Lanthanum improves salt tolerance of maize seedlings

In this study, the effects of lanthanum were investigated on contents of pigments, chlorophyll (Chl) fluorescence, antioxidative enzymes, and biomass of maize seedlings under salt stress. The results showed that salt stress significantly decreased the contents of Chl and carotenoids, maximum photochemical efficiency of PSII (F_v/F_m), photochemical quenching (q_P), and quantum efficiency of PSII photochemistry (Φ_PSII), net photosynthetic rate (P_N), and biomass. Salt stress increased nonphotochemical quenching (q_N), the activities of ascorbate peroxidase, catalase, superoxide dismutase, glutathione peroxidase, and the contents of malondialdehyde and hydrogen peroxide compared with control. Pretreatment with lanthanum prior to salt stress significantly enhanced the contents of Chl and carotenoids, F_v/F_m, q_P, q_N, Φ_PSII, P_N, biomass, and activities of the above antioxidant enzymes compared with the salt-stressed plants. Pretreatment with lanthanum also significantly reduced the contents of malondialdehyde and hydrogen peroxide induced by salt stress. Our results suggested that lanthanum can improve salt tolerance of maize seedlings by enhancing the function of photosynthetic apparatus and antioxidant capacity.     

Simultaneous gas exchange and fluorescence measurements indicate differences in the response of sunflower,bean and maize to water stress

Gas exchange and fluorescence measurements of attached leaves of water stressed bean, sunflower and maize plants were carried out at two light intensities (250 mol quanta m^-2s^-1 and 850 mol quanta m^-2s^-1). Besides the restriction of transpiration and CO₂ uptake, the dissipation of excess light energy was clearly reflected in the light and dark reactions of photosynthesis under stress conditions. Bean and maize plants preferentially use non-photochemical quenching for light energy dissipation. In sunflower plants, excess light energy gave rise to photochemical quenching. Autoradiography of leaves after photosynthesis in ¹⁴CO₂ demonstrated the occurrence of leaf patchiness in sunflower and maize but not in bean. The contribution of CO₂ recycling within the leaves to energy dissipation was investigated by studies in 2.5% oxygen to suppress photorespiration. The participation of different energy dissipating mechanisms to quanta comsumption on agriculturally relevant species is discussed.Abbreviations F_o minimal fluorescence - F_m maximal fluorescence - F_p peak fluorescence - g leaf conductance - P_N net CO₂ uptake - q_N coefficient of non-photochemical quenching - q_P coefficient of photochemical quenching     

Effects of zinc and soil moisture on photosynthetic rate and chlorophyll fluorescence parameters of maize

Effects of zinc [0 and 5.0 mg Zn kg⁻¹ (soil)] on photosynthetic rate (P_N), and chlorophyll fluorescence in leaves of maize (Zea mays L.) cv. Zhongdan 9409 seedlings grown under different soil moisture regimes (40–45 % and 70–75 % of soil saturated water content) were studied. Zn application did not enhance maize plant adaptation to drought stress. The relative water content and the water potential of leaves were not affected by Zn treatment. Moreover, The P_N of drought-stressed plants was not improved by Zn supply. The increases of plant biomass, stomatal conductance and quantum yield of photosystem 2 due to Zn addition were notable in well-watered plants.     

低温胁迫下丛枝菌根真菌对玉米光合特性的影响

利用盆栽试验,在15 ℃和5 ℃低温胁迫下研究了丛枝菌根（AM）真菌对玉米生长、叶绿素含量、叶绿素荧光和光合作用的影响.结果表明：低温胁迫抑制了AM真菌的侵染;接种AM真菌的玉米地上部和地下部干物质量、相对叶绿素含量高于不接种植株.与非菌根玉米相比,菌根玉米具有较高的最大荧光（F_m）、可变荧光（F_v）、最大光化学效率（F_v/F_m）和潜在光化学效率（F_v/F_o）及较低的初始荧光（F_o）,并且在5 ℃处理中差异显著.接种AM真菌使玉米叶片的净光合速率（P_n）和蒸腾速率（T_r）显著增强;低温胁迫下,菌根植株的气孔导度（G_s）显著高于非菌根植株;而胞间CO₂浓度（C_i）显著低于非菌根植株.表明AM真菌可通过提高叶绿素含量及改善叶片叶绿素荧光和光合作用来减轻低温胁迫对玉米植株造成的伤害,提高玉米耐受低温的能力,进而提高玉米的生物量,促进玉米生长.     

