外源SNP对干旱胁迫下不同马铃薯品种叶片抗氧化酶活性的影响

以正常水分状态、轻度干旱胁迫、中度干旱胁迫和重度干旱胁迫下的马铃薯抗旱品种‘底西瑞’和干旱敏感品种‘大西洋’ 植株为材料,于现蕾期采用0（对照）和0.01 mmol·L^-1 SNP分别喷施各处理植株,对不同处理下2个品种的植株形态、叶片超氧阴离子和H₂O₂含量以及抗氧化酶活性进行比较分析,探讨外源SNP对干旱状态下马铃薯的生理应答机制,为马铃薯的抗旱栽培提供新的技术理论支持。结果显示：（1）SNP喷施对重度水分胁迫下马铃薯植株的正常生长具有一定的保护作用。（2）在干旱胁迫条件下,马铃薯叶片POD活性在品种‘底西瑞’中增加而在品种‘大西洋’中降低,超氧阴离子含量和H₂O₂含量以及CAT和APX活性在各品种中均增加,但超氧阴离子含量和H₂O₂含量增加程度与胁迫程度无关。（3）抗旱品种‘底西瑞’在干旱胁迫下的超氧阴离子含量低于干旱敏感品种‘大西洋’,而其POD、CAT和APX活性则高于‘大西洋’; 0.01 mmol·L^-1SNP处理未改变马铃薯叶片中超氧阴离子和H₂O₂含量随土壤水分的变化趋势,但改变了‘大西洋’叶片中SOD、POD、CAT活性以及‘底西瑞’叶片中APX活性的变化趋势。（4）外源喷施0.01 mmol·L^-1SNP降低了‘底西瑞’在中度和重度胁迫下以及‘大西洋’在轻度和中度胁迫下超氧阴离子含量,提高了干旱胁迫下‘底西瑞’和‘大西洋’的POD和APX活性。研究表明,POD、CAT和APX可作为马铃薯水分胁迫下的应答以及品种抗旱性的筛选指标,外源SNP可通过诱导增强干旱胁迫下马铃薯的抗氧化酶活性来提高其抗旱性。     

谷氨酸脱羧酶放射测量法的改良及应用

用NaOH代替苯乙胺作为¹⁴CO₂的吸附剂,改进谷氨酸脱羧酶（GAD）活性的放射测定方法,结果发现NaOH为吸附剂组内变异系数为9.6%, 以苯乙胺为吸附剂组内变异系数为31.9%;以NaOH为吸附剂72 h后测量其放射活性仍稳定不变,以苯乙胺为吸附剂者1 h后放射性活性即下降47%,6 h后已降低至本底水平;¹⁴CO₂重吸收实验亦证明以苯乙胺为吸附剂吸附的¹⁴CO₂ 6 h内已有80%以上重新被NaOH吸附;以NaOH作为吸附剂测定GAD的活性,在0.39～17.8 mg脑组织样品范围内GAD量与¹⁴CO₂生成量之间有线性关系.NaOH代替苯乙胺作为¹⁴CO₂的吸附剂测定GAD的活性其灵敏度提高1.66倍.用此方法测定组织和细胞内GAD活性证明其具有良好的重复性和稳定性,值得推广应用.     

肋果沙棘幼苗抗氧化酶系统及总黄酮对UV-B辐射增强的响应

该文以青藏高原特有木本植物肋果沙棘为材料,在62 μW·cm^-2的UV-B辐射强度下,分别测定处理了0～6 d幼苗叶片的氧化损伤程度、抗氧化系统酶活性和总黄酮含量及其抗氧化活性,以探究肋果沙棘对UV-B辐射的生理生态响应机制。结果表明:随UV-B辐射时间的增加,肋果沙棘幼苗过氧化氢含量(H₂O₂)和膜脂过氧化产物(MDA)显著增加; 抗氧化系统酶中过氧化氢酶(CAT)活性显著升高; 过氧化物酶(POD)、抗坏血酸过氧化物酶(APX)活性呈先降后升的趋势,且活性均显著低于对照,超氧化物歧化酶(SOD)活性无明显变化; 总黄酮含量随辐射时间的积累显著增加,作为非酶抗氧化物质的总黄酮对1,1-二苯基-2-苦基肼(DPPH)的清除率与其含量变化具有显著正相关关系。综上结果表明,肋果沙棘幼苗在抵御该辐射产生的氧化损伤中,通过提高CAT活性及增加总黄酮含量来抵御辐射造成的氧化损伤     

