Cotton Leaf Senescence can be Delayed by Nitrophenolate Spray Through Enhanced Antioxidant Defence System

Leaf senescence is an oxidative process, and most of the catabolic processes involved in senescence are propagated irreversibly once initiated. An experiment was conducted to test the hypothesis that nitrophenolates (Atonik, a plant growth regulator) spray can delay the leaf senescence through reduced oxidative damage. Atonik 3.75 g a.i. ha^?1 was sprayed during boll filling stage on cotton, and the senescence process was evaluated by quantifying total chlorophyll contents, photosynthetic rate, Fv/Fm ratio, various reactive oxygen species (ROS) content, antioxidant content and antioxidant enzyme activity from 90 days after sowing (DAS) to 130 DAS. The result indicated that nitrophenolate spray reduced the hydrogen peroxide (H₂O₂), superoxide anion (O₂^?) accumulation, lipid peroxidation (malondialdehyde), lipoxygenase activity and membrane permeability over unsprayed control. The antioxidant enzyme activity (superoxide dismutase, SOD; ascorbate peroxidase, APX; peroxidase, POX; glutathione peroxidase, GSH‐Px) were significantly increased by the nitrophenolate spray. POX (118.1 %) and GSH‐Px (143.3 %) activities were enhanced to a higher level compared to APX (8.5 %) activity at 130 DAS. Enhanced accumulation of ascorbate (144.9 %), phenol (154.7 %) and proline (50 %) was seen in nitrophenolate‐sprayed plants compared with unsprayed control plants at 130 DAS. Ascorbate content is increased by higher dehydroascorbate reductase enzyme activity. Ascorbate was thus able to replenish reducing equivalents to phenoxyl radicals resulting in an increase in phenolic compounds. The increased phenolic acid content may be involved in scavenging the ROS produced during senescence process. The higher level of reduced ascorbate and low level of endogenous H₂O₂ in the leaves may be the prerequisite for delayed leaf senescence in the nitrophenolate‐sprayed plants. Based on the present work, it can be concluded that nitrophenolate‐sprayed plants can postpone the leaf senescence by peroxide/phenolic/ascorbate system which is involved in scavenging the ROS produced during leaf senescence.     

Growth,Physiological Attributes and Antioxidant Enzyme Activities in Soybean Seedlings Treated With or Without Silicon Under UV‐B Radiation Stress

Silicon (Si) can increase plant defence systems against abiotic and biotic stress, but there is little information on UV‐B radiation stress alleviation by Si for field crops. Using soybean (Glycine max (L.) Mell) seedlings, we determined how Si may mediate UV‐B radiation stress by studying changes in biomass, physiological attributes and antioxidants’ activities. The seedlings were raised with 0, 1.70 and 2.55 mm of Si in the growth chamber under ambient, ambient +2.7 kJ m^?2 day and ambient +5.4 kJ m^?2 day of UV‐B radiation. As expected, plants suffered severe growth limitations under UV‐B radiation, but Si alleviated these limitations through improvements in leaf area (LA) and root‐to‐shoot ratio (R/S). The UV‐B radiation stress reduced the LA by 73.9–94.7%, total dry weight (TDW) by 11.8–36.6% and R/S by 9.2–30.2% but induced the activities of soluble protein by 18.4–21.0%, catalase (CAT) by 22.7–54.2%, superoxide dismutase (SOD) by 31.9–63.1%, and peroxidases (POD) by as much as 162.9–381.6%. Further confirmation of stress alleviations by Si was noted from reductions in these stress signals (antioxidant activities) under UV‐B radiation: CAT decreased significantly by 78.3–79.4%, SOD by 5.3–7.2% and POD by 49.9–61.9% in silicon‐treated UV‐B stressed soybean.     