Effects of altered phosphoenolpyruvate carboxylase activities on transgenic C3 plant Solanum tuberosum

Phosphoenolpyruvate carboxylase (PEPC) genes from Corynebacterium glutamicum (cppc), Escherichia coli (eppc) or Flaveria trinervia (fppc) were transferred to Solanum tuberosum. Plant regenerants producing foreign PEPC were identified by Western blot analysis. Maximum PEPC activities measured in eppc and fppc plants grown in the greenhouse were doubled compared to control plants. For cppc a transgenic plant line could be selected which exhibited a fourfold increase in PEPC activity. In the presence of acetyl-CoA, a known activator of the procaryotic PEPC, a sixfold higher activity level was observed. In cppc plants grown in axenic culture PEPC activities were even higher. There was a 6-fold or 12-fold increase in the PEPC activities compared to the controls measured in the absence or presence of acetyl-CoA, respectively. Comparable results were obtained by transient expression in Nicotiana tabacum protoplasts. PEPC of C. glutamicum (PEPC C.g.) in S. tuberosum leaf extracts displays its characteristic K _m(PEP) value. Plant growth was examined with plants showing high expression of PEPC and, moreover, with a plant cell line expressing and antisense S. tuberosum (anti-sppc) gene. In axenic culture the growth rate of a cppc plant cell line was appreciably diminished, whereas growth rates of an anti-sppc line were similar or slightly higher than in controls. Malate levels were increased in cppc plants and decreased in antisense plants. There were no significant differences in photosynthetic electron transport or steady state CO₂ assimilation between control plants and transformants overexpressing PEPC C.g. or anti-sppc plants. However, a prolonged dark treatment resulted in a delayed induction of photosynthetic electron transport in plants with less PEPC. Rates of CO₂ release in the dark determined after a 45 min illumination period at a high proton flux density were considerably enhanced in cppc plants and slightly diminished in anti-sppc plants. When CO₂ assimilation rates were corrected for estimated rates of mitochondrial respiration in the light, the electron requirement for CO₂ assimilation determined in low CO₂ was slightly lower in transformants with higher PEPC, whereas transformants with decreased PEPC exhibited an appreciably elevated electron requirement. The CO₂ compensation point remained unchanged in plants (cppc) with high PEPC activity, but might be increased in an antisense plant cell line. Stomatal opening was delayed in antisense plants, but was accelerated in plants overexpressing PEPC C.g. compared to the controls.Abbreviations CO₂ compensation point - CO₂ quantum efficiency of CO₂ assimilation - _PSII quantum efficiency of photosystem II electron transport - A CO₂ assimilation rate - C_i intercellular CO₂ concentration; e, electron - PFD photon flux density - Q_A primary quinone electron acceptor of photosystem II - Q_N non-photochemical chlorophyll a fluorescence quenching - q_P photochemical chlorophyll a fluorescence quenching     

Non-photochemical quenching of chlorophyll a fluorescence in isolated chloroplasts under conditions of stressed photosynthesis

Non-photochemical quenching of chlorophyll a fluorescence after short-time light, heat and osmotic stress was investigated with intact chloroplasts from Spinacia oleracea L. The proportions of non-photochemical fluorescence quenching (q _N ) which are related (q _E ) and unrelated (q _I ) to the transthylakoid proton gradient (pH) were determined. Light stress resulted in an increasing contribution of q _Ito total q _N.The linear dependence of q. _Eand pH, as seen in controls, was maintained. The mechanisms underlying this type of quenching are obviously unaffected by photoin-hibition. In constrast, q _Ewas severely affected by heat and osmotic stress. In low light, the response of q _Eto changes in pH was enhanced, whereas it was reduced in high light. The data are discussed with reference to the hypothesis that q _Eis related to thermal dissipation of excitation energy from photosystem II. It is shown that q _Eis not only controlled by pH, but also by external factors.Abbreviations and symbols 9-AA 9-aminoacridine - F _o basic chlorophyll fluorescence - F _o variable chlorophyll fluorescence - L ₂ saturating light pulse - PS photosystem - q _E pH-dependent, non-photochemical quenching of fluorescence - q _I pH-independent, non-photochemical quenching - q _N entire non-photochemical quenching - q _Q photochemical quenching     

