Phytoplasma [Stolbur-subgroup (Bois Noir-BN)] infection inhibits photosynthetic pigments, ribulose-1,5-bisphosphate carboxylase and photosynthetic activities in field grown grapevine (Vitis vinifera L. cv. Chardonnay) leaves

In this work we have studied the influence of phytoplasma-induced grapevine yellows (yellowing) on some features of the thylakoids from field grown grapevine (Vitis vinifera L. cv. Chardonnay) leaves. Changes in photosynthetic pigments, soluble proteins, ribulose-1,5-bisphosphate carboxylase, photosynthetic activities and thylakoid membrane proteins were investigated. The level of total chlorophyll and carotenoids, on a unit fresh weight basis, showed a progressive decrease in phytoplasma infected leaves. Similar results were also observed for total soluble proteins and ribulose-1,5-bisphosphate carboxylase activity. When various photosynthetic activities were followed in isolated thylakoids, phytoplasma infection caused marked inhibition of whole chain and photosystem (PS) II activity. Smaller inhibition of PSI activity was observed even in severely infected leaves. The artificial exogenous electron donors, DPC and NH₂OH significantly restored the PSII activity in both mild and severely infected leaves. The same results were obtained when F_v/ F_m was evaluated by Chl fluorescence measurements. The marked loss of PS II activity in infected leaves was evidently due to the loss of 33, 28–25, 23, 17 and 10 kDa polypeptides. This conclusion was confirmed by immunological studies showing that the content of the 33 kDa protein of the water-splitting complex was diminished significantly in infected leaves. Phytoplasma infection induced a fast degradation of LHCP II which became visible as yellowish colour in leaves.     

Physiological response of yellow vein mosaic virus-infected bhendi [Abelmoschus esculentus] leaves

The effect of okra yellow vein mosaic virus infection on PS II efficiency and thylakoid membrane protein changes in field-grown bhendi (Abelmoschus esculentus) leaves was studied. The degree of virus infection was determined by means of the ratio of variable to maximum chlorophyll fluorescence (F_v/F_m). Changes in photosynthetic pigments, soluble proteins, nitrate reductase, photosynthetic activities and thylakoid membrane proteins were investigated. Okra yellow vein mosaic virus infection resulted in the reduction of photosynthetic pigments, soluble proteins, ribulose-1, 5-bisphosphate carboxylase activity and nitrate reductase activity. Virus infection caused marked inhibition of PS II activity. The artificial electron donors, diphenyl carbazide and hydroxylamine significantly restored the loss of PS II activity in infected leaves. The marked loss of PS II activity in infected leaves may be due to the loss of 47, 33, 28–25, 17 and 15 kDa polypeptides. It is concluded that the yellow vein mosaic virus infection inactivates the donor side of PS II.     

Effects of Erysiphe pisi on protein profiles and ribulose-1,5-bisphosphate carboxylase content in resistant and susceptible pea (Pisum sativum × Pisum fulvum) plants

Journal of Plant Diseases and Protection - The fate of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was examined in leaves of both resistant and susceptible plants from a Pisum sativum...     

The influence of <Emphasis Type="Italic">Aphanomyces cochlioides</Emphasis> on selected physiological processes in sugar beet leaves and yield parameters

