施氮和大气CO2浓度升高对小麦旗叶光合电子传递和分配的影响

采用开顶式气室盆栽培养小麦,设计2个大气CO2浓度(正常:400 μmol.mol-1;高:760 μmol·mol-1)、2个氮素水平(0和200 mg·kg-1土)的组合处理,通过测定小麦抽穗期旗叶氮素和叶绿素浓度、光合速率(Pn)-胞间CO2浓度(C1)响应曲线及荧光动力学参数,来测算小麦叶片光合电子传递速率等,研究了高大气CO2浓度下施氮对小麦旗叶光合能量分配的影响.结果表明:与正常大气CO2浓度相比,高大气CO2浓度下小麦叶片氮浓度和叶绿素浓度降低,高氮处理的小麦叶片叶绿素a/b升高.施氮后小麦叶片PSⅡ最大光化学效率(Fv/Fm)、PSⅡ反应中心最大量子产额(Fv'/Fm')、PSⅡ反应中心的开放比例(qr)和PSⅡ反应中心实际光化学效率(φPSⅡ)在大气CO2浓度升高后无明显变化,虽然叶片非光化学猝灭系数(NPQ)显著降低,但PSⅡ总电子传递速率(JF)无明显增加;不施氮处理的Fv'/Fm'、φPSⅡ和NPQ在高大气CO2浓度下显著降低,尽管Fv/Fm和qp无明显变化,JF仍显著下降.施氮后小麦叶片JF增加,参与光化学反应的非环式电子流传递速率(Jc)明显升高.大气CO2浓度升高使参与光呼吸的非环式电子流传递速率(J0)、Rubisco氧化速率(V0)、光合电子的光呼吸/光化学传递速率比(J0/Jc)和Rubisco氧化/羧化比(V0/Vc)降低,但使Jc和Rubisco羧化速率(Vc)增加.因此,高大气CO2浓度下小麦叶片氮浓度和叶绿素浓度降低,而增施氮素使通过PSⅡ反应中心的电子流速率显著增加,促进了光合电子流向光化学方向的传递,使更多的电子进入Rubisco羧化过程,Pn显著升高.     

低温弱光胁迫对日光温室栽培杏树光系统功能的影响

以温室栽培的金太阳杏为材料,测定了金太阳杏叶片光合速率(P_n)、光系统Ⅱ(PSⅡ)光下实际光化学效率(Φ_PSⅡ)、光化学猝灭系数(q_P)和开放的PSⅡ反应中心的激发能捕获效率(F_v^′/F_m^′), 探讨了低温弱光(7 ℃、200 μmol·m^-2·s^-1 PFD)对叶片光系统Ⅰ(PSⅠ)和PSⅡ的抑制作用.结果表明：温室栽培的金太阳杏叶光合作用的最适温度在25 ℃左右.光下7 ℃的低温可使叶片净光合速率(P_n)大幅下降,造成激发压(1-q_P)增大,进而引起光抑制.低温弱光条件使PSⅠ和PSⅡ功能受到破坏,与单纯低温胁迫(7 ℃,黑暗)处理相比,经低温、弱光(7 ℃, 200 μmol·m^-2·s^-1PFD)胁迫2 h后,PSⅠ活性下降了28.26%,而PSⅡ最大光化学效率（F_v/F_m）没有发生显著变化,表明低温弱光条件下PSⅠ比PSⅡ 更易发生光抑制.     

Inhibition of photosystem II of spinach by the respiration inhibitors piericidin A and thenoyltrifluoroacetone

The effects of several respiration inhibitors on photosystem II (PS II) were investigated. Among the agents tested, piericidin A and thenoyltrifluoroacetone (TTFA) inhibited the photosynthetic electron transport of spinach as measured from chlorophyll (Chl) fluorescence parameters (Fm'-F)/Fm' and Fv/Fm. Using specific donors and acceptors of electrons, we identified the sites of inhibition in and around the PS II complex; the site of inhibition by TTFA was between QA, primary quinone acceptor in PS II, and QB, secondary quinone acceptor, in the acceptor side of P680, the reaction center Chl of PS II, while inhibition by piericidin A of the acceptor side was downstream of Q(B), out of the PS II complex. Both agents also inhibited the donor side of P680, probably between tyrosine-161 of the reaction center protein of PS II and P680.     

