Effect of Cd on Photosynthesis and Transpiration of Excised Leaves of Corn and Sunflower

Detached corn and sunflower leaves exposed to various concentrations of Cd, supplied as CdCl₂, exhibit reduced photosynthesis and transpiration. The reduction is dependent on the concentration of CdCl₂ solution and generally becomes more pronounced with time. In sunflower, net photosynthesis and transpiration are completely inhibited within 45 min after the introduction of 18 mM Cd. Within two hours net photosynthesis is reduced to 40% and 70% of maximum after the introduction of 9 and 4.5 mM Cd respectively. In corn the trend of photo-synthetic response to Cd is similar to that in sunflower except that the inhibition in corn is more pronounced at all treatment levels. A strong linear relationship between photosynthesis and transpiration inhibition is obtained in both species suggesting that Cd contamination induces stomatal closure.     

Thickness Fluctuations in Veins of Corn and Sunflower detected by a Linear Transducer

A short-stroke linear displacement transducer has been usedto detect changes in thickness of veins of corn and sunflower.After illumination, continual fluctuations in thickness occurred,some 2–5 min and others 15–20 min apart and from0.05% to 2% of the total vein thickness in amplitude. Thesechanges, often erratic and usually not simultaneous at points4.0 cm apart, seem to be related to changes in water potentialin the tissue. The short-term fluctuations were not due to majorstomatal oscillations but could be due to local variations intranspiration, or to fluctuations in photosynthate moving throughthe veins, for they often resembled the patterns of radioactivitydetected after feeding ¹¹CO₂or ¹¹C-sucrose. The magnitude offluctuation increased as the diameter of vein increased, andthus could be useful in monitoring changes of water potentialin leaves.     

Inhibition of Respiration of Yeast by Photosynthesis Inhibiting Herbicides

In order to elucidate the mode of action of some herbicides, effect of several anilide type herbicides on the respiration of yeast cells was studied. The results obtained were as follows: 1) DCPA (3,4-dichloropropionanilide) and DCMU (3-(3,4-dichlorophenyl)-1,1- dimethylurea), the powerful inhibitors of the Hill reaction in photosynthesis, inhibited the oxygen uptake of yeast cells at low concentrations. 2) DCPA and DCMU inhibited the enzymic reduction of cytochrome-c by the yeast cell-free preparation, but not the reduction of dye. 3) The oxidation of cytochrome-b was inhibited in the yeast cells treated with DCPA or DCMU.     

Sink-Regulation of Photosynthesis in Relation to Temperature in Sunflower and Rape

Stimulation of the rate of photosynthesis at 2·0 kPaO₂ in comparison with 21 kPa O₂ and carbohydrate accumulationover 4h were measured during exposure of sunflower (Helianthusannuus L.) and rape (Brassica napus L.), grown at 30 °Cand 13 °C, to temperatures between 7 °C and 35 °C.The effect of reducing source: sink ratio by shading on theresponse of photosynthetic rate to temperature was also determined.Stimulation of photosynthesis by 2·0 kPa O₂ in comparisonwith 21 kPa O₂ decreased over 4 h at cool temperatures in sunflowerplants grown at 30 °C but not in rape grown at 30 °C.Stimulation did not decrease over 4 h in plants grown at 13CC. Sucrose was the main carbohydrate accumulated over 4 h;its accumulation increased with decreasing temperature. Starchaccumulation either decreased or remained the same with decreasingtemperature. In plants grown at 30 °C more carbohydrateaccumulated between 8 °C and 21 °C in sunflower thanin rape, but more carbohydrate accumulated at 30 °C in rapethan in sunflower. In plants grown at 13 °C much less carbohydrateaccumulated between 13 °C and 23 °C than in plants grownat 30 °C. Photosynthetic rate in plants grown at 30 °Cexposed to between 20 °C and 35 °C over 32 h (14 h light-10h dark-8 h light), declined over 32 h at 20 °C and 25 °Cin sunflower and at 20 °C in rape. This fall over 32 h,especially at 20 °C in sunflower, was significantly reducedby shading the rest of the plant. Shading had little effecton photosynthetic rate above 25 °C. The work confirms thatlow temperature imposes a sink-limitation on photosynthesiswhich occurs at higher temperatures in sunflower than in rape.This limitation may be relieved by decreasing the source:sinkratio. Key words: Sunflower, rape, photosynthesis, carbohydrates, sink demand, temperature     