Alterations in some physiological processes in source leaves of sugar beet—such as chlorophyll and carbohydrate concentrations, stomatal conductance, rate of net photosynthesis and transpiration, and activity of the photosynthetic apparatus during root interaction with Aphanomyces cochlioides, were investigated. The influence of time of infection on plant health, yield quality and quantity was also examined. Plants were infected at different times of their growth period: on the sowing day and 4 or 8 weeks after sowing. A variation treatment, with non-pelleted seeds infected on the sowing day, was also analyzed. The experiment showed that development of disease symptoms depends on the time of infection and seed protection. A significant root yield decrease was observed in case of late infection, as compared to the yield of plants infected on the sowing day. The fresh weight of leaves was significantly increased where there was late infection. The infected plants showed a lower content of K⁺, Na⁺ and α-amino-N than did the controls. Infection by A. cochlioides induced chlorophyll degradation mostly in older leaves with the occurrence of natural senescence processes. Chlorophyll fluorescence parameters indicated that the photosynthetic apparatus of younger leaves was more sensitive to pathogen infection, when compared to older ones. The photochemical efficiency of photosystem II was reduced in young leaves mainly due to disturbance of the water-splitting system. In plants grown from non-pelleted seeds a strong impairment of PSII was observed only in those leaves which developed during early pathogen infection. In young leaves of plants infected in the fourth week after sowing, inhibition of the rate of net photosynthesis was correlated with the increase in intercellular CO₂ concentration, indicating some disturbance in the carbon assimilation phase. In mature leaves of late infected plants the reduction of photosynthesis net rate was associated with a decrease of stomatal conductance and an increase of diffusion resistance to CO₂ and H₂O, which was also the cause of the transpiration rate inhibition. When the leaves developed during early infection, an increase of specific leaf weight and accumulation of carbohydrates was observed. In mature leaves of non-protected plants infected on the sowing day, the recovery of all physiological processes was observed together with a diminution of disease symptoms.     

Alteration of photosynthetic performance and source–sink relationships in wheat plants infected by Pyricularia oryzae

Two experiments were carried out to assess the changes associated with photoassimilate production and partitioning in the source–sink relationship of flag leaves and spikes of wheat plants infected with Pyricularia oryzae, the causal agent of blast. Flag leaves and spikes were inoculated at 10 and 20 days after anthesis (daa) with a conidial suspension of P. oryzae. Analysis of chlorophyll a fluorescence using maximal photosystem II quantum efficiency (F_v?F_m), fraction of energy absorbed that is used in photochemistry (YII), quantum yield of non‐regulated energy dissipation (Y(NO)) and quantum yield of regulated energy dissipation (Y(NPQ)), showed an impairment of the photosynthetic performance in both infected flag leaves and spikes, coupled with reduced concentrations of chlorophyll a  + b and carotenoids. Compared to non‐inoculated controls, there was lower capacity for CO₂ fixation by RuBisCO in the infected flag leaves. Similarly, in the infected flag leaves and grains (obtained from infected spikes), there were lower concentrations of soluble sugars, while the hexoses‐to‐sucrose ratio increased in infected flag leaves. Compared to non‐inoculated controls, infected flag leaves showed lower sucrose phosphate synthase (SPS) activity and lower expression of the sucrose synthesis (SuSy) gene, while higher expression and activity of acid invertases also occurred. At the advanced stages of fungal infection, the concentration of starch in grains decreased but remained high for the infected flag leaves. There were reductions in ADP‐glucose pyrophosphorylase activity and the expression of ADP‐glucose pyrophosphorylase genes and a down‐regulation of β‐ and α‐amylase expression at the advanced stages of fungal infection on flag leaves and spikes. In conclusion, the effect of blast on both grain quality and yield can be associated with alterations in both production and partitioning of carbohydrates during the grain filling process.     

