Heat Emission as a Protective Mechanism against High-irradiance Stress in Sugar Maple Leaves

High-irradiance (HI) induced changes in heat emission, fluorescence, and photosynthetic energy storage (EST) of shade grown sugar maple (Acer saccharum Marsh.) saplings were followed using modulated photoacoustic and fluorescence spectroscopic techniques. HI-treatment at 900-4400 µmol m-2 s-1 for 15 min caused an increase in heat emission and a decrease in EST. In some leaves, HI-treatment of 900 µmol m-2 s-1 for 1 min induced a rapid increase in heat emission with a marginal decrease in EST. Parallel to the increase in heat emission, there was a decrease in fluorescence, and this phenomenon was reversible in darkness. Quenching of thermal energy dissipation and a recovery in EST were observed during the first 15 min after the HI-treatment. This down-regulation of photochemical activity and its recovery may be one of the photoprotective mechanisms in shade grown sugar maple plants. The increase in thermal energy dissipation was greater in the red absorbing long wavelength (640-700 nm) region than in the blue absorbing short wavelength region of photosynthetically active excitation radiation. The photochemical activity was affected more in short wavelengths (400-520 nm) than in the long wavelength region of the spectrum. This can be due to the migration of light-harvesting chlorophyll (Chl) a/b protein complex from photosystem (PS) 2 to PS1 and/or to the disconnection of carotenoid pool from Chls in the pigment bed of photosynthetic apparatus.     

Nano-TiO2 Improve the Photosynthesis of Tomato Leaves under Mild Heat Stress

Nano-TiO₂ has been reported to promote photosynthesis in some crops; however, the mechanism behind this action remains unknown. In this research, the effects of nano-TiO₂ on leaf photosynthesis under mild heat stress were investigated. Results showed that the net photosynthetic rate, conductance to H₂O, and transpiration rate of tomato leaves increased after application of an appropriate concentration of nano-TiO₂. Nano-TiO₂ also significantly decreased the minimum chlorophyll fluorescence and relative electron transport in leaves. Under mild heat stress, Nano-TiO₂ increased regulated photosystem II (PS II) energy dissipation and decreased non-regulated PS II energy dissipation. These results indicate that nano-TiO₂ plays a positive role in promoting photosynthesis in tomato leaves under mild heat stress.     

Induction kinetics of heat emission before and after photoinhibition in cotyledons of Raphanus sativus

Heat emitted during non-radiative de-excitation was determined in vivo by the photoacoustic method. The dependence of the photoacoustic signal on the length of the pulses (modulation frequency) of the excitation light and the effect of continuous light, which saturates photosynthesis but does not directly contribute to the signal, are described. The induction kinetic of heat emission measured with intact leaves differed only slightly from the induction kinetic of fluorescence (Kautsky effect) detected in parallel. The photoacoustic signal at high modulation frequencies (279 Hz), which represents the signal of heat emission, and the photoacoustic signal at low modulation frequencies (17 Hz), interpreted as a signal of pulsed oxygen evolution superimposed on the heat emission, were measured with leaves before and after photoinhibition. It was demonstrated that after photoinhibition the decrease in fluorescence yield and in photosynthetic activity (here detected as photoacoustic signal at 17 Hz) are paralleled by an increase in the yield of non-radiative deexcitation (photoacoustic signal at 279 Hz). The increase of heat emission, which has been hypothized for photoinhibited leaves, could now be proved by measuring the induction kinetics of the photoacoustic signal.     