Differential Response of Southern US Rice (Oryza sativa L.) Cultivars to Ultraviolet‐B Radiation

Elevated ultraviolet‐B (UV‐B; between 290 and 320 nm) radiation, because of depletion of the stratospheric ozone layer, is one of the major environmental factors influencing plant metabolic processes and yield. The southern US rice cultivars contribute greatly towards US rice production, but the effects of elevated UV‐B radiation on these cultivars are not well known. The objective of this study was to determine the effects of elevated UV‐B radiation on leaf photosynthetic rate (P_n), membrane stability, pollen viability, phenolic concentration and yield of eight commercially popular southern US rice cultivars (five inbred cultivars and three hybrids). Plants were grown in a temperature‐controlled greenhouse in Beaumont, TX, USA, and were exposed to UV‐B radiation of 0, 8 or 16 kJ m^?2 day^?1 for 90 days. For most of the cultivars, plants grown under 8 or 16 kJ UV‐B radiation showed significant decreases in P_n, membrane stability, pollen viability, and yield compared with the plants grown under an UV‐B‐free environment, whereas there was a significant increase in leaf phenolic concentration under 16 kJ UV‐B radiation. The hybrid ‘Clearfield XL729’ performed best among the selected southern US rice cultivars under 16 kJ UV‐B radiation.     

In UV‐B Irradiated Rice High Levels of Mn Limits Chloroplasts Injury Triggered by Oxidative Stress

The molecular mechanism to control the oxidative burst exerted by Mn accumulation in rice (Oryza sativa L.) plants, grown in hydroponics containing 0.5, 2 and 8 mg l^–1 Mn and irradiated with a total biological effective UV‐B irradiation of 20.825 kJ m^?2, was investigated in the chloroplasts at the 15th and 21st days after germination. In both experimental periods, Mn accumulation kinetics in the leaves and in the chloroplast lamellae displayed overall increases. Coupled to higher membrane selectivity, superoxide production and acyl lipids peroxidation in the thylakoids decreased, prompting upper rates of the Hill and Mehler reactions. Connected to UV‐B irradiation, higher accumulated Mn in thylakoids was found to be chelated in a 36.5 kDa protein, with Mn/protein ratio of about 1 and high content of Gln, Asp, Glu, Leu and Gly, being its EPR spectrum characteristic of high‐spin Mn(II), in a S = 5/2 ground state. As this protein exhibited enzymatic catalysis of superoxide dismutation, it was concluded that, under UV‐B irradiation, the high internal tolerance of Oryza sativa L. to Mn during the vegetative growth also triggers the synthesis of a manganese protein that mimics superoxide dismutase functioning, therefore furnishing an additional intimate protection against oxidative stress.     

Differential Cl−/Salt Tolerance and NaCl‐Induced Alternations of Tissue and Cellular Ion Fluxes in Glycine max,Glycine soja and their Hybrid Seedlings

The salt‐sensitive Glycine max N23674 cultivar, the salt‐born Glycine soja BB52 population, and their hybrid 4076 strain (F₅) selected for salt tolerance generation by generation were used as the experimental materials in this study. First, the effects of NaCl stress on seed germination, tissue damage, and time‐course ionic absorption and transportation were compared. When qualitatively compared with seed germination appearance in culture dishes, and tissue damages on roots or leaves of seedlings, or quantitatively compared with the relative salt injury rate, the inhibition on N23674 was all the most remarkable. After the exposure of 140 mm NaCl for 1 h, 4 h, 8 h, 12 h, 2 days and 4 days, the content of Cl^? gradually increased in the roots and leaves of seedlings of BB52, 4076 and 23674. Interestingly, the extents of the Cl^? rise in roots of the three experimental soybean materials were BB52 > 4076 > N23674, whereas those in leaves were just on the contrary. Secondly, by using the scanning ion‐selective electrode technique (SIET), fluxes of Na⁺ and Cl^? in roots and protoplasts isolated from roots and leaves were also investigated among the three experimental soybean materials. After 140 mm NaCl stress for 2, 4 and 6 days, and when compared with N23674, slighter net Cl^? influxes were observed in root tissue and protoplasts of roots and leaves of BB52 and 4076 seedlings, especially at the cellular protoplast level. The results indicate that with regard to the ionic effect of NaCl stress, Cl^? was the main determinant salt ion for salt tolerance in G. soja, G. max and their hybrid, and the difference in their Cl^?/salt tolerance is mainly attributed to the capacity of Cl^? restriction to the plant above‐ground parts such as leaves.     

