David C. Fork

Chlorophyll A fluorescence transient as an indicator of water potential of leaves

Abstract The ratio of the maximum (P level) to the minimum (O level) chlorophyll a fluorescence, measured at 685 nm, decreased from a value of about 4-1 as Nerium oleander plants were water stressed (water potential of leaves decreasing from −8 bars to −9 bars). Furthermore, this change was reversed to a large degree when water-stressed plants were re-watered. No measurable effect was observed on the O level. A similar relationship between the P/O ratio and the water potential was also observed in the leaves of Atriplex triangularis and Tolmiea menziesii. These data indicate that water stress inhibits the electron donation (or the water oxidation) side of photosystem II (PSII).

Electron transport and photophosphorylation by Photosystem I in vivo in plants and cyanobacteria.

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 b6-f complex. An increased demand for ATP lowers the transthylakoid pH gradient, activates the b6-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 photophosphorylation will depend on experiments made on living cells and measurements of cyclic photophosphorylation in vivo.

STATE I‐STATE II TRANSITIONS IN THE THERMOPHILIC BLUE‐GREEN ALGA (CYANOBACTERIUM) Synechococcus lividus*

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.

Quenching of chlorophyll fluorescence by quinones in algae and chloroplasts

Abstract 1. A number of quinones and substituted quinones quenched strongly chlorophyll fluorescence in Swiss chard chloroplasts and in the intact algae, Ulva lobata and Porphyra perforata . Quenching occurred in the absence as well as in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea. 2. Among the quinones found to have a high quenching activity were 2,3,5,6-tetramethylbenzoquinone, 2-methyl-1,4-naphthoquinone, 5-hydroxy-1,4-naphthoquinone, phenanthrenequinone, and 1,2-dihydroxyanthraquinone. Reduced quinones had no or little quenching activity. 3. The quinones tested quenched less strongly the initial fluorescence, observed immediately upon illumination, than the subsequent rise of fluorescence yield during illumination. For the most active compounds, the concentrations needed for 50% quenching of the initial and the subsequent increase of fluorescence were about 70 and 15 μM, respectively. 4. The kinetics of fluorescence quenching at different light intensities and concentrations of quencher and absence of stimulation of O 2 evolution indicate that the quenchers do not stimulate photosynthetic electron transport but interact directly with chlorophyll molecules of photosystem 2 by formation of traps for the excitation energy. 5. In agreement with the assumption that the site of action of the quinones is near system 2, a number of these compounds inhibited light-induced cytochrome reduction in Porphyra in vivo . However, for most compounds no quantitative relation was found between the extent of inhibition of cytochrome reduction or of the Hill reaction in chloroplasts and the activity in quenching chlorophyll fluorescence.

Light adaptation of cyclic electron transport through Photosystem I in the cyanobacterium Synechococcus sp. PCC 7942

Photosystem I-driven cyclic electron transport was measured in intact cells of Synechococcus sp PCC 7942 grown under different light intensities using photoacoustic and spectroscopic methods. The light-saturated capacity for PS I cyclic electron transport increased relative to chlorophyll concentration, PS I concentration, and linear electron transport capacity as growth light intensity was raised. In cells grown under moderate to high light intensity, PS I cyclic electron transport was nearly insensitive to methyl viologen, indicating that the cyclic electron supply to PS I derived almost exclusively from a thylakoid dehydrogenase. In cells grown under low light intensity, PS I cyclic electron transport was partially inhibited by methyl viologen, indicating that part of the cyclic electron supply to PS I derived directly from ferredoxin. It is proposed that the increased PSI cyclic electron transport observed in cells grown under high light intensity is a response to chronic photoinhibition.

THE APPLICATION OF PHOTOACOUSTIC TECHNIQUES TO STUDIES OF PHOTOSYNTHESIS

The photoacoustic (PA)? technique is becoming an increasingly popular method in photosynthesis research. The PA effect, which is the production of sound by modulated light, was first described by Alexander Graham Bell’ but was not applied to studies of photosynthesis until the work of Malkin and Cahen.2,3 In studies of photosynthesis, the PA technique serves to quantify the conversion of absorbed light into heat in a photosynthetic sample. By difference, the fraction of absorbed light energy that is utilized in photochemistry may also be quantified. A similar use of heat measurement to quantify photosynthetic efficiency was made by Arnold in the 1 9 3 0 ~ ~ when he used the Callendar “radio balance,” an instrument designed to detect heat production by radioactive substances, to measure the small heat differences between photosynthetically active and inactive leaves, a phenomenon that had been predicted by Spoehr in 1926.5 Arnold found that intact leaves stored light energy with an efficiency of 35%, a value that he considered at that time to be far too low and that he did not publish until much later.6 Photocaustic measurements of the present day confirm that the overall efficiency of photosynthesis in intact samples is in fact, about 35%,’ a value that may also be derived theoreti ~ a l l y . ~ . ~ The growing interest in PA as applied to photosynthesis research is reflected by the number of reviews that have appeared recently and that provide an account of the progress in this field.1s2z The two previous Yearly Reviews of Balasubramanian and RaoZ3 and Braslavskyz4 appearing in this journal have summarized applications of the PA technique to the study of biological systems in general. This review limits itself to applications of this technique to studies of photosynthesis. Another review by Malkin and CanaaniZS is appearing in this year’s issue of the Annual Review of Plant Physiology.

