A photoacoustic study of water infiltrated leaves

Photoacoustic measurements of photosynthetic energy storage were conducted on water infiltrated pea and sugar maple leaves. The samples were vacuum infiltrated with pure water or with a suitable buffer. The use of such methodology permitted an accurate determination of the energy storage parameter at low modulation frequencies, where in non-infiltrated leaves oxygen evolution dominates the photoacoustic signal and does not allow energy storage measurements. Differences between infiltration media were not essential, however the use of pure water as infiltration medium sometimes caused instability of the measured energy storage, particularly at longer experimental time. Values of energy storage in individual samples ranged mostly between 0.2 to 0.35. Measured as a function of the modulation frequency, energy storage was found to be constant from about 10 to 200 Hz for pea leaves. In sugar maple leaves, the energy storage slightly increased between 100 and 500 Hz. Obtaining an accurate value for energy storage also allowed an accurate estimation of the O₂ evolution contribution to the photoacoustic signal of an unfiltrated leaf. In a maple leaf its frequency dependence showed only the effect of diffusion in the entire frequency range (10–500 Hz). Energy storage transients were observed after long periods (ca. 1/4-2 hrs) of dark adaptation upon the transition to light. In this case the initial energy storage was roughly about 1/2 that of the steady state value indicating strong PS I activity, while PS II was transiently incompetent. Energy-storage increased during illumination in a way to correspond to photosynthetic induction events as previously measured by fluorescence and O₂ evolution. Transients in energy storage were also found following high light to low light transitions (i.e., switch off of the saturating background light), that paralleled similar transients in oxygen evolution, showing initial transient inactivation followed by progressive reactivation of PS II.Abbreviations ES energy storage - PA photoacoustic(s) - PTR photothermal radiometry     

Photosynthetic energy storage efficiency,oxygen evolution and chloroplast movement

When there is a saturating supply of dissolved carbon available, photosynthetic energy storage efficiency (ES) varies linearly with light fluence rate (I) for both Vallisneria americana and Pisum sativum leaves. The frequently reported hyperbolic relationship between ES and I occurs only when low levels of dissolved carbon are present in the medium. The linear relationship has its origin in intracellular events and implies that two heat-producing processes limit the value of ES. The rate of one process varies as I and the other varies as I². The rates of both processes were changed after a 2 hour exposure to 400 μmol photons m⁻² s⁻¹ of red light, speeding up the process that depends linearly on I and slowing the other. Illumination for 1 hour with 100 μmol photons m⁻² s⁻¹ of blue (but not red) light moves many chloroplasts from the periclinal to the anticlinal cell walls [Inoue and Shibata (1973) Planta 114: 341–358]. Blue light exposure of V. americana leaf sections (a) reduced the rate of oxygen evolution under light-limiting conditions by about 22%; (b) increased the value of ES by an amount dependent on the light fluence rate; and (c) decreased the slope of (ES v I). The slope change indicated that light absorption had fallen by 26% after blue light exposure. The rate of oxygen evolution (V) was measured under light-limiting conditions with leaf sections in which the chloroplasts had been immobilised after blue or red light exposure. With both red and blue-exposed leaf sections, V fell by about 50% after exposure to 1 hour of 1250 μmol photons m⁻² s⁻¹ of white light. Thus accumulation of chloroplasts on anticlinal walls did not protect the leaf from photoinactivation by a high light fluence rate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Estimation of oxygen evolution by marine phytoplankton from measurement of the efficiency of Photosystem II electron flow

The relation between photosynthetic oxygen evolution and Photosystem II electron transport was investigated for the marine algae t Phaeodactylum tricornutum, Dunaliella tertiolecta, Tetraselmis sp., t Isochrysis sp. and t Rhodomonas sp.. The rate of Photosystem II electron transport was estimated from the incident photon flux density and the quantum efficiency of Photosystem II electron transport as determined by chlorophyll fluorescence. The relation between the estimated rate of Photosystem II electron transport and the rate of oxygen evolution was investigated by varying the ambient light intensity. At limiting light intensities a linear relation was found in all species. At intensities approaching light saturation, the relation was found to deviate from linearity. The slope of the line in the light-limited range is species dependent and related to differences in absorption cross-section of Photosystem II. The observed non-linearity at high irradiances is not caused by photorespiration but probably by a Mehler-type of oxygen reduction. The relationship could be modelled by including a redox-state dependent oxygen uptake. In the diatom t Phaeodactylum tricornutum, the photochemical efficiency of dark adapted open Photosystem II centers was found to be temperature-dependent with an optimum near 10°C.     