Effects of Nitrogen Nutrition on Photosynthesis in Cd-treated Sunflower Plants

Increased nitrogen supply stimulates plant growth and photosynthesis.Since it was shown that heavy metals may cause deficienciesof essential nutrients in plants the potential reversal of cadmiumtoxicity by increased N nutrition was investigated. The effectson photosynthesis of low Cd (0, 0.5, 2 or 5 mmol m^-3) combinedwith three N treatments (2, 7.5 or 10 mol m^-3) were examinedin young sunflower plants. Chlorophyll fluorescence quenchingparameters were determined at ambient CO₂and at 100 or 800 µmolquanta m^-2 s^-1. The vitality index (R_fd) decreased approx. three-timesin response to 5 mmol m^-3Cd, at 2 and 10 mol m^-3N. The maximumphotochemical efficiency of PSII reaction centres (F_v/ F_m) wasnot influenced by Cd or N treatment. The highest Cd concentrationdecreased quantum efficiency of PSII electron transport (_II)by 30%, at 2 and 10 mol m^-3N, mostly due to increased closureof PSII reaction centres (q_P). Photosynthetic oxygen evolutionrates at saturating CO₂were decreased in plants treated with5 mmol m^-3Cd, at all N concentrations. The results indicatethat Cd treatment affected the ribulose-1,5-bisphosphate (RuBP)regeneration capacity of the Calvin cycle more than other processes.At the same time, the amounts of soluble and ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco) protein increased with Cd treatment.Decreased photosynthesis, but substantially increased Rubiscocontent, in sunflower leaves under Cd stress indicate that asignificant amount of Rubisco protein is not active in photosynthesisand could have another function. It is shown that optimal nitrogennutrition decreases the inhibitory effects of Cd in young sunflowerplants. Copyright 2000 Annals of Botany Company Helianthus annuus L., cadmium, nitrogen, photosynthesis, Rubisco, sunflower     

Inhibition of Photosynthesis in Certain Algae by Extreme Red Light

This paper shows that in Porphyridium cruentum and in Chlorella pyrenoidosa (but apparently not in Anacystis nidulans) “extreme red” light (> 720 mμ) can inhibit photosynthesis produced by “far red” light (up to 720 mμ). From the action spectrum of this phenomenon, it appears that an unknown pigment with an absorption band around 745 mμ must be responsible for it.     

Photosynthesis by isolated chloroplasts. Inhibition by dl-glyceraldehyde of carbon dioxide assimilation

Photosynthetic carbon assimilation and associated CO(2)-dependent O(2) evolution by chloroplasts isolated from pea shoots and spinach leaves is almost completely inhibited by 10mm-dl-glyceraldehyde. The inhibitor is without appreciable effect on photosynthetic electron transport, photophosphorylation, the carboxylation of ribulose 1,5-diphosphate or the reduction of 3-phosphoglycerate, but apparently blocks the conversion of triose phosphate into ribulose 1,5-diphosphate.     

The Effect of Temperature on Photosynthesis and Carbon Fluxes in Sunflower and Rape

Sunflower (Helianthus annuusL.) and oilseed rape (Brassica napusL.) were grown at constant temperatures of 30 ?C (warm) and13 ?C (cold). Maximal rates of photosynthesis between 5 ?C and35 ?C were at higher temperatures in sunflower than rape. Photosyntheticrate over 4 h at the growth temperature declined in warm-andcold-grown rape and cold-grown sunflower, but remained constantin warm-grown sunflower. The stimulation of photosynthesis by2.0 kPa O₂ compared to 21 kPa O₂ declined with decreasing temperature.At 10 ?C in warm-grown rape photosynthesis was insensitive to2.0 kPa O₂. However, sensitivity to low O₂ continued at 10 ?Cin warm-grown sunflower. Carbohydrates accumulated in the cold,particularly fructose, glucose and sucrose in warm-grown sunflowertransferred to 13 ?C. By monitoring changes of ¹⁴C in leaves after the assimilationof ¹⁴CO₂, the rates of carbon export from leaves, pool sizesand carbon fluxes between them were estimated. The transferof warm- and cold-grown rape to 13 ?C and 30 ?C, respectively,had little effect on these parameters over 22 h. However, exportof carbon from sunflower leaves at 13 ?C was markedly less thanat 30 ?C, irrespective of the growth temperature, due to slowerexport from the transport pool. The rapid suppression of carbonexport at 13 ?C in warm-grown sunflower may be due to inhibitedtranslocation rather than reduced sink demand in the cold. It is concluded that assimilate utilisation is more depressedin the cold than is photosynthesis; this imposes a greater restrictionon biomass production in sunflower than in rape. Key words: Sunflower, rape, temperature, photosynthesis, carbon fluxes     

