Relationship between quantum efficiency of PSII and cold-induced plant resistance to fungal pathogens

The aim of the presented work was to study whether the efficiency of photosynthesis may influence resistance of hardened plants to disease. Seedlings of spring barley, meadow fescue and winter oilseed rape were chilled at 5 °C for 2, 4 or 6 weeks and at these deadlines the changes in cell membrane permeability (expressed as electrolyte leakage), chlorophyll fluorescence (initial fluorescence - F₀, maximal fluorescence - F_m, quantum yield of PSII - F_v/F_m) and net photosynthesis rate (F_N) were measured. Also, the influence of cold on the degree of plant resistance to economically important pathogens -Bipolaris sorokiniana or Phoma lingam was estimated. Two, four or six week-hardened plants were artificially infected: barley and fescue by B. sorokiniana, and oilseed rape by P. lingam. Hardening at 5 °C stimulated resistance of barley, fecue and rape to their specific pathogens. Six-week long acclimation was the most effective for plant resistance. Cold significantly changed cell membrane permeability and decreased chlorophyll fluorescence (F₀, F_m and F_v/F_m) of all studied plant species, while net photosynthesis rate was found to decrease only in barley. The results indicate that cold-induced resistance of plants to pathogens was correlated with a decrease in cell membrane permeability. In the case of fescue and barley a significant connection between the quantum yield of PSII and their resistance to B. sorokiniana was shown. Additionally, the resistance of barley to fungus was depended on net photosynthesis rate. In general this research shows that the efficiency of photosynthesis may be used as an indicator of plant resistance to disease.     

Effects of the interaction between ozone and carbon dioxide on gas exchange,ascorbic acid content,and visible leaf symptoms in rice leaves

Tropospheric ozone (O₃) decreases photosynthesis, growth, and yield of crop plants, while elevated carbon dioxide (CO₂) has the opposite effect. The net photosynthetic rate (P _N), dark respiration rate (R _D), and ascorbic acid content of rice leaves were examined under combinations of O₃ (0, 0.1, or 0.3 cm³ m⁻³, expressed as O⁰, O^0.1, O^0.3, respectively) and CO₂ (400 or 800 cm³ m⁻³, expressed as C⁴⁰⁰ or C⁸⁰⁰, respectively). The P _N declined immediately after O₃ fumigation, and was larger under O^0.3 than under O^0.1. When C⁸⁰⁰ was combined with the O₃, P _N was unaffected by O^0.1 and there was an approximately 20 % decrease when the rice leaves were exposed to O^0.3 for 3 h. The depression of stomatal conductance (g _s) observed under O^0.1 was accelerated by C⁸⁰⁰, and that under O^0.3 did not change because the decline under O^0.3 was too large. Excluding the stomatal effect, the mesophyll P _N was suppressed only by O^0.3, but was substantially ameliorated when C⁸⁰⁰ was combined. Ozone fumigation boosted the R _D value, whereas C⁸⁰⁰ suppressed it. An appreciable reduction of ascorbic acid occurred when the leaves were fumigated with O^0.3, but the reduction was partially ameliorated by C⁸⁰⁰. The degree of visible leaf symptoms coincided with the effect of the interaction between O₃ and CO₂ on P _N. The amelioration of O₃ injury by elevated CO₂ was largely attributed to the restriction of O₃ intake by the leaves with stomatal closure, and partly to the maintenance of the scavenge system for reactive oxygen species that entered the leaf mesophyll, as well as the promotion of the P _N.     

Physiological responses of Fagus sylvatica L. exposed to low levels of ozone in open-top chambers

Four-year-old beech seedlings were fumigated with three levels of ozone for 2 consecutive years in open-top chambers. During the second growth season different physiological measurements were conducted before and during daily fumigation. A 25–40% decrease in net photosynthesis was seen during fumigation, whereas no differences were detected before fumigation in July. In August lasting effects in net photosynthesis were seen. The apparent quantum yield was decreased after fumigation. Stomatal conductance was generally decreased during fumigation, but transpiration was reduced relatively less than net photosynthesis indicating a lower water use efficiency of the trees exposed to ozone. Chlorophyll fluorescence (F_v/F_m) showed additive reductions in relation to ozone and light.     

