Seismic Stress Responses of Soybean to Different Photosynthetic Photon Flux

Physical agitation applied as periodic seismic stress (shaking)reduced stem clongation, leaf expansion, and biomass accumulationby vegetative soybeans. Level of photon flux (PPF) influencedthe type and extent of plant response to mechanical stress.Plant parts responded differently as PPF varied between 135and 592 µmol m^–2 S^–1. Stem length was significantlyreduced by seismic stress at 135 µmol m^–2 s^–1but this effect was insignificant at higher PPFs. Reduced stemlength resulted from an inhibition of internode elongation.Stem diameter was unaffected by stress at the PPFs tested. Incontrast to effects on stem elongation, leaf area was insensitiveto stress treatments at 135 µmol m^–2 S^–1 butwas progressively inhibited by stress as PPF increased. Statisticallysignificant reductions in shoot f. wt and d. wt by seismic stressoccurred only at 295 µmol m^–2 S^–1. Root biomassaccumulation was not affected by seismic stress at any PPF usedin this study. Glycine max (L.) Merr. cv. Century 84, mechanical stress, photosynthetic photon flux, seismic stress, soybean     

Use of the Plastochron Index to Evaluate Effects of Light, Temperature and Nitrogen on Growth of Soya Bean (Glycine max L. Merr.)

The plastochron index (PI) has been compared with leaf growthand biomass accumulation in young soya bean plants of severalcultivars that were grown in controlled environments with differentirradiance levels and durations, temperatures, and nitrogen(N) regimes. Increasing the photoperiod from 10 to 16 h day^–1 increasedthe plastochron rate (PR) and the proportion of axillary growth.Doubling the photosynthetic photon flux density (PPFD) to 1000µmol m^–2S^–1, increased PR and the proportionof roots to total plant weight, but decreased the proportionof stems plus petioles to total. In a series of experiments,the plants were grown in an 8 h photoperiod at constant temperaturesof 17, 20, 26 or 32 °C. As temperature increased, PR increased,but the duration of leaf expansion decreased. Leaves were largestat 20 and progressively smaller at 26, 32 and 17 °C. Biomasswas greatest for a given PI at 20 °C and decreased in theorder of 26, 32, and 17 °C. The proportion of axillary growthalso was greatest at 20 °C. When plants were grown in a15 h photoperiod at temperatures from 17.1 to 26.6 °C, leafsize continued to increase up to the highest temperature. At17 °C, the PR in the 15 h photoperiod (PPFD 390 µmol;m^–2S^–1) was about threefold greater than in 8 h(500 µmol m^–2 S^–1); biomass accumulation perday was about fivefold greater. Increasing N from 3 to 36 mMincreased PR about 10 per cent, altered biomass partitioningamong plant parts, and increased the biomass of the plants.The NO₂ form of N markedly stimulated axillary growth as comparedwith the NH₄⁺ form. Environment or cultivar had little influenceon the duration of leaf expansion in terms of PI. Cultivarsdid not differ consistently in biomass production and allocationin the different environments. Glycine max (L.) Merrill, soybean, soya bean, plastochron index, leaf development, growth analysis, partitioning, light, nitrogen, temperature     

Response of young barley plants to CO2 enrichment

Barley (Hordeum vulgare L. cv. Digger) was grown for 22 d inenclosed chambers with a CO₂ enrichment of 35, 155, 400 or 675µmol CO₂ mol1. CO₂ enrichment increased photosyntheticcapacity in the plants grown at either of the two highest levelsof pCO₂. A CO₂ enrichment of 675µmol CO₂ caused a significantincrement of shoot dry weight, whereas no changes were observedin fresh weight, chlorophyll or protein levels. At a light intensityof 860µmol m^–2s^–1 CO₂ enrichment caused photosyntheticcapacity to increase by 250%, whereas no effect was observedat 80 µmol m^–2 s^–1. Over time, photosynthesisdecreased by 70% independent of CO₂. A time-dependent increasein the level of extractable fructose was observed whereas totalextractable carbohydrate only changed slightly. Key words: Carbohydrates, CO₂ enrichment, Hordeum vulgare, photosynthesis, respiration     

