Effect of copper deficiency on photosynthetic electron transport in wheat plants

Copper deficiency in wheat ( Triticum aestivum L. cv. Nazareno Stramppeli) markedly affects photosynthetic activity. Flag leaves of copper-deficient plants showed a 50% reduction of the photosynthetic rate expressed as mg CO₂ dm⁻²h⁻¹. The activities of PSI and PSII, determined for isolated chloroplasts, as well as fluorescence measurements on intact leaves of copper-deficient plants, indicated a low activity of photosynthetic electron transport. Ribulose bisphosphate carboxylase/oxygenase (Rubisco) activity was not affected by copper deficiency but copper deficiency affected the chloroplast ultrastructure, especially at the level of grana, where a disorganization of thylakoids is evident.     

Inhibition of cyanobacterial photosynthetic activity by natural ketones

Microbial volatiles have a significant impact on the physiological functions of prokaryotic and eukaryotic organisms. Various ketones are present in volatile mixtures produced by plants, bacteria, and fungi. Our earlier results demonstrated the inhibitory effects of soil bacteria volatiles, including ketones, on cyanobacteria. In this work, we thoroughly examined the natural ketones, 2‐nonanone and 2‐undecanone to determine their influence on the photosynthetic activity in Synechococcus sp. PCC 7942. We observed for the first time that the ketones strongly inhibit electron transport through PSII in cyanobacteria cells in vivo. The addition of ketones decreases the quantum yield of primary PSII photoreactions and changes the PSII chlorophyll fluorescence induction curves. There are clear indications that the ketones inhibit electron transfer from Q_A to Q_B, electron transport at the donor side of PSII. The ketones can also modify the process of energy transfer from the antenna complex to the PSII reaction center and, by this means, increase both chlorophyll fluorescence quantum yield and the chlorophyll excited state lifetime. At the highest tested concentration (5 mM) 2‐nonanone also induced chlorophyll release from Synechococcus cells that strongly indicates the possible role of the ketones as detergents.     

Survey of gene expression in winter rye during changes in growth temperature,irradiance or excitation pressure

Previous comparisons of winter rye plants (Secale cereale L. cv. Musketeer) grown in a combination of specific temperature (degrees C)/irradiance (micromol m(-2) s(-1)) regimes (20/50; 20/250; 20/800; 5/50; 5/250) revealed (1) that photosynthetic acclimation to low temperature mimics photosynthetic acclimation to high light because both conditions result in comparable reduction states of photosystem II (PSII), that is, comparable PSII excitation pressure; (2) that the relative redox state of PSII also appears to regulate a specific cold acclimation gene, Wcs19. In order to identify additional genes regulated differentially by either low temperature, irradiance or excitation pressure, we initiated a detailed analysis of gene expression. We identified and characterized 42 differentially expressed genes from wheat and rye. Based on their patterns of regulation under the five growth conditions employed, 37 of the cDNAs could be classified into four groups: genes regulated by PSII excitation pressure, low temperature, growth irradiance and interaction between growth temperature and irradiance. Partial sequence analyses revealed that several of these genes encode known chloroplastic proteins such as ELIPs, transketolase, carbonic anhydrase and Mg-chelatase. However, five of the genes could not be classified unambiguously into any one of these four categories. The implications of these results and the limitations of the experimental design are discussed in terms of larger-scale genomic studies designed to understand the interactions of multiple abiotic stresses to which a plant may be exposed when examining regulation of gene expression.     

Calcium controls the assembly of the photosynthetic water-oxidizing complex: a cadmium(II) inorganic mutant of the Mn4Ca core

