Growth Response of a Chrysanthemum Crop to the Environment. III, Effects of Radiation and Temperature on Dry Matter Partitioning and Photosynthesis

Vegetative crops of chrysanthemum were grown for 5 or 6 weekperiods in daylit assimilation chambers. Crop responses to differentradiation levels and temperatures were analysed into effectson dry matter partitioning, specific leaf area, leaf photosynthesisand canopy light interception. The percentage of newly formed dry matter partitioned to theleaves was almost constant, although with increasing radiationor decreasing temperature, a greater percentage of dry matterwas partitioned to stem tissue at the expense of root tissue.There was a positive correlation between the percentage of drymatter in shoot material and the overall carbon: dry matterratio. Canopy photosynthesis was analysed assuming identical behaviourfor all leaves in the crop. Leaf photochemical efficiency wasonly slightly affected by crop environment. The rate of grossphotosynthesis per unit leaf area at light saturation, P_A (max),increased with increasing radiation integral, but the same parameterexpressed per unit leaf dry matter, P_w (max) was almost unaffectedby growth radiation. In contrast, P_A (max) was hardly affectedby temperature but P_w (max) increased with increasing growthtemperature. This was because specific leaf area decreased withdecreasing temperature and increased with decreasing radiation.There was a positive correlation between canopy respirationintegral and photosynthesis integral, and despite a four-foldchange in crop mass during the experiments, the maintenancecomponent of canopy respiration remained small and constant. Canopy extinction coefficient showed no consistent variationwith radiation integral but was negatively correlated with temperature.This decrease in the efficiency of the canopy at interceptingradiation exactly cancelled the increase in specific carbonassimilation rate that occurred with increasing growth temperature,giving a growth rate depending solely on the incident lightlevel. Chrysanthemum, dry matter partitioning, photosynthesis, specific leaf area     

Effect of Temperature and Nitrogen Supply on the Growth of Perennial Ryegrass and White Clover. 1. Carbon and Nitrogen Economies of Mixed Swards at Low Temperature

Simulated mixed swards of perennial ryegrass (Lolium perenneL. cv. S23) and white clover (Trifolium repens L. cv. S100)were grown from seed under a constant 10°C day/8°C nighttemperature regime and their growth, and carbon and nitrogeneconomies examined. The swards received a nutrient solution,every second day, which contained either high (220 µgg^–1) or low (40 µg g^–1) nitrate N. The High-N swards had rates of canopy photosynthesis and drymatter production (over the linear phase of growth) similarto those previously shown by mixed swards at high temperature.The Low-N swards grew more slowly; canopy photosynthesis, ata given LAI, was similar to that at High-N but lower LAI's weresustained. Clover increased its contribution to total carbonuptake and total dry weight throughout the period in the Low-Ntreatment and, despite the fact that grass took up most of theavailable nitrate, clover maintained a consistently higher Ncontent by virtue of N₂-fixation. At High-N, grass dominated throughout the measurement period.Earlier, when plants grew as spaced individuals, clover grewless well than grass, but once the canopy was closed it hada similar relative growth rate and thus maintained a steadyproportion of total sward dry weight. It is proposed that earlyin the development of the crop, leaf area production is thelimiting factor for growth, and that in this respect cloveris adversely affected by low temperature relative to grass.Later, as the LAI of the crop builds up, and the canopy becomesfully light intercepting, net canopy photosynthesis plays amore dominant role and here the higher photosynthetic rate perunit leaf area of the clover is crucial. Trifolium repens, white clover, Lolium perenne, perennial ryegrass, low temperature, nitrogen, photosynthesis     