The effects of light and temperature on photosynthate partitioning in Antarctic freshwater phytoplankton

The effects of temperature and radiation flux on the partitioningof photosynthetically fixed carbon into four intracellulai metabolicpools was investigated for natural phytoplankton assemblagesfrom an Antarctic freshwater lake. At ambient temperature, proteinsynthesis was saturated at low photon flux densities (30–40µmol m^–2 s^–1) and above this flux fixed carbonwas increasingly stored as lipid and polysaccharide. Increasingtemperature raised both the saturated rate of protein synthesisand the photon flux at which saturation occurred. There wasa corresponding decline in the accumulation of reserve products,particularly at low radiation fluxes. The consequences of thispattern of uptake for the phytoplankton is discussed.     

The Growth and Development of the Wheat Apex: The Effects of Photoperiod on Spikelet Production and Sucrose Concentration in the Apex

Spring wheat (Triticum aestivum cv. Warimba) plants were grownin a controlled environment (20°C) in two photoperiods (8or 16 h). In the first instance, plants were maintained in eachof the photoperiods from germination onwards at the same irradiance(375 µE m^–2 s^–1). In the second case, allplants were grown in a long photoperiod until 4 days after double-ridgeinitiation when half the plants were transferred to a shortphotoperiod with double the irradiance (16 h photoperiod at225 or 8 h at 475 µE ^–2 s^–1). The rates of growth and development of the apices were promotedby the longer photoperiod in both experiments. Shoot dry weightgain was proportional to the total light energy received perday whereas the dry weight of the shoot apex increased withincreasing photoperiod even when the total daily irradiancewas constant. The principal soluble carbohydrate present in the shoot apexwas sucrose, although low concentrations of glucose and fructosewere found in the apices of long photoperiod plants late indevelopment. Sucrose concentration was invariably greater inthe slow-growing apices of short photoperiod plants, but roseto approach this level in the long photoperiod plants when theterminal spikelet had been initiated. Triticum aestivum, wheat, apex, spikelet initiation, photoperiod, flower initiation     

The Relationship between Sucrose and Starch during 'Dark' Export from Leaves of Uniculm Barley

Uniculm barley plants were grown in 8 h photoperiods at a quantumflux density of 655 µE m^–2 s^–1. Groups ofplants were transferred to four different light environmentsfor one 8 h photoperiod (106, 270, 665, and 975 µE m^–2s^–1) and harvested at intervals throughout the succeedingdark period for subsequent carbohydrate analysis of the youngestmature leaf. Sucrose was the predominant carbohydrate in the leaves (attaininga level of c. 100 mg dm^–2 after 8 h at 975 µE m^–2s^–1) but starch was also of significance (20 mg dm^–2after 8 h at 975 µE m^–2 s^–1). During the dark period, following a photoperiod at the threehighest light levels (270, 665, and 975 µE m^–2 s^–1),sucrose was exported first while the starch level remained fairlyconstant. When the-sucrose level fell to 15–20 mg dm^–2starch degradation began. This critical sucrose level was reachedearlier in those plants subjected to lower quantum flux densitiesduring the preceding photoperiod. The delay in the remobilizationof starch suggests an important regulatory mechanism which maybe dependent upon the sucrose level. At 106 µE m^–2s^–1 the sucrose level rose to only 10 mg dm^–2. Herethere was no discernible delay in the depletion of sucrose orstarch.     

Effect of elevated CO2 on the utilization of light energy in Nothofagus fusca and Pinus radiata

