Maize (Zea mays L.) planted at higher density utilizes dynamic light more efficiently.

In nature, plants are exposed to a dynamic light environment. Fluctuations in light decreased the photosynthetic light utilization efficiency (PLUE) of leaves, and much more severely in C4 species than in C3 species. However, little is known about the plasticity of PLUE under dynamic light in C4 species. Present study focused on the influence of planting density to the photosynthesis under dynamic light in maize (Zea mays L.), a most important C4 crop. In addition, the molecular mechanism behind photosynthetic adaptation to planting density were also explored by quantitative proteomics analysis. Results revealed that as planting density increases, maize leaves receive less light that fluctuates more. The maize planted at high density (HD) improved the PLUE under dynamic light, especially in the middle and later growth stages. Quantitative proteomics analysis showed that the transfer of nitrogen from Rubisco to RuBP regeneration and C4 pathway related enzymes contributes to the photosynthetic adaptation to lower and more fluctuating light environment in HD maize. This study provides potential ways to further improve the light energy utilization efficiency of maize in HD.

Has breeding altered the light environment, photosynthetic apparatus, and photosynthetic capacity of wheat leaves?

Whether photosynthesis has improved with increasing yield in major crops remains controversial. Research in this area has often neglected to account for differences in light intensity experienced by cultivars released in different years. Light intensity is expected to be positively associated with photosynthetic capacity and the resistance of the photosynthetic apparatus to high light but negatively associated with light-utilization efficiency under low light. Here, we analyzed the light environment, photosynthetic activity, and protein components of leaves of 26 winter wheat cultivars released during the past 60 years in China. Over time, light levels on flag leaves significantly decreased due to architectural changes, but photosynthetic rates under high or low light and the resistance of the photosynthetic apparatus to high light remained steady, contrary to expectations. We propose that the difference between the actual and expected trends is due to breeding. Specifically, breeding has optimized photosynthetic performance under high light rather than low light. Moreover, breeding selectivity altered the stoichiometry of several proteins related to dynamic photosynthesis, canopy light distribution, and photoprotection. These results indicate that breeding has significantly altered the photosynthetic mechanism in wheat and its response to the light environment. These changes likely have helped increase wheat yields.

Photoprotection by mitochondrial alternative pathway is enhanced at heat but disabled at chilling.

The mitochondrial alternative pathway (AP) represents an important photoprotective mechanism for the chloroplast, but the temperature sensitivity of its photoprotective role is unknown. In this study, using the aox1a Arabidopsis mutant, the photoprotective role of the AP was verified under various temperatures, and the mechanism underlying the temperature sensitivity of the AP's photoprotective role was clarified. It was observed that the photoprotective role of the AP increased with rising temperature but was absent at low temperature. The photoprotective role of the AP was severely reduced under non-photorespiratory conditions. Disturbance of the AP inhibited the conversion of glycine to serine in mitochondria, which may restrain upstream photorespiratory metabolism and aggravate photoinhibition. With rising temperatures, photorespiration accelerated and the restraint of photorespiration caused by disturbance of the AP also increased, determining the temperature sensitivity of the AP's photoprotective role. We also verified that not only the AP but also the cytochrome pathway in mitochondria contributes to photoprotection by maintaining photorespiration.

Dynamic light caused less photosynthetic suppression, rather than more, under nitrogen deficit conditions than under sufficient nitrogen supply conditions in soybean.

BACKGROUND: Plants are always exposed to dynamic light. The photosynthetic light use efficiency of leaves is lower in dynamic light than in uniform irradiance. Research on the influence of environmental factors on dynamic photosynthesis is very limited. Nitrogen is critical for plants, especially for photosynthesis. Low nitrogen (LN) decreases ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and thus limits photosynthesis. The decrease in Rubisco also delays photosynthetic induction in LN leaves; therefore, we hypothesized that the difference of photosynthetic CO2 fixation between uniform and dynamic light will be greater in LN leaves compared to leaves with sufficient nitrogen supply. RESULTS: To test this hypothesis, soybean plants were grown under low or high nitrogen (HN), and the photosynthetic gas exchange, enzyme activity and protein amount in leaves were measured under uniform and dynamic light. Unexpectedly, dynamic light caused less photosynthetic suppression, rather than more, in LN leaves than in HN leaves. The underlying mechanism was also clarified. Short low-light (LL) intervals did not affect Rubisco activity but clearly deactivated fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase), indicating that photosynthetic induction after a LL interval depends on the reactivation of FBPase and SBPase rather than Rubisco. In LN leaves, the amount of Rubisco decreased more than FBPase and SBPase, so FBPase and SBPase were present in relative excess. A lower fraction of FBPase and SBPase needs to be activated in LN leaves for photosynthesis recovery during the high-light phase of dynamic light. Therefore, photosynthetic recovery is faster in LN leaves than in HN leaves, which relieves the photosynthetic suppression caused by dynamic light in LN leaves. CONCLUSIONS: Contrary to our expectations, dynamic light caused less photosynthetic suppression, rather than more, in LN leaves than in HN leaves of soybean. This is the first report of a stress condition alleviating the photosynthetic suppression caused by dynamic light.