Acceleration of leaf senescence inFagus sylvatica L. by low levels of tropospheric ozone demonstrated by leaf colour,chlorophyll fluorescence and chloroplast ultrastructure

From April 1988 to October 1991 3-year-old seed propagated beech (Fagus sylvatica L.) trees were exposed in open-top chambers to four different levels of air pollution: (1) charcoal filtered air, (2) ambient air, (3) ambient air plus 30 nl 1^-1 ozone during the summer, and (4) ambient air plus 30 nl 1^-1 ozone during the summer and 20 nl 1^-1 SO₂ and NO₂ during the winter. Leaf colour was studied in the autumns of 1989 and 1991 and a close relationship between ozone dose and premature senescence was found. A correlation also exists between the colour groups and chlorophyll fluorescence (F_v/F_m). Ozone fumigation increases the size and speeds up the development of the plastoglobules. This is described using an index based on the volume of plastoglobules as a percentage of chloroplast volume. The index was significantly higher for ozone fumigated plants than for control plants during August to November 1989. According to all three methods it is concluded that low levels of ozone accelerate leaf senescence processes inF. sylvatica. There are indications that leaves of the first and the second flush react differently to the ozone treatment. Irrespective of the ozone treatment a special cell wall structure, probably a local suberization, is confined to the subsidiary cells in leaves of the first flush.     

Interactions between nitrogen fertilization and ozone in watermelon cultivar Reina de Corazones in open-top chambers. Effects on chlorophyll a fluorescence, lipid peroxidation, and yield

Watermelon (Citrillus lanatus) plants were grown for two consecutive years in open-top chambers with three different ozone concentrations (O₃-free air, O₃ ambient, and air with additional O₃; CFA, NFA, and NFA+O₃) and three nitrogen fertilizer concentrations [0, 14.0, and 29.6 g N per pot; N0, N1, and N2). There was an interaction between ozone and N fertilizer for the major parameters studied. O₃ and N2 treatments led to a significant decrease in maximum efficiency of photosystem 2 (PS2) photochemistry (F_v/F_m), and induced a significant decrease in the actual quantum yield of PS2 (Φ_PS2), due mainly to the increased closure of PS2 reaction centres (q_P) and to an increase in the non-photochemical quenching (NPQ). On the other hand, these plants exhibited an increased susceptibility to photoinhibition, which could be associated with an increased fraction of reduced Q_A. An increase in lipid peroxidation indicated that damage was occurring at the membrane levels. High N concentration enhanced the detrimental effects of ozone on the fluorescence parameter induction and lipid peroxidation. All these negative alterations led to a decreased yield.     

Changes of net photosynthesis,antioxidant enzyme activities,and antioxidant contents of <Emphasis Type="Italic">Liriodendron tulipifera</Emphasis> under elevated ozone

Liriodendron tulipifera was exposed to gradually elevated ozone concentrations of 100–300 μg kg⁻¹ in the naturally irradiated environment chamber. During 15 d of exposure to O₃, net photosynthetic rate (P _N) decreased and there was large difference between the control (C) and treatment with ozone (OT), while there was no significant difference in water use efficiency. Total chlorophyll content as well as the value of fluorescence parameter F_v/F_m decreased, while antioxidant enzyme activities related to ascorbate-glutathione cycle increased after 15 d of OT. Unchanged contents of ascorbate and glutathione indirectly suggest that the species hastened the antioxidant’s oxidization/reduction cycle using enzymes instead of expanding their pool against oxidative stress.     

