Photooxidative damage to the photosynthetic apparatus during chilling

Short periods of chilling in the presence of light (up to 6 h: 1°C; 270 W/m²) decreased the subsequent apparent photosynthesis and chlorophyll fluorescence of leaf discs of Cucumis sativus L. cv. Kleine Groene Scherpe. The extent of the injury depended on the duration of the chilling pretreatment. After 6 h the subsequent apparent photosynthesis even reached a negative value, and it increased only slightly during the next 2 1/2 h. The decrease of apparent photosynthesis was not a consequence of increased dark respiration but was of photooxidative origin since the presence of both light and oxygen was required. Preincubation in the light for 2 h at 20°C sensitized leaf discs to subsequent photooxidation during chilling. Prompt and delayed chlorophyll fluorescence decreased simultaneously after chilling and light treatment. The corresponding decrease of photosynthesis and chlorophyll fluorescence is discussed in relation to primary photooxidative damage to the photosystems in the chloroplast membrane.     

Effect of cinnamate on nitrate reductase activity in isolated cucumber cotyledons

The effect of cinnamic acid on in vivo nitrate reductase activity and protein content in cucumber cotyledons was studied. Cinnamate increased in vivo nitrate reductase activity and also the total protein content at lower concentrations (0.01–0.1 mM). Higher concentration, however, proved inhibitory. The effect of cinnamate on nitrate reductase activity has been discussed.     

Changes of polyamines and ethylene in cucumber seedlings in response to chilling stress

Cucumber ( Cucumis sativus L. cv. Victory) seedlings were exposed to chilling at 5°C and endogenous levels of polyamines and 1-aminocyclopropane-1-carboxylic acid (ACC) were measured after chilling and after warming at 20°C. The level of spermidine was higher in the chilled seedlings than in the non-chilled seedlings. Treatment with a plant bioregulator, (2RS,3RS)-1-(4-cholorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pentan-3-ol (paclobutrazol), reduced the chilling injury and the levels of spermidine in the chilled seedlings. The levels of ACC and production of ethylene showed sharp increases after warming following exposure to chilling. These increases were suppressed by the application of aminooxyacetic acid (AOA). However, AOA treatment did not reduce chilling injury or affect the levels of polyamines in the tissue. These data indicate that the increase in ACC and ethylene is a response of the tissue to the chilling exposure and is not a cause of the injury. The data also suggest that the syntheses of polyamines and ethylene are not competitive with each other even under chilling stress conditions.     

Nitrate reductase in cotyledons of cucumber seedlings as affected by nitrate, phytochrome and calcium

Regulation by the active form of phytochrome (P_FR) and the effect of Ca²⁺ was examined with nitrate reductase (NR) in etiolated cucumber ( Cucumis sativus cv. Beilpuig). Nitrate reductase activity (NRA) was studied in excised cotyledons of cucumber seedlings grown in distilled water and in darkness for seven days at 24 ± 0.5°C. All experiments were performed in the dark and a dim green safelight was used during analyses. In etiolated cucumber cotyledons NRA was induced by nitrate and a brief irradiation (15 min) with red light (R) resulted in 62% increase in NRA. This effect was nullified when R was followed immediately by a brief (5 min) far-red light (FR). NRA also showed a semidian (12 h) rhythmicity. Both P_FR, and nitrate effects were age dependent. Calcium seemed to be involved since the phytochrome effect was only observed when calcium was supplied in the external solution. The effect of R on NRA depended on the period of calcium nitrate incubation. An external supply of calcium ionophore mimicked the effect of R and, if supplied to R-irradiated cotyledons, produced a higher NR level than that caused by R alone. This suggested that intracellular free calcium was involved.     

