Light-Dependent Ammonium Ion Toxicity of Rice Leaves in Response to Phosphinothricin Treatment

Ammonium ion accumulation in detached rice leaves treated with phosphinothricin (PPT), an inhibitior of glutamine synthetase (GS), was investigated in the light and darkness. PPT treatment increased NH₄ ⁺ content and induced toxicity in rice leaves in the light but not in darkness, suggesting the importance of light in PPT-induced NH₄ ⁺ toxicity in detached rice leaves. PPT treatment in the light resulted in a decrease of activities of the cytosolic form of GS and the chloroplastic form of GS. The photosynthetic electron transport inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea reduced NH₄ ⁺ accumulation induced by PPT in the light. In darkness, PPT-induced NH₄ ⁺ accumulation and toxicity were observed in the presence of glucose or sucrose.     

Dependency of Nitrate Reduction on Soluble Carbohydrates in Primary Leaves of Barley under Aerobic Conditions

Nitrate reduction was studied as a function of carbohydrate concentration in detached primary leaves of barley (Hordeum vulgare L. cv Numar) seedlings under aerobic conditions in light and darkness. Seedlings were grown either in continuous light for 8 days or under a regimen of 16-hour light and 8-hour dark for 8 to 15 days. Leaves of 8-day-old seedlings grown in continuous light accumulated 4 times more carbohydrates than leaves of plants grown under a light and dark regimen. When detached leaves from these seedlings were supplied with NO₃⁻ in darkness, those with the higher levels of carbohydrates reduced a greater proportion of the NO₃⁻ that was taken up. In darkness, added glucose increased the percentage of NO₃⁻ reduced up to 2.6-fold depending on the endogenous carbohydrate status of the leaves. Both NO₃⁻ reduction and carbohydrate content of the leaves increased with age. Fructose and sucrose also increased NO₃⁻ reduction in darkness to the same extent as glucose. Krebs cycle intermediates, citrate and succinate, did not increase NO₃⁻ reduction, whereas malate slightly stimulated it in darkness.

In light, 73 to 90% of the NO₃⁻ taken up was reduced by the detached leaves; therefore, an exogenous supply of glucose had little additional effect on NO₃⁻ reduction. The results indicate that in darkness the rate of NO₃⁻ reduction in primary leaves of barley depends upon the availability of carbohydrates.

     

Light-Dark Changes in Proline Content of Barley Leaves under Salt Stress

Proline accumulation in leaves of barley (Hordeum vulgare L. cv. Alfa) seedlings treated with 150 mM NaCl was promoted in the light and suppressed in the dark. The light/dark changes of proline content was enhanced with each 12 h light/12 h dark cycle and the proline content increased steadily. Root and shoot concentrations of Na⁺ and Cl^– in salt treated plants increased about 10 to 25 times as compared to the control. The content of these ions and the content of malondialdehyde were higher in the shoot of seedlings exposed to salt stress for 4 d in the light in comparison with the seedlings exposed to NaCl for 4 d in darkness. Light stimulated both ions and proline accumulation in the leaves and has no effect in the roots. Oxygen uptake was higher in the seedlings kept 4 d in the light which have higher endogenous free proline content. Chlorophyll fluorescence measurements showed that the photochemical activity of PS 2 slightly decreased as a result of salt stress and was not influenced by light regimes during plant growth.     

Relationship between ammonium accumulation and senescence of detached rice leaves caused by excess copper

The possibility that ammonium (NH ₄ ⁺ ) accumulation is linked to the senescence of detached rice (Oryza sativa) leaves induced by copper (Cu) was investigated. CuSO₄ was effective in promoting senescence of detached rice leaves. Both CuSO₄ and CuCl₂ induced NH ₄ ⁺ accumulation in detached rice leaves, indicating that NH ₄ ⁺ accumulation is induced by copper. Sulfate salts of Mg, Mn, Zn, and Fe were ineffective in inducing NH ₄ ⁺ accumulation in detached rice leaves. The senescence of detached rice leaves induced by Cu was found to be prior to NH ₄ ⁺ accumulation. Free radical scavengers, such as glutathione and thiourea, inhibited senescence caused by Cu and at the same time inhibited Cu-induced NH ₄ ⁺ accumulation. The current results suggest that NH ₄ ⁺ accumulation is not associated with senescence induced by Cu, but is part of the overall expression of oxidative damage caused by an excess of Cu. Evidence was presented to show that copper-induced ammonium accumulation in detached rice leaves is attributed to a decrease in glutamine synthetase activity and an increase in reduction of nitrate.     

