Effects of heat and high irradiance stress on energy dissipation of photosystem II in low irradiance-adapted peanut leaves

To increase crop yields and not to compete for land with food crops, intercropping agricultural cultivation approach was introduced into cultivation of peanut (Arachis hypogaca L.). This approach improves the total yield of the crop per unit area, but decreases the yield of a single crop compared with mono-cropped agricultural cultivation approach. In wheat-peanut relay intercropping system, peanut plants would suffer heat and high light (HI) stress after wheat harvest. In the present work, peanut seedlings were cultivated in low light to simulate wheat-peanut relay intercropping environments. Upon exposure to heat and HI stress, energy dissipation in PSII complexes was evaluated by comparing those cultivated in low irradiance conditions with the mono-cropped peanut. The maximal photochemical efficiency of PSII (F_v/F_m) and the net photosynthetic rate (P_n) decreased markedly in relay-cropped peanut (RP) after heat and HI stress, accompanied by higher degree of PSII reaction center closure (1–qP). After heat and HI stress, higher antioxidant enzyme activity and less ROS accumulation were observed in mono-cropped peanut (MP) seedlings. Meanwhile, higher content of D1 protein and higher ratio of (A + Z)/(V + A + Z) were also detected in MP plants under such stress. These results implied that heat and HI stress could induce photoinhibition of PSII reaction centers in peanut seedlings and both xanthophyll cycle-dependent thermal energy dissipation and the antioxidant system were down-regulated in RP compared to classical monocropping systems after heat and high irradiance stress.     

Involvement of betacyanin in chilling-induced photoinhibition in leaves of <Emphasis Type="Italic">Suaeda salsa</Emphasis>

Seeds of Suaeda salsa were cultured in dark for 3 d and betacyanin accumulation in seedlings was promoted significantly. Then the seedlings with accumulated betacyanin (C+B) were transferred to 14/10 h light/dark and used for chilling treatment 15 d later. Photosystem 2 (PS2) photochemistry, D1 protein content, and xanthophyll cycle during the chilling-induced photoinhibition (exposed to 5 °C at a moderate photon flux density of 500 μmol m⁻² s⁻¹ for 3 h) and the subsequent restoration were compared between the C+B seedlings and the control (C) ones. The maximal efficiency of PS2 photochemistry (F_v/F_m), the efficiency of excitation energy capture by open PS2 centres (F_v′/F_m′), and the yield of PS2 electron transport (Φ_PS2) of the C+B and C leaves both decreased during photoinhibition. However, smaller decreases in F_v/F_m, F_v′/F_m′, and Φ_PS2 were observed in the C+B leaves than in C ones. At the same time, the deepoxidation state of xanthophyll cycle, indicated by (A+Z)/(V+A+Z) ratio, increased rapidly but the D1 protein content decreased considerably during the photoinhibition. The increase in rate of (A+Z)/(V+A+Z) was higher but the D1 protein turnover was slower in C+B than C leaves. After photoinhibition treatment, the plants were transferred to a dim irradiation (10 μmol m⁻² s⁻¹) at 25 °C for restoration. During restoration, the chlorophyll (Chl) fluorescence parameters, D1 protein content, and xanthophyll cycle components relaxed gradually, but the rate and level of restoration in the C+B leaves was greater than those in the C leaves. The addition of betacyanins to the thylakoid solution in vitro resulted in similar changes of F_v/F_m, D1 protein content, and (A+Z)/(V+A+Z) ratio during the chilling process. Therefore, betacyanin accumulation in S. salsa seedlings may result in higher resistance to photoinhibition, larger slowing down of D1 protein turnover, and enhancement of non-radiative energy dissipation associated with xanthophyll cycle, as well as in greater restoration after photoinhibition than in the control when subjected to chilling at moderate irradiance.     

