Photosynthesis of Ears and Flag Leaves of Wheat and Barley

Immediately after anthesis ears of spring wheat absorbed lessthan 0.5 mg CO₂, per hour in daylight and later evolved CO₂,in the light and in the dark. The rate of apparent photosynthesisof the combined flag-leaf lamina and sheath and peduncle (collectivelycalled flag leaf) of two spring wheat varieties, Atle and JufyI, was 3–4 mg per hour; the rates of the flag leaf andthe ear of two spring barleys, Plumage Archer and Proctor, wereeach about 1 mg per hour. The gas exchange of ears and flag leaves between ear emergenceand maturity accounted for most of the final grain dry weight.The CO₂, fixed by the wheat ear was equivalent to between 17and 30 per cent of the grain weight, but more than this waslost by respiration, so assimilation in the flag leaf was equivalentto 110–20 per cent of the final grain weight. In barley,photosynthesis in the flag leaf and the net CO₂ uptake by theear each provided about half of the carbohydrate in the grain. Barley ears photosynthesized more than wheat ears because oftheir greater surface, and flag leaves of wheat photosynthesizedmore than those of barley because they had more surface anda slightly greater rate of photosynthesis per dm².     

Varietal Differences in Photosynthesis of Ears and Leaves of Barley

Rates of apparent photosynthesis of ears and of the combinedflag leaf and sheath and peduncle of Proctor barley grown inpots or in the field were similar to those of Plumage Archer,or slightly smaller when the dimensions of the ear and leafarea of Proctor were less than those of Plumage Archer. Thephotosynthesis rate of the ear—about 1.0 mg. CO₂ per earper hour—was similar or slightly less than the rate ofthe flag leaf and sheath and peduncle. These rates of photosynthesisindicated that 40-50 per cent, of the carbohydrate in the grainwas provided by photosynthesis in the shoot and about 40 percent, by photosynthesis in the ear. The total CO₂ fixed by theear was equivalent to about 60 per cent, of the grain weight,20 per cent, being lost by respiration. Shading the ear underestimatedthe total amount of CO₂ fixed by the ear and decreased dry weightof grain per ear of both Proctor and Plumage Archer by 26 percent., as in pots. The contribution of ear photosynthesis toyield of grain per acre was greater for Proctor than for PlumageArcher because Proctor had more ears. The rate of apparent photosynthesis per dm.² of leaves of Proctorwas similar to that of Plumage Archer both before and afterear emergence. Before ear emergence, the photosynthesis rateof a particular leaf decreased linearly with time and was slowerfor lower than for higher leaves on the shoot. Respiration ratesper g. dry weight of ears of Proctor and Plumage Archer weresimilar; in one experiment the leaves of Proctor respired slightlyfaster than those of Plumage Archer.     

Photosynthesis of Lamina and Sheath of Barley Leaves

Apparent photosynthesis, in mg. CO₂ absorbed per dm.² per hour,of the sheath and enclosed stem of a barley leaf was about 50per cent. of that of the lamina of the same leaf, when the photosynthesizingarea was measured as one side of the lamina and the outer exposedsurface of the sheath. Apparent photosynthesis of a particularlamina or sheath was about 70 per cent. of that of the one aboveon the same stem. Respiration per dm.², though not per g. dry weight, of sheathwith enclosed stem was greater than of lamina in one experimentdone with low-intensity illumination so that true rates of photosynthesisof lamina and sheath were similar. Differences in respirationrates per unit area of laminae and sheaths probably accountedfor most of the greater apparent photosynthesis of the formerin other experiments done with higher intensity illumination. It is suggested that for growth-analysis studies the size ofthe photosynthetic system of cereals should be measured as thatof one side of the leaf laminae plus the outer surface of thecombined leaf sheaths. In the later stages of growth the surfacearea of exposed stem and peduncle should also be included.     

The Effect of an Oxygen-free Atmosphere on Net Photosynthesis and Transpiration of Barley (Hordeum vulgare L.) and Wheat (Triticum aestivum L.) Leaves

Stomata of barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) leaves failed to open in the light and close in the dark or respond to changes in the CO₂ concentration of the atmosphere in either light or dark when the leaves were in an O₂-free atmosphere. In contrast, the expected responses to environmental changes were found in atmospheres containing 1.5% O₂. It appears that O₂ is necessary for both opening and closing of wheat and barley stomata.     

