Adaptation of the Photosynthetic Apparatus in Maize Leaves as a Result of Nitrogen Limitation : Relationships between Electron Transport and Carbon Assimilation

In maize (Zea mays L., cv Contessa), nitrogen (NO₃⁻) limitation resulted in a reduction in shoot growth and photosynthetic capacity and in an increase in the leaf zeaxanthin contents. Nitrogen deficiency had only a small effect on the quantum yield of CO₂ assimilation but a large effect on the light-saturated rate of photosynthesis. Linear relationships persisted between the quantum yield of CO₂ assimilation and that of photosystem II photochemistry in all circumstances. At high irradiances, large differences in photochemical quenching and nonphotochemical quenching of Chl a fluorescence as well as the ratio of variable to maximal fluorescence (Fv/Fm) were apparent between nitrogen-deficient plants and nitrogen-replete controls, whereas at low irradiances these parameters were comparable in all plants. Light intensity-dependent increases in nonphotochemical quenching were greatest in nitrogen-deficient plants as were the decreases in Fv/Fm ratio. In nitrogen-deficient plants, photochemical quenching decreased with increasing irradiance but remained higher than in controls at high irradiances. Thermal dissipative processes were enhanced as a result of nitrogen deficiency (nonphotochemical quenching was elevated and Fv/Fm was lowered) allowing PSII to remain relatively oxidised even when carbon metabolism was limited via nitrogen limitation.     

Photoinhibition in Chilling Stressed Wheat and Maize

At chilling stress, the contents of photosynthetic pigments decreased significantly in maize, but in wheat the contents of chlorophyll (Chl) remained unchanged whereas the contents of total carotenoids (Car) increased. In both species the contents of + carotene and lutein + lutein-5,6-epoxide remained unaffected, but the de-epoxidation state involving the components of the xanthophyll cycle increased. Under chilling stress the photosynthetic electron transport also displayed a general failure in maize but in wheat only photosystem (PS) 2 coupled to the water oxidation complex was inhibited. Moreover, in stressed maize the quinone pool decreased, while the low and high potential forms of cytochrome b ₅₅₉ increased. In wheat only the contents of cytochrome b _559LP decreased. Peroxidation of acyl lipids in the chloroplast lamellae became more distinct in chilling stressed maize but could also be detected in wheat. Thus in chilling stressed maize prevails an impairment of the acceptor site of PS2 while in wheat photodamage is restricted to the electron donation pathway from water to P680 or to the oxygen evolving complex.     

Effect of Elevated Temperatures on the Activity of Alternative Pathways of Photosynthetic Electron Transport in Intact Barley and Maize Leaves

The light-induced rise in chlorophyll fluorescence and the subsequent decay of fluorescence in darkness were measured in barley and maize leaves exposed to heat treatment. The redox conversions of the photosystem I primary donor P700, induced by far-red light, were also monitored from the absorbance changes at 830 nm. After heating of leaves at temperatures above 40°C, the ratio of variable and maximum fluorescence decreased for leaves of both plant species, indicating the inhibition of photosystem II (PSII) activity. A twofold reduction of this ratio in barley and maize leaves was observed after heating at 45.3 and 48.1°C, respectively, which suggests the higher functional resistance of PSII in maize. The amplitude of the slow phase in the dark relaxation of variable fluorescence did not change after the treatment of barley and maize leaves at temperatures up to 48°C. In leaves treated at 42 and 46°C, the slow phase of dark relaxation deviated from an exponential curve. The relaxation kinetics included a temporary increase in fluorescence to a peak about 1 s after turning off the actinic light. Unlike the slow component, the fast and intermediate phases in the dark relaxation of variable fluorescence disappeared fully or partly after the treatment of leaves at 46°C. The photooxidation of P700 in heat-treated leaves was saturated at much higher irradiances of far-red light than in untreated leaves. At the same time, the dark reduction of P700⁺ was substantially accelerated after heat treatment. The data provide evidence that the heating of leaves stimulated the alternative pathways of electron transport, i.e., cyclic transport around photosystem I and/or the donation of electrons to the plastoquinone pool from the reduced compounds located in the chloroplast stroma. The rate of alternative electron transport after the heat treatment was higher in maize leaves than in barley leaves. It is supposed that the stimulation of alternative electron transport, associated with proton pumping into the thylakoid, represents a protective mechanism that prevents the photoinhibition of PSII in leaves upon a strong suppression of linear electron transport in chloroplasts exposed to heat treatment.     

