Growth of Perennial Ryegrass (Lolium perenne L.) Exposed to a Low Concentration of Sulphur Dioxide

The influence of a low concentration of SO₂ on the growth andsulphur status of perennial ryegrass (Lolium perenne L.) cv.S23 was examined in a system of specially designed growth chambers.For plants grown without additions of sulphate to the soil,erposure to ifitered air containing 50 µg m^–3 SO₂alleviated sulphur deficiency that developed during a sequenceof successive harvests, and gave increased yield. The plantshad higher contents of organic and inorganic sulphur than thosegrown with neither sulphate nor SO₂. Plants grown with addedsulphate showed no signs of sulphur deficiency and althoughexposure to SO₂ increased their sulphur content it had no effecton yield. The coefficient of transpiration (g water transpiredg^–1 dry wt. of shoots) was reduced when plants which otherwisehad an inadequate supply of sulphur were exposed to SO₂; forplants with adequate supplies of sulphate, it was similar whetherthey were grown with or without the addition of SO₂ to the air.The results are discussed in relation to other known reactionsof plants to SO₂.     

Changes in Growth and Quality Characteristics of Lucerne (Medicago sativa L.) in Response to Sulphur Dioxide Exposure under Field Conditions

The effects of SO₂ on some growth and quality characteristicsof lucerne (Medicago sativa L.) were investigated by exposingplants to mean SO₂ concentrations of 215, 78 or 2.8 µgm_–3 in open-top chambers for 166 d. Plants exposed to215 µg m_–3 had significantly lower shoot and rootweights compared with plants exposed to 78 µg m^–3,but not compared with control plants. Exposure to 215 or 78µg m ^–3 increased the plant shoot: root ratio, buthad no effect on leaf area. During the middle of the fumigationperiod, relative growth rate and net assimilation rate werehighest in plants exposed to 215 fig m, but these later fellbelow control values, and plants exposed to 78 µg m^–3had the highest relative growth rate and net assimilation rate.As the duration of exposure increased, an initial SO₂-inducedstimulation of growth may have developed to toxicity at thehighest SO₂ exposure. Exposure to SO₂ depressed L-ascorbic acid concentrations inleaves, had no effect on foliar protein or starch concentrations,and increased the specific energy of shoots and plant sulphurconcentrations. The effect of SO₂ on L-ascorbic acid concentrationsmay suggest a mechanism for reduced freezing tolerance of plantsafter exposure to SO₂. Key words: SO₂, Medicago sativa L., Growth     

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.     

The Influence of the Concentration of Gaseous Ammonia on its Uptake by the Leaves of Italian Ryegrass, with and without an Adequate Supply of Nitrogen to the Roots

Whitehead, D. C. and Lockyer, D. R. 1986. The influence of theconcentration of gaseous ammonia on its uptake by the leavesof Italian ryegrass, with and without an adequate supply ofnitrogen to the roots.—J. exp. Bot. 38: 818–827. Plants of Italian ryegrass (Lolium multiflorum Lam.) were grownin pots of soil with two rates of ¹⁵N-labclled nitrate, oneproviding adequate, and the other less than adequate, N formaximum growth. After 25 d in a controlled environment cabinet,the plants were transferred to chambers and exposed for 33 dto NH₃in the air at one of nine concentrations ranging from14 to 709 µg NH₃ m^–3. Increasing the concentrationof NH₃ in the air increased the dry weight of the shoots ofplants grown at the lower but not the higher rate of nitrate.The content of total N in the plant shoots (% dry weight) waslinearly related to NH₃ concentration; at 709 µg NH₃ andin both sets of plants it was more than double the content at14 µg NH₃ m^–3. Calculations, based on ¹⁵N enrichment,indicated that the amount of N taken up from the NH₃ per unitleaf area increased linearly with increasing concentration ofNH₃ in the air uptake (µg dm^–2 h^–1) = 0.1009xat the lower rate of nitrate and 0-0829x at the higher rateof nitrate, where x is the concentration of NH₃ in the air expressedas µg NH₃m^–3. The proportion of the total plant N that was derived from theNH₃ ranged from 4?0% at a concentration of 14 µg NH₃ m^–3with the higher rate of nitrate addition to 77?5% at a concentrationof 709 µg m^–3 with the lower rate of nitrate addition.The proportions of the total N in the water-insoluble proteinof the leaf tissue that were derived from nitrate and gaseousNH₃ were similar to the proportions in the whole leaf material. Key words: Ammonia, nitrogen, leaf sorption, Lolium multiflorum     

