Inhibition of Pigment Biosynthesis in Cucumber Cotyledons by Isoxaflutole

Isoxaflutole [5-cyclopropyl-4-(2-methylsulphonyl-4-trifluromethylbenzoyl)isoxazole] is a new preemergence herbicide for broad-spectrum weed control in maize. The effect of isoxaflutole on chlorophyll (Chl) and carotenoid (Car) biosynthesis was investigated in cucumber (Cucumis sativus L.) cotyledons. Etiolated tissue was incubated with 5 mM isoxaflutole for 24 h and irradiated (60 mol m^-2 s^-1). The irradiation for 3 h did not reduce Chl a, Chl b, and Car contents, but after a 28-h irradiation the contents of Chl a and Car decreased by 35 and 15 %, respectively, and the content of Chl b increased by 24 %. Increasing the concentration of isoxaflutole beyond 5 mM resulted in reduction of Chl a (71 %), Chl b (20 %), and Car (31 %) contents. Similarly, increase in irradiance from 60 to 180 mol m^-2 s^-1 resulted in larger reduction of Chl and Car contents. Exogenously supplied 5-aminolevulinic acid did not reverse the isoxaflutole-inhibited Chl synthesis, whereas an exogenously supplied homogentisic acid lactone reversed the inhibition of pigment synthesis due to isoxaflutole.     

Analysis of UDP-Sugar Content in Cucumber Cotyledons in Relation to Growth Rate

UDP-sugar content in cucumber cotyledons grown with or withoutadded zeatin was measured by a newly established method usinghigh performance liquid chromatography and gas-liquid chromatography.With zeatin the growth rate was twice that of the control, andUDP-sugar content was 1.5-fold that of the control. (Received June 6, 1984; Accepted September 27, 1984)     

Delay of Senescence of Detached Cucumber Cotyledons by Triadimefon

Changes in contents of reactive oxygen species (O₂ ⁻ and H₂O₂) and non-enzymatic antioxidants, activities of antioxidant enzymes and lipid peroxidation were investigated during senescence of detached cucumber cotyledons dipped in water (control) and 20 mg dm⁻³ triadimefon (TDM). O₂ ⁻ and H₂O₂ accumulation and lipid peroxidation were observed during senescence of cucumber cotyledons, which coincided with a drop in the contents of carotenoids (Car) and ascorbic acid (AsA), and the activities of superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), and an increase in the activity of peroxidase (POD). However, TDM could significantly inhibit the accumulation of O₂ ⁻ and H₂O₂, and lipid peroxidation by preventing the decrease of CAT, APX, Car and AsA and the increase of POD, while TDM had little effect on SOD activity during the senescence. Therefore we can draw a conclusion that TDM protects the membrane system and retards the senescence of detached cucumber cotyledons. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Ammonium and Potassium Ions on Some Physiological and Biochemical Processes of Excised Cucumber Cotyledons

Incubation of excised cucumber cotyledons (Cucumis sativus L.) with NH₄Cl solutions exceeding 0.001 M inhibited their greening, fresh weight increases, and incorporation of ¹⁴C-leucine into insoluble N compounds. The absorption of ¹⁴C-leucine during incubation and retention of moisture by the excised cotyledons after incubation were greatly diminished by the NH₄Cl treatments. Treatment with KCl solutions of the same concentrations as the NH₄Cl stimulated the greening, fresh weight increases, and the absorption and incorporation of ¹⁴C-leucine. Desiccation of cotyledons stored at 5°C for 10 days was inhibited by dilute KCl solutions. The toxicity of NH₄Cl was alleviated by KCl solutions at low concentrations.     

Studies on Phospholipid-synthesizing Enzyme Activities during the Growth of Etiolated Cucumber Cotyledons

The enzymatic incorporation of sn-glycerol 3-phosphate into lipid by extracts of cucumber (Cucumis sativus) cotyledons showed an absolute requirement for ATP (saturation 2 mM). The incorporation was stimulated 4-fold by 0.2 mM oleate. Ethyldiaminetetraacetate stimulated the incorporation at concentrations below 1 mM and inhibited at higher concentrations. Mg²⁺ did not affect the reaction. Triton X-100 and Cutscum inhibited the reaction, while a third detergent, Span 80, was stimulatory. p-Mercuribenzoate was inhibitory. The enzymatic reaction has a pH optimum in the range of 8.8 to 9.6. The Michaelis constant was 112 μM for sn-glycerol 3-phosphate. The major amount of product was phosphatidic acid, the remainder was diacylglycerol, monoacylglycerol, and an unknown phospholipid.     

Stimulation by Ethylene of Chlorophyll Biosynthesis in Dark-grown Cucumber Cotyledons

Five-day-old etiolated cucumber (Cucumis sativus L. var. Alpha Green) cotyledons produced more chlorophyll over a 4-hour illumination period after a prolonged exposure (12 to 72 hours) in the dark to ethylene concentrations ranging from 0.1 to 10 μl/l. Intact seedlings and excised cotyledons responded in the same way to this treatment. This effect does not involve a shortening of the lag phase of chlorophyll accumulation. Exposure of cotyledons to ethylene during the illumination period did not produce the same stimulatory effect on chlorophyll synthesis and, under certain conditions, chlorophyll synthesis was slightly inhibited by exposure to ethylene in the light.     