Red beech (Nothofagus fusca (Hook. F.) Oerst.; Fagaceae) andradiata pine (Pinus radiata D. Don; Pinaceae) were grown for16 months in large open-top chambers at ambient (37 Pa) andelevated (66 Pa) atmospheric partial pressure of CO₂, and incontrol plots (no chamber). Summer-time measurements showedthat photosynthetic capacity was similar at elevated CO₂ (lightand CO₂-saturated value of 17.2 µmol m^–2 s^–1for beech, 13.5 µmol m^–2 s^–1 for pine), plantsgrown at ambient CO₂ (beech 21.0 µmol^–2 s^–1,pine 14.9 µmol m^–2s^–1) or control plants grownwithout chambers (beech 23.2 µmol m^–2 s^–1,pine 12.9 µmol m^–2 s^–1). However, the higherCO₂ partial pressure had a direct effect on photosynthetic rate,such that under their respective growth conditions, photosynthesisfor the elevated CO₂ treatment (measured at 70 Pa CO₂ partialpressure: beech 14.1 µmol m^–2 s^–1 pine 10.3)was greater than in ambient (measured at 35 Pa CO₂: beech 9.7µmol m^–2 s^–1, pine 7.0 µmol m^–2s^–1) or control plants (beech 10.8 µmol m^–2s^–1, pine 7.2 µmol m^–2 s^–1). Measurementsof chlorophyll fluorescence revealed no evidence of photodamagein any treatment for either species. The quantity of the photoprotectivexanthophyll cycle pigments and their degree of de-epoxidationat midday did not differ among treatments for either species.The photochemical efficiency of photosystem II (yield) was lowerin control plants than in chamber-grown plants, and was higherin chamber plants at ambient than at elevated CO₂. These resultssuggest that at lower (ambient) CO₂ partial pressure, beechplants may have dissipated excess energy by a mechanism thatdoes not involve the xanthophyll cycle pigments. Key words: Carotenoids, chlorophyll fluorescence, photosynthesis, photoinhibition, photoprotection, xanthophyll cycle     

Effect of Water Stress on Gas Exchange in Fronds of the Ostrich Fern (Matteuccia struthiopteris (L.) Todaro)

Experiments were conducted in a gas exchange system to examinethe effect of a water stress, induced by –200 kPa polyethyleneglycol (PEG), on carbon dioxide and water vapour flux, fronddiffusive resistance, intercellular carbon dioxide concentration,carbon dioxide residual resistance and frond water potentialin the ostrich fern (Matteuccia struthiopteris (L.) Todaro).Measurements were taken 1 d after the application of PEG. Themeasurements were made on young fronds (8 d old) and maturefronds (20–24 d old) at PPFD's (Photosynthetic PhotonFlux Density) from 0–1400 µmol m^–22 s^–1.Water stress decreased the net photosynthesis rate in maturefronds at PPFD's of 210 µmol m^–2 s^–1 or greaterand increased the net photosynthesis rate below 210 µmolm^–2 s^–1 in young fronds. The increase in net photosynthesisin stressed young fronds was associated with a significant reductionin the dark respiration rate. Water stress and decreasing PPFD'sincreased frond diffusive resistance. Carbon dioxide concentrationin the intercellular spaces decreased with increasing frondage and PPFD's up to 200 µmol m^–2 s^–1. Theresidual resistance to carbon dioxide flux was not significantlyaffected by either frond age or water stress. Frond water potentialwas significantly lower in mature fronds than in young fronds. Key words: Matteuccia struthiopteris, Water relations, Photosynthesis, Dark respiration     

Long Term Effects of High CO2 Concentration on Photosynthesis of Water Hyacinth (Eichhornia crassipes (Mart.) Solms)

The photosynthetic response to CO₂ concentration, light intensityand temperature was investigated in water hyacinth plants (Eichhorniacrassipes (Mart.) Solms) grown in summer at ambient CO₂ or at10000 µmol(CO₂) mol^–1 and in winter at 6000 µmol(CO₂)mol^–1 Plants grown and measured at ambient CO₂ had highphotosynthetic rate (35 µmo1(CO₂) m^–2 s^–1),high saturating photon flux density (1500–2000) µmolm^–2 s^–1 and low sensitivity to temperature in therange 20–40 °C. Maximum photosynthetic rate (63 µmol(CO₂)m^–2 s^–1) was reached at an internal CO₂ concentrationof 800 µmol mol^–1. Plants grown at high CO₂ in summerhad photosynthetic capacities at ambient CO₂ which were 15%less than for plants grown at ambient CO₂, but maximum photosyntheticrates were similar. Photosynthesis by plants grown at high CO₂and high light intensity had typical response curves to internalCO₂ concentration with saturation at high CO₂, but for plantsgrown under high CO₂ and low light and plants grown under lowCO₂ and high light intensity photosynthetic rates decreasedsharply at internal CO₂ concentrations above 1000 µmol^–1. Key words: Photosynthesis, CO₂, enrichment, Eichhornia crassipes     