干旱-复水条件下氮素对高粱光合特性及抗氧化代谢的影响

采用盆栽试验，以持绿性品系高粱B35和非持绿性高粱品系三尺三为试验材料，设置两个施氮处理（每盆0和6 g尿素），在灌浆期干旱-复水条件下测定光合特性及抗氧化代谢指标。结果表明，氮素显著提升B35和三尺三在干旱胁迫下净光合速率（Pn）、胞间CO₂浓度（Ci）和蒸腾速率（Tr），提高了光系统II（PSII）反应中心活性；干旱胁迫下，施氮显著提高磷酸烯醇式丙酮酸羧化酶（PEPCase）活性，施氮使B35和三尺三PEP羧化酶分别提高了29.17%和25.66%, 而二磷酸核酮糖羧化酶（RuBPCase）活性对氮素不敏感。与三尺三相比，氮素对B35的光合能力的促进作用更加明显。干旱胁迫下氮素显著提升了超氧化物歧化酶（SOD）和过氧化物酶（POD）活性，B35和三尺三的SOD活性分别提高了25.56%和17.07%，POD活性分别提高了48.97%和76.62%。B35抗氧化酶的活性均高于三尺三。同时，氮素降低了B35和三尺三丙二醛（MDA）含量。复水后，氮素显著提升B35和三尺三Pn、Tr和PEP羧化酶活性，Pn升高幅度分别为33.66%和60.01%，Tr升高幅度分别为36.59%和41.57%，PEP羧化酶活性升高幅度分别为23.47%和18.64%，同时显著降低了初始荧光值（Fo），Fo降低幅度分别为18.50%、10.98%。施氮有利于提高复水后的B35和三尺三抗氧化酶活性，降低细胞膜脂过氧化程度。复水后，与三尺三相比，B35的光合特性和抗氧化酶活性较高。两个施氮处理的B35产量均高于三尺三，氮素使B35和三尺三生物产量分别增加9.73%和10.08%，籽粒产量分别增加24.47%和21.79%。氮素调节气孔导度及光系统Ⅱ活性，降低干旱对光合机构的损伤；复水后，氮素通过提高光系统Ⅱ活性，提高光合酶活性，光合性能提升。干旱及复水条件下，施氮提升抗氧化酶活性，减轻膜脂过氧化的损伤。氮素有利于干旱及复水条件下B35和三尺三光合特性及抗氧化酶系统能力的提升。     

Accumulation of Pathogenesis-Related Proteins in the Apoplast of a Susceptible Cultivar of Apple (Malus domestica cv. Elstar) after Infection by Venturia inaequalis and Constitutive Expression of PR Genes in the Resistant Cultivar Remo

Leaves of apple (Malus domestica cv. Elstar) were infected with a cloned isolate of the apple scab Venturia inaequalis. The intercellular washing fluid (IWF) of these plants was collected and the variation in the composition of proteins in the IWF was analysed by SDS-PAGE and two-dimensional gel electrophoresis during and after the infection with V. inaequalis, the causal agent of apple scab. The subsequent analysis of induced proteins by electron spray ionization quadrupole time of flight mass spectroscopy revealed the presence of -1,3-glucanase, chitinase, thaumatin-like protein and a cysteine-like protease in M. domestica leaves infected by V. inaequalis. These results were confirmed by immunoblotting with antibodies against some of these proteins. Moreover, a non-specific lipid transfer protein was identified in uninfected leaves: the amount declined to a non-detectable level within the first week after infection by V. inaequalis. The analysis of the IWF of M. domestica cv. Remo, bearing resistances to apple scab, powdery mildew and fire blight, showed a protein pattern comparable to that of the IWF from V. inaequalis infected leaves from cultivar Elstar indicating the constitutive production at least of some of the pathogenesis-related proteins in the resistant cultivar.     

Inhibition of photosystem II in spinach chloroplasts by trachyloban-19-oic acid

The effect of trachyloban-19-oic acid isolated from Iostephane heterophylla (Asteraceae), was investigated on several photosynthetic activities in spinach thylakoids. The results indicated that this compound inhibited ATP synthesis and uncoupled electron transport, as well as basal and phosphorylating electron flow. Therefore, trachyloban-19-oic acid acts as Hill reaction inhibitor. This compound did not affect photosystem I activity but inhibited uncoupled photosystem II electron flow from H₂O to DCPIP, and has not effect on electron flow from H₂O to SiMo, indicating that the site of inhibition of this compound is at the level of Q_A-Q_B. Chlorophyll a fluorescence measurements confirm the behavior of this diterpene as Q_A-Q_B inhibitor, and in the other hand, this results indicate that a perturbation in the thylakoid membranes at the level of LHC II occurs.     

