Comparison of Triazine-Resistant and -Susceptible Biotypes of Senecio vulgaris and Their F(1) Hybrids

The relationship of triazine resistance to decreased plant productivity was investigated in Senecio vulgaris L. F₁ reciprocal hybrids were developed from pure-breeding susceptible (S) and resistant (R) lines. The four biotypes (S, S × R, R, R × S) were compared in terms of atrazine response, electron transport, carbon fixation, and biomass production. Atrazine response, carbon fixation rate, and PSII and whole-chain electron transport rates of hybrids were nearly identical to those of their respective maternal parents. Significant differences occurred between the two susceptible (S, S × R) and two resistant (R, R × S) biotypes in atrazine response (I₅₀), carbon fixation rate, and PSII and whole-chain electron transport rates; PSI rates were identical in all four biotypes. Coupled and uncoupled, whole-chain electron transport rates of thylakoids of the two susceptible biotypes were approximately 50% greater than those of the two resistant biotypes at photon flux densities greater than 215 micromoles per square meter per second. Carbon exchange rates of the two susceptible biotypes were 23% greater than those of the two resistant biotypes. Hybrid biotypes (S × R, R × S) were not identical to their maternal parents in biomass production. The S, S × R, and R × S plants all achieved greater biomass than R plants. These results suggest that while the resistance mutation influences thylakoid performance, reduced productivity of triazine-resistant plants cannot be ascribed solely to decreases in electron transport or carbon assimilation rates brought about by the altered binding protein. Since the F₁ hybrids differed from their maternal parents only in nuclear genes, it appears that the detrimental effects of the triazine resistance mutation on plant growth may be attenuated by interactions of the plastid and nuclear genomes.     

The Effect of High Temperatures on the Photosynthetic Activity of Intact Barley Leaves at Low and High Irradiance

Light curves of CO₂ fixation by barley seedling leaves preliminarily heated at 30–43°C for 5 min were measured. The slope of the linear part of the light curve decreased after leaf heating at temperatures above 35°C; whereas, at a high light level, the photosynthesis rate decreased only at temperatures of 40°C and higher. The linear relationships between the photosynthetic CO₂-fixation rate and a photon flux density up to 1400 mol/(m² s) were found in leaves preheated at 42°C; this indicates the strong nonphotochemical dissipation of absorbed light quanta. The lowering of the oxygen concentration from 21 to 1% led to a CO₂ fixation maximum quantum yield and a photosynthesis-rate increase at the highest light intensity in leaves preheated at temperatures above 40°C as compared to the control leaves. Nevertheless, the linear relationship between the photosynthetic CO₂ fixation and the light intensity was found in leaves heated at 42°C at O₂ concentrations of both 21 and 1%. The latter fact suggests that the proton gradient of the thylakoid membrane, which causes an increase in the nonphotochemical dissipation of the quanta absorbed, could also be formed due to the cyclic electron transport over photosystem I.     

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.     

Photosynthetic Electron Transport During Senescence of the Primary Leaves of Phaseolus vulgaris L.: I. NON-CYCLIC ELECTRON TRANSPORT

Coupled, non-cyclic electron transport was measured for chloroplastsisolated from the primary leaves of Phaseolus vulgaris. Preparationsfrom young, fully expanded leaves gave good rates of electrontransport, but the rates obtained decreased by approximately80% during leaf senescence. Higher rates of electron transportwere recorded for chloroplasts isolated from primary leaveswhich had regreened following removal of the remainder of theshoot. With preparations from leaves of all ages, photophosphorylationwas coupled to electron transport with a mean P/2e ratio ofapproximately 1.3. No evidence was obtained for inactivationof chloroplasts from older leaves during isolation or assay,and it is suggested that the decrease in rate of electron transportover the period of senescence, and its increase during regreening,were consequences of changes in the composition and physicalproperties of the thylakoid membrane which occur in vivo. Thedecrease in rate of non-cyclic electron transport may be importantin limiting the rate of photosynthesis in the senescing leaves.     