高大气CO2浓度下氮素对小麦叶片光能利用的影响

关于氮素对高大气CO₂浓度下C₃植物光合作用适应现象的调节机理已有较为深入的研究, 但对其光合作用适应现象的光合能量转化和分配机制缺乏系统分析。该文以大气CO₂浓度和施氮量为处理手段, 通过测定小麦(Triticum aestivum)抽穗期叶片的光合作用-胞间CO₂浓度响应曲线以及荧光动力学参数来测算光合电子传递速率和分配去向, 研究了长期高大气CO₂浓度下小麦叶片光合电子传递和分配对施氮量的响应。结果表明, 与正常大气CO₂浓度处理相比, 高大气CO₂浓度下小麦叶片较多的激发能以热量的形式耗散, 增施氮素可使更多的激发能向光化学反应方向的分配, 降低光合能量的热耗散速率; 大气CO₂浓度升高后小麦叶片光化学淬灭系数无明显变化, 高氮叶片的非光化学猝灭降低而低氮叶片明显升高, 施氮促进PSII反应中心的开放比例, 降低光能的热耗散; 高大气CO₂浓度下高氮叶片通过PSII反应中心的光合电子传递速率(J_F)较高, 而且参与光呼吸的非环式电子流速率(J₀)显著降低, 较正常大气CO₂浓度处理的高氮叶片下降了88.40%, 光合速率增加46.47%; 高大气CO₂浓度下小麦叶片J_F-J₀升高而J₀/J_F显著下降, 光呼吸耗能被抑制, 更多的光合电子分配至光合还原过程。因此, 大气CO₂浓度增高条件下, 小麦叶片激发能的热耗散速率增加, 但增施氮素后小麦叶片PSII反应中心开放比例提高, 光化学速率增加, 进入PSII反应中心的电子流速率明显升高, 光呼吸作用被抑制, 光合电子较多地进入光化学过程, 这可能是高氮条件下光合作用适应性下调被缓解的一个原因。     

Effects of Exogenous Putrescine on Chlorophyll Fluorescence Imaging and Heat Dissipation Capacity in Cucumber (Cucumis sativus L.) Under Salt Stress

The objective of this study was to identify the effects of exogenous putrescine on photosynthetic performance and heat dissipation capacity in cucumber seedlings under salt stress. The stress of 75 mM NaCl for 7 days caused a significant decrease in net photosynthetic rate (P _N ). The experiment employed a chlorophyll fluorescence imaging technique and demonstrated that the maximal quantum yield of photosystem II photochemistry (Fv/Fm) and the actual photochemical efficiency of photosystem II (ΦPSII) were reduced by salt stress. Moreover, salt stress markedly reduced the photochemical quenching coefficient (qP) and non-photochemical quenching coefficient (qN), and significantly increased non-regulated heat dissipation (ΦNO). However, stressed plants supplied with exogenous putrescine exhibited higher P _N and ΦPSII, which indicated that putrescine can alleviate the detrimental effects on photosynthesis induced by salt stress. Putrescine sprayed on stressed plants significantly enhanced the regulated energy dissipation (ΦNPQ) and decreased ΦNO. Application of exogenous putrescine also changed the levels of xanthophyll cycle components and further enhanced the de-epoxidation state of xanthophyll cycle pigments under salt stress. Under control conditions, putrescine exerted little influence on the photosynthetic parameters in cucumber leaves. In conclusion, the application of exogenous putrescine may improve the heat dissipation capacity by promoting the xanthophyll cycle to reduce the damage caused by excess excitation energy, thus enhancing the salt tolerance of cucumber seedlings.     

Functioning of photosystems I and II in pea leaves exposed to heat stress in the presence or absence of light