Inhibition of Photosystem II in Isolated Chloroplasts by Lead

Inhibition of photosynthetic electron transport in isolated chloroplasts by lead salts has been demonstrated. Photosystem I activity, as measured by electron transfer from dichlorophenol indophenol to methylviologen, was not reduced by such treatment. However, photosystem II was inhibited by lead salts when electron flow was measured from water to methylviologen and Hill reaction or by chlorophyll fluorescence. Fluorescence induction curves indicated the primary site of inhibition was on the oxidizing side of photosystem II. That this site was between the primary electron donor of photosystem II and the site of water oxidation could be demonstrated by hydroxylamine restoration of normal fluorescence following lead inhibition.     

Ozone Inhibition of Photosynthesis in Chlorella sorokiniana

Exposure of Chlorella sorokiniana (07-11-05) to ozone inhibits photosynthesis. In this study, the effects of ozone on O₂ evolution and fluorescence yields are used to characterize this inhibition. At an ozone dose of about 3 micromoles delivered to 2 × 10⁹ cells, the photosynthetic rate of the cells is inhibited 50%, as indicated by a decrease in bicarbonate-stimulated O₂ evolution (control rate, 1.4 ± 0.3 × 10⁻¹⁵ moles per cell per minute).     

Effect of Verticillium dahliae on Photosynthesis, Leaf Expansion and Senescence of Field-grown Sunflower

On the basis of known sunflower (Helianthus annuus L.) responsesto soil water deficit, it is proposed that the effect of thefungus Verticillium dahliae Klebahn on plant leaf area precedesand is greater than its effect on leaf photosynthesis and stomatalconductance. To test this hypothesis, we measured shoot andleaf area growth, leaf photosynthetic rate, stomatal conductanceand disease symptoms in a field experiment including hybridsof high (Sankol) and low (Dekasol 3900) susceptibility to V.dahliae. Plants inoculated with V. dahliae and controls werecompared. We also investigated the effect of V. dahliae on keycomponents of plant leaf area, leaf expansion and senescence,in inoculated and control plants of Sankol and Toba, a hybridof intermediate susceptibility to V. dahliae. Reduction in plantleaf area caused by V. dahliae was first detected 31 d afterinoculation (DAI), when visual symptoms of disease in inoculatedplants were slight (Sankol) or absent (Dekasol 3900). Reductionin leaf photosynthesis was first observed 66 DAI; stomatal conductanceand leaf dark respiration were both unaffected by V. dahliaeduring the whole experiment. In comparison with controls, V.dahliae reduced seasonal duration of plant leaf area by 25%in Dekalb 3900 and by 55% in Sankol, whereas the average reductionin leaf photosynthetic rate was 9%. In correspondence with thereduction in leaf area duration, inoculation reduced shoot drymatter of mature Sankol by 50%. In both experiments, less leafexpansion accounted for most of the early reduction in plantleaf area; as the disease progressed, increasing senescencealso contributed to reduced plant leaf area. It is concludedthat the response of sunflower to V. dahliae resembled the responseof the plant to soil water deficit: (1) plant leaf area, ratherthan leaf photosynthetic rate, accounted for the reduction ingrowth in mass; and (2) reduced leaf expansion early in theseason and faster leaf senescence in older plants accountedfor the decrease in plant leaf area. Copyright 2000 Annals ofBotany Company Helianthus annuus, Verticillium dahliae, allometry, apical dominance, drought, leaf expansion, leaf senescence, photosynthesis, stomatal conductance, growth     

Photorespiration and Oxygen Inhibition of Photosynthesis in Chlorella pyrenoidosa

The inhibition of photosynthesis by O₂ in air-grown Chlorella pyrenoidosa was investigated using three experimental techniques (artificial leaf, aqueous method, and O₂ electrode) to measure carbon assimilation. CO₂ response curves were determined under different O₂, pH, and temperature conditions. Regardless of the experimental technique and condition, O₂ inhibition was not evident until a concentration of 50% was reached; V_max values were reduced whereas K_m (CO₂) values were unaffected by the increasing O₂ concentration. The response of photosynthesis to O₂ was independent of CO₂ and HCO₃⁻ concentrations as well as temperature. Relative rates of photosynthesis showed a 4 to 5% stimulation in 2% O₂, a 12% inhibition in 50% O₂, and a 24% inhibition in 100% O₂. The inhibition by 50% O₂ was still reversible after 20 minutes exposure whereas 100% O₂ caused irreversible inhibition after only 4 minutes.     