Exposure of Leaves of Cucumis sativus L. to Low Temperatures in the Light Causes Uncoupling of Thylakoids II. Non-Destructive Measurements with Intact Leaves

To examine the effects of chilling of leaves of cucumber (Cucumissativus L.) in moderate light on the coupling state of thylakoidsin situ, changes in fluorescence, changes in light scatteringand flash-induced changes in absorbance at 518 nm were examinedin intact leaves. After chilling of leaves at 5?C in the lightfor 5 h, the non-photochemical quenching of fluorescence, ameasure of energisation of thylakoids, was largely suppressed.The treatment also caused a suppression of light-induced changesin the light scattering by leaves, which depends on the formationof a pH gradient across thylakoid membranes. When thylakoidswere prepared by very gentle methods from the leaves chilledin the light, through a step of preparation of intact chloro-plasts,the transport of electrons from H₂O to ferricyanide was uncoupled,being insensitive to an uncoupler, methylamine. These data provide consistent evidence that the thylakoids areuncoupled in situ by the chilling of leaves in the light and,as a consequence of the uncoupling, the energisation of themembranes is suppressed. However, the decay of the flash-inducedchange in absorbance at 518 nm in leaves was not markedly acceleratedby the treatment. The thylakoids isolated from leaves chilledin the light, which were in the uncoupled state, also did notshow a rapid decay, unless an efficient uncoupler such as gramicidinwas added. These results suggest that even a considerable uncouplingof thylakoids, brought about by chilling of leaves in the light,is not sufficient to cause a marked acceleration of the decayof the flash-induced change in absorbance at 518 nm. Therefore,analysis at 518 nm is not always a sensitive method for assessingthe coupling state of thylakoids. (Received July 1, 1991; Accepted October 4, 1991)     

Changes in Chlorophyll Content and Organization During Senescence of the Primary Leaves of Phaseolus vulgaris L. in Relation to Photosynthetic Electron Transport

A large decrease was observed in the chlorophyll content ofthe primary leaves of Phaseolus vulgaris during senescence.Chloroplasts isolated from mature and senescent leaves gavevery similar light saturation curves for electron transportreactions involving either PS I or PS II, indicating that theaverage number of chlorophyll molecules associated with eachreaction centre did not change during senescence. It is concludedthat the reaction centres ceased to function at the same timeas, or perhaps before, their antenna chlorophylls were lostfrom the thylakoid membrane, and that the percentage decreasein the number of functional reaction centres per leaf was atleast as great as the percentage decrease in the leaf chlorophyllcontent. The chlorophyll-protein composition of thylakoid membrane preparationswas examined by electrophoresis of samples treated with sodiumdodecyl sulphate. In older leaves a smaller proportion of thechlorophyll applied to polyacrylamide gels was associated withthe P700- chlorophyll a-protein complex. There was also a declinein emission at 734 nm in the 77 °K fluorescence spectrumof intact leaf tissue during senescence. These results indicatethat older leaves contained a smaller proportion of chlorophyllsassociated with PS I, and this is consistent with the decreaseobserved in the leaf chlorophyll a/b ratio during senescence.The effect of these changes in chlorophyll content on the capacityof the chloroplast to carry out photosynthetic electron transportis discussed.     

Photosynthetic Electron Transport During Senescence of the Primary Leaves of Phaseolus vulgaris L.: III. KINETICS OF CHLOROPHYLL FLUORESCENCE EMISSION FROM INTACT LEAVES

The kinetics of 685 nm chlorophyll fluorescence emission weremeasured at 20 °C following illumination of primary leavesof P. vulgaris. During foliar senescence, a large reductionwas observed in the maximal level of fluorescence emission (P)of the induction curve, normalized with respect to the minimallevel (O), and in the time taken to reach P. This suggests thatfewer plastoquinone (PQ) molecules were able to accept electronsfrom each photosystem two (PS II) reaction centre in older leaves.Measurements of fluorescence emission at 77 °K indicatedthat the primary photochemical quantum yield of the PS II reactioncentres remained constant during senescence. The redox stateof the PQ pool was estimated throughout the induction curveat 20 °C. In both mature and senescent leaves PQ was highlyreduced at P. There followed a reoxidation of PQ in the matureleaves, but in the old leaves the PQ pool remained reduced.This indicates that the rate of electron flow from PQ to photosystemone (PS I) decreased considerably during senescence. Fluorescencewas quenched from P to a steady state level (T) in leaves ofall ages, and this was associated with a redistribution of excitationin favour of PS I. Since, in senescent leaves, changes in theredox state of PQ were absent, it is suggested that quenchingresulted from the generation of proton and ion gradients acrossthe thylakoid membranes, and the synthesis of ATP.     

Effects of Different Inorganic Nitrogen Sources on Photosynthetic Carbon Metabolism in Primary Leaves of Non-nodulated Phaseolus vulgaris L

Young bean plants (Phaseolus vulgaris L. var Saxa) were fed with three different types of inorganic nitrogen, after being grown on nitrogen-free nutrient solution for 8 days. The pattern of ¹⁴CO₂ fixation was investigated in photosynthesizing primary leaf discs of 11-day-old plants (3 days with nitrogen source) and in a pulse-chase experiment in 13-day-old plants (5 days with nitrogen source).