A Battery-Operated Instrument for Non-destructive Measurements of Photosynthesis and Transpiration of Ears and Leaves of Cereals using 14CO2 and a Lithium Chloride Hygrometer

The design and evaluation of a battery-operated cuvette suitablefor non-destructive measurements of photosynthesis and transpirationin the field are described. The ear or leaf of wheat was sealedin a clear acrylic tube of 2 cm diameter and ¹⁴CO₂-enrichedair was circulated in the closed system which had a total volumeof 1.17 x 10^–3 m³; a boundary layer conductance of 5.2cm s^–1 was measured with an ear in the tube. Photosynthesiswas calculated from the decrease in activity of 2.5 cm3 samplesof air taken from the chamber at 15, 60, 120, and 180 s afterassimilation began. The gas samples were injected into scintillationvials containing 0.5 cm³ of strong base and subsequently countedin a scintillation counter. Transpiration was calculated fromthe time required to increase the humidity of the circulatinggas by a known amount as measured by a lithium chloride hygrometer.The cuvette was flushed between measurements and recharged froma cylinder of air enriched with ¹⁴CO₂; ten measurements an hourwere possible with two operators. Comparisons between the lossin activity of the gas samples and the activity gained by astainless steel mesh saturated with strong base, or activityextracted from exposed leaves, indicated the method of gas-samplingwas sensitive, reproducible, and accurate. Transpiration ofears of wheat measured with the lithium chloride hygrometerin the closed cuvette gave comparable values to those obtainedwhen the same ears were measured in an open system with a dewpointhygrometer. Formulae for calculating photosynthesis and transpirationare given.     

Photosynthesis and Transpiration as a Function of Gaseous Diffusive Resistances in Orange Leaves

The CO₂ and H₂O vapour exchange of single attached orange, Citrus sinensis (L.), leaves was measured under laboratory conditions using infrared gas analysis. Gaseous diffusive resistances were derived from measurements at a saturating irradiance and at a leaf temperature optimum for photosynthesis. Variation in leaf resistance (within the range 1.6 to 60 s cm^-1) induced by moisture status, or by cyclic oscillations in stomatal aperture, was associated with changes in both photosynthesis and transpiration. At low leaf resistance (ri less than 10 s cm^-1) the ratio of transpiration to photosynthesis declined with reduced stomatal aperture, indicating a tighter stomatal control over H₂O vapour loss than over CO₂ assimilation. At higher leaf resistance (ri greater than 10 s cm^-1) changes in transpiration and photosynthesis were linearly related, but leaf resistance and mesophyll resistance were also positively correlated, so that strictly stomatal control of photosynthesis became more apparent than real. This evidence, combined with direct measurements of CO₂ diffusive resistances (in a -O₂ gas stream) emphasised the presence of a significant mesophyll resistance; i.e., an additional and rate limiting resistance to CO₂ assimilation over and above that encountered by H₂O vapour escaping from the leaf.     

A Portable System for Measuring the Photosynthesis and Transpiration of Graminaceous Leaves

A portable system has been developed for measuring rates ofphotosynthesis and transpiration of Graminaceous leaves; itcomprises a transparent chamber, into which the leaves are sealed,and a gas supply. The chamber is made from poly 4-methylpent-1-ene(PMP), chosen for its transparency and small water absorption.The leaf is sealed into the chamber by an inflatable rubberseal. The chamber contains a humidity sensor, photocell, thermistor,and a fan which is positioned to give efficient gas-mixing andrapid thermal equilibration. The chamber is surrounded by aPerspex tube to absorb part of the incident infrared radiation.Another fan blows ambient air between the chamber and the Perspextube to counteract solar heating of the chamber wall. The gas supply to the chamber comes from a gas cylinder viaa dilutor. The dilutor contains four small orifices arrangedin parallel. Each orifice is mounted in the inlet of a two-waygas valve: one way goes to the leaf chamber directly, the othervia a CO₂ absorber. The total gas flow-rate is independent ofvalve position, and the CO₂ concentration entering the chambercan be varied from zero to the cylinder concentration by switchingthe valves. The system has been tested both in the laboratory and in thefield, with satisfactory results.     