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.

Systemic signalling in photosynthetic induction of Rumex†K-1 (Rumex patientia × Rumex tianschaious) leaves.

The rapid induction of photosynthesis is critical for plants under light-fleck environment. Most previous studies about photosynthetic induction focused upon single leaf, but they did not consider the systemic integrity of plant. Here, we verified whether systemic signalling is involved in photosynthetic induction. Rumex K-1 (Rumex patientia × Rumex tianschaious) plants were grown under light-fleck condition. After whole night dark adaptation, different numbers of leaves (system leaf or SL) were pre-illuminated with light, and then the photosynthetic induction of other leaves (target leaf or TL) was investigated. This study showed that the pre-illumination of SL promoted photosynthetic induction in TL. This promotion was independent of the number of SL, the light intensity on SL and the distance between SL and TL, indicating that this systemic signalling is non-dose-dependent. More interestingly, the photosynthetic induction was promoted by only the pre-illumination of morphological upper leaf rather than the pre-illumination of morphological lower leaf, indicating that the transfer of this signal is directional. The results showed that the transfer of this systemic signalling depends upon the phloem. This systemic signalling helps plants to use light energy more efficiently under light flecks.

Effects of Environmental and Non-Environmental Factors on Dynamic Photosynthetic Carbon Assimilation in Leaves under Changing Light.

Major research on photosynthesis has been carried out under steady light. However, in the natural environment, steady light is rare, and light intensity is always changing. Changing light affects (usually reduces) photosynthetic carbon assimilation and causes decreases in biomass and yield. Ecologists first observed the importance of changing light for plant growth in the understory; other researchers noticed that changing light in the crop canopy also seriously affects yield. Here, we review the effects of environmental and non-environmental factors on dynamic photosynthetic carbon assimilation under changing light in higher plants. In general, dynamic photosynthesis is more sensitive to environmental and non-environmental factors than steady photosynthesis, and dynamic photosynthesis is more diverse than steady photosynthesis. Finally, we discuss the challenges of photosynthetic research under changing light.

The mitochondrial alternative oxidase pathway protects the photosynthetic apparatus against photodamage in Rumex K-1 leaves.

BACKGROUND: It is known that excess reducing equivalents in the form of NADPH in chloroplasts can be transported via shuttle machineries, such as the malate-oxaloacetate (OAA) shuttle, into the mitochondria, where they are efficiently oxidised by the mitochondrial alternative oxidase (AOX) respiratory pathway. Therefore, it has been speculated that the AOX pathway may protect plants from photoinhibition, but the mechanism by which this protection occurs remains to be elucidated. RESULTS: The observation that the malate-OAA shuttle activity and the AOX pathway capacity increased markedly after intense light treatment in Rumex K-1 leaves indicates that excess NADPH was transported from the chloroplasts and oxidised by the AOX pathway. The inhibition of the AOX pathway by salicylhydroxamic acid (SHAM) caused the over-reduction of the photosystem I (PSI) acceptor side, as indicated by the increases in the extent of reduction of P700+. Furthermore, the photosynthetic linear electron flow was restricted, which was indicated by the decreases in the PSII electron transport rate (ETR) and the photosynthetic O2 evolution rate. The restriction of the photosynthetic linear electron flow, which generates the thylakoid ΔpH, inevitably decreased the de-epoxidation of the xanthophyll cycle (ΔPRI). Therefore, the induction of non-photochemical quenching (NPQ) was suppressed when the AOX pathway was inhibited. The effect of the inhibition of the AOX pathway on NPQ induction was less at 20 mM NaHCO3 than at 1 mM NaHCO3. The suppression of NPQ induction by the inhibition of the AOX pathway was also observed during the induction phase of photosynthesis. In addition, the inhibition of the AOX pathway increased the accumulation of hydrogen peroxide (H2O2), suggesting that the AOX pathway functions as an antioxidant mechanism. CONCLUSIONS: The inhibition of the AOX pathway resulted in the rapid accumulation of NADPH in the chloroplasts, which caused the over-reduction of the PSI acceptor side. Furthermore, the restriction of the photosynthetic linear electron flow due to the inhibition of the AOX pathway limited the generation of the thylakoid ΔpH and suppressed the induction of NPQ. Therefore, the mitochondrial AOX pathway protected the photosynthetic apparatus against photodamage by alleviating the over-reduction of the PSI acceptor side and accelerating the induction of NPQ in Rumex K-1 leaves.