Acclimation of thermotolerant algae to light and temperature interaction1

Here, we explore the responses of photosynthesis and related cellular processes in the thermotolerant microalga Micractinium sp. acclimated to limiting and saturating irradiances combined with elevated temperatures, using a novel computer-controlled multi-sensor system. This system allows for the monitoring of online values of oxygen exchange during photosynthesis and respiration with high accuracy. Micractinium sp. cells showed maximum growth and net oxygen production rates under the optimal temperature of 25°C regardless of the light acclimation conditions. Our results show that the upper thermal threshold for Micractinium sp. photosynthesis and growth ranges between 35°C and 40°C. This microalga exhibited stable photosynthetic efficiency and effective non-photochemical quenching (NPQ) under saturating light, and was more susceptible to temperature change when acclimated to limiting light levels. These results demonstrate that the acclimation of thermotolerant microalgae to saturating light helps to enhance the thermal tolerance of PSII. This feature results from enhanced heat stability of PSII photochemistry and oxygen evolution.     

Rapid screening for heat tolerance in Phaseolus species using the photoacoustic technique

The potential utility of photoacoustic measurements in vivo for rapid estimation of heat tolerance was investigated in seven Phaseolus genotypes with known and contrasting heat resistance. When small leaf discs of 1 cm diameter were heated at 40.5°C, both relative quantum yield for oxygen evolution (as estimated by the ratio of the amplitude of the oxygen evolution signal (Aox) to the amplitude of the photothermal signal (Apt)) and photochemical energy storage were reduced drastically within a few minutes in the heat sensitive genotypes. In contrast, in the genotypes processing heat tolerance characteristics, these photoacoustic parameters remained constant for at least 20 min. Photochemical energy storage appeared to be more resistant towards heat stress conditions than oxygen evolution, suggesting differential heat sensitivity of PS2 compared to PS1. Within the heat resistant species, we were able to further select for high temperature tolerance when the rate of decrease of Aox/Apt was measured in leaf discs incubated at a higher temperature (42°C). The results suggest that the photoacoutic technique could be used as a rapid and easy screening test for heat tolerance in crop plants.     

Studies on thermoluminescence,delayed light emission and oxygen evolution from photosynthetic materials: UV effects

The effect of ultraviolet light on thermoluminescence, oxygen evolution and the slow component of delayed light has been investigated in chloroplasts and Pothos leaves. All peaks including peak V (48°C) were inhibited by UV. However, the peak at 48°C which was induced by DCMU was enhanced following UV irradiation of chloroplasts at ambient temperature (23°C) whereas peak II (-12°C) and peak III (10°C) which were also induced by DCMU were inhibited. Chloroplasts treated with DCMU and dark incubated for several minutes at ambient temperature prior to recording of glow curves have also shown enhancement of peak at 48°C. A slow component of delayed light and photosystem II activity of chloroplasts were inhibited by UV whereas photosystem I activity was marginally affected. These results corroborate involvement of photosystem II in generating thermoluminescence and slow components of delayed light in photosynthetic materials.Abbreviations DCIP Dichlorophenol Indophenol - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCQ 2,6 Dichloro-p-benzoquinone - DLE delayed light emission - MOPS Morpholino propane sulfonic acid - PSI Photosystem I - PS II Photosystem II - TL thermoluminescence     

Up-regulation of sucrose phosphate synthase in rice grown under elevated CO2 and temperature

