Increased susceptibility to photoinhibition in pre-existing needles experiencing low temperature at spring budbreak in Sakhalin spruce (Picea glehnii) seedlings

The seasonal changes in photosynthetic properties in 1-year-old needles of Sakhalin spruce ( Picea glehnii ) were measured using the chlorophyll fluorescence technique at various temperatures (5, 10, 20, 25 and 30°C). In the course of seasonal change, a temporary decrease in the quantum yield of PSII electron transport (Φ_PSII) was observed just before budbreak. A decline in photochemical quenching ( q _P) was observed at the same time as that of Φ_PSII but only at the two lowest temperatures (5 and 10°C). Photochemical efficiency of open PSII ( F _v'/ F _m') also declined just before budbreak at 25 and 30°C. An increase in thermal energy dissipation as indicated by a decrease in F _v'/ F _m' before budbreak was not significant at lower temperatures (5 and 10°C) in spite of the declines in q _P. This implies that thermal energy dissipation necessitated by the decline in Φ_PSII might not be sufficiently strong to prevent a decline in q _P at lower temperatures. On the other hand, at higher temperatures no decline was observed in q _P because Φ_PSII decreased to a relatively small extent, therefore thermal energy dissipation is sufficient in coping with the excessive energy accumulation in PSII. Seedlings of Sakhalin spruce exposed to ambient air temperature below 10°C before budbreak exhibited photoinhibition indicated by a decrease in the maximal photochemical efficiency of PSII ( F _v/ F _m) after an overnight dark adaptation. The present study suggests that 1-year-old shoots of Sakhalin spruce have an increased susceptibility to photoinhibition at low temperature just before budbreak.     

Photosynthesis is improved by exogenous glycinebetaine in salt-stressed maize plants

The effects of exogenous application of glycinebetaine (GB) (10 m M ) on growth, leaf water content, water use efficiency, photosynthetic gas exchange, and photosystem II photochemistry were investigated in maize plants subjected to salt stress (50 and 100 m M NaCl). Salt stress resulted in the decrease in growth and leaf relative water content as well as net photosynthesis and the apparent quantum yield of photosynthesis. Stomatal conductance, evaporation rate, and water use efficiency were decreased in salt-stressed plants. Salt stress also caused a decrease in the actual efficiency of PSII ( Φ _PSII), the efficiency of excitation energy capture by open PSII reaction centers ( F _v'/ F _m'), and the coefficients of photochemical quenching ( q _P) but caused an increase in non-photochemical quenching (NPQ). Salt stress showed no effects on the maximal efficiency of PSII photochemistry ( F _v/ F _m). On the other hand, in salt-stressed plants, GB application improved growth, leaf water content, net photosynthesis, and the apparent quantum yield of photosynthesis. GB application also increased stomatal conductance, leaf evaporation rate, and water use efficiency. In addition, GB application increased Φ _PSII, F _v'/ F _m', and q _P but decreased NPQ. However, GB application showed no effects on F _v/ F _m. These results suggest that photosynthesis was improved by GB application in salt-stressed plants and such an improvement was associated with an improvement in stomatal conductance and the actual PSII efficiency.     

Photosystem II inhibition by moderate light under low temperature in intact leaves of chilling-sensitive and -tolerant plants

Photosystem II (PSII) activity was examsined in leaves of chilling-sensitive cucumber ( Cucumis sativus L.), tomato ( Lycopersicum esculentum L.), and maize ( Zea mays L.), and in chilling-tolerant barley ( Hordeum vulgare L.) illuminated with moderate white light (300 µmol m⁻² s⁻¹) at 4°C using chlorophyll a fluorescence measurements. PSII activity was inhibited in leaves of all the four plants as suggested by the decline in F _v/ F _m, 1/ F _o − 1/ F _m, and F _v/ F _o values. The changes in initial fluorescence level ( F _o), F _v/ F _m, 1/ F _o − /1/ F _m, and F _v/ F _o ratios indicate a stronger PSII inhibition in cucumber, maize and tomato plants. The kinetics of chlorophyll a fluorescence rise showed complex changes in the magnitudes and rise of O-J, J-I, and I-P phases caused by photoinhibition. The selective suppression of the J-I phase of fluorescence rise kinetics provides evidence for weakened electron donation from the oxidizing side, whereas the accumulation of reduced Q_A suggests damage to the acceptor side of PSII. These findings imply that the process of chilling-induced photoinhibition involves damage to more than one site in the PSII complexes. Furthermore, comparative analyses of the decline in F _v/ F _o and photooxidation of P700 explicitly show that the extent of photoinhibitory damage to PSII and photosystem I is similar in leaves of cucumber plants grown at a low irradiance level.     