The Fine Structure of Senescing Cotyledons of Cucumber

ABSTRACT Changes in fine structure have been followed in mesophyll cellsof attached and detached cucumber cotyledons from maturity at16–20 days to an advanced stage of senescence at 50 days.From about 25 days in the attached cotyledons, the chloroplastthylakoids gradually disappear and large osmiophilic globulesaccumulate in the stroma, which itself is reduced in density.At the same time the ribosomes are lost, first those free inthe ground plasm, followed by those attached to the endoplasmicreticulum (E.R.). The E.R. later vesiculates and breaks down.The mitochondria become smaller and ultimately contain few cristae,but they remain morphologically intact until a very late stage.The tonoplast breaks down from 35 days and the plasma membranefrom 40 days, the latter probably resulting in cell death. The pattern of changes in cotyledons excised at 16 days andsubsequently floated or stood in water is similar to that inattached cotyledons except that the chloroplast breakdown ismore rapid and the ribosome loss is less rapid and complete.Breakdown of the tonoplast is delayed to 40 days and the plasmamembrane remains intact. These observations are discussed in relation to possible senescencemechanisms.     

Inter-organ Control of Greening in Etiolated Cucumber Cotyledons

Inter-organ control of greening in etiolated cucumber (Cucumis sativus L. cv. Aonagajibae) cotyledons was investigated. Four- or six-day-old excised or intact etiolated cucumber cotyledons were illuminated under aerobic conditions. Excised cotyledons without hypocotyl hooks produced chlorophyll without a prolonged lag phase and the rate of chlorophyll formation was not depressed if they were illuminated immediately after excision. If the excised cotyledons were incubated in the dark before illumination, chlorophyll accumulation at the end of 6 h of continuous illumination was remarkably lowered as the dark period lengthened, especially in 6-day-old cotyledons. The rapid loss of chlorophyll-forming capacity of excised cotyledons during dark preincubation suggests a stimulatory effect of hypocotyls on the greening in cotyledons. The treatment of excised cotyledons with bleeding sap in the dark for 18 h resulted in the promotion of chlorophyll formation during subsequent continuous illumination. Partial fractionation of bleeding sap with organic solvents and paper chromatography indicates that the active substances showed the same behavior as cytokinins. These facts add weight to the hypothesis that cytokinins from roots flow into cotyledons and stimulate greening.     

Control of Mitochondrial Activities by Phytochrome in Cotyledons of Dark-Grown Cucumber Seedlings

Mitochondria isolated from cotyledons of dark-grown cucumberseedlings after illumination with white light showed an increasedcapacity for oxidation of malate. This increase is regulatedby phytochrome, since the rate of oxidation was increased bybrief pulses of red light, the effects of which were reversedby brief pulses of far-red light. Similarly, increases in therates of oxidation of citrate and -ketoglutarate were observedin mitochondria isolated from illuminated cotyledons. By contrast,rates of succinate-, exogenous NADH-, durohydroquinone- andascor-bate-dependent consumption of O₂ were little affectedby the light treatment. Exogenous NAD⁺stimulated the oxidationof malate and -ketoglutarate by mitochondria from dark-grown,control cotyledons, while it did not enhance the rates of oxidationof these substrates by mitochondria isolated from light-treatedcotyledons. The activities of mitochondrial enzymes (malateand succinate dehydrogenase, NAD⁺-malic enzyme and cytochromeoxidase) were not different between illuminated and dark-growncotyledons. Neither cycloheximide nor chloramphenicol inhibitedthe increase in the rate of oxidation of malate. (Received October 24, 1988; Accepted May 24, 1989)     

The Yellowing of Attached and Detached Cucumber Cotyledons

Measurements of various constituents and the activities of variousenzymes have been made in attached and detached cotyledons ofCucumis sativus. The complex changes observed suggest that thereis a reproducible pattern. Comparison of results for attachedand detached material shows that, although both become yellow,the other biochemical changes associated with yellowing arenot the same.     

The Inhibition of Flowering by Non-Induced Cotyledons of Pharbitis nil

Inhibitory effects on flowering of a non-induced cotyledon havebeen examined in Pharbitis nil seedlings. The photoperiodicinduction of one cotyledon was accomplished by wrapping it inaluminium foil for 13 to 15 h while the seedling remained inthe light. The presence of the other cotyledon in the lightblocked this inductive stimulus. The timing of its inhibitoryeffect suggested that its action was to block the expressionof the inductive stimulus, presumably at the shoot apex. Byvarying the area of the non-induced cotyledon parallel inhibitoryeffects were shown on export of stimulus and of ¹⁴C-labelledassimilate to the apex from the induced cotyledon. Thus, partof the inhibition was by interference with assimilate/stimulusco-transport in the phloem. However, an additional inhibitoryeffect was also evident and for this second component therewas no relationship between assimilate and stimulus transport.This latter inhibition was generated by brief light interruptionsof darkness given to one cotyledon only whilst the other waswrapped. The control treatment, removal of the unwrapped cotyledon,did not alter flowering compared to seedlings with intact, darkenedcotyledons. Thus, these studies show that the brief night interruptionsacted to trigger a photoperiodically sensitive inhibitor notto block induction. The implications of these findings are discussedin relation to models of time measurement in the photoperiodiccontrol of flowering. (Received March 20, 1989; Accepted November 16, 1989)     