小麦叶片光合作用与其渗透调节能力的关系

在缓慢干旱条件下,小麦叶片渗透调节能力在一定范围内随胁迫程度的加剧而增加,而在快速干旱下,渗透调节能力丧失。小麦叶片通过渗透调节使光合速率和气孔导度对水分胁迫的敏感性降低,叶片维持较高的电子传递能力、RuBP羧化酶活性和叶绿体光合能量转换系统活性,并推迟了小麦叶片光合速率受气孔因素限制向叶肉细胞光合活性限制转变的时间。     

Senescence of Flag Leaves and Ears of Wheat Hastened by Methyl Jasmonate

Treatment of flag leaves and ears of wheat plants with MJ (jasmonic acid methylester) (10⁻⁵ and 10⁻⁴ m) did not increase ethylene production, but it did accelerate senescence as indicated by the loss of chlorophyll. MJ also caused the closure of stomata, and consequently the rates of transpiration and photosynthesis decreased. Early maturity shortened the grain filling period, so the thousand grain weight was lower. Although ethylene elicited the same physiologic effects, the syndrome of senescence by MJ is independent of the former. We conclude that senescence and death in wheat are far from being elucidated; however, MJ and ethylene seem to participate in the phenomenon. Received July 10, 1997; accepted January 5, 1998     

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.     

Effect of Temperature on Photosynthesis and Photorespiration of Wheat Leaves

Wheat plants were grown in a controlled environment with daytemperatures of 18 ?C and with 500 µ Einsteins m^–28^–1 of photosynthetically active radiation for 16 h. Beforeanthesis and 2 to 3 weeks after, rates of net photosynthesiswere measured for leaves in 2 or 21% O₂ containing 350 vpm CO₂at 13, 18, 23, and 28 ?C and with 500 µEinsteins m^–2s^–1 of photosynthetically active radiation. Also, underthe same conditions of light intensity and temperature, therates of efflux of CO₂ into CO₂-free air were measured and,for mature flag leaves 3 to 4 weeks after anthesis, gross andnet photosynthesis from air containing 320 vpm ¹⁴CO₂ of specificactivity 39?7 nCi µmol^–1. When the O₂ concentration was decreased from 21 to 2% (v/v)the rate of net photosynthesis increased by 32 per cent at thelowest temperature and 54 per cent at the highest temperature.Efflux of CO₂ into CO₂-free air ranged from 38 per cent of netphotosynthesis at 13 ?C to 86 per cent at 28 ?C. Gross photosynthesis,measured by the ¹⁴C assimilated during 40 s, was greater thannet photosynthesis by some 10 per cent at 13 ?C and 17 per centat 28 ?C. These data indicate that photorespiration was relativelygreater at higher temperatures.     

Rubisco Quantitation in Leaves of Different Barley Varieties by Enzyme-Linked Immunosorbent Assay

An enzyme-linked immunosorbent assay (ELISA) for the quantitationof Rubisco was developed. The amount of Rubisco in the primaryleaves often different barley varieties was determined. It variedfrom 15 to 50 per cent of the total soluble protein and from2·5 to 10 mg g^–1 fresh weight. The carboxylaseand oxygenase activities of the extracts were consistent withthe results of the immunoenzymatic assay. The variability ofthe Rubisco content among varieties suggests genetic determination. Key words: Rubisco, ELISA, Hordeum vulgare     

Comparisons between Photosynthesis and Transpiration in Birch

Changes in photosynthesis and transpiration of a potted birch seedling were simultaneously followed in the field using two infrared gas analyzers. Wet and dry temperature measurements alone explained 81% of the variation in the transpiration rate when the plant was not suffering from water deficit. During drought and the period after the water balance had been restored, net photosynthesis decreased more distinctly than transpiration. This result was in accordance with our previous results on the after-effects of water deficit on photosynthesis, and it was also interpreted as evidence for at least partly separate control mechanisms for photosynthesis and transpiration.     