Rice (Oryza sativa L. cv. IR-30) was grown season-long in outdoor, controlled-environment chambers at 33 Pa CO₂ with day/night/paddy-water temperatures of 28/21/25 °C, and at 66 Pa CO₂ with five different day/night/paddy-water temperature regimes (25/18/21, 28/21/25, 31/24/28, 34/27/31 and 37/30/34 °C). Sucrose phosphate synthase (SPS) activities in leaf extracts at 21, 48 and 81 days after planting (DAP) were assayed under saturating and selective (limiting) conditions. Diel SPS activity data indicated that rice SPS was light regulated; with up to 2.2-fold higher rates during the day. Throughout the growth season, leaf SPS activities were up-regulated in the CO₂-enriched plants, averaging 20 and 12% higher than in ambient-CO₂ grown plants in selective and saturating assays, respectively. Similarly, SPS activities increased 2.4% for each 1 °C rise in growth temperature from 25 to 34 °C, but de creased 11.5% at 37 °C. Leaf sucrose content was higher, and mirrored SPS activity better, than starch, although starch was more responsive to CO₂ treatment. Leaf sucrose and starch contents were significantly higher throughout the season in plants at elevated CO₂, but the N content averaged 6.5% lower. Increasing growth temperatures from 25 to 37 °C caused a linear decrease (62%) in leaf starch content, but not in sucrose. Consequently, the starch:sucrose ratio declined with growth temperature. The data are consistent with the hypothesis that the up-regulation of leaf SPS may be an acclimation response of rice to optimize the utilization and export of organic-C with the increased rates of inorganic-C fixation in elevated CO₂ or temperature growth regimes.     

Growth and photosynthetic responses of wheat plants grown in space.

Growth and photosynthesis of wheat (Triticum aestivum L. cv Super Dwarf) plants grown onboard the space shuttle Discovery for 10 d were examined. Compared to ground control plants, the shoot fresh weight of space-grown seedlings decreased by 25%. Postflight measurements of the O2 evolution/photosynthetic photon flux density response curves of leaf samples revealed that the CO2-saturated photosynthetic rate at saturating light intensities in space-grown plants declined 25% relative to the rate in ground control plants. The relative quantum yield of CO2-saturated photosynthetic O2 evolution measured at limiting light intensities was not significantly affected. In space-grown plants, the light compensation point of the leaves increased by 33%, which likely was due to an increase (27%) in leaf dark-respiration rates. Related experiments with thylakoids isolated from space-grown plants showed that the light-saturated photosynthetic electron transport rate from H2O through photosystems II and I was reduced by 28%. These results demonstrate that photosynthetic functions are affected by the microgravity environment.     

Mitochondrial Contribution to Photosynthetic Metabolism (A Study with Barley (Hordeum vulgare L.) Leaf Protoplasts at Different Light Intensities and CO2 Concentrations)

An oligomycin concentration that specifically inhibits oxidative phosphorylation was added to isolated barley (Hordeum vulgare L.) leaf protoplasts at various irradiances and carbon dioxide concentrations. At saturating as well as low light intensities, photosynthetic oxygen evolution was decreased as a result of the oligomycin treatment, whereas no effect was observed at intermediate light intensities. This was the same for photorespiratory and nonphotorespiratory conditions. These results were confirmed by measurements of fluorescence quenching under the same conditions. Metabolite analysis in the presence of oligomycin revealed a drastic decrease in the mitochondrial and cytosolic ATP/ADP ratios, whereas there was little or no effect on the chloroplastic ratio. Concomitantly, sucrose phosphate synthase activity was reduced. Under high irradiances, this inhibition of sucrose synthesis by oligomycin apparently caused a feedback inhibition on the Calvin cycle and the photosynthetic activity. Under low irradiances, a feedback regulation compensated, indicating that light was more limiting than the activity of regulative enzymes. Thus, the importance of mitochondrial respiratory activity might be different in different metabolic situations. At saturating light, the oxidation of excess photosynthetic redox equivalents is required to sustain a high rate of photosynthesis. At low light, the supply of ATP to the cytosol might be required to support biosynthetic reactions.     