The toxicity induced by phosphinothricin and methionine sulfoximine in rice leaves is mediated through ethylene but not abscisic acid

The possible role of ethylene and abscisic acid (ABA) in regulating thetoxicity of detached rice leaves induced by phosphinothricin (PPT) andmethionine sulfoximine (MSO), both known to be glutamine synthetase (GS)inhibitors, was studied. During 12 h of incubation, PPT and MSOinhibited GS activity, accumulated NH₄ ⁺ and inducedtoxicity of detached rice leaves in the light but not in darkness. PPT and MSOtreatments also resulted in an increase of ethylene production and ABA contentin a light dependent way. Addition of fluridone, an inhibitor of ABAbiosynthesis, reduced ABA content in rice leave but did not preventNH₄ ⁺ toxicity of rice leaves induced by PPT and MSO.Cobalt ion, an inhibitor of ethylene biosynthesis, affected PPT- andMSO-inducedtoxicity of detached rice leaves but had no effect on PPT- and MSO-inducedNH₄ ⁺ accumulation. Results suggest that ethylene but notABA may be responsible for PPT- and MSO-induced toxicity of detached riceleaves.     

Role of Light in the Appearance of Glutamine Synthetase in Leaves of Pennisetum glaucum

Leaves of Pennisetum [Pennisetum glaucum (L) HHB 67] seedlings contained two isozymes of glutamine synthetase (GS, EC 6.3.1.2): cytosolic GS1 and chloroplastic GS2. Leaves of seedlings grown in light for seven days contained about twofold higher GS activity than etiolated leaves. In both light and dark grown seedlings, total GS, GS1 and GS2 activity declined with plant age with more pronounced effect in leaves of etiolated seedlings, and GS2 declined at a much faster rate than GS1. Exposure of etiolated seedlings to light markedly enhanced GS1 and GS2 activity. This increase in activity was not affected by cycloheximide, precluding light dependent de novo synthesis of the enzyme. Treatment of etiolated seedlings with photosynthetic inhibitor, dichlorophenyl dimethyl urea (DCMU) inhibited light dependent appearance of GS. Exogenous supply of sucrose to dark grown seedlings greatly increased the GS activity in dark. These results suggest that light-mediated stimulation in activity of GS in Pennisetum leaves is dependent on photosynthetic reaction.     

Effects of Juglone on Growth of Muskmelon Seedlings with Respect to Physiological and Anatomical Parameters

The effect phosphinothricin (PPT), an inhibitor of glutamine synthetase (GS), on proline accumulation in detached rice leaves was investigated. During 12 h incubation, PPT inhibited GS activity and induced accumulation of NH₄ ⁺, and accumulation of proline in the light but not in darkness. Proline accumulation caused by PPT in the light was related to protein hydrolysis, and increase in the contents of precursors of proline, ornithine and arginine. Abscisic acid accumulation was not required for proline accumulation in PPT-treated rice leaves. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effects of 2,2′-bipyridine and other metal chelators on the conversion of 1-aminocyclopropane-1-carboxylic acid to ethylene in rice

Effects of metal chelators, 2,2-bipyridine, 8-hydroxyquinoline and 1,10-phenenthroline, on the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to ethylene in detached leaves of light-grown rice (Oryza sativa) seedlings and detached shoots of etiolated rice seedlings were investigated. Metal chelators strongly inhibited the in vivo ACC oxidase activity in detached leaves and detached etiolated shoots. This inhibition could be partially recovered by Fe²⁺. Our results support the notion that Fe²⁺ is an essential cofactor for the conversion of ACC to ethylene in vivo.Abbreviations ACC 1-aminocyclopropane-1-carboxylic acid - BP 2,2-bypyridine - HQ 8-hydroxylquinoline - MJ methyl jasmonate - PA 1,10-phenanthroline - Put putrescine     

Comparative uptake of nitrate by intact seedlings of C3 (barley) and C4 (corn) plants: Effect of light and exogenously supplied sucrose