Abnormalities Induced in Barley Ears by Gibberellic Acid

Five varieties of spring barley were sprayed with gibberellicacid (GA) at various growth stages from the one-leaf to thefive-leaf stage. In short days a number of abnormalities ofthe ear were found. These abnormalities are described in themature ear, and in the case of the branched ear, the ontogenyof the abnormality is shown. The relationship of the abnormalitiesto the environment, to the stage at which the plant was treated,and to the variety is analysed. The role of CA in inducing these abnormal features is discussedand their resemblance to abnormalities induceed geneticallyand by enviromental manipulations is pointed out.     

The Influence of Temperature on Photosynthesis and Transpiration in ten Temperate Grass Varieties Grown in four Different Environments

The influence of temperature on photosynthesis and transpirationwas studied in ten varieties of Lolium perenne, L. multiflorum,Dactylis glomerata, and Festuca arundinacea from three climaticorigins grown in three different controlled environments (15?C, 72 W m^-2 visible irradiation, 16-h photoperiod; 25 ?C, 72W m^-2 visible irradiation, 16-h photoperiod; and 25 ?C, 180W m^-2 visible irradiation, 16-h photoperiod) and in the glasshousein July/August. The optimum temperature for photosynthesis was influenced primarilyby growth environment; growth at low temperature (15 ?C) resultedin a low optimum temperature, which differed little from varietyto variety. The maximum CO₂-exchange rate was influenced bygrowth environment and by variety. Within a variety, plantsgrown at higher light intensity or lower temperature had a greaterCO₂-exchange rate. Seven varieties showed a negative correlationbetween the optimum leaf temperature and the maximum CO₂-exchangerate. Activation energies for photosynthesis were influencedby growth environment only. There were marked varietal differences in the values of leafresistances (r_a + r_t) obtained from transpiration data at theoptimum leaf temperature for CO₂ exchange. In Lolium, and Dactylisthe Mediterranean varieties had higher leaf resistances thanthe Northern varieties with the maritime varieties intermediate.In general the Dactylis varieties had higher resistances thanthe corresponding Lolium and Festuca varieties. Only at highgrowth temperatures was (r_a+r_l) insensitive to temperature;otherwise an activation energy of about 10 kcal/mole was observed.A negative correlation was found between mean varietal diffusionresistances (r_a+r_l), and corresponding maximum CO₂-exchangerates.     

Nitrate Nutrition and Temperature Effects on Wheat: Photosynthesis and Photorespiration of Leaves

Lawlor, D. W., Boyle, F. A., Young, A. T., Keys, A. J. and Kendall,A. C. 1987. Nitrate nutrition and temperature effects on wheat:photosynthesis and photorespiration of leaves.—J. exp.Bot. 38: 393–408. Photosynthetic and photorespiratory carbon dioxide exchangeby the third leaf of spring wheat (Triticum aestivum cv. Kolibri),was analysed for plants grown at 13/10 °C (day/night temperature)and 23/18 °C with two rates of nitrate fertilization (abasal rate, — N, and a 4-fold larger rate, +N) and, insome experiments, with two photon fluxes. Net photosynthesiswas greatest at the time of maximum lamina expansion, and forleaves grown with additional nitrate. Maximum rate of photosynthesis,carboxylation efficiency and photochemical efficiency at maturitywere slightly decreased by nitrate deficiency but photosystemactivity was similar under all conditions. As leaves aged, photosynthesisand photochemical efficiency decreased; carboxylation efficiencydecreased more than photochemical efficiency particularly withbasal nitrate. Low oxygen increased the carboxylation and photochemicalefficiencies, and increased the maximum rate of assimilationby a constant proportion in all treatments. Photorespiration,measured by CO₂ efflux to CO₂-free air, by ¹⁴CO₂ uptake, andfrom compensation concentration, was proportional to assimilationin all treatments. It was greater, and formed a larger proportionof net photosynthesis, when measured in warm than in cold conditionsbut was independent of growth conditions. Assimilation was relatedto RuBPc-o activity in the tissue. Relationships between photosynthesis,photorespiration and enzyme complement are discussed. Key words: Wheat, leaves, nitrate nutrition, temperature effect, photosynthesis, photorespiration