棉花花铃期低温对叶片PSI和PSII光抑制的影响

选用陆地棉(Gossypium hirsutum L.)品种新陆早45号,在室外盆栽至开花结铃期后,移至人工气候室,模拟新疆棉花花铃期易出现的低温逆境条件,设置处理T(16℃/10℃,昼/夜),以常温(30℃/18℃,昼/夜)处理作为对照,采用叶绿素荧光和P700同步测定技术,研究低温对棉花花铃期叶片光合机构PSII能量分配、PSI氧化还原状态及环式电子传递流的影响。结果表明,与对照相比,低温处理显著降低了棉花叶片PSII光适应状态下最大光化学量子产量(F_v'/F_m')、光化学猝灭系数(qP)和PSII有效光化学量子产量[Y(II)],并使PSII非调节性能量耗散[Y(NO)]和调节性能量耗散[Y(NPQ)]量子产量显著升高,诱导PSII发生光抑制。低温引起棉花叶片光合机构PSI受体侧限制[Y(NA)]显著下降和供体侧限制[Y(ND)]显著升高,但未引起有效的PSI复合体含量(P_m)显著降低,表明与PSII相比,棉花叶片PSI对低温不敏感。此外,低温引起环式电子传递量子产量[Y(CEF)]以及与PSII实际量子产量比率的[Y(CEF)/Y(II)]显著升高,进一步表明在低温下,光破坏防御机制中环式电子传递流对棉花PSI、PSII起着重要的保护作用,是主要的光破坏防御机制。非光化学热耗散(NPQ)和调节性非光化学热耗散[Y(NPQ)]与[Y(CEF)]具有显著的正相关关系,表明低温引起棉花花铃期叶片PSII反应中心过度关闭产生过剩的激发能,造成了PSII可逆的光抑制,环式电子传递流的响应及较高的调节性能量耗散共同保护了棉花叶片PSI和PSII免受光抑制的损伤,这可能是棉花叶片PSI对低温不敏感的重要原因。     

The Key Physiological Response to Alkali Stress by the Alkali‐Resistant Halophyte Puccinellia tenuiflora is the Accumulation of Large Quantities of Organic Acids and into the Rhyzosphere

Eight‐week‐old seedlings of Puccinellia tenuiflora were stressed by exposure to 1 : 1 molar ratio mixtures either of the two neutral salts NaCl and Na₂SO₄ or of the two alkali salts, NaHCO₃ and Na₂CO₃. To identify the physiological mechanisms involved in this plant’s resistance to alkali stress, the relative growth rates, the quantities and compositions of organic acids accumulated and secreted through the roots into the rhyzosphere, the concentrations of inorganic ions, proline and other solutes accumulating in the shoots were measured. The results show that the organic acid constituents in the shoots and roots were much the same. These were predominantly malic acid, oxalic acid, citric acid and succinic acid. The total concentration of organic acids in the shoots increased strongly with increasing alkali stress. However, these either did not increase or they decreased slightly with increasing salt stress. Of the four organic acids, the concentration difference between salt‐ and alkali‐stressed plants was most striking for citric acid. This became the dominant organic acid component under alkali stress. Results show that proline is the main organic osmolyte, whereas the contribution of betaine to osmotic adjustment is insignificant under either salt or alkali stress. The main organic acid accumulated was not only an important organic osmotic regulator, but also an important negative charge contributor, playing important roles in ionic balance and pH adjustment. The concentrations of Na⁺, K⁺, Cl^? and of organic acid were 80.7% of all solutes under salt stress. The concentrations of Na⁺, K⁺, Cl^? and of organic acid were 85.4% of all solutes under alkali stresses. The ionic balance was disrupted by the strong increase in Na⁺ content under alkali stress. This perhaps explains why large amounts of the organic acids were accumulated. The organic acid concentration in the roots was lower than in the shoots. The roots secreted citric acid into the rhyzosphere only under alkali stress, secretion of the other organic acids was not detected. Therefore, citric acid secreted from the roots probably plays an important role in pH adjustment in the rhyzosphere of P. tenuiflora.     