Effect of Cd on growth, photosynthetic gas exchange, and chlorophyll fluorescence of wild and Cd-sensitive mutant rice

Growth, photosynthetic gas exchange, and chlorophyll fluorescence characteristics were investigated in wild type (WT) and Cd-sensitive mutant rice (Oryza sativa L.) plants using 50 μM Cd treatment for 12 d followed by a 3-d recovery. Under Cd stress, net dry mass and pigment contents were significantly lower in the mutant plants than in the WT. The mutant had lower net photosynthetic rate (P _N), transpiration rate (E), and stomatal conductance (g _s) than WT rice, however, it had higher intercellular CO₂ concentration (C _i), indicating that non-stomatal factors accounted for the inhibition of P _N. Maximal photochemical efficiency of photosystem 2 (F_v/F_m), effective quantum yield of PS2 (Φ_PS2), and photochemical quenching (q_P) decreased much in the mutant under Cd stress. Cd content in roots and leaves of the mutant was significantly higher than those in the WT. Hence Cd toxicity was associated with the marked increases in Cd contents of plant tissue. After the recovery for 3 d, the WT rice had higher capacity to recover from Cd injury than the mutant.     

Persistent effects of low concentrations of SO2 and NO2 on photosynthesis in Scots pine (Pinus sylvestris) needles

In an open-field experiment, 50-year-old trees of Scots pine (Pinus sylvestris L.) were fumigated with low concentrations of SO₂ and NO₂ (10–15 nl I^?1) during the growing season in four consecutive years (1988 to 1991). Results from the autumn and early winter of 1991 and 1992 are presented. The maximum photochemical efficiency of photosystem II (PSII), as indicated by the ratio of variable to maximum fluorescence (F_v/F_M) was assessed in current and one-year-old needles from the top and the bottom of the canopy. Furthermore, simultaneous measurements of photosynthetic O₂ evolution and chlorophyll fluorescence were made in current-year needles at 20°C. In general, the F_v/F_M ratio as well as the gross rate of O₂ evolution in needles of fumigated trees was not significantly different from that in needles of control trees during the fumigation period. However, both current and one-year-old needles sampled in November and December 1991 from the top of the canopy of fumigated trees had significantly lower F_v/F_M values than corresponding needles of control trees. Similar differences in F_v/F_M correlated with the treatments were observed in needles from the bottom of the canopy, indicating that the depression of F_v/F_M in needles of fumigated trees was not due to an increased susceptibility to photoinhibition. In 1992, when no fumigation occurred, differences in F_v/F_M between the treatments were not significant during autumn and early winter. The gross rate of O₂ evolution at high irradiances was significantly lower in current-year needles of fumigated trees sampled in November and December 1991 than in those of control trees. Furthermore, a nearly identical linear relationship between the quantum yield of PSII electron transport determined from chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PPFD) was found during autumn and early winter. This appeared to be largely a result of changes in the thermal energy dissipation within PSII. The observed differences in photosynthetic characteristics correlated with the different treatments after the fumigation period is suggested to be mainly caused by increased sensitivity of the needles of fumigated trees to low and subfreezing temperatures. However, current-year needles of fumigated trees tended to have a lower N content than those of control trees, which may partly explain the differences in gross photosynthesis between fumigated and control trees.     

Photosynthesis and yield responses of ozone-polluted winter wheat to drought

Winter wheat (Triticum aestivum L. cv. Jingdong 8) was exposed to short-term high ozone treatment after anthesis and then was either well irrigated with soil water content (SWC) of 80–85 % (O₃+W) or drought treated (SWC 35–40 %, O₃+D). Short-term ozone exposure significantly decreased irradiance-saturated net photosynthetic rate (P _N) of winter wheat. Under good SWC, P _N of the O₃-treated plant was similar to that of control on 2 d after O₃-exposure (6 DAA), but decreased significantly after 13 DAA, indicating that O₃ exposure accelerated leaf senescence. Meanwhile, green flag leaf area was reduced faster than that of control. As a result, grain yield of O₃+W was significantly decreased. P _N of O₃+D was further notably decreased and green flag leaf area was reduced more than that in O₃+W. Consequently, substantial yield loss of O₃+D was observed compared to that of O₃+W. Although P _N was significantly positively correlated with stomatal conductance, it also had notable positive correlation with the maximum photochemical efficiency in the dark adapted leaves (F_v/F_m), electron transport rate (ETR), photochemical quenching (q_P), as well as content of chlorophyll, suggesting that the depression of P _N was mainly caused by non-stomatal limitation. Hence optimal soil water condition should be considered in order to reduce the yield loss caused by O₃ pollution.     

