Apparatus for the Simultaneous Measurement of Water Vapour and Carbon Dioxide Exchanges of Single Leaves

Equipment is described which delivers air with concentrationsof CO₂ and water vapour closely controlled in the ranges 0 to2500 ppm and 5 to 15 mb respectively, at flow rates of up to10 1 min^-1, to each four leaf chambers. The leaf temperatureis controlled to ±0.5 °C and, with a light intensityof 0.3 cal cm^-2 min^-1 visible radiation (0.4 to 0.7 µm)leaf temperature can be maintained at 17.5 °C.The apparatusused to measure the concentration differences between the watervapour and CO₂ entering and leaving the leaf chamber (used tocalculate transpiration, photosynthetic, and respiration rates)is described in detail.Results of tests, which show the necessityfor mounting a fan within the leaf chamber, are reported.Typicallight- and CO₂-response curves are given for kale leaves (Brassicaoleracca var. acephala) and an attempt is made to quantify theerrors in the measurement of photosynthesis and transpiration.     

Photosynthesis and Diffusion Conductance of the Valencia Orange Fruit under Field Conditions

CO₂ uptake and diffusion conductance of Valencia orange fruits(Citrus sinensis L. Osbeck) were measured in the field duringthe growing season of 1977/78 to ascertain if, as in the leaf,stomata control photosynthesis and transpiration under changingenvironmental conditions. Measurements were made on 15 yearold trees grown in a sandy loam soil and receiving either adry or a wet treatment. Fruit diffusive conductance was measuredwith a modified water vapour diffusion conductance meter andgross photosynthesis was measured with a ¹⁴CO₂ uptake meter.Photosynthetically active radiation (PAR) was measured witha quantum sensor. Fruits exposed to light assimilated CO₂ ata rate which was 25–50% of that assimilated by leaves.The uptake was dependent on fruit size, PAR, chlorophyll content,and on diffusive conductance of the fruit epidermis. Epidermalconductance showed a diurnal trend which was similar in shapeto that of the leaf except in the late afternoon. Cuticularconductance of the fruit was calculated and ranged between 0.22and 0.30 mm s^–1. It was speculated that the CO₂ uptakeby the fruit could support the growth of flavedo cell layerswhen exposed to light. Dry soil caused an increase in the ¹⁴CO₂uptake by fruit possibly caused by the increased potential areaof the stomatal opening per unit of fruit surface area.     

Humidity Pretreatment Affects the Responses of Stomata and CO2 Assimilation to Vapor Pressure Difference in C3 and C4 Plants

Experiments were carried out to investigate the long-term influenceof humidity on the short-term responses of stomata and CO₂ assimilationto vapor pressure difference in Oryza sativa (rice, C₃ species)and Panicum maximum (green panic, C₄ species). Plants were grownfor four weeks in growth chambers set at 35% and 85% relativehumidity at 25C air temperature, 38+2 Pa CO₂ partial pressureand 1,700µmol m^-2s^-1 photon flux density. Soil was saturatedwith water in both humidity treatments. Low humidity pretreatmentscaused low leaf conductance and low rates of transpiration andCO₂ assimilation in O. sativa, but small changes in stomatalresponses to humidity and in CO₂ assimilation were found inP. maximum. From the short-term gas exchange experiments, itwas noted that the responsiveness of leaf conductance to vaporpressure difference were affected by humidity pretreatmentsin O. sativa, whereas unaffected in P. maximum. In O. sativameasurements of CO₂ assimilation as a function of internal CO₂partial pressure (A-Ci curve) indicated that low humidity pretreatmentsreduced the CO₂ assimilation at high internal CO₂ partial pressure,but the initial slope of the A-Ci curve was unaffected. Furthermore,plant characteristics such as total dry weight and leaf areaof plants subjected to low umidity were lower than plants subjectedto high humidity. The reductions in O. sativa, however, werelarger than in P. maximum. Stomatal frequency from low humiditygrown plant was higher than that from high humidity grown plantsin both species although there is no significant difference.The data indicated that if the short term inhibition of netCO₂ assimilation at a high vapor pressure difference was imposedduring vegetative growth, the photosynthetic biochemistry andthe resultant plant growth were largely depressed in O. sativa,a C₃ species. (Received May 26, 1992; Accepted November 2, 1992)     

A Rapid Field Method for Measuring Photosynthesis with Labelled Carbon Dioxide

A portable field apparatus for the measurement of photosynthesisis described. Short pulses (20 s) of air containing ¹⁴CO₂ areapplied to circular leaf areas of 1 cm², enclosed in a transparentminiature chamber; the radioactivity of the leaf areas is measuredby liquid scintillation, using a new in-vial wet combustiontechnique. Typical daily courses of photosynthesis in maizeand cotton are presented.     