Water Relation, Photosynthetic Ability and Growth of Thai Jasmine Rice (Oryza sativa L. ssp. indica cv. KDML 105) to Salt Stress by Application of Exogenous Glycinebetaine and Choline

Rice reportedly possesses a very low capacity to accumulate glycinebetaine (Glybet), but may be accumulated by the exogenous application of Glybet or Choline (Cho) as an alternative way to improve its salt‐tolerant ability. The aim of this research was to determine whether Glybet accumulation could be induced in Thai jasmine rice by the exogenous application of Glybet and Cho, and to determine the effects of Glybet and Cho treatment on various growth parameters of seedlings cultured under salt‐stress conditions. Thai jasmine rice seeds were aseptically germinated in vitro on solidified Murashige–Skoog media, supplied with either Glybet or Cho in the culture media for 12 days and then treated with 342 mm NaCl (salt stress) for 4 days. GlyBet content, water relation, photosynthetic capabilities and growth characteristics of salt‐stressed seedlings were measured. The addition of Glybet or Cho to plant culture media containing 342 mm NaCl resulted in increased accumulation of Glybet in rice seedlings. Increased Glybet accumulation was strongly associated with a high efficiency of water usage (r = 0.96), which in turn correlated with increased maximum quantum yield of PSII (F_v/F_m) (r = 0.86). Moreover, the pigment concentrations of seedlings cultured under salt stress were maintained by a function of Glybet, led to high efficiency of photochemical and non‐photochemical quenching of PSII as well as to exhibit on net photosynthetic rate. Thus, our results suggest that the addition of either Glybet or Cho to the plant growth media can improve growth performance under salt stress conditions by increasing the salt tolerance of Thai jasmine rice. The exogenous application of Glybet and/or Cho to culture media may be an effective method of improving resistance to salt stress via the promotion of Glybet accumulation with in rice seedlings.     

Methyl jasmonate alleviated salinity stress in soybean

We studied the role of methyl jasmonate (MeJA) in alleviating NaCl-induced salt stress on soybean growth and development in hydroponics medium. Soybean seedlings were exposed to 60 mM NaCl stress for 2 weeks, 24 h after the application of 20 and 30 μM MeJA. NaCl stress induced a significant reduction in plant growth, endogenous bioactive gibberellin (GA4), photosynthesis and transpiration rate, while a marked increase in the endogenous abscisic acid (ABA) and proline contents were recorded. MeJA application greatly mitigated the adverse effects of NaCl on soybean growth and endogenous hormones. MeJA significantly increased ABA levels, while the endogenous amount of GA4 was reduced by the application of NaCl. Our study revealed that MeJA counteracted the negative effects of NaCl stress on plant growth, chlorophyll content, leaf photosynthetic rate, leaf transpiration rate, and proline content.     

Characterization of Barley Leaf Tolerance to Drought Stress by Chlorophyll Fluorescence and Electron Paramagnetic Resonance Studies