Cooperation of xanthophyll cycle with water-water cycle in the protection of photosystems 1 and 2 against inactivation during chilling stress under low irradiance

The xanthophyll cycle and the water-water cycle had different functional significance in chilling-sensitive sweet pepper upon exposure to chilling temperature (4 °C) under low irradiance (100 µmol m⁻² s⁻¹) for 6 h. During chilling stress, effects of non-photochemical quenching (NPQ) on photosystem 2 (PS2) in dithiothreitol (DTT) fed leaves remained distinguishable from that of the water-water cycle in diethyldithiocarbamate (DDTC) fed leaves. In DTT-fed leaves, NPQ decreased greatly accompanied by visible inhibition of the de-epoxidized ratio of the xanthophyll cycle, and maximum photochemical efficiency of PS2 (F_v/F_m) decreased markedly. Thus the xanthophyll cycle-dependent NPQ could protect PS2 through energy dissipation under chilling stress. However, NPQ had a slighter effect on photosystem 1 (PS1) in DTT-fed leaves than in DDTC-fed leaves, whereas effects of the water-water cycle on PS1 remained distinguishable from that of NPQ. Inhibiting superoxide dismutase (SOD) activity increased the accumulation of , the oxidation level of P700 (P700⁺) decreased markedly relative to the control and DTT-fed leaves. Both F_v/F_m and NPQ changed little in DDTC-fed leaves accompanied by little change of (A+Z)/(V+A+Z). This is the active oxygen species inducing PS1 photoinhibition in sweet pepper. The water-water cycle can be interrupted easily at chilling temperature. We propose that during chilling stress under low irradiance, the xanthophyll cycle-dependent NPQ has the main function to protect PS2, whereas the water-water cycle is not only the pathway to dissipate energy but also the dominant factor causing PS1 chilling-sensitivity in sweet pepper.This research was supported by the State Key Basic Research and Development Plan of China (G1998010100), the Natural Science Foundation of China (30370854), and the open project from Key Lab of Crop Biology of Shandong Province.     

Calmodulin stimulation of unidirectional calcium uptake by the endoplasmic reticulum of barley aleurone

The role of calmodulin (CaM) in gibberellic acid (GA₃)-stimulated Ca²⁺ uptake was investigated in endomembranes isolated from aleurone cells of barley (Hordeum vulgare L.). Unidirectional Ca²⁺ -uptake activity of endoplasmic reticulum (ER) was higher in membranes isolated from aleurone layers treated for 16 h with GA₃ and Ca²⁺ compared with those isolated from layers incubated in Ca²⁺ alone. However, the level of uptake from Ca²⁺-treated tissue could be stimulated to that of the GA₃-treated cells by applying exogenous CaM which increased the V _max of the Ca²⁺ transporter approximately threefold. Calcium uptake in ER from GA₃-treated tissue was inhibited by the CaM antagonist W7 in 50% of experiments, whereas the activity in membranes from non-GA₃-treated tissue was unaffected. Treatment with GA₃ also led to a twofold increase in CaM levels in aleurone layers within 4–6 h, paralleling the time course of the stimulation of Ca²⁺ uptake and preceding the stimulation of α-amylase secretion. We propose that the elevation of Ca²⁺ uptake into the ER induced by GA₃ may be coordinated and regulated by elevated levels of membrane-associated CaM and this may regulate Ca²⁺-dependent α-amylase synthesis in the lumen of the ER.     

Constitutive accumulation of zeaxanthin in tomato alleviates salt stress-induced photoinhibition and photooxidation

Zeaxanthin (Z) has a role in the dissipation of excess excitation energy by participating in non‐photochemical quenching (NPQ) and is essential in protecting the chloroplast from photooxidative damage. To investigate the physiological effects and functional mechanism of constitutive accumulation of Z in the tomato at salt stress‐induced photoinhibition and photooxidation, antisense‐mediated suppression of zeaxanthin epoxidase transgenic plants and the wild‐type (WT) tomato were used. The ratio of Z/(V + A + Z) and (Z + 0.5A)/(V + A + Z) in antisense transgenic plants were maintained at a higher level than in WT plants under salt stress, but the value of NPQ in WT and transgenic plants was not significantly different under salt stress. However, the maximal photochemical efficiency of PSII (Fv/Fm) and the net photosynthetic rate (Pn) in transgenic plants decreased more slowly under salt stress. Furthermore, transgenic plants showed lower level of hydrogen peroxide (H₂O₂), superoxide anion radical (O₂^??) and ion leakage, lower malondialdehyde content. Compared with WT, the content of D1 protein decreased slightly in transgenic plants under salt stress. Our results suggested that the constitutive accumulation of Z in transgenic tomatoes can alleviate salt stress‐induced photoinhibition because of the antioxidant role of Z in the scavenging quenching of singlet oxygen and/or free radicals in the lipid phase of the membrane.     

