Measuring the Canopy Net Photosynthesis of Glasshouse Crops

A null balance method is described for measuring net photosynthesisof mature canopies of cucumber and other protected crops overperiods of 10 min in a single-span glasshouse (c. 9m x 18m inarea). Accuracy of control of the CO₂ concentration in the greenhouseatmosphere is within ±10 vpm of the normal ambient level(c. 350 vpm). The amounts of CO₂ used in canopy net photosynthesisare measured with linear mass flowmeters accurate to within±0.80g. The total errors incurred in measuring canopynet photosynthesis at an ambient CO₂ level are estimated tobe of the order of ± 1·2% in bright light (350W m^–2, PAR)and ±3·6% in dull light (100W m^–2, PAR). Measurements of the rates of net photosynthesis of a maturecanopy of a cucumber crop were made at near-ambient CO₂ concentrationsover a range (0–350 W m^–2) of natural light fluxdensities. A model of light absorption and photosynthesis applicableto row crops was used to obtain a net photosynthesis versuslight response curve for the cucumber crop. At a light fluxdensity of 350 W m^–2 the fitted value of canopy net photosynthesiswas 2.65 mg CO₂ m^–2s^–1 (equivalent to over 95 kgCO₂ ha^–1h^–1). The results are discussed in relationto the need for CO₂ supplements to avoid depletion in both ventilatedand unventilated glasshouses during late spring and summer. Key words: Glasshouse crops, cucumber, measurement, canopy photosynthesis, light, CO₂     

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.     

Microclimate, Photosynthesis and Growth of Lucerne (Medicago sativa L.). I. Microclimate and Photosynthesis

Measurements of microclimate and photosynthesis of lucerne var.Europe were made in the field during the spring of 1976. Themaximum rate of canopy gross photosynthesis (14.3 g CO₂ m^–2h^–1, I = ) was 2.5 times greater than that of S 24 perennialryegrass at the same LAI. This difference was due to differencesin individual leaf photosynthesis. The photosynthetic rate ofthe youngest fully expanded leaf of lucerne remained constantthroughout the experimental period at 3.6 g CO₂ m^–2 h^–1(300 W m^–2). Measurements of soil water potential profiles indicated thatlucerne extracted water from the soil to a depth of at least800 mm, with a region of maximum uptake between 400 and 600mm. This capability, with a moderate mean leaf resistance of460 s m^–1, conferred a high assimilation efficiency onlucerne, with a mean water use efficiency of 34 g H₂O lost pergram of carbohydrate assimilated, compared with 200 g H₂O pergram of carbohydrate for S 24. Medicago sativa L, lucerne, photosynthesis, assimilation efficiency     

Transfer of Mass and Energy in a Cassava Community (Manihot esculenta Crantz cv. Cubana) 2. CO2 Exchange in a Savanna Climate

The energy balance approach was applied to calculate the CO₂flux above and within a cassava community, growing during asavanna wet season. Data of the response of CO₂ exchange todiurnal changes in the savanna environment were integrated toa growth analysis of the cassava crop. The carbon budget of the entire community was calculated atdifferent development stages. Results indicate that CO₂ uptakein cassava appears to follow a linear net radiation responsecurve, dependent on crop age. The maximum net CO₂ uptake decreasedfrom 0.195 MJ m^–2 day^–1 at maximum leaf area development(August) to 0.028 MJ m^–1 day-1 2 months later. These ratesrepresent 41 and 19 per cent gross assimilation. Data of energy conversion efficiency show that at a maximumleaf area development, the crop fixed 2.2 and 0.9 per cent Rtas gross (_g) and net photosynthesis (n) respectively. As theseason proceeded,n decreased to 0.1, whereas g decreased to0.7, which indicates that dry matter lost by respiration isone of the determining factors in the seasonal trend of efficiencychanges. The comparison of growth characteristics calculated for cv.Cubana, growing during consecutive years in the same experimentalsite, indicate that yearly variability in dry matter accumulationis due to the wide range of environmental conditions presentduring the savanna wet season. Operational factors acting depressivelyon the cassava carbon budget and affecting dry matter productionwere also analysed. Manihot esculenta Crantz, cassava, microclimate, carbon budget, carbon dioxide fluxes     