两个玉米品系叶片光合速率差异的生理机制初探

田间生长的两个玉米品系"双105"和"40×44"叶片净光合速率(Pn)存在显著差异。在不同的生长阶段,"40×44"Pn值均比"双105"高10%~32%,而且PSII光化学效率(Fv/Fm和△F/Fm')也较高,表现为植株较高,叶片较大,但叶绿素含量较低。尽管"双105"品系具有较低的光合速率和较低的气孔导度(Gs),但其胞间CO₂浓度(Ci)却略高于"40×44"。因此,这两个玉米品系叶片光合速率的差异并非气孔因素,而可能源于光合机构的光反应系统。     

模拟酸雨胁迫下硅对髯毛箬竹光合特性的影响

以髯毛箬竹为试验材料,研究了模拟酸雨胁迫(pH 3.0)下硅对髯毛箬竹叶片叶绿素相对含量、光合作用日变化以及叶绿素荧光特性等的影响.结果表明:20和100 mg·L^-1 Na₂SiO₃预处理可以不同程度地抑制酸雨胁迫下髯毛箬竹叶片叶绿素含量的显著下降,且以100 mg·L^-1浓度处理效果最佳,叶绿素含量可提高22.7%,而高浓度(500 mg·L^-1)预处理则无缓解作用-酸雨胁迫下,髯毛箬竹光合“午休”现象加重,日平均净光合速率(P_n)、气孔导度(G_s)和气孔限制值(L_s)明显降低,而胞间CO₂浓度(C_i)增大,经过20～100 mg·L^-1硅预处理后,P_n、G_s、L_s不同程度增加,而C_i有所降低,且以100 mg·L^-1硅预处理效果最佳,日平均P_n增加39.2%.酸雨胁迫下,箬竹PSⅡ最大荧光(F_m)、最大光化学效率(F_v/F_m)、潜在活性(F_v/F_o)、有效光化学效率(F_v′/F_m′)、最大荧光产额(F_m′)、光化学淬灭系数(q_P)、非光化学淬灭系数(q_N)、PSII实际光化学效率(Φ_PSⅡ)降低,暗适应和光适应下的最小荧光产额F_o、F_o′ 则升高;而100 mg·L^-1硅预处理明显抑制了胁迫下各荧光参数的变化,F_v/F_m、F_v/F_o、F_v′/F_m′和Φ_PSⅡ分别增加35.2%、146.2%、55.0%和24.3%.说明外源适宜浓度硅预处理能有效地缓解酸雨胁迫导致的髯毛箬竹光合活性下降和光合系统损伤,从而提高胁迫下植物的光合能力.     

Photosynthetic utilization of radiant energy by CAM Dendrobium flowers

14CO2 fixation was observed in orchid Dendrobium flowers; its rate decreased with the flower development. Chlorophyll (Chl) fluorescence in different developmental stages of flowers was compared to other green plant parts (leaf, inflorescence stalk, and fruit capsule). The photochemical efficiency of photosystem 2 (PS2) (Fv/Fm) of a leaf was 14-21 % higher than that of a mature flower perianth (sepal, petal, and labellum) which had a much lower total Chl content and Chl a/b ratio. A higher quantum yield of PS2 (PS2) than in the mature flowers was observed in all green parts. Flower sepals had higher Chl content, Chl a/b ratio, and Fv/Fm values than the petal and labellum. During flower development the Chl content, Chl a/b ratio, Fv/Fm, and qN decreased while PS2 and qP remained constant. An exposure of developing flowers to irradiances above 50 µmol m-2 s-1 resulted in a very drastic drop of PS2 and qP, and a coherent increase of qN as compared to other green plant organs. A low saturation irradiance (PFD of 100 µmol m-2 s-1) and the increase in qN in the flower indicate that irradiation stress may occur since there is no further protection when the flower is exposed to irradiances above 100 µmol m-2 s-1. A low Chl/carotenoid ratio in mature flower perianth as a consequence of Chl content reduction in the course of flower development suggests a relief of irradiation stress via this mean.     