Adaptations of Photosynthetic Electron Transport,Carbon Assimilation,and Carbon Partitioning in Transgenic Nicotiana plumbaginifolia Plants to Changes in Nitrate Reductase Activity

Transgenic Nicotiana plumbaginifolia plants that express either a 5-fold increase or a 20-fold decrease in nitrate reductase (NR) activity were used to study the relationships between carbon and nitrogen metabolism in leaves. Under saturating irradiance the maximum rate of photosynthesis, per unit surface area, was decreased in the low NR expressors but was relatively unchanged in the high NR expressors compared with the wild-type controls. However, when photosynthesis was expressed on a chlorophyll (Chl) basis the low NR plants had comparable or even higher values than the wild-type plants. Surprisingly, the high NR expressors showed very similar rates of photosynthesis and respiration to the wild-type plants and contained identical amounts of leaf Chl, carbohydrate, and protein. These plants were provided with a saturating supply of nitrate plus a basal level of ammonium during all phases of growth. Under these conditions overexpression of NR had little impact on leaf metabolism and did not stimulate growth or biomass production. Large differences in photochemical quenching and nonphotochemical quenching components of Chl a fluorescence, as well as the ratio of variable to maximum fluorescence, (FV/FM), were apparent in the low NR expressors in comparison with the wild-type controls. Light intensity-dependent increases in nonphotochemical quenching and decreases in FV/FM were greatest in the low NR expressors, whereas photochemical quenching decreased uniformly with increasing irradiance in all plant types. Nonphotochemical quenching was increased at all except the lowest irradiances in the low NR expressors, allowing photosystem II to remain oxidized on its acceptor side. The relative contributions of photochemical and nonphotochemical quenching of Chl a fluorescence with changing irradiance were virtually identical in the high NR expressors and the wild-type controls. Zeaxanthin was present in all leaves at high irradiances; however, at high irradiance leaves from the low NR expressors contained considerably more zeaxanthin and less violaxanthin than wild-type controls or high NR expressors. The leaves of the low NR expressors contained less Chl, protein, and amino acids than controls but retained more carbohydrate (starch and sucrose) than the wild type or high NR expressors. Sucrose phosphate synthase activities were remarkably similar in all plant types regardless of the NR activity. In contrast phosphoenolpyruvate carboxylase activities were increased on a Chl or protein basis in the low NR expressors compared with the wild-type controls or high NR expressors. We conclude that large decreases in NR have profound repercussions for photosynthesis and carbon partitioning within the leaf but that increases in NR have negligible effects.     

Relationship between Photosynthetic Electron Transport and Stromal Enzyme Activity in Pea Leaves : Toward an Understanding of the Nature of Photosynthetic Control

The responses of the quantum efficiencies of photosystem (PS) II and PSI measured in vivo simultaneously with estimations of the activities and activation states of NADP-malate dehydrogenase, chloroplast fructose-1,6-bisphosphatase, and ribulose-1,5-bisphosphate carboxylase were used to study the relationship between electron transport and carbon metabolism. The effects of varying irradiance and CO₂ partial pressure on the relationship between the quantum efficiencies of PSI and II, and the activity of these enzymes shows that the interrelationships vary according to the limitations placed on the system. The relationship between the quantum efficiencies of PSII and PSI was linear in most situations. In response to increasing irradiance, the activity of all three enzymes increased. In the case of NADP-malate dehydrogenase this increase was well correlated with the estimated flux of electrons through PSI and PSII. The other two enzymes showed a more complex relationship with the estimated flux of electrons through both photosystems. These relationships are consistent with the known interactions between these stromal enzymes and the thylakoids. The response to varying CO₂ partial pressure is more complex. The efficiencies of PSI and II declined with decreasing CO₂ partial pressure and the activity of each enzyme varied uniquely. However, there are clear correlations between the activities of the enzymes and the flux of electrons through the photosystems. In contrast to the data obtained under conditions of varying irradiance, there is clear evidence of photosynthetic control of electron transport when the CO₂ concentration is varied.     

