三叶橡胶光合作用能力和抗氧化系统以及单萜类物质对茉莉酸的响应

用茉莉酸（JA）处理三叶橡胶,分别在JA处理1、24、48、72、96、120h后,分析三叶橡胶光合作用能力和抗氧化系统活性以及单萜类物质的变化。结果发现,与对照相比,用JA处理的三叶橡胶的叶片最大净光舍速率和气孔导度显著下降,光系统Ⅱ潜在最大光化学效率和内在光化学效率也明显下降。光合作用能力的下降可能是由于JA处理导致的叶绿素、类胡萝卜素、可溶性蛋白质含量逐渐下降的结果。然而,JA处理却导致过氧化氢含量逐渐增加,过氧化氢含量的增加激活了抗氧化系统,结果导致超氧化物歧化酶、过氧化氢酶、过氧化物酶活性上升,抗坏血酸-谷胱甘肽循环加强。但是,α-蒎烯、β-蒎烯、桧烯以及总单萜这些潜在的抗氧化物的含量却先增加,而后逐渐下降。     

低温胁迫对广玉兰幼苗光合及叶绿素荧光特性的影响

以广玉兰幼苗为材料,对其进行0,-4℃、-8℃,-12℃和-16℃ 5个梯度低温处理,研究了广玉兰幼苗的光合作用与叶绿素荧光特性变化.结果表明:随着温度降低,广玉兰幼苗叶片的净光合速率(Pn)、蒸腾速率(Tr)、气孔导度(G3)、叶肉细胞间隙CO2浓度(Ci)、叶绿素含量均逐步降低,最大光化学效率(Fv/Fm)、潜在光化学效率(Fv/F0)、最大量子产额(Yield)、光合电子传递速率(ETR)、光化学猝灭系数(qp)也均同时下降,而初始荧光(F0)、非光化学猝灭系数(qN)则逐步上升.可见,低温胁迫除导致光合作用的气孔抑制外,还直接损伤光合机构使PSⅡ反应中心失活,引起其光能原初捕捉能力和光能同化率减弱,增加了通过热辐射消耗的光能比例,最终导致广玉兰幼苗光合作用能力减弱.     

γ-氨基丁酸(GABA)对低氧胁迫下甜瓜幼苗光合作用和叶绿素荧光参数的影响

采用营养液水培方法,研究了低氧胁迫下外源γ-氨基丁酸(GABA)对甜瓜幼苗光合色素含量、光合作用及叶绿素荧光参数的影响.结果表明:低氧胁迫导致甜瓜幼苗光合色素含量显著下降,光合作用降低;外源GABA能显著提高正常通气和低氧胁迫下甜瓜幼苗的光合色素含量、净光合速率、气孔导度、胞间CO2浓度、CO2羧化效率、最大光化学效率、光化学猝灭系数、表观光合电子传递速率和PSⅡ光合电子传递量子效率,而气孔限制值、初始荧光和非光化学猝灭系数显著降低,GABA在低氧胁迫下的提高效果更明显;同时添加GABA和GABA转氨酶抑制剂γ-乙烯基-γ-氨基丁酸(VGB)处理显著降低了低氧胁迫下GABA对甜瓜幼苗光合特性的缓解效果.     

2,4-表油菜素内酯对高温胁迫下甜瓜幼苗抗氧化酶活性和光合作用的影响

以耐热甜瓜品种红绿早脆和热敏感品种白玉香为材料,在人工气候箱内采用基质栽培法,研究了2,4-表油菜素内酯(EBR)处理对高温胁迫下甜瓜幼苗抗氧化酶活性和光合作用的影响.结果显示:EBR能有效促进高温胁迫下甜瓜幼苗的生长,增加保护酶活性,增强植株抗氧化能力,使脯氨酸和可溶性蛋白含量升高,丙二醛(MDA)含量下降,能有效缓解高温胁迫引起的膜脂过氧化伤害;同时,光合指标净光合速率(Pn)和气孔导度(Gs)升高,胞间CO2浓度(Ci)和蒸腾速率(Tr)下降;叶绿素荧光参数初始荧光(F0)下降,最大光化学效率(Fv/Fm)、光系统Ⅱ光能捕获效率(Fv′/Fm′)和实际光化学效率(ФPSⅡ)升高,且热敏感品种白玉香变化幅度大于耐热品种红绿早脆.研究结果说明,EBR有利于甜瓜幼苗在高温胁迫下抗氧化酶活性的提高和对光能的捕获与转换,促进了甜瓜幼苗的生长,降低高温胁迫对甜瓜幼苗的抑制作用,且对热敏感品种白玉香效果大于耐热品种红绿早脆.     