The Diffusive Conductivity of the Stomata of Wheat Leaves

A leaf chamber (described in detail) was used alternately witha resistance porometer to measure resistance to viscous flowof air through the leaf, and with a diffusion porometer to measurethe differential diffusive flow of hydrogen and air (V_H–V_A)through the leaf and the component of hydrogen flow (V'_H) movingstraight across the leaf. The resistance of the mesophyll isneeded for interpretation: estimates by three different methodsfor viscous flow did not agree very well, but two differentmethods for diffusive flow gave good agreement. For wheat leaves,only very large errors are important. Formal analysis is in three appendixes: I. Interpretation ofviscous and diffusive flow in small pores involves some problemsin molecular physics, complicated by the particular geometryof the wheat stoma. With some uncertainty, formal expressionsare derived for the viscous resistance of a single stoma, rv,and for the resistances to diffusion of hydrogen and air, andof water vapour and carbon dioxide, all expressed as r_s persquare centimetre of leaf surface. The analysis for hydrogen/airis the most uncertain; that for water vapour and carbon dioxideis more reliable. II. An indication is given of the flow characteristicsof the leaf-chamber system, from which rv can be derived, andof the basis for estimating mesophyll resistance. III. The methodof converting estimates of r_s into estimates of V_H–V_Aand V'_H is given. The results presented are expressed as nearly as possible interms of the quantities which were measured. For five leavesthe dependence of V_H–V_A on V'_H agrees well with theoreticalpredictions; the dependence of V_H–V_A (and V'_H) on rv,on average, agrees well with prediction, but involves the assumptionthat the stomata get shorter as they close. The agreement isgood enough to suggest that the formal expressions for r_s interms of stomatal dimensions and molecular gas constants arereliable enough to be carried forward into future transpirationand assimilation studies. The minimum value of ra for watervapour (c. 3 sec cm⁺¹) is close to values found elsewhere bydifferent techniques. At very small stomatal openings there was a large deviationfrom predicted behaviour, such as would occur if the imposedexcess air pressure further closed the stomata during viscousflow experiments.     

The Effects of Low Concentrations of Sulphur Dioxide on Stomatal Conductance and Epidermal Cell Survival in Field Bean (Vicia faba L.)

Exposure of 3 week old field bean plants to concentrations ofSO₂ from 50–500 µg m^-3 induced comparable 20–25%increases in mean leaf diffusive conductance regardless of whetherthe diffusive conductances were obtained by porometric measurementor calculation from gas exchange data. The stomatal conductancesof the adaxial and abaxial leaf surfaces were both increasedby exposure to SO₂. Microscopic examination of epidermal strips from control andpolluted plants revealed that the stomatal opening observedin treated plants was associated with a sharp reduction in theproportion of living epidermal cells adjacent to the stomata.The proportion of surviving adjacent epidermal cells was invariablysmaller on the lower epidermis and appeared to decrease as theSO₂ concentration was raised from 50 to 500 µg m^–3.Although the guard cells appeared to be undamaged at concentrationsbelow 200 µg m^–3, structural disorganization ordeath of one or both guard cells was observed frequently ator above 500 µg m^–3. The results are discussed in relation to the controversy concerningthe effects of SO₂ on stomatal aperture.     

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.     