Effects of Ozone Fumigation on Photosynthesis and Membrane Permeability in Leaves of Spring Barley,Meadow Fescue,and Winter Rape

Photosynthetica - Seedlings of spring barley, meadow fescue, and winter rape were fumigated with 180 μg kg−1 of ozone for 12 d, and effect of O3 on photosynthesis and cell membrane...     

CO2 assimilation, xanthophyll cycle pigments and PSII efficiency in pumpkin plants as affected by ozone fumigation

CO₂ assimilation, xanthophyll cycle pigments and PSII efficiency were analyzed in two different ages of pumpkin leaves (Cucurbita pepo L. cv. Ambassador) exposed to 150 nmol mol^-1 of ozone (5 days, 5 h day^-1). Gas-exchange measurements revealed a reduction in CO₂ assimilation and stomatal conductance, accompanied by an increase in the intercellular CO₂ concentration both in young and in mature leaves as compared to their respective controls. In both leaves, F₀ remained unchanged, while F_m and the F_v/F_m ratio decreased after O₃ fumigation, indicating that ozone may induce an alteration in the capability of photosystem II (PSII) to reduce the primary acceptor Q_A. In the mature leaves the photochemical quenching (q_p) was significantly lowered by the pollutant, but this was not the case in the young leaves where q_p did not change. In both mature and young ozonated pumpkin leaves, the development of non-photochemical quenching caused a decrease in the PSII photochemical rate, as shown by the correlation between F_v/F_m and the de-epoxidation state of dark-adapted leaves. Decreases in the F_v/F_m ratio are generally attributed to damage to the PSII reaction centre, apart from the down-regulation of the capacity of PSII electron transport. While in young ozonated leaves the decrease in the F_v/F_m ratio was not associated with damage to the D1 protein, in mature ozonated pumpkin leaves, the decrease in the F_v/F_m was accompanied by a significant decline in the D1 content. In conclusion, ozone exposure induces alterations in the light reactions of photosynthesis in both young and mature leaves. However, in young leaves the engagement of the xanthophyll cycle appears to counteract ozone effects against the photosynthetic apparatus as demonstrated by the absence of damage to the D1 protein. On the other hand, the loss of D1 protein in mature fumigated leaves suggests that the activation of the xanthophyll cycle is not sufficient to prevent photoinhibition, probably because a physiological state of senescence adds to the oxidative stress.     

Changes in Chlorophyll Fluorescence in Maize Plants with Imposed Rapid Dehydration at Different Leaf Ages

A comparison of the effects of a rapidly imposed water deficit with different leaf ages on chlorophyll a fluorescence and gas exchange was performed in maize (Zea mays L.) plants. The relationships between photosynthesis and leaf relative turgidity (RT) and ion leakage were further investigated. Leaf dehydration substantially decreased net photosynthetic rate (A) and stomatal conductance (G _s), particularly for older leaves. With dehydration time, F _v /F _m maintained a relatively stable level for youngest leaves but significantly decreased for the older leaves. The electron transport rate (ETR) sharply decreased with intensifying dehydration and remained at lower levels during continuous dehydration. The photochemical quenching of variable chlorophyll fluorescence (q _P) gradually decreased with dehydration intensity for the older leaves but increased for the youngest leaves, whereas dehydration did not affect the nonphotochemical chlorophyll fluorescence quenching (NPQ) for the youngest leaves but remarkably decreased it for the older leaves. The leaf RT was significantly and positively correlated with its F _v /F _m, ETR, and q _P, and the leaf ion leakage was significantly and negatively correlated with F _v /F _m and NPQ. Our results suggest that the photosynthetic systems of young and old leaves decline at different rates when exposed to rapid dehydration.     