食用黑粉菌侵染对茭白植株抗氧化系统和叶绿素荧光的影响

研究了茭白(Zizania latifolia Turcz.)受食用黑粉菌(Ustilago esculenta)侵染后的生长、抗氧化特性和叶绿素荧光的变化.结果表明,食用黑粉菌侵染导致植株的株高、叶长、叶宽、叶片厚度、根长、茎鲜重、根鲜重和地上部鲜重下降,但分蘖数提高;同时提高了茭白叶片的抗坏血酸过氧化物酶(APX)、过氧化氢酶(CAT)、过氧化物酶(POD)、谷胱甘肽还原酶(GR)、超氧化物歧化酶(SOD)的活性和过氧化氢(H2O2)含量,但却引起超氧自由基(O2-)产生速率和丙二醛(MDA)含量降低.食用黑粉菌侵染提高了叶片的PSⅡ实际光化学效率(ΦPSⅡ)和电子传递速率(ETR),同时降低了非光化学猝灭(NPQ),但对PSⅡ最大光化学量子产量(Fv/Fm)的影响不大.     

基于光子学技术研究弱光胁迫早期拟南芥光合损伤机理

弱光限制植物的光合作用,降低了光合作用效率,造成农业产量下降.本文主要研究了弱光处理早期,拟南芥光合作用相关指标的变化.研究中发现在弱光处理的早期,植株生长表型和最大光化学效率（Fv/Fm）没有明显变化,实际光化学效率Y（Ⅱ）以及光系统电子传递效率（ETR）下降较明显.此外,弱光处理原生质体,利用2 ＇,7＇-二氯二氢荧光素二乙酯（dichlorofluorescin diacetate,H2DCF-DA）染色,共聚焦显微镜观察,发现细胞中有较明显的活性氧（ROS）合成,且定位于叶绿体.该研究结果为植物弱光耐受性的研究提供理论依据.     

γ-氨基丁酸（GABA）对低氧胁迫下甜瓜幼苗光合作用和叶绿素荧光参数的影响

采用营养液水培方法,研究了低氧胁迫下外源γ-氨基丁酸（GABA）对甜瓜幼苗光合色素含量、光合作用及叶绿素荧光参数的影响.结果表明：低氧胁迫导致甜瓜幼苗光合色素含量显著下降,光合作用降低;外源GABA能显著提高正常通气和低氧胁迫下甜瓜幼苗的光合色素含量、净光合速率、气孔导度、胞间CO₂浓度、CO₂羧化效率、最大光化学效率、光化学猝灭系数、表观光合电子传递速率和PSⅡ光合电子传递量子效率,而气孔限制值、初始荧光和非光化学猝灭系数显著降低,GABA在低氧胁迫下的提高效果更明显;同时添加GABA和GABA转氨酶抑制剂γ-乙烯基-γ-氨基丁酸（VGB）处理显著降低了低氧胁迫下GABA对甜瓜幼苗光合特性的缓解效果.     

模拟氮沉降对紫苏叶挥发油主要成分的影响

我国为世界三大高氮沉降区之一,氮沉降严重影响了植物生长发育。该研究采用喷施硝酸铵(NH4NO3)模拟氮沉降,分析了不同浓度氮沉降作用下紫苏叶中紫苏醛、D-柠檬烯、α-蒎烯等3种挥发油成分的变化规律。结果表明:随喷施氮盐浓度不断提高,紫苏叶挥发油的3种主要成分含量均有显著下降趋势;氮盐浓度升至0.044 mol·L~-1时,紫苏醛、D-柠檬烯、α-蒎烯的含量降至最低,之后趋于稳定;氮盐浓度对3种挥发油成分含量的比例也有影响;不同氮盐浓度处理下,3种挥发油成分的变异系数不同,紫苏醛的变异系数为0.692 9,D-柠檬烯的变异系数为0.460 1,而α-蒎烯的变异系数为0.271 6,即紫苏醛含量变化最大,α-蒎烯含量最为稳定。大气氮沉降浓度对紫苏叶挥发油主要成分含量有显著影响,随氮盐浓度不断提高,紫苏醛、D-柠檬烯、α-蒎烯等3种挥发油成分含量呈降低趋势,尤以紫苏醛含量的降低最为剧烈。氮沉降增加对紫苏叶有效成分含量有降低的作用。     