Relationship Between Photosystem 2 Electron Transport and Photosynthetic CO2 Assimilation Responses to Irradiance in Young Apple Tree Leaves

The responses to irradiance of photosynthetic CO₂ assimilation and photosystem 2 (PS2) electron transport were simultaneously studied by gas exchange and chlorophyll (Chl) fluorescence measurement in two-year-old apple tree leaves (Malus pumila Mill. cv. Tengmu No.1/Malus hupehensis Rehd). Net photosynthetic rate (P _N) was saturated at photosynthetic photon flux density (PPFD) 600-1 100 (mol m^-2 s^-1, while the PS2 non-cyclic electron transport (P-rate) showed a maximum at PPFD 800 mol m^-2 s^-1. With PPFD increasing, either leaf potential photosynthetic CO₂ assimilation activity (F_d/F_s) and PS2 maximal photochemical activity (F_v/F_m) decreased or the ratio of the inactive PS2 reaction centres (RC) [(F_i – F_o)/(F_m – F_o)] and the slow relaxing non-photochemical Chl fluorescence quenching (q_s) increased from PPFD 1 200 mol m^-2 s^-1, but cyclic electron transport around photosystem 1 (RFp), irradiance induced PS2 RC closure [(F_s – F_o)/F_m – F_o)], and the fast and medium relaxing non-photochemical Chl fluorescence quenching (q_f and q_m) increased remarkably from PPFD 900 (mol m^-2 s^-1. Hence leaf photosynthesis of young apple leaves saturated at PPFD 800 mol m^-2 s^-1 and photoinhibition occurred above PPFD 900 mol m^-2 s^-1. During the photoinhibition at different irradiances, young apple tree leaves could dissipate excess photons mainly by energy quenching and state transition mechanisms at PPFD 900-1 100 mol m^-2 s^-1, but photosynthetic apparatus damage was unavoidable from PPFD 1 200 mol m^-2 s^-1. We propose that Chl fluorescence parameter P-rate is superior to the gas exchange parameter P _N and the Chl fluorescence parameter F_v/F_m as a definition of saturation irradiance and photoinhibition of plant leaves.     

Photosynthetic Electron Transport and Carbon Metabolism during Altered Source/Sink Balance in Single-Rooted Soybean Leaves

The single-rooted leaf of soybean (Glycine max L. Merr.) wasused to study electron transport and various reactions involvedin carbon metabolism under conditions of altered source/sinkbalance. When the leaves grown for 5 d under a regime of 10h of light per day (the standard growth condition, 500–580µmol photons m²s^–1) were treated for 6 or 7 d withcontinuous light to alter the source/sink balance of photosynthates(the sink limit condition), the photo-synthetic rate decreasedto 54% of that in leaves grown under the standard condition.The decrease in photosynthesis due to the treatment with continuouslight was associated with a large increase in the resistanceto the movement of CO₂ from surfaces of mesophyll cell wallsto intercellular sites of photosynthesis. The treatment alsocaused large increases in levels of NADPH and ATP and in theratio of ATP to ADP in the leaves. The levels of ribulose 1,5-bisphosphateand dihydroxyacetone phosphate in the treated leaves rose to460% and 160% of those in the control leaves, respectively,while the level of 3-phosphoglycerate in the former fell to80% of the level in the latter. The treatment also caused thelevels of almost all of the other phosphorylated intermediatesin photosynthetic carbohydrate metabolism, with the exceptionof fructose 6-phos-phate and adenosine diphosphate glucose,to increase by 110% to 200%. These results indicate that, underthe sink limit condition, the significant decrease in photosyntheticactivity was due to inactivation of photosynthetic carbohydratemetabolism rather than to a decline in photoelectron transport.Our observations also suggest that the accumulation of variousphosphorylated intermediate results in a limitation of levelsof Pj in chloroplast stroma and that, under such conditions,the activity of ribulose 1,5-bisphosphate carboxylase is reducedand there is a subsequent decrease in photosynthetic activity. (Received August 8, 1988; Accepted April 19, 1989)     