Deciphering the 820 nm signal: redox state of donor side and quantum yield of Photosystem I in leaves

By recording leaf transmittance at 820 nm and quantifying the photon flux density of far red light (FRL) absorbed by long-wavelength chlorophylls of Photosystem I (PS I), the oxidation kinetics of electron carriers on the PS I donor side was mathematically analyzed in sunflower (Helianthus annuus L.), tobacco (Nicotiana tabacum L.) and birch (Betula pendula Roth.) leaves. PS I donor side carriers were first oxidized under FRL, electrons were then allowed to accumulate on the PS I donor side during dark intervals of increasing length. After each dark interval the electrons were removed (titrated) by FRL. The kinetics of the 820 nm signal during the oxidation of the PS I donor side was modeled assuming redox equilibrium among the PS I donor pigment (P700), plastocyanin (PC), and cytochrome f plus Rieske FeS (Cyt f + FeS) pools, considering that the 820 nm signal originates from P700⁺ and PC⁺. The analysis yielded the pool sizes of P700, PC and (Cyt f + FeS) and associated redox equilibrium constants. PS I density varied between 0.6 and 1.4 μmol m⁻². PS II density (measured as O₂ evolution from a saturating single-turnover flash) ranged from 0.64 to 2.14 μmol m⁻². The average electron storage capacity was 1.96 (range 1.25 to 2.4) and 1.16 (range 0.6 to 1.7) for PC and (Cyt f + FeS), respectively, per P700. The best-fit electrochemical midpoint potential differences were 80 mV for the P700/PC and 25 mV for the PC/Cyt f equilibria at 22 °C. An algorithm relating the measured 820 nm signal to the redox states of individual PS I donor side electron carriers in leaves is presented. Applying this algorithm to the analysis of steady-state light response curves of net CO₂ fixation rate and 820 nm signal shows that the quantum yield of PS I decreases by about half due to acceptor side reduction at limiting light intensities before the donor side becomes oxidized at saturating intensities. Footnote: This revised version was published online in August 2006 with corrections to the Cover Date.     

Combined influence of light and temperature on growth rates of <Emphasis Type="Italic">Nannochloropsis oceanica</Emphasis>: linking cellular responses to large-scale biomass production

The interaction effects between irradiance and temperature on growth rates ofNannochloropsis oceanicawere determined in both laboratory cultures and large-scale tubular photobioreactors. Growth responses were investigated in 48 batch cultures subjected to crossing light/temperature gradients ranging from 34–80μmol photons m⁻²s⁻¹and 14.5–35.7^∘C respectively. Comparisons were made to growth responses observed in production systems (200L biofences) operated in climate-regulated greenhouses with controlled temperature and artificial light gradients. Cellular responses showed increasing specific growth rates as a function of temperature, with a peak at 25–29^∘C, after which the growth became increasingly unstable. The optimum temperature for growth increased with higher light intensities up to approximately 28^∘C at 80μmol photons m⁻²s⁻¹. At low light intensities the specific growth rate was less affected by temperature. The maximum daily production measured in the biofence systems increased proportionally with irradiation and reached approximately 0.7gL⁻¹d⁻¹at 1030μmol photons m⁻²s⁻¹average daily radiation for a culture temperature of 24^∘C. This corresponds to a daily yield of 140g per day in a 200L biofence system. When specific growth rates for the biofence cultures were measured at different densities and plotted against temperature, results showed a peak with the 24^∘C temperature treatment. This peak became less pronounced as the density increased in the cultures. This is consistent with the laboratory results; increasing cell density in the biofence cultures resulted in less average light cell⁻¹, which produced the same temperature dependent response as seen by reducing the external irradiance exposure for the dilute laboratory cultures.     