Oxygen evolution and respiration of the cyanobacterium Synechocystis sp. PCC 6803 under two different light regimes applying light/dark intervals in the time scale of minutes

The photosynthetic performance of the cyanobacterium Synechocystis sp. PCC 6803 exposed to intermittent light was studied by measuring oxygen evolution, respiration and the fluorescence parameters for maximum efficiency of excitation energy capture by photosystem II (PSII) reaction centres ( F _v/ F _m), PSII quantum yield (ΔF/ F _m ¹) and non-photochemical quenching (NPQ). Cultures were pre-acclimated to constant light conditions. Block and sinusoidal light regimes were tested using four photon-flux densities (PFDs) applied in light/dark intervals of 1:1, 5:5 and 10:10 min. Light use was higher under the sinusoidal light regime compared with the block regime. The accumulated gross photosynthesis of the cyanobacterium was lower under intermittent light conditions compared with predictions from the photosynthesis-irradiance curve (PI curve). The respiration rates were similar for all light/dark intervals tested. However, the respiration slightly increased with increasing oxygen production for both block and sinusoidal light regime. NPQ, ΔF/ F _m' and F _v/ F _m depended on the PFD rather than on the duration of the light/dark intervals tested, and there was no detected influence of the two applied light regimes.     

The effect of soil temperature on the bud phenology, chlorophyll fluorescence, carbohydrate content and cold hardiness of Norway spruce seedlings

The effects of soil temperature on the shoot phenology, carbohydrate dynamics, chlorophyll fluorescence and cold hardiness of 4-year-old Norway spruce seedlings ( Picea abies L. Karst.) were studied. The experiment was carried out under controlled conditions in the Joensuu dasotrons. Air conditions were similar but soil temperatures differed by treatments (9, 13, 18 and 21°C) during the second growing period in the dasotrons. The after-effects of the treatments were investigated during the third growing period following the treatments. Low soil temperature increased the starch content of needles and delayed the loss of starch at the end of the growing season. The photochemical efficiency ( F _v/ F _m) of the PSII of the current-year needles was reduced at the lowest soil temperature. The cold hardiness of needles correlated with the soluble sugar content. The differences in soil temperature had no effect on the timing of bud burst. No after-effects from the treatments were observed during the third growing period in the dasotrons.     

Ecophysiological analysis of two arctic sedges under reduced root temperatures

Shoot physiological activity in arctic vascular plants may be controlled by low soil temperatures. While leaves may be exposed to moderate temperatures during the growing season, root temperatures often remain near freezing. In this study, two tundra sedges, Eriophorum vaginatum and Carex bigellowii , were subjected to reduced soil temperatures, and photosynthetic parameters (light saturated photosynthesis A _max, variable to maximal fluorescence and F _v/ F _m stomatal conductance) and abscisic acid concentrations were determined. Stomatal conductance and A _max for both E. vaginatum and C. bigellowii strongly decreased with declining soil temperatures. Decreasing soil temperature, however, impacted F _v/ F _m to a much lesser degree. Root and leaf ABA concentrations increased with decreasing root temperature. These observations support the contention that soil temperature is a significant photosynthetic driving factor in arctic sedges exposed to variable root and shoot temperatures. Because these two species comprise approximately 30% of the vascular ground cover of wet tussock tundra, the soil temperature responses of these sedges potentially scale up to significant effects on ecosystem carbon exchange.     

Growth and photosynthesis of virus-infected and virus-eradicated orchid plants exposed to different growth irradiances under natural tropical conditions