Acclimation in Seedlings of a Tropical Tree, Bischofia javanica, Following a Stepwise Reduction in Light

Acclimation of fully developed leaves of Bischofia javanicaBlume to shadelight was examined. Seedlings were grown undersimulated daylight (1000 µmol m^–2 s^–1), thentransferred to a simulated shadelight (40 µmol m^–2s^–1). When a high-light leaf was transferred to low light, large negativenet photosynthetic rates (P_m) were recorded. This decrease wasrapid, but within 7 d the rate increased and became equal tothe low-light control leaf. These changes in photosynthesisdid not follow the pattern of changes in stomatal conductance(g_s). Transfer to the low light resulted in a dramatic decreasein leaf weight per unit area (L_w), and most of the decreasesin L_w occurred within 3 d of transfer when the P_m of the transferredleaf was well below that of the low-light control leaf. There was a significant decrease in chlorophyll a in the transferredleaf without an appreciable change in chlorophyll b resultingin a large decrease in the chlorophyll a to chlorophyll b ratio.Leaf chlorophylls per unit area were higher in the transferredleaf than the low-light control leaf. Maximum photosyntheticrate in the transferred leaf was decreased by 40% compared tothat for the high-control leaf, but was almost at the same extenthigher than the low-light control leaf The results are discussedin the context of carbon gain capacity of its seedlings underlight-limiting forest understorey habitats. Bischofia, chlorophylls, light, photosynthesis, shade acclimation, tree seedlings, tropical tree     

The Effect of Current Photosynthesis on the Origin of Translocates in Old Tomato Leaves

The rate of carbon transport from an old tomato leaf (54 days),grown at 80 W m^–2, was measured under light flux densitiesbetween 7 and 90 W m^–2. Under low light, the rate of carbontransport over a 6 h period was about 1 mg C dm^–2 h^–1,well in excess of the concurrent photosynthetic rate. The lossfrom these leaves of ¹⁴C-leaf assimilate which was fixed beforethe experimental period amounted to 62 per cent of the totalinitial uptake and was higher than that from leaves with higherconcurrent photosynthetic rates. The higher loss of ¹⁴C fromleaves with low photosynthetic rates was due to a greater contributionof ¹⁴C from the starch and residue fractions. The rate of transportappeared to be determined by the concentration of the labilesucrose, not the total sucrose concentration. In comparisonwith young fully-expanded tomato leaves (Ho, 1976) the sizeof the labile sucrose pool appeared to decrease with age. Thephotosynthesistranslocation coefficient was low (k₁k₂=0•21)for an old tomato leaf. Based on these results a scheme of carbonpartitioning in relation to translocation is proposed. Criteriafor assessing the efficiency of translocation in leaves arediscussed.     

The Seasonal Pattern of Growth and Production of a Temperate C4 Species, Cyperus longus

Growth and production of the temperate C₄ species Cyperus longusL. was measured throughout a growing season in an establishedplot in Eastern Ireland. The maximum standing live biomass reachedwas 2·5 kg m^–2. Estimates of unit leaf rate (ULR)and leaf area index (LAI) were made. The product of these quantitiesgave the crop growth rate (CGR) each week. C. longus was foundto maintain high values of LAI throughout the summer, with amaximum value of about 13 in early August. CGR reached a peakin early July. The optimum LAI was 11·6. Temperaturesat five levels in the plant canopy, and the amount of solarradiation intercepted by the canopy were measured continuouslyduring the summer. The mean daily rate of leaf extension waspositively correlated with the mean daily air temperature abovethe canopy but the temperature coefficient of the process waslow compared with other temperate species. The percentage ofsolar radiation intercepted by the canopy increased rapidlyin early summer, and canopy closure had occurred by mid-June.Rates of net photosynthesis were measured on young and old leafmaterial in situ at the time of peak LAI. In young leaves themaximum rates of net photosynthesis were higher than those publishedfor a range of temperate C₃ species, but similar to those foundin another temperate C₄ species, Spartina townsendii. Key words: C₄ photosynthesis, leaf growth, productivity     