Two kinds of barley genotypes with various water‐stress tolerances, tolerant Cam/B1 and sensitive Maresi, were subjected to 10‐day soil‐drought stress in seedling and flag leaf developmental phases. After this time, both genotypes regardless of the growth stage showed a decrease in quantum yield of PSII photochemistry (Φ_PSII) upon stress treatment; however, this effect was stronger in the sensitive plants than in the tolerant ones. The drought stress in the flag leaf stage was associated with an increase in superoxide dismutase (SOD) level in both genotypes, whereas in seedlings, this effect was observed only for Maresi. The activity of other enzymes (catalase and peroxidase) was changed only in small degree. An increase in proline levels and activities of Δ¹‐pyrroline‐5‐carboxylate synthetase (P5CS) and ornithine delta‐aminotransferase (OAT) were observed independently of genotype and the phase of plant development, whereas the activity pyruvate dehydrogenase (PDH) decreased in tolerant genotype. Moreover, changes in the concentration of monocarbohydrates (glucose and fructose) and dicarbohydrates (saccharose, raffinose and maltose) were found: in seedlings, the amount of all soluble sugars increased, while in flag leaves decreased. The drought treatment resulted in a drop in starch level in the tolerant genotype, but in the sensitive one, the content of this substance increased in both developmental stages. EPR studies allowed the determination of the amount and character of organic radicals present in leaves. In control conditions, the content of these radical species was higher in the sensitive genotype than in tolerant one and decreased upon water stress, with the exception of flag leaves of the sensitive plant. Simulation procedure revealed four types of signals in the EPR spectra. One of them was attributed to a chlorophyll a cation and decreased upon drought. The second, ascribed to semiquinone radicals, reflected the redox balance disturbed by water deficit. The two remaining signals were connected with carbon‐centred radicals situated in the carbohydrate matrix. Their number was correlated with starch concentration.     

Response of Oryzacystatin I Transformed Tobacco Plants to Drought,Heat and Light Stress

Transformed tobacco plants expressing a rice cysteine proteinase inhibitor (OC‐I) and non‐transformed plants were grown in a controlled environment and subjected to various stresses. Two‐month‐old transformed and non‐transformed plants were exposed for 5 days to drought conditions by withholding watering. High temperature (40 °C) was applied additionally at day 6th for 5 h either individually or in combination with drought. All stress treatments were applied under low (150 μmol m^?2 s^?1 PPFD) and high light intensity (HL) of 1000 μmol m^?2 s^?1 PPFD to determine if OC‐I expression might provide protection under combination of stresses usually existing in nature. Drought stress led to diminution in leaf relative water content, photosynthesis inhibition, decrease in chlorophyll content and accumulation of malondialdehyde and proline. Heat stress alone did not affect the plants significantly, but intensified the effect of drought stress. HL intensity further increased the proline content. OC‐I transformed plants grown under low light intensity had significantly higher total superoxide dismutase and guaiacol peroxidase activities as well as their isoforms than non‐transformed control plants under non‐stress and stress conditions. Catalase activity was not highly affected by OC‐I expression. Results indicate that OC‐I expression in tobacco plants provides protection of the antioxidative enzymes superoxide dismutase and guaiacol peroxidise under both non‐stress and stress conditions.     

Methyl Jasmonate‐Induced Alteration in Lipid Peroxidation,Antioxidative Defence System and Yield in Soybean Under Drought

Methyl jasmonate (MeJA), a plant‐signalling molecule, is involved in an array of plant development and the defence responses. This study was conducted to explore the role of exogenous MeJA application in alleviating the adversities of drought stress in soybean (Glycine max L. Merrill.). Soybean plants were grown under normal conditions until blooming and were then subjected to drought by withholding irrigation followed by foliar application of (50 μm ) MeJA. Drought stress substantially suppressed the yield and yield‐related traits, whereas it accelerated the membrane lipid peroxidation. Nonetheless, substantial increase in activities of enzymatic antioxidants (superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT)), proline, relative water contents (RWC) with simultaneous decrease in membrane lipid peroxidation was observed in MeJA‐treated plants under drought. These beneficial effects led to improvement in biological and grain yield, and harvest index under drought. Interestingly, MeJA application was also useful under well‐watered conditions. These results suggest the involvement of MeJA in improving the drought tolerance of soybean by modulating the membrane lipid peroxidation and antioxidant activities.     