Responses of virus-infected (VI) and virus-eradicated (VE) Oncidium Gower Ramsey orchid plants grown under 30% of prevailing solar radiation and those transferred from 30 to 60% and 100% of prevailing solar radiation were studied under natural tropical conditions. Plants grown under 30% of prevailing solar radiation suffered lower leaf and floral production and reduced photosynthesis. When the irradiance was increased to 60% of prevailing solar radiation, enhancement of leaf and floral production and photosynthetic capacities were achieved. However, when the plants were transferred from 30 to 100% of prevailing solar radiation, the growth and photosynthetic capacities of the plants were significantly reduced. All plants exhibited a midday depression in photosynthetic CO₂ assimilation ( A ), stomatal conductance ( g _s) and F _v/ F _m ratio . The degree of midday depression of these parameters was not only associated with high temperatures and high irradiances but also with virus infection. Midday F _v/ F _m ratio depression indicated that dynamic photo-inhibition occurred in all plants grown under all three light conditions. However, chronic photo-inhibition, measured by pre-dawn F _v/ F _m ratio and chlorophyll content, occurred only in those plants transferred from 30 to 100% of prevailing solar radiation. Hence, it is concluded that the VI Oncidium Gower Ramsey was more susceptible to high irradiance than the VE plants.     

High-Resolution Chlorophyll Fluorescence Imaging Serves as a Non-Invasive Indicator to Monitor the Spatio-Temporal Variations of Metabolism during the Day-Night Cycle and during the Endogenous Rhythm in Continuous Light in the CAM Plant Kalanchoë daigremontiana

Abstract: Chlorophyll fluorescence imaging is a powerful tool to monitor temporal and spatial dynamics of photosynthesis and photosynthesis-related metabolism. In this communication, we use high resolution chlorophyll fluorescence imaging techniques under strictly controlled conditions to quantify day courses of relative effective quantum yield (φ_PSII) of an entire leaf of the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana at different light intensities. Careful interpretation of the combined gas exchange and fluorescence data, in combination with micro malate samples, allow the interpretation of underlying metabolic properties, such as leaf internal CO₂ concentration (ci_CO2) and energy demand of the cells. Spatial variations of φ_PSII, which occur as running wave fronts at the transition from phase III to phase IV of CAM, may reflect spatial differences of ci_CO2, which are preserved in the tightly packed mesophyll cells of K. daigremontiana. An endogenous rhythm is driven by a master switch which mediates between malate storage and malate release to and from the vacuole, however, using fluorescence techniques, four different metabolic states can be distinguished which also account for the activity of phosphoenolpyruvate carboxylase.     

Changes in in vivo chlorophyll fluorescence quenching in lichen thalli as a function of water content and suggestion of zeaxanthin-associated photoprotection

The function of photosystem II (PSII) during desiccation was investigated via analysis of Chl a fluorescence emission in thalli from Parmelia quercina (Willd.) Vainio, Parmelia acetabulum (Necker) Duby, Ramalina farinacea (L.) Ach., Pseudevernia furfuracea (L.) Zopf., and Evernia prunastri (L.) Ach. Water loss followed the same exponential pattern in all these species, the half time being dependent on species. Desiccation affected the fluorescence parameters. Dark-adapted maximum fluorescence (F_m), instantaneous fluorescence (F_o) and the ratio of variable (F_m–F_o) to F_m were dependent on water content and decreased in two distinct phases: a slow and apparently linear phase, followed by a more steep decline at low water content. Actual PSII photochemical yield (φ_PSII), non-photochemical quenching (NPQ), efficiency of photon capture (φ_exc), and photochemical quenching (q_p) remained nearly constant until 30% relative water content (RWC), decreasing rapidly thereafter. In contrast, increased NPQ appeared to occur only at water content values lower than 20%. Treatment of thalli with dithiothreitol (DTT) effectively reduced NPQ during desiccation and increased susceptibility to photoinhibition caused by exposure to high light as measured by dark recovery of the F_vF_m ratio. HPLC analysis showed that the level of the de-epoxidized xanthophyll cycle pigments antheraxanthin (Anth) and zeaxanthin (Zea) increased during lichen desiccation. The results point towards the existence of a photoprotective mechanism with the involvement of Zea and Anth in non-radiative dissipation of the desiccation-induced excess of energy.     

Rootstock-mediated changes in xylem ionic and hormonal status are correlated with delayed leaf senescence, and increased leaf area and crop productivity in salinized tomato