Photosynthetic activity of poikilochlorophyllous desiccation tolerant plant <Emphasis Type="Italic">Reaumuria soongorica</Emphasis> during dehydration and re-hydration

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as Chl content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub during dehydration and rehydration. The net photosynthetic rate (P _N), maximum photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m), quantum efficiency of non-cyclic electron transport of PS2, and Chl content decreased, but non-photochemical quenching of fluorescence and carotenoid content increased in stems with the increasing of drought stress. 6 d after re-hydration, new leaves budded from stems. In the re-watered plants, the chloroplast function was restored and Chl a fluorescence returned to a similar level as in the control plants. This improved hydraulic adjustment in plant triggered a positive effect on ion flow in the tissues and increased shoot electrical admittance. Thus R. soongorica plants are able to sustain drought stress through leaf abscission and keep part of Chl content in stems.     

Investigations on the electron transfer processes in Photosystem II: New insights by chlorophyll fluorescence decay measurements with additional saturating light pulses

Electron transfer processes in leaves were investigated by chlorophyll fluorescence decay measurements. A fast chlorophyll fluorescence decay was observed in the intact state, reflecting normal electron transfer in Photosystem II. After treatment with DCMU a slow chlorophyll fluorescence decay was measured due to blocked electron transfer after the primary quinone Q_A. Additional saturating light pulses, one between each two measuring pulses, were used to completely reduce Q_A of the intact leaf: the chlorophyll fluorescence decay became similar to that of a DCMU treated leaf. A decreased electron donation rate to the reaction centre P680 was obtained after treatment with hydroxylamine. The intensity of the additional saturating light pulses was not sufficient to reduce all Q_A under this condition and only a small increase of the average chlorophyll fluorescence decay time occurred. Following our previous paper [Berg et al. (1997) Photosynthetica 34, in press], we investigated the effects of water stress with the additional saturating light pulses. An almost complete reduction of Q_A was possible after water stress started. A small, but systematic shortening of the slow chlorophyll fluorescence decay followed, up to a relative loss of leaf mass of 80%. At this time a rapid shortening of the chlorophyll fluorescence decay occurred, caused by an electron deficiency at the donor site of PS II. Additional saturating light pulses had no effects on the chlorophyll fluorescence decay any more, revealing a radiationless recombination between the reduced primary quinone Q and the oxidized reaction centre P680⁺.     

Induction of water deficit tolerance in wheat due to exogenous application of plant growth regulators: membrane stability,water relations and photosynthesis

Our experiment was carried out in order to explore effects of plant growth regulators (PGR; thidiazuron, paclobutrazol, and ascorbic acid) on physiological traits of wheat genotypes under water surplus and deficit conditions. Study revealed that relative water content, membrane stability index, chlorophyll content, photosynthetic rate (P_N), and maximal quantum yield of PSII improved with PGRs application across the genotypes both under irrigation and water stress. The response of HD 2733 genotype was more positive toward PGRs treatment as compared to other genotypes under water stress. Higher P_N and chlorophyll contents were observed in HD 2987 followed by C 306 genotype under water-stress conditions. Moreover, Rubisco small subunit (SSU) expression was lower in wheat genotypes under water stress as compared to irrigated conditions. Application of PGRs led to upregulation of SSU under water stress, while no significant change was found in Rubisco level and activity under irrigated condition in dependence on PGRs treatments. Yield-related traits showed also significant reduction under water-stress conditions, while application of PGRs enhanced the yield and its components. Results indicated that the PGRs exhibited a positive interaction and synergetic effect on water stressed wheat plants in terms of photosynthetic machinery and yield.     

Effects of Pre-Treatments with Abscisic Acid and/or Benzyladenine on Gas Exchange of French Bean, Sugar Beet, and Maize Leaves During Water Stress and After Rehydration

Net photosynthetic rate (P_N), transpiration rate (E), and stomatal conductance (g_s) during water stress and after rehydration were measured in Phaseolus vulgaris, Beta vulgaris, and Zea mays. Immediately before imposition of water stress by cessation of watering, plants were irrigated with water (control), 100 M abscisic acid (ABA), and/or 10 M N⁶-benzyladenine (BA). In all three species, application of ABA decreased g_s, E, and P_N already 1 h after application. However, during water stress g_s, E, and P_N in plants pre-treated with ABA remained higher than in plants pre-treated with water. Positive effects of ABA application were observed also after rehydration. In contrast, the effects of pre-treatment with BA were species-specific. While in bean plants BA application ameliorated negative effect of water stress, only very slight effects were observed in maize, and in sugar beet BA even aggravated the effects of water stress.     