Na+ Retention in the Root is a Key Adaptive Mechanism to Low and High Salinity in the Glycophyte,Talinum paniculatum (Jacq.) Gaertn. (Portulacaceae)

Talinum paniculatum is an important leafy vegetable and medicinal plant, used in many parts of South America, Africa and Asia. Its adaptation to abiotic stress has received little attention and therefore worthy of interest, especially as environmental conditions are rendering arable lands increasingly unfavourable for agriculture. Therefore, this study was undertaken to examine the influence of salt stress on the vegetative growth of the plant by subjecting seedlings to 0, 25, 50, 100, 200 and 300 mm NaCl stress for 10 days. The dry weight, ion concentrations, relative water content, oxidative damage, proline, osmotic potential and some antioxidants were determined. The plants were found to retain Na⁺ mainly in the root, with less affected leaf K⁺ concentration, and consequently very low shoot Na⁺/K⁺ ratios (<0.2) under all the stress treatments. The proline content significantly increased under the 100–300 mm treatments (18‐ to 244‐fold), with a corresponding significant reduction in osmotic potential and hence high osmotic adjustment. The antioxidant enzyme activities and non‐enzyme antioxidants showed significant increase only under the highest salinity. Taken together, these results suggest that shoot Na⁺ exclusion is characteristic of this plant and is mainly responsible for its adaptation to low salinity.     

Role of Nitrogen and Gibberellin (GA3) in the Regulation of Enzyme Activities and in Osmoprotectant Accumulation in Brassica juncea L. under Salt Stress

Salt stress alters a wide array of plant metabolic mechanisms. Different strategies of the application of nutrients and phytohormones are required to overcome the adverse effects of salt stress. The main objective of the present study was to determine if added nitrogen (N) and gibberellin (GA₃) in growth medium could alleviate the adverse effects of salt stress on plant metabolism. Two‐week‐old plants were fed with: (i) 0 mm NaCl + 0 mg N kg^?1 sand + 0 m GA₃ (control), (ii) 100 mm NaCl + 0 mg N kg^?1 sand + 0 m GA₃, (iii) 0 mm NaCl + 40 mg N kg^?1 sand + 0 m GA₃, (iv) 0 mm NaCl + 0 mg N kg^?1 sand + 10^?5 m GA₃, (v) 100 mm NaCl + 40 mg N kg^?1 sand + 0 m GA₃, (vi) 100 mm NaCl + 0 mg N kg^?1 sand + 10^?5 m GA₃, (vii) 100 mm NaCl + 40 mg N kg^?1 sand + 10^?5 m GA₃. Growth and physio‐biochemical attributes i.e. shoot length, leaf area, fresh weight, dry weight, net photosynthetic rate, stomatal conductance, malondialdehyde concentration, electrolyte leakage, total chlorophyll concentration, nitrate reductase and carbonic anhydrase activities, proline and glycinebetaine concentration, leaf – N, potassium (K) and sodium (Na) concentration and K/Na ratio were affected by NaCl treatment. But application of N or GA₃ alone as well as in combination proved beneficial in alleviating the adverse effects of salt stress on these growth and physio‐biochemical parameters. However, N applied with GA₃ proved more effective than N and GA₃ applied alone. The results revealed that combined application of N and GA₃ may ameliorate most of the attributes and prove to be a physiological remedy to increase the tolerance against the ill effects of salt stress in Brassica.     

Response of Rice (Oryza sativa L.) Tillering to Sub-ambient Levels of Ultraviolet-B Radiation

The negative impact of enhanced ultraviolet‐B (UV‐B) radiation has been demonstrated for rice, but few studies have examined the effects of low UV‐B radiation on rice growth and physiology. The objective of this study was to determine the effects of low UV‐B radiation on rice physiology with special emphasis on tiller initiation and development in two popular US rice cultivars, ‘Cocodrie’ and ‘Clearfield 161 (CL161)’. Plants received no natural UV‐B radiation because of the UV‐absorption characteristics of the greenhouse glass, hence UV‐B was artificially supplied by supplemental UV‐B lighting. Plants were grown in soil collected from research plots, and were exposed to UV‐B radiation of 0, 4 (sub‐ambient) or 8 kJ m⁻² day⁻¹ (ambient) for 29 days in one experiment and 87 days in a second experiment. ‘Cocodrie’ had no differential response among 0, 4 and 8 kJ m⁻² day⁻¹ UV‐B treatments for all the parameters measured except for plant height, which showed 5% decrease at ambient UV‐B conditions. For ‘CL161’ sub‐ambient UV‐B radiation exposure decreased vegetative tiller production (25%) and total panicle dry weight (15%). The decrease in tiller production was not due to a decrease in carbohydrate content or increase in auxin content under the low UV‐B radiation.     