Tomato crop productivity under salinity can be improved by grafting cultivars onto salt-tolerant wild relatives, thus mediating the supply of root-derived ionic and hormonal factors that regulate leaf area and senescence. A tomato cultivar was grafted onto rootstocks from a population of recombinant inbred lines (RILs) derived from a Solanum lycopersicum  ×  Solanum cheesmaniae cross and cultivated under moderate salinity (75 m m NaCl). Concentrations of Na⁺, K⁺ and several phytohormones [abscisic acid (ABA); the cytokinins (CKs) zeatin, Z; zeatin riboside, ZR; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC)] were analysed in leaf xylem sap in graft combinations of contrasting vigour. Scion leaf area correlated with photosystem II (PSII) efficiency ( F _v/ F _m) and determined fruit productivity. Xylem K⁺ (but not Na⁺), K⁺/Na⁺, the active CK Z, the ratio with its storage form Z/ZR and especially the ratio between CKs and ACC (Z/ACC and Z + ZR/ACC) were positively loaded into the first principal component (PC) determining both leaf growth and PSII efficiency. In contrast, the ratio ACC/ABA was negatively correlated with leaf biomass. Although the underlying physiological mechanisms by which rootstocks mediate leaf area or chlorophyll fluorescence (and thus influence tomato salt tolerance) seem complex, a putative potassium–CK interaction involved in regulating both processes merits further attention.     

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.     

Persistence of photosynthetic components and photochemical efficiency in ears of water-stressed wheat (Triticum aestivum)

Ear photosynthesis may be an important source of C for grain growth in water-stressed plants of cereals. The main objectives of this work were to determine the stability of the photosynthetic apparatus and the photochemical efficiency of ears in plants subjected to post-anthesis drought. Plants of wheat ( Triticum aestivum L. cv. Granero INTA) were grown in pots under a rain shelter and subjected to water stress (soil water potential around −0.6 to −0.8 MPa) starting 4  days after anthesis. Post-anthesis drought substantially accelerated the loss of chlorophyll, Rubisco and the light-harvesting complex of photosystem II (LHCII) in the flag leaf, but the degradation of these photosynthetic components was much less affected by water deficit in awns and ear bracts. Quantum yield of PSII (Φ_PSII) decreased in leaves of water-stressed plants. In contrast, ear bracts had a higher Φ_PSII than leaves, and Φ_PSII of ear bracts did not decrease at all in response to drought. Removing the grains immediately before fluorescence measurements (less than 30 min) slightly reduced Φ_PSII, indicating that CO₂ supplied by grain respiration may contribute to the high photochemical efficiency of ears in droughted plants. However, other factors may be involved in maintaining high Φ_PSII, since even in the absence of grains Φ_PSII remained much higher in ear bracts than in the flag leaf. The relative stability of ear photosynthetic components and their relatively high photochemical efficiency may help to maintain ear photosynthesis during the grain filling period in droughted plants.     

ON-LINE MONITORING OF CHLOROPHYLL FLUORESCENCE TO ASSESS THE EXTENT OF PHOTOINHIBITION OF PHOTOSYNTHESIS INDUCED BY HIGH OXYGEN CONCENTRATION AND LOW TEMPERATURE AND ITS EFFECT ON THE PRODUCTIVITY OF OUTDOOR CULTURES OF SPIRULINA PLATENSIS (CYANOBACTERIA)

The saturating pulse fluorescence technique was applied to study photoinhibition of photosynthesis in outdoor cultures of the cyanobacterium Spirulina platensis (Nordstedt) Geitler strain M2 grown under high oxygen and low temperature stress. Diurnal changes in maximum photochemical yield (F_v/F_m), photon yield of PSII (ΔF/F 'm), and nonphotochemical quenching (qN) were measured using a portable, pulse-amplitude–modulated fluorometer. When solar irradiance reached the maximum value, the F _v/F_m and ΔF/F 'm ratios of the Spirulina cultures grown under high oxygen stress decreased by 35% and 60%, respectively, as compared with morning values. The depression of the F_v/F_m and ΔF/F 'm ratios reached 55% and 84%, respectively, when high oxygen stress was combined with low temperature (i.e. 10° C below the optimal value for growth). Photoinhibition reduced the daily productivity of the culture grown under high oxygen stress by 33% and that of the culture grown under high oxygen–low temperature stress by 60%. Changes in the biomass yield of the cultures correlated well with changes in the daily integrated value of the estimated electron transport rate through the PSII (ΔF/F 'm × photon flux density). The results indicate that on-line chlorophyll fluorescence measurement is a powerful tool for assessing the photosynthetic performance of outdoor Spirulina cultures.     