光波动频率影响黄瓜幼苗光合作用的机制

自然条件下环境光强往往是波动的,但波动光变化频率影响植物光合作用的机制尚不清楚.为了探讨植物光合作用对波动光频率的响应及机制,本文以黄瓜为材料,对植物生长、叶绿素含量、气体交换、叶绿素荧光以及抗氧化酶进行了研究.结果显示,相对于弱光(T4),强光下(T1, T2和T3)黄瓜株高、生物量、叶面积和比叶重均明显较高,但波动频率增加(T2, T3)能够导致这些参数值降低.强光与弱光处理相比其叶绿素总量较低,且随着波动光频率的提高叶绿素含量轻微下降.强光下的光合速率和气孔导度均高于弱光,不过随着强光波动频率增加,两者呈下降趋势.荧光诱导动力学的结果显示,尽管各处理间光系统Ⅱ(PSⅡ)最大光化学效率没有明显差异,但波动频率较大时黄瓜的PSⅡ的电子传递活性略有降低;而且增加强光波动频率还导致光系统Ⅱ天线转化效率(Fv′/Fm′)明显降低和非光化学猝灭(NPQ)大幅增加.此外,强光下黄瓜的酶促抗氧化系统的主要酶活性高于弱光,但波动光频率提高能够降低其活性.因此,提高波动光的频率不仅导致黄瓜光合能力下降,还导致其主要抗氧化酶的活性降低,所以增强的热耗散可能是其应对波动光下过剩激发能的重要机制.此外,本文还对黄瓜适应波动强光和稳态强光的差异进行了讨论.     

巨尾桉不同树龄及染虫状态桉叶油的成分分析

为了考察树龄、虫害与巨尾桉叶油成分的相互关系,更好的开发桉树资源,采用水蒸气蒸馏法提取不同树龄(4~6年生)巨尾桉叶油,用GC-MS法对巨尾桉叶油主要化学成分进行鉴定,分析不同树龄和染虫状态的桉叶油成分变化规律。结果表明,随着树龄增加,桉叶油总量逐年增加;生长年限不同,桉叶油中物质的组成及相对含量存在差异。主要成分相对含量变化情况为:桉油醇、乙酸松油酯含量随树龄逐年上升;d-柠檬烯、α-蒎烯相对含量在4到5年达到峰值,5年后开始下降;莰烯、β-蒎烯和α-松油醇逐年下降。被云斑天牛感染的林地,桉油醇、α-蒎烯、β-蒎烯、莰烯呈相对下降趋势,而d-柠檬烯、α-松油醇、乙酸松油酯的含量上升。     

根系温度对光核桃幼苗光合机构热稳定性的影响

以光核桃(Prunus mira)幼苗为材料, 通过控制根系温度研究了根系温度变化与叶片脱落酸(ABA)的关系及其对光合机构热稳定性的影响。结果表明: 1)环境高温(37和40 ℃)胁迫下保持根系温度适宜((25±2) ℃)时, 幼苗叶片相对含水量(Relative water content, RWC)下降较少, 但叶片ABA含量低, 超氧化物歧化酶(SOD)、抗坏血酸过氧化物酶(APX)、过氧化物酶(POD)和过氧化氢酶(CAT)活性低, 过氧化氢(H₂O₂)含量和膜质过氧化水平(丙二醛(Malondialdehyde, MDA)浓度)提高, 最大光化学效率(F_v/F_m)下降程度较大; 2)而同等环境高温(37和40 ℃)条件下根系温度逐步升高时, 幼苗叶片RWC降低, 叶片ABA含量增加, SOD、APX、POD、CAT活性高, H₂O₂含量高, MDA生成量低, F_v/F_m降低程度较小。与37 ℃相比, 40 ℃处理条件下各生理指标变化趋势相似, 但差异加大。因此认为: 高温胁迫条件下, 根系温度适宜时RWC高, 但导致光合机构伤害较重; 根系感受高温胁迫能够增加叶片ABA含量, 有助于保护光合机构、提高光合机构的抗热性。     

Drought and Oxidative Load in the Leaves of C3 Plants: a Predominant Role for Photorespiration?