An Examination of the Value of 14C-urea as a Source of 14CO2 for Studies of Assimilate Distribution

Twenty-four hours after leaf 3 of a plant of Lolium multiflorumLam, was supplied with a droplet of ¹⁴C-urea and the plant enclosedin a polyethylene bag with an untreated plant, there were significantamounts of radiocarbon recovered from the untreated plant. Theleaf treated with ¹⁴C-urea was the major source of ¹⁴C leakagebut significant losses were also recorded from other parts ofthe plant. Reducing the humidity within the bag decreased theamount of ¹⁴CO₂ which escaped. Losses of radiocarbon from CO₂-treated plants were very low compared with those from urea-treatedplants but the pattern of assimilate distribution within thetwo types of plants was very similar. The possible causes ofthese effects are considered and the usefulness of ¹⁴C-ureaas a source of ¹⁴CO₂ discussed.     

Factors Affecting Uptake of Radioactive Phosphorus by Leaves and its Translocation to other Parts of the Plant

The rate of uptake of ³²P from labelled NaH₂PO₄ solutions sprayedon to one leaf of swedes (Brassica napus) or French beans (Phaseolusvulgaris) was rapid during the first few hours and fell to zeroafter 4 days. ²²P was detected in the root after 3 hours andcontinued to move out of the treated leaf for at least 6 daysafter application. A larger fraction of the applied ³²P wasabsorbed from repeated than from a single spraying. Swedes absorbed more ³²P from a single application to the lowersurface than to the upper surface of the leaf. Doubling theconcentration of the spray caused a small increase in the percentageof applied ²²P that was absorbed. Absorption by French-beanleaves decreased slightly when the area sprayed with a constantamount of ³²P was doubled, and decreased with increasing ageof leaf. Increasing the phosphorus supply to the roots of swedesaffected neither the initial rate nor the total amount of ³²Puptake by the leaves but decreased the quantity of ³²P thatwas translocated out of the treated leaf. Increasing the relative humidity of the air around the plantsalso increased³²P uptake. Shading usually decreased uptake andalways decreased translocation. Rewetting the leaf to whichthe ³²P had been applied, with water or sucrose solution, hadvariable effects. The significance of the results is discussed.     

Measurement of the rate of CO2 solubilization in leaf tissue 4

A novel method for the measurement of the rate of CO₂ exchangecaused by solubilization and bicarbonate formation in leavesis presented. The method is based on the measurement of changesin O² concentration, caused by the uptake or evolution of CO₂from the gas flow as a result of solubilization in or desolubilizationfrom leaf tissue, by means of a zirconium-oxide O₂ analyser.The advantages of the new method are a fast response and a lackof interference by the CO₂ contained in the leaf chamber andleaf intercellular spaces. The method can be used for the investigationof the kinetics of CO₂ transport from leaf intercellular spacesto chloroplasts and the role of carbonic anhydrase in this transportand the investigation of mechanistic and ecological aspectsof buffering, proton translocation and other processes controllingpH in chloroplasts of intact leaves. Key words: Leaf, carbon dioxide, pH.     

The Growth Rate of Successive Leaves of Wheat Plants in Relation to Sugar and Protein Concentrations in the Extension Zone

The growing part of a wheat leaf (the extension zone) is loocatedat the leaf base and following from this it was proposed thatthe absolute leaf extension rate (L_er) can be partitioned intotwo components: the length of the extension zone (L_ez) and therelative extension rate of that extension zone (R_ez). R_ez isan appropriate measure of the efficiency of leaf growth forcomparing different leaves. This model of a wheat leaf was thenused to investigate whether differences in growth rates betweensuccessive leaves on wheat plants were due to differences inhexose sugar or protein concentrations within the extensionzone. Measurements were done in an irrigated field crop suppliedwith 0, 3, 10, or 30 kg N ha^–1 per week. The mean values of L_er at 15 °C increased with leaf numberand with nitrogen supply as did values for L_ez. In contrastR_ez at 15 °C declined from 0.9 d^–1 for the first leavesto 0.3 d^–1 for the flag leaf. Nitrogen supply had littleeffect on R_ez. A separate measure of the efficiency of leafgrowth, the responsiveness of L_er to temperature (measured asthe slope of the temperature response curve), also decreasedwith leaf number by the same order as R_ez and was similarlyunaffected by nitrogen supply. The protein concentrations in the extension zones of the firstleaves were around 40 mg g^–1 fr. wt. and this declinedto approximately 20 mg g^–1 fr. wt for leaves emergingafter tillers emerged and remained low thereafter. Protein concentrationswere not correlated with the external supply of nitrogen. Hexosesugar concentrations followed a reverse pattern of increasingin the later order leaves and these also increased as nitrogensupply decreased. Both R_ez and the responsiveness of L_er to temperature were positivelycorrelated with the protein concentration, the relationshipin each case being described by a rectangular hyperbola equation(P < 0.01), and negatively correlated with hexose concentrations.It was concluded that internal competition between growing pointsfor reduced nitrogen caused the observed effects. However, differencesin protein concentrations may not simply reflect differencesin enzyme concentrations; rather these differences may indicatechanges in some other character such as cell numbers.     