Perturbation of the catalytic inorganic core (Mn4Ca1OxCly) of the photosystem II-water-oxidizing complex (PSII-WOC) isolated from spinach is examined by substitution of Ca2+ with cadmium(II) during core assembly. Cd2+ inhibits the yield of reconstitution of O2-evolution activity, called photoactivation, starting from the free inorganic cofactors and the cofactor-depleted apo-WOC-PSII complex. Ca2+ affinity increases following photooxidation of the first Mn2+ to Mn3+ bound to the 'high-affinity' site. Ca2+ binding occurs in the dark and is the slowest overall step of photoactivation (IM1-->IM1* step). Cd2+ competitively blocks the binding of Ca2+ to its functional site with 10- to 30-fold higher affinity, but does not influence the binding of Mn2+ to its high-affinity site. By contrast, even 10-fold higher concentrations of Cd2+ have no effect on O2-evolution activity in intact PSII-WOC. Paradoxically, Cd2+ both inhibits photoactivation yield, while accelerating the rate of photoassembly of active centres 10-fold relative to Ca2+. Cd2+ increases the kinetic stability of the photooxidized Mn3+ assembly intermediate(s) by twofold (mean lifetime for dark decay). The rate data provide evidence that Cd2+ binding following photooxidation of the first Mn3+, IM1-->IM1*, causes three outcomes: (i) a longer intermediate lifetime that slows IM1 decay to IM0 by charge recombination, (ii) 10-fold higher probability of attaining the degrees of freedom (either or both cofactor and protein d.f.) needed to bind and photooxidize the remaining 3 Mn2+ that form the functional cluster, and (iii) increased lability of Cd2+ following Mn4 cluster assembly results in (re)exchange of Cd2+ by Ca2+ which restores active O2-evolving centres. Prior EPR spectroscopic data provide evidence for an oxo-bridged assembly intermediate, Mn3+(mu-O2(-))Ca2+, for IM1*. We postulate an analogous inhibited intermediate with Cd2+ replacing Ca2+.     

Desiccation-time limits of photosynthetic recovery in Equisetum hyemale (Equisetaceae) spores

The chlorophyllous spores of Equisetum survive desiccation, yet cannot tolerate this quiescent state for more than ~2 wk. The hypothesis that spore viability of Equisetum hyemale L. is limited by inhibition of photosynthetic recovery was tested using chlorophyll a fluorescence and oxygen-exchange analyses. Experimental spores were desiccated at 2% relative humidity and 25C for time periods of 24 h, 1 wk, and 2 wk, and then rehydrated at 200 mmol photons/m2s (PAR) and 25C for up to 24 h. Spores desiccated for 24 h recovered photosynthetic competence very rapidly during rehydration, reaching the O2 compensation point in 6.3 ~ 0.3 (mean +/- SE) min. Recovery of photosynthetic performance of spores desiccated for 1 wk was slower, as judged by significantly slower increases of (1) photochemical efficiency of photosystem (PS) II, (2) PS II quinoneB-reducing center concentration, (3) quinoneB concentration, (4) water-oxidation activity, (5) rate of light-induced O2 evolution, and (6) apparent quantum yield of net O2 exchange. Photosystem-II and whole-spore photosynthetic competence of 2-wk desiccated spores was increasingly impaired, and did not recover during rehydration. Origin fluorescence yield and dark respiration were not affected by desiccation time following rehydration. The results suggest that the extremely short viability of disseminated spores of Equisetum hyemale is due to the inability to recover losses of water oxidation and photosystem II-core function following 2 wk of desiccation.     

Rapid photosynthetic adaptation to heat stress triggered in potato leaves by moderately elevated temperatures

Photosystem II (PSII) is considered to be one of the most thermolabile aspects of photosynthesis. In vivo measurements of chlorophyll fluorescence and photosynthetic oxygen evolution in 25°C-grown potato leaves (cv. Haig) indicated that the threshold temperature T_c above which PSII denatures was indeed rather low–about 38°C–with temperatures higher than Tc causing a rapid and irreversible loss of PSII activity. The present study demonstrates the existence of adaptive processes which rapidly adjust the in vivo thermal stability of PSII in response to temperature increase. Transfer of potato leaves from 25°C to temperatures slightly lower than T_c (between 30 and 35°C) was observed to cause an upward shift of the Tc value without any appreciable loss of PSII activity. This increase in PSII thermotolerance was substantial (around +5°C in the Haig cultivar), rapid (with a half-time of ～20 min) and slowly reversible at 25°C (>24h). As a consequence, high temperatures (e.g. 40°C) which caused a complete and irreversible inhibition of the PSII function had very little effect in 35°C-treated leaves, thus suggesting that the above-described PSII changes could be of prime importance for the plant's behaviour in the field. Accordingly, the rise in Tc at 35°C was much larger (+8°C) in Sahel, a stress-resistant potato variety, than in the heat-sensitive Haig cultivar.     