Effect of Benzylaminopurine on Rehydration of Bean Plants after Water Stress

The possibility of improving the recovery of plant photosynthesis after water stress by cytokinin-induced stimulation of stomatal opening or delay of leaf senescence was tested. The 6-benzylaminopurine (BAP) in concentrations 1 and 10 M was applied to the substrate (sand + nutrient solution) or sprayed on primary leaves of 14-d-old Phaseolus vulgaris L. plants sufficiently supplied with water or water-stressed for 4 d. The later ones having relative water content decreased to 69 % were fully rehydrated during the following three days. Parameters of photosynthesis and water relations were measured in primary leaves of 7-, 10-, 14-, and 17-d-old plants. Application of 1 M BAP slightly delayed leaf senescence: in 17-d-old control plants, net photosynthetic rate (P_N) and chlorophyll (Chl) content, and when sprayed on leaves also some of Chl a fluorescence kinetic parameters of BAP-treated leaves were slightly higher than those of untreated leaves. Both types of application of 1 M BAP slightly improved recovery of plants during rehydration after water stress in terms of increased g_ad, g_ab and P_N, i.e., parameters which were markedly decreased by mild water stress. However, contents of Chl a, Chl b and carotenoids and parameters of Chl a fluorescence kinetic were not markedly affected by mild water stress and after rehydration were not stimulated by 1 M BAP. 10 M BAP had mostly negative effects on the parameters measured.     

The carotenoid zeaxanthin and ‘high-energy-state quenching’ of chlorophyll fluorescence

The possibility that zeaxanthin mediates the dissipation of an excess of excitation energy in the antenna chlorophyll of the photochemical apparatus has been tested through the use of an inhibitor of violaxanthin de-epoxidation, dithiothreitol (DTT), as well as through the comparison of two closely related organisms (green and blue-green algal lichens), one of which (blue-green algal lichen) naturally lacks the xanthophyll cycle. In spinach leaves, DTT inhibited a major component of the rapidly relaxing high-energy-state quenching' of chlorophyll fluorescence, which was associated with a quenching of the level of initial fluorescence (F₀) and exhibited a close correlation with the zeaxanthin content of leaves when fluorescence quenching was expressed as the rate constant for radiationless energy dissipation in the antenna chlorophyll. Green algal lichens, which possess the xanthophyll cycle, exhibited the same type of fluorescence quenching as that observed in leaves. Two groups of blue-green algal lichens were used for a comparison with these green algal lichens. A group of zeaxanthin-free blue-green algal lichens did not exhibit the type of chlorophyll fluorescence quenching indicative of energy dissipation in the pigment bed. In contrast, a group of blue-green algal lichens which had formed zeaxanthin slowly through reactions other than the xanthophyll cycle, did show a very similar response to that of leaves and green algal lichens. Fluorescence quenching indicative of radiationless energy dissipation in the antenna chlorophyll was the predominant component of high-energy-state quenching in spinach leaves under conditions allowing for high rates of steady-state photosynthesis. A second, but distinctly different type of high-energy-state quenching of chlorophyll fluorescence, which was not inhibited by DTT (i.e., it was zeaxanthin independent) and which is possibly associated with the photosystem II reaction center, occurred in addition to that associated with zeaxanthin in leaves under a range of conditions which were less favorable for linear photosynthetic electron flow. In intact chloroplasts isolated from (zeaxanthin-free) spinach leaves a combination of these two types of rapidly reversible fluorescence quenching occurred under all conditions examined.Abbreviations DTT dithiothreitol - F₀ (or F₀) yield of instantaneous fluorescence at open PS II reaction centers in the dark (or during actinic illumination) - F_M (or F_M) yield of maximum fluorescence induced by a saturation pulse of light in the dark (or during actinic illumination) - F_V (or F_V) yield of variable fluorescence induced by a saturating pulse of light in the dark (or during actinic illumination) - k _D rate constant for radiationless energy dissipation in the antenna chlorophyll - SV Stern-Volmer equation - PFD photon flux density - PS I photosystem I - PS II photosystem II - Q_A acceptor of photosystem II - q_N coefficient of nonphotochemical chlorophyll fluorescence quenching - q_P coefficient of photochemical chlorophyll fluorescence quenching     