Effects of CO2 and Photosynthetic Photon Flux on Yield, Gas Exchange and Growth Rate of Lactuca Sativa L. 'Waldmann's Green'

Knight, S. L. and Mitchell, C. A. 1988. Effects of CO₂ and photosyntheticphoton flux on yield, gas exchange and growth rate of Lactucasativa L. ‘Waldmann’s Green'.—J. exp. Bot.39: 317–328. Enrichment of CO₂ to 46 mmol m^–3 (1 000 mm³ dm^–3)at a moderate photosynthetic photon flux (PPF) of 450 µmolm^–2 s^–1 stimulated fresh and dry weight gain oflettuce leaves 39% to 75% relative to plants at 16 mmol m^–3CO₂ (350 mm³ dm^–3). Relative growth rate (RGR) was stimulatedonly during the first several days of exponential growth. ElevatingCO₂ above 46 mmol m^–3 at moderate PPF had no further benefit.However, high PPF of 880–900 µmol m^–2 s^–1gave further, substantial increases in growth, RGR, net assimilationrate (NAR) and photosynthetic rate (Pn), but a decrease in leafarea ratio (LAR), at 46 or 69 mmol m^–3 (1000 or 1500 mm³dm^–3) CO², the differences being greater at the higherCO₂ level. Enrichment of CO₂ to a supraoptimal level of 92 mmolm^–3 (2000 mm³ dm^–3) at high PPF increased leaf areaand LAR, decreased specific leaf weight, NAR and Pn and hadno effect on leaf, stem and root dry weight or RGR relativeto plants grown at 69 mmol m^–3 CO₂ after 8 d of treatment.The results of the study indicate that leaf lettuce growth ismost responsive to a combination of high PPF and CO₂ enrichmentto 69 mmol m^–3 for several days at the onset of exponentialgrowth, after which optimizing resources might be conserved. Key words: Photosynthesis, relative growth rate, CO₂ enrichment     

Effects of elevated CO2 concentration and climate-warming on photosynthesis during winter in Lolium perenne

Long-term effects of atmospheric carbon dioxide concentration(ambient or 700 µmol mol^–1) and air temperature(simulation of field conditions or + 4 C) on leaf photosyntheticrate were examined in Lolium perenne L. cv. Vigor, exposed tonatural illumination during winter. Photosynthetic capacitywas compared over a range of air temperatures and photon fluxdensities of photosynthetically active radiation which wererepresentative of winter climate (5–15 C and 0–500µmol m^–2s^–1), with CO₂ level during measurementsimilar to that during the experimental period. Long-term exposureto increased air temperature reduced leaf CO₂ fixation capacityby 23% (averaged over all measurement conditions), resultingfrom a decline in lightsaturated uptake rate, but not in incident-lightquantum efficiency. CO₂- stimulation was largely absent in plantsgrown in ambient temperature, but pronounced in plants grownunder +4 C, where it compensated for two-thirds of the 23%drop. This enhancing effect of elevated CO₂ level on leaf CO₂uptake rate observed in the warmer treatment, was strongly dependenton measurement temperature, increasing from 5% at 5 C, to upto 32% at 15 C. Measurements of chlorophyll fluorescence anddry matter corresponded with the observed changes in assimilationcapacity, which could not be attributed to a deteriorated nitrogenstatus of the leaves as there was a similar N content on anarea basis. Several hypotheses are considered to explain theobserved CO₂-temperature interactions. Key words: Acclimation, chlorophyll fluorescence, elevated CO₂ level, global warming, low temperature     

The Responses of Net Photosysthesis to Light and Temperature in Spartina townsendii(sensu lato), a C4 Species from a Cool Temperate Climate

