Potassium alleviates over-reduction of the photosynthetic electron transport chain and helps to maintain photosynthetic function under salt-stress.

Potassium ions enhance photosynthetic tolerance to salt stress. We hypothesized that potassium ions, by minimizing the trans-thylakoid proton diffusion potential difference, can alleviate over-reduction of the photosynthetic electron transport chain and maintain the functionality of the photosynthetic apparatus. This study investigated the effects of exogenous potassium on the transcription level and activity of proteins related to the photosynthetic electron-transport chain of tobacco seedlings under salt stress. Salt stress retarded the growth of seedlings and caused an outflow of potassium ions from the chloroplast. It also lowered qP (indicator of the oxidation state of QA , the primary quinone electron acceptor in Photosystem II (PSII) and YPSII (average photochemical yield of PSII in the light-adapted state) while increasing YNO+NF (nonregulatory energy dissipation in functional and nonfunctional PSII), accompanied by decreased expression of most light-harvesting, energy-transduction, and electron-transport genes. However, exogenous potassium prevented these effects due to NaCl. Interestingly, lincomycin (an inhibitor of the synthesis of chloroplast-encoded proteins in PSII) significantly diminished the alleviation effect of exogenous potassium on salt stress. We attribute the comprehensive NaCl-induced downregulation of transcription and photosynthetic activities to retrograde signaling induced by reactive oxygen species. There probably exist at least two types of retrograde signaling induced by reactive oxygen species, distinguished by their sensitivity to lincomycin. Exogenous potassium appears to exert its primary effect by ameliorating the trans-thylakoid proton diffusion potential difference via a potassium channel, thereby accelerating ATP synthesis and carbon assimilation, alleviating over-reduction of the photosynthetic electron transport chain, and maintaining the functionality of photosynthetic proteins.

The energy cost of repairing photoinactivated photosystem II: an experimental determination in cotton leaf discs.

Photosystem II (PSII), which splits water molecules at minimal excess photochemical potential, is inevitably photoinactivated during photosynthesis, resulting in compromised photosynthetic efficiency unless it is repaired. The energy cost of PSII repair is currently uncertain, despite attempts to calculate it. We experimentally determined the energy cost of repairing each photoinactivated PSII in cotton (Gossypium hirsutum) leaves, which are capable of repairing PSII in darkness. As an upper limit, 24 000 adenosine triphosphate (ATP) molecules (including any guanosine triphosphate synthesized at the expense of ATP) were required to repair one entire PSII complex. Further, over a 7-h illumination period at 526-1953 µmol photons m-2 s-1 , the ATP requirement for PSII repair was on average up to 4.6% of the ATP required for the gross carbon assimilation. Each of these two measures of ATP requirement for PSII repair is two- to three-fold greater than the respective reported calculated value. Possible additional energy sinks in the PSII repair cycle are discussed.

Mehler reaction plays a role in C3 and C4 photosynthesis under shade and low CO2.

Alternative electron fluxes such as the cyclic electron flux (CEF) around photosystem I (PSI) and Mehler reaction (Me) are essential for efficient photosynthesis because they generate additional ATP and protect both photosystems against photoinhibition. The capacity for Me can be estimated by measuring O2 exchange rate under varying irradiance and CO2 concentration. In this study, mass spectrometric measurements of O2 exchange were made using leaves of representative species of C3 and C4 grasses grown under natural light (control; PAR ~ 800 µmol quanta m-2 s-1) and shade (~ 300 µmol quanta m-2 s-1), and in representative species of gymnosperm, liverwort and fern grown under natural light. For all control grown plants measured at high CO2, O2 uptake rates were similar between the light and dark, and the ratio of Rubisco oxygenation to carboxylation (Vo/Vc) was low, which suggests little potential for Me, and that O2 uptake was mainly due to photorespiration or mitochondrial respiration under these conditions. Low CO2 stimulated O2 uptake in the light, Vo/Vc and Me in all species. The C3 species had similar Vo/Vc, but Me was highest in the grass and lowest in the fern. Among the C4 grasses, shade increased O2 uptake in the light, Vo/Vc and the assimilation quotient (AQ), particularly at low CO2, whilst Me was only substantial at low CO2 where it may contribute 20-50% of maximum electron flow under high light.

The half-life of the cytochrome bf complex in leaves of pea plants after transfer from moderately-high growth light to low light.