青海芨芨草属一新变种

与原变种的区别：花序紧缩；颖宽披针形，顶端较钝；基盘钝圆，长0.5mm；外稃顶端没有较长的毛。     

Chloride inhibition of spinach nitrate reductase

Initial rate studies of spinach (Spinacia oleracea L.) nitrate reductase showed that NADH:nitrate reductase activity was ionic strength dependent with elevated ionic concentration resulting in inhibition. In contrast, NADH:ferricyanide reductase was markedly less ionic strength dependent. At pH 7.0, NADH:nitrate reductase activity exhibited changes in the V_max and K_m for NO₃⁻ yielding V_max values of 6.1 and 4.1 micromoles NADH per minute per nanomoles heme and K_m values of 13 and 18 micromolar at ionic strengths of 50 and 200 millimolar, respectively. Control experiments in phosphate buffer (5 millimolar) yielded a single K_m of 93 micromolar. Chloride ions decreased both NADH:nitrate reductase and reduced methyl viologen:nitrate reductase activities, suggesting involvement of the Mo center. Chloride was determined to act as a linear, mixed-type inhibitor with a K_i of 15 millimolar for binding to the native enzyme and 176 millimolar for binding to the enzyme-NO₃⁻ complex. Binding of Cl⁻ to the enzyme-NO₃⁻ complex resulted in an inactive E-S-I complex. Electron paramagnetic resonance spectra showed that chloride altered the observed Mo(V) lineshape, confirming Mo as the site of interaction of chloride with nitrate reductase.     

黄瓜花粉母细胞减数分裂过程中微核仁现象的研究

在黄瓜(Cucumis sativus L．)高代自交系花粉母细胞减数分裂过程中,首次观察到了微核仁现象。在被观察的430个花粉母细胞(PMCs)中,有279个含有微核仁,占64．9％。微核仁在PMCs中数目变化范围在1～7之间。从细线期到四分孢子期都可观察到微核仁,其数量和大小在分裂的不同时期有所差异,但在第一次和第二次减数分裂时期内的变化趋势相似。在微核仁的周期性变化过程中,染色体的行为正常,并没有发现染色体畸变或染色体桥等现象。将此高代自交材料与3个商品种对照,分别在不同的季节种植在不同的地点,证明微核仁仅在此高代自交系上发生,且不受栽培季节和环境的影响。     

A double enhancement of greening in the cotyledons of cucumber seedlings by excision and red light

Cotyledons excised without the hypocotyl hook from 6-day-old etiolated cucumber ( Cucumis sativus L. var. Elem) seedlings accumulated a significantly higher amount of chlorophyll than cotyledons excised with hooks or intact cotyledons. It was found that maximum ehancement of greening was achieved after 2 h of dark incubation following excision. Pretreatments with red light effected an additive rise in chlorophyll level in subsequent white light after a dark incubation, suggesting that the effects of excision and phytochrome on greening act independently. Etiolated seedlings were variously dissected before greening and it was found that enhancement occurred only when cotyledons were excised at the level of the hypocotyl hook or above it. Similar results were obtained when the dissected plants were pre-treated with red light.     

Kinetics of substrate inhibition of periplasmic nitrate reductase

Periplasmic nitrate reductase catalyzes the reduction of nitrate into nitrite using a mononuclear molybdenum cofactor that has nearly the same structure in all enzymes of the DMSO reductase family. In previous electrochemical investigations, we found that the enzyme exists in several inactive states, some of which may have been previously isolated and mistaken for catalytic intermediates. In particular, the enzyme slowly and reversibly inactivates when exposed to high concentrations of nitrate. Here, we study the kinetics of substrate inhibition and its dependence on electrode potential and substrate concentration to learn about the properties of the active and inactive forms of the enzyme. We conclude that the substrate-inhibited enzyme never significantly accumulates in the EPR-active Mo(+ V) state. This conclusion is relevant to spectroscopic investigations where attempts are made to trap a Mo(+ V) catalytic intermediate using high concentrations of nitrate.     

Phytochrome-mediated development of glycine oxidation by mitochondria in cucumber cotyledons

The development of glycine oxidation activity in mitochondria in etiolated cucumber ( Cucumis sativus L., cv. Shinfushinari) cotyledons is regulated by phytochrome. This conclusion is based on two lines of evidence. 1. The oxidation activity was increased by continuous illumination of far-red light. 2. It was also increased by brief red light pulses, the effect of which was reversed by brief far-red light pulses. The light-induced increase in glycine oxidation and in glycine decarboxylase (EC 2.1.2.10) activity in the cotyledons was inhibited by cycloheximide, but not by chloramphenicol. While glycine oxidation activity continued to increase during light-illumination for 20 h, malate oxidation activity increased for 6 to 8 h after illumination and decreased thereafter. This transient increase in the activity of malate oxidation was also induced by red light pulses and the effect of the red light was reversed by far-red light pulses.     