Photoadaptation in Antarctic phytopfankton: variations in growth rate, chemical composition and P versus I curves

The response of phytoplankton to variations in the light regimewas studied during the VULCAN and ACDA cruises in the Antarctic.Unenriched batch cultures of 12–19 days' duration reachedchl concentrations of 10–50 µg^–1 and exhibitedexponential growth rates, with the maximal rate being 0.41 doubl,day^–1. Ice edge algae exhibited maximum growth rates atphoton flux densities (PFD) of 30–100 µE m^–2S^–1and the growth rate was reduced by about 30% at 500–1000µE m^–2S^–1 The chl/C ratio ranged between 0.004and 0.018, with the lowest ratios at PFDs above 500 µEm^–2S^–1 chl/C ratios were also below maximum at PFDsbelow 40–50 µE m^–2S^–1 The C:N:P ratioswere close to the Redfield ratios; the Si/C ratio averaged 0.16(atoms), and the ATP/C ratio averaged from 0.0024 to 0.0050in different culture senes. When thawed after having been frozenfor 10 days, shade-adapted cultures were in a much better conditionthan sun-adapted ones. P versus I data showed that the maximumassimilation number varied from 0.75 to 4.4 µg C (µgchl)^–1h^–1. It varied inversely with the chl/C ratio;therefore the maximum carbon turnover rate varied little betweensamples (0.024/0.035 h^–1). Low biomass communities exhibitedrelatively high values for (the initial slope of P versus Icurves), low values for 1_sat (160–330 µE m^–2S^–1),and they were susceptible to photoinhibition. In contrast, communitiesdominated by Odontella weissflogii exhibited low values for, a high value for I_sat (560 µE m^–2S^–1 andthey tolerated high PFDs. The photo-adaptational status of thephytoplankton in natural water samples is discussed relativeto the profile of water column stability and mixing processes.     

Partition of 14C Assimilate between Organs and Fractions of Contrasting Varieties of Carrot during Initiation of the Storage Root

This work examines the differences in partition and activityof ¹⁴C in two varieties of carrot (Daucus carota L.) contrastingin shoot to storage root ratio at maturity. Plants were grownin a controlled environment of 20 ?C and 500 µmol m^–2s^–1. During initiation of the storage root (10–25d from sowing) plants were exposed to ¹⁴CO₂ for 1 h and theradioactivity in ethanol-soluble and -insoluble fractions ofshoots, storage and fibrous roots estimated at various timesup to 48 h after exposure. Between 35% and 40% of radioactivityinitially present in the plants was respired during the first24 h and 25–35% of that remaining after 24 h was foundin the roots, depending on age. The proportion found in thestorage region remained fairly constant between 15 and 25 dand was smaller than at 10 d. In the variety with a larger proportionof storage root at maturity (cv. Super Sprite), there was agreater proportion of label in both ethanol-soluble and -insolublefractions of the storage region soon after storage root initiationhad begun than in the variety with a smaller proportion of storageroot at maturity (cv. Kingston). There was no varietal differencein specific activities of the storage roots, but fibrous rootsof cv. Super Sprite showed a greater specific activity thanin cv. Kingston. Differences in shoot to storage root ratiomay thus be associated with characteristics of the fibrous roots.Partition and specific activities are discussed in relationto the initiation and development of the storage organ. Key words: Daucus carota, carrot, assimilate, partition, ¹⁴C, storage root     

Quantum Flux Density as a Factor Controlling the Rate of Growth, Carbohydrate Partitioning and Wood Structure of Betula pubescens Seedlings