The content of cytochrome (cyt) bf complex is the main rate-limiting factor that determines light- and CO2-saturated photosynthetic capacity. A study of the half-life of the cyt f content in leaves was conducted whereby Pisum sativum L. plants, grown in moderately high light (HL), were transferred to low light (LL). The cyt f content in fully-expanded leaves decreased steadily over the 2 weeks after the HL-to-LL transfer, whereas control leaves in HL retained their high contents. The difference between the time courses of HL-to-LL plants and control HL plants represents the time course of loss of cyt f content, with a half-life of 1.7 days, which is >3-fold shorter than that reported for tobacco leaves at constant growth irradiance using an RNA interference approach (Hojka et al. 2014). After transfer to LL (16h photoperiod), pea plants were re-exposed to HL for 0, 1.5h or 5h during the otherwise LL photoperiod, but the cyt f content of fully-expanded leaves declined practically at the same rate regardless of whether HL was re-introduced for 0, 1.5h or 5h during each 16h LL photoperiod. It appears that fully-expanded leaves, having matured under HL, were unable to increase their cyt f content when re-introduced to HL. These findings are relevant to any attempts to maintain a high photosynthetic capacity when the growth irradiance is temporarily decreased by shading or overcast weather.

Photoinhibition and photoinhibition-like damage to the photosynthetic apparatus in tobacco leaves induced by pseudomonas syringae pv. Tabaci under light and dark conditions.

BACKGROUND: Pseudomonas syringae pv. tabaci (Pst), which is the pathogen responsible for tobacco wildfire disease, has received considerable attention in recent years. The objective of this study was to clarify the responses of photosystem I (PSI) and photosystem II (PSII) to Pst infection in tobacco leaves. RESULTS: The net photosynthetic rate (Pn) and carboxylation efficiency (CE) were inhibited by Pst infection. The normalized relative variable fluorescence at the K step (W k) and the relative variable fluorescence at the J step (V J) increased while the maximal quantum yield of PSII (F v/F m) and the density of Q A-reducing PSII reaction centers per cross section (RC/CSm) decreased, indicating that the reaction centers, and the donor and acceptor sides of PSII were all severely damaged after Pst infection. The PSI activity decreased as the infection progressed. Furthermore, we observed a considerable overall degradation of PsbO, D1, PsaA proteins and an over-accumulation of reactive oxygen species (ROS). CONCLUSIONS: Photoinhibition and photoinhibition-like damage were observed under light and dark conditions, respectively, after Pst infection of tobacco leaves. The damage was greater in the dark. ROS over-accumulation was not the primary cause of the photoinhibition and photoinhibition-like damage. The PsbO, D1 and PsaA proteins appear to be the targets during Pst infection under light and dark conditions.

Acclimation of leaves to low light produces large grana: the origin of the predominant attractive force at work.

Photosynthetic membrane sacs (thylakoids) of plants form granal stacks interconnected by non-stacked thylakoids, thereby being able to fine-tune (i) photosynthesis, (ii) photoprotection and (iii) acclimation to the environment. Growth in low light leads to the formation of large grana, which sometimes contain as many as 160 thylakoids. The net surface charge of thylakoid membranes is negative, even in low-light-grown plants; so an attractive force is required to overcome the electrostatic repulsion. The theoretical van der Waals attraction is, however, at least 20-fold too small to play the role. We determined the enthalpy change, in the spontaneous stacking of previously unstacked thylakoids in the dark on addition of Mg(2+), to be zero or marginally positive (endothermic). The Gibbs free-energy change for the spontaneous process is necessarily negative, a requirement that can be met only by an increase in entropy for an endothermic process. We conclude that the dominant attractive force in thylakoid stacking is entropy-driven. Several mechanisms for increasing entropy upon stacking of thylakoid membranes in the dark, particularly in low-light plants, are discussed. In the light, which drives the chloroplast far away from equilibrium, granal stacking accelerates non-cyclic photophosphorylation, possibly enhancing the rate at which entropy is produced.

Modulating the light environment with the peach 'asymmetric orchard': effects on gas exchange performances, photoprotection, and photoinhibition.