Effects of Lipoxygenase on the Activities of Photosynthetic Electron Transport in Cucumber Leaves

Photosynthesis and electron transport activity decreased with leaf aging, and however, lipoxygenase (Lox) activity increased correspondingly. Soybean Lox-1 inhibited significantly PSⅡ electron transport activity of chloroplasts isolated from cucumber (Cucumis sativus L. ) cotyledon. But the inhibition could be eliminated by the addition of propyl gallate (PG) or 3, 3, 4, 5, 7-pentahydroxyflavon (PF). The inhibition of PSⅠ activity by soybean Lox-1 was enhanced in the presence of 3, 4, dichlorophenyl-1, 1-dimethylurea (DCMU) or 2, 5-dibromothymoquinone (DBMIB), bfft could be restored to its original level when PG was added. Addition of 2, 2-diphenylcarbonic dihydrazide (DPC) to the mixture of isolated chloroplasts and Lox-1, PSⅡ activity resumed obviously. Chlorophyll a fluorescence study showed that Fm was decreased by Lox-1 and resumed slightly by DPC. Based on the above results, it was suggested that Lox might act at least on three sensitive sites located on Q, PQ and the oxidative side of PSⅠ . The bleaching of chlorophyll and carotenoid stimulated by Lox-l, and the inhibition of PSⅠ electron transport activity by active oxygen might be. one of the important reasons to explaine the effect of Lox on the function of photosynthetic membrane.     

低温与光对黄瓜幼苗子叶光合电子传递活性的影响

25～30℃和30 μmol m~(-2)s~(-1)光下培养的黄瓜幼苗,在黑暗下经 1～7℃处理24h或5℃处理24～72h,光合电子传递活性受不同程度的抑制;其抑制部位主要在PSⅡ氧化侧;随温度的降低和时间的延长,抑制部位可发展至PSⅡ及之后的电子递体上,但尚未影响PSⅠ的活性。160μmol m~(-2)s~(-1)的光强加重低温对电子传递活性的抑制,光强越高,则加重的程度越高;抑制部位从PSⅡ氧化侧发展至PSⅡ反应中心以及PSⅠ。     

烟草叶片中呼吸电子传递途径在缓解叶绿体PSⅡ光抑制中的作用

前期研究发现线粒体交替氧化酶(AOX)呼吸途径对叶绿体光系统II(PSII)的光抑制有明显的缓解作用。线粒体内另一条呼吸途径——细胞色素氧化酶(COX)呼吸途径是否也具有光保护作用尚不清楚。该文通过荧光快速诱导动力学和荧光淬灭分析,解析了烟草(Nicotiana tabacum)叶片中COX途径对PSII光保护的贡献及其与AOX途径的关系。结果表明,强光处理后PSII活性在所有叶片中均下降。AOX途径受抑明显加速了叶片PSII活性的下降。而当COX途径受抑后,叶片PSII活性的下降与水处理的对照叶片无明显差异。当AOX途径与COX途径同时受抑时,叶片PSII活性的下降比单独抑制AOX途径时更严重。此外,呼吸电子传递受抑均导致叶片非光化学淬灭(NPQ)增加,AOX途径受抑导致的NPQ上调比COX途径受抑时更明显,AOX和COX途径同时受抑时NPQ的增幅最大。上述结果表明,烟草叶片中COX途径和AOX途径均参与PSⅡ的光保护。当COX途径受抑时,其光保护功能可以被AOX途径和NPQ补偿,而AOX途径的光保护作用不能被COX途径和NPQ完全补偿。     