Ammonium caused, in contrast to nitrate nutrition, a higher level of ¹⁴C incorporation into sugar phosphates but a lower incorporation of label into malate, glycolate, glycerate, aspartate, and alanine. The labeling kinetics of glycine and serine were little changed by the nitrogen source. Ammonium feeding also produced an increase in the ratio of extractable activities of ribulose-1,5-bisphosphate carboxylase to phosphoenolpyruvate carboxylase and an increase in dark respiration and the CO₂ compensation concentration. Net photosynthesis was higher in plants assimilating nitrate.

The results point to stimulated turnover of the photosynthetic carbon reduction cycle metabolites, reduced phosphoenolpyruvate carboxylation, and altered turnover rates within the photosynthetic carbon oxidation cycle in ammonium-fed plants. Mechanisms of the regulation of primary carbon metabolism are proposed and discussed.

     

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.     

Carbon Partitioning in Mature Leaves of Pepper: Effects of Transfer to High or Low Irradiance

Grange, R. I. 1987. Carbon partitioning in mature leaves ofpepper: Effects of transfer to high or low irradiance.—J.exp. Bot. 38: 77–83. Pepper plants were grown at an irradiance of either 55 W m^–2or 90 W m^–2 PAR. Changes in net photosynthesis, carbonexport, starch and sugar contents in a single mature leaf weremeasured at intervals for 8 d following transfer of plants betweenthe two irradiances. On transfer from low to high irradiance,the net photosynthesis rate increased immediately but exportrate increased only slowly, to a maximum after 3 d. While assimilationexceeded export more starch and sucrose accumulated in the dayand remained in the leaf at the end of each night. Hexose contentsat the end of night remained low and constant, but the daytimemaximum rose during the first 2 d from transfer, thereafterreturning to pre-transfer contents. Following transfer of leaves from high to low irradiance starchpresent in the leaf provided sufficient reserves to maintainthe rate of export for one day. Subsequently, the sucrose contentfell and the export rate declined to near that in leaves grownin low irradiance. Sucrose and hexose accumulation following transfer from lowto high irradiance suggests a limitation to export ‘downstream’from sucrose synthesis, probably in the loading step from mesophyllto phloem. Key words: Pepper, export, starch, loading     

夜间高温胁迫对水稻叶片光合机构的影响

The net photosynthetic rate (Pn), the apparent quantum yield (AQY), the photochemical efficiency of PSⅡ(Fv/Fm), the quantum yield of PSⅡ electron transport (Ф PSⅡ) and the coefficient of photochemical quenching (qp) were decreased in rice (Oryza sativa L.) leaves under high nocturnal temperature (42±1) ℃ stress, but the relative reduction state of PSⅡ (1-qp) was increased. With the increment of stress time, the chlorophyll content and the binding degree of chlorophyll-protein complex declined gradually, the O2(superoxide radical) production rate and H2O2 content in leaves elevated. The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) increased for 2-3 days under high temperature stress yet decreased afterwards. The results showed that the photosynthetic efficiency declined in rice leaves under high nocturnal temperature stress with concurrent emergence of the symptom of oxidative damage. The gradual increase of the SOD, POD and CAT activities, as well as that of the ratio of Pr (photo-respiration rate) to Pr+Pn and non-photochemical quenching of chlorophyll fluorescence (NPQ) indicates that those mechanisms related to the changes of these parameters may play an important role in protecting rice leaves from oxidative damage under high nocturnal temperature stress.     

Starch Accumulation and Photosynthetic Activity in Primary Leaves of Bean (Phaseolus vulgaris L.)

The effects of starch accumulation on photosynthesis and chloroplastultrastructure were studied in primary leaves of bean (Phaseolusvulgaris L. cv. Bulgarian). De-topping the shoot above the primaryleaf node, caused over an 8-day period, a considerable increasein the photosynthetic activity of the primary leaves, despitethe fact that a large quantity of starch had accumulated intheir chloroplasts. The accumulation of starch was greater inthe chloroplasts of spongy cells in comparison with that ofthe palisade cells. Initiation of starch grains was observedmainly in the peripheral part of the chloroplast, distant fromthe cell wall. As a result, most of the starch was accumulatedclose to the inner part of the cell, leaving a considerablemass of the chloroplast near the cell wall free of starch. Theaccumulation of starch was accompanied by the destruction, deformationand disorientation of grana and thylakoids. It is concludedthat the accumulation of starch is not inevitably a limitingfactor in photosynthesis and the results cast doubt on the hypothesisthat starch accumulation or dissipation is the main factor involvedin the regulation of photosynthesis. Phaseolus vulgaris L, bean, photosynthesis, starch accumulation, chloroplast ultrastructure     