The ability of Betula pubescens seedlings to acclimate to arange of quantum flux densities is examined, paying particularattention to the quantity, rate and type of wood produced. Betulapubescens seedlings were germinated and grown for 2 months ata quantum flux density of 440 µmol m^–2 s^–1,then transferred to 440, 244 or 89 µmol m^–2 s^–1. Seedlings transferred to reduced fluence rates were able tosurvive and grow. Acclimation was associated with an increasedallocation of carbon to stem elongation and leaf-area production,this occurred at the expense of root storage carbohydrate androot growth. The rate of wood production was shown to be directlyrelated to quantum flux density. Stem wood structure at smallquantum flux density showed an increase in proportion of fibres,while root wood structure in this regime showed a greater proportionof vessels. In general the activity of phenylalanine ammonia-lyase(PAL) showed little significant difference between treatments.The implications of these changes in wood structure are discussedand related to wood production and PAL activity. Betula pubescens Ehrh., birch., shade tolerance, carbohydrate partitioning, relative growth rate, wood structure, phenylalanine-ammonia-lyase     

Influence of Elevated CO2 and Temperature on the Photosynthesis and Respiration of White Clover Dependent on N2 Fixation

Single clonal plants of white clover (Trifolium repens L) grownfrom explants in a Perlite rooting medium, and dependent fornitrogen on N₂ fixation in root nodules, were grown for severalweeks in controlled environments which provided two regimesof CO₂, and temperature 23/18 °C day/night temperaturesat 680 µmol mol^–1 CO₂, (C680), and 20/15 °Cday/night temperatures at 340 µmol mol^–1 CO₂ (C340)After 3–4 weeks of growth, when the plants were acclimatedto the environmental regimes, leaf and whole-plant photosynthesisand respiration were measured using conventional infra-red gasanalysis techniques Elevated CO₂ and temperature increased ratesof photosynthesis of young, fully expanded leaves at the growthirradiance by 17–29%, despite decreased stomatal conductancesand transpiration rates Water use efficiency (mol CO₂ mol H₂O^–1)was also significantly increased Plants acclimated to elevatedCO₂, and temperature exhibited rates of leaf photosynthesisvery similar to those of C340 leaves ‘instantaneously’exposed to the C680 regime However, leaves developed in theC680 regime photosynthesised less rapidly than C340 leaves whenboth were exposed to a normal CO₂, and temperature environmentIn measurements where irradiance was varied, the enhancementof photosynthesis in elevated CO₂ at 23 °C increased graduallyfrom approx 10 % at 100 µmol m^–1 s^–1 to >27 % at 1170 µmol m^–2 s^–1 In parallel, wateruse efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 In parallel,water use efficiency increased by 20–40 % at 315 µmolm^–2 s^–1 In parallel, water use efficiency increasedby 20–40 % at 315 µmol m^–2 s^–1 to approx100 % at the highest irradiance Elevated CO₂, and temperatureincreased whole-plant photosynthesis by > 40 %, when expressedin terms of shoot surface area or shoot weight No effects ofelevated CO₂ and temperature on rate of tissue respiration,either during growth or measurement, were established for singleleaves or for whole plants Dependence on N₂, fixation in rootnodules appeared to have no detrimental effect on photosyntheticperformance in elevated CO₂, and temperature Trifolium repens, white clover, photosynthesis, respiration, elevated CO₂, elevated temperature, water use efficiency, N₂ fixation     

Light-driven movements of the trifoliate leaves of bean (Phaseolus vulgaris L.). Activity of blue light and red light

The puhrinule of the terminal leaflet in the trifoliate leafof bean (Phaseolus vulgaris L.) responds to its continuous exposureto directional overhead light by increasing the elevation ofits attached lamina. Blue light drives this response, but theeffectiveness of unfiltered white light equalled, or exceededthe effectiveness of blue light at equivalent irradiances (200–800µmol m^–2 s^–1). Adding red light to blue lightenhanced the initial rate of response, and increased its steady-state.These effects of red light increased with irradiance. Adding200–800 µmol m^–2 s^–1 red light to 50µmol m^–2 s^–1 blue light was more effectivein enhancing the initial rate of response than adding blue lightat equivalent irradiances, whereas added blue light was moreeffective in increasing the steady-state. In continuous bluelight the initial (maximal) angular velocity of laminar reorientation,as well as the eventual steady-state of the response increasedlinearly with log PFD (up to 800 µmol m^–2s^–2).Laminar reorientation also took place in continuous red lightby itself, and the angular velocity of the response was initiallyhigh, then became considerably slower. The initial phase wasapparently independent of irradiance up to PFD 100 µmolm^–2 s^–1 but increased progressively with log PFDat higher irradiances. During the second phase, the rate increasedlinearly with irradiance, becoming saturated at PFD 200 µmolm^–2 s^–1. Key words: Phaseolus, phototropism, pulvinule, spectral dependence, trifoliate leaf movements     