Photoinhibition in tropical forest understorey species with short- and long-lived leaves

1. Shade-tolerant species that inhabit the understorey have a range of leaf lifetimes (from 1 to 8 years), which may indicate a variety of strategies for dealing with increases in light associated with tree-fall gaps. We hypothesized that species with long-lived leaves should be more tolerant of an increase in light levels than species with short-lived leaves.
2. In understorey plants of 12 shade-tolerant rain-forest species, photoinhibition, measured as a reduction in the chlorophyll fluorescence parameter F _v/ F _m when leaf discs were exposed to 1h at 1000μmol m^–2s^–1, was greater in species with short-lived leaves than species with long-lived leaves.
3. Less photoinhibition in species with long-lived leaves was not associated with higher levels of non-photochemical dissipation (NPQ) of absorbed light, but may be the result of a higher yield of photosystem II compared with short-lived leaves.
4. Thus, species with long-lived leaves are more tolerant of abrupt increases in light that occur when tree-fall gaps are formed than species with short-lived leaves.
5. Discs from leaves of all species growing in tree-fall gaps had higher levels of NPQ, yield of photosystem II and more rapid recovery from photoinhibition than leaves developed in the understorey; however, there were no differences among species with short- and long-lived leaves.     

Effects of Phospholipases on the Kinetic Properties of Rat Striatal Membrane-Bound Tyrosine Hydroxylase

Abstract: Rat striatal tyrosine hydroxylase can be isolated in both a soluble and a synaptic membrane-bound form. The membrane-bound enzyme, which exhibits lower K _ms for both tyrosine (7 μ M ) and reduced pterin cofactor (110 μ M ) relative to the soluble enzyme (47 μ M and 940 μ M , respectively), can be released from the membrane fraction with mild detergent, and concomitantly its kinetic properties revert to those of the soluble enzyme. Treatment of membrane-bound tyrosine hydroxylase with C. perfringens phospholipase C increased the K _m of the enzyme for tyrosine to 27 μ M and the V _max by 60% without changing the K _m for cofactor. In contrast, treatment of membrane-bound tyrosine hydroxylase with V. russelli phospholipase A₂ increased the K _m for tyrosine to 48 μ M increased the V _max and increased the K _m for cofactor to 560 μ M . The enzyme remained bound to the membrane fraction following both phospholipase treatments. Addition of phospholipids to treated enzyme could partially reverse the effects of phospholipase A₂ treatment, but not the effects of phospholipase C treatment. The kinetic properties of phospholipase-treated, detergent-solubilized tyrosine hydroxylase were identical to those of the control solubilized enzyme. Tyrosine hydroxylase appears to interact with synaptic membrane components to produce at least two separately determined consequences for the kinetic properties of the enzyme.     

Aldehyde Dehydrogenases in Rat Brain. Subcellular Distribution and Properties

Abstract: Kinetic studies suggested the presence of several forms of NAD-dependent aldehyde dehydrogenase (ALDH) in rat brain. A subcellular distribution study showed that low- and high- K _m activities with acetaldehyde as well as the substrate-specific enzyme succinate semialdehyde dehydrogenase were located mainly in the mitochondrial compartment. The low- K _m activity was also present in the cytosol (<20%). The low- K _m activity in the homogenate was only 10–15% of the total activity with acetaldehyde as the substrate. Two K _m values were obtained with both acetaldehyde (0.2 and 2000 μ m ) and 3,4-dihydroxyphenylacetaldehyde (DOPAL) (0.3 and 31 μ m ), and one K _m value with succinate semialdehyde (5 μ m ). The main part of the aldehyde dehydrogenase activities with acetaldehyde, DOPAL, and succinate semialdehyde, but only little activity of the marker enzyme for the outer membrane (monoamine oxidase, MAO), was released from a purified mitochondrial fraction subjected to sonication. Only small amounts of the ALDH activities were released from mitochondria subjected to swelling in a hypotonic buffer, whereas the main part of the marker enzyme for the intermembrane space (adenylate kinase) was released. These results indicate that the ALDH activities with acetaldehyde, DOPAL and succinate semialdehyde are located in the matrix compartment. The low- K _m activity with acetaldehyde and DOPAL, but not the high- K _m activities and succinate semialdehyde dehydrogenase, was markedly stimulated by Mg²⁺ and Ca²⁺ in phosphate buffer. The low- and high- K _m activities with acetaldehyde showed different pH optima in pyrophosphate buffer.     