Effects of Low Concentrations of Sulphur Dioxide on Net Photosynthesis and Dark Respiration of Vicia faba

Rates of net photosynthesis, P_N, and dark respiration of Viciafaba plants were measured in the laboratory in clean air andin air containing up to 175 parts 10^–9 (500 µg m^–3)SO₂. At all SO₂ concentrations exceeding 35 parts 10^–9,P_N was inhibited compared with clean air. At light saturation,the magnitude of inhibition depended on SO₂ concentration butat low irradiances the inhibition was independent of concentration.Dark respiration rates increased substantially, independentof concentration. When exposures continued for up to 3 days,P_N returned to clean air values about 1 h after fumigation ceased:dark respiration recovered after one photoperiod. There wereno visible injuries. Reviewing possible mechanisms responsible for the inhibitionof P_N, it is suggested that SO₂ competes with CO₂ for bindingsites in RuBP carboxylase. Analysis of resistance analoguesdemonstrates that SO₂ altered both stomatal and internal (residual)resistances. A model of crop photosynthesis shows the implications of theobserved responses for the growth of field crops in which plantsare assumed to respond like laboratory plants. Photosynthesisof the crop would be less sensitive than that of individualplants to SO₂ concentration. Daily dry matter accumulation ofhypothetical ‘polluted crops’ would be substantiallyless than clean air values but would vary relatively littlewith SO₂ concentration. It is concluded that physiological basesexist to account for observed reductions in growth of plantsat very low SO₂ concentrations, and that thresholds for plantresponses to SO₂ require reassessment.     

Responses of CO2 assimilation to changes in irradiance: laboratory and field data and a model for beans (Phaseolus vulgaris L.)

The responses of net CO₂ assimilation to sudden changes in irradiancewere studied in Phaseolus vulgaris L. in the laboratory andthe field. For irradiance changes between 50 µmol m^–2s^–1 to 350 µmol m^–2 s^–1 in the laboratory,assimilation rate increased with half-times of 2.7 and 4.1 minin well-watered and water-stressed plants, respectively. Ina field experiment with a change in irradiance from 400 to 1200µmol m^–2 s^–1 the response was faster (half-time=c.1.2 min). In all cases when irradiance was returned to a lowvalue, assimilation declined rapidly with a half-time of approximately1 min, which approached the time resolution of the gas-exchangesystem. The corresponding changes in stomatal conductance in responseto both increasing and decreasing irradiance were much slowerthan the assimilation responses, indicating that biochemicalprocesses, rather than CO₂ supply, primarily determined theactual rate of assimilation in these experiments. The conceptof stomatal limitation to photosynthesis is discussed in relationto these results. A simple model for assimilation in a fluctuating light environmentis proposed that depends on a steadystate light response curve,an ‘induction lag’ on increasing irradiance, andan induction-state memory. The likely importance of taking accountof such induction lags in natural canopy microclimates is considered. Key words: Models, Phaseolus vulgaris, photosynthetic induction, CO₂ assimilation, stomatal limitation, sunflecks, water stress     

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     

Modification of Stomatal Conductance by Sulphur Dioxide

The mechanism of SO₂-induced changes in stomatal conductance(g) of alder was examined to determine if SO₂ affects guardcell function directly or indirectly through the SO₂-inducedchanges in photosynthesis. During experimental fumigations at SO₂ concentrations of 3–3µmol m^–3 (0.08 µl l^–1), stomatal closurepreceded declines in net photosynthetic rate (A), indicatingthat SO₂ can directly affect guard cells. From these and otherstudies it appears that the sequence of A and g responses maybe influenced by SO₂ concentration as well as by species. Fumigation with SO₂ did not cause increases in g, even whenthe intercellular substomatal CO₂ concentration (c_i) was reducedby 50 µmol mol^–1. Increases in g are not attributableto SO₂ effects on the CO₂-based stomatal control system. Key words: Air pollution, Alnus serrulata, gas exchange, stomata, sulphur dioxide     

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.     