Photosynthetic Units and Carbon Assimilation in Leaves of Grain Sorghum under Different Light Intensities

The Cyt f and P700 contents in leaves of three Sorghum, varietieswere measured, in relation to their carbon assimilation, underdifferent light intensities during growth. At the maximum irradiationused (1,800 µE m^–2 s^–1) the ratio of P700to Cyt f was close to unity, whereas under low irradiation (450µE m^–2 s^–1) the ratio of P700 to Cyt f rangedfrom two to three. A strikingly positive correlation existedbetween the P700 contents of the leaves and their rates of carbondioxide fixation, dry matter production and Cyt f contents,only when the plants were grown under high light intensities.The P700 content of the leaves in plants grown under low irradiationwas unrelated to the contents of Cyt f. Thus, at a high lightintensity there is a close relationship between the Cyt f andP700 levels, but at low intensities the amounts of electroncarriers and the reaction centre are independent. (Received March 7, 1983; Accepted August 24, 1983)     

The Effect of Heat Shock on the Capacity of Wheat Plants to Restore Their Photosynthetic Electron Transport after Photoinhibition or Repeated Heating

The shoots of 16-day-old spring wheat plants (Triticum aestivumL., cv. Albidum 29) were subjected to heat shock (HS) at 40, 41, or 43°C for 10 min. The activity of the Hill reaction in chloroplasts isolated immediately after HS was 83, 61, and 30% of the initial value, respectively. The activity of the Hill reaction was also estimated after plant return to the initial growth conditions for one day. It was completely restored after heating at 40°C and achieved 82 and 30–33% of the initial level after heating at 41 and 43°C, respectively. Thereafter, the shoots were heated repeatedly at 42, 43, or 43.5°C for 10 min, and the activity of the Hill reaction was measured immediately or one day after this heating. Immediately after the second heating, the activity decreased again as compared to its value before heating. The percent of inhibition of the Hill reaction was similar in the control plants not subjected to preliminary HS and HS-treated plants independently of the temperature used. However, after one-day growth under normal conditions, the activity of the Hill reaction was restored almost completely in HS-treated plants but not more than by 10% in the control plants. The conclusion is that different mechanisms underlie the development of the tolerance to HS and recovery. Some plants were tested for the effect of HS (40°C) on their tolerance to photoinhibition. The degree of the Hill reaction inhibition after plant exposure to the light of 65–75 klx for 3 h was essentially similar in detached leaves from the HS-treated and unheated plants and comprised about 40% of the activity before light stress. After the leaves were returned to the low-light conditions for 3 h, the Hill reaction was restored and attained about 75% of that before photoinhibition in both HS-treated and untreated plants. The lack of the HS-induced stimulation of the Hill reaction recovery after photoinhibition is evidently related to the fact that heating and excess light damage different sites of photosystem II, which implies the different pathways for the recovery of its functional activity.     

Responses of Photosynthetic Electron Transport to Drought and Re-watering in Two Maize Genotypes

Russian Journal of Plant Physiology - Effects of drought stress on photosynthesis have been well-documented. However, photosynthetic electron transport process in response to combined drought...     

冷锻炼对甜椒叶片光合作用及其低温光抑制的影响

以冷敏感植物甜椒 (CapsicumannuumL .)抗冷性不同的两个品种为试材 ,利用CIRAS 1光合测定系统和FMS2调制式荧光仪 ,在控温控光条件下分析比较了冷锻炼苗与未经锻炼苗的叶片光合特性、叶绿素荧光参数对温度的响应。结果表明 ,随着温度的降低 ,无论是否经过锻炼 ,低温主要通过抑制碳同化能力来影响光合作用 ,并使光能过剩 ,导致低温光抑制。提高环境CO2 浓度以增强暗反应对光能的利用 ,低温光抑制减轻。 5d的亚适温锻炼过程中甜椒叶片已发生一定程度的光抑制 ,但锻炼苗叶片能在低温下维持较高的光系统II光化学效率(ФPSII)、光化学猝灭系数 (qP)和光适应下光系统II最大光化学效率 (Fv′/Fm)值 ;冷锻炼提高了两品种低温下对光抑制的抗性 ,而且对抗冷品种的作用效果更明显     