Function of mitochondria during the transition of barley protoplasts from low light to high light

Mitochondrial contribution to photosynthetic metabolism during the transition from low light (25–100 μmol quanta m⁻² s⁻¹, limiting photosynthesis) to high light (500 μmol quanta m⁻² s⁻¹, saturating photosynthesis) was investigated in protoplasts from barley (Hordeum vulgare) leaves. After the light shift, photosynthetic oxygen evolution rate increased rapidly during the first 30–40 s and then declined up to 60–70 s after which the rate increased to a new steady-state after 80–110 s. Rapid fractionation of protoplasts was used to follow changes in sub-cellular distribution of key metabolites during the light shift and the activation state of chloroplastic NADP-dependent malate dehydrogenase (EC 1.1.1.82) was measured. Although oligomycin (an inhibitor of the mitochondrial ATP synthase) affected the metabolite content of protoplasts following the light shift, the first oxygen burst was not affected. However, the transition to the new steady-state was delayed. Rotenone (an inhibitor of mitochondrial complex I) had similar, but less pronounced effect as oligomycin. From the analysis of metabolite content and sub-cellular distribution we suggest that the decrease in oxygen evolution following the first oxygen burst is due to phosphate limitation in the chloroplast stroma. For the recovery the control protoplasts can utilize ATP supplied by mitochondrial oxidative phosphorylation to quickly overcome the limitation in stromal phosphate and to increase the content of Calvin cycle metabolites. The oligomycin-treated protoplasts were deficient in cytosolic ATP and thereby unable to support Calvin cycle operation. This resulted in a delayed capacity to adjust to a sudden increase in light intensity.     

Simultaneous detection of photosynthetic energy storage and oxygen evolution in leaves by photothermal radiometry and photoacoustics

We have measured simultaneously the photothermal radiometry and the photoacoustic signals from intact leaves. We have confirmed that while the former senses that part of the modulated absorbed radiation not used in photosynthesis, but converted into heat, the latter, at low modulation frequencies, senses not only this heat but also the modulated oxygen evolution resulting from photosynthesis in the leaf. When photosynthetic activity is saturated upon additional excitation with strong non-modulated light, the photothermal radiometry signal increases (virtually all absorbed modulated light being converted into heat), while at the same time the photoacoustic signal decreases, because virtually no modulated oxygen evolution occurs any more. At higher modulation frequencies the behaviour of the photoacoustic signal closely follows that of the photothermal radiometry signal. We have used combined photothermal radiometry / photoacoustic measurements to estimate directly the yield of chemical energy storage in various plant species which applies for different times after excitation. Measurement of light saturation curves for wheat and Siberian pea bush leaves and of action spectra for the latter confirm the similarity between photothermal radiometry and high-frequency photoacoustic signals, and their difference from the low-frequency photoacoustic signal. Combined use of photothermal radiometry (or high-frequency photoacoustics) and low-frequency photoacoustics can thus provide more information than any one method alone. Experiments on intact chloroplasts and on a blue-green alga demonstrate that photothermal radiometry and photoacoustic methodologies can also be used for these tissues.     

Studies on photoacoustic uptake signals in tobacco leaves under high carbon dioxide levels

Photoacoustic signals were measured in expanded tobacco leaves, exposed to a controlled atmosphere by being only partly enclosed within the photoacoustic cell. It was aimed to corroborate the conjecture of Reising and Schreiber (Photosynthesis Research 42: 65-73, 1994) that under exceptionally high CO2 levels (ca. 1–5%) the photobaric uptake contribution reflects CO2 uptake induced by light dependent stromal alkalinization. This is shown here by: (1) the shallower damping of the uptake signal vs. the modulation frequency, compared to a normal oxygen evolution signal; (2) the partial inhibition of the uptake signal under 5% CO2 by nigericin; (3) the complete absence of uptake signals under 5% CO2 in a carbonic-anhydrase-deficient mutant, which gave rather a normal oxygen evolution signal. The photoacoustic signals from the wild type and the transgenic tobacco in air could not be distinguished, indicating that the CO2 uptake signal is negligible under this condition. Uptake photobaric signals were also measured in modulated far-red light (ca. 715–750 nm), following addition of white background light (in light limiting intensity). In normal tobacco under 5% CO2, the background light induced an uptake transient, lasting about a minute, then declining to a low steady level. Significantly smaller transients were obtained under normal air, and in the carbonic-anhydrase deficient mutant also under 5% CO2. Extrapolation to zero frequency of the signal damping vs. modulation frequency, in both tobacco genotypes, suggests however similar magnitudes of the uptake transients. On the other hand, no proportional steady-state uptake was observed for the last two cases. Presumably, the steady uptake under 5% CO2 in modulated far-red light reflects CO2 solubilization, while it is an open question whether the transient could be partly contributed also by oxygen photoreduction by PS I (Mehler reaction). It is reasoned that, under conditions of low light, the respiratory activity results in accumulation of CO2 in the photoacoustic cell, which is sufficient to induce an uptake phenomenon, giving a more satisfactory interpretation for the so-called 'low light state' [Cananni and Malkin (1984) Biochim Biophys Acta 766: 525–532].     