Phytoplasmas change the source–sink relationship of field-grown sweet cherry by disturbing leaf function

Changes in the function of field-grown sweet cherry leaves infected with phytoplasma were evaluated through the analysis of photosynthesis, respiration, carbohydrates and hormones. Phytoplasmal infection caused witches' broom, small leaves, leaf yellowing and leaf rolling. The photosynthesis of infected leaves was considerably reduced, and they were unable to produce sufficient carbohydrates for their own needs. In contrast, the starch content of infected leaves was significantly increased. These results demonstrate the change in the role of infected leaves from sources to sinks. Further analysis revealed that the photosynthetic decline was related to a significant decrease in photosynthetic pigments and to marked inactivation of photosystem II (PSII). Furthermore, the loss of PSII function was due to a decrease in chlorophyll content, reduction and closure of active reaction centers, and decline in photochemical efficiency.     

Inhibitors of N-linked glycosylation induce systemic acquired resistance in cucumber

Localized treatment of cucumber (Cucumis sativus L. cv. Wisonsin) cotyledons with inhibitors of N-glycosylation such as tunicamycin or amphomycin resulted in systemic acquired resistance in the first leaf to the fungal pathogen Colletotrichum lagenarium. Resistance was maximal as early as 2 days after application and best results were observed when the inhibitor was used at 100 μ . The same treatment also induced salicylic acid accumulation as well as the expression of chitinase and a PR1-like protein. The systemic effect is not caused by the transport of tunicamycin, since tunicamycin was not detected in the leaves. Within 2 h after application tunicamycin inhibited N-glycosylation, but not protein synthesis as indicated by labelling experiments. The amount of large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase decreased after tunicamycin treatment and after pathogen inoculation and the expression of BiP, a protein localized in the endoplasmic reticulum was enhanced. The activation of defense reactions seems to be dependent and sensitive to N-linked glycosylation.     

Effect of some herbicides on CO2 fixation,intermediates pattern,and ruDP-carboxylase and FDPase activities of spinach chloroplasts

We have studied the inhibitory effect of the herbicides phenmediphan, chloroxuron, dinoseb, dichlobenil, dicamba, 2,4-D, 2,4-DB, and 2,4-DP on photosynthetic CO₂ fixation and on the level of intermediates of the CO₂ assimilation cycle by isolated chloroplasts, as well as their in vitro activities on the enzymatic systems ribulose-1,5-diphosphate carboxylase and fructose-1,6-bisphosphatase. Phenmedipham showed the strongest inhibition of CO₂ assimilation, with an I₅₀ of 0.05 μM, followed by chloroxuron and dinoseb, with a 50% inhibition in the range of 0.5–1 μM. A weaker inhibitory effect, with an I₅₀ of 50 μM, is promoted by 2,4-DB, whereas dicamba and 2,4-DP showed this inhibition at 100 μM; dichlobenil and 2,4-D were completely ineffective. In the presence of phenmedipham and chloroxuron, the $\text{trioses-}\text{P}\text{P}\text{-glycerate}$ ratio showed a sharp decrease, which means an inhibition of the P-glycerate reduction step by a low NADPH synthesis; a low ratio is also promoted by 2,4-D, but it may be a consequence of induced collateral metabolic pathways of P-glycerate. Dinoseb showed a 25% inhibition of ribulose-1,5-diphosphate carboxylase activity in the concentration range of 10–100 μM and an I₅₀ of 50 μM of the fructose-1,6-bisphosphatase. Thus these effects could contribute, in addition to the photochemical ones, to an explanation of the dinoseb inhibition of CO₂ assimilation by isolated chloroplasts. The other herbicides tested showed a weak or no effect on these enzyme systems.     