Cross-talks between Ca<Superscript>2+</Superscript>/CaM and H<Subscript>2</Subscript>O<Subscript>2</Subscript> in abscisic acid-induced antioxidant defense in leaves of maize plants exposed to water stress

Here we examined whether Ca²⁺/Calmodulin (CaM) is involved in abscisic acid (ABA)-induced antioxidant defense and the possible relationship between CaM and H₂O₂ in ABA signaling in leaves of maize (Zea mays L.) plants exposed to water stress. An ABA-deficient mutant vp5 and its wild type were used for the experimentation. We found that water stress enhanced significantly the contents of CaM and H₂O₂, and the activities of chloroplastic and cytosolic superoxide dismutase (SOD), ascorbate peroxidase (APX) and glutathione reductase (GR), and the gene expressions of the CaM1, cAPX, GR1 and SOD4 in leaves of wild-type maize. However, the increases mentioned above were almost arrested in vp5 plants and in the wild-type plants pretreated with ABA biosynthesis inhibitor tungstate (T), suggesting that ABA is required for water stress-induced H₂O₂ production, the enhancement of CaM content and antioxidant defense. Besides, we showed that the up-regulation of water stress-induced antioxidant defense was almost completely blocked by pretreatment with Ca²⁺ inhibitors, CaM antagonists and reactive oxygen (ROS) manipulators. Moreover, the analysis of time course of CaM and H₂O₂ production under water stress showed that the increase in CaM content preceded that of H₂O₂. These results suggested that Ca²⁺/CaM and H₂O₂ were involved in the ABA-induced antioxidant defense under water stress, and the increases of Ca²⁺/CaM contents triggered H₂O₂ production, which inversely affected the contents of CaM. Thus, a cross-talk between Ca²⁺/CaM and H₂O₂ may play a pivotal role in the ABA signaling.     

Activity of the yeast vacuolar TRP channel TRPY1 is inhibited by Ca2+–calmodulin binding

Transient receptor potential (TRP) cation channels, which are conserved across mammals, flies, fish, sea squirts, worms, and fungi, essentially contribute to cellular Ca²⁺ signaling. The activity of the unique TRP channel in yeast, TRP yeast channel 1 (TRPY1), relies on the vacuolar and cytoplasmic Ca²⁺ concentration. However, the mechanism(s) of Ca²⁺-dependent regulation of TRPY1 and possible contribution(s) of Ca²⁺-binding proteins are yet not well understood. Our results demonstrate a Ca²⁺-dependent binding of yeast calmodulin (CaM) to TRPY1. TRPY1 activity was increased in the cmd1–6 yeast strain, carrying a non–Ca²⁺-binding CaM mutant, compared with the parent strain expressing wt CaM (Cmd1). Expression of Cmd1 in cmd1–6 yeast rescued the wt phenotype. In addition, in human embryonic kidney 293 cells, hypertonic shock-induced TRPY1-dependent Ca²⁺ influx and Ca²⁺ release were increased by the CaM antagonist ophiobolin A. We found that coexpression of mammalian CaM impeded the activity of TRPY1 by reinforcing effects of endogenous CaM. Finally, inhibition of TRPY1 by Ca²⁺–CaM required the cytoplasmic amino acid stretch E₃₃–Y₉₂. In summary, our results show that TRPY1 is under inhibitory control of Ca²⁺–CaM and that mammalian CaM can replace yeast CaM for this inhibition. These findings add TRPY1 to the innumerable cellular proteins, which include a variety of ion channels, that use CaM as a constitutive or dissociable Ca²⁺-sensing subunit, and contribute to a better understanding of the modulatory mechanisms of Ca²⁺–CaM.     