Characterization of the programmed cell death induced by metabolic products of Alternaria alternata in tobacco BY-2 cells.

Alternaria alternata has received considerable attention in current literature and most of the studies are focused on its pathogenic effects on plant chloroplasts, but little is known about the characteristics of programmed cell death (PCD) induced by metabolic products (MP) of A. alternata, the effects of the MP on mitochondrial respiration and its relation to PCD. The purpose of this study was to explore the mechanism of MP-induced PCD in non-green tobacco BY-2 cells and to explore the role of mitochondrial inhibitory processes in the PCD of tobacco BY-2 cells. MP treatment led to significant cell death that was proven to be PCD by the concurrent cytoplasm shrinkage, chromatin condensation and DNA laddering observed in the cells. Moreover, MP treatment resulted in the overproduction of reactive oxygen species (ROS), rapid ATP depletion and a respiratory decline in the tobacco BY-2 cells. It was concluded that the direct inhibition of the mitochondrial electron transport chain (ETC), alternative pathway (AOX) capacity and catalase (CAT) activity by the MP might be the main contributors to the MP-induced ROS burst observed in tobacco BY-2 cells. The addition of adenosine together with the MP significantly inhibited ATP depletion without preventing PCD; however, when the cells were treated with the MP plus CAT, ROS overproduction was blocked and PCD did not occur. The data presented here demonstrate that the ROS burst played an important role in MP-induced PCD in the tobacco BY-2 cells.

Mitochondrial alternative oxidase pathway protects plants against photoinhibition by alleviating inhibition of the repair of photodamaged PSII through preventing formation of reactive oxygen species in Rumex K-1 leaves.

The purpose of this study was to explore how the mitochondrial AOX (alternative oxidase) pathway alleviates photoinhibition in Rumex K-1 leaves. Inhibition of the AOX pathway decreased the initial activity of NADP-malate dehydrogenase (EC 1.1.1.82, NADP-MDH) and the pool size of photosynthetic end electron acceptors, resulting in an over-reduction of the photosystem I (PSI) acceptor side. The over-reduction of the PSI acceptor side further inhibited electron transport from the photosystem II (PSII) reaction centers to the PSII acceptor side as indicated by an increase in V(J) (the relative variable fluorescence at J-step), causing an imbalance between photosynthetic light absorption and energy utilization per active reaction center (RC) under high light, which led to the over-excitation of the PSII reaction centers. The over-reduction of the PSI acceptor side and the over-excitation of the PSII reaction centers enhanced the accumulation of reactive oxygen species (ROS), which inhibited the repair of the photodamaged PSII. However, the inhibition of the AOX pathway did not change the level of photoinhibition under high light in the presence of the chloroplast D1 protein synthesis inhibitor chloramphenicol, indicating that the inhibition of the AOX pathway did not accelerate the photodamage to PSII directly. All these results suggest that the AOX pathway plays an important role in the protection of plants against photoinhibition by minimizing the inhibition of the repair of the photodamaged PSII through preventing the over-production of ROS.

Wheat embryo globulin nutrients ameliorate d-galactose and aluminum chloride-induced cognitive impairment in rats.