The effect of light and exogenously supplied sucrose on NO₃ ⁻ uptake was studied in 9-day-old intact C₃ (barley) and C₄ (corn) seedlings. The seedlings used were uninduced for nitrate uptake system (i.e. had never seen nitrogen during germination and growth) and were exposed to continuous light for 3 days to avoid any diurnal variation and to load the seedlings fully with photosynthates. The uptake assay was conducted either in light or in darkness. Prior to assay, seedlings were treated with darkness or light for 24 h. Accordingly, four sets of seedlings, i.e. pretreated with light and assayed in light (LL); pretreated and assayed in darkness (DD); pretreated with light and assayed in darkness (LD); and pretreated with darkness and assayed in light (DL) were formed. Barley exhibited 55% higher NO₃ ⁻ uptake than corn during light (LL) and 91% higher during darkness (DD). Shifting barley seedlings from light to dark (LD) or dark to light (DL) for uptake assay, did not affect NO₃ ⁻ uptake, i.e. in LD the uptake was similar to LL and in DL it was similar to DD. However, in corn, the light conditions during the assay determined the uptake regardless of the conditions during the period preceding the assay. One percent sucrose in the medium increased NO₃ ⁻ uptake by 31% in barley and 70% in corn during light (LL). The corresponding increase during darkness (DD) was 38% in both barley and corn. Removal of the corn residual endosperm decreased NO₃ ⁻ uptake by 40% during darkness. Etiolated seedlings (those having never seen light) of both barley and corn were able to take up significant amount of NO₃ ⁻ during darkness. Externally supplied sucrose in the assay medium of etiolated seedlings increased the NO₃ ⁻ uptake to about 4 and 2 fold in barley and corn, respectively. The data presented here provide evidence that: 1. In intact seedlings, light per se is not obligatory for NO₃ ⁻ uptake and that the carbohydrate supply may mimic light. 2. Light affected the NO₃ ⁻ uptake differently in barley and corn. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Relation between Accumulation of Abscisic Acid and Proline in Detached Rice Leaves

The relation between abscisic acid (ABA) and proline accumulation was investigated in detached rice (Oryza sativa L.) leaves. In darkness, proline content increased about 2-, 2,5- and 6-fold after 24, 48 and 72 h. ABA content reached maximum after 48 h. In the light, proline content remained almost unchanged until 48 h and subsequently increased slightly. ABA content in the light was lower than in darkness, but the maximum was also after 48 h. During 12-h exposure to decreased air humidity, proline content gradually increased, but ABA content increased about 25-fold after 4 h and declined thereafter. Exogenous application of ABA resulted in an increase in proline content in detached rice leaves under both light and darkness.     

In Vivo Nitrate Reduction in Roots and Shoots of Barley (Hordeum vulgare L.) Seedlings in Light and Darkness

In vivo NO₃⁻ reduction in roots and shoots of intact barley (Hordeum vulgare L. var Numar) seedlings was estimated in light and darkness. Seedlings were placed in darkness for 24 hours to make them carbohydrate-deficient. During darkness, the leaves lost 75% of their soluble carbohydrates, whereas the roots lost only 15%. Detached leaves from these plants reduced only 7% of the NO₃⁻ absorbed in darkness. By contrast, detached roots from the seedlings reduced the same proportion of absorbed NO₃⁻, as did roots from normal light-grown plants. The rate of NO₃⁻ reduction in the roots accounted for that found in the intact dark-treated carbohydrate-deficient seedlings. The rates of NO₃⁻ reduction in roots of intact plants were the same for approximately 12 hours, both in light and darkness, after which the NO₃⁻ reduction rate in roots of plants placed in darkness slowly declined. In the dark, approximately 40% of the NO₃⁻ reduction occurred in the roots, whereas in light only 20% of the total NO₃⁻ reduction occurred in roots. A lesser proportion was reduced in roots because the leaves reduced more nitrate in light than in darkness.     

Toxicity in leaves of rice exposed to cadmium is due to hydrogen peroxide accumulation