Gas Exchange and Chlorophyll Fluorescence in Symbiotic and Non-Symbiotic Ryegrass Under Water Stress

The symbiotic association of endophyte fungus, Neotyphodium lolii, and ryegrass improves the ryegrass resistance to drought. This is shown by a 30 % increase in the number of suckers in infected plants (E+), compared to plants lacking endophyte (E−), and by a higher water potential in the E+ than E− plants. The E+ plants have higher stomatal conductance (g _s), transpiration rate, net photosynthetic rate (P _N), and photorespiratory electron transport rate than the E− plants. The maximal photochemical efficiency (F_v/F_m) and the actual photochemical efficiency (Φ_PS2) are not affected by the endophyte fungus. The increase in P _N of the E+ plants subjected to water stress was independent from internal CO₂ concentration. An increased P _N was observed in E+ plants also in optimal water supply. Hence the drought resistance of E+ plants results in increased g _s, P _N, and photorespiratory electron transport rate. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chlorophyll a Fluorescence Emission,Xanthophyll Cycle Activity,and Net Photosynthetic Rate Responses to Ozone in Some Foliose and Fruticose Lichen Species

The lichens Parmelia quercina, Parmelia sulcata, Evernia prunastri, Hypogymnia physodes, and Anaptychia ciliaris were exposed to ozone (O₃) in controlled environment cuvettes designed to maintain the lichens at optimal physiological activity during exposure. Measurements of gas exchange, modulated chlorophyll (Chl) fluorescence, and pigment analysis were conducted before and after exposure to 300 mm³ (O₃) m^–3, 4 h per d for 14 d. No changes in the efficiency of photosystem 2 (PS2) photochemistry, the reduction state of Q_A, or the electron flow through PS2, measured by Chl fluorescence, were detected in any of the five lichen species studied. Additionally, neither photosynthetic CO₂ assimilation nor xanthophyll cycle activity or photosynthetic pigment concentration were affected by high O₃ concentrations. Thus the studied lichen species have significant capacities to withstand oxidative stresses induced by high concentration of O₃.     

Leaf Gas Exchange and Chlorophyll Fluorescence Parameters in Phaseolus Vulgaris as Affected by Nitrogen and Phosphorus Deficiency

The effects of N and P deficiency, isolated or in combination, on leaf gas exchange and fast chlorophyll (Chl) fluorescence emission were studied in common bean cv. Negrito. 10-d-old plants grown in aerated nutrient solution were supplied with high N (HN, 7.5 mol m⁻³) or low N (LN, 0.5 mol m⁻³), and also with high P (HP, 0.5 mol m⁻³) or low P (LP, 0.005 mol m⁻³). Regardless of the external P supply, in LN plants the initial fluorescence (F₀) increased 12 % in parallel to a quenching of about 14 % in maximum fluorescence (F_m). As a consequence, the variable to maximum fluorescence ratio (F_v/F_m) decreased by about 7 %, and the variable to initial fluorescence ratio (F_v/F₀) was lowered by 25 % in relation to control plants. In LP plants, F_v/F_m remained unchanged whilst F_v/F₀ decreased slightly as a result of 5 % decline in F_m. Under N deficiency, the net photosynthetic rate (P _N) halved at 6 d after imposition of treatment and so remained afterwards. As compared to LN plants, P _N declined in LP plants latter and to a less extent. From 12 d of P deprivation onwards. P _N fell down progressively to display rates similar to those of LN plants only at the end of the experiment. The greater P _N in LP plants was not reflected in larger biomass accumulation in relation to LN beans. In general, P and N limitation affected photosynthesis parameters and growth without showing any synergistic or additive effect between deficiency of both nutrients. This revised version was published online in June 2006 with corrections to the Cover Date.     