Inhibition of Dark CO(2) Fixation and Photosynthesis in Leaf Discs of Corn Susceptible to the Host-specific Toxin Produced by Helminthosporium maydis, Race T

The host-specific toxin produced by Helminthosporium maydis, race T, causes 50% inhibition of dark fixation of ¹⁴CO₂ by leaf discs of susceptible (Texas male sterile) corn when it is diluted to approximately 1/10,000 of the volume of the original fungus culture filtrate. Dilutions of 1/10 or less are required for equivalent inhibition of discs prepared from resistant (N) corn. Root growth and photosynthesis were considerably less sensitive (dilution values 1/3000 and 1/1200, respectively), as was leakage of ¹⁴C induced by toxin from preloaded discs. Based on literature values for dilutions causing ion leakage or inhibition of mitochondrial oxidation, toxin dilutions several orders of magnitude greater bring about inhibition of dark CO₂ fixation. Preincubation of discs in light increased sensitivity of dark fixation to toxin and an effect of light on symptom development was shown. Phosphoenolypruvate carboxylase activity in extracts of roots or leaves was not affected by toxin nor was the enzyme level altered in excised leaves treated with toxin. Inhibition of dark fixation of CO₂ provides a bioassaay for race T toxin which is both reliable and rapid.     

Stomatal and Mesophyll Limitations of Photosynthesis in Phosphate Deficient Sunflower, Maize and Wheat Plants

The effects of phosphate deficiency on the composition and photosyntheticCO₂ assimilation rates of fully expanded leaves of sunflower,maize and wheat plants are described. The regulation of photosynthesisby stomatal and mesophyll characteristics of leaves of differentphosphate status is analysed and related to structure. Phosphatedeficient leaves had small concentrations of inorganic phosphate,Pi, in the tissue water. Rate of photosynthesis in leaves andstomatal conductance were smaller in plants grown with inadequatephosphate when measured under any given light intensity or CO₂partial pressure. Despite the decrease in stomatal conductance(and without evidence of patchy stomatal closure), the relativestomatal limitation of photosynthesis was similar in the plantsgrown with deficient or abundant phosphate. However, the mesophyllcapacity for photosynthesis was greatly limited by phosphatedeficiency. Leaves deficient in phosphate had larger numbersof small size cells per unit leaf area than leaves with adequatephosphate. The total soluble protein content of leaves decreasedwith phosphate deficiency in all three species; however, theleaf chlorophyll content was decreased only in sunflower andmaize and not in wheat. These results suggest that stomatalconductance did not restrict the CO₂ diffusion rate, ratherthe metabolism of the mesophyll was the limiting factor. Thisis shown by poor carboxylation efficiency and decreased apparentquantum yield for CO₂ assimilation, both of which contributedto the increase in relative mesophyll limitation of photosynthesisin phosphate deficient plants. Key words: Apparent quantum yield, carboxylation efficiency, phosphate nutrition, photosynthesis, stomatal and mesophyll limitation     

Investigation of Arbuscular mycorrhizal Fungus and EDTA Efficiencies on Lead Phytoremediation by Sunflower in a Calcareous Soil

Lead (Pb) contamination of soils is a widespread problem. Mycorrhizal inoculation and synthetic chelators such as ethylenediaminetetraacetic acid (EDTA) may be useful for improving phytoremediation efficiency in Pb-contaminated soils. A greenhouse experiment was performed to study the influence of inoculation with arbuscular mycorrhizal fungus (AMF), Glomus mosseae, and addition of EDTA on phytoremediation of Pb by sunflowers (Helianthus annuus) in a calcareous soil. The experiment was a completely randomized design in a factorial arrangement with five levels of Pb, two levels of mycorrhizal treatments, and two levels of EDTA. Inoculation increased root colonization as Pb levels increased, but the addition of EDTA decreased it. Shoot and root dry matter yields increased by inoculation; however, they decreased with EDTA and Pb levels in co-application treatments. Pb concentration in shoots was significantly higher than that in roots, indicating a translocation factor greater than 1. Inoculation or addition of EDTA significantly increased Pb in roots and its translocation to shoots. The uptake index (UI) value increased in co-application of EDTA and AMF and the individual application of them; it is, therefore, concluded that both AMF and EDTA are effective in phytoremediation of Pb by sunflowers in the studied soil.     