Although active oxygen species are produced at high rates inboth the chloroplasts and peroxisomes of the leaves of C₃ plants,most attention has focused on the potentially damaging consequencesof enhanced chloroplastic production in stress conditions suchas drought. This article attempts to provide quantitative estimatesof the relative contributions of the chloroplast electron transportchain and the glycolate oxidase reaction to the oxidative loadplaced on the photosynthetic leaf cell. Rates of photorespiratoryH₂O₂ production were obtained from photosynthetic and photorespiratoryflux rates, derived from steady-state leaf gas exchange measurementsat varying irradiance and ambient CO₂. Assuming a 10 % allocationof photosynthetic electron flow to the Mehler reaction, photorespiratoryH₂O₂ production would account for about 70 % of total H₂O₂ formedat all irradiances measured. When chloroplastic CO₂ concentrationrates are decreased, photorespiration becomes even more predominantin H₂O₂ generation. At the increased flux through photorespirationobserved at lower ambient CO₂, the Mehler reaction would haveto account for more than 35 % of the total photosynthetic electronflow in order to match the rate of peroxisomal H₂O₂ production.The potential signalling role of H₂O₂ produced in the peroxisomesis emphasized, and it is demonstrated that photorespiratoryH₂O₂ can perturb the redox states of leaf antioxidant pools.We discuss the interactions between oxidants, antioxidants andredox changes leading to modified gene expression, particularlyin relation to drought, and call attention to the potentialsignificance of photorespiratory H₂O₂ in signalling and acclimation.     

外源NO对南方红豆杉幼苗光合色素及抗氧化酶的影响

以4年生南方红豆杉幼苗为实验材料,通过对南方红豆杉幼苗喷施不同浓度外源一氧化氮（NO）供体硝普钠溶液（0、0.01、0.1、0.5和1 mmol·L^-1SNP）,测定光合色素含量、抗氧化酶活性、丙二醛（MDA）含量和过氧化氢（H₂O₂）含量等生理指标,以探讨不同浓度外源NO对南方红豆杉叶片光合色素和抗氧化酶的影响。结果表明：喷施低浓度（0.01、0.1 mmol·L^-1）SNP可显著提高南方红豆杉叶片的叶绿素a、叶绿素b、类胡萝卜素和总叶绿素含量,增加叶绿素a/b的比值,而喷施高浓度（0.5、1 mmol·L^-1）SNP降低了叶片的光合色素含量。随着外源NO供体浓度的增加,叶片过氧化氢酶(CAT)活性显著增加,过氧化物酶(POD)活性先增加后降低。此外,处理前期,低浓度SNP处理明显提高了抗坏血酸过氧化物酶(APX)活性,而高浓度SNP处理显著降低了APX活性,处理后期APX活性随SNP浓度的增加而显著下降。喷施低浓度SNP可有效提高超氧化物歧化酶(SOD)活性和增加可溶性蛋白含量,降低MDA和H₂O₂的含量,而喷施高浓度SNP显著增加了MDA和H₂O₂的含量。因此,低浓度的SNP（<0.5 mmol·L^-1）处理南方红豆杉幼苗,可增加其叶绿素含量,提高抗氧化酶活性,降低MDA和H₂O₂的含量,而高浓度的SNP（≥0.5 mmol·L^-1）处理会降低叶绿素含量,提高H₂O₂含量,增加细胞膜质过氧化程度,从而对南方红豆杉幼苗造成一定伤害。     

Systemic induction of H2O2 in pea seedlings pretreated by wounding and exogenous jasmonic acid