Na+, K+ and Cl- in Xylem Sap Flowing to Shoots of NaCl-Treated Barley

Munns, R. 1985. Na⁺, K⁺ and Cl^– in xylem sap flowing toshoots of NaCl-treated barley.—J. exp. Bot. 36: 1032–1042. Na⁺, Cl^– and K⁺ concentrations were measured in xylemsap obtained by applying pressure to the roots of decapitatedbarley plants grown at external [NaCl] of 0, 25, 50, 100, 150and 200 mol m^–3. For any given NaCl treatment, ion concentrationsin the xylem sap were hyperbolically related to the flux ofwater. Ion concentrations in sap collected at very low volumefluxes (without applied pressure) were 5–10 times higherthan in sap collected at moderate fluxes (under pressure). Fora given moderate volume flux, Na+ concentration in the xylemsap, [Na⁺]_x, was only 4.0 mol m^–3 at external [NaCl] of25–150 mol m^–3, and increased to 7.0 mol m^–3at 200 mol m^–3. [Cl-]x showed a similar pattern. Thisshows there would be little difference in the rate of uptaketo the shoot of plants at 25–150 mol m^–3 externalNaCl and indicates little change even at 200 mol m^-3 NaCl becausetranspiration rates would be much lower. Thus the reduced growthof the shoot of plants at high NaCl concentrations is not dueto higher uptake rates of Na⁺ or Cl^–. The fluxes of Na⁺, Cl^– and K^– increased non-linearlywith increasing volume flux indicating little movement of saltin the apoplast. The flux of K⁺ increased even when [K⁺]_x wasgreater than external [K⁺], indicating that membrane transportprocesses modify the K⁺ concentration in the transpiration streamas it flows through the root system. Key words: -Xylem sap, Na⁺, K⁺, Cl^– fluxes, salinity, barley     

The Carbon Economy of Clonal Plants of Trifolium repens L.

Fluxes of carbon between sources and sinks were quantified forclonal plants of Trifolium repens L. (cv. Blanca) in two glasshouseexperiments. Carbon sources were (a) leaves on the parent (=main)stolon apex, or (b) leaves on either young or old branches,and the major sinks of interest were the parent stolon apex,branches, and the adventitious root arising at the same parentstolon node as a young source branch. Defoliation treatmentswere applied to the parent stolon and/or branches (excludingsource branches). Carbon moved freely from the parent stolon to branches and vice-versa;these bidirectional exchanges of C provided important supplementarysources of carbohydrate for the sinks and buffered them againstthe effects of defoliation. Young branches exported more C tothe parent plant (mean=6.3µmol d^–1) than they importedfrom leaves on the parent stolon (5·2µmol d^–1)which, in turn, exceeded the amount fixed by leaves on the branchand utilized within the branch itself (2·7µmold^–1). In contrast, the C economy of old branches was largelyself-contained with, on average, 25·4µmol d^–1exported to the parent plant, 1·8µmol d^–1imported from the parent, and 63·0µmol d^–1fixed and utilized by the branch itself. Thus the growth ofyoung branches was immediately reduced by removal of parentstolon leaves, but old branches were unaffected. An estimated 42% of the C utilized by the main stolon apex originatedfrom branches, while by far the largest proportion (84%) ofthe C used for growth of young nodal roots originated from theassociated branch and not from leaves on the parent stolon towhich the root was directly attached. Key words: Trifolium repens, clonal growth, carbon economy, physiological integration, defoliation     