Effect of Gibberellic Acid on the Distribution of Products of Photosynthesis in Sunflower

Single leaves on growing sunflower plants were allowed to assimilate¹⁴CO₂. Gibberellic acid was applied to the same leaf or to theterminal bud or the roots, and the distribution of assimilated¹⁴C was determined at intervals of 1–96 h. Gibberellicacid had no significant effect on initial distribution of ¹⁴Cduring the period of rapid export from the leaf, but enhancedre-export from the roots after translocation from the leaf hadvirtually ceased. Most of the ¹⁴C exported from the roots accumulatedin the shoot tip. The site of application of the hormone wasof relatively minor importance. Wherever it was applied themajor effect was enhancement of movement from the roots to theshoot tip. Application to the terminal bud was most effectivein this respect. There was no evidence that gibberellic aciddirectly affected the transport system, but the data supportthe hypothesis that it increases the strength of the sink inthe shoot tip. Helianthus annuus L., sunflower, assimilate transport, gibberellic acid, phloem transport     

The Effect of Cooling on Photosynthesis, Amounts of Carbohydrate and Assimilate Export in Sunflower

Sunflower plants (Helianthus annuus L.) grown at 30°C werecooled to 13°C in the light in atmospheric CO² or low CO²,or in darkness. Photosynthetic rate at 30°C after coolingwhole plants in atmospheric CO² for 12 h during a photoperiodwas significantly lower than at the start of the photoperiodcompared to plants cooled at low CO², those cooled in the darkand those maintained at 30°C. Amounts of sucrose, hexosesand starch in leaves at 13°C increased throughout a 14 hphotoperiod to levels higher than in leaves at 30°C, whereamounts of sucrose and hexoses were stable or falling after4 h. Carbohydrate accumulation at 13°C during this photoperiodwas more than twice that at 30°C. After three photoperiodsand two dark periods at 13°C carbohydrate levels in leaveswere still as high as at the end of the first photoperiod, butless carbohydrate accumulated during the photoperiods than duringthe first photoperiod, and more was partitioned as starch. Amountsof soluble carbohydrate in roots were greater after 14 h at13°C than in roots of plants at 30°C. Loss of ¹⁴C fromleaves at 30°C as a proportion of ¹⁴CO2 fixed by them at30°C, decreased after exposure of plants to 13°C inthe light for 30 min prior to ¹⁴CO²feeding. Results indicatean effect of cold on the transport process that was light-dependent.It is inferred that the reduction in the proportion of ¹⁴C lostfrom leaves after 10 h cooling was due to reduced sink demand,whereas the rise in the proportion of ¹⁴C lost from leaves after24 h reflects reduced photosynthetic rate. The coincidence ofreduced photosynthetic rate with raised carbohydrate levelsin leaves maintained at 30°C throughout, whilst the restof the plant was cooled to 13°C in the light implies feedbackinhibition of photosynthesis. This may reduce the imbalancebetween source and sink in sunflower during the first days oflong-term cooling. Key words: Temperature, carbon export, carbohydrates, photosynthesis, sunflower     

Biphasic Inhibition of Photosynthesis in Powdery Mildewed Barley

A careful restudy of the photosynthetic process of barley (Hordeum vulgare) infected with powdery mildew (Erysiphe graminis f. sp. hordei) is reported. Unlike previous reports, which indicated a stimulation in infected host tissue photosynthesis during early stages of the disease followed by a rapid decline in activity, this study observed no stimulation but instead a biphasic inhibition in infected host photosynthesis under physiological concentrations of CO₂. Under high CO₂ (1.0%) a stimulation was observed during early stages of infection. The inhibition in low CO₂ and stimulation in high CO₂ of infected host photosynthesis was shown to occur in healthy host tissue when in the presence of 10⁻²m α-hydroxy-2-pyridinemethanesulfonic acid. The possible significance of these similar results is discussed.     

The Rate of Photorespiration during Photosynthesis and the Relationship of the Substrate of Light Respiration to the Products of Photosynthesis in Sunflower Leaves

Single attached leaves of sunflower (Helianthus annus L. “Mennonite”) were supplied ¹⁴CO₂ of constant specific radioactivity in gas mixtures containing various CO₂ and O₂ concentrations. The ¹⁴CO₂ and CO₂ fluxes were measured concurrently in an open system using an ionization chamber and infrared gas analyzer.