盐与碱胁迫对燕麦离子平衡和有机酸含量的影响

以燕麦品种‘白燕2号’为材料,试验分别设置0、50、100、150、200 mmol/L盐胁迫(NaCl∶Na₂SO₄=5∶1)和碱胁迫(NaHCO₃∶Na₂CO₃=5∶1)处理的温室内盆栽试验,观测燕麦植株生长速率、植株含水率、叶片离子含量及叶片各类有机酸含量,分析不同盐胁迫、碱胁迫对燕麦离子平衡的影响,并比较燕麦对两类胁迫的适应性差异。结果显示：(1)燕麦植株生长速率和植株含水率在低浓度(50和100 mmol/L)盐胁迫下均升高,而高浓度(150和200 mmol/L)盐胁迫下则降低;燕麦植株生长速率和植株含水率均随碱胁迫浓度增加而降低;在相同胁迫浓度下,碱胁迫对植株生长率、植株含水率的影响大于盐胁迫。(2)燕麦叶片K⁺、Ca²⁺、Mg²⁺、H₃PO^-₄、NO^-₃ 含量均随盐、碱浓度升高而降低,而Na⁺、Cl^-、SO^2-₄含量在盐、碱胁迫下均大幅上升;200 mmol/L盐、碱胁迫下,Na⁺ 含量分别较对照增加367.15%和518.41%,Cl^- 含量分别较对照增加785.07%和52.59%,SO^2-₄ 分别较对照增加142.01%和52.86%。(3)200 mmol/L盐、碱胁迫下,有机酸分别较对照增加74.52%和1 232.34%;碱胁迫及高浓度盐胁迫下燕麦叶片的柠檬酸、乌头酸、琥珀酸和苹果酸含量均高于对照,且乌头酸是燕麦响应盐胁迫、碱胁迫的主要有机酸成分,柠檬酸和琥珀酸略有变化,而甲酸、乙酸、乳酸、苹果酸、草酸含量均相对较低。研究表明,碱胁迫对燕麦植株生长速率、植株含水率、叶片离子含量及叶片各类有机酸含量的影响大于盐胁迫;盐胁迫与碱胁迫均引起燕麦叶片阳离子(Na⁺)大量积累,而K⁺、Ca²⁺、Mg²⁺、H₃PO^-₄及NO^-₃吸收受阻;燕麦叶片在盐胁迫下主要通过积累Cl^-调节叶片离子平衡,而碱胁迫下主要通过积累有机酸来调节离子平衡;有机酸是燕麦叶片响应碱胁迫的特异代谢物,其中乌头酸是其有机酸的主要成分。     

NO和H2O2诱导大豆根尖和边缘细胞耐铝反应的作用

 NO和H₂O₂是参与植物抗非生物胁迫反应的重要信号分子, 为了确定NO和H₂O₂在大豆(Glycine max)根尖和根边缘细胞(root border cells, RBCs)耐铝反应中的作用及其相互关系, 以‘浙春3号’大豆为材料, 研究了铝毒胁迫下大豆根尖内源NO和H₂O₂的变化, 以及外源NO和H₂O₂诱导大豆根尖和RBCs的耐铝反应。结果表明, 50 μmol·L^–1 Al处理48 h显著抑制大豆根的伸长, 提高Al在根尖的积累, 同时显著增加根尖内源NO和H₂O₂含量。施加0.25 mmol·L^–1外源NO供体亚硝基铁氰化钠(Na₂[Fe(CN)₅NO]·2H₂O, sodium nitroprusside, SNP)和0.1 mmol·L^–1H₂O₂, 能有效地缓解Al对大豆根伸长的抑制、根尖Al积累和RBCs 的死亡, 该缓解作用可以被0.05 mmol·L^–1 NO清除剂2-(4- 羧基苯)-4,4,5,5- 四甲基咪唑-1- 氧-3- 氧化物, 钾盐(C₁₄H₁₆N₂O₄·K, carboxy-PTIO, cPTIO)和150 U·mL^–1 H₂O₂清除酶(catalase, CAT)逆转。并且外源NO能够显著促进根尖H₂O₂的积累, 而外源H₂O₂对根尖NO的含量无显著影响。这表明NO和H₂O₂是诱导大豆根尖及RBCs耐铝反应的两种信号分子, NO可能通过调控H₂O₂的形成, 进而诱导大豆根尖及RBCs的耐铝反应。     