Culture on sugar medium enhances photosynthetic capacity and high light resistance of plantlets grown in vitro

The significance of photosynthetic photon flux (PPF) and sugar feeding for the production of plants in vitro is only poorly understood. Nicotiana tabacum L. plantlets were grown photoautotrophically and photomixotrophically (3% sucrose) at two different PPFs (60 µmol m⁻² s⁻¹ and 200 µmol m⁻² s⁻¹) to investigate the effect of these culture parameters on photosynthetic performance and growth. Photomixotrophically‐grown plantlets showed an increase in carbohydrate content, mainly in glucose and fructose. Plant growth, dry matter accumulation and total leaf area were higher under photomixotrophic than photoautotrophic conditions. Not only biomass formation but also photosynthesis was positively affected by exogenous sucrose; the chlorophyll (Chl) content and the light‐saturated rate of photosynthetic oxygen evolution were higher in photomixotrophic plantlets. Photoinhibition occurred in plantlets that were grown photoautotrophically at the higher PPF. It became apparent as a loss in Chl content and photochemical efficiency. Photoinhibited plantlets showed a decrease in the D2/LHCII and CP47/LHCII ratios, suggesting a preferential loss of proteins from the photosystem II (PSII) core. The increased content of xanthophyll cycle pigments in photoinhibited plantlets indicated that also protective mechanisms were activated. Photomixotrophic growth of the plantlets prevented the occurrence of photoinhibitory symptoms. Therefore, we conclude that culture on sugar medium increases not only the photosynthetic potential but also the high light resistance of plantlets grown in vitro.     

Effects of acute and chronic UV-B exposure on a green alga: a continuous culture study using a computer-controlled dynamic light regime

The green alga Selenastrum capricornutum was grown in a specially developed continuous culture system to study long-term effects of chronic UV-B exposure. The new system improves upon previous laboratory culture approaches. It is demonstrated that short-term experiments underestimate UV-B effects. It is also shown that photoinhibition cannot explain the effects under chronic exposure. Under both nutrient-replete and phosphorus-limiting conditions a UV-B mediated delay in cell division rate and an increase in the cellular content of proteins, carbohydrates and chlorophyll a was measured. Transition experiments showed that complete acclimatisation to UV-B took several cell cycles. DNA damage appears to be the major cause of the observed long-term UV-B effects.     

Adjustment of photosynthetic activity to drought and fluctuating light in wheat

Drought is a major cause of losses in crop yield. Under field conditions, plants exposed to drought are usually also experiencing rapid changes in light intensity. Accordingly, plants need to acclimate to both, drought and light stress. Two crucial mechanisms in plant acclimation to changes in light conditions comprise thylakoid protein phosphorylation and dissipation of light energy as heat by non-photochemical quenching (NPQ). Here, we analyzed the acclimation efficacy of two different wheat varieties, by applying fluctuating light for analysis of plants, which had been subjected to a slowly developing drought stress as it usually occurs in the field. This novel approach allowed us to distinguish four drought phases, which are critical for grain yield, and to discover acclimatory responses which are independent of photodamage. In short-term, under fluctuating light, the slowdown of NPQ relaxation adjusts the photosynthetic activity to the reduced metabolic capacity. In long-term, the photosynthetic machinery acquires a drought-specific configuration by changing the PSII-LHCII phosphorylation pattern together with protein stoichiometry. Therefore, the fine-tuning of NPQ relaxation and PSII-LHCII phosphorylation pattern represent promising traits for future crop breeding strategies.     