Gene Loci in Maize Influencing Susceptibility to Chilling Dependent Photoinhibition of Photosynthesis

Variation in tolerance in chilling-dependent photoinhibition has been associated with a wide range of traits in comparative physiological studies. A sweet corn (Zea mays L.) population of 214 F_2:3 families previously mapped to near-saturation with 93 RFLP DNA markers were subjected to low temperature and high-light events prior to measurement of the maximum dark-adapted quantum efficiency of PS II (F_v/F_m), to identify loci associated with variation in chilling-dependent photoinhibition. In the first assay with ten families varying in seedling growth and germination, significant differences were observed among families in their response to and recovery from exposure to high light at low temperature. All the 214 F_2:3 families from this population were then evaluated for tolerance of chilling-dependent photoinhibition in a controlled environment and then in three replicated trials in the field, each following naturally occurring chilling events during spring. The measured effects on F_v/F_m were analyzed with software that mapped segregating loci that regulate trait expression and linked to genetic markers (PLABQTL). QTL 3.096 (i.e. 96 cM on chromosome three) was consistently identified in both controlled environment and in the mean of the three field trails. Another QTL at 8.025, described the greatest percentage of total phenotypic variance (ca. 10%) for the mean reduction in F_v/F_m of all three periods of measurement in the field. A third QTL (4.136) showed a highly significant association in the third field trial. These three QTLs were closely associated with genes that have been mechanistically related to photoinhibition tolerance and repair. The results suggest that the ratio of F_v/F_m is an approach that may be used in establishing marker-assisted breeding for improved tolerance to chilling of maize in the light and in turn better early-season growth in cool temperate climates.     

Effect of Methionine Sulfoximine on Photosynthetic Carbon Metabolism in Wheat Leaves

Methionine sulfoximine (MSO) greatly reduced the carbon dioxideexchange rate (CER) of detached wheat (Triticum aestivvm L.cv Roland) leaves in 21% O₂, but only slightly reduced it in2% O₂. A supply of 50 mM NH₄Cl had little effect on the CERirrespective of the O₂ concentration. A simultaneous additionof glutamine and MSO protected against the inhibition of photosynthesisto a considerable extent and caused the accumulation of moreNH₃ than did the addition of MSO alone. Fixation of ¹⁴CO₂ in wheat leaves was inhibited by MSO treatmentin 22% O₂, and there was decreased incorporation of ¹⁴G intoamino acids and sugars and increased label into acid fractions.The addition of MSO and glutamine together eliminated the effectof MSO on the photosynthetic ¹⁴CO₂ fixation pattern. NH₄Cl stimulatedthe synthesis of amino acids from ¹⁴CO₂, especially the synthesisof serine in 22% O₂. Our observations show that factors other than the uncouplingof photophosphorylation by accumulated NH₃ may be responsiblefor the early stage of photosynthesis inhibition by MSO underphotorespiratory conditions. ¹Present address: Department of Agricultural Chemistry, KyushuUniversity, Fukuoka 812 Japan. ²Also at U.S. Department of Agriculture, Agricultural ResearchService, Urbana, Illionois 61801, U.S.A. (Received September 13, 1983; Accepted February 2, 1984)     

The Effect of Nitrate and Ammonium Feeding on Carbon Dioxide Assimilation in Maize

The effect of nitrogen nutrition on the pattern of ¹⁴CO₂ fixationduring photosynthesis by young maize plants was studied. Nitratefeeding increased the incorporation of ¹⁴C into malate and certainamino acids (particularly aspartate) and decreased that intosucrose and starch. Ammonium nutrition caused similar, but notidentical, changes, the major difference being a much smallereffect on incorporation into malate. Tungstate applied to theplants prior to nitrate almost completely inhibited the formationof nitrate reductase and eliminated most, but not all, of theeffects of nitrate on the fixation pattern. The results obtainedare discussed in terms of: (i) more reduced nitrogen as substratefor amino acid synthesis; (ii) the changes in pH and ionic balancecaused by the process of nitrate reduction to ammonium; and(iii) specific enzyme activation or inhibition by nitrate orammonium ions.     