The productivity of fruit trees is a linear function of the light intercepted, although the relationship is less tight when greater than 50% of available light is intercepted. This paper investigates the management of light energy in peach using the measurement of whole-tree light interception and gas exchange, along with the absorbed energy partitioning at the leaf level by concurrent measurements of gas exchange and chlorophyll fluorescence. These measurements were performed on trees of a custom-built 'asymmetric' orchard. Whole-tree gas exchange for north-south, vertical canopies (C) was similar to that for canopies intercepting the highest irradiance in the morning hours (W), but trees receiving the highest irradiance in the afternoon (E) had the highest net photosynthesis and transpiration while maintaining a water use efficiency (WUE) comparable to the other treatments. In the W trees, 29% and 8% more photosystems were damaged than in C and E trees, respectively. The quenching partitioning revealed that the non-photochemical quenching (NPQ) played the most important role in excess energy dissipation, but it was not fully active at low irradiance, possibly due to a sub-optimal trans-thylakoid DeltapH. The non-net carboxylative mechanisms (NC) appeared to be the main photoprotective mechanisms at low irradiance levels and, probably, they could facilitate the establishment of a trans-thylakoid DeltapH more appropriate for NPQ. These findings support the conclusion that irradiance impinging on leaves may be excessive and can cause photodamage, whose repair requires energy in the form of carbohydrates that are thereby diverted from tree growth and productivity.

Recovery of photoinactivated photosystem II in leaves: retardation due to restricted mobility of photosystem II in the thylakoid membrane.

The functionality of photosystem II (PS II) following high-light pre-treatment of leaf segments at a chilling temperature was monitored as F(v)/F(m), the ratio of variable to maximum chlorophyll fluorescence in the dark-adapted state and a measure of the optimal photochemical efficiency in PS II. Recovery of PS II functionality in low light (LL) and at a favourable temperature was retarded by (1) water stress and (2) growth in LL, in both spinach and Alocasia macrorrhiza L. In spinach leaf segments, water stress per se affected neither F(v)/F(m) nor the ability of the adenosine triphosphate (ATP) synthase to be activated by far-red light for ATP synthesis, but it induced chloroplast shrinkage as observed in frozen and fractured samples by scanning electron microscopy. A common feature of water stress and growth of plants in LL is the enhanced anchoring of PS II complexes, either across the shrunken lumen in water-stress conditions or across the partition gap in larger grana due to growth in LL. We suggest that such enhanced anchoring restricts the mobility of PS II complexes in the thylakoid membrane system, and hence hinders the lateral migration of photoinactivated PS II reaction centres to the stroma-located ribosomes for repair.

Granal stacking of thylakoid membranes in higher plant chloroplasts: the physicochemical forces at work and the functional consequences that ensue.

The formation of grana in chloroplasts of higher plants is examined in terms of the subtle interplay of physicochemical forces of attraction and repulsion. The attractive forces between two adjacent membranes comprise (1) van der Waals attraction that depends on the abundance and type of atoms in each membrane, on the distance between the membranes and on the dielectric constant, (2) depletion attraction that generates local order by granal stacking at the expense of greater disorder (i.e. entropy) in the stroma, and (3) an electrostatic attraction of opposite charges located on adjacent membranes. The repulsive forces comprise (1) electrostatic repulsion due to the net negative charge on the outer surface of thylakoid membranes, (2) hydration repulsion that operates at small separations between thylakoid membranes due to layers of bound water molecules, and (3) steric hindrance due to bulky protrusions of Photosystem I (PSI) and ATP synthase into the stroma. In addition, specific interactions may occur, but they await experimental demonstration. Although grana are not essential for photosynthesis, they are ubiquitous in higher plants. Grana may have been selected during evolution for the functional advantages that they confer on higher plants. The functional consequences of grana stacking include (1) enhancement of light capture through a vastly increased area-to-volume ratio and connectivity of several PSIIs with large functional antenna size, (2) the ability to control the lateral separation of PSI from PSII and, therefore, the balanced distribution of excitation energy between two photosystems working in series, (3) the reversible fine-tuning of energy distribution between the photosystems by State 1-State 2 transitions, (4) the ability to regulate light-harvesting via controlled thermal dissipation of excess excitation energy, detected as non-photochemical quenching, (5) dynamic flexibility in the light reactions mediated by a granal structure in response to regulation by a trans-thylakoid pH gradient, (6) delaying the premature degradation of D1 and D2 reaction-centre protein(s) in PSII by harbouring photoinactived PSIIs in appressed granal domains, (7) enhancement of the rate of non-cyclic synthesis of adenosine triphosphate (ATP) as well as the regulation of non-cyclic vs. cyclic ATP synthesis, and (8) the potential increase of photosynthetic capacity for a given composition of chloroplast constituents in full sunlight, concomitantly with enhancement of photochemical efficiency in canopy shade. Hence chloroplast ultrastructure and function are intimately intertwined.