Fluorimetric, photoacoustic, polarographic and absorbance techniques were used to measure in situ various functional aspects of the photochemical apparatus of photosynthesis in intact pea leaves (Pisum sativum L.) after short exposures to a high temperature of 40 ° C. The results indicated (i) that the in-vivo responses of the two photosystems to high-temperature pretreatments were markedly different and in some respects opposite, with photosystem (PS) II activity being inhibited (or down-regulated) and PSI function being stimulated; and (ii) that light strongly interacts with the response of the photosystems, acting as an efficient protector of the photochemical activity against its inactivation by heat. When imposed in the dark, heat provoked a drastic inhibition of photosynthetic oxygen evolution and photochemical energy storage, correlated with a marked loss of variable PSII-chlorophyll fluorescence emission. None of the above changes were observed in leaves which were illuminated during heating. This photoprotection was saturated at rather low light fluence rates (around 10 W · m^–2). Heat stress in darkness appeared to increase the capacity for cyclic electron flow around PSI, as indicated by the enhanced photochemical energy storage in far-red light and the faster decay of P ₇₀₀ ⁺ (oxidized reaction center of PSI) monitored upon sudded interruption of the far-red light. The presence of light during heat stress reduced somewhat this PSI-driven cyclic electron transport. It was also observed that heat stress in darkness resulted in the progressive closure of the PSI reaction centers in leaves under steady illumination whereas PSII traps remained largely open, possibly reflecting the adjustment of the photochemical efficiency of undamaged PSI to the reduced rate of photochemistry in PSII.Abbreviations B₁ and B₂ fraction of closed PSI and PSII reaction centers, respectively - ES photoacoustically measured energy storage - F_o, F_m and F_s initial, maximal and steady-state levels of chlorophyll fluorescence - P₇₀₀ reaction center of PSI - PS (I, II) photosystem (I, II) - V = (F_s – F_o)/(F_m – F_o) relative variable chlorophyll fluorescence We wish to thank Professor R. Lannoye (ULB, Brussels) for the use of this photoacoustic spectrometer and Mrs. M. Eyletters for her help.     

Photosynthesis Regulation by Glucohexaose Through Redox Changes in Cucumis sativus

To investigate the effects of glucohexaose (P6) on cucumber, leaf CO₂ assimilation, chlorophyll fluorescence parameters, chlorophyll content, and carbohydrate metabolism were examined in cucumber plants. The net photosynthetic rate (P _n ) of cucumber leaves was enhanced after being treated with 10 μg mL^?1 P6. The increase was correlated with increases in transpiration rate (E) and stomatal conductance (G _s), whereas the intercellular CO₂ concentration (C _i) was not different from the control plants. Chlorophyll content, absorption of light energy per unit area (ABS/CS), capture of light energy per unit area (TR_o/CS), quantum yield of electron transport per unit area (ET_o/CS), maximum photochemical efficiency of PSII (φP _o), quantum yield of photosynthetic institution electron transfer (φE _o), probability of other electron acceptors that captured exciton-transferred electrons to the electronic chain which exceeds QA (ψ _o), number of reaction centers per unit leaf area (RC/CS_o), and the performance index on absorption basis (PI_ABS) were improved, but heat dissipation per unit area (DI_o/CS) and maximum quantum yield of non-chemical quenching (φD _o) were reduced. In addition, increases in sucrose, soluble sugars, and starch contents were observed in P6-treated plants. However, H₂O₂ scavenger (DMTU) or NADPH oxidase inhibitor (DPI) pretreatment significantly abolished the effect of P6 on photosynthesis. The results demonstrated that ROS played a critical role in P6-induced photosynthesis. The increase in chlorophyll content together with efficient light absorption, transmission, and conversion in P6-treated plants is important for increasing photosynthesis.     

Light-induced heat production correlated with fluorescence and its quenching mechanisms

Light-induced heat produced by the non-radiative decay represents one way of de-excitation after excitation by light absorption. It was detected in vivo with cotyledons of radish seedlings (Raphanus sativus L.) by measuring the photoacoustic signal at a modulation frequency of 279 Hz. During the induction kinetic of photosynthesis the photoacoustic signal, the chlorophyll fluorescence as well as the photochemical and the non-photochemical quenching of fluorescence were simultaneously determined in order to get information about the correlation of heat production, fluorescence and its quenching mechanisms. Our results demonstrate that the changes of the photoacoustic signal can in most cases be related directly or indirectly to changes in the photochemical activity. However the kinetic of the photoacoustic signal differs from that of the fluorescence and from that of the non-photochemical quenching. This indicates that the sum of energy dissipation processes resulting in the production of light-induced heat and measured by the high-frequency photoacoustic signal must be taken into account when judging photosynthetic activity.Abbreviations LED light-emitting diode - PA photoacoustic - PAM pulse-amplitude-modulated     