Regulation of photosynthetic electron-transport in Phaseolus vulgaris L., as determined by room-temperature chlorophyll a fluorescence

The regulation of photosystem II (PSII) by light-, CO₂-, and O₂-dependent changes in the capacity for carbon metabolism was studied. Estimates of the rate of electron transport through PSII were made from gas-exchange data and from measurements of chlorophyll fluorescence. At subsaturating photon-flux density (PFD), the rate of electron transport was independent of O₂ and CO₂. Feedback on electron transport was observed under two conditions. At saturating PFD and low partial pressure of CO₂, p(CO₂), the rate of electron transport increased with p(CO₂). However, at high p(CO₂), switching from normal to low p(O₂) did not affect the net rate of photosynthetic CO₂ assimilation but the rate of electron-transport decreased by an amount related to the change in the rate of photorespiration. We interpret these effects as 1) regulation of ribulose-1,5-bisphosphatecarboxylase (RuBPCase, EC 4.1.1.39) activity to match the rate of electron transport at limiting PFD, 2) regulation of electron-transport rate to match the rate of RuBPCase at low p(CO₂), and 3) regulation of the electron-transport rate to match the capacity for starch and sucrose synthesis at high p(CO₂) and PFD. These studies provide evidence that PSII is regulated so that the capacity for electron transport is matched to the capacity for other processes required by photosynthesis, such as ribulose-bisphosphate carboxylation and starch and sucrose synthesis. We show that at least two mechanisms contribute to the regulation of PSII activity and that the relative engagement of these mechanisms varies with time following a step change in the capacity for ribulose-bisphosphate carboxylation and starch and sucrose synthesis. Finally, we take advantage of the relatively slow activation of deactivated RuBPCase in vivo to show that the activation level of this enzyme can limit the rate of electron transport as evidenced by increased feedback on PSII following a step change in p(CO₂). As RuBPCase as activated, the feedback on PSII declined.Abbreviations and symbols J_C electron-transport rate calculated from CO₂-assimilation measurements - J_F electron-transport rate calculated from fluorescence parameters - PFD photon-flux density - q_E energy-dependent quenching - PSII photosystem II - q_Q Q-dependent quenching - QY quantum yield - RuBPCase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) C.I.W. publication No. 1015     

Depth distribution of photosynthetic pigments and diatoms in the sediments of a microtidal fjord

The depth distribution of photosynthetic pigments and benthic marine diatoms was investigated in late spring at three different sites on the Swedish west coast. At each site, sediment cores were taken at six depths (7–35 m) by scuba divers. It was hypothesized that (1) living benthic diatoms constitute a substantial part of the benthic microflora even at depths where the light levels are <1% of the surface irradiance, and (2) the changing light environment along the depth gradient will be reflected in (a) the composition of diatom assemblages, and (b) different pigment ratios. Sediment microalgal communities were analysed using epifluorescence microscopy (to study live cells), light microscopy and scanning electron microscopy (diatom preparations), and HPLC (photosynthetic pigments). Pigments were calculated as concentrations (mg m^–2) and as ratios relative to chlorophyll a. Hypothesis (1) was accepted. At 20 m, the irradiance was 0.2% of surface irradiance and at 7 m, 1%. Living (epifluorescent) benthic diatoms were found down to 20 m at all sites. The cell counts corroborated the diatom pigment concentrations, decreasing with depth from 7 to 25 m, levelling out between 25 and 35 m. There were significant positive correlations between chlorophyll a and living (epifluorescent) benthic diatoms and between the diatom pigment fucoxanthin and chlorophyll a. Hypothesis (2) was only partly accepted because it could not be shown that light was the main environmental factor. A principal component analysis on diatom species showed that pelagic forms characterized the deeper locations (25–35 m), and epipelic–epipsammic taxa the shallower sites (7–20 m). Redundancy analyses showed a significant relationship between diatom taxa and environmental factors – temperature, salinity, and light intensities explained 57% of diatom taxa variations.     

Spectral analysis on origination of the bands at 437 nm and 475.5 nm of chlorophyll fluorescence excitation spectrum in Arabidopsis chloroplasts

Chlorophyll fluorescence has been often used as an intrinsic optical molecular probe to study photosynthesis. In this study, the origin of bands at 437 and 475.5 nm in the chlorophyll fluorescence excitation spectrum for emission at 685 nm in Arabidopsis chloroplasts was investigated using various optical analysis methods. The results revealed that this fluorescence excitation spectrum was related to the absorption characteristics of pigment molecules in PSII complexes. Moreover, the excitation band centred at 475.5 nm had a blue shift, but the excitation band at 437 nm changed relatively less due to induction of non‐photochemical quenching (NPQ). Furthermore, fluorescence emission spectra showed that this blue shift occurred when excitation energy transfer from both chlorophyll b (Chl b) and carotenoids (Cars) to chlorophyll a (Chl a) was blocked. These results demonstrate that the excitation band at 437 nm was mainly contributed by Chl a, while the excitation band at 475.5 nm was mainly contributed by Chl b and Cars. The chlorophyll fluorescence excitation spectrum, therefore, could serve as a useful tool to describe specific characteristics of light absorption and energy transfer between light‐harvesting pigments. Copyright © 2015 John Wiley & Sons, Ltd.     