Long-term plum pox virus infection produces an oxidative stress in a susceptible apricot, Prunus armeniaca, cultivar but not in a resistant cultivar

The effect of plum pox virus (PPV) infection on the response of some antioxidant enzymes was studied in two apricot cultivars, which behaved differently against PPV infection: cultivar Real Fino (susceptible) and cultivar Stark Early Orange (cv. SEO, resistant). In the susceptible cultivar, PPV produced a decrease in Φ _PSII, F '_v/ F '_m and Q _p. PPV infection produced a drop in p -hydroxy mercury benzoic acid (pHMB)-sensitive ascorbate peroxidase, dehydroascorbate reductase and peroxidase in the soluble fraction from susceptible plants, whereas in the resistant apricot cultivar, pHMB-insensitive ascorbate peroxidase, monodehydroascorbate reductase, glutathione reductase and superoxide dismutase increased. However, catalase decreased in the soluble fractions from both infected cultivars. Long-term PPV infection also produced a decrease in the chloroplastic ascorbate–glutathione cycle enzymes only in the susceptible plants. As a consequence of PPV infection, an oxidative stress, indicated by an increase in lipid peroxidation and in protein oxidation, was produced only in the leaves from the susceptible cultivar which was also monitored by the diaminobenzidine peroxidase-coupled H₂O₂ probe. The loss of Φ _PSII, indicative of activated oxygen species production, and the decrease in the levels of antioxidant enzymes in chloroplasts from susceptible plants could be responsible for the chlorosis symptoms observed. The results suggest that the higher antioxidant capacity showed by cv. SEO could be a consequence of a systemic acquired resistance induced by PPV penetration in stem tissue at the graft site and could be related, among other factors, to their resistance to PPV.     

Midday depression of photosynthesis and effects of mist spray in citrus

Diurnal variations of gas exchange, chlorophyll a fluorescence and some related biochemical characteristics in sun-acclimated mature citrus leaves of mist-sprayed (treatment) and unsprayed (control) trees were compared on sunny days during summer to identify the environmental and physiological factors limiting carbon gain in citrus tree canopies. At midday, net photosynthesis and maximal photochemical efficiency of photosystem II ( F _v/ F _m) in citrus leaves decreased significantly under control conditions, but the decrease was mitigated by mist spraying. Although the content of malondialdehyde, hydrogen peroxide and activities of antioxidant enzymes increased at midday in both mist-sprayed and control leaves, they were much higher in control leaves than in mist-sprayed leaves. The level of D1 protein decreased significantly in control leaves at midday and then was partly recovered later, while that in treated leaves changed to a much lesser extent because of alleviation of photoinhibition by mist spraying. Both the fast and the slow phases of millisecond-delayed light emissions in treated citrus leaves were higher than those in control leaves, indicating that mist spraying protects the normal operation of the photosynthetic apparatus in leaves. Mist spraying also reduced leaf temperatures and the ratio of air to leaf vapour pressure deficit (ALVPD), leading to increases in stomatal conductance ( g _s) and alleviation of photoinhibition at midday. It is concluded that the decline of leaf g _s under high-ALVPD conditions in summer is an important factor contributing to midday depression of photosynthesis in citrus, and mist spraying is effective in alleviating midday depression of photosynthesis in citrus leaves.     

Alleviation of photoinhibition by calcium supplement in salt-treated Rumex leaves

By analysis of gas exchange and chlorophyll fluorescence, the effects of NaCl treatment and supplemental CaCl₂ on photosynthesis, photosystem II (PSII) photochemistry and photoinhibition were investigated in Rumex leaves. Photosynthesis in Rumex leaves was strongly inhibited by 200 m M NaCl treatment. Such inhibition of photosynthesis was ameliorated by CaCl₂ supplement. Neither NaCl treatment nor CaCl₂ supplement had any significant effects on the PSII primary photochemical reaction in dark-adapted leaves. In light-adapted leaves, however, 200 m M NaCl treatment significantly decreased photochemical quenching (q_p), efficiency of excitation energy capture by open PSII reaction centers (F_V'/F_M') and quantum yield of PSII electron transport (Φ_PSII). These decreases in q_p, F_V'/F_M' and Φ_PSII were mitigated by CaCl₂ supplement with the maximum of its effect appearing at a concentration of 8 m M CaCl₂. A similar mitigating effect was shown in 200 m M NaCl-treated Rumex leaves when susceptibility of PSII to photoinhibition was determined under high irradiance. It is suggested that the mitigation of photoinhibition in NaCl-treated leaves is because of the amelioration of inhibition of photosynthesis.