遮荫对黑茶藨子叶片形态结构和光合特性的影响

以黑茶藨子为试验材料,通过人工遮荫的方式,研究了不同遮荫强度（全光照、20%和60%遮荫）和遮荫时间（初期、中期和后期）对黑茶藨子叶片形态解剖结构和光合特性的影响。结果表明：（1）20%、60%遮荫处理的叶片厚度均显著低于全光照处理,上下表皮厚度则高于全光照处理;随着遮荫强度的加大,栅栏组织与海绵组织厚度比值大小均呈减少趋势,随着遮荫时间的延长,呈增加趋势。（2）遮荫初期和中期,20%遮荫处理的叶片净光合速率（P_n）比全光照和60%遮荫处理的值略高,但差异并不显著（P>0.05）,其蒸腾速率（T_r）、水分利用效率（WUE）也略有增加;但是,60%遮荫处理的叶片净光合速率（P_n）显著低于全光照处理（P<0.05）。遮荫后期,60%遮荫处理的叶片净光合速率（P_n）显著高于全光照处理,且水分利用效率（WUE）、气孔导度（G_s）、蒸腾速率（T_r）均显著高于全光照和20%遮荫处理。综上所述,遮荫处理导致叶片厚度减少,海绵组织比例增加,有利于减少透射光中的光量子丧失,提高光能利用效率;生长早期和中期作20%轻度遮荫处理,后期作60%中度遮荫处理能提高植株对光和水分的利用效率、维持较高的光合速率、延缓衰老。     

Photosynthetic Performance of Ginkgo biloba L. Grown Under High and Low Irradiance

Diurnal variation in net photosynthetic rate (P _N) of three-year-old plants of Ginkgo biloba was studied under open, O (receiving full sunlight), net-shade, NS (40 % of photosynthetically active radiation, PAR), or greenhouse, G (25 % PAR) conditions. In all three conditions, P _N was higher in morning along with stomatal conductance (g _s), and intercellular CO₂ concentration (C _i), while leaf temperature and vapour pressure deficit were low. The O-plants exhibited a typical decline in P _N during midday, which was not observed in NS-plants. This indicated a possible photoinhibition in O-plants as the ratio of variable to maximum fluorescence (F_v/F_m) and photosystem 2 (PS2) yield (_PS2) values were higher in the NS- and G-plants. On the contrary, stomatal density and index, chlorophyll a/b ratio, leaf thickness, and density of mesophyll cells were greater in O-plants. Further, higher P _N throughout the day along with higher relative growth rate under NS as compared to O and G suggested the better efficiency of Ginkgo plants under NS conditions. Therefore, this plant species could be grown at 40 % irradiance to meet the ever-increasing demand of leaf and also to increase its export potential.     

The Genetics of Evolutionary Change in Senecio vulgaris L.: A QTL Mapping Approach

Abstract: The cosmopolitan weed Senecio vulgaris var. vulgaris is likely to have originated from the non-weedy S. vulgaris ssp. denticulatus from which it differs by showing no seed dormancy, by completing its life cycle from germination to seed formation much faster and by lacking ray florets. An F2 generation of 120 individuals obtained through selfing of one hybrid individual between var. vulgaris and ssp. denticulatus was used to construct a linkage map based on RAPD polymorphisms and the presence or absence of ray florets. This linkage map was used for a QTL analysis of 12 characters distinguishing the two taxa. For seven of these 12 characters, three significant QTLs could be found. One of these QTLs controls the speed of development, height of plants, leaf number, number of lateral branches and number of outer involucral bracts. A second QTL, located in the same linkage group, coincides with the ray floret locus and controls the number of disc florets. Plant height and leaf number are controlled by a third QTL in a different linkage group. Considering earlier evidence on the genetics of seed dormancy, it is argued that probably only three chromosomal regions, or even genetic loci, control seed dormancy, speed of development and presence or absence of ray florets as the ecologically most important differences between var. vulgaris and ssp. denticulatus. These findings have important implications for the genetics of evolutionary change and speciation.     