The production of H₂O₂ in detached rice leaves of Taichung Native 1 (TN1) caused by CdCl₂ was investigated. CdCl₂ treatment resulted in H₂O₂ production in detached rice leaves. Diphenyleneiodonium chloride (DPI) and imidazole (IMD), inhibitors of NADPH oxidase (NOX), prevented CdCl₂-induced H₂O₂ production, suggesting that NOX is a H₂O₂-genearating enzyme in CdCl₂-treated detached rice leaves. Phosphatidylinositol 3-kinase inhibitors wortmanin (WM) or LY294002 (LY) inhibited CdCl₂-inducted H₂O₂ production in detached rice leaves. Exogenous H₂O₂ reversed the inhibitory effect of WM or LY, suggesting that phosphatidylinositol 3-phosphate is required for Cd-induced H₂O₂ production in detached rice leaves. Nitric oxide donor sodium nitroprusside (SNP) was also effective in reducing CdCl₂-inducing accumulation of H₂O₂ in detached rice leaves. Cd toxicity was judged by the decrease in chlorophyll content. The results indicated that DPI, IMD, WM, LY, and SNP were able to reduce Cd-induced toxicity of detached rice leaves. Twelve-day-old TN1 and Tainung 67 (TNG67) rice seedlings were treated with or without CdCl₂. In terms of Cd toxicity (leaf chlorosis), it was observed that rice seedlings of cultivar TN1 are Cd-sensitive and those of cultivar TNG67 are Cd-tolerant. On treatment with CdCl₂, H₂O₂ accumulated in the leaves of TN1 seedlings but not in the leaves of TNG67. Prior exposure of TN1 seedlings to 45^oC for 3 h resulted in a reduction of H₂O₂ accumulation, as well as Cd tolerance of TN1 seedlings treated with CdCl₂. The results strongly suggest that Cd toxicity of detached leaves and leaves attached to rice seedlings are due to H₂O₂ accumulation.     

Chlorophyll turnover in etiolated greening barley transferred to darkness: Isotopic (1-14C glutamic acid) evidence of dark chlorophyll synthesis in the absence of chlorophyll accumulation

Synthesis of chlorophyll was initiated in 5- to 6-day-old dark-grown barley (Hordeum vulgare L. cv. Clipper)seedlings by exposing them to light in the presence of 1-¹⁴ C glutamic acid supplied via the roots.The plants were then returned to darkness. At the end of light treatment (_T) and after 7 or 18 h dark treatment chlorophylls a and b were extracted, quantified (μgleaf¹). purified by HPLC to their magnesium-free derivatives (pheophytin a and b) and their molar radioactivities determined. After 2 h exposure to light followed by 6 h illumination in the presence of 1-¹⁴ C glutamic acid, seedlings had accumulated 4-7 nmol chlorophyll leaf¹ and had incorporated between 900-1 350 Bq (g fresh weight)¹ of radioactive label into the chlorophyll pool. When seedlings were transferred to darkness, label continued to be incorporated and after 18 h the radioactivity of the chlorophyll pool had increased by 300-700 Bq (g fresh weight)¹. Net chlorophyll content, however, remained constant during dark treatment. The increase in radioactivity of the chlorophyll pool in darkness represented the difference between a net increase of label incorporated into chlorophyll a and a small loss of label from chlorophyll b. The absence of measurable radioactivity in the phytol moiety of labelled chlorophyll a, extracted at the endof dark treatment, demonstrated thatincorporation of label was into the tetrapyrrole moiely of chlorophyll and not into the phytol chain. Light-independent incorporation of 1-¹⁴ C glutamic acid into chlorophyll of greening barley seedlings transferred to darkness indicates that chlorophyll synthesis continues when light is withheld. We interpret the net gain in radioactivity of chlorophyll in darkness, in the absence of a net gain in chlorophyll content, to chlorophyll turnover i.e. to simultaneous synthesis and breakdown of chlorophyll when etiolated greening barley seedlings are transferred to darkness.     

Light inhibition of the conversion of 1-aminocyclopropane-1-carboxylic acid to ethylene in leaves is mediated through carbon dioxide

The mechanism of light-inhibited ethylene production in excised rice (Oryza sativa L.) and tobacco (Nicotiana tabacum L.) leaves was examined. In segments of rice leaves light substantially inhibited the endogenous ethylene production, but when CO₂ was added into the incubation flask, the rate of endogenous ethylene production in the light increased markedly, to a level which was even higher than that produced in the dark. Carbon dioxide, however, had no appreciable effect of leaf segments incubated in the dark. The endogenous level of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, was not significantly affected by lightdark or CO₂ treatment, indicating that dark treatment or CO₂exerted its effect by promoting the conversion of ACC to ethylene. This conclusion was supported by the observations that the rate of conversion of exogenously applied ACC to ethylene was similarly inhibited by light, and this inhibition was relieved in the presence of CO₂. Similar results were obtained with tobacco leaf discs. The concentrations of CO₂ giving half-maximal activity was about 0.06%, which was only slightly above the ambient level of 0.03%. The modulation of ACC conversion to ethylene by CO₂ or light in detached leaves of both rice and tobacco was rapid and fully reversible, indicating that CO₂ regulates the activity, but not the synthesis, of the enzyme converting ACC to ethylene. Our results indicate that light inhibition of ethylene production in detached leaves is mediated through the internal level of CO₂, which directly modulates the activity of the enzyme converting ACC to ethylene.Abbreviation ACC 1-aminocyclopropane-1-carboxylic acid Recipient of a Republic of China National Science Council Fellowship     