Carbon dioxide and water vapour exchanges for single attachedleaves of the temperate C₄ grass Spartina townsendii were measuredunder controlled environment conditions in an open gas-exchangesystem. The responses of net photosynthesis, stomatal resistance,and residual resistance to leaf temperature and photon fluxdensity are described. The light and temperature responses ofnet photosynthesis in S. townsendii are compared to informationon these responses in both temperate C₃ grasses and sub-tropicalC₄ grasses. Adaptation of photosynthesis in this C₄ speciesto a cool temperate climate is indicated both by the light andtemperature responses of net photo-synthesis. Unlike the C₄grasses examined previously, significant rates of net photosynthesiscan be detected at leaf temperatures below 10?C. Rates of netphotosynthesis equal or exceed those reported for temperateC₃ grasses at all of the temperature (5–40?C) and photonflax density (13–2500µmol m^–2 s^–1) conditionsexamined. Maximum rates of net photosynthesis in S. townsendiiare almost double those reported for C₃ herbage grasses. Unliketemperate C₃ grasses, the major limitation to net photosynthesisat low leaf temperatures (10?C and below) is the stomatal resistance,showing that the low residual resistance characteristic of C₄species is maintained in S. townsendii even at low leaf temperatures.     

The Effect of Vapour Pressure Deficit on Carbon Isotope Discrimination in the Desert Shrub Larrea tridentata(Creosote Bush)

Larrea tridentata (creosote bush) seedlings were subjected tothree regimens of atmospheric humidity in a growth chamber experiment.Relative humidity was varied to achieve daytime vapour pressuredeficits (VPD) during growth of 29, 48 and 77 kPa. Photosyntheticgas exchange, carbon isotope composition and biomass productionwere measured after 8–10 weeks of treatment. Whereas stomatalconductance (g) declined linearly with increasing ambient VPD,CO₂ assimilation rate (A) was not measurably affected by changesin ambient VPD. This resulted in a decrease in intrinsic wateruse efficiency (ratio of CO₂ assimilation to stomatal conductance;A/_g) with increasing VPD. Leaf carbon isotope discrimination(A) was negatively correlated (r² = 088) with A/g ratios. Carbonisotope discrimination also correlated positively with ratiosof internal (C₁) to ambient (c_a) CO₂ levels determined by gasexchange measurements (c₁/c). The ratio of c₁ to c_a was lowerat higher VPD levels. Leaf biomass decreased with increasingambient VPD and correlated positively with. Root to leaf biomassratio increased at higher VPD levels and correlated negativelywith. Key words: Larrea tridentata, vapour pressure deficit, carbon isotope discrimination, intrinsic water-use efficiency     

A Humidity-induced Convective Throughflow Ventilation System Benefits Annona squamosa L. Explants and Coconut Calloid

A simple apparatus is described for generating pressurized throughflowventilation in plant tissue culture vessels. No pumps or gas-cylindersare required and the flow is driven by humidity-induced diffusionacross microporous membranes. In the experiments described,pressurized flows of sterile humidified air were supplied atrates of up to 1 ml min^-1and these had beneficial effects onleaf survival and production inAnnona cuttings and on calloidform in coconut. Ethylene (ethene) was removed more quicklyfrom the pressure-flow ventilated culture vessels (t ₅₀, 0.4–0.7h) than from those aerated by diffusion through conventionalpolypropylene membranes (t ₅₀, 1.6–2.4 h). InAnnona cuttings leaf production was greatly increased andethylene-induced leaf fall considerably delayed when culturedwith the forced as opposed to diffusion-based ventilation ofthe vessels. With throughflow ventilation, coconut calloid was more convolutedthan under wholly diffusive aeration and had a smooth distinctepidermal surface and clearly defined sub-epidermal meristematicnodules. It resembled freshly initiated calloid from which regenerationof plantlets via somatic embryogenesis can be obtained. Underwholly diffusive aeration, calloid developed a rough, relativelyundifferentiated surface, more haustorial (i.e. cotyledonary)in appearance, and characteristic of cultures where regenerationpotential has been lost. It is suggested that other benefits of the pressurized throughflowventilation may be the removal of volatiles such as ethanoland acetaldehyde, the removal of excess carbon dioxide at nightand its improved supply during the day, and sustained oxygenconcentrations at levels close to atmospheric both night andday. Abscission; callus; ethylene; explants; tissue-culture; growth; ventilation     