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     

Control of the Acclimation of Photosynthesis to Light and Temperature in Relation to Partitioning of Photosynthate in Developing Soybean Leaves

Photosynthetic acclimation was examined by exposing third trifoliolateleaves of soybeans to air temperatures of 20 to 30°C andphotosynthetic photon flux densities (PPFD) of 150 to 950µmolphotons m^–2 s^–1 for the last 3 d before they reachedmaximum area. In some cases the environment of the third leafwas controlled separately from that of the rest of the plant.Photosynthesis, respiration and dry mass accumulation were determinedunder the treatment conditions, and photosynthetic capacity,and dry mass and protein content were determined at full expansion.Photosynthetic capacity, the light-saturated rate of net carbondioxide exchange at 25°C and 34 Pa external partial pressureof carbon dioxide, could be modified between 21 and 35 µmolCO₂ m^–2 s^–1 by environmental changes after leaveshad become exporters of photosynthate. Protein per unit leafmass did not differ between treatments, and photosynthetic capacityincreased with leaf mass per unit area. Photosynthetic capacityof third leaves was affected by the PPFD incident on those leaves,but not by the PPFD on other leaves on the plant. Photosyntheticcapacity of third leaves was affected by the temperature ofthe rest of the plant, but not by the temperature of the thirdleaves. Photosynthetic capacity was linearly related to carbondioxide exchange rate in the growth regimes, but not to daytimePPFD. At high PPFD, and at 25 and 30°C, mass accumulationwas about 28% of the mass of photosynthate produced. At lowerPPFD, and at 20°C, larger percentages of the photosynthateproduced accumulated as dry mass. The results suggest that photosynthatesupply is an important factor controlling leaf structural growthand, consequently, photosynthetic acclimation to light and temperature. Key words: Glycine max (L.) Merr., photosynthesis, temperature acclimation, light acclimation, photosynthate partitioning     

Stimulation of Net Photosynthesis in Cucumber Leaf Discs: Effect of K-Glyoxylate and KCL

Incubation of leaf discs of Cucumis sativus in 15 mol m^–3K-glyoxylate (pH 4.6) doubled the rate of net photosynthesisat limiting CO₂ or HCO^–₃ compared with discs floated ondistilled water. When both control and treated discs were incubatedin Mes-TMAOH buffer at pH 5.0, K-glyoxylate still increasednet photosynthesis by as much as 70%. The tetramethylammoniumsalt (TMA-glyoxylate) was without effect but KCl enhanced netphotosynthesis. Both KCl and K-glyoxylate increased stomatalaperture at pH 5.0. At pH 7.5 (Mops-TMAOH), neither stomatalaperture or photosynthesis was altered by K-glyoxylate, KClor TMA-glyoxylate. None of these salts was found to stimulatephotosynthesis in isolated cucumber mesophyll cells over a rangeof pH values although the cells incorporated as much ¹⁴C-glyoxylateas did leaf discs. The data suggest that enhanced photosynthesisin leaf discs is not due directly to a stimulation of mesophyllcell photosynthesis but rather is a consequence of increasedCO₂ availability and decreased stomatal resistance at low pHin the presence of potassium. Key words: Cucumber, Photosynthesis, Potassium glyoxylate     

Effect of NaCI Salinity on Flows and Partitioning of C, N, and Mineral Ions in Whole Plants of White Lupin, Lupinus albusL.