The flexible interrelation between AOX respiratory pathway and photosynthesis in rice leaves.

Alternative respiratory pathway was investigated in rice seedlings grown under total darkness, light/dark cycle, or continuous light. The capacity of the alternative pathway was relatively higher in leaves that had longer light exposure. An analysis of rice AOX1 multigene family revealed that AOX1c, but not AOX1a and AOX1b, had a light-independent expression. The alternative oxidase (AOX) inhibitor, salicylhydroxamic acid (SHAM, 1mM), inhibited nearly 68% of the capacity of the alternative pathway in leaves grown under different light conditions. The plants grown under different light periods were treated with SHAM and then were exposed to illumination for 4h. The transition from dark to 4h of light stimulated the capacity of alternative pathway in etiolated rice seedlings and in those grown under light/dark cycle, whereas the capacity of the alternative pathway was constant in seedlings grown under continuous light with additional 4h of illumination. Etiolated leaves did not show any CO(2) fixation after 4h of illumination, and the increase in chlorophyll content was delayed by the SHAM pretreatment. When seedlings grown under light/dark cycle were moved from dark and exposed to 4h of light, increases in chlorophyll content and CO(2) fixation rate were reduced by SHAM. Although these parameters were stable in plants grown under continuous light, SHAM decreased CO(2) fixation rate but not the chlorophyll content. These results indicate that the role and regulation of AOX in light are determined by the developmental stage of plant photosynthetic apparatus.     

Cadmium toxicity of rice leaves is mediated through lipid peroxidation

Oxidative stress, in relation to toxicity of detached rice leaves,caused by excess cadmium was investigated. Cd content inCdCl₂-treated detached rice leaves increased with increasingdurationof incubation in the light. Cd toxicity was followed by measuring the decreasein chlorophyll and protein. CdCl₂ was effective in inducing toxicityand increasing lipid peroxidation of detached rice leaves under both light anddark conditions. These effects were also observed in rice leaves treated withCdSO₄, indicating that the toxicity was indeed attributed to cadmiumions. Superoxide dismutase (SOD), ascorbate peroxidase (APOD), and glutathionereductase (GR) activities were reduced by excess CdCl₂ in the light.The changes in catalase and peroxidase activities were observed inCdCl₂-treated rice leaves after the occurrence of toxicity in thelight. Free radical scavengers reduced CdCl₂-induced toxicity and atthe same time reduced CdCl₂-induced lipid peroxidation and restoredCdCl₂-decreased activities of SOD, APOD, and GR in the light. Metalchelators (2,2-bipyridine and 1,10-phenanthroline) reducedCdCl₂ toxicity in rice leaves in the light. The reduction ofCdCl₂ toxicity by 2,2-bipyridine (BP) is closely associatedwith a decrease in lipid peroxidation and an increase in activities ofantioxidative enzymes. Furthermore, BP-reduced toxicity of detached riceleaves,induced by CdCl₂, was reversed by adding Fe²⁺ orCu²⁺, but not by Mn²⁺ or Mg²⁺.Reduction of CdCl₂ toxicity by BP is most likely mediated throughchelation of iron. It seems that toxicity induced by CdCl₂ mayrequire the participation of iron.     