准噶尔荒漠早春短命植物的光合特性及生物量分配特点

准噶尔荒漠分布的早春短命植物不仅具有十分独特的生物学特点,而且在荒漠植物群落演替、物种多样性维持及土壤改良与防治水土流失等方面具有重要的生态学价值。该文运用Li-6400开放式气体交换光合作用测定系统,对分布于准噶尔荒漠的16种早春短命植物生长盛期的净光合速率(P_n)、蒸腾速率(T_r)、水分利用效率(WUE)等特征进行了测定,并对其中7种植物与生长相关的生物量分配特征进行了分析。结果表明:1)16种植物的最大P_n、 最大T_r及WUE分别为8.07~35.96 μmol CO₂·m^-2·s^-1、3.16~29.64 mmol H₂O·m^-2·s^-1、0.54~4.26 μmol CO₂·mmol^-1H₂O;种间最大P_n与最大气孔导度(Stomatal conductance, G_s)之间存在正相关关系,其相关系数为0.77(p<0.05),线性回归斜率为26.36 μmol·mmol^-1;从光合速率对胞间CO₂浓度及光量子通量密度的响应曲线来看,这类植物的表观CO₂补偿点均在4~5 Pa之间(28~30 ℃),表观羧化效率为0.64~1.86 μmol CO₂·m^-2·s^-1·Pa^-1,表观量子效率为0.05~0.06。2)从生物量分配来看,所测植物的个体生物量为0.05~0.39 g;单株总叶面积为 3.24~51.40 cm²;单位叶面积干重为0.40~0.77 g·m^-2,根在总生物量中所占比例为5.72%~19.43%,单株叶面积比在2.92~9.00 m²·kg^-1之间。种间根所占生物量的比与对应的WUE之间的比较分析结果表明,二者之间存在显著的正相关关系,其相关系数r为0.93(p<0.01)。这些结果表明,所观测的早春短命植物具有典型的C₃植物特征,相比其它类型的荒漠植物具有较高的单位叶面积P_n、高T_r及低WUE,并且在生长发育过程中表现出很低的根/地上生物量比、较高的叶面积比和单位叶面积干重,说明它们具有相对高的生长速率,这与其生长发育节律相一致,反映了它们与准噶尔荒漠环境相适应的特点。     

Photosynthesis by developing embryos of oilseed rape (Brassica napus L.)

The aim of this study was to assess the photosynthetic potentialof developing seeds of oilseed rape (Brassica napus L.) andto compare photosynthetic properties of embryo plastids withthose of leaf chloroplasts from the same species. Measurementsof CO₂-dependent O₂ evolution show that developing seeds ofB. napus are photosynthetically active in vitro. Essentially,all of the photosynthetic activity of the developing seed isaccounted for by the embryo. The rate of photosynthesis by developingembryos increased until the onset of desiccation, after whichit declined, so that by maturity embryos were no longer photosyntheticallyactive. Photosynthetic activity was positively correlated withchlorophyll content throughout development. Comparison of thephotosynthetic characteristics of leaf and embryo chloroplastsrevealed that rates of uncoupled electron transport were 2.5-foldgreater in those from the embryo. Light-saturated rates of CO₂-dependentO₂ evolution, per unit chlorophyll, and CO₂ saturation pointswere similar for chloroplasts from both tissues. However, light-saturationpoints and chlorophyll a/b ratios were lower for embryo thanfor leaf choroplasts. Embryos and embryo chloroplasts also containedconsiderably less ribulose 1,5-bisphosphate carboxylase/oxygenaseprotein per unit total protein, than leaves. Although excisedembryos were capable of high rates of CO₂-dependent O₂ evolution(90–100 mol mg^–1 chlorophyll h^–1) under asaturating photosynthetic photon flux density (PPFD), low transmittanceof light through the silique wall (30%), together with the highPPFD required to achieve light compensation points in developingseeds (500 mol m^–2 s^–1), suggests that photosynthesisin vivo is unlikely to make a net contribution to carbon economyunder normal environmental conditions. Key words: Embryo, development, photosynthesis, chloroplast, Brassica napus L.     