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     

Early photosynthetic response of Arabidopsis thaliana to temperature and salt stress conditions

Temperature changes and salt accumulation are among the most common abiotic factors affecting plants in agricultural and natural ecosystems. The different responses of plants to these factors have been widely investigated in previous works. However, detailed mechanism of the early photosynthetic response (first 24 h) has been poorly studied. The aim of the work was to monitor the early response of adult Arabidopsis thaliana plants exposed to different thermal (cold and heat) and salt conditions. Detailed evaluation of the efficiency of photosystem II was done, and the various routes of energy output as well as measurements of the contents of H₂O₂, proline, and photosynthetic pigments at different times during the first 24 h of treatment were examined. The conditions used in the study were those that caused a weak stress with time of exposure. Cold-treated plants showed the most continuous inhibitory effect on photosynthetic activity, with a fast metabolic slowdown (reduced PSII efficiency and decreased pigment contents), although they also demonstrated clear acclimation responses (increased heat dissipation and protein content). Heat-treated plants showed a late but stronger effect on photosynthesis with significantly increased quantum yield of nonregulated energy dissipation (??_NO) and H₂O₂ content at the last measurements. Finally, salt-induced oxidative stress (increased H₂O₂ content), decreased PSII efficiency and pigment content.     

O2-dependent inhibition of photosynthetic capacity in intact isolated chloroplasts and isolated cells from spinach leaves illuminated in the absence of CO2

When isolated intact chloroplasts or cells from spinach (Spinacia oleracea L.) leaves are incubated in the light in the absence of CO₂, their capacity for subsequent CO₂-dependent photosynthetic oxygen evolution is drastically decreased. This inhibition is light and oxygen-dependent and can be prevented by addition of bicarbonate. It is concluded that the normal dissipation of photosynthetic energy by carbon assimilation and in processes related to photorespiration is an essential condition for the physiological stability of illuminated intact chloroplasts and cells.Abbreviation chl chlorophyll     

Increased photosynthetic activities and thermostability of photosystem II with leaf development of elm seedlings (Ulmus pumila) probed by the fast fluorescence rise OJIP

Experiments were conducted to investigate the photosynthetic activity and thermostability of photosystem II (PSII) in elm seedling (Ulmus pumila) leaves from initiation to full expansion. During leaf development, photosynthesis, measured as CO₂ fixation, increased gradually and reached a maximum value when leaves were fully developed. In parallel with the increase of carbon assimilation, chlorophyll content increased. The chlorophyll a fluorescence measurements showed that the maximum quantum yield of PSII primary photochemistry (φ_po), the efficiency with which the energy of trapped excitons is converted into the electron transport beyond Q_A (Ψ_o) and the quantum yield of electron transport beyond Q_A (φ_Eo) increased gradually. The low light experiments confirmed these results independently. When subjected to heat stress, young leaves exhibited progressively lower φ_po and maximal fluorescence (F_m) values with considerably higher minimal fluorescence (F_o) than mature leaves, demonstrating that PSII in newly initiating leaves is more sensitive to heat stress. Further analysis revealed that PSII structure in newly initiating leaves showed a robust alteration under heat stress, which was reflected by the clear K phase in the OJIP curves. Therefore, we suggest that the enhanced thermostability of PSII in the case of leaf growth might be associated with an improvement of the stability of the oxygen-evolving complex (OEC) to heat stress during leaf development.     

SO(2) Effect on Photosynthetic Activities of Intact Sugar Maple Leaves as Detected by Photoacoustic Spectroscopy

Short-term (4 hours) effect of different concentrations of SO₂ fumigation on in vivo photochemical activities of sugar maple (Acer saccharum Marsh.) leaves was investigated using photoacoustic spectroscopy. The relative quantum yield of O₂ evolution (ratio of O₂ signal to the photothermal signal) and photochemical energy storage are increased by 0.05 microliter per liter of SO₂. This increase is more pronounced in 5 to 7 year old saplings than in 3 month old seedlings. Both oxygen-relative quantum yield and energy storage of seedlings are inhibited by increased concentrations of SO₂ and the inhibition is concentration dependent. The inhibition is greater in seedlings than in saplings at 2 microliters per liter of SO₂, indicating the more susceptible nature of seedlings. The present study indicates a concentration dependent differential effect of SO₂ on photochemical activities of sugar maple leaves.     