水分胁迫下葡萄糖对小麦幼苗光合作用和相关生理特性的影响

以15%聚乙二醇(PEG-6000)模拟水分胁迫,以不同浓度外源葡萄糖(Glc)处理小麦幼苗,探讨外源Glc对水分胁迫下小麦幼苗生长发育和光合特性的影响。结果表明,水分胁迫显著降低了小麦叶片水势和光合作用,抑制植株的生长,而水分胁迫下外源Glc处理能明显增加叶片水势和光合色素含量,并使水分胁迫和水分胁迫后复水处理条件下,小麦幼苗叶片的净光合速率(P_n)、气孔导度(G_s)胞间CO₂浓度(C_i)和叶片水分利用效率(WUE)显著升高,而使蒸腾速率(T_r)下降。同时,外源Glc处理显著提高了水分胁迫下叶片中可溶性糖和脯氨酸的积累,促进不定根和侧根的生长,植株干重比单一干旱处理提高14.32%~40.39%。由此表明,水分胁迫下外源Glc通过促进小麦根系生长和提高叶组织的渗透调节能力,改善叶片的水分状况,提高了叶片的光合功能,促进小麦幼苗的生长,降低了水分胁迫对小麦幼苗生长的抑制作用。     

室外盆栽条件下盐胁迫对甜高粱光系统II活性的影响

室外盆栽条件下, 设置2个NaCl浓度(100 mmol L^–1和200 mmol L^–1), 调查盐胁迫对甜高粱光合特性和光系统II (PSII)活性的影响。结果表明,叶片Na⁺离子含量与Na⁺/K⁺比随盐浓度增加和处理时间延长而增加。净光合速率(P_n)、光系统II开放反应中心天线转化效率(F_v¢/F_m¢)、光化学猝灭系数(q_P)和光系统II实际光化学效率(Φ_PSII)随盐浓度的增加而降低,非光化学猝灭(NPQ)随盐浓度增加而增加;100 mmol L^–1处理组的P_n、F_v¢/F_m¢、q_P和Φ_PSII随处理时间延长有所恢复,但200 mmol L^–1处理组无此现象。光系统II (PSII)最大光化学效率(F_v/F_m)在100 mmol L^–1 NaCl处理时影响较小,但在200 mmol L^–1 NaCl处理时明显下降。短期盐胁迫未影响荧光诱导动力学曲线,而200 mmol L^–1 NaCl处理5 d后荧光诱导动力学曲线O-K和O-J相上升。进一步研究证明,PSII的失活速率在两个盐浓度下均无明显变化,而修复速率在200 mmol L^–1盐浓度处理5 d后降低明显。因此,认为室外盆栽条件下盐胁迫造成甜高粱碳同化能力降低并改变PSII激发能分配;叶片Na⁺离子含量的大幅增加会导致PSII活性下降及光抑制,这与PSII失活速率无关,主要是失活PSII修复速率受抑制的结果。这对理解户外盐胁迫条件下C₄作物的光抑制机制具有一定意义。     

Glycinebetaine Improved Photosynthesis in Canola under Salt Stress: Evaluation of Chlorophyll Fluorescence Parameters as Potential Indicators