Wheat embryo globulin nutrient (WEGN), with wheat embryo globulin (WEG) as the main functional component, is a nutritional combination that specifically targets memory impairment. In this study, we explored the protective role of WEGN on Alzheimer's disease (AD)-triggered cognitive impairment, neuronal injury, oxidative stress, and acetylcholine system disorder. Specifically, we established an AD model via administration of d-galactose (d-gal) and Aluminum chloride (AlCl3) for 70 days, then on the 36th day, administered animals in the donepezil and WEGN (300, 600, and 900 mg/kg) groups with drugs by gavage for 35 days. Learning and memory ability of the treated rats was tested using the Morris water maze (MWM) and novel object recognition (NOR) test, while pathological changes and neuronal death in their hippocampus CA1 were detected via HE staining and Nissl staining. Moreover, we determined antioxidant enzymes by measuring levels of superoxide dismutase (SOD), malondialdehyde (MDA), and glutathione peroxidase (GSH-Px) in serum, cortex, and hippocampus, whereas changes in the acetylcholine system were determined by evaluating choline acetyltransferase (ChAT), and acetylcholinesterase (AChE) activities, as well as choline acetylcholine (Ach) content. Results revealed that rats in the WEGN group exhibited significantly lower escape latency, as well as a significantly higher number of targeted crossings and longer residence times in the target quadrant, relative to those in the model group. Notably, rats in the WEGN group spent more time exploring new objects and exhibited lower damage to their hippocampus neuron, had improved learning and memory activity, as well as reversed histological alterations, relative to those in the model group. Meanwhile, biochemical examinations revealed that rats in the WEGN group had significantly lower MDA levels and AChE activities, but significantly higher GSH, SOD, and ChAT activities, as well as Ach content, relative to those in the model group. Overall, these findings indicate that WEGN exerts protective effects on cognitive impairment, neuronal damage, oxidative stress, and choline function in AD rats treated by d-gal/AlCl3.

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To improve light environment, photosynthetic capacity, and thus the yield of maize, the effects of directional planting on light distribution in canopy and photosynthetic characteristics of ear leaves, as well as the performance of PSII that closely related with photosynthetic characteristics and reflected by the rapid chlorophyll fluorescence kinetic curves were examined in Zhengdan 958 maize variety. The results showed that the orientation of leaves remarkably affected photosynthetically active radiation (PAR) interception of ear leaves, with PAR interception of ear leaves in southward treatment being 271.8% higher than that under northward treatment. The orientation of leaves affec-ted photosynthetic light use efficiency of ear leaves under high and low light conditions. The southward treatment increased net photosynthetic rate (Pn) under saturated light in ear leaves, indicating that the use efficiency to high light was enhanced in leaves of southward treatment. In contrast, the northward treatment increased the apparent quantum yield (α) of ear leaves, indicating leaves in southward treatment adapted the light-limited environment. During the early stage after anthesis, the performance of PSII electron donor side and electron acceptor side was significantly improved, and thus enhanced the performance of PSII reaction center (PIABS) and fluorescence photochemical quenching coefficient (Ψo) in ear leaves of southward treatment. The increase of quantum yield of electron transfer (φEo) indicated the enhancement of transfer performance of electrons from photosystem 2 (PSII) to photosystem 1 (PSI) in leaves of southward treatment. The photosynthetic performance of ear leaves showed a trend of southward > eastward > westward > northward during the early stage after anthesis. Forty days after anthesis, the use efficiency to high light decreased in ear leaves of southward treatment, but the ear leaves of southward treatment showed high use efficiency to low light, which changed the trend of photosynthetic performance of ear leaves to northward > westward > eastward > southward. In summary, northward and eastward treatments improved the light distribution in canopy, the PAR interception of ear leaves, the capacity of photosynthesis and dry matter production, and consequently increased the yield of summer maize.

Mechanisms by which Bisphenol A affect the photosynthetic apparatus in cucumber (Cucumis sativus L.) leaves.

Bisphenol A (BPA), a widely distributed pollutant, suppresses photosynthesis in leaves. In previous studies on higher plants, the plants were treated by BPA through irrigation to root. This method cannot distinguish whether the BPA directly suppresses photosynthesis in leaves, or indirectly influences photosynthesis through affecting the function of root. Here, only the leaves but not the roots of cucumber were infiltrated with BPA solution. The photosystem II and I (PSII, PSI) were insensitive to BPA under darkness. BPA aggravated the PSII but not the PSI photoinhibition under light. BPA also inhibited CO2 assimilation, and the effect of BPA on PSII photoinhibition disappeared when the CO2 assimilation was blocked. The H2O2 accumulated in BPA-treated leaves under light. And the BPA-caused PSII photoinhibition was prevented under low (2%) O2. We also proved that the BPA-caused PSII photoinhibition depend on the turnover of D1 protein. In conclusion, this study proved that BPA could directly suppress photosynthesis in leaves, however, BPA does not damage PSII directly, but inhibits CO2 assimilation and over-reduces the electron transport chain under light, which increases the production of reactive oxygen species (H2O2), the over-accumulated ROS inhibits the turnover of D1 protein and consequently aggravates PSII photoinhibition.