皇竹草活性氧代谢对阿特拉津胁迫的响应特征

采用水培实验研究了4个浓度（5、10、20、40 mg·L^-1）除草剂阿特拉津胁迫下,皇竹草（Pennisetum hydridum）叶片内超氧阴离子生成速率、过氧化氢（H₂O₂）含量、超氧化物歧化酶（SOD）活性、过氧化氢酶（CAT）活性、过氧化物酶（POD）活性、丙二醛（MDA）含量、原生质膜透性的变化,探讨皇竹草对阿特拉津的抗性及其生理机制。结果显示:（1）低浓度（5、10 mg·L^-1）的阿特拉津胁迫使皇竹草叶片内超氧阴离子生成速率和CAT活性升高,却使H₂O₂含量及SOD和POD活性降低,但随着培养时间的延长,培养液中阿特拉津浓度的降低导致上述指标又有恢复到正常水平的趋势;而高浓度（40 mg·L^-1）的阿特拉津胁迫则使皇竹草叶片内H₂O₂含量、SOD、POD和CAT活性持续降低。（2）在各胁迫浓度下持续胁迫10 d后,皇竹草叶片内MDA含量开始逐渐升高,并且升高幅度随着胁迫浓度的提高而明显增加,但各胁迫浓度下叶片原生质膜相对透性未见明显的变化。研究表明,皇竹草可能通过活性氧等信号分子调控自身保护酶系统的活性来缓解阿特拉津造成的伤害,从而对低浓度（5、10 mg·L^-1）的阿特拉津胁迫表现出较强抗性。     

He-Ne激光预处理对镉胁迫下小麦幼苗生理特性的影响

用He-Ne激光（波长632.8 nm,辐射剂量5.43 mW/mm²）对萌动小麦种子辐照5 min,待幼苗长至一叶一心时,用150 μmol/L CdCl₂溶液进行胁迫处理,研究He-Ne激光预处理对镉（Cd²⁺）胁迫下小麦幼苗生长发育和生理特性的影响。结果显示:He-Ne激光预处理能显著降低Cd²⁺胁迫下小麦幼苗中丙二醛（MDA）、过氧化氢（H₂O₂）含量及超氧自由基（O^-·₂）产生速率,显著提高幼苗叶片超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)、抗坏血酸氧化酶（APX）活性,并使叶片抗氧化物质谷胱甘肽(GSH)和抗坏血酸(AsA)含量以及幼苗株高、根长和干重增加。研究表明,He-Ne激光预处理可有效缓解镉胁迫对小麦幼苗生长的抑制作用,并通过促进其幼苗中酶类和非酶类抗氧化剂的产生,有效减少镉胁迫产生的脂质过氧化物含量,从而提高其耐镉性。     

外源H2O2对盐胁迫下小白菜种子萌发和幼苗生理特性的影响

以小白菜"甜脆青"为试材,研究不同浓度（5、10、25、50和100 mmol/L）过氧化氢（H₂O₂）浸种处理对100 mmol/L NaCl胁迫下小白菜（Brassica chinensis L.）种子萌发、幼苗生长及生理特性的影响。结果表明：100 mmol/L NaCl胁迫明显抑制小白菜种子的萌发状况和幼苗生长,发芽势、发芽指数、活力指数及幼苗根和芽长度和鲜重均明显降低,根和芽中CAT的活性及K⁺含量明显受到抑制,渗透调节物质、活性氧和MDA含量显著增加。不同浓度H₂O₂浸种处理提高了NaCl胁迫下小白菜种子发芽势、发芽指数和活力指数,促进小白菜根和芽的生长,增强了NaCl胁迫下根和芽中SOD、CAT和APX的活性及K⁺含量,降低O₂^-·产生速率及H₂O₂和MDA含量,进一步促进脯氨酸和可溶性糖含量的增加,降低体内Na⁺含量。其中以10 mmol/L H₂O₂处理缓解盐胁迫效果最好,明显缓解NaCl胁迫对小白菜种子萌发和幼苗生长的抑制。     