C3 and CAM Photosynthetic Characteristics of the Submerged Aquatic Macrophyte Littorella uniflora: Regulation of Leaf Internal CO2 Supply in Response to Variation in Rooting Substrate Inorganic Carbon Concentration

The relationships between CO₂ concentrating mechanisms, photosyntheticefficiency and inorganic carbon supply have been investigatedfor the aquatic macrophyte Littorella uniflora. Plants wereobtained from Esthwaite Water or a local reservoir, with thelatter plants transplanted into a range of sediment types toalter CO₂ supply around the roots. Free CO₂ in sediment-interstitial-waterranged from 1–01 mol m^–3 (Esthwaite), 0.79 mol m^–3(peat), 0.32 mol m^–3 (silt) and 0–17 mol m^–3(sand), with plants maintained under PAR of 40 µmol m^–2s^–1. A comparison of gross morphology of plants maintained underthese conditions showed that the peat-grown plants with highsediment CO₂ had larger leaf fresh weight (0–69 g) andtotal surface area (223 cm² g^–1 fr. wt. including lacunalsurface area) than the sand-grown plants (0.21 g and 196 cm²g^–1 fr. wt. respectively). Root fresh weights were similarfor all treatments. In contrast, leaf internal CO₂ concentration[CO₂], was highest in the sand-grown plants (2–69 molm^–3, corresponding to 6.5% CO₂ in air) and lowest inthe Esthwaite plants (1–08 mol m^–3). Expressionof CAM in transplants was also greatest in the low CO₂ regime,with H⁺ (measured as dawn-dusk titratable acidity) of 50µmolg^– fr. wt., similar to Esthwaite plants in natural sediment.Assuming typical CAM stoichiometry, decarboxylation of malatecould account largely for the measured [CO₂]₁ and would makea major contribution to daytime CO₂ fixation in vivo. A range of leaf sections (0–2, 1–0, 5–0 and17–0 mm) was used to evaluate diffusion limitation andto select a suitable size for comparative studies of photosyntheticO₂ evolution. The longer leaf sections (17.0 mm), which weresealed and included the leaf tip, were diffusion-limited witha linear response to incremental addition of CO₂ and 1–0mol m^–3 exogenous CO₂ was required to saturate photosynthesis.Shorter leaf sections were less diffusion-limited, with thegreatest photosynthetic capacity (36 µmol O₂ g^–1 fr. wt. h^–1) obtainedfrom the 1.0 mm size and were not infiltrated by the incubatingmedium. Comparative studies with 1.0 mm sections from plants grown inthe different sediment types revealed that the photosyntheticcapacity of the sand-grown plants was greatest (45 µmolO₂ g^–1 fr. wt. h^–1) with a K_0.5 of 80 mmol m^–3.In terms of light response, saturation of photosynthesis intissue slices occurred at 850–1000 µmol m^–2s^–1 although light compensation points (6–11 µmolm^–2s^–1) and chlorophyll a: b ratios (1.3) were low.While CO₂ and PAR responses were obtained using varying numbersof sections with a constant fresh weight, the relationshipsbetween photosynthetic capacity and CO₂ supply or PAR were maintainedwhen the data were expressed on a chlorophyll basis. It is concludedthat under low PAR, CO₂ concentrating mechanisms interact inintact plants to maintain saturating CO₂ levels within leaflacunae, although the responses of the various components ofCO₂ supply to PAR require further investigation. Key words: Key words-Uttorella uniflora, internal CO₂ concentration, crassulacean acid metabolism, root inorganic carbon supply, CO₂ concentrating mechanism     

Intraspecific variation in the light/dark modulation of ribulose 1,5-bisphosphate carboxylase-oxygenase activity in soybean

A comparative study was made of the inhibition of ribulose-1,5-bisphosphatecarboxylase-oxygenase (Rubisco) amongst six cultivars of Glycinemax L. Merr., associated with synthesis of 2-carboxyarabinitol1-phosphate (CA1P) during darkness. Significantly lower meanvalues of dark inhibition of Rubisco were observed in soybeancv. Davis than in cvs Bragg, Cobb, Hardee, Gordon, and Kirby.The CA1P synthesis/degradation cycle during dark/light transitionsremained operational in cv. Bragg plants grown at low irradiance(40 µmol photons m–2 s–1). However, CA1P synthesisand degradation rates were slower in the dark (t_0.5 = 240 versus25 min), and light (t_0.5 = 20 versus 3.8 min) respectively,as compared to plants grown at higher irradiance (550 µmolphotons m^–2 s^–1). In addition, the activation stateof Rubisco in low-light-grown plants showed only a small declineafter a transition to darkness. We conclude that (a) cultivar-dependentvariation occurs amongst soybeans with respect to CAlP regulationof Rubisco, and (b) soybeans acclimated to low irradiance maydepend more on CA1P synthesis/degradation to regulate Rubisco,and less on changes in the enzyme activation state. Key words: Activation state, Glycine max, photosynthesis, Rubisco, 2-carboxyarabinitol 1-phosphate     