Sulfite inhibition of photochemical activity of intact pea leaves

Sulfite treatment of pea leaf disks in light caused a significant decrease in the relative quantum yield of photosynthetic oxygen evolution and energy storage (ES) as measured by photoacoustic (PA) spectroscopy. The inhibition was concentration dependent and was less in darkness than in light, indicating light-dependent inhibitory site(s) on the photosynthetic electron transport chain. Further, in darksulfite-treated leaves, the energy storage was more affected than the relative quantum yield of oxygen evolution, suggesting that photophosphorylation and/or cyclic electron transport around PS I are sites of sulfite action in darkness. The R_fd values, the ratio of fluorescence decrease (fd) to the steady-state fluorescence (fs), decreased significantly in leaves treated with sulfite in light but were not affected in dark-treated ones, confirming the photoacoustic observations. Similarly, the ratio of variable fluorescence (F_v) to maximum fluorescence (F_m), a measure of PS II photochemical efficiency, was affected by sulfite treatment in light and not changed by treatment in darkness. An attempt was made to explain the mechanism of sulfite action on photosynthetic electron transport in light and in darkness.Abbreviations A_PT amplitude of photothermal signal - A_ox amplitude of oxygen signal - ES energy storage - fd fluorescence decrease - fs steady-state fluorescence - F_m maximum fluorescence - F_v variable fluorescence - PA photoacoustic(s)     

Profiles of photosynthetic oxygen-evolution within leaves of Spinacia oleracea

Oxygen evolution was measured from mesophyll tissues in spinach leaves using a photoacoustic technique. The photosynthetic capacity of individual cell layers was measured by directing microscopic beams of light, 40 μm wide, to cells exposed within a leaf cross section. The resulting profile for oxygen-evolution potential was relatively flat, indicating a uniform capacity for photosynthesis in leaf mesophyll tissues. Two experimental approaches were used to estimate the photosynthetic performance of individual mesophyll cell layers when white light was applied to the adaxial leaf surface. These experiments indicated that oxygen was produced relatively uniformly across the mesophyll and that oxygen evolution increased with irradiance of the white light applied to the leaf surface. The measured profiles for oxygen evolution and capacity are flatter than previous measurements of profiles of fixed carbon and estimates of profiles for absorbed light within spinach leaves.     

Photoacoustic detection of photosynthetic oxygen evolution from leaves. Quantitative analysis by phase and amplitude measurements

The photoacoustic signal from an intact leaf was analyzed as a vectorial summation of photothermal and photosynthetic oxygen-evolution contributions. A method is outlined to estimate each contribution separately. The amplitude of the oxygen-evolution component relative to that of the photothermal singnal decreases as the modulation frequency increases due to two processes which specifically damp the oxygen-evolution modulation: (1) diffusion of oxygen from the chloroplasts to the cell boundary, and (2) electron-transfer reactions occurring between the photochemical act and oxygen evolution. The effects of the two processes are well separated and are observed over different ranges of modulation frequency. Analysis of the data leads to a consistent estimation of the oxygen diffusion coefficient and also to a preliminary idea on the limiting time constant on the donor side of Photosystem II. The dependence of the photoacoustic oxygen-evolution signal on the intensity of added nonmodulated background light is used to construct the light saturation curve of (gross) Photsynthesis, with an estimation of the ratio maximal rate / maximal quantum yield. The photoacoustic method is distinguished by its sensitivity and rapidity (a single measurement takes approx. 1 s), far better than any other method to measure gross photosynthesis. The only disadvantage is in the fact that the quantum yield of oxygen evolution is determined in a relative basis only. Attempts to calibrate the photoacoustic measurements in an absolute sense are underway.     