鸟氨酸脱羧酶活性微量测定法

本文采用一种简单,微量反应系统,根据 ¹⁴C-鸟氨酸释放的 ¹⁴CO₂量测定鸟氨酸脱羧酶(ODC)的活性,酶反应在置于液闪计数瓶内的玻璃小管中进行,释放的 ¹⁴CO₂被瓶内滤纸片上的海胺吸收。实验结果表明,加酸释放 ¹⁴CO₂后30分钟 ¹⁴CO₂吸收已达最大值,且吸收量与释放量成正比,酶反应测定证明 ¹⁴CO₂释放速度在40分钟内保持恒定。ODC活性与酶浓度呈线性关系,此方法不仅用于ODC活性测定,而且亦可用于其他脱羧酶活性的测定。     

外源NO对增补UV-B辐射下兴安落叶松幼苗叶片光合色素和叶绿素荧光特性的影响

在增强UV-B辐射下,以3年生兴安落叶松幼苗为实验材料,研究了外源NO供体硝普钠(Sodium nitroprusside,SNP)对幼苗的光合色素（Chla、Chlb和Car）和叶绿素荧光参数的影响。方差分析结果表明0.5 mmol·L^-1的SNP对增补UV B胁迫下的兴安落叶松幼苗产生显著影响。0.5 mmol·L^-1的SNP能够显著抑制增补UV-B辐射后光合色素、F_v/F_m、Φ_PSⅡ、F_v′/F_m′和qP的明显下降以及Chla /Chlb、F_o和NPQ的升高。表明了外源NO能够减轻UV-B辐射胁迫下兴安落叶松幼苗光合反应中心的生理损伤,从而增强兴安落叶松幼苗对增补UV-B辐射胁迫环境的适应能力。     

Effect of UV-B radiation on the growth and antioxidant enzymes of Antarctic sea ice microalgae <Emphasis Type="Italic">Chlamydomonas</Emphasis> sp. ICE-L

The effect of ultraviolet-B (UV-B) radiation on Antarctic phytoplankton has become an attractive ecological issue as a result of annual springtime ozone depletion. The effects of UV-B radiation on the growth and antioxidant enzymes were investigated using Antarctic sea ice microalgae Chlamydomonas sp. ICE-L as the material in this study. The results demonstrated that UV-B radiation could notably inhibit the growth, especially at high UV-B radiation intensity (70 μW cm⁻²). Malondialdehyde and O₂ ^·− content in ICE-L increased rapidly in early days (1–3 days) exposed to UV-B radiation enhancement, then decreased rapidly. In the stress of UV-B radiation enhancement, the superoxide dismutase, peroxidase and Catalase activities of 1–4 days in ICE-L were obviously higher than those in the control, and their activities became higher at high UV-B radiation intensity (70 μW cm⁻²). These enzymes activity of 7 days would kept stable at low UV-B radiation intensity (35 μW cm⁻²), but kept high level at high UV-B radiation intensity (70 μW cm⁻²). However, the ascorbate peroxidase activity in ICE-L kept stable under the stress of UV-B radiation enhancement. The above experimental results indicated that the antioxidant enzyme system played an important role in the adaptation of Antarctic ice microalgae under the UV-B radiation change of Antarctic ecosystems.     

Protective role of selenium in wheat seedlings subjected to enhanced UV-B radiation

Selenium (Se) is beneficial for some plants and is able to increase resistance and antioxidant capacity of plants subjected to stressful environment. In this work, the effects of enhanced ultraviolet-B (UV-B) radiation, Se supply, and their combination on growth and physiological traits of wheat (Triticum aestivum L., cv. Han NO.7086) seedlings were studied. The objective was to elucidate whether Se could alleviate the expected adverse effects of UV-B stress on seedlings. UV-B treatment caused a marked decline in growth parameters and total chlorophyll content and changed biomass allocation between aboveground and underground parts, which led to an increase in the root/shoot ratio. UV-B treatment also increased MDA content and the rate of superoxide radical (O₂^·−) production, although it increased some antioxidant (proline, phenolic compounds, and flavonoids) content and activity of antioxidant enzymes (peroxidase, superoxide dimutase, catalase (CAT)). Se treatment only increased total chlorophyll content and CAT activity. Compared with UV-B treatment alone, the combined treatment with UV-B and Se induced a significant increase in the biomass, total chlorophyll content, antioxidant content, and activity of antioxidant enzymes, and an evident decrease in MDA content and the rate of O₂^·− production. The results of this study demonstrated that Se alleviated the damage caused by UV-B to wheat seedlings to some extent by increasing antioxidant enzyme activity and antioxidant content.     