Extracellular ATP functions in alleviating the decrease of PSII photochemistry caused by the infection of Xanthomonas campestris pv. phaseoli

Extracellular ATP (eATP) can function as a signalling molecule to regulate a wide range of cellular processes. This present work investigated the role of eATP in mediating the change of PSII (photosystem II) photochemistry of the tissues of Phaseolus vulgaris leaves infected with Xanthomonas campestris pv. phaseoli (Xcp). Infection of the leaves with Xcp caused a significant decrease in the the PSII maximal photochemical efficiency (F_v/F_m), the maximum quantum efficiency of PSII photochemistry at illumination (F_v′/F_m′), the PSII operating efficiency (Φ_PSII), the rate of non‐cyclic electron transport through PSII (ETR), photochemical quenching (qP), and eATP level in the tissues of the infected leaves. At the same time, the levels of non‐photochemical quenching (qN) and the quantum yield of regulated energy dissipation of PSII (Y(NPQ)) were significantly increased. Application of exogenous ATP at 0·2 mm to uninfected leaves had no significant effect on any of the chlorophyll fluorescence parameters being measured. However, in the tissues of infected leaves, the application of exogenous ATP alleviated the decreases of the F_v/F_m, F_v′/F_m′, Φ_PSII, ETR, qP and eATP level, and also abolished the increases of qN and Y(NPQ). These results suggest that the change of PSII photochemistry by pathogen infection could be mediated by eATP.     

Structure-activity relationships of triazinone herbicides on resistant weeds and resistant Chlamydomonas reinhardtii

Weeds resistant to the s-triazine herbicide atrazine also show resistance to the triazinone herbicide metribuzin. However, with highly lipophilic triazinones, thylakoids isolated from atrazine-resistant Amaranthus retroflexus (mutation at position Ser₂₆₄ of the photosystem II D-1 reaction centre protein) in general show a higher pI₅₀ value in photosystem II electron transport than those from the wild type (i.e. negative cross-resistance; ‘supersensitivity’). A quantitative structure–activity relationship (QSAR) can be established, wherein the lipophilicity of the compound plays a major role. In in-vivo experiments, it was found that the triazinone DRW2698 killed resistant Amaranthus retroflexus and Chenopodium album whereas the wild type was almost unaffected. Triazinones were further investigated in five different mutants of Chlamydomonas rheinhardtii (mutations in the D-1 protein at positions Ser₂₆₄, Ala₂₅₁, Leu₂₇₅, Phe₂₅₅, and Val²¹⁹). Inhibitory activity of all triazinones was generally enhanced in the Phe₂₅₅ mutant but decreased in the Val₂₁₉ mutant. In the other mutants, biological activity was decreased when position 3 of the triazinone was substituted by CH₃, OCH₃, SCH₃, NHCH₃ or N(CH₃)₂. However, negative cross-resistance was again observed when this position was occupied by free thiol. It is therefore suggested that these two groups of triazinones orient themselves differently within the herbicide binding niche of the photosystem II D-1 protein.     

Role of Cylindrosporium-type conidia of Mycosphaerella pomi (Pass.) Lindau: cause of Brooks fruit spot of apple, as an infection source in apple orchards

Ascospores of Mycosphaerella pomi, the pathogen of Brooks fruit spot of apple, were produced in pseudothecia on previously infected and overwintered apple leaves from late April through early August in Aomori Prefecture, Japan. In June 2003, the ascospores were germinating and producing Cylindrosporium-type conidia on apple fruit and leaf surfaces in an orchard. After ascospores were sprayed on apple leaves, Cylindrosporium-type conidia developed on the leaf surfaces. Such Cylindrosporium-type conidia caused typical symptoms of Brooks fruit spot on apple trees after inoculations. These results suggested that the Cylindrosporium-type conidia also serve as an infection source, in addition to the ascospores, for Brooks fruit spot in apple orchards.     