Induction of chilling tolerance in cucumber (Cucumis sativus) seedlings by endogenous and applied ethanol

Five-day-old etiolated cucumber ( Cucumis sativus L.) seedlings cv. Marketmore held at 2°C for 72 h developed chilling injury, resulting in desiccation and collapse of the hypocotyl tissue and eventual plant death. Hypoxia-induced accumulation of ethanol and acetaldehyde led to tolerance of subsequent chilling, as evidenced by continued hypocotyl growth and freedom from injury. Attenuated accumulation of volatiles by applied bisulfite reduced the development of hypoxia-induced chilling tolerance in seedlings. In seedlings held in normoxia cold tolerance was induced by applied ethanol vapors, whereas acetaldehyde had a marginal effect, suggesting that hypoxia-induced cold tolerance may arise from the accumulation and activity of ethanol. Cold tolerance was also induced by exposure of seedlings to volatile anesthetics including n -propanol, n -butanol, chloroform and halothane, suggesting that ethanol activity may result from fluidization of membrane lipids. This view is consistent with results which showed that ethanol activity was not associated with lipid metabolism. However, development of cold tolerance in ethanol-enriched tissues was time dependent, indicating that ethanol activity probably also entails biosynthetic event(s).     

Relationship between potassium fertilisation and nitrate assimilation in leaves and fruits of cucumber (Cucumis sativus) plants

The effect of application of different potassium rates on some parameters of nitrate metabolism and yield in cucumber plants (Cucumis sativus) was studied. All plants were grown under controlled conditions in an experimental greenhouse. The treatments consisted of applications of K⁺ at three rates in the form of K₂SO₄ (Kl: 0.075 mg ml^?1, K2: 0.15 mg ml^?1, and K3: 0.30 mg ml^?1). The results showed a positive effect of higher K⁺ fertilisation (0.30 mg ml^?1) on uptake, translocation and reduction of NO₃^? in leaves compared with the lowest K⁺ rate. In addition, the higher K⁺ rates strengthened the translocation of organic nitrogenous compounds (amino acids) towards the fruit, thereby perhaps also enhancing the maximal commercial yield. In conclusion, for improved cucumber cultivation under greenhouse conditions, 0.15 mg ml^?1 of K⁺ gave maximal yield, while the application of 0.30 mg ml^?1 increased the metabolism and efficient utilisation of NO₃^?.     

Modifications of the activity of nitrate reductase from cucumber roots

The regulatory properties of NADH-dependent nitrate reductase (NR) in desalted root extracts from hydroponically grown cucumber (Cucumis sativus L.) seedlings were examined. The lowest activity of NR was detected in extracts incubated with Mg²⁺ and ATP. An inhibitory effect of Mg-ATP was cancelled in the presence of staurosporine (the protein kinase inhibitor) and completely reversed after addition of ethylenediaminetetraacetate (EDTA) as well as AMP into reaction mixture. Reactivation of enzyme due to AMP presence, contrary to the chelator-dependent NR activation, was sensitive to microcystin LR (the protein phosphatase inhibitor). Above results indicated that the nitrate reductase in cucumber roots was regulated through reversible phosphorylation of enzyme protein. A drop in the activity of NR was also observed after incubation of enzyme at low pH. At low pH, the presence of ATP alone in the incubation medium was sufficient to inactivate NR, indicating that H⁺ can substitute the Mg²⁺ in formation of an inactive complex of enzyme. ATP-dependent inactivation of NR at low pH was prevented by staurosporine and reversed by AMP. However, AMP action was not altered by microcystin LR suggesting that in low pH the nucleotide induced reactivation of NR is not limited to the protein phosphorylation.     

Homeostasis in cucumber plants during low temperature stress

Shoots, roots and whole plants of Cucumis sativus L. cv. Inspektowy Warszawski were subjected to different temperatures. The magnitude and direction of alterations of growth depended on the organ that was submitted to stress conditions. Changes in soluble protein level in leaves and in roots were inversely correlated with changes of total organic nitrogen level in both organs. In spite of differentiated temperature conditions between plant organs, the allometric coefficients and the ratio between maintenance respiration in shoot and in roots remained constant. Such constancy may be treated as an indicator of homeostasis of the whole-plant system. The results point to the importance of both shoot and roots in the response of the plant to temperature stress.     