Protective effect of nitric oxide against oxidative stress under ultraviolet-B radiation.

The response of bean leaves to UV-B radiation was extensively investigated. UV-B radiation caused increase of ion leakage, loss of chlorophyll, and decrease of the maximum efficiency of PSII photochemistry (Fv/Fm) and the quantum yield of PSII electron transport (PhiPSII) of bean leaves. H2O2 contents and the extent of thylakoid membrane protein oxidation increased, indicated by the decrease of thiol contents and the increase of carbonyl contents with the duration of UV-B radiation. Addition of sodium nitroprusside, a nitric oxide (NO) donor, can partially alleviate UV-B induced decrease of chlorophyll contents, Fv/Fm and PhiPSII. Moreover, the oxidative damage to the thylakoid membrane was alleviated by NO. The potassium salt of 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, a specific NO scavenger, arrested NO mediated protective effects against UV-B induced oxidative damage. Incubation of thylakoid membrane with increasing H2O2 concentrations showed a progressive enhancement in carbonyl contents. H2O2 contents were decreased in the presence of NO under UV-B radiation through increased activities of superoxide dismutases, ascorbate peroxidases, and catalases. Taken together, the results suggest that NO can effectively protect plants from UV-B damage mostly probably mediated by enhanced activities of antioxidant enzymes.     

Nitric Oxide Alleviates Oxidative Damage Induced by Enhanced Ultraviolet-B Radiation in Cyanobacterium

To study the role of nitric oxide (NO) on enhanced ultraviolet-B (UV-B) radiation (280–320 nm)-induced damage of Cyanobacterium, the growth, pigment content, and antioxidative activity of Spirulina platensis-794 cells were investigated under enhanced UV-B radiation and under different chemical treatments with or without UV-B radiation for 6 h. The changes in chlorophyll-a, malondialdehyde content, and biomass confirmed that 0.5 mM sodium nitroprusside (SNP), a donor of nitric oxide (NO), could markedly alleviate the damage caused by enhanced UV-B. Specifically, the biomass and the chlorophyll-a content in S. platensis-794 cells decreased 40% and 42%, respectively under enhanced UV-B stress alone, but they only decreased 10% and 18% in the cells treated with UV-B irradiation and 0.5 mM SNP. Further experiments suggested that NO treatment significantly increased the activities of superoxide dismutase (SOD) and catalase (CAT), and decreased the accumulation of O ₂ ⁻ in enhanced UV-B-irradiated cells. SOD and CAT activity increased 0.95- and 6.73-fold, respectively. The accumulation of reduced glutathione (GSH) increased during treatment with 0.5 mM SNP in normal S. platensis cells, but SNP treatment could inhibit the increase of GSH in enhanced UV-B-stressed S. platensis cells. Thus, these results suggest that NO can strongly alleviate oxidative damage caused by UV-B stress by increasing the activities of SOD, peroxidase, CAT, and the accumulation of GSH, and by eliminating O ₂ ⁻ in S. platensis-794 cells. In addition, the difference of NO origin between plants and cyanobacteria are discussed.     

Changes in morphological, photosynthetic and physiological responses of Mono Maple seedlings to enhanced UV-B and to nitrogen addition