Effect of supercooling and freezing on photosynthesis in freezing tolerant leaves of Afroalpine ‘giant rosette’ plants

Summary The effect of supercooling and freezing on the photosynthetic capability of representatives of the permanent frost hardy giant rosette plants Dendrosenecio keniodendron, D. brassica and Lobelia telekii, of the tropical alpine regions was investigated with the non-invasive chlorophyll a fluorescence technique. While supercooling, normal chlorophyll a fluorescence kinetics exhibiting the sequence 0, I, (D), P, S, M, were recorded, however with some retardation of both, the fast and the slow characteristics as compared to those obtained at day-time temperature. As long as the leaves remained unfrozen, the rise of the variable fluorescence F from the level 0 to P was inversely related to a drop of the temperature from about 0°C to-8°C. The increase of F with lower temperature is understood to result from a decrease of the velocity of the quenching reactions while photoreduction of the primary electron acceptor appeared to be unimpeded. The second fluorescence maximum (M), usually interpreted to indicate the commencement of the biochemical reactions of photosynthesis was consistenly to be observed during supercooling. Fluoescence induction kinetics of frozen leaves showed only fast rise to presumably F _max which was not followed by a significant decay for as long as 4 min. The lack of substantial quenching indicates that in the freeze-dehydrated state neither reoxidation of the primary acceptor nor energetization of the thylakoid membrane was accomplished. This effect however was immediately and fully reserved upon thawing of the leaves when the usual fluorescence induction kinetics as well as normal rates of CO₂-uptake were observed. Thus the permanent frost-hardy afroalpine plants do not exhibit any even short-term memory effect of the nocturnal frost on such a delicate process as is photosynthesis.     

The quenching characteristics of potassium iridic chloride and their meaning for the origin of chlorophyll fluorescence components

To understand the origins of the different lifetime components of photosystem 2 (PS2) chlorophyll (Chl) fluorescence we have studied their susceptibility to potassium iridic chloride (K₂IrCl₆) which has been shown to bleach antenna pigments of photosynthetic bacteria (Loach et al. 1963). The addition of K₂IrCl₆ to PS2 particles gives rise to a preferential quenching of the variable Chl fluorescence (F_v). At concentrations lower than 20 M, this is brought about mainly by a decrease in the yield, but not in the lifetime, of the slowest component when all the PS2 reaction centres are closed (F_M). The yield of the middle and fast decays are not significantly altered. This type of quenching is not seen with DNB. The iridate-induced quenching of the initial fluorescence level (F₀) is due to a proportional decrease in the yield and lifetime of the three components and correlates with the observed modification in the relative quantum yield of oxygen evolution. In this concentration range a bleaching of Chl a is seen. At higher iridate levels, greater than 20 M, a proportional decrease in the lifetimes and yields of the three kinetic components is seen at F_M. These changes are associated with a carotenoid bleaching. In isolated light harvesting Chl a/b complexes of PS2 (LHC2), iridate addition converts a 4 ns decay into a 200 ps emission and both types of bleaching are observed. By also measuring the rate of PS2 trap closure versus iridate concentration, we have discussed the results in terms of excitation energy transfer.Abbreviations DNB m-dinitrobenzene - F_M maximum Chl fluorescence - F₀ initial fluorescence - F_v variable fluorescence - I pheophytin a primary electron acceptor of PS2 - P₆₈₀ chlorophyll a of photochemical centre - PS2 photosystem 2 - Q_A primary stable electron acceptor of PS2 - Chl chlorophyll - LHC2 light harvesting Chl a/b complex of PS2 - MES 2(N-morpholino) ethanesulfonic acid - DCMU 3-(3-4-dichlorophenyl) 1-1 dimethylurea - PPBQ phenyl-p-benzo-quinone - BBY PS2-enriched membranes prepared as in Berthold et al. (1981) - Q₄₀₀ PS2 electron acceptor with a midpoint potential of 400 mV     

Increase in photosynthesis of maize hybrids (<Emphasis Type="Italic">Zea mays</Emphasis> L.) at suboptimal temperature (15 °C) by selection of parental lines on the basis of chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence measurements