Salinity is a widespread soil problem limiting productivity of oilseed crops worldwide. Reduction in growth generally associated with decline in photosynthesis. Accumulation of osmo‐protectants such as glycinebetaine (GB) permits the plant to cope with salt stress. This study was aimed to understand the mechanism by which exogenous GB application improves photosynthetic capacity and salt tolerance in two diverse lines of canola differing in salt tolerance using chlorophyll fluorescence technique. Glycinebetaine was applied as foliarly or through rooting medium to two canola lines Dunkeld and Cyclone that were grown under non‐saline or saline conditions. Root‐applied GB caused adverse effects whereas foliar application of GB was effective in improving growth of canola cultivars. However, the response of salt‐tolerant cultivar Dunkeld was stronger than that of salt sensitive Cyclone. Foliar application of GB improved accumulation of proline and plant water status. Application of GB enhanced the photosynthetic CO₂ fixation, stomatal conductance (g_s) and water‐use efficiency. The tolerant lines Dunkeld had more responsive to GB application. Chlorophyll fluorescence measurements (F_o, F_m, F_v, F_o/F_m, F_v/F_o and F_v/F_m ratios) revealed that salt stress reduced energy trapping efficiency by damaging oxygen evolving complex, over reduction of Q_A resulting in occurrence of chronic photoinhibition. However, exogenous GB protected the oxygen evolving centre of PSII and maintains activity of PSII. Although root‐applied GB adversely affected the growth of canola plants, it did not have any adverse effect on PSII photochemistry. Moreover, fluorescence parameters could provide a rapid means for determining salt tolerance in canola and could be a rapid and sensitive test to identify genotypes highly tolerant to salt stress.     

Decay of the Chloroplast Pool of Ascorbate Switches on the Oxidative Burst in UV‐B‐Irradiated Rice

A parallel was drawn between UV‐B irradiation and the oxidative burst in the chloroplast lamellae of rice plants (Oryza sativa L. cv. Safari). After irradiation with a total biological effective UV‐B of 20.825 kJ m^?2 between the 15th and 21st days following germination, it was found that superoxide, hydroxyl radicals and hydrogen peroxide increased significantly in stressed leaves. The levels of ascorbate and the kinetics of superoxide dismutase, glutathione reductase, dehydroascorbate reductase, ascorbate peroxidase and catalase became increasingly inhibited. The developing oxidative burst also diminished photosynthetic pigments and galactolipids accumulation, while the de‐epoxidation state of xanthophylls and lipids peroxidation increased. Following grana disorganisation, the accumulation of chloroplast polypeptides having 72/69, 33/32, 28/26, 22/20 and 18/16 kDa decreased, but the relative proportion of the 49/46 kDa polypeptides increased. The 69/65 kDa polypeptides also diminished on the 15th day. These alterations implicated the failure of both photosystems functioning, but the inhibition of the Hill reactions coupled to the water splitting complex was uppermost. In the 28th day, the chloroplast lamellae of leaves grown after UV‐B exposure revealed a general recovery from oxidative burst and a subsequent higher stacking of thylakoids. It is concluded that UV‐B irradiation becomes lethal when ascorbate oxidation in the xanthophyll cycle limits the antioxidant enzymes kinetics, triggering chloroplast proteolysis and acyl lipid peroxidation because of reactive oxygen species propagation.     

Interactive Effects of High Boron and NaCl Stresses on Subcellular Localization of Chloride and Boron in Wheat Leaves

Salinity and boron (B) toxicity often occur simultaneously and may have interactive effects on plant responses. This study aims at further investigating long‐term effects of salinity, B toxicity and their combination on wheat plants. Plants grown in hydroponics were treated with 2.5 μm H₃BO₃ (control), 75 mm NaCl, 200 μm H₃BO₃ or 75 mm NaCl + 200 μm H₃BO₃ and were analysed on a subcellular level 6 weeks after germination. Shoot fresh and dry weight (DW), water content, transpiration rate and osmolality were reduced, while Na⁺ and Cl^? concentrations increased under salinity stress. However, Cl^? was reduced in all compartments by an additional application of B toxicity. At adequate B supply, NaCl increased apoplastic and symplastic soluble B concentrations, whereas the total B content remained unchanged. At high B level, however, soluble and total B was reduced by additional salt stress. Despite this antagonistic effect of a combined salt and B toxicity stress on Cl^? and B concentrations, an additive effect was observed regarding shoot fresh weight reduction. Our results confirm an alleviating effect of the combined stresses on toxic ion concentrations, which did not prevent additive growth reductions.