A strategy of Ca2+ alleviating Na+ toxicity in salt-treated Cyclocarya paliurus seedlings: photosynthetic and nutritional responses

Cyclocarya paliurus seedlings were subjected to 85?mM NaCl and 0, 6, 12 or 18?mM Ca(NO₃)₂ treatments to study changes in plant growth, photosynthetic parameters and distribution and/or accumulation of organic and inorganic solutes. Na⁺ toxicity symptoms were observed in plants non-treated with Ca(NO₃)₂, while 12?mM Ca(NO₃)₂ supplementation produced a significant promotion of shoot growth; meanwhile chlorophyll content, photosynthetic rate and optimum quantum yield of photosystem II (PSII), represented by the F_v/F_m ratio and pigments content as well as proline and soluble sugars, significantly increased. Ca(NO₃)₂ supply increased K⁺ and Ca²⁺ concentration, whereas the Na⁺ transport to the shoot was inhibited. There was a strong increase in the K⁺/Na⁺ ratio in shoot of Ca(NO₃)₂-treated plants. X-Ray microanalysis of roots showed that K⁺, Ca²⁺ and Na⁺ accumulated mainly in the epidermal cells and cortical cells of roots with 12?mM Ca(NO₃)₂ supply, and low accumulation was observed in stelar parenchyma, indicating exogenous Ca²⁺ possibly induced or strengthened effects of Casparian bands on ion transport. These results suggest that Ca(NO₃)₂ supplement increased inorganic and organic solutes accumulation in shoot and leaf, and restricted Na⁺ transport to the shoot by reinforcing barrier effects for attenuating salt injuries in plants, which could be a strategy of Ca²⁺ alleviating Na⁺ toxicity in C. paliurus seedlings subjected to salt stress.     

Interplay between Calmodulin and Phosphatidylinositol 4,5-Bisphosphate in Ca2+-induced Inactivation of Transient Receptor Potential Vanilloid 6 Channels

The epithelial Ca²⁺ channel transient receptor potential vanilloid 6 (TRPV6) undergoes Ca²⁺-induced inactivation that protects the cell from toxic Ca²⁺ overload and may also limit intestinal Ca²⁺ transport. To dissect the roles of individual signaling pathways in this phenomenon, we studied the effects of Ca²⁺, calmodulin (CaM), and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) in excised inside-out patches. The activity of TRPV6 strictly depended on the presence of PI(4,5)P₂, and Ca²⁺-CaM inhibited the channel at physiologically relevant concentrations. Ca²⁺ alone also inhibited TRPV6 at high concentrations (IC₅₀ = ∼20 μm). A double mutation in the distal C-terminal CaM-binding site of TRPV6 (W695A/R699E) essentially eliminated inhibition by CaM in excised patches. In whole cell patch clamp experiments, this mutation reduced but did not eliminate Ca²⁺-induced inactivation. Providing excess PI(4,5)P₂ reduced the inhibition by CaM in excised patches and in planar lipid bilayers, but PI(4,5)P₂ did not inhibit binding of CaM to the C terminus of the channel. Overall, our data show a complex interplay between CaM and PI(4,5)P₂ and show that Ca²⁺, CaM, and the depletion of PI(4,5)P₂ all contribute to inactivation of TRPV6.     

The role of Ca2+ ions in modulating changes induced in bean plants by an excess of Cu2+ ions. Chlorophyll fluorescence measurements

Ca²⁺-dependent influence of excess Cu²⁺ on the photosynthetic akpparatus monitored through chlorophyll fluorescence measurements was investigated in runner bean plants (Phaseolus coccineus L. cv. Pie kny Ja?) at three different growth stages. It was observed that the toxic effect of excess Cu²⁺ on plants depends both on their growth stages and the Ca²⁺ content in the medium. Increased Ca²⁺ content limits Cu²⁺ action on plants at their initial growth stage (I) through: stabilization of the PSII complex (increase of the ratio of variable to minimal fluorescence [F_v/F₀]), improved electron flow and reoxidative processes of the quinone primary electron acceptor of PSII (Q_A) (increase of quantum yield of PSII electron transport [φ_e] and photochemical quenching of fluorescence [q_P] values) and elimination of nonphotochemical energy dissipation (decrease of nonphotochemical fluorescence quenching from the Stern-Volmer equation [NPQ] and fraction of the absorbed light energy not used for photochemistry [LNU] values). At this growth stage excess Cu²⁺ decreases the rates of Q_A reduction as a result of decreased PSII activity at its donor side only at lower Ca²⁺ level. At the intermediate growth stage (II) the plants were less sensitive to Cu²⁺ treatment and also to changed Ca²⁺ content. A weakening of some photochemical processes by excess Cu²⁺ could be observed only at a higher Ca²⁺ dose. At the final growth stage of plants (III) Ca²⁺ ions exerted a decisively different effect on the mechanism of excess Cu²⁺ action on bean plants, visualized by decreased PSII stabilization and utilization of absorbed light energy at increased Ca²⁺ content in the medium.     