Differential influence of herbicide treatments on activity and kinetic parameters of C4 photosynthetic enzymes from Zea mays

The recommended field dose of rimsulfuron, imazethapyr, alachlor, atrazine or fluometuron differentially reduced shoot fresh and dry weight of 10-day-old maize seedlings as well as leaf protein content during the following 12 days. These reductions seemed consistent during the whole period with fluometuron, atrazine and alachlor but appeared to be nullified by the 5th day of treatment with rimsulfuron and imazethapyr. On the other hand, all herbicides mostly provoked significant inhibitions in specific activities of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), malate dehydrogenase (MDH, EC 1.1.1.82), pyruvate phosphate dikinase (PPDK, EC 2.7.9.1) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) in leaves during the first 2 days. Thereafter, the inhibition was recovered in samples treated with rimsulfuron and imazethapyr, leveled off with alachlor but consistently augmented with atrazine and fluometuron. The kinetic characterization showed that rimsulfuron or imazethapyr unchanged V_max of all enzymes in vitro, however, V_max of PEPC, PPDK and Rubisco were decreased in vivo. Nevertheless, atrazine or fluometuron substantially reduced V_max of all enzymes while alachlor showed a reduction in this value of PEPC, MDH and Rubisco. Thus atrazine, fluometuron and, to a lower extent, alachlor reduced concentrations of all enzymes as well as rimsulfuron and imazethapyr for only Rubisco. On the contrary, K_m values of all enzymes were progressively increased by all herbicides indicating that the different herbicides altered the structural integrity of all enzymes. These findings conclude that rimsulfuron or imazethapyr competitively inhibited MDH but revealed mixed inhibition to PEPC, PPDK and Rubisco. Atrazine or fluometuron revealed mixed inhibitions to all enzymes whereas alachlor seemed to be either a competitive inhibitor to PPDK or a mixed inhibitor to PEPC, MDH and Rubisco.     

Proteomic analysis of a compatible interaction between Pisum sativum (pea) and the downy mildew pathogen Peronospora viciae

A proteomic approach was used to identify host proteins altering in abundance during Peronospora viciae infection of a susceptible cultivar of pea (Pisum sativum cv. Livioletta). Proteins were extracted from fully developed pea leaflets at 4 days post-inoculation, before visible symptoms were apparent. Cytoplasmic proteins and membrane- and nucleic acid-associated proteins from infected and control leaves were examined using two-dimensional difference gel electrophoresis. The majority of proteins had a similar abundance in control and infected leaves; however, several proteins were altered in abundance and twelve were found to have increased significantly in the latter. These proteins were selected for either matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry or electro-spray ionisation quadrupole time-of-flight tandem mass spectrometry analysis following trypsin digestion, with sequence identity being assigned to eight of the proteins. These included the ABR17 stress-response protein, the pathogen-induced PI176 protein, three photosynthetic proteins, a glycine-rich RNA binding protein and two glyceraldehyde 3-phosphate dehydrogenases (cytosolic and chloroplastic) which can be induced by a range of abiotic and biotic stresses in many plant species. The possible roles of these proteins in the response of the pea plant during P. viciae infection are discussed. This study represents the first proteomic analysis of downy mildew infection of pea leaves, and provides the basis for further work to elucidate molecular mechanisms of compatibility in P. viciae infections.     

Long-term effects of herbicides on the photosynthetic apparatus II. Investigations on bentazone inhibition

Non-cyclic electron transport by isolated chloroplasts from the alga Bumilleriopsis is inhibited by bentazone, while the activity of photosystem I is not affected. Chloroplast material isolated from cells grown for several days in the presence of bentazone also shows inhibition of photosystem II activity, similar to the decreased cellular photosynthesis. A slow partial recovery is possible, whereas it is fast and complete after exposure of cells or isolated chloroplasts to the herbicide for some hours only. Irreversibility of longterm inhibition could be a metabolic process which brings about the binding of bentazone (or an active derivative) to the thylakoids in vivo, but not in vitro.     