Plants of Lupinus albus L., cv. Ultra, were grown hydroponicallywith NO₃^–-nutrition for 51 d under control (0.05 mol m^–3Na⁺ and 10 mol m^–3 Cl^–) and saline (40 mol m^–3NaCI) conditions. Plants were harvested 41 and 51 d after germinationand analysed for content and net increment of C, N and the mineralcations K⁺, Na⁺, Mg²⁺, and Ca²⁺ and the anions Cl^–, NOJ,malate, phosphate, and SO₄^2–. Roots, stem interaodes,petioles and leaflets were analysed separately. During the studyperiod net photosynthesis, respiratory losses of CO₂ from shootand root and the composition of the spontaneously bleeding phloemsap and the root pressure xylem exudate were also determined.Using molar ratios of C over N in the transport fluids, incrementsof C and N, and photosynthetic gains as well as respiratorylosses of C, the net flows of C and N in the xylem and phloemwere then calculated as in earlier studies (Pate, Layzell andMcNeill, 1979a). Knowing the carbon flows, the ratios of ionto carbon in the phloem sap, and ion increments in individualorgans, net flows of K⁺, Na⁺, and Cl^– over the study periodwere also calculated. Salt stress led to a general decrease of all partial componentsof C and N partitioning indicating that inhibitions were notdue to specific effects of NaCI salinity on photosynthesis oron NO₃ uptake. However, there were differences between variouslyaged organs, and net phloem export of nitrogenous compoundsfrom ageing leaves was substantially enhanced under saline conditions.In addition, NO₃^–reduction in the roots was specificallyinhibited. Uptake and xylem transport of K⁺ was more severelyinhibited than photosynthetic carbon gain or NO₃^– uptakeby the root. K⁺ transport in the phloem was even more severelyrestricted under saline conditions. Na⁺ and Cl^– flowsand uptake, on the other hand, were substantially increasedin the presence of salt and, in particular, there were thenmassive flows of Na^– in the phloem. The results are discussedin relation to the causes of salt sensitivity of Lupinus albus.The data suggest that both a restriction of K⁺ supply and astrongly increased phloem translocation of Na⁺ contribute tothe adverse effects of salt in this species. Restriction ofK⁺ supply occurs by diminished K⁺ uptake and even more by reducedK⁺ cycling within the plant. Key words: Lupinus albus, salt stress, phloem transport, xylem transport, partitioning, carbon, nitrogen, K⁺, Na⁺, CI^–     

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     

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.     

Effects of Temperature and Light on the Carbon Budget of Young Cucumber Plants Studied by Steady-State Feeding with 14CO2

Shishido, Y., Challa, H. and Krupa, J. 1987. Effects of temperatureand light on the carbon budget of young cucumber plants studiedby steady-state feeding with ¹⁴CO°₂J. exp. Bot. 38: 1044–1054. The effect of temperature on the fate of ¹⁴C assimilated insteady-state by the expanding third leaf of cucumber seedlingswas studied at irradiances of either 30 or 75 W m^–2 (PAR)with a daylength of 8 h. The irradiance did not affect the relativedistribution of ¹⁴C assimilated by the source leaf between growth,respiration and export. In the range 15–30°C risesin temperature generally increased the proportion of carbonexported. The average rate of carbon exported during the nightwas about half the rate in the day. About 45% of the exportedcarbon was lost by respiration. The distribution pattern ofcarbon exported during the day differed considerably from thatof carbon exported during the night. The intensity of irradiance did not affect the proportion oflabelled carbon recovered from the roots. Thus the decreasedshoot/root ratio generally observed with increased irradianceis not directly controlled by carbohydrate supply. We found that the distribution patterns of exported ¹⁴C do notnecessarily represent the real carbon distribution, due to differencesin specific activity of imported carbon of individual organs.Consequently distribution patterns of ¹⁴C observed in experimentswith one source leaf have to be considered with caution. Key words: Carbon budget, ¹⁴C, ¹⁴C steady-state feeding, translocation, respiration, assimilate distribution, cucumber, temperature     

Growth at elevated CO2 leads to down-regulation of photosynthesis and altered response to high temperature in Quercus suber L. seedlings