Short-term responses of photosynthetic membrane lipids and photochemical efficiency in plants of <Emphasis Type="Italic">Phaseolus vulgaris</Emphasis> and <Emphasis Type="Italic">Vigna unguiculata</Emphasis> submitted to high irradiance

Primary leaves of young plants of common bean (Phaseolus vulgaris cv. Carioca and Negro Huasteco) and cowpea (Vigna unguiculata Walp cv. Epace 10) were exposed to high irradiance (HI) of 2 000 μmol m⁻² s⁻¹ for 10, 20, and 30 min. The initial fluorescence (F₀) was nearly constant in response to HI in each genotype except for Carioca. A distinct reduction of maximum fluorescence (F_m) was clearly observed in stressed genotypes of beans after 20 min followed by a slight recovery for the longer stress times. In common bean, the maximum quantum yield (F_v/F_m) was reduced slowly from 10 to 30 min of HI. In cowpea, only a slight reduction of F_v/F_m was observed at 20 min followed by recovery to normal values at 30 min. HI resulted in changes in the photochemical (q_P) and non-photochemical (q_N) quenching in both species, but to a different extent. In cowpea plants, more efficiency in the use of the absorbed energy under photoinhibitory conditions was related to increase in q_P and decrease in q_N. In addition, lipid peroxidation changed significantly in common bean genotypes with an evident increase after 20 min of HI. Hence the photosynthetic apparatus of cowpea was more tolerant to HI than that of common bean and the integrity of cowpea cell membranes was apparently maintained under HI.     

Constitutive and herbivore-inducible glucosinolate concentrations in oilseed rape (Brassica napus) leaves are not affected by Bt Cry1Ac insertion but change under elevated atmospheric CO2 and O3

Glucosinolates are plant secondary compounds involved in direct chemical defence by cruciferous plants against herbivores. The glucosinolate profile can be affected by abiotic and biotic environmental stimuli. We studied changes in glucosinolate patterns in leaves of non-transgenic oilseed rape (Brassica napus ssp. oleifera) under elevated atmospheric CO₂ or ozone (O₃) concentrations and compared them with those from transgenic for herbivore-resistance (Bacillus thuringiensis Cry1Ac endotoxin), to assess herbivory dynamics. Both elevated CO₂ and O₃ levels decreased indolic glucosinolate concentrations in transgenic and non-transgenic lines, whereas O₃ specifically increased the concentration of an aromatic glucosinolate, 2-phenylethylglucosinolate. The herbivore-inducible indolic glucosinolate response was reduced in elevated O₃ whereas elevated CO₂ altered the induction dynamics of indolic and aliphatic glucosinolates. Herbivore-resistant Bt plants experienced minimal leaf damage after target herbivore Plutella xylostella feeding, but exhibited comparatively similar increase in glucosinolate concentrations after herbivory as non-transgenic plants, indicating that the endogenous glucosinolate defence was not severely compromised by transgenic modifications. The observed differences in constitutive and inducible glucosinolate concentrations of oilseed rape under elevated atmospheric CO₂ and O₃ might have implications for plant–herbivore interactions in Brassica crop-ecosystems in future climate scenarios.     

Deriving room temperature excitation spectra for photosystem I and photosystem II fluorescence in intact leaves from the dependence of <Emphasis Type="Italic">F</Emphasis><Subscript>V</Subscript>/<Emphasis Type="Italic">F</Emphasis><Subscript>M</Subscript> on excitation wavelength