Progressive degradation of serum samples limits proteomic biomarker discovery

Preanalytical variables play a key role in discovery of biomarkers. Although the effect of several preanalytical variables on the mass spectral profiles has been studied extensively, little is known about long-term storage of serum samples. This is important because samples used in case-control or epidemiological studies are usually stored for a long time before analysis. Here we evaluated long-term storage effects on mass spectral peak patterns of serum peptides extracted using weak cation exchange magnetic beads. For this, 20 serum samples stored at −80 °C were divided equally into two groups based on their storage time. We found that intensities of 26 mass spectral peaks significantly varied between these two groups. Intensities of these peaks significantly correlated with storage time. Genetic algorithm-based models generated using these 26 peaks could classify 63 additional samples into these two groups with 100% and 96% accuracy, respectively. We also show that storing samples for 10 months at −80 and −20 °C results in the appearance/disappearance or intensity variation of peaks, some of which were previously reported as disease biomarkers.     

Limitation of Photosynthesis by Carbon Metabolism : I. Evidence for Excess Electron Transport Capacity in Leaves Carrying Out Photosynthesis in Saturating Light and CO(2)

It has been investigated how far electron transport or carbon metabolism limit the maximal rates of photosynthesis achieved by spinach leaves in saturating light and CO₂. Leaf discs were illuminated with high light until a steady state rate of O₂ evolution was attained, and then subjected to a 30 second interruption in low light, to generate an increased demand for the products of electron transport. Upon returning to high light there is a temporary enhancement of photosynthesis which lasts 15 to 30 seconds, and can be up to 50% above the steady state rate of O₂ evolution. This temporary enhancement is only found when saturating light intensities are used for the steady state illumination, is increased when low light rather than darkness is used during the interruption, and is maximal following a 30 to 60 seconds interruption in low light. Decreasing the temperature over the 10 to 30°C range led to the transient enhancement becoming larger. The temporary enhancement is associated with an increased ATP/ADP ratio, a decreased level of 3-phosphoglycerate, and increased levels of triose phosphate and ribulose 1,5-bisphosphate. Since electron transport can occur at higher rates than in steady state conditions, and generate a higher energy status, it is concluded that leaves have a surplus electron transport capacity in saturating light and CO₂. From the alterations of metabolites, it can be calculated that the enhanced O₂ evolution must be accompanied by an increased rate of ribulose 1,5-bisphosphate regeneration and carboxylation. It is suggested that the capacity for sucrose synthesis ultimately limits the maximal rates of photosynthesis, by restricting the rate at which inorganic phosphate can be recycled to support electron transport and carbon fixation in the chloroplast.     

Photoacoustic and fluorescence measurements of the chilling response and their relationship to carbon dioxide uptake in tomato plants

The response of tomato plants to various chilling treatments was studied using two approaches for the measurement of photosynthetic activity. One involved the use of a portable fluorometer for the measurement of in-vivo chlorophyll fluorescence, while the other employed a newly introduced photoacoustic system which allowed changes in oxygen evolution to be followed in a leaf disc. A strong correlation was found between results obtained by each system and those obtained by a conventional open gas-exchange system for the determination of CO₂ uptake. Both systems of measurements could readily distinguish between the effects of chilling in the dark (at 3° C for 18 h) and chilling at high photon flux density (2000 mol m^-2 s^-1 for 5h at 5° C). Chilling in the dark had practically no effect on the quantum yield of oxygen evolution, chlorophyll fluorescence or CO₂ uptake, while chilling at excessively high photon flux density resulted in a sharp reduction (50–70%) in the quantum yields obtained. The results support the view that photosystem II cannot be the primary site of damage by chilling in the dark, although it is significantly affected by chilling at high light intensity.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - PA photoacoustic - PFD photon flux density - PSII photosystem II