Changes in the cyanobacterial photosynthetic apparatus during acclimation to macronutrient deprivation.

When the cyanobacterium Synechococcus sp. Strain PCC 7942 is deprived of an essential macronutrient such as nitrogen, sulfur or phosphorus, cellular phycobiliprotein and chlorophyll contents decline. The level of β-carotene declines proportionately to chlorophyll, but the level of zeaxanthin increases relative to chlorophyll. In nitrogen- or sulfur-deprived cells there is a net degradation of phycobiliproteins. Otherwise, the declines in cellular pigmentation are due largely to the diluting effect of continued cell division after new pigment synthesis ceases and not to net pigment degradation. There was also a rapid decrease in O2 evolution when Synechococcus sp. Strain PCC 7942 was deprived of macronutrients. The rate of O2 evolution declined by more than 90% in nitrogen- or sulfur-deprived cells, and by approximately 40% in phosphorus-deprived cells. In addition, in all three cases the fluorescence emissions from Photosystem II and its antennae were reduced relative to that of Photosystem I and the remaining phycobilisomes. Furthermore, state transitions were not observed in cells deprived of sulfur or nitrogen and were greatly reduced in cells deprived of phosphorus. Photoacoustic measurements of the energy storage capacity of photosynthesis also showed that Photosystem II activity declined in nutrient-deprived cells. In contrast, energy storage by Photosystem I was unaffected, suggesting that Photosystem I-driven cyclic electron flow persisted in nutrient-deprived cells. These results indicate that in the modified photosynthetic apparatus of nutrient-deprived cells, a much larger fraction of the photosynthetic activity is driven by Photosystem I than in nutrient-replete cells.

Reversible inhibition of photochemistry of photosystem II by Ca2+ removal from intact cells of Anacystis nidulans

Depletion of Ca2+ from Anacystis nidulans produces an inhibition of O2 evolution that is accompanied both at 39°C and 77 K by a loss of chlorophyll fluorescence of variable yield. This indicates that Ca2+‐depletion causes disruption of normal photosystem II function, manifested by the disappearance of photoreduction of Q. Delayed light emission in the ms time range is also eliminated in Ca2+‐depleted cells, which confirms that Ca2+ removal prevents charge separation and recombination in reaction centers of photosystem II. Readdition of Ca2+ to depleted cells restores fully the fluorescence of variable yield and delayed light emission, as well as O2 evolution. Thus, Ca2+ may be a required component for photosystem II in A. nidulans.

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☆

Abstract 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.

Light distribution, transfer and utilization in the marine red alga Porphyra perforata from photoacoustic energy-storage measurements

Abstract Light energy utilization in Porphyra perforata was monitored by the photoacoustic method in different conditions of illumination. Auxiliary chlorophyll a fluorescence measurements were made to estimate the fraction of open photosystem II (PS II) reaction centers. These measurements allowed a consistent quantitation of excitation distribution and transfer from PS II to PS I under the physiological conditions used. Maximum energy storage was obtained with modulated light absorbed almost exclusively by the phycobilins (light 2). Modulated light absorbed by chlorophyll a (light I) gave much smaller energy storage (about 1 3 of the maximum), which could be enhanced to the maximum by addition of background light 2. Addition of increasing intensities of background light 1 to modulated light 2 did not initially induce any effect and then decreased the energy storage to about half of the maximum. From the above results and with simple mathematical modelling, numbers were obtained for light distribution and energy transfer parameters. From the enhancement saturation curves of the effect of background light 2 on the energy storage in modulated light 1 we conclude that in state 1 light 2 is exclusively absorbed in PS II and that there is no energy transfer to PS I from open PS II reaction centers. From the value of the energy storage for light 2 in state 1 and the degree of openness of PS II reaction centers it is possible to conclude that energy transfer to PS I occurs from closed PS II reaction centers with a probability approaching 1. In state 2 light 2 is distributed more evenly (approximately in a ratio PS II PS I of 0.55:0.45) either by energy transfer via PS II from open PS II reaction centers, or by direct interactions of the phycobilins and PS I. Comparison of the maximum fluorescence values in the two states favors the second possibility. Energy transfer from PS II units with closed reaction centers occurs again in state 2 with a probability approaching 1. Comparison of energy utilization and oxygen evolution in light 1 relative to light 2 and the inhibitory effect of DCMU, which is complete in light 2 but only partial in light 1, suggests the existence of two types of PS I units: one type is engaged in electron transfer from PS II and the other type specializes in cyclic electron flow. The above quantitative analysis allows to estimate the ratio of the two types of PS I unit to be roughly about 0.3:0.7, respectively.