The effects of growth at elevated CO₂ on the response to hightemperatures in terms of carbon assimilation (net photosynthesis,stomatal conductance, amount and activity of Rubisco, and concentrationsof total soluble sugars and starch) and of photochemistry (forexample, the efficiency of excitation energy captured by openphotosystem II reaction centres) were studied in cork oak (Quercussuber L.). Plants grown in elevated CO₂ (700 ppm) showed a down-regulationof photosynthesis and had lower amounts and activity of Rubiscothan plants grown at ambient CO₂ (350 ppm), after 14 monthsin the greenhouse. At that time plants were subjected to a heat-shocktreatment (4 h at 45C in a chamber with 80% relative humidityand 800–1000 mol m^–2 s^–1 photon flux density).Growth in a CO₂-enriched atmosphere seems to protect cork oakleaves from the short-term effects of high temperature. ElevatedCO₂ plants had positive net carbon uptake rates during the heatshock treatment whereas plants grown at ambient CO₂ showed negativerates. Moreover, recovery was faster in high CO₂-grown plantswhich, after 30 min at 25C, exhibited higher net carbon uptakerates and lower decreases in photosynthetic capacity (A_max aswell as in the efficiency of excitation energy captured by openphotosystem II reaction centres (F_vJF_m than plants grown atambient CO₂. The stomata of elevated CO₂ plants were also lessresponsive when exposed to high temperature. Key words: Elevated CO₂, temperature, acclimation, photosynthesis, Quercus suber L.     

A System for Measuring Effects of Sulphur Dioxide on Gas Exchange of Plants

Apparatus is described for exposing plants to low concentrationsof SO₂ (50–500µg m^–3 in air) and for measuringeffects on photosynthesis, dark respiration, and transpiration.Temperature, humidity, and irradiance in the chambers were controlledindependently, and fans ensured that leaf boundary layer resistanceswere low. Experiments with plants of Vicia faba in clean andpolluted air showed that: (i) a depression of net photosynthesisby 50 µg m^–3 SO₂ depended on boundary layer resistanceand on irradiance; (ii) stomatal resistance was increased ordecreased by 50 µg m^–3 SO₂ when relative humidityin the chambers was low (35% r.h., 22 °C) or high (50% r.h.,22 °C) respectively.     

Leaf Development in Lolium temulentum: Photosynthesis and Photosynthetic Proteins in Leaves Senescing under Different Irradiances

Changes in carbon fixation rate and the levels of photosyntheticproteins were measured in fourth leaves of Lolium temulentumgrown until full expansion at 360 µmol quanta m^–2s^–1 and subsequently at the same irradiance or shadedto 90 µmol m^–2 s^–1. Ribulose-1,5-bisphosphatecarboxylase/oxygenase (Rubisco), light-harvesting chlorophylla/b protein of photosystem II (LHCII), 65 kDa protein of photosystemI (PSI), cytochrome f (Cytf) and coupling factor 1 (CF₁) declinedsteadily in amount throughout senescence in unshaded leaves.In shaded leaves, however, the decrease in LHCII and the 65kDa protein was delayed until later in senescence whereas theamount of Cyt f protein decreased rapidly following transferto shade and was lower than that of unshaded leaves at the earlyand middle stages of senescence. Decreases in the Rubisco andCF₁ of shaded leaves occurred at slightly reduced rates comparedwith unshaded leaves. These results indicate that chloroplastproteins in fully-expanded leaves are controlled individually,in a direction appropriate to acclimate photosynthesis to agiven irradiance during senescence. (Received August 20, 1992; Accepted January 5, 1993)     

Temporal Patterns of Uptake, Flow and Utilization of Nitrate, Reduced Nitrogen and Carbon in a Leaf of Salt-Treated Castor Bean (Ricinus communis L.)