Pea seedlings (Pisum sativum L.) were used as materials to test the timings and compartments of hydrogen peroxide (H₂O₂) triggered by wounding and exogenous jasmonic acid (JA). The results showed that H₂O₂ could be systemically induced by wounding and exogenous JA. H₂O₂ increased within 1 h and reached the peak 3–5 h after wounding in either the wounded leaves or the unwounded leaves adjacent to the wounded ones and the inferior leaves far from the wounded ones. After this, H₂O₂ decreased and recovered to the control level 12 h after wounding. The activities of antioxidant enzymes, however, were rapidly increased by wounding. Diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, could significantly inhibit H₂O₂ burst that was mediated by wounding and exogenous JA. Assay of H₂O₂ subcellular location showed that H₂O₂ in response to wounding and exogenous JA was predominantly accumulated in plasma membrane, cell wall and apoplasmic space. Numerous JA (gold particles) was found via immunogold electron microscopy to be located in cell wall and phloem zones of mesophyll cell after wounding.     

土壤水分和氮素有效性对刺槐幼苗叶片活性氧产生和清除的影响

采用施加氮肥和人工控水的方法,以一年生刺槐幼苗为材料进行盆栽实验,探讨提高土壤氮素含量对不同土壤水分条件下刺槐幼苗叶片中活性氧产生和清除的影响。结果表明:(1)相同氮素水平下,降低土壤水分含量引起刺槐生物量和叶片光合色素含量降低,而过氧化氢(H2O2)含量升高;抗氧化酶系统中的超氧化物歧化酶(SOD)和过氧化物酶(POD)活性和过氧化氢酶(CAT)活性不同程度降低;抗氧化剂中抗坏血酸(ASA)含量和还原型谷胱甘肽(GSH)含量均有所提高;MDA含量逐渐升高,而同期细胞膜相对电导率显著升高。(2)相同水分条件下,提高土壤氮素水平显著提高了刺槐幼苗叶片光合色素含量,同时也一定程度提高了总生物量,显著降低了H2O2含量;SOD、POD和CAT活性不同程度升高;ASA含量和GSH含量则表现出不同程度下降;相对电导率显著降低同时MDA含量一定程度降低。因此,增加土壤氮素有效性可显著提高刺槐幼苗叶片光合色素含量,显著抑制活性氧的产生,一定程度提高总生物量和抗氧化酶活性,降低膜脂过氧化程度,从而有利于缓解干旱引起的伤害。     

外源H_2S对NO_3~-胁迫下番茄幼苗生理生化特性的影响

采用营养液水培方法,通过外源施加H2S供体NaHS(100μmol/L),研究了信号分子H2S对100mmol/L NO3-胁迫下番茄幼苗生理生化特性的影响。结果表明:(1)NO3-胁迫下,随着处理时间的延长,番茄幼苗的株高、根长、鲜重和干重显著降低,叶绿素(a、b)含量、净光合速率、气孔导度、蒸腾速率均显著降低,而胞间CO2浓度以及丙二醛(MDA)、H2O2含量增加,超氧化物歧化酶(SOD)、过氧化物酶(POD)、过氧化氢酶(CAT)和抗坏血酸过氧化物酶(APX)活性显著降低,抗坏血酸(AsA)和还原性谷胱甘肽(GSH)含量显著降低。(2)与NO3-胁迫处理相比,外源NaHS处理1、3、5d后,番茄幼苗的株高、根长、鲜重和干重显著增加,叶绿素(a、b)含量、净光合速率、气孔导度、蒸腾速率均显著升高,而胞间CO2浓度显著降低;MDA和H2O2含量降低,SOD、POD、CAT和APX活性显著增强,AsA和GSH含量显著增加,而且幼苗的硝酸还原酶、谷氨酰胺合成酶、谷氨酸合酶的活性显著增强;L-半胱氨酸脱巯基酶活性和内源H2S含量增加。研究认为,外源H2S可能通过提高抗氧化物酶的活性和增加抗氧化物质含量来缓解NO3-对番茄幼苗造成的伤害,从而增强其对NO3-胁迫耐性。     

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.     

