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.     

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.     

Proline, Ornithine, Arginine and Glutamic Acid Contents in Detached Rice Leaves

The effects of water stress on the contents of proline, ornithine, arginine and glutamic acid in detached rice leaves were examined. In water stressed leaves, the content of proline was elevated to a content approximately 8-, 14- and 17-fold higher than in control leaves after 4, 8 and 12 h, respectively. We also observed that omithine and arginine contents were much higher under water stress than in control leaves. However, the content of glutamic acid in water stressed leaves was higher after 4 and 8 h and lower after 12 h than that in control leaves.     

Abscisic Acid and Ethylene Interact in Rice Spikelets in Response to Water Stress During Meiosis

This study investigated whether and how the interaction between abscisic acid (ABA) and ethylene is involved in the regulation of rice (Oryza sativa L.) spikelet sterility when subjected to water stress during meiosis. Two rice cultivars, HA-3 (drought-resistant) and WY-7 (drought-susceptible), were used and subjected to well-watered and water-stressed (WS) treatments during meiosis (15–2 days before heading). Leaf water potentials of both cultivars markedly decreased during the day as a result of the WS treatment, but panicle water potentials remained constant. The percentage of sterile spikelets in WS plants was increased by 49.7% for WJ-7 but only 12.7% for HA-3. ABA, ethylene, and 1-aminocyclopropane-1-carboxylic acid were all enhanced in spikelets by the water stress, but ethylene was enhanced more than ABA in WY-7 when compared with that in HA-3. Spikelet sterility was significantly reduced when ABA or amino-ethoxyvinylglycine, an inhibitor of ethylene synthesis, was applied to the panicles of WS plants at the early meiosis stage. Application of ethephon, an ethylene-releasing agent, or fluridone, an inhibitor of ABA synthesis, had the opposite effect, and sterility was increased. The results suggest that antagonistic interactions between ABA and ethylene may be involved in mediating the effect of water stress on spikelet fertility. A higher ratio of ABA to ethylene would be a physiologic trait of rice adaptation to water stress.     

Involvement of Lipid Peroxidation in Water Stress-Promoted Senescence of Detached Rice Leaves

Role of lipid peroxidation and antioxidative enzymes (catalase, peroxidase, superoxide dismutase, ascorbate peroxidase and glutathione reductase) in water stress-promoted senescence of detached rice leaves was investigated. The senescence was followed by measuring the decrease in protein content. Increased lipid peroxidation was closely correlated with senescence in water stressed leaves. Decrease in superoxide dismutase activity was evident 8 h after beginning of water stress. However, decreased catalase, peroxidase, and ascorbate peroxidase activity was observed only when senescence was observed. Glutathione reductase was not affected by water stress. Free radical scavengers retarded water stress-enhanced senescence.     

Effects of Irradiance and Copper on the Activity of Ascorbate Oxidase in Detached Rice Leaves

The effects of copper on the activity of ascorbic acid oxidasc (AAO) in detached rice leaves under both light and dark conditions and in etiolated rice seedlings were investigated. CuSO₄ increased AAO activity in detached rice leaves in both light and darkness, however, the induction in darkness was higher than in the light. In the absence of CuSO₄, irradiance (40 μmol m^-2 s^-1) resulted in a higher activity of AAO in detached rice leaves than dark treatment. Both CuSO₄ and CuCl₂ increased AAO activity in detached rice leaves, indicating that AAO is activated by Cu. Sulfate salts of Mg, Mn, Zn and Fe were ineffective in activating AAO in detached leaves. CuSO₄ was also observed to increase AAO activity in the roots but not in shoots of etiolated rice seedlings. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of Abscisic Acid,Salicylic Acid,Ethylene and Hydrogen Peroxide in Thermotolerance and Recovery for Creeping Bentgrass