遮荫对半夏叶片光合色素与保护酶活性的影响

对半夏进行不同程度遮荫处理,研究了其生长过程中叶组织的光合色素含量、叶绿素a/b值、保护酶体系(SOD、POD、CAT)活性和丙二醛(MDA)含量的变化及其生理特性.结果表明:不同处理的半夏在生长过程中,光合色素的含量呈先升后降的趋势;与全光照相比,遮荫处理的叶绿素含量较高,叶绿素a/b值较低,类胡萝卜素含量在生长发育中早期较低而后期较高;SOD、POD、CAT活性呈由低到高再降低的趋势,MDA含量持续升高;遮荫处理的SOD、POD活性和MDA含量均低于全光照处理,CAT活性则表现为先高于后低于全光照处理.适度的遮荫可以提高半夏叶绿素的含量,降低膜系统受到的伤害,改善半夏的倒苗状况,为半夏叶片的生长提供较好的环境.     

Formation of the Photosynthetic Electron Transport System during the Early Phase of Greening in Barley Leaves

The development of photochemical activity in isolated plastids during the early phase of greening of 5-day-old etiolated barley seedlings was studied and related to the appearance of chlorophyll-protein complexes. Photochemical activities of PSI (DCIPH₂ → MV) and PSII (H₂O → DCIP, DPC → DCIP) appeared at 1 and 1.5 hours after the onset of illumination, respectively. However, PSI + PSII activity (H₂O → MV, H₂O → NADP) appeared at 4 hours. The functional plastoquinone pool was noticed, at the latest, from 4 hours. Chloroplast preparations from seedlings of 1 h of greening showed O₂ uptake upon illumination in the absence of MV (−MV activity). This activity peaked at 2 hours of greening, then fell to zero by 6 hours. In contrast to the −MV activity, MV-Hill activity began to increase at 2 hours. Although PSI activity appeared at 1 hour, it failed to reduce ferredoxin until 2 hours. NADP began to be photoreduced at 4 hours in accordance with the appearance of the ferredoxin:NADP reductase activity. After formation of PSI and PSII, electron transport systems between them and between PSI and NADP developed in coordination with each other. Thus, the whole electron transport from water to NADP began to operate at 4 hours.     

Relationship between CO2 Assimilation, Photosynthetic Electron Transport, and Active O2 Metabolism in Leaves of Maize in the Field during Periods of Low Temperature

Measurements of the quantum efficiencies of photosynthetic electron transport through photosystem II (_PSII) and CO₂ assimilation (_CO2) were made simultaneously on leaves of maize (Zea mays) crops in the United Kingdom during the early growing season, when chilling conditions were experienced. The activities of a range of enzymes involved with scavenging active O₂ species and the levels of key antioxidants were also measured. When leaves were exposed to low temperatures during development, the ratio of _PSII/_CO2 was elevated, indicating the operation of an alternative sink to CO₂ for photosynthetic reducing equivalents. The activities of ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, glutathione reductase, and superoxide dismutase and the levels of ascorbate and α-tocopherol were also elevated during chilling periods. This supports the hypothesis that the relative flux of photosynthetic reducing equivalents to O₂ via the Mehler reaction is higher when leaves develop under chilling conditions. Lipoxygenase activity and lipid peroxidation were also increased during low temperatures, suggesting that lipoxygenase-mediated peroxidation of membrane lipids contributes to the oxidative damage occurring in chill-stressed leaves.