Purification of NADPH-specific indole-3-acetaldehyde reductase from Cucumis sativus by two-dimensional native polyacrylamide gel electrophoresis

NADPH-specific indole-3-acetaldehyde (IAAId) reductase from cucumber ( Cucumis sativus L. 相似文献   

Prior temperature exposure affects subsequent chilling sensitivity

The chilling sensitivity of small discs or segments of tissue excised from chillingsensitive species was significantly altered by prior temperature exposure subsequent to holding the tissue at chilling temperatures as measured by a number of physiological processes sensitive to chilling. This temperature conditioning was reversible by an additional temperature exposure before chilling, and mature-green and red-ripe tomato tissue exhibit similar chilling sensitivities. Exposing pericarp discs excised from tomato fruit (Lycopersicon esculentum Mill. cv. Castelmart), a chilling-sensitive species, to temperatures from 0 to 37°C for 6 h before chilling the discs at 2.5°C for 4 days significantly altered the rate of ion leakage from the discs, but had no effect on the rate of ion leakage before chilling and only a minimal effect on discs held at a non-chilling temperature of 12°C. Exposing chillingsensitive tissue to temperatures below that required to induce heat-shock proteins but above 20°C significantly increased chilling sensitivity as compared to tissue exposed to temperatures between 10 and 20°C. Rates of ion leakage after 4 days of chilling at 2.5°C were higher from fruit and vegetative tissue of chilling-sensitive species (Cucumis sativus L. cv. Poinsett 76, and Cucurbita pepo L. cv. Young Beauty) that were previously exposed for 6 h to 32°C than from similar tissue exposed to 12°C. Exposure to 32 and 12°C had no effect on the rate of ion leakage from fruit tissue of chilling tolerant species (Malus domestica Borkh. cv. Golden Delicious, Pyrus communis L. cv. Bartlett). Ethylene and CO₂ production were higher and lycopene synthesis was lower in chilled tomato pericarp discs that were previously exposed for 6 h to 32°C than the values from tissue exposed to 12°C for 6 h before chilling. Increased chilling sensitivity induced by a 6 h exposure to 32°C could be reversed by subsequent exposure to 12°C for 6 h.     

Immunohistochemical localization of the stress-related anionic peroxidase in germinating cucumber seeds

The distribution of the stress-related anionic peroxidase in the course of cucumber (Cucumis sativus L.) seed germination was determined by tissue printing and immunoblotting. Of the three molecular forms of cucumber stress-related anionic peroxidase, the form PRX 1 was temporally accumulated in developing seedlings. Up to 6 d of germination PRX 1 was localized mainly in roots. As germination progressed, the immunoreactive PRX 1 signal was found in the transition zone between roots and stem, as well as in the lower epidermis of expanding cotyledons at the midrib. This revised version was published online in July 2006 with corrections to the Cover Date.     

黄瓜花芽启始分化的形态解剖研究

苗龄 6d的黄瓜幼苗 ,在第一节位叶腋处花芽原基开始启动分化。花芽分化时间早、速度快、节位低、同步性好。诱导黄瓜开花的因素可能不是光和夜低温 ,其开花特性类似于自主开花植物     

Hydroxylamine inhibition of the nitrate reductase complex from Amaranthus

Nitrate reductase from Amaranthus viridis is similar to nitrate reductase from other plant sources. NH₂OH inhibits nitrate reduction from NADH by the nitrate reductase complex, but it does not inhibit either the NADH-dehydrogenase activity or nitrate reduction from reduced flavin mononucleotides. The inhibition observed was non-competitive with nitrate when the enzyme was pre-incubated with NH₂OH and NADH, and competitive with nitrate without pre-incubation. The K_i values for NH₂OH were 5 μM and 30 μM with or without pre-incubation respectively.     

Uptake and translocation of calcium in cucumber

Uptake and translocation of Ca²⁺(⁴⁵Ca) were compared with water translocation in 12-day old intact plants and excised roots of cucumber ( Cucumis sativus L. var. Cilla), which had been cultivated in nutrient solution. No immediate reduction of Ca²⁺ uptake was found when water translocation was reduced by excision of the shoot. In the presence of 2,4-dinitrophenol Ca²⁺ translocation was reduced in the intact plants while water translocation was unchanged. It is suggested that regulation of Ca²⁺ uptake is primarily achieved in the root. The DNP-sensitive mechanism of Ca²⁺ uptake was associated with the root and probably represented transport through the endodermis into the stele.