Photosystem II energy use,non-photochemical quenching and the xanthophyll cycle in Sorghumbicolor grown under drought and free-air CO2 enrichment(FACE) conditions

The present study was carried out to test the hypothesis thatelevated atmospheric CO₂ (Ca) will alleviate over‐excitationof the C₄ photosynthetic apparatus and decrease non‐photochemicalquenching (NPQ) during periods of limited water availability. Chlorophyll a fluorescencewas monitored in Sorghum bicolor plants grown under a free‐aircarbon‐dioxide enrichment (FACE) by water‐stress (Dry) experiment.Under Dry conditions elevated Ca increased the quantum yield ofphotosystem II (φPSII) throughout the day throughincreases in both photochemical quenching coefficient (q_p)and the efficiency with which absorbed quanta are transferred toopen PSII reaction centres (F_v′/F_m′).However, in the well‐watered plants (Wets) FACE enhanced φPSIIonly at midday and was entirely attributed to changes in F_v′/F_m′_. Underfield conditions, decreases in φPSII under Dry treatmentsand ambient Ca corresponded to increases in NPQ but the de‐epoxidation stateof the xanthophyll pool (DPS) showed no effects. Water‐stress didnot lead to long‐term damage to the photosynthetic apparatus asindicated by φPSII and carbon assimilation measuredafter removal of stress conditions. We conclude that elevated Caenhances photochemical light energy usage in C₄ photosynthesisduring drought and/or midday conditions. Additionally,NPQ protects against photo‐inhibition and photodamage. However,NPQ and the xanthophyll cycle were affected differently by elevatedCa and water‐stress.     

Antisense-mediated suppression of tomato zeaxanthin epoxidase alleviates photoinhibition of PSII and PSI during chilling stress under low irradiance

A tomato (Lycopersicon esculentum Mill.) zeaxanthin epoxidase gene (LeZE) was isolated and antisense transgenic tomato plants were produced. Northern, southern, and western blot analyses demonstrated that antisense LeZE was transferred into the tomato genome and the expression of LeZE was inhibited. The ratio of (A+Z)/(V+A+Z) in antisense transgenic plants was maintained at a higher level than in the wild type (WT) plants under high light and chilling stress with low irradiance. The value of non-photochemical quenching (NPQ) in WT and transgenic plants was not affected during the stresses. The oxidizable P700 and the maximal photochemical efficiency of PSII (F_v/F_m) in transgenic plants decreased more slowly at chilling temperature under low irradiance. These results suggested that suppression of LeZE caused zeaxanthin accumulation, which was helpful in alleviating photoinhibition of PSI and PSII in tomato plants under chilling stress.     

Analysis of ΔpH and the xanthophyll cycle in NPQ of the Antarctic sea ice alga Chlamydomonas sp. ICE-L

