or of Mg'--induced changes of chlorophyll a fluorescence.It is suggested that the transitions of the hyperfine splitting of the ESR signal, electron transport, and the configurational change, as well as the appearance of the maximum of chlorophyll a fluorescence, in the thylakoid membranes of Anacystis nidulans are all related to the transition of the physical phase of membrane lipids between the liquid crystalline state and the mixed liquid crystal-solid state. It has been well demonstrated that the physical phase of lipids of biological membranes plays an important role in the physiological function of membranes (45,47). The proper functioning of biological membranes requires the presence of the smectic liquid crystalline state in which rotational and translational movements of lipid and protein molecules in the membrane are possible (29). It has been established in model membranes that the temperature of phase transition between the liquid crystalline and the solid states depends on the lipid species as well as the fatty acid composition (7,10,27,57); the higher the degree of unsaturation of fatty acids, the lower the phase transition temperature. The same relationship between the phase transition temperature and fatty acid composition also appears in the biological membranes of Escherichia coli (44,50,59), Mycoplasma laidlawii (14, 35, 55, 56), mitochondria (30, 32, 47), and yeast (13). Drastic changes of physiological activities of the biological membranes are observed at the phase transition temperatures. This is seen as changes in growth, transport, and respiration in E. coli (44,59) and in phosphorylation, respiration, and conformational changes in mitochondria (24,31,32,48

High growth temperatures induced a substantial increase in the thermal stability of the photosynthetic apparatus of Atriplex lentiformis. shrub that occurs in the hot interior deserts of California and possesses the C4 pathway of photosynthesis. Previous experiments in both the field and laboratory have shown that growth temperature has a substantial effect on the photosynthetic performance of these plants (19)(20)(21). Growth at high temperatures resulted in a substantially greater photosynthetic temperature optimum, increased rates of CO2 uptake at high temperatures, but decreased rates at low temperatures as compared to the responses of low temperature-grown plants. Differences in photosynthetic performance at low temperatures appear to be related principally to changes in RuDP carboxylase activity in the leaves (20). In this paper, we show that an important factor leading to a higher temperature optimum for CO2 uptake and the ability to maintain high CO2 uptake rates at high temperatures is an increase in the thermal stability of the photosynthetic apparatus of the high temperature-grown plants as compared to that of low temperature-grown plants, and we report studies of the physiological basis of this increased tolerance to high temperatures.The high summer temperatures of some deserts, which can often exceed 40 C and may reach 50 C or higher, can impose a severe constraint upon plant growth and productivity, even when adequate water is available. These temperatures are well above the thermal optimum for CO2 uptake for temperate zone plants as well as species predominantly active during the cooler seasons in the desert (3,8). It is becoming evident, however, that at least some summer active desert species have physiological adaptations that allow for high rates of CO2 uptake at high temperatures (7,21). For example, Tidestromia oblongifolia has a thermal optimum for CO2 uptake which exceeds 40 C. This species, however, is not capable of growth at cool temperatures and is not photosynthetically active during the mild spring and winter months (5).In contrast to species predominantly active during one season, evergreen desert species that are active throughout the year must possess a broad thermal tolerance or must be capable of acclimation to the seasonal changes in the temperature regime. Recent experiments have shown that many desert species that are active under seasonally variable temperatures possess a capacity to acclimate to the prevailing temperature regime (8, 16). Despite these observations, however, there is little in the literature concerning the mechanisms underlying temperature acclimation, particularly those involved in high temperature responses.Atriplex lentiformis (Torr.) Wats. is a phreatophytic evergreen

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