茄子光系统Ⅱ的热胁迫特性

以耐热性较弱的黑贝一号圆茄和耐热性较强的黑贝二号圆茄为试材,热胁迫处理后采用植物效率仪PEA进行快速叶绿素荧光诱导曲线及其参数测定.结果表明：当温度高于40 ℃,PSⅡ结构受热胁迫影响较为敏感,表现为初始荧光F_o缓慢上升;PSⅡ原初光化学效率F_v/F_m和ΔF/F_m′大幅度下降,且黑贝二号F_v/F_m的半衰时间T₅₀和ΔF/F_m′的半衰温度t₅₀分别大于黑贝一号.较高的热胁迫剂量（48℃处理5 min或44℃处理20～30min）下,快速荧光诱导动力学曲线呈现OKJIP型,在700μs处出现与放氧复合体失活有关的K相.黑贝一号在44 ℃下处理20 min才有K相出现,黑贝二号则晚10 min出现.与35℃相比,在48℃,特别是在52℃的较高剂量热胁迫下,Strasser能量流动模型参数中的DI_o/RC有大幅度地增加,体现了热耗散对PSⅡ的较强保护能力.随着热胁迫温度的升高和热胁迫时间的延长,两品种的无活性中心F_vi/Fv显著增加.     

温州蜜柑叶片光合作用光抑制的保护机理

晴天条件下,使用便携式调制荧光仪和分光光度计观察了温州蜜柑叶片光合作用光抑制发生过程中几个主要荧光参数（初始荧光F0、最大荧光Fm、PSⅡ的光化学效率Fv/Fm、非光化学猝灭qN及其快相qNf和慢相qNs）、电子传递速率（ETR）和玉米黄素相对含量的日变化,结果表明,随着光强的增强,ETR、qN及其qNr与qNs以及玉米黄素相对含量升高,Fv/Fm、Fm和F0下降。用DTT处理后,qNs较对照明显下降,F0较对照明显上升,可以认为,柑橘在光合作用日变化中存在依赖于叶黄素循环和类囊体膜质子梯度两种非辐射能量耗散方式,而且它们在防御光破坏方面起着重要作用。     

Rat liver 5''-nucleotidase

Synopsis Under assay conditions such that there is minimal interference by lysosomal acid phosphatase, the dephosphorylation of nucleoside-5-monophosphates (AMP or UMP) by rat-liver homogenates at alkaline pH values is attributable to a Mg²⁺-dependent enzyme (5-nucleotidase, EC 3.1.3.5). It shows a bimodal distribution between the nuclear and the microsomal fractions, different from that for glucose-6-phosphatase, and also differs in its response to deoxycholate. There is no evidence of an endogenous inhibitor or of enzyme latency. No clear evidence for the existence of more than one 5-nucleotidase has come from the approaches adopted. These include differential centrifugation, trial of different ions as activators and inhibitors, determination of pH curves, mixed-substrate assays, column chromatography, and the study of liver regeneration (which depresses the activity) and of hepatocarcinogenesis. Results obtained with maleate, Pb²⁺ ions and Ni²⁺ ions have a bearing on histochemical and clinical work. The acid phosphatase activity of lysosome-rich fractions towards -glycerophosphate, 5-AMP, 5-UMP and 2-(3-)UMP, at pH values near 5.0 with no divalent cations added, seems to be due to a single enzyme or group of isoenzymes. The effects of certain anions and cations on this lysosomal activity, and on microsomal 5-nucleotidase activity, depend on the concentration of the substrate as well as on its nature.A. A. El-Aaser is on leave from the Faculty of Medicine, University of Cairo     

Transfer of axonally transported phospholipids into myelin isolated from the rabbit optic pathway

The contribution of the axonal transport to the biosynthesis of myelin phospholipids was investigated in the rabbit optic pathway. A double labeling technique was used. The same animals were injected with one isotope intravitreally and the other intraventricularly. This procedure allows double labeling of the optic nerves, optic tracts, lateral geniculate bodies (LGB), and superior colliculus (SC). The precursors simultaneously injected were: [1-¹⁴C]palmitate (15 Ci intravitreally in both eyes or 50 Ci intraventricularly) and [2-³H]glycerol (50 Ci intravitreally in both eyes or 100 Ci intraventricularly). Twenty four hours and 10 days after the injections, myelin was purified from pooled optic nerves and optic tracts as well as from pooled LGBs or SCs. The phospholipids were extracted and then separated by thin-layer chromatography; the specific radioactivity of the various classes of phospholipids was determined. Using both administration routes of¹⁴C-or³H-precursors, the distribution of label and specific radioactivity of myelin phospholipids in the retina and in all other optic structures were very similar. Phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine + phosphoinositol were preferentially labeled with both precursors. These results suggest that, in the rabbit optic pathway the phospholipids synthesized in the retinal ganglion cells and transported along the axons, could undergo transaxonal transfer into myelin.     