Effects of cherry leaf spot on photosynthesis in tart cherry 'Montmorency' foliage

Results described here span a total of three field seasons and quantitatively depict the effects of an economically important fungal pathogen (Blumeriella jaapii) on tart cherry (Prunus cerasus 'Montmorency') leaf physiology. For the first time, leaf photosynthesis, stomatal conductance (g(s)), maximum ribulose-1,5-bisphosphate carboxylation rate (V(cmax)), and maximum electron transport (J(max)) were measured as functions of visible cherry leaf spot disease (CLS) severity. Defined as the proportion of chlorotic and necrotic tissue per leaf, CLS severity was estimated from leaves of mature 'Montmorency' trees in 2007, 2008, and 2009. Briefly, as visible disease severity increased, all of the leaf-level physiological parameters decreased significantly (P < 0.01) and disproportionately. Thus, the effects of visible symptoms on leaf photosynthetic metabolic function encroached upon asymptomatic tissue as well. Impairment of photosynthetic metabolism in 'Montmorency' tart cherry leaves due to CLS appears to be mediated through disproportionately large perturbations in g(s), V(cmax), and J(max). These findings offer a new perspective on the amount of damage that this serious disease can inflict.     

Photosynthetic and antioxidant responses of Mexican lime (Citrus aurantifolia) plants to Citrus tristeza virus infection

The effect of Citrus tristeza virus (CTV) infection on photosynthetic activity and antioxidant metabolism was analysed in plants of the highly susceptible citrus genotype Mexican lime (Citrus aurantifolia). Two virus isolates differing in their virulence (the severe T318 and the mild T385) were used in the experiments. CTV infection caused a reduction in photosynthetic capacity in infected plants. This limitation was mainly due to a reduction in the carboxylative efficiency whereas the limitation of CO₂ diffusion through the stoma had lower impact. The virus did not damage the antennae and did not reduce the efficiency of light harvesting complexes. Oxidative damage occurred in infected plants, as evidenced by the increase in malondialdehyde levels. Indeed, CTV infection caused an increase in ascorbate peroxidase activity in new shoots developed in infected plants during the 2 years of the experiment. Data suggest that the H₂O₂ removal machinery was not damaged as a result of stress but the defence mechanism was overwhelmed with time due to the continuing pressure of biotic stress.     

干旱胁迫对3种苹果砧木叶片光合、叶绿体超微结构和抗氧化系统的影响

以新疆野苹果（Malus sieversii Roem.（XJ））、垂丝海棠（Malus halliana Koehne（CS））和山定子（Malus baccata Borkh.（SDZ））3种一年生苹果砧木实生苗为材料，采用盆栽控水的方法设置正常供水（75%~80% RWC）和干旱胁迫（45%~50% RWC）2种水分处理，研究干旱胁迫下叶片光合特性、叶绿体超微结构、丙二醛（MDA）含量、超氧阴离子（O^—[KG-1][JX*3]·[JX-*3]₂）产生速率以及抗氧化酶活性的变化规律，并利用主成分分析（PCA）对3种砧木进行抗旱性综合评价。结果表明：干旱胁迫抑制了3种苹果砧木的光合作用，但干旱胁迫下CS的净光合速率（Pn）、气孔导度（Gs）、蒸腾速率（Tr）、PSII最大光能转化效率（Fv/Fm）、潜在光化学活性（Fv/Fo）及光化学猝灭系数（qP）的降幅均显著小于其他两种砧木，胞间CO₂浓度（Ci）和非光化学猝灭系数（NPQ）的增幅显著高于其他两种砧木；干旱胁迫下，3种砧木超微结构受到不同程度的伤害，其中CS叶片的超微结构损伤较小，能较好地保持细胞结构的完整性；干旱胁迫下3种砧木的SOD和CAT活性先升高后降低，POD活性逐渐增加至21 d趋于稳定，MDA含量和O^—[KG-1][JX*3]·[JX-*3]₂产生速率持续升高。PCA结果显示：2个主成分的方差贡献率达到98.502%，干旱胁迫下CS的综合得分最高。因此，干旱胁迫下，垂丝海棠能保持叶绿体结构的完整性，激活抗氧化酶系统，清除氧化产物而保持较高的光合能力。