大气臭氧浓度升高对水稻叶片膜脂过氧化及保护酶活性的影响

采用开顶式气室（open top chambers,OTCs）装置,以水稻品种“3694繁”(Oryza sativa L.)为材料,研究3种处理：过滤大气（CF,O₃浓度约为20 nl·L^-1）、环境大气（NF,O₃浓度约为40 nl·L^-1）和高浓度O₃（EO,O₃浓度约为75 nl·L^-1）下叶片可溶性蛋白质含量、膜脂过氧化程度与主要保护酶活性的变化.结果表明：过滤大气与环境大气处理之间各个指标无显著差异.与CF处理相比,高浓度O₃处理条件下水稻叶片中可溶性蛋白含量显著下降,而过氧化氢（H₂O₂）和抗坏血酸（ASA）含量显著增加;超氧化物歧化酶（SOD）、过氧化氢酶（CAT）、过氧化物酶（POD）活性升高,最大升高幅度分别为93.7%、39.9%和312.4%;抗坏血酸过氧化物酶（APX）活性呈先升高后下降趋势; 不同O₃处理条件下叶片中丙二醛（MDA)含量无显著变化.表明抗氧化系统可有效抑制水稻叶片中的O₃胁迫引起的膜质过氧化作用,说明水稻品种“3694繁”对O₃胁迫有一定抗性.     

Resistance of Photosynthesis to Hydrogen Peroxide in Algae

The effects of H₂O₂ on the photosynthetic fixation of CO₂ andon thiol-modulated enzymes involved in the photosynthetic reductionof carbon in algae were studied in a comparison with those inchloroplasts isolated from spinach leaves. In both systems,H₂O₂-scavenging enzymes were inhibited by addition of 0.1 mMNaN₃ 1 h prior to the addition of H₂O₂. A concentration (10^-4M) of H₂O₂ caused strong inhibition of the CO₂ fixation by intactspinach chloroplasts, as observed by Kaiser [(1976) Biochim.Biophys. Acta 440: 476], but not that by Euglena and Chlamydomonascells. The same results were also obtained with cells of thecyanobacteria Synechococcus PCC 7942 and Synechocystis PCC 6803in the presence of 1 mM hydroxylamine. These results indicatethat algal photosynthesis is rather resistant to H₂O₂. The insusceptibilityto H₂O₂ of thiolmodulated enzymes, namely, fructose-1,6-bisphosphatase,NADP-glyceraldehyde-3-phosphate dehydrogenase, and ribulose-5-phosphatekinase, was also observed in the chloroplasts of Euglena andChlamydomonas and in cyanobacterial cells. It seems likely thatthe resistance of photosynthesis to H₂O₂ is due in part to theinsusceptibility of the algal thiol-modulated enzymes to H₂O₂. (Received April 22, 1995; Accepted June 29, 1995)     

Formation of Hydrogen Peroxide and Oxygen Dependence of Photosynthetic CO2 Assimilation by Intact Chloroplasts

1. Hydrogen peroxide excretion by photosynthesizing intact spinachchloroplasts was determined. The rates were dependent on theoxygen concentration and on the ATP/NADPH requirement of thefinal electron acceptor. Upon CO₂ assimilation a maximum rateof 0.9 µmol H₂O₂/mg chlorophyll/hr and half saturationat 7.5 x 10^–5 M O₂ were found. Excretion of H₂O₂ was considerablyreduced upon photosynthetic reduction of glycerate 3-phosphateor oxaloacetate.
2. Light- and HCO₃-saturated CO₂ assimilationwas inhibited bymore than 50% by anaerobic conditions, whereuponquantum efficiencywas also drastically decreased. However,no anoxic influencewas detected with glycerate 3-phosphateas the terminal electronacceptor and the quantum requirementwith this acceptor wasnot increased by anaerobiosis. Thus theenhancing effect ofoxygen on CO₂ assimilation was ascribedto an improvement ofphotosynthetic ATP supply.
3. Since thestimulation of anaerobic photosynthetic CO₂ assimilationbyoxygen was markedly greater than the concomitant increaseinH₂O₂ evolution, photosynthetic oxygen reduction alone isnotsufficient to produce the required additional ATP for theobservedenhanced CO₂ assimilation. But it provides a meansto avoidthe over-reduction of photosynthetic electron carriersand thusenables aerobic cyclic photophosphorylation. This supportsthehypothesis that cyclic photophosphorylation is not an alternativeto ATP formation by "pseudocyclic" electron transport, but ratherthat it depends on the latter.

(Received January 5, 1981; Accepted March 9, 1981)