两种化感物质对茄子幼苗光合作用及叶绿素荧光参数的影响

采用基质栽培,研究不同浓度的2,6-二叔丁基苯酚及邻苯二甲酸二甲酯对茄子幼苗光合作用及叶绿素荧光参数的影响.结果表明,两种化感物质使茄子叶片叶绿素含量、光合速率（P_n）、气孔导度（G_s）下降,细胞间隙CO₂浓度（C_i）先降低后升高;使茄子叶片的初始荧光(F_o)增加,PSⅡ最大光化学效率(F_v/F_m)、光系统Ⅱ光合电子传递量子效率(Ф_PSⅡ)、光化学猝灭系数(q_P)、天线转化效率(F_v^' /F_m^′)降低.2,6-二叔丁基苯酚使茄子叶片非光化学猝灭系数(q_N)先升高后降低,邻苯二甲酸二甲酯使q_N低于对照,对茄子叶片光合机构造成了伤害.     

Reduced levels of cytochrome b 6/f in transgenic tobacco increases the excitation pressure on Photosystem II without increasing sensitivity to photoinhibition in vivo

We have examined tobacco transformed with an antisense construct against the Rieske-FeS subunit of the cytochromeb ₆ f complex, containing only 15 to 20% of the wild-type level of cytochrome f. The anti-Rieske-FeS leaves had a comparable chlorophyll and Photosystem II reaction center stoichiometry and a comparable carotenoid profile to the wild-type, with differences of less than 10% on a leaf area basis. When exposed to high irradiance, the anti-Rieske-FeS leaves showed a greatly increased closure of Photosystem II and a much reduced capacity to develop non-photochemical quenching compared with wild-type. However, contrary to our expectations, the anti-Rieske-FeS leaves were not more susceptible to photoinhibition than were wild-type leaves. Further, when we regulated the irradiance so that the excitation pressure on photosystem II was equivalent in both the anti-Rieske-FeS and wild-type leaves, the anti-Rieske-FeS leaves experienced much less photoinhibition than wild-type. The evidence from the anti-Rieske-FeS tobacco suggests that rapid photoinactivation of Photosystem II in vivo only occurs when closure of Photosystem II coincides with lumen acidification. These results suggest that the model of photoinhibition in vivo occurring principally because of limitations to electron withdrawal from photosystem II does not explain photoinhibition in these transgenic tobacco leaves, and we need to re-evaluate the twinned concepts of photoinhibition and photoprotection.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlophenyl)-1,-dimethylurea - Fo and Fo minimal fluorescence when all PS II reaction centers are open in dark- and light-acclimated leaves, respectively - Fm and Fm maximal fluorescence when all PS II reaction centers are closed in dark- and light-acclimated leaves, respectively - Fv variable fluorescence (Fm-Fo) in dark acclimated leaves - Fv variable fluorescence (Fm-Fo) in lightacclimated leaves - NPQ non-photochemical quenching of fluorescence - PS I and PS II Photosystem I and II - P680 primary electron donor of the reaction center of PS II - PFD photosynthetic flux density - Q_A primary acceptor quinone of PS II - q_p photochemical quenching of fluorescence - V+A+Z violaxanthin+antheraxanthin+zeaxanthin     

Effects of Strong Light and Active Oxygen on Photosynthesis in Soybean

With the use of chlorophyll fluorescence technique, it was found that the net photosynthetic oxygen evolution rate decreased after strong light (2 000 μmol· m-2·2-1 ) treatment for two hours in soybean ( Glycine max L. ) leaves. The chlorophyll fluorescence parameters, Fm/Fo, Fv/Fm, ФPSII, qp and qN decreased along with the increase of light intensity. In strong light, exogenous active oxygen H202、·OH and 'O2 were harmful to soybean leaves. The destruction of 'O2 and·OH to leaves was most evident, as was shown that Fv/Fm and PS H decreased significantly. The antioxidants DABCO, mannitol, ascorbate and histidine protected the leaves, but weakly, from strong light. In darkness, the SOD inhibitor sodium diethyldithiocar- bamate (DDC) had no significant effect on Fm/Fo and Fv/Fm, but NAN,, the ascorbate peroxidase (APX)inhibitor, significantly decreased Fm/Fo, Fv/Fm and ФPS II. In strong light, however, beth DDC and NaN3 reduced the above-mentioned fluorescence parameters, but NaN3 was more effective than DDC. The results suggested that photoinhibition did take place in soybean leaves under strong light, and it was related to active oxygen in vivo.     