Inhibition of Sulfur Use by Sulfite Ion in Thiobacillus ferrooxidans

In Thiobacillus ferrooxidans AP19-3, elemental sulfur is oxidized by the cooperation of three enzymes, namely, hydrogen sulfide: ferric ion oxidoreductase (SFORase), sulfite: ferric ion oxidoreductase, and iron oxidase. Sulfite ions are one of the products when elemental sulfur is oxidized by SFORase. Under the conditions in which sulfite ions are accumulated in the cells, use of sulfur as an energy source by this strain was strongly inhibited. So the mechanism of inhibition by sulfite ions in T. ferrooxidans AP19-3 was studied. The activities of SFORase and iron oxidase were completely inhibited by 0.8 mm and 1.5 mm NaHSO₃, respectively. ¹⁴CO₂ uptake into washed intact cells was also completely inhibited by 1mm NaHSO₃ when ferrous ion or elemental sulfur was used as an energy source. However, the activities of ribulose-1,5-bisphosphate carboxylase, phosphoribulokinase, and ribosephosphate isomerase measured with a cell-free extract were not inhibited by NaHSO₃ at 1 mm, indicating that sulfite ions didn’t inhibit key enzymes of the Calvin cycle. Since the activity of CO₂ uptake into washed intact cells was absolutely dependent on Fe^{2 +} - or S0-oxidation, mechanism of inhibition of sulfur use by sulfite ions is proposed as follows: sulfite ions inhibit SFORase and iron oxidase, as a result T. ferrooxidans AP19-3 can not obtain a carbon source for CO₂ fixation and stops cell growth on sulfur-salts medium.     

Infection of Cucumber Cotyledons by Cucumber Mosaic Virus and the Participation of Chlorophyllase in the Development of Chlorotic Lesions

Cotyledon infection of cucumber (Cucumis sativus L. cultivarImproved Telegraph) results in abnormal growth and the appearanceof chlorotic lesions leading to general chlorosis of the cotyledons.The disease causes increased chlorophyll degradation and chlorophyllaseactivity. Virus-induced perturbations in chlorophyllase activityare discussed in relation to current concepts of the role ofthis enzyme in vivo.     

Regulation of Mitochondrial Function and Biogenesis in Cucumber (Cucumis sativus L.) Cotyledons during Early Seedling Growth

The aim of this work was to characterize the respiratory metabolism of the greening cotyledons of cucumber (Cucumis sativus L.) during early seedling growth and to investigate how this is integrated with changes in mitochondrial biogenesis and function. In light-grown cotyledons, lipid mobilization extended from germination to 6 days postimbibition, reaching a maximum at 3 to 4 days postimbibition. The rate of dark oxygen uptake reached a maximum at 2 days postimbibition in dark-grown and 3 days postimbibition in light-grown cotyledons. Development of photosynthetic capacity occurred from 4 to 7 days postimbibition. In dark-grown cotyledons, lipid mobilization extended beyond 7 days postimbibition, and there was no greening or acquisition of photosynthetic competence. Measurements of mitochondrial function indicated that the respiratory capacity of the tissue changed such that during lipid mobilization there was a much greater capacity for the operation of the nondecarboxylating portion of the citric acid cycle (succinate to oxaloacetate), whereas during the development of photosynthetic function the activity of the remainder of the cycle (oxaloacetate to succinate) was induced. Comparison of the maximum capacities for mitochondrial substrate oxidations in vitro with the rates of in vivo substrate oxidations, predicted from the rate of lipid breakdown, indicated that mitochondria in this tissue operate at or below state 4 rates, suggesting limitation by both availability of ADP and substrate.     

激动素在黄瓜子叶器官分化中的作用

采用在培养初期分阶段供应激动素 (KT)的方法 ,研究了其对黄瓜子叶器官分化的影响 ,并对全程供应和不供应KT下子叶中内源多胺和内源激素的变化进行了动态测定。结果表明 :1)供应KT使不定根分化率下降 ,不影响营养芽的分化 ,而对于花芽分化则是必需的。2 )供应KT使多胺中腐胺 (Put)含量变化最为显著 ,在培养第 2、 6d出现峰值 ,在第 4d呈低谷 ,对照则在第 4d出现峰值。 3)供应KT使培养 2d的子叶中吲哚乙酸 (IAA)和脱落酸(ABA)含量显著降低 ,4d时IAA含量显著升高。据上结果 ,就这些生理变化和花芽分化之间是否存在相关进行了充分讨论。