Contribution of the Alternative Respiratory Pathway to PSII Photoprotection in C3 and C4 Plants.

The mechanism by which the mitochondrial alternative oxidase (AOX) pathway contributes to photosystem II (PSII) photoprotection is in dispute. It was generally thought that the AOX pathway protects photosystems by dissipating excess reducing equivalents exported from chloroplasts through the malate/oxaloacetate (Mal/OAA) shuttle and thus preventing the over-reduction of chloroplasts. In this study, using the aox1a Arabidopsis mutant and nine other C3 and C4 plant species, we revealed an additional action model of the AOX pathway in PSII photoprotection. Although the AOX pathway contributes to PSII photoprotection in C3 leaves treated with high light, this contribution was observed to disappear when photorespiration was suppressed. Disruption or inhibition of the AOX pathway significantly decreased the photorespiration in C3 leaves. Moreover, the AOX pathway did not respond to high light and contributed little to PSII photoprotection in C4 leaves possessing a highly active Mal/OAA shuttle but with little photorespiration. These results demonstrate that the AOX pathway contributes to PSII photoprotection in C3 plants by maintaining photorespiration to detoxify glycolate and via the indirect export of excess reducing equivalents from chloroplasts by the Mal/OAA shuttle. This new action model explains why the AOX pathway does not contribute to PSII photoprotection in C4 plants.

Characterization of photosynthetic gas exchange in leaves under simulated adaxial and abaxial surfaces alternant irradiation.

Previous investigations on photosynthesis have been performed on leaves irradiated from the adaxial surface. However, leaves usually sway because of wind. This action results in the alternating exposure of both the adaxial and abaxial surfaces to bright sunlight. To simulate adaxial and abaxial surfaces alternant irradiation (ad-ab-alt irradiation), the adaxial or abaxial surface of leaves were exposed to light regimes that fluctuated between 100 and 1,000 µmol m(-2) s(-1). Compared with constant adaxial irradiation, simulated ad-ab-alt irradiation suppressed net photosynthetic rate (Pn) and transpiration (E) but not water use efficiency. These suppressions were aggravated by an increase in alternant frequency of the light intensity. When leaves were transferred from constant light to simulated ad-ab-alt irradiation, the maximum Pn and E during the high light period decreased, but the rate of photosynthetic induction during this period remained constant. The sensitivity of photosynthetic gas exchange to simulated ad-ab-alt irradiation was lower on abaxial surface than adaxial surface. Under simulated ad-ab-alt irradiation, higher Pn and E were measured on abaxial surface compared with adaxial surface. Therefore, bifacial leaves can fix more carbon than leaves with two "sun-leaf-like" surfaces under ad-ab-alt irradiation. Photosynthetic research should be conducted under dynamic conditions that better mimic nature.

Light Suppresses Bacterial Population through the Accumulation of Hydrogen Peroxide in Tobacco Leaves Infected with Pseudomonas syringae pv. tabaci.

Pseudomonas syringae pv. tabaci (Pst) is a hemibiotrophic bacterial pathogen responsible for tobacco wildfire disease. Although considerable research has been conducted on the tobacco plant's tolerance to Pst, the role of light in the responses of the photosystems to Pst infection is poorly understood. This study aimed to elucidate the underlying mechanisms of the reduced photosystem damage in tobacco leaves due to Pst infection under light conditions. Compared to dark conditions, Pst infection under light conditions resulted in less chlorophyll degradation and a smaller decline in photosynthetic function. Although the maximal quantum yield of photosystem II (PSII) and the activity of the photosystem I (PSI) complex decreased as Pst infection progressed, damage to PSI and PSII after infection was reduced under light conditions compared to dark conditions. Pst was 17-fold more abundant in tobacco leaves under dark compared to light conditions at 3 days post inoculation (dpi). Additionally, H2O2 accumulated to a high level in tobacco leaves after Pst infection under light conditions; although to a lesser extent, H2O2 accumulation was also significant under dark conditions. Pretreatment with H2O2 alleviated chlorotic lesions and decreased Pst abundance in tobacco leaves at 3 dpi under dark conditions. MV pretreatment had the same effects under light conditions, whereas 3-(3,4-dichlorophenyl)-1,1-dimethylurea pretreatment aggravated chlorotic lesions and increased the Pst population. These results indicate that chlorotic symptoms and the size of the bacterial population are each negatively correlated with H2O2 accumulation. In other words, light appears to suppress the Pst population in tobacco leaves through the accumulation of H2O2 during infection.