Photosynthetic performance and fluorescence in relation to antenna size and absorption cross-sections in rye and barley grown under normal and intermittent light conditions

The size of the Photosystem II light harvesting antenna and the absorption cross-sections of PS I (_PSI) and PS II (_PSII) were examined in relation to photosynthetic performance fluorescence. Wild-type (WT) rye (Secale cereale) and barley (Hordeurn vulgare) as well as the barley chlorophyllb-less chlorina F2 mutant were grown under control and intermittent light (IML) conditions. (_PSII) in control barley F2 was similar to IML grown WT rye and barley, which, in turn was 2.5 to 3.5 times smaller than for control WT plants. In contrast, _PSI was similar for all control plants. This was 2.5 to 4 times larger than for IML-grown WT plants. IML-grown barley mutant plants had the smallest absorption cross-sections. Photosynthetic light response curves revealed that the barley chlorina F2-mutant had rates of oxygen evolution on a per leaf area basis that were only slightly lower than control WT rye and barley while IML-grown plants had strongly reduced photosynthetic performance. Convexity () for control barley chlorina F2-mutants was equal to the WT controls (0.6–0.7), while all IML-grown plants had a of 0. This indicates that, in contrast to control barley mutants, IML-plants were limited by PS II turn-over rates at all irradiances. However, on a per leaf Chl-basis the IML-grown plants exhibited the highest photosynthetic rates. Thus, the comparatively poor photosynthetic rates for IML-grown plants on a per leaf area basis were not due to less efficient photosynthetic reaction centers, but may rather be due to an increased limitation from PS II turn-over and a reduction in the number of reaction centers per leaf area.     

β-structures of polypeptides with L- and D-residues

Summary A series of five alternating poly(leucyl-lysyl) samples with varying amounts of L- and D-residues randomly distributed along the chain, but evenly shared out amongst leucyl and lysyl residues were synthesized by condensation of a mixture of the four diastereoisomeric dipeptidep-nitro-phenylesters. Their behavior in aqueous solution at various ionic strengths was studied by infrared spectroscopy which allowed measurement of the total amount of-structures, and by circular dichroism which gives the excess of L-residues over D-residues in the same structures. Comparison with the properties of the all L-poly(Lys-Leu-Lys-Leu) shows that incorporation of a few D-residues in a L-chain seems to reduce the width of the-sheets obtained in presence of salt. Higher proportions of D-isomers prevent the coil transition from occurring when the ionic strength is increased except for segments containing at least 6 to 7 adjacent residues of the same configuration.     

极小种群海南假韶子叶片光合特性和叶绿素荧光参数的动态变化

探讨海南省极小种群物种海南假韶子(Paranephelium hainanensis)的光合生理特性,以期为该物种的保护和利用提供科学依据。应用Li-COR便携式光合仪和FC1000-H便携式植物荧光成像系统测定海南假韶子的光合特性、叶绿素荧光参数的动态变化。结果表明:海南假韶子成年树和林下幼苗在7月和11月时净光合速率(Pn)和蒸腾速率(Tr)的日变化均呈明显"双峰"曲线,成年树的最大Pn达9.24μmol·m^-2·s^-1,最大Tr为3.34μmol·m^-2·s^-1,其光合能力显著强于幼苗;海南假韶子成年树初始荧光值(F0)在7月和11月最大值分别为323.04和135.87,Fm在7月和11月最大值分别为502.78和307.76,但幼苗的F0和Fm均低于成年树;成年树Fv/Fm在7月呈单谷型,在11月时呈先降后升再降的日变化趋势;相关性分析结果表明,在7月,成年树的Pn与Tr、Fv/Fm呈极显著正相关,幼苗Pn与F0呈极显著正相关;在11月,成年树的Pn与Tr、Fm呈极显著正相关,与Fv/Fm呈显著负相关;幼苗的Pn与Fm呈显著正相关。以上结果表明,海南假韶子为阳性树种,在7月和11月表现出不同的光合特性和叶绿素荧光参数来适应环境,水分可能是引起海南假韶子光合速率和叶绿素荧光参数动态变化的关键因子。     

Biochemical and photochemical changes in response to triacontanol in rice (Oryza sativa L.)