Development of photosynthetic apparatus and respiration in pea seedlings during greening as influenced by toxic concentration of lead

Detached leaves of 14 day-old dark-grown pea seedlings were immersed with their cut ends either in water (control) or in 20 mM Pb(NO₃)₂ solution. They were exposed to continuous illumination during 24 and 48 h. The formation of PSII primary photochemistry in thylakoids was determined in vivo by measuring changes in values of parameters of chlorophyll a fast fluorescence kinetics: Fo, Fm, Fv, Fv/Fm and t 1/2. The amount of lead accumulation in leaves, content of chlorophylls and carotenoids and rates of CO₂ uptake in light and evolution in darkness (Pn-net photosynthesis and DR - dark respiration respectively) were determined. It has been found that with the exception of Fo, values of Fv, Fm and Fv/Fm were reduced by Pb²⁺. The values of t 1/2 were significantly larger in Pb²⁺ treated leaves. Decrease in the chlorophyll a fluorescence parameters was paralleled with the strong inhibition by this metal the biosynthesis of chlorophyll a and b but less of the carotenoids. Pb²⁺ drastically reduced Pn but had a stimulatory action on DR after 24 h and small inhibition of DR after 48 h exposure of leaves to this metal. As a consequence, after 48 h of greening the ratio of DR/Pn of control leaves was 0.45 whereas in Pb²⁺ treated leaves 2.7. It is proposed that DR in leaves plays a protective role against damage of Pn by Pb²⁺. Protection can be due to the supply the respiratory derived reductant and ATP to carry out cell metabolism upon reduced photosynthesis.     

Inhibition of photosynthetic activities under slow water stress measured in vivo by the photoacoustic method

A slow water stress over several days was imposed on tobacco plants (Nicotiana tabacum L. var. Xanthi) by withholding water from the soil. Photosynthesis was measured in leaves from those water-stressed plants by the photoacoustic method. Slow drought induced marked changes in the photoacoustic signals, which were largely similar to those observed previously in leaves subjected to rapid desiccation in air (over 3–4 h), reflecting two simultaneous changes: 1) Modification of the heat and oxygen diffusion characteristics of the leaves due to changes in their anatomical structure [shown by the change in the slope of the plot of the oxygen (A_OX) to photothermal signal (A_PT) ratio vs the square root of the modulation frequency]; 2) Inhibition of gross photosynthesis measured by the extrapolation of the A_OX/A_PT ratio to zero frequency. However, in contrast to rapid water stress in detached leaves, where it was shown that mainly the oxidizing side of photosystem II (PS II) was damaged, we found a slower and more complex phenomenology having largely biphasic kinetics. During the first 6 days, there was a strong reduction in the photochemical energy storage, but the inhibition of oxygen evolution was relatively mild. The Emerson enhancement in state 1 dropped considerably, indicating lowering of the apparent absorption cross-section of PS II. Fluorescence measurements suggest that PS II reaction center itseIf may be the primary site of the damage. PS I activity, judged by cytochrome f photooxidation, remained largely intact. The subsequent days were associated with a further spectacular decrease in the oxygen evolution quantum yield with both photosystems damaged. The photochemical energy storage continued to decrease further. The Emerson enhancement ratio of the remaining activities in both State 1 and 2 showed a marked increase, indicating the reestablishment of a strong imbalance in the distribution of excitation energy within the photochemical apparatus in favor of PS II. All the photoacoustic changes observed in response to drought were completely reversible within 2–3 days upon rewatering of the soil.     