Non-photochemical fluorescence quenching (NPQ) is mainly associated with the transthylakoid proton gradient (ΔpH) and xanthophyll cycle. However, the exact mechanism of NPQ is different in different oxygenic photosynthetic organisms. In this study, several inhibitors were used to study NPQ kinetics in the sea ice alga Chlamydomonas sp. ICE-L and to determine the functions of ΔpH and the xanthophyll cycle in the NPQ process. NH₄Cl and nigericin, uncouplers of ΔpH, inhibited NPQ completely and zeaxanthin (Z) was not detected in 1 mM NH₄Cl-treated samples. Moreover, Z and NPQ were increased in the samples containing N,N’-dicyclohexyl-carbodiimide (DCCD) under low light conditions. We conclude that ΔpH plays a major role in NPQ, and activation of the xanthophyll cycle is related to ΔpH. In dithiothreitol (DTT)-treated samples, no Z was observed and NPQ decreased. NPQ was completely inhibited when NH₄Cl was added suggesting that part of the NPQ process is related to the xanthophyll cycle and the remainder depends on ΔpH. Moreover, lutein and β-carotene were also essential for NPQ. These results indicate that NPQ in the sea ice alga Chlamydomonas sp. ICE-L is mainly dependent on ΔpH which affects the protonation of PSII proteins and de-epoxidation of the xanthophyll cycle, and the transthylakoid proton gradient alone can induce NPQ.     

Evidence for the possible involvement of calmodulin in regulation of steady state levels of Hsp90 family members (Hsp87 and Hsp85) in response to heat shock in sorghum

Pharmacological studies, using Ca²⁺ channel blockers (LaCl₃ and verapamil) and calmodulin (CaM) antagonists (CPZ and W7), were carried out to understand the role of Ca²⁺/CaM in the regulation of heat shock-induced expression of Hsp90 (Hsp87 and Hsp85) and Hsp70 (Hsp75 and Hsp73) members in sorghum. It was observed that the expression of both Hsp87 and Hsp85 proteins was decreased in presence of Ca²⁺ channel blockers and CaM antagonists, under both control and heat stress conditions, as contrary to the steady state levels of Hsp75 and Hsp73, which were not affected significantly under similar conditions. Further, the exposure of sorghum seedlings to geldanamycin, a specific inhibitor of Hsp90, resulted in induction of Hsp87 and Hsp85 in the absence of heat shock also. This study provides the first evidence suggesting that in plants, the in vivo expression of Hsp90 (Hsp87 and Hsp85) is likely to be modulated by Ca²⁺/CaM under normal and thermal stress conditions. The likely implications of these findings are discussed.Key words: calmodulin, calmodulin-binding proteins, heat shock proteins, sorghum     

Ferredoxin-quinone reductase benefits cyclic electron flow around photosystem 1 in tobacco leaves upon exposure to chilling stress under low irradiance

The function of chloroplast ferredoxin quinone reductase (FQR)-dependent flow was examined by comparing a wild type tobacco and a tobacco transformant (ΔndhB) in which the ndhB gene had been disrupted with their antimycin A (AA)-fed leaves upon exposure to chilling temperature (4 °C) under low irradiance (100 μmol m⁻² s⁻¹ photon flux density). During the chilling stress, the maximum photochemical efficiency of photosystem (PS) 2 (F_v/F_m) decreased markedly in both the controls and AA-fed leaves, and P700⁺ was also lower in AA-fed leaves than in the controls, implying that FQR-dependent cyclic electron flow around PS1 functioned to protect the photosynthetic apparatus from chilling stress under low irradiance. Under such stress, non-photochemical quenching (NPQ), particularly the fast relaxing NPQ component (q_f) and the de-epoxidized ratio of the xanthophyll cycle pigments, (A+Z)/(V+A+Z), formed the difference between AA-fed leaves and controls. The lower NPQ in AA-fed leaves might be related to an inefficient proton gradient across thylakoid membranes (ΔpH) because of inhibiting an FQR-dependent cyclic electron flow around PS1 at chilling temperature under low irradiance.     