Changes in net photosynthesis, respiration, transpiration andcontents of total C, NO₃-N and reduced N were followed throughoutthe life of leaf 6 of nitrate-dependent plants of castor beanexposed to moderate salinity stress (71 mol m^–3 NaCl).Salt treatment was applied for measuring mineral flows in aparallel study (Jeschke and Pate, 1991b). Concurrent measurementswere made of solute composition and C: N molar ratios and concentrationsof reduced N and collected NO₃-N in phloem sap bleeding fromshallow incisions in the top and at the base of petioles andin xylem exudates from flaps of proximal leaf midribs followingpressurization of the root system. The resulting data were usedto construct empirical models of the respective economies ofC, total N, NO₃ and reduced N for a sequence of defined phasesof leaf life. Water use efficiency increased 3-fold from emergenceto a maximum of 1·5 mmol CO₂ mol^–1 H₂O before decliningto 0·5 mmol CO₂ mol^–1 H₂O at senescence. Xylemmolar ratios of C:N varied from 1·2–2·8,with nitrate always a smaller component than reduced N. Phloemsap C:N increased from 10–40 with leaf expansion and wasthen maintained in the range of 40–50 until falling steeplyto 20 at leaf senescence. Nitrate comprised less than 1% oftotal N in all phloem sap samples. The models of C uptake, flow,and utilization showed a major role of phloem import and thenincreasingly of laminar photosynthesis in providing C for leafgrowth. The carbon budget was thereafter characterized by ratesof phloem export closely matched to net rates of CO₂ fixationby the lamina. Corresponding data for total N depicted an earlymajor role of both xylem and phloem import, but the eventualdominance of xylem import as the N source for leaf growth. Cyclingof N by xylem to phloem exchange commenced before the leaf hadachieved maximum N content, and was the major contributor tophloem export until leaf senescence when mobilized N providedmost exported N. The nitrate economy of the leaf was characterizedby early establishment of tissue pools of the ion in the petioleand to a lesser extent in the lamina, continued high rates ofnitrate reduction in the lamina but negligible assimilationin the petiole, and a release through xylem of previously accumulatedNO₃ from petiole to lamina. Related data for reduced N illustratedthe much greater importance of this form of N than nitrate intransport, storage and cycling of N at all stages of leaf andpetiole life. Xylem to phloem interchanges of reduced N in petiolewere minimal in comparison with cycling through the lamina.The ratio of CO₂ reduction to NO₃ reduction in the lamina wasat first low (57 mol mol^–1) increasing to a peak valueof 294 during mature leaf functioning before declining to 190during the presenescence phase of leaf development. This patternreflected age-related effects on water use efficiency, changesin NO₃ levels in the xylem stream entering the lamina, and therelatively low photosynthetic performances of very young andsenescent laminae. Key words: Ricinus communis, leaf development, phloem transport, xylem transport, carbon, nitrogen, nitrate, reduced nitrogen, nitrate reduction, partitioning     

Photosynthesis of Stands of Tomato, Cucumber and Sweet Pepper Measured in Greenhouses under Various CO2-concentrations

The rate of net photosynthesis (P) of whole plant stands oftomato (Lycopersicon esculentum Mill.), cucumber (Cucumis sativusL.) and sweet pepper (Capsicum annuum L.) was measured in sixlong-term experiments in large greenhouses under normal operatingconditions and CO₂-concentrations between 200 and 1200 µmolmol^-1. The objective was to quantify the responses to lightand carbon dioxide and to obtain data sets for testing simulationmodels. The method of measuring canopy photosynthesis involvedan accurate estimation of the greenhouse CO₂ balance, usingnitrous oxide (N₂O) as tracer gas to determine, on-line, theexchange rate between greenhouse and outside air. The estimatedrelative error in the observed P was about ± 10%, exceptthat higher relative errors could occur under particular conditions. A regression equation relating P to the photosynthetically activeradiation, the CO₂ concentration and the leaf area index explained83-91% of the variance. The main canopy photosynthesis characteristicscalculated with the fitted regression equations were: canopyP_max 5-9 g m^-2 h^-1 CO₂ uptake; ratio P_max/LAI 1·5-3 gm^-2 h^-1; light compensation point 32-86 µmol s^-1 m^-2;light use efficiency (quantum yield) at low light 0·06-0·10µmol µmol^-1 and CO₂ compensation point 18-54 µmolmol^-1. The results were related to the prevailing conditions.Copyright1994, 1999 Academic Press Canopy photosynthesis, Capsicum annuum L., carbon dioxide, CO₂, CO₂ balance, CO₂ use efficiency, cucumber, _{Cucumis sativus} L., glasshouse, greenhouse, light use efficiency, Lycopersicon esculentum Mill., sweet pepper, tomato, tracer gas     

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