Ammonium ion, ethylene, and NaCl-induced senescence of detached rice leaves

The possibility that NH₄ ⁺ accumulation is linkedto the senescence of detached rice (Oryza sativa) leavesinduced by NaCl was investigated. NaCl was effective in promoting senescenceandin increasing NH₄ ⁺ content of detached rice leaves.NaCl-promoted senescence is mainly due to the effect of both Na⁺ andCl^- ions. NaCl had no or slight effect on relative water content,suggesting that an osmotic effect is unlikely to be a major factor contributingto senescence of these leaves. NaCl-induced NH₄ ⁺accumulation was due to enhanced nitrate reduction and decreased glutaminesynthetase activity. Exogenous NH₄Cl, which caused an accumulationofNH₄ ⁺ in detached rice leaves, also promoted senescence.Itwas found that an increase in NH₄ ⁺ content preceded theoccurrence of senescence caused by NaCl. Results also show that NaCl-promotedsenescence is unlikely to be due to the lack of glutamate, glutamine,aspartate,and asparagine. The current results suggest that NH₄ ⁺accumulation is linked to NaCl-induced rice leaf senescence. Since ethylene isknown to be a potent promoter of leaf senescence, we also investigated the roleof ethylene in the regulation of NH₄ ⁺-promoted senescenceof detached rice leaves. NaCl or NH₄Cl treatment resulted in adecrease of ethylene production. Evidence was presented to show thatNH₄ ⁺ accumulation in detached rice leaves does not changetissue sensitivity to ethylene. Clearly, the possible involvement of ethyleneinNH₄ ⁺-promoted senescence is excluded.     

The Metabolism of Oat Leaves during Senescence: IV. The Effects of alphaalpha'-Dipyridyl and other Metal Chelators on Senescence

The senescence of the first leaves of light-grown Avena seedlings when detached and placed in the dark is inhibited by α, α′-dipyridyl and α, α′, α″-tripyridyl at concentrations between 10⁻⁵ and 10⁻⁴ M. Five other chelating agents exert similar inhibiting effects at concentrations 3 to 30 times higher. The senescence of etiolated leaves, as shown by loss of carotenoid and protein, is similarly inhibited. Ethylene-diaminetetraacetate has a similar effect in the dark, though only at 10 mM and above, but in the light it causes bleaching of chlorophyll. It is deduced that an iron-containing system plays an essential part in the initiation of the senescence process.     

Effects of Short-term Red Radiation and Choline Compounds on Cytokinin Content, Chlorophyll Accumulation and Photomorphogenesis in Wheat Seedlings

Effects of choline compounds (2-chloroethyltrimethylammonium chloride and 2-ethyltrimethylammonium chloride) as well as red radiation (R) pulse on the dynamics of cytokinin changes, growth and chlorophyll (a + b) accumulation were studied during the growth and greening of etiolated wheat seedlings (Triticum aestivum L., var. Mironovskaya-808). The seedlings were grown for 120 h in the dark and then exposed for 72 h to white light. Pre-treatment of caryopses with cholines and pre-irradiation of etiolated seedlings with R inhibited elongation of both coleoptile and first leaf; but the same factors accelerated these growth responses when seedlings were exposed to white light. Chlorophyll (Chl) accumulation and the first leaf appearance from coleoptile were accelerated by the pre-treatments as well. Far-red radiation (FR) reversed all effects of R but choline pre-treatment eliminated partly R/FR photoreversibility. Two compounds with high cytokinin activity (tested on a fresh weight basis by the bioassay with Amaranthus caudatus L.) were found in shoots and first leaves. One of them had R_f, UV absorbance spectrum and the biological activity similar to N⁶-(Δ²-isopentenyl)adenosine. Another cytokinin-like substance was not identified with the used standards. Stimulation of greening by R pulse and cholines was accompanied with accelerated accumulation of both cytokinin-like substances. We conclude that the influence of R and cholines on the concentration of substances with cytokinin activities detected in the leaves might be involved in the stimulation of Chl accumulation.     

Ammonium Ion, Ethylene, and Abscisic Acid in Polyethylene Glycol-treated Rice Leaves

Polyethylene glycol (PEG)-treatment decreased chlorophyll and protein contents and increased NH₄ ⁺ content due to decreased glutamine synthetase activity in detached rice leaves. PEG-treatment also increased abscisic acid (ABA) content and decreased ethylene production. Addition of fluridone, an inhibitor of ABA biosynthesis, reduced ABA content in rice leaves but did not prevent chlorophyll and protein loss in rice leaves induced by PEG. Silver thiosulfate, an inhibitor of ethylene action, was effective in preventing PEG-promoted chlorophyll and protein loss, but had no effect on PEG-induced NH₄ ⁺ accumulation. The current results suggest that NH₄ ⁺ accumulation in rice leaves induced by PEG increases leaf sensitivity to ethylene, which in turn results in an enhancement of chlorophyll and protein loss. This revised version was published online in July 2006 with corrections to the Cover Date.