We tested the usefulness of chlorophyll a fluorescence quenching analysis for the selection of maize parental inbred lines able to produce F₁ hybrids with a high CO₂ assimilation rate during growth at suboptimal temperature. Fifty inbred lines, grown at 15 °C, showed at 6 °C a broad genetic variability regarding the quantum yield of photosynthetic electron transport (_PS2). A decrease of _PS2 in sensitive lines was caused more by reduction of the efficiency of excitation energy capture by open photosystem 2 (PS2) reaction centres (F_v'/F_m') than by a drop in photochemical quenching (q_P). Selected inbred lines with the highest (H) and the lowest (L) values of _PS2 were used for separate crossings in a diallelic arrangement. Twenty-one of H×H hybrids and 21 of the L×L hybrids were grown at 15 °C. The H×H hybrids showed at suboptimal temperature a significantly higher transport of photosynthetic electrons than the L×L hybrids at lower (400) as well as at higher [800 mol(photon) m^–2 s^–1] irradiance. The mean net photosynthetic rate (P _N) in H×H and L×L hybrids amounted to 8.4 and 5.8 (second leaf) and 8.5 and 7.6 mol(CO₂) m^–2 s^–1 (third leaf), respectively. Among the best 20 hybrids with regard to P _N (values larger than the average) of second leaves, as many as 15 were derived from H lines (75 % of hybrids), whereas among the best 21 hybrids with regard to P _N of the third leaves, 16 were derived from H lines (76 % of hybrids). The intensive P _N of H×H hybrids was most often accompanied by less water lost via transpiration in relation to photosynthesis than in the hybrids of L lines. Hence an analysis of chlorophyll a fluorescence quenching enables the selection of inbred lines, which can produce hybrids with improved CO₂ fixation and with efficient water management during growth at suboptimal temperature.     

Effects of rhizobia inoculation and nitrogen fertilization on photosynthetic physiology of soybean

Plant growth, contents of photosynthetic pigments, photosynthetic gas exchange, and chlorophyll (Chl) fluorescence in soybean [Glycine max (L.) Merr. cv. Heinong37] were investigated after it was inoculated with Sinorhizobium fredii USDA191 or treated with 5 mM (NH₄)₂SO₄ (N5) and 30 mM (NH₄)₂SO₄ (N30), respectively. In the plants following N5 fertilization, not only plant biomass, leaf area, and Chl content, but also net photosynthetic rate (P _N), stomatal conductance (g _s), carboxylation efficiency (CE), maximum photochemical efficiency (F_v/F_m) of photosystem 2 (PS2), and quantum yield of PS2 (Φ_PS2) were markedly improved as compared with the control plants. There were also positive effects on plant growth and plant photosynthesis after rhizobia inoculation, but the effects were much less than those of N5 fertilization. For N30 plants there were no significant positive effects on plant growth and photosynthetic capacity. Plant biomass, P _N, and g _s were similar to those of N-limited (control) plants. Φ_PS2 and photochemical quenching (q_P) were obviously declined while content of carotenoids and non-photochemical quenching (q_N) were significantly enhanced in N30 treated plants. This indicated that excess N supply may cause some negative effects on soybean plants.     

Changes in Chlorophyll Fluorescence During the Course of Photoperiod and in Response to Drought in Casuarina equisetifolia Forst. and Forst.

The effects of drought and the diurnal changes in photosynthetic electron transport were studied in non-nodulated plants of Casuarina equisetifolia. The induction of fluorescence showed a slightly higher I step in water-stressed than control plants, and the time from the start of irradiation to the P step of induction was significantly shortened by drought. The quantum efficiency of photosystem 2 (PS2) in the dark-adapted state (F_v/F_m) was generally not affected by drought, whereas it decreased during the central hours of the day. The decrease in quantum yield of PS2 electron transport (₂) in water-stressed plants was associated with decreases in the photochemical efficiency of open (oxidised) PS2 centres (F_v'/F_m') and increases in non-photochemical quenching (q_N) rather than with increased closure of PS2 centres (lowered photochemical quenching, q_P). In contrast, the changes in quantum yield of electron transport during the day were related to changes in q_P rather than in F_v'/F_m'. When chlorophyll fluorescence was measured at the same irradiance during the day, a greater q_N was observed at the end of the drying cycle than after watering, and early and late in the photoperiod than in the central hours of the day. The greater q_N at the beginning and end of the day did not prevent an increase in energy not used photochemically nor dissipated non-photochemically. Drought did not affect this excess of photon energy.     