Triacontanol (TRIA) increased the contents of total chlorophyll (Chl), Chl a and Chl b by 25.1%, 26.1% and 22.4% respectively 4 h after treatment in rice seedlings. The minimal fluorescence (F₀), the maximal fluorescence (Fm) and Fv/Fm were also higher in TRIA-treated plants. In actinic light, other Chl fluorescence parameters were measured at different photon flux densities (PFD) to construct light response curves of the quantum yield of PSII electron transport (_PSII), light response curves of photochemical quenching (qp), and light response curves of non-photochemical quenching (qN), respectively. The _PSII and qp declined with the increasing PFD with a higher level present in TRIA-treated plants. The qN increased with the increasing PFD with a lower level present in TRIA-treated plants. Two-dimensional gel electrophoresis indicated a protein expression difference between TRIA-treated materials and the controls at the total-soluble-protein level. Rubisco was 30% higher in TRIA-treated plants than in controls. The quantity of other proteins was unchanged in response to TRIA. These data provide biochemical and photochemical evidence for the effects of TRIA on photosynthesis.     

草莓叶片光合作用对强光的响应及其机理研究

用便携式调制叶绿素荧光仪和光合仪研究了强光下草莓叶片荧光参数及表观量子效率的变化.结果表明,Fm、Fv/Fm、PSⅡ无活性反应中心数量和Q_A的还原速率在强光下降低,在暗恢复时升高;而PSⅡ反应中心非还原性Q_B的比例在强光下增加,在暗恢复时降低.上述荧光参数的变化幅度均以强光胁迫或暗恢复的前10 min最大.强光下Φ_PSII、ETR和qP先升高后降低,但qN先大幅度降低,然后小幅回升.强光处理4 h后,丰香和宝交早生的表观量子效率(AQY)分别降低了20.9%和37.5%;qE(能量依赖的非光化学猝灭)为NPQ(非光化学猝灭)的最主要成分.强光胁迫下丰香的Fo、Fm、Fv/Fm、Φ_PSII、ETR和AQY的变化幅度均明显比宝交早生小.DTT处理后,草莓叶片的Fm和Fv/Fm明显降低,Fo显著升高.可以认为,依赖叶黄素循环和类囊体膜质子梯度两种非辐射能量耗散在草莓叶片防御光损伤方面起着重要作用,丰香的光合机构比宝交早生更耐强光.     

高、低温胁迫对牡丹叶片PSⅡ功能和生理特性的影响

以牡丹‘肉芙蓉’离体叶片为试材,以25 ℃为对照,研究了强光(1400 μmol·m-2·s-1)下高温(40℃)和低温(15℃)处理对牡丹叶片PSⅡ光化学活性和生理特性的影响.结果表明:随处理时间的延长,各处理叶片的PSⅡ最大光化学效率(Fv/Fm)、PSⅡ实际光量子效率(φPsⅡ)和光下开放的PSⅡ反应中心激发能捕获效率(Fv’/Fm’)均持续降低.暗恢复4h后,对照和15℃处理叶片的Fv/Fm基本上完全恢复,而40℃处理叶片仅恢复到处理前的75.5％,即使15 h后也不能完全恢复;强光下40℃处理使PSⅠ和PSⅡ间的激发能分配严重偏离平衡状态.强光下40 ℃处理抑制了超氧化物歧化酶活性,加剧了O2、H2O2、丙二醛的产生,导致叶绿素和可溶性蛋白含量不断下降.说明强光下40℃高温胁迫对牡丹叶片光合机构造成了不可逆的破坏,而15℃低温处理对其光合机构的影响相对较弱.     

Different Relationship Between Electron Transport and CO2 Assimilation in two Zea mays Cultivars as Influenced by Increasing Irradiance

Gas exchange and fluorescence parameters were measured simultaneously in two Zea mays L. cultivars (Liri and 121C D8) to assess the relationship between the quantum yield of electron transport (_PS2) and the quantum yield of CO₂ assimilation (_CO2) in response to photosynthetic photon flux density (PPFD). The cv. Liri was grown under controlled environmental conditions in a climate chamber (CC) while cv. 121C D8 was grown in CC as well as outdoors (OT). By exposing the two maize cultivars grown in CC to an increasing PPFD, higher photosynthetic and photochemical rates were evidenced in cv. Liri than in cv. 121C D8. In Liri plants the _PS2/_CO2 ratio increased progressively up to 27 with increasing PPFD. This suggests that the reductive power was more utilised in non-assimilatory processes than in CO₂ assimilation at high PPFD. On the contrary, by exposing 121C D8 plants to increasing PPFD, _PS2/_CO2 was fairly constant (around 11–13), indicating that the electron transport rate was tightly down regulated by CO₂ assimilation. Although no significant differences were found between _PS2/_CO2 of the 121C D8 maize grown under CC and OT by exposing them to high PPFD, the photosynthetic rate and photochemical rates were higher in OT maize plants.