Comparison of Photosynthetic Performance in Triazine-Resistant and Susceptible Biotypes of Amaranthus hybridus

The rate of CO₂ reduction in the S-triazine-resistant biotype of smooth pigweed (Amaranthus hybridus L.) was lower at all levels of irradiance than the rate of CO₂ reduction in the susceptible biotype. The intent of this study was to determine whether or not the lower rates of CO₂ reduction are a direct consequence of the same factors which confer triazine resistance. The quantum yield of CO₂ reduction was 23 ± 2% lower in the resistant biotype of pigweed and the resistant biotype of pigweed had about 25% fewer active photosystem II centers on both a chlorophyll and leaf area basis. This quantum inefficiency of the resistant biotype can be accounted for by a decrease in the equilibrium constant between the primary and secondary quinone acceptors of the photosystem II reaction centers which in turn would lead to a higher average level of reduced primary quinone acceptor in the resistant biotype. Thus, the photosystem II quantum inefficiency of the resistant biotype appears to be a direct consequence of those factors responsible for triazine resistance but a caveat to this conclusion is discussed. The effects of the quantum inefficiency of photosystem II on CO₂ reduction should be overcome at high light and therefore cannot account for the lower light-saturated rate of CO₂ reduction in the resistant biotype. Chloroplast lamellar membranes isolated from both triazine-resistant and triazine-susceptible pigweed support equivalent rates of whole chain electron transfer and these rates are sufficient to account for the rate of light-saturated CO₂ reduction. This observation shows that the slower transfer of electrons from the primary to the secondary quinone acceptor of photosystem II, a trait which is characteristic of the resistant biotype, is nevertheless still more rapid than subsequent reactions of photosynthetic CO₂ reduction. Thus, it appears that the lower rate of light-saturated CO₂ reduction of the resistant biotype is not limited by electron transfer capacity and therefore is not a direct consequence of those factors which confer triazine resistance.     

Energy and Electron Transfer Systems of Chlamydomonas reinhardi: II. Two Cyclic Pathways of Photosynthetic Electron Transfer in the Pale Green Mutant

Light- and oxygen-induced changes of cytochromes f, b₅₆₃, and b₅₅₉ and ferredoxin-flavoprotein were studied by a double beam spectrophotometer with combinations of inhibitors and lowered temperatures in the whole cells of the pale green mutant of Chlamydomonas reinhardi (ATCC 18302). At room temperature, the steady state changes of cytochrome f and ferredoxin-flavoprotein are small, but at low temperature slightly above 0 C, they are clearly defined. Phenylmercuric acetate inhibits photoreduction of ferredoxin-flavoprotein and cytochrome f simultaneously but not that of cytochrome b₅₆₃. 2-Heptyl-4-hydroxyquinoline-N-oxide shows a crossover point between cytochromes f and b₅₆₃ and partially inhibits photoreduction of cytochrome f. Two cyclic pathways operating in C. remhardi are postulated: (a) photosystem I → x → b₅₆₃ → f → photosystem I; and (b) photosystem I → x → ferredoxin-flavoprotein → f → photosystem I.     

Effect of the Pool Size of Stromal Reductants on the Alternative Pathway of Electron Transfer to Photosystem I in Chloroplasts of Intact Leaves

The effect of elevated temperature on electron flow to plastoquinone pool and to PSI from sources alternative to PSII was studied in barley (Hordeum vulgare L.) and maize (Zea mays L.) leaves. Alternative electron flow was characterized by measuring variable fluorescence of chlorophyll and absorption changes at 830 nm that reflect redox changes of P700, the primary electron donor of PSI. The treatment of leaves with elevated temperature resulted in a transient increase in variable fluorescence after cessation of actinic light. This increase was absent in leaves treated with methyl viologen (MV). The kinetics of P700⁺ reduction in barley and maize leaves treated with DCMU and MV exhibited two exponential components. The rate of both components markedly increased with temperature of the heat pretreatment of leaves when the reduction of P700⁺ was measured after short (1 s) illumination of leaves. The acceleration of both kinetic components of P700⁺ reduction by high-temperature treatment was much less pronounced when P700⁺ reduction rate was measured after illumination of leaves for 1 min. Since the treatment of leaves with DCMU and MV inhibited both the electron flow to PSI from PSII and ferredoxin-dependent cycling of electrons around PSI, the accelerated reduction of P700⁺ indicated that high temperature treatment activated electron flow to PSII from reductants localized in the chloroplast stroma. We conclude that the lesser extent of activation of this process by elevated temperature after prolonged illumination of heat-inhibited leaves is caused by depletion of the pool stromal reductants in light due to photoinduced electron transfer from these reductants to oxygen.