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     

On the causes of interspecific differences in the growth-irradiance relationship for phytoplankton. II. A general review

The causes of interspecific differences in the µ-l relationshipare examined in the context of a mechanistic model which relatesµ to irradiance in terms of six factors:, k_c photosyntheticquotient (PQ), Chl a:C, respiration and excretion. The effectof cell size on the light saturated growth rate is also considered.It is shown that photosynthetic efficiency and PQ exhibit remarkablylittle interspecific variability, and average 0.024 ±0.005 µg C(µg Chl a)^–1 h^–1 (µEm^–2 s^–1)^–1 and 1.5 ± 0.2 mol 02 molC^–1 (when NO₃^– is the nitrogen source) respectively.Two useful relationships were derived: (i) between growth efficiency,_g and Chl a:C at µ. = 0; (ii) between the compensationintensity, I_c and the Chl a-specific maintenance respirationrate. Both relationships were independent of temperature anddaylength. Species best adapted to growth at low light werefound to exhibit high Chl a:C ratios and low maintenance respirationrates. As a group, diatoms were consistently the best adaptedfor growth at low irradiance. Chiorophytes, haptophytes, chrysophytesand cryptophytes were intermediate in their performance at lowirradiance. Dinoflagellates exhibited extreme diversity, withspecies spanning the spectrum from very good performance atlow irradiance to very poor. A new µ_max-cell carbon relationshipis given based on growth rates normalized to 15°C. Evidenceis presented to show that noise in this relationship can besignificantly reduced by using only carbon-specific growth ratesand using only data for species grown at the same daylength.     

Growth of Ryegrass (Lolium perenne L.) Exposed to SO2: I. EFFECTS ON PHOTOSYNTHESIS AND RESPIRATION

The effects of exposure to SO₂ (50 and 400 µg m^–3SO₂) on the growth, photosynthesis, and respiration of perennialryegrass (Lolium perenne L.) cv. S23 were examined in two successivegrowth periods of 29 and 22 d. At the higher concentration ofSO₂, there was some visible injury of the leaves and specificleaf area was reduced, but yield, net photosynthesis, and darkrespiration of the plants were not significantly affected byexposure. The treatment was also without effect on the transpirationcoefficient of the plants and their number of tillers. The plantsexposed to the lower concentration of SO₂ showed no signs ofinjury and did not differ in any of the measured characteristicsfrom plants grown in SO₂-free air. Content of S in the shootsincreased linearly with the concentration of SO₂, the additionalS being found in the sulphate fraction whilst organic S wasunchanged. The results are discussed in relation to earlierfindings that yield of ryegrass exposed to SO₂ may be reducedwithout visible signs of injury.     

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     

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     

Growth Responses of Two Solanum Species to Contrasting Temperatures and Irradiance Levels: Relations to Photosynthesis, Dark Respiration and Chlorophyll Fluorescence