Ultraviolet-B Radiation (UV-B) Relieves Chilling-Light-Induced PSI Photoinhibition And Accelerates The Recovery Of CO2 Assimilation In Cucumber (Cucumis sativus L.) Leaves.

Ultraviolet-B radiation (UV-B) is generally considered to negatively impact the photosynthetic apparatus and plant growth. UV-B damages PSII but does not directly influence PSI. However, PSI and PSII successively drive photosynthetic electron transfer, therefore, the interaction between these systems is unavoidable. So we speculated that UV-B could indirectly affect PSI under chilling-light conditions. To test this hypothesis, the cucumber leaves were illuminated by UV-B prior or during the chilling-light treatment, and the leaves were then transferred to 25 °C and low-light conditions for recovery. The results showed that UV-B decreased the electron transfer to PSI by inactivating the oxygen-evolving complex (OEC), thereby protecting PSI from chilling-light-induced photoinhibition. This effect advantages the recoveries of PSI and CO2 assimilation after chilling-light stress, therefore should minimize the yield loss caused by chilling-light stress. Because sunlight consists of both UV-B and visible light, we suggest that UV-B-induced OEC inactivation is critical for chilling-light-induced PSI photoinhibition in field. Moreover, additional UV-B irradiation is an effective strategy to relieve PSI photoinhibition and yield loss in protected cultivation during winter. This study also demonstrates that minimizing the photoinhibition of PSI rather than that of PSII is essential for the chilling-light tolerance of the plant photosynthetic apparatus.

Water Status Related Root-to-Shoot Communication Regulates the Chilling Tolerance of Shoot in Cucumber (Cucumis sativus L.) Plants.

Although root-to-shoot communication has been intensively investigated in plants under drought, few studies have examined root-to-shoot communication under chilling. Here we explored whether root-to-shoot communication contributes to the chilling-light tolerance of cucumber shoots and clarified the key signal involves in this communication. After leaf discs chilling-light treatment, the photoinhibitions of Photosystem I (PSI) and Photosystem II (PSII) were similar in leaf discs of two cucumber varieties (JY-3 and JC-4). When the whole plants, including roots, were chilled under light, the photosynthetic performances in JC-4 leaves decreased more seriously than that in JY-3 leaves. However, when the water status of leaves was maintained by warming roots or floating the attached leaves on water, the PSII activity and amount of PSI in the leaves of the two varieties were similar after chilling-light treatment. In addition, the differences of PSII activities and amount of PSI between the two varieties under whole plant chilling-light treatment were independent of ABA pretreatment. Above results indicate that (1) the better water status in leaves under chilling contributes to the higher chilling tolerance of JY-3; (2) the water status, rather than an ABA signal, dominates root-to-shoot communication under chilling and the chilling tolerance of cucumber shoot.

The mechanism by which NaCl treatment alleviates PSI photoinhibition under chilling-light treatment.

The effects of chilling-light stress combined with additional stress on PSI and PSII photoinhibition and their interrelationship have not been known. To explore whether NaCl affects the PSI and PSII photoinhibition and their interrelationship under chilling-light treatment, the PSI and PSII activities were studied under chilling-light with or without NaCl treatment. The results showed that the extent of PSI and PSII photoinhibition both increased under chilling-light, while NaCl aggravated PSII photoinhibition and severely damaged cytochrome b6/f complex but alleviated PSI photoinhibition. Moreover, DCMU had a similar effect as NaCl in this study, which indicates that NaCl alleviated PSI photoinhibition through reducing electrons transported to PSI. It was also showed that the increased damage to PSII by NaCl did not depend on the inhibition of PSII repair and PSI electron transportation. In conclusion, NaCl alleviated PSI photoinhibition by inhibiting electron transport from PSII under chilling-light conditions. In addition, PSII photoinhibition was not affected by PSI photoinhibition because of a full inhibition of PSII repair by chilling-light treatment. We also speculate that NaCl aggravates PSII photoinhibition by enhancing the damage instead of inhibiting the repair of it under chilling-light conditions.