Acclimation of Lolium temulentum to enhanced carbon dioxide concentration

Acclimation of single plants of Lolium temulentum to changing[CO₂] was studied on plants grown in controlled environmentsat 20°C with an 8 h photoperiod. In the first experimentplants were grown at 135 µ;mol m^–2 s^–1 photosyntheticphoton flux density (PPFD) at 415µl l^–1 or 550µll^–1 [CO₂] with some plants transferred from the lowerto the higher [CO₂] at emergence of leaf 4. In the second experimentplants were grown at 135 and 500 µmol m^–2 s^–1PPFD at 345 and 575 µl l^–1 [CO₂]. High [CO₂] during growth had little effect on stomatal density,total soluble proteins, chlorophyll a content, amount of Rubiscoor cytochrome f. However, increasing [CO₂] during measurementincreased photosynthetic rates, particularly in high light.Plants grown in the higher [CO₂] had greater leaf extension,leaf and plant growth rates in low but not in high light. Theresults are discussed in relation to the limitation of growthby sink capacity and the modifications in the plant which allowthe storage of extra assimilates at high [CO₂]. Key words: Lolium, carbon dioxide, photosynthesis, growth, stomatal density     

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.     

The Response of Bean Plants to UV-B Radiation Under Different Irradiances of Background Visible Light

Plants of Phaseolus vulgaris L. (cv. Stella) were grown in controlledconditions under three different irradiances of visible lightwith or without UV-B (280–320nm) radiation. The biologicallyeffective UV-B radiation (UV-B_BE) was 6.17 kJ m^–2 d^–1,and simulated a c. 5% decrease in stratospheric ozone at 55.7?N,13.4?E. The photon flux densities of the photosyntheticallyactive radiation (PAR, 400–700 nm) were either 700 µmolm^–2–1 (HL), 500, µmol m^–2 s^–1(ML) or 230 µmol m^–2 s^–1 PAR (LL). Under highlight (HL) conditions plus UV-B radiation, bean plants appearedmost resistant to the enhanced levels of UV-B radiation, andresponded only by increasing leaf thickness by c. 18%. A smallincrease in UV screening pigments was also observed. Both thelower irradiances (ML and LL) increased the sensitivity of theplants to UV-B radiation. Changes in leaf structure were alsoobserved. Photosystem II was inhibited under ML and LL togetherwith UV-B radiation, as determined by Chi fluorescence inductionand calculation of the fluorescence half-rise times. Leaf reflectivitymeasurements showed that the amount of PAR able to penetrateleaves of UV-B treated plants was reduced, and that a possiblecorrelation may exist between the reduced PAR levels, loss ofChi and lowered photosynthetic activity, especially for LL +UV-Bgrown plants, where surface reflection from leaves was highest.Changes in leaf chlorophyll content were mostly confined toplants grown under LL + UV-B, where a decrease of c. 20% wasfound. With regard to protective pigments (the carotenoids andUV screening pigments) plants subjected to different visiblelight conditions responded differently. Among the growth parametersmeasured, there was a substantial decrease in leaf area, particularlyunder LL + UV-B (c. 47% relative to controls), where leaf dryweight was also reduced by c. 25%. Key words: Chlorophyll fluorescence induction, bean, flavonoids, Phaseolus vulgaris, reflectance, UV-B radiation     

Growth of Ryegrass (Lolium perenne L.) Exposed to SO2: III. EFFECTS ON FREE AND STORAGE CARBOHYDRATE CONCENTRATIONS

Exposure of ryegrass (Lolium perenne L.) cv. S23 to 0, 50, and400 µg m^–3 SO₂ for an initial 29 d (first harvest),and for an additional 22 d period of regrowth (second harvest),resulted in distinct alterations in carbohydrate metabolismat each harvest. At the first harvest, exposure to 50 µgm^–3 increased concentrations of free and total carbohydrates,whereas exposure to 400 µg m^–3 resulted in concentrationshardly different from those in control plants. At both SO₂ concentrations,more assimilate was retained as free carbohydrate rather thanas storage carbohydrate. Comparison of assimilate distributionat the end of the light, and at the end of the dark period atthe first harvest led to the conclusion that light-mediatedmetabolism is more sensitive to SO₂ exposure than dark metabolism,and that assimilate distribution might be controlled by at leasttwo processes exhibiting different SO₂ sensitivities.