Potato production in the tropical lowlands during the rainyseason is constrained by high temperature and low irradiance.This study examined the effect of these two variables on drymatter production and allocation, using plant growth, leaf anatomy,gas exchange and chlorophyll fluorescence measurements. Plantsof two clones, Solanum goniocalyx cv. Garhuash Huayro (GH) andDTO-33, a heat tolerant clone of S. tuberosum x S. phureja,were grown in growth chambers at 33/25 °C or 20/10 °Cday/night temperature. At each temperature, plants were grownin either 12 h high irradiance (430–450 µmol m^–2s^–1 PAR) or 12 h low irradiance (250–280 µmolm^–2 s^–1) both with a 6–h photoperiod extensionof 6 µmol m^–2 s^–1. Plants were harvested after10 d (initial harvest) and after 20 d (final harvest). By theend of the study DTO-33 had produced more dry matter and hadtuberized, whereas GH had a greater leaf area ratio (LAR) andspecific leaf area (SLA). The highest relative growth rate (RGR)was at low temperature and low irradiance, possibly due to acombination of thin leaves with a large surface area. At thehigh temperature, low irradiance had the opposite effect, producingthe lowest net assimilation rate (NAR) and lowest RGR. Bothtuber number and weight were markedly reduced by high temperature.Low irradiance, in combination with high temperature, producedvirtually no tubers. Stomatal density, which was greater onGH than in DTO-33, was increased at high temperature. When measuredat 30 °C both clones, especially DTO-33, showed heat-adaptationin terms of ability to maintain a high rate of net photosynthesisat 30 °C. Plants grown at high irr-adiance and low temperaturehad the lowest net photosynthetic rate at 30 °C. Concurrentmeasurements of chlorophyll fluorescence indicated that onlythe initial (O) fluorescence parameter was affected. The dataconfirm the field observation that reduction in potato growthat high temperature can be aggravated by lower irradiance. Thisreduction is associated with a reduced leaf area and NAR. Growth analysis, heat adaptation, light     

Absorption of SO2 by Pecan (Carya illinoensis (Wang) K. Koch) and Alfalfa (Medicago sativa L.) and its Effect on Net Photosynthesis

Absorption rates of SO₂ by pecan (Carya illinoensis (Wang) K.Koch) leaflets exposed to 2.6, 5.2, and 7.8 mg SO₂ m^–3were measured over a 2 h period. SO₂ was rapidly absorbed bythe leaflets in all treatments during the initial 30–50min; the rate of uptake decreased to a rather constant levelthereafter. Total SO₂ absorbed during the 2 h period was 15.6,25.6, and 38.9 nmol cm^–2 for the low, medium, and highSO₂ concentrations, respectively. Reductions in net photosyntheticrates were proportional to ambient SO₂ concentrations and totalSO₂ absorbed. Partial photosynthetic recovery occurred in alltreatments during a 2 h post-treatment period and full recoveryoccurred during a 12 h dark period. Exposure to SO₂ resultedin slight increases in stomatal and boundary layer resistancesto CO₂ and substantial increases in residual resistances. Absorptionrates of SO₂ by alfalfa (Medicago saliva L.) exposed to 5.2mg SO₂m^–3 for 1 h were approximately double those of pecanexposed to the same ambient SO₂ concentration. Alfalfa net photosyntheticrates were reduced 74% after 1 h exposure to 5.2 mg SO₂ m^–3while a depression of 42% occurred in pecan.     

The Uptake of Gaseous Ammonia by the Leaves of Italian Ryegrass

Lockyer, D. R. and Whitehead, D. C. 1986. The uptake of gaseousammonia by the leaves of Italian ryegrass.—J. exp. Bot.37: 919–927. Plants of Italian ryegrass (Lolium multiflorum Lam.) grown insoil with two rates of added ¹⁵N-labelled nitrate were exposed,in chambers, for 40 d to NH₃ in the air at concentrations of16, 118 and 520 µg m^–3. At the highest concentrationof NH₃, this source provided 47?3% of the total nitrogen inplants grown with the lower rate of nitrate addition (100mgN kg^–1 dry soil) and 35?2% with the higher rate (200mgN kg^–1 dry soil) At the intermediate concentration ofNH₃, the contributions to total plant N were 19?6% and 10?8%,respectively, at low and high nitrate while, at the lowest concentrationof NH₃, they were 5?1% and 32%. Most of the N derived from theNH₃ remained in the leaves, but some was transported to theroots. The amount of N derived from the NH³ that was presentin the leaves was not reduced by washing the leaves in waterat pH 5?0 before harvesting, indicating that the N was assimilatedby the plant and not adsorbed superficially. Rates of uptakeof NH₃ per unit leaf area ranged from 1?7 µg dm^–2h^–1 at a concentration of 16 µg m^–3 to 29?0µg dm^–2 h^–1 at a concentration of 520 µgm^–3 and with the lower rate of nitrate addition. Increasingthe supply of nitrate to the roots slightly reduced the rateof uptake of NH₃ per unit leaf area. Uptake of N from the higherrate of nitrate was reduced at the highest concentration ofNH₃ in the air. Key words: Ammonia, nitrogen, leaf sorption, Lolium multiflorum