Abscisic acid (ABA), salicylic acid (SA), ethylene, and hydrogen peroxide (H₂O₂) may be involved in the regulation of plant responses to heat stress. The objective of this study was to determine whether these signaling molecules are involved in survival at high temperatures in creeping bentgrass (Agrostis stolonifera). We investigated the effects of treatment with ABA, SA, H₂O₂, and ACC (an ethylene precursor) on physiological damage occurring in creeping bentgrass during heat stress (35 °C for 1 month). We also compared the effects of chemical application and the induction of thermotolerance using moderate heat stress (30 °C for 24 h). All of the pre-treatments (heat or chemical) resulted in increased tolerance to prolonged heat stress (1 month) compared to control plants. All treated samples showed more green leaves, decreased membrane leakage and reduced oxidative damage compared to control plants. We then measured changes in the endogenous concentration of these chemical components during heat stress (35 °C) and during recovery after a stress treatment (cooling back to 20 °C). An oxidative burst was detected 5 min after the initiation of heat treatment, with the increase in H₂O₂ being detected primarily in the apoplast of the cells in both leaf and root tissues. Free SA was detected only an hour after the initiation of heat stress, and concentration remained low subsequently. Neither ABA nor ethylene concentrations rose during heat stress, but the concentration of both increased during subsequent cooling. These results suggest that the signaling components of interest are involved in thermotolerance in creeping bentgrass, but that the different chemicals are likely to be involved in separate signaling pathways. An oxidative burst and SA may be bona fide heat stress signals, but ABA and ethylene appear to be involved in signaling pathways in response to recovery from heat stress in this species.     

Ammonium in relation to proline accumulation in detached rice leaves

Ammonium accumulation in relation to prolineaccumulation in detached rice leaves under stressconditions was investigated. Ammonium accumulation indark-treated detached rice leaves preceded prolineaccumulation. Ammonium accumulation caused by waterstress coincided closely with proline accumulation indetached rice leaves. Exogenous NH₄Cl andmethionine sulfoximine (MSO), which caused anaccumulation of ammonium in detached rice leaves,increased proline content. It was found that prolinein NH₄Cl- or MSO-treated rice leaves is lessutilized than in water-treated rice leaves (controls). These results are in agreement with the observationthat a decrease in proline utilization contributes tothe accumulation of proline in dark-treated and waterstressed rice leaves. Although ammonium contentincreased in Cd- and Cu-treated rice leaves, theincrease in ammonium content was only observed afterthe increase in proline content.     

The Effect of Polyethylene Glycol on Proline Accumulation in Rice Leaves

The regulation of proline accumulation in polyethylene glycol (PEG, –1.5 MPa) treated rice leaves was investigated. PEG treatment resulted in a decrease in relative water content, indicating that PEG treatment caused water stress in rice leaves. Proline accumulation caused by PEG was related to protein hydrolysis, an increase in ornithine--amino- transferase activity, an increase in the content of ammonia, and an increase in the contents of the precursors of proline biosynthesis, glutamic acid, ornithine, and arginine. Results also show that abscisic acid accumulation is not required for proline accumulation in PEG-treated rice leaves.     

Interaction of Cytokinins and Abscisic Acid During Regulation of Stomatal Opening in Bean Leaves

Effects of benzyladenine (BA) and abscisic acid (ABA) applied separately or simultaneously on parameters of gas exchange of Phaseolus vulgaris L. leaves were studied. In the first two experimental sets) 100 M ABA and 10 M BA were applied to plants sufficiently supplied with water. Spraying of leaves with ABA decreased stomatal conductance (g _s) and in consequence transpiration rate (E) and net photosynthetic rate (P _N) already 1 h after application, but 24 h after application the effect almost disappeared. 10 M BA slightly decreased gas exchange parameters, but in simultaneous application with ABA reversed the effect of ABA. Immersion of roots into the same solutions markedly decreased gas exchange parameters and 24 h after ABA application the stomata were completely closed. The effect of ABA was ameliorated by simultaneous BA application, particularly after 1-h treatment. In the third experimental set, plants were pre-treated by immersing roots into water, 1 M BA, or 100 M ABA for 24 h and then the halves of split root system were dipped into different combinations of 1 M BA, 100 M ABA, and water. In plants pre-treated with ABA all gas exchange parameters were small and they did not differ in plants treated with H₂O+H₂O, H₂O+BA, or BA+BA. In plants pre-treated with BA or H₂O, markedly lower values of P _N were found when both halves of roots were immersed in ABA. Further, the effects of pre-treatment of plants with water, 1 M BA, 100 M ABA, or ABA+BA on the development of water stress induced by cessation of watering and on the recovery after rehydration were followed. ABA markedly decreased gas exchange parameters at the beginning of the experiment, but in its later phase the effect was compensated by delay in development of water stress. BA also delayed development of water stress and increased P _N in water-stressed leaves. BA reversed the effect of ABA at mild water stress. Positive effects of BA and ABA pre-treatments were observed also after rehydration.     