The F ₀ and F _M level fluorescence from a wild-type barley, a Chl b-less mutant barley, and a maize leaf was determined from 430 to 685 nm at 10 nm intervals using pulse amplitude-modulated (PAM) fluorimetry. Variable wavelengths of the pulsed excitation light were achieved by passing the broadband emission of a Xe flash lamp through a birefringent tunable optical filter. For the three leaf types, spectra of F _V/F _M (=(F _M − F ₀)/F _M) have been derived: within each of the three spectra of F _V/F _M, statistically meaningful variations were detected. Also, at distinct wavelength regions, the F _V/F _M differed significantly between leaf types. From spectra of F _V/F _M, excitation spectra of PS I and PS II fluorescence were calculated using a model that considers PS I fluorescence to be constant but variable PS II fluorescence. The photosystem spectra suggest that LHC II absorption results in high values of F _V/F _M between 470 and 490 nm in the two wild-type leaves but the absence of LHC II in the Chl b-less mutant barley leaf decreases the F _V/F _M at these wavelengths. All three leaves exhibited low values of F _V/F _M around 520 nm which was tentatively ascribed to light absorption by PS I-associated carotenoids. In the 550–650 nm region, the F _V/F _M in the maize leaf was lower than in the barley wild-type leaf which is explained with higher light absorption by PS I in maize, which is a NADP-ME C₄ species, than in barley, a C₃ species. Finally, low values of F _V/F _M at 685 in maize leaf and in the Chl b-less mutant barley leaf are in agreement with preferential PS I absorption at this wavelength. The potential use of spectra of the F _V/F _M ratio to derive information on spectral absorption properties of PS I and PS II is discussed.     

毛竹对大气臭氧胁迫的生理响应变化

以毛竹1年生盆栽苗为材料,运用开顶式气室(OTCs)模拟环境背景大气O3浓度(AA,40～45 nL·L-1)和高O3浓度(EO,92～106 nL·L-1)情景,分析毛竹叶片光合生理、脂质过氧化及抗氧化酶等主要生理指标的变化,为气候变化背景下的竹林培育应对策略提供理论依据.结果显示:(1)EO较AA在同一处理时间的毛竹叶片O3通量均显著升高,且二处理的叶片O3通量均随着处理时间的延长呈升高趋势.(2) EO较AA的光合速率(Pn)、气孔导度(Gs)和可溶性糖含量均显著下降,且叶片叶绿素(ChD含量、胞间CO2浓度(Ci)显著下降的时间点分别出现在EO处理的60 d和92 d,可溶性蛋白在处理60 d后显著升高;随处理时间的延长,EO的叶片Pn、Ci、Chl含量均呈下降趋势,可溶性糖和可溶性蛋白含量呈先升高后降低的趋势;Pn下降由气孔限制因素引起.(3)超氧自由基(O2)含量、丙二醛(MDA)含量、相对电导率分别在处理29 d、60 d、60 d后均显著升高,且随着处理时间的延长呈升高趋势.(4)超氧化物歧化酶(SOD)活性在高浓度O3处理60 d时显著升高,后显著下降,而POD活性均显著升高,且SOD和POD活性均随着处理时间呈先升高后降低的趋势.研究表明,毛竹对大气高O3胁迫存在着短时间的主动生理生化适应,但长期高O3胁迫会对毛竹造成严重的过氧化伤害,从而影响毛竹的正常生长.     

Rehydration of Sugar Beet Plants after Water Stress: Effect of Cytokinins

The possibility to improve the recovery of sugar beet plants after water stress by application of synthetic cytokinins N⁶-benzyladenine (BA) or N⁶-(m-hydroxybenzyl)adenosine (HBA) was tested. Relative water content (RWC), net photosynthetic rate (P_N), transpiration rate (E), stomatal conductance (g_s), chlorophyll (Chl) a and Chl b contents, and photosystem 2 efficiency characterized by variable to maximal fluorescence ratio (F_v/F_m) were measured in control plants, in water-stressed plants, and after rehydration (4, 8, 24, and 48 h). Water stress markedly decreased parameters of gas exchange, but they started to recover soon after irrigation. Application of BA or HBA to the substrate or sprayed on leaves only slightly stimulated recovery of P_N, E, and g_s in rehydrated plants, especially during the first phases of recovery. Chl contents decreased only under severe water stress and F_v/F_m ratio was not significantly affected by water stress applied. Positive effects of BA or HBA application on Chl content and F_v/F_m ratio were mostly not observed.