Photoinhibition and xanthophyll cycle activity in bayberry (Myrica rubra) leaves induced by high irradiance

The effect of high irradiance (HI, photosynthetically active photon flux density of 1 300 μmol m⁻² s⁻¹) on net photosynthetic rate (P _N), chlorophyll fluorescence parameters, and xanthophyll cycle components were studied in fruit tree bayberry leaves. HI induced the photoinhibition and inactivation of photosystem 2 (PS2) reaction centres (RCs), which was characterized by decreased P _N, maximum yield of fluorescence after dark adaptation (F_m), photochemical efficiency of PS2 (F_v/F_m) and quantum yield of PS2 (Φ_PS2), and increased reduction state of Q_A (1-q_P) and non-photochemical quenching (NPQ). Initial fluorescence (F₀) showed a decrease after the first 2 h, and subsequently increased from the third hour exposure to HI. Furthermore, a greater increase in the ratio (F_i-F₀)/(F_p-F₀) which is an expression of the proportion of the Q_B non-reducing PS2 centres, whereas a remarked decrease in the slope of F_i to F_p which represents the rate of Q_A reduction was observed in leaves after HI exposure. Additionally, HI caused an increase in the pool size of the xanthophyll cycle pigments and sustained elevated contents of zeaxanthin (Z), antheraxanthin (A), and de-epoxidation state (DES) at the end of the irradiation period. During HI, decreased F_m, F_v/F_m, Φ_PS2, NPQ, slope of F_i to F_p, V+A+Z, and DES, and increased F₀, 1-q_P, ratio (F_i-F₀)/(F_p-F₀), and V were observed in dithiothreitol (DTT)-fed leaves compared to control ones under the same conditions. Hence photoinhibition caused by HI in bayberry was probably attributed to inactivation of PS2 RCs, and photoprotection from photodamage were mainly related to the xanthophyll cycle-dependent heat dissipation in excess photons.     

Effects of paraquat-induced oxidative stress on the neuronal plasma membrane Ca-ATPase

Oxidative stress leads to the disruption of calcium homeostasis in brain neurons; however, the direct effects of oxidants on proteins that regulate intracellular calcium ([Ca²⁺]_i) are not known. The calmodulin (CaM)-stimulated plasma membrane Ca²⁺-ATPase (PMCA) plays a critical role in regulating [Ca²⁺]_i. Our previous in vitro studies showed that PMCA present in brain synaptic membranes is readily inactivated by a variety of reactive oxygen species (ROS). The present studies were conducted to determine the vulnerability of PMCA to ROS generated in neurons as would probably occur in vivo. Primary cortical neurons were exposed to paraquat (PQ), a redox cycling agent that generates intracellular ROS. Low concentrations of PQ (5–10 μM) increased PMCA basal activity by two-fold but abolished its sensitivity to CaM. Higher concentrations (25–100 μM) inhibited both components of PMCA activity. Immunoblots showed the formation of high-molecular-weight PMCA aggregates. Additionally, PMCA showed evidence of proteolytic degradation. PMCA proteolysis was prevented by a calpain inhibitor, suggesting a role for calpain. Our findings suggest that PMCA is a sensitive target of oxidative stress in primary neurons. Inactivation of this Ca²⁺ transporter under prolonged oxidative stress could alter neuronal Ca²⁺ signaling.     

Exogenous Calcium Alleviates Low Night Temperature Stress on the Photosynthetic Apparatus of Tomato Leaves

The effect of exogenous CaCl₂ on photosystem I and II (PSI and PSII) activities, cyclic electron flow (CEF), and proton motive force of tomato leaves under low night temperature (LNT) was investigated. LNT stress decreased the net photosynthetic rate (Pn), effective quantum yield of PSII [Y(II)], and photochemical quenching (qP), whereas CaCl₂ pretreatment improved Pn, Y(II), and qP under LNT stress. LNT stress significantly increased the non-regulatory quantum yield of energy dissipation [Y(NO)], whereas CaCl₂ alleviated this increase. Exogenous Ca²⁺ enhanced stimulation of CEF by LNT stress. Inhibition of oxidized PQ pools caused by LNT stress was alleviated by CaCl₂ pretreatment. LNT stress reduced zeaxanthin formation and ATPase activity, but CaCl₂ pretreatment reversed both of these effects. LNT stress caused excess formation of a proton gradient across the thylakoid membrane, whereas CaCl₂ pretreatment decreased the said factor under LNT. Thus, our results showed that photoinhibition of LNT-stressed plants could be alleviated by CaCl₂ pretreatment. Our findings further revealed that this alleviation was mediated in part by improvements in carbon fixation capacity, PQ pools, linear and cyclic electron transports, xanthophyll cycles, and ATPase activity.