In the southeast of the Qinghai-Tibetan Plateau of China, Mono Maple is a common species in reforestation processes. The paper mainly investigated the changes in morphological, photosynthetic and physiological responses of Mono Maple seedlings to UV-B radiation, nitrogen supply and their combination. The experimental design included two levels of UV-B treatments (ambient UV-B, 11.02 KJ m⁻² day⁻¹; enhanced UV-B, 14.33 KJ m⁻² day⁻¹) and two nitrogen levels (0; 20 g N m⁻² a⁻¹)—to determine whether the adverse effects of UV-B on plants are eased by nitrogen supply. Enhanced UV-B caused a marked decline in growth parameters, net photosynthetic rate, and photosynthetic pigments, whereas it induced an increase in reaction oxygen species (hydrogen peroxide accumulation and the rate of superoxide radical production) and malondialdehyde content. Enhance UV-B also induced an increase in antioxidant compounds of Mono Maple, such as UV-B absorbing compounds, proline content, and activities of antioxidant enzymes (peroxidase, superoxide dimutase and catalase). On the other hand, nitrogen supply caused an increase in some growth parameters, net photosynthetic rate, photosynthetic pigments and antioxidant compounds (peroxidase, proline content and UV-B absorbing compounds), and reduced the content of reaction oxygen species (H₂O₂ accumulation, the rate of O₂⁻ production) and malondialdehyde content under ambient UV-B. However, under enhanced UV-B, nitrogen supply inhibited some growth parameters, and increased H₂O₂ accumulation, the rate of O₂⁻ production and MDA content, though proline content, UV-B absorbing compounds and activities of POD and SOD increased. These results implied that enhanced UV-B brought harmful effects on Mono Maple seedlings and nitrogen supply made plants more sensitive to enhanced UV-B, though increased some antioxidant activity.     

Counteractive action of nitric oxide on the decrease of nitrogenase activity induced by enhanced ultraviolet-B radiation in cyanobacterium

The experimental enhancement of UV-B radiation resulted in damage to chlorophyll-a in Spirulina platensis 794, and the degree of this damage was modified by chemical treatments. The addition of 0.5 mM sodium nitroprusside (SNP), a donor of nitric oxide (NO), to cultures of Spirulina platensis 794 could markedly alleviate the damage to chlorophyll-a caused by enhanced ultraviolet-B radiation. Exposure of N₂-fixing cyanobacterium Spirulina platensis 794 to enhanced ultraviolet-B radiation resulted in an intensity-dependent inhibition of nitrogenase activity. In cultured cells that were treated with 0.5 mM SNP and enhanced UV-B for 6 h, nitrogenase activity increased by 47.3% compared with UV-B treated control cells. SNP apparently counteracted the decrease in nitrogenase activity caused by UV-B stress. NAC (a free radical scavenger) significantly increased nitrogenase activity, but PTIO (a nitric oxide scavenger) decreased nitrogenase activity in UV-B treated S. platensis 794. Thus, the free radical scavenger NAC and NO may counteract the effects of enhanced UV-B radiation. The activity of UV-B-inhibited nitrogenase did not recover upon transfer of exposed cells to fluorescent light, suggesting that the inhibition may be due to specific inactivation of the enzyme. By experimentally manipulating the inhibitors of photosystem-II activity, it was demonstrated that nitrogenase activity in cyanobacterium S. platensis 794 is limited by the amount of reductant and ATP. This result further confirmed that nitrogenase activity requires a continued and abundant supply of suitable reductant and ATP for conversion of N₂ to NH₃. The effects of UV-B treatment on nitratase activity were also examined, and enhanced UV-B radiation increased nitratase activity. In addition, enhanced UV-B in combination with SNP and NAC resulted in significant increases in the activity of nitratase.     

The Regulation of Exogenous Jasmonic Acid on UV-B Stress Tolerance in Wheat

Enhanced ultraviolet-B radiation (UV-B, 280?C320?nm) is recognized as one of the environmental stress factors that cannot be neglected. Jasmonic acid (JA) is an important signaling molecule in a plant??s defense against biotic and abiotic stresses. To determine the role of exogenous JA in the resistance of wheat to stress from UV-B radiation, wheat seedlings were exposed to 0.9?kJ?m^?2?h^?1 UV-B radiation for 12?h after pretreatment with 1 and 2.5?mM JA, and the activity of antioxidant enzymes, the level of malondialdehyde (MDA), the content of UV-B absorbing compounds, photosynthetic pigments, and proline and chlorophyll fluorescence parameters were measured. The results of two-way ANOVA illustrated that the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), MDA level, anthocyanin and carotenoid (Car) content, and almost all chlorophyll fluorescence parameters were significantly affected by UV-B, JA, and UV-B?×?JA (P?<?0.05) [the maximal efficiency of photosystem II photochemistry (F _v/F _m) was not affected significantly by UV-B radiation]. Duncan??s multiple-range tests demonstrated that UV-B stress induced a significant reduction in plant photosystem II (PSII) function and SOD activity and an increased level of membrane lipid peroxidation, indicative of the deleterious effect of UV-B radiation on wheat. JA pretreatment obviously mitigated the detrimental effect of UV-B on PSII function by increasing F _v/F _m, reaction centers?? excitation energy capture efficiency (F _v??/F _m??), effective photosystem II quantum yield (??PSII), and photosynthetic electron transport rate (ETR), and by decreasing nonphotochemical quenching (NPQ) of wheat seedlings. Moreover, the activity of SOD and the content of proline and anthocyanin were provoked by exogenous JA. However, the MDA level was increased and Car content was decreased by exogenous JA with or without the presence of supplementary UV-B, whereas the contents of chlorophyll and flavonoids and related phenolics were not affected by exogenous JA. Meanwhile, exogenous JA resulted in a decrease of CAT and POD activities under UV-B radiation stress. These results partly confirm the hypothesis that exogenous JA could counteract the negative effects of UV-B stress on wheat seedlings to some extent.     