Peroxidation of lipids and growth inhibition induced by UV-B irradiation

Cotyledons excised from dark-grown seedlings of cucumber (Cucumis sativus L.) were cultured in vitro under UV radiation at different wavelengths, obtained by passage of light through cut-off filters with different transmittance properties. Growth and the synthesis of chlorophyll (Chl) in cotyledons were inhibited and malondialdehyde was accumulated upon irradiation at wavelengths below 320 nm. Exogenous application of scavengers of free radicals reversed the growth inhibition induced by UV-B. Measurement of the fluorescence of Chl a suggested that electron transfer in photosystems was affected by UV-B irradiation. On the basis of these results, the involvement is postulated of active species of oxygen in damages to thylakoid membranes and the growth inhibition that are induced by UV-B irradiation.Abbreviations Chl chlorophyll - F_m maximal fluorescence (dark) - F_m maximal fluorescence (light) - F_v variable fluorescence (dark) - F_v variable fluorescence (light) - MDA malondialdehyde - O₂ Superoxide radical - PS photosystem - q_N non-photochemical quenching of fluorescence - q_P photochemical quenching of fluorescence - UV-B_BE biologically effective UV-B radiation - WL(T = 0.5) wavelength at which 50% transmittance occurs     

Effects of water stress on photosystem II photochemistry and its thermostability in wheat plants

Modulated chlorophyll fluorescence, rapid fluorescence induction kinetics and the polyphasic fluorescence transients (OJIP) were used to evaluate PSII photochemistry in wheat plants exposed to water stress and/or heat stress (25-45C). Water stress showed no effects on the maximal quantum yield of PSII photochemistry (Fv/Fm), the rapid fluorescence induction kinetics, and the polyphasic fluorescence transients in dark-adapted leaves, indicating that water stress had no effects on the primary photochemistry of PSII. However, in light-adapted leaves, water stress reduced the efficiency of excitation energycapture by open PSII reaction centres (F'v/F'm) and the quantum yield of PSII electron transport (PSII), increased the non-photochemical quenching (qN) and showed no effects on the photochemical quenching (qP). This suggests that water stress modified the PSII photochemistry in the light-adapted leaves and such modifications may be a mechanism to down-regulate the photosynthetic electron transport to match a decreased CO2 assimilation. In addition, water stress also modified the responses of PSII to heat stress. When temperature was above 35C, thermostability of PSII was strongly enhanced in water-stressed leaves, which was reflected in a less decrease in Fv/Fm, qP, F'v/F'm, and PSII in water-stressed leaves than in well-watered leaves. There were no significant variations in the above fluorescence parameters between moderately and severely water-stressed plants, indicating that the moderate water-stressed plants, indicating that the moderate water stress treatment caused the same effects on thermostability of PSII as the severe treatment. It was found that increased thermostability of PSII may be associated with an improvement of resistance of the O2-evolving complex and the reaction centres in water-stressed plants to high temperature.Key words: Chlorophyll fluorescence, heat stress, photosystem II photochemistry, water stress, wheat (Tritium aestivum L.).      

Gas Exchange and Chlorophyll Fluorescence Response to Simulated Rainfall in Hedysarum fruticosum var. mongolicum

The response of gas exchange and chlorophyll fluorescence along with changes in simulated rainfall were studied in water stressed plants Hedysarum fruticosum var. mongolicum (H.f.m.). Net photosynthetic rate (P _N), stomatal conductance (gs), leaf water potential (_leaf), and apparent carboxylation efficiency (P _N/C _i) were significantly increased with the increase of rainfall. However, they did not change synchronously. The complete recovery of both P _N and P _N/C _i appeared 3 d after watering while g _s and _leaf were recovered 1 d after treatment. Gas exchange characters increased sharply from 5 to 15 mm rainfall and then maintained steady state with increasing rainfall. During the initial phase of water recovery, photosystem 2 (PS2) activity was not affected and its complete recovery occurred also 3 d after rainfall. Hence the recovery of P _N was attributed to both opening of stomata and increase in carboxylation efficiency. Furthermore, PS2 activity was really impaired by water stress and could recover to the normal status when the water stress disappeared.