In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply

Diurnal time courses of net CO₂ assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O₂ concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O₂ to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO₂ assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R₁) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.     

Photoinhibition of photosynthesis in vivo: The role of protein turnover in photosystem II

Divergent theories on the mechanism behind, and the nature of, photoinhibition are discussed, especially in relation to observations made in higher plant leaves. Comparisons are made with 'lower' plant groups and results of in vivo and in vitro experiments are considered. Irradiance-induced mechanisms involved in the regulation of PSII function and structure are discussed in connection with turnover of the DI protein. A model is presented in which a structural change in DI protein facilitates the formation of a population of dissipative PSII centres that do not participate in linear electron transport to PSI. We suggest a sophisticated regulatory mechanism whereby this variable PSII function is controlled with respect to both incident light and biochemical demand, a control which relies on feedback from both light and dark reactions.     

高温伤害光合机构原初位点的研究进展

本文介绍了高温伤害光合机构原初位点的研究进展,分析了不同观点产生的可能原因,为进一步研究高温对植物光合作用的影响提供思路。     

Adaptive radiation of photosynthetic physiology in the Hawaiian lobeliads: dynamic photosynthetic responses

Hawaiian lobeliads have radiated into habitats from open alpine bogs to densely shaded rainforest interiors, and show corresponding adaptations in steady-state photosynthetic light responses and associated leaf traits. Shaded environments are not uniformly dark, however, but punctuated by sunflecks that carry most of the photosynthetically active light that strikes plants. We asked whether lobeliads have diversified in their dynamic photosynthetic light responses and how dynamic responses influence daily leaf carbon gain. We quantified gas exchange and dynamic light regimes under field conditions for ten species representing each major Hawaiian sublineage. Species in shadier habitats experienced shorter and less numerous sunflecks: average sunfleck length varied from 1.4 ± 1.7 min for Cyanea floribunda in shaded forest understories to 31.2 ± 2.1 min for Trematolobelia kauaiensis on open ridges. As expected, the rate of photosynthetic induction increased significantly toward shadier sites, with assimilation after 60 s rising from ca. 30% of fully induced rates in species from open environments to 60% in those from densely shaded habitats. Uninduced light use efficiency—actual photosynthesis versus that expected under steady-state conditions—increased from 10 to 70% across the same gradient. In silico transplants—modeling daily carbon gain using one species’ photosynthetic light response in its own and other species’ dynamic light regimes—demonstrated the potential adaptive nature of species differences: understory Cyanea pilosa in its light regimes outperformed gap-dwelling Clermontia parviflora, while Clermontia in its light regimes outperformed Cyanea. The apparent crossover in daily photosynthesis occurred at about the same photon flux density where dominance shifts from Cyanea to Clermontia in the field. Our results further support our hypothesis that the lobeliads have diversified physiologically across light environments in Hawaiian ecosystems and that those shifts appear to maximize the carbon gain of each species in its own environment. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Changes in photosystem II function during senescence of wheat leaves

Analyses of chlorophyll fluorescence were undertaken to investigate the alterations in photosystem II (PSII) function during senescence of wheat ( Triticum aestivum L. cv. Shannong 229) leaves. Senescence resulted in a decrease in the apparent quantum yield of photosynthesis and the maximal CO₂ assimilation capacity. Analyses of fluorescence quenching under steady‐state photosynthesis showed that senescence also resulted in a significant decrease in the efficiency of excitation energy capture by open PSII reaction centers (F'_v/F'_m) but only a slight decrease in the maximum efficiency of PSII photochemistry (F'_v/F'_m). At the same time, a significant increase in non‐photochemical quenching (q_N) and a considerable decrease in photochemical quenching (q_P) were observed in senescing leaves. Rapid fluorescence induction kinetics indicated a decrease in the rate of Q_A reduction and an increase in the proportion of Q_B‐non‐reducing PSII reaction during senescence. The decrease in both F'_v/F'_m and q_P explained the decrease in the actual quantum yield of PSII electron transport ((φ_PSII). We suggest that the modifications in PSII function, which led to the down‐regulation of photosynthetic electron transport, would be in concert with the lower demand for ATP and NADPH in the Calvin cycle which is often inhibited in senescing leaves.