UV-B induced stress responses in three rice cultivars

UV-B responses of three rice (Oryza sativa L.) cultivars (Sasanishiki, Norin 1 and Surjamkhi) with different photolyase activity were investigated. Carbon dioxide assimilation data support that Sasanishiki was less sensitive to UV-B than Norin 1 and Surjamkhi. UV-B radiation sharply decreased the content of Rubisco protein in Surjamkhi and has no effect in Sasanishiki. The photochemical activities of photosystem (PS) 1 and PS 2 was slightly affected by UV-B treatment. The content of H₂O₂ and the activities of antioxidant enzymes, catalase (CAT), peroxides (POX) and superoxide dismutase (SOD) were enhanced after UV-B treatment. The activities of CAT and POX isoenzymes in Sasanishiki were more enhanced by UV-B radiation than those in Norin 1 and Surjamkhi.     

The combined effects of CO2 concentration and enhanced UV-B radiation on faba bean. 3. Leaf optical properties,pigments, stomatal index and epidermal cell density

Seedlings of Vicia faba L. (cv. Minica) were grown in a factorial experiment in a greenhouse. The purpose of the study was to determine whether CO₂ enrichment and supplemental UV-B radiation affect leaf optical properties and whether the combined effects differ from single factor effects. Seedlings were grown at either 380 mol mol^-1 or 750 mol mol^-1 CO₂ and at four levels of UV-B radiation. After 20 and 40 days of treatment, absorptance, transmittance and reflectance of photosynthetically active radiation (PAR) were measured on the youngest fully developed leaf. On the same leaf, the specific leaf area on a fresh weight basis (SLA_fw), chlorophyll content, UV-B absorbance, transmittance of UV light and stomatal index were measured. UV-B radiation significantly increased PAR absorptance and decreased PAR transmittance. The increased PAR absorptance can be explained by an increased chlorophyll content in response to UV-B radiation. Leaf transmittance of UV radiation decreased with increasing UV-B levels mainly caused by increased absorbance of UV absorbing compounds. UV-B radiation decreased both the stomatal density and epidermal cell density of the abaxial leaf surface, leaving the stomatal index unchanged. Effects of CO₂ enrichment were less pronounced than those of UV-B radiation. The most important CO₂ effect was an increase in stomatal density and epidermal cell density of the adaxial leaf surface. The stomatal index was not affected. No interaction between CO₂ and UV-B radiation was found. The results are discussed in relation to the internal light environment of the leaf.     

Competition and sensitivity of wheat and wild oat exposed to enhanced UV-B radiation at different densities under field conditions

The influence of enhanced UV-B radiation (approximating a 15% ozone layer reduction) on competitive interaction between spring wheat (Triticum aestivum) and wild oat (Avena fatua) was examined in the field. The density-dependent mortality of both wheat and wild oat did not exhibit a significant difference between control and UV-B treatment conditions. A relatively high degree of competitive stress enhanced the effects of UV-B stress on biomass reduction. The relative competitive status of wheat in terms of total biomass increased under UV-B enhancement while it decreased when based upon grain production. Shifts in competitive balance occurred with significant changes in total biomass, especially when plants grew at higher densities in monocultures and mixtures. The sensitivity of wild oat to intensification of UV-B radiation at higher densities in mixtures was greater than that at lower densities. At all densities examined, wheat grown in mixture was significantly less sensitive to UV-B radiation than that in monoculture, and just the opposite for wild oat. The density of monocultures did not alter the response index (RI) of wheat and wild oat to enhanced UV-B radiation.