Thermal dissipation during photosynthetic induction and subsequent dark recovery as measured by photoacoustic signals

The thermal photoacoustic signal (279 Hz) and the chlorophyll (Chl) fluorescence of radish cotyledons (Raphanus sativus L.) were measured simultaneously. The signals were recorded during a photosynthetic induction with actinic radiation of different quantum fluence rates [20, 200, and 1200 μmol(PAR-quantum) m^-2 s^-1]. The rise of these signals upon irradiation saturating photosynthesis was followed in the steady state of the induction and during the subsequent dark-recovery (i.e., in dark periods of 1, 5, 15, and 45 min after the induction). From these values various parameters (e.g., quantum yield, photochemical loss, different types of quenching coefficients) were calculated. The results show that heat dissipation detected by photoacoustic measurements is neither low, constant, nor always parallel to Chl fluorescence. Therefore, the thermal signal should always be measured in order to fully understand the way leaves convert energy taken up by PAR absorption. This helps in the interpretation of photosynthesis under different natural and anthropogenic conditions (stress and damage effects). This revised version was published online in September 2006 with corrections to the Cover Date.     

Gas Exchange Analysis of the Relative Importance of Stomatal and Biochemical Factors in Photosynthetic Induction in Alocasia macrorrhiza

When leaves of Alocasia macrorrhiza adapted to 10 micromole quanta per square meter per second were transferred to 500 micromole quanta per square meter per second, the rate of photosynthetic CO₂ assimilation increased for over 45 minutes. For the first 10 to 15 minutes, increases in both stomatal conductance and the leaf's photosynthetic capacity were responsible for the increase in assimilation rate. Thereafter, continuing increases in stomatal conductance were almost entirely responsible for further increases in assimilation rate. When conductances were initially high, assimilation rates 1 minute after the increase in photon flux density could be more than six times as high as for similar leaves with initially low conductance. Further increases in assimilation rate in these leaves with high conductance were predominantly due to increases in the induction state at the biochemical level and followed an exponential time course. When stomatal conductances were initially low, then increases in conductance were predominantly responsible for the increases in assimilation rate, with both following a sigmoidal time course. In these leaves, it was important to also consider the effect of cuticular water loss on the calculation of the intracellular partial pressure of CO₂, and an assessment of the relative importance of stomatal conductance differed considerably from one that did not include cuticular water loss.     

Photoacoustic measurements of the energy-conversion efficiency of photosynthesis in thalli of the green alga Bryopsis maxima

The efficiency of photosynthetic energy conversion in thalli of the green alga Bryopsis maxima was studied with the photoacoustic technique. Photosynthetic O₂ evolution did not interfere with the photoacoustic measurements in this material, most probably owing to its coenocytic cellular organization. The energy yield (defined as the fraction of absorbed photon energy that is stored in photosynthetic products or intermediates relative to the total absorbed photon energy) was estimated from the photoacoustic signals by applying the background-illumination method to obtain a reference without the photochemical capacity (Lasser-Ross, N., Malkin, S. and Cahen, D. (1980) Biochim. Biophys. Acta 593, 330–341). With the monitoring light modulated at 60 Hz, photon energy is mainly stored by redox changes in electron-transport chains because the energy yield was strongly reduced by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea and heat treatment of the thalli, whereas KCN, an inhibitor of CO₂ reduction, had no effect, and because a significant lowering of the energy yield occurred in the presence of methyl viologen but the effect of the Photosystem I acceptor was largely reversed on the addition of an uncoupler, methylamine. The maximum energy yield of 0.4 that was obtained with a saturating background light and with a sufficiently weak monitoring light modulated at 100 Hz is explained in terms of electron transfer from electron-donor pools to acceptor pools of the two photosystems with the quantum yield close to unity. A lowering of the modulation frequency decreased the energy yield, indicating that less energy is stored in more stable intermediates.     