NaCl-Induced Changes in Putrescine Content and Diamine Oxidase Activity in Roots of Rice Seedlings

The effects of NaCl on putrescine (Put) content and diamine oxidase (DAO) activity in roots of rice seedlings were examined. NaCl treatment lowered the content of Put and increased the activity of DAO in roots. Our current results indicate that Cl^– is not required for NaCl-induced decline in Put content and increase in DAO activity in roots. Put content in roots of rice seedlings exposed to NaCl is possibly regulated by DAO activity.     

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.     

内源抗坏血酸对水稻种子萌发及幼苗生长的影响

为了解内源抗环血酸在水稻(Oryza sativa)种子中的作用,以野生型品种‘中花11’(ZH-11)、抗坏血酸合成关键酶GLDH基因的上调(超表达)株系GO-2及下调(干涉)株系GI-2为材料,研究了抗环血酸含量对其种子萌发和幼苗生长的影响。结果表明,GO-2株系的种子萌发率比野生型ZH-11高6%,比干涉型GI-2高60%。外源添加1 mmol L–1抗坏血酸后,干涉型GI-2萌发率提高了22%,GO-2及ZH-11则没有明显增加。GO-2株系在幼苗的根长、株高、分蘖数和鲜重等指标上均高于ZH-11和GI-2。实时荧光定量测定结果表明,GO-2株系叶片的GLDH基因表达量显著上调,而GI-2株系则显著下调。这说明抗坏血酸有助于维持水稻种子活力和促进水稻种子发芽和幼苗生长。     

Effects of Abscisic Acid and Water Stress on the Senescence of Detached Rice Leaves

The effects of abscisic acid (ABA) and water stress on senescence and enzyme activities of oxygen scavenging enzymes of detached rice leaves were compared. Exogenously applied ABA exhibited water stress-like effects by promoting senescence, by decreasing the activities of catalase, peroxidase, ascorbate peroxidase and superoxide dismutase. It seems that the effects of water stress on senescence and enzyme activities are possibly mediated through increased content of endogenous ABA. This revised version was published online in July 2006 with corrections to the Cover Date.     

Brassinolide Effect on Growth of Apical Meristems, Ethylene Production, and Abscisic Acid Content in Potato Tubers

Treatment of intact potato (Solanum tuberosum L., cv. Nevskii) tubers with 24-epibrassinolide (EB) resulted in prolonged deep dormancy, increased production of ethylene and higher contents of free and bound abscisic acid (ABA) in buds. EB at the most efficient concentration 0.021 mg dm^–3, applied immediately after tuber harvest, inhibited sprouting by 36 – 38 d, increased ethylene formation after 1 and 7 d of storage by almost 300 and 150%, respectively, and increased the content of both free and bound ABA during the whole period of storage (on average by about 80%). Electron microscopic and morphometric studies showed that EB brings about a decrease in cell volume in tunica and all types of meristems and an increase in the number of vacuoles, accompanied by a decrease in their volume.     

In Vitro Selection for Salt Tolerance in Rice

In six cultivars of rice (Oryza sativa L.), Pusa Basmati 1, Basmati 370, Type III, Pant Dhan 4, CSR 10 and Pokkali, embryogenic callus growth, plant regeneration, and proline and total protein contents were studied under salt stress (on agar solidified media containing 0, 0.5, 1.0, 1.5 and 2.0 % NaCl). Four weeks after inoculation the callus fresh mass decreased with increasing salt concentration in all the six cultivars. The regeneration frequency in salt stressed callus was also lower as compared to control. 15 d and 30 d after inoculation proline content increased several fold whereas total protein content decreased markedly with increase in salt concentration.     

Amelioration of NaCl Stress by Arginine in Rice Seedlings: Changes in Endogenous Polyamines

Effects of NaCl (0.1 – 0.2 M) alone or in combination with 1 mM arginine on growth and endogenous polyamine (PA) content have been observed in two cultivars of rice differing in NaCl stress tolerance. The germination, seedlings fresh mass and water content decreased with increase in salinity in both the cultivars. This inhibition was partially alleviated by application of arginine. Cv. CSR-27 exhibited relatively better germination than cv. Bas-370 at different salinities. Total PA content increased in both the cultivars under NaCl stress alone and in combination with arginine. Putrescine to spermidine and spermine ratio was higher in NaCl-treated seedlings being more in cv. Bas-370 as compared to cv. CSR-27 and the ratio reversed to almost control level when arginine was applied along with NaCl.