Effect of Elevated CO2 on Stomatal Density and Distribution in a C4 Grass and a C3 Forb under Field Conditions

Two common tallgrass prairie species, Andropogon gerardii, thedominant C₄ grass in this North American grassland, and Salviapitcheri, a C₃ forb, were exposed to ambient and elevated (twiceambient) CO₂ within open-top chambers throughout the 1993 growingseason. After full canopy development, stomatal density on abaxialand adaxial surfaces, guard cell length and specific leaf mass(SLM; mg cm^-2) were determined for plants in the chambers aswell as in adjacent unchambered plots. Record high rainfallamounts during the 1993 growing season minimized water stressin these plants (leaf xylem pressure potential was usually >-1·5 MPa in A. gerardii) and also minimized differencesin water status among treatments. In A. gerardii, stomatal densitywas significantly higher (190 ± 7 mm^-2; mean ±s.e.) in plants grown outside of the chambers compared to plantsthat developed inside the ambient CO₂ chambers (161 ±5 mm^-2). Thus, there was a significant 'chamber effect' on stomataldensity. At elevated levels of CO₂, stomatal density was evenlower (P < 0·05; 121 ± 5 mm^-2). Most stomatawere on abaxial leaf surfaces in this grass, but the ratio ofadaxial to abaxial stomatal density was greater at elevatedlevels of CO₂. In S. pitcheri, stomatal density was also significantlylower when plants were grown in the open-top chambers (235 ±10 mm^-2 outside vs. 140 ± 6 mm^-2 in the ambient CO₂ chamber).However, stomatal density was greater at elevated CO₂ (218 ±12 mm^-2) compared to plants from the ambient CO₂ chamber. Theratio of stomata on adaxial vs. abaxial surfaces did not varysignificantly in this herb. Guard cell lengths were not significantlyaffected by growth in the chambers or by elevated CO₂ for eitherspecies. Growth within the chambers resulted in lower SLM inS. pitcheri, but CO₂ concentration had no effect. In A. gerardii,SLM was lower at elevated CO₂. These results indicate that stomataland leaf responses to elevated CO₂ are species specific, andreinforce the need to assess chamber effects along with treatmenteffects (CO₂) when using open-top chambers.Copyright 1994, 1999Academic Press Andropogon gerardii, elevated CO₂, Salvia pitcheri, stomatal density, tallgrass prairie     

Leaf uptake of nitrogen dioxide (NO2) in a tropical wet forest: implications for tropospheric chemistry

Tropical forest soils are known to emit large amounts of reactive nitrogen oxide compounds, often referred to collectively as NO_y (NO_y = NO + NO₂ + HNO₃ + organic nitrates). Plants are known to assimilate and emit NO_y and it is therefore likely that plant canopies affect the atmospheric concentration of reactive nitrogen compounds by assimilating or emitting some fraction of the soil-emitted NO_y. It is crucial to understand the magnitude of the canopy effects and the primary environmental and physiological controls over NO_y exchange in order to accurately quantify regional NO_y inventories and parameterize models of tropospheric photochemistry. In this study we focused on nitrogen dioxide (NO₂), which is the component of NO_y that most directly catalyzes the chemistry of O₃ dynamics, one of the most abundant oxidative species in the troposphere, and which has been reported as the NO_y species that is most readily exchanged between plants and the atmosphere. Leaf chamber measurements of NO₂ flux were measured in 25 tree species growing in a wet tropical forest in the Republic of Panama. NO₂ was emitted to the atmosphere at ambient NO₂ concentrations below 0.53-1.60 ppbv (the NO₂ compensation point) depending on species, with the highest rate of emission being 50 pmol m^-2 s^-1 at <0.1 ppbv. NO₂ was assimilated by leaves at ambient NO₂ concentrations above the compensation point, with the maximum observed uptake rate being 1,550 pmol m^-2 s^-1 at 5 ppbv. No seasonal variation in leaf NO₂ flux was observed in this study and leaf emission and uptake appeared to be primarily controlled by leaf nitrogen and stomatal conductance, respectively. When scaled to the entire canopy, soil NO emission rates to the atmosphere were estimated to be maximally altered ᆧ% by the overlying canopy.