Limiting Factors in Photosynthesis: V. Photochemical Energy Supply Colimits Photosynthesis at Low Values of Intercellular CO(2) Concentration

Although there is now some agreement with the view that the supply of photochemical energy may influence photosynthetic rate (P) at high CO₂ pressures, it is less clear whether this limitation extends to P at low CO₂. This was investigated by measuring P per area as a function of the intercellular CO₂ concentration (C_i) at different levels of photochemical energy supply. Changes in the latter were obtained experimentally by varying the level of irradiance to normal (Fe-sufficient) leaves of Beta vulgaris L. cv F58-554H1, and by varying photosynthetic electron transport capacity using leaves from Fe-deficient and Fe-sufficient plants. P and C_i were determined for attached sugar beet leaves using open flow gas exchange. The results suggest that P/area was colimited by the supply of photochemical energy at very low as well as high values of C_i. Using the procedure developed by Perchorowicz et al. (Plant Physiol 1982 69:1165-1168), we investigated the effect of irradiance on ribulose bisphosphate carboxylase (RuBPCase) activation. The ratio of initial extractable activity to total inducible RuBPCase activity increased from 0.25 to 0.90 as leaf irradiance increased from 100 to 1500 microeinsteins photosynthetically active radiation per square meter per second. These data suggest that colimitation by photochemical energy supply at low C_i may be mediated via effects on RuBPCase activation.     

Combined effects of cadmium and ozone on photosynthesis of Lycopersicon esculentum

Tomato (Lycopersicon esculentum Mill. cv. Pearson) plants were grown in growth chambers for 25 days with cadmium (Cd) and then exposed briefly to ozone (O₃). Gas exchange, chlorophyll a fluorescence, and pigment composition were analysed in leaves at the end of the treatment to assess the effects of a single pollutant and their combination on photosynthesis. The CO₂ assimilation rate was dramatically reduced in plants subjected to the combined treatment, while the single effect of Cd appeared less severe than that of O₃. The decline of CO₂ photoassimilation found in all O₃-exposed plants was attributed to both stomatal and nonstomatal limitations. Tomato plants seemed to detoxify Cd to a great extent, but this resulted in growth suppression. In response to O₃ exposure, the plants protected their photosystems by heat dissipation of excess energy via the xanthophyll cycle. Cd combined with O₃ affected adversely this cycle resulting in an increase in photosynthetic performance under the same experimental light conditions.     

Zeaxanthin and the Induction and Relaxation Kinetics of the Dissipation of Excess Excitation Energy in Leaves in 2% O(2), 0% CO(2)

The relationship between the carotenoid zeaxanthin, formed by violaxanthin de-epoxidation, and nonphotochemical fluorescence quenching (q_NP) in the light was investigated in leaves of Glycine max during a transient from dark to light in 2% O₂, 0% CO₂ at 100 to 200 micromoles of photons per square meter per second. (a) Up to a q_NP (which can vary between 0 and 1) of about 0.7, the zeaxanthin content of leaves was linearly correlated with q_NP as well as with the rate constant for radiationless energy dissipation in the antenna chlorophyll (k_D). Beyond this point, at very high degrees of fluorescence quenching, only k_D was directly proportional to the zeaxanthin content. (b) The relationship between zeaxanthin and k_D was quantitatively similar for the rapidly relaxing quenching induced in 2% O₂, 0% CO₂ at 200 micromoles of photons per square meter per second and for the sustained quenching induced by long-term exposure of Nerium oleander to drought in high light (B Demmig, K Winter, A Krüger, F-C Czygan [1988] Plant Physiol 87: 17-24). These findings suggest that the same dissipation process may be induced by very different treatments and that this particular dissipation process can have widely different relaxation kinetics. (c) A rapid induction of strong nonphotochemical fluorescence quenching within about 1 minute was observed exclusively in leaves which already contained a background level of zeaxanthin.     

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)