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     

Dry Matter Production in a Tomato Crop: Measurements and Simulation

Simulation of dry matter production by the explanatory glasshousecrop growth model SUKAM (Gijzen, 1992, Simulation Monographs),based on SUCROS87 (Spitters, Van Keulen and Van Kraalingen,1989, Simulation and systems management in crop protection),was validated for tomato. In the model, assimilation rates arecalculated separately for shaded and sunlit leaf area at differentcumulative leaf area in the canopy, taking into account thedifferent interception of direct and diffuse components of light.Daily crop gross assimilation rate (P_gd) is computed by integrationof these rates over total crop leaf area and over the day. Leafphotochemical efficiency and potential gross assimilation rateat saturating light depend on temperature and CO₂ concentrationand are approximated as being identical in the whole canopy.Crop growth results from P_gd minus maintenance respiration rate(R_m; dependent on temperature and crop dry weight), multipliedby the conversion efficiency (carbohydrates to structural drymatter; C_f). Growth experiments (periodic destructive harvest) with differentplanting dates and plant densities and two data-sets from commerciallygrown crops, were used for model validation. Hourly averagesfor global radiation outside the glasshouse, glasshouse temperatureand CO₂ concentration, together with measured leaf area index,dry matter distribution (for calculation of C_f) and organ dryweights (for calculation of R_m) were the inputs to the model. Dry matter production (both level and dynamic behaviour) wassimulated reasonably well for most experiments, but final drymatter production was under-estimated by about 27% for the commerciallygrown crops. At low irradiance and with large crop dry weight,growth rate was under-estimated, probably as a result of over-estimationof R_m. This could almost completely explain the large under-estimationfor the commercially grown crops, which had large dry weight.Final dry matter production was over-estimated by 7-11% if dailyaverages instead of hourly input of climatic data were used. It is concluded that SUKAM is a reliable model for simulatingdry matter production in a tomato crop, except for those situationswhere R_m has a large influence on crop growth rate (low irradianceand large crop dry weight). An improved estimate of R_m wouldtake into account the influence of metabolic activity. A preliminaryattempt to relate maintenance costs to relative growth rate(a measure for metabolic activity), showed promising results.Copyright1995, 1999 Academic Press Crop growth, dry matter production, glasshouse, maintenance respiration, metabolic activity, model, relative growth rate, respiration, simulation, tomato, model validation     

Canopy Photosynthesis and Crop Growth Rate of Eight Temperate Forage Grasses

The rates of canopy and individual leaf photosynthesis, ratesof growth of shoots and roots, and the extinction coefficientfor light of eight temperate forage grasses were determinedin the field during early autumn. Canopy gross photosynthesiswas calculated as net photosynthesis plus dark respiration adjustedfor temperature using a Q₁₀ = 2. The relationships between canopygross photosynthesis and light intensity were hyperbolic, andthe initial slopes of these curves indicated that light wasbeing utilized efficiently at low light intensities. The initialslope depended on the distribution of light in the canopy andthe quantum efficiency of the individual leaves. The maximumrate of canopy gross photosynthesis reflected the maximum rateof individual leaf photosynthesis. Although the maximum rateof canopy gross photosynthesis was correlated with crop growthrate, there was no significant relationship between daily grossphotosynthesis and crop growth rate. Indeed, daily gross photosynthesisvaried by only 22 per cent, whereas the daily growth of shootsand roots varied by 120 per cent. This poor correlation is influencedby a negative correlation (P < 0.01) between the maximumrate of canopy gross photosynthesis and the initial slope ofthe curve relating canopy gross photosynthesis and light intensity.Difficulties in the interpretation of measurements of dark respirationappeared to confound attempts to relate daily net photosynthesisto crop growth rate, individual leaf photosynthesis, and theextinction coefficient for light.     

A Generic Equation for Nitrogen-limited Leaf Area Index and its Application in Crop Growth Models for Predicting Leaf Senescence

Appropriate quantification of leaf area index (LAI) is importantfor accurate prediction of photosynthetic productivity by cropgrowth models. Estimation of LAI requires accurate modellingof leaf senescence. Many models use empirical turnover coefficients,the relative leaf-death rate determined from frequent fieldsamplings, to describe senescence during growth. In this paper,we first derive a generic equation for nitrogen-determined photosyntheticallyactive LAI (LAI_N), and then describe a method of using thisequation in crop growth models to predict leaf senescence. Basedon the theory that leaf-nitrogen at different horizons of acanopy declines exponentially, LAI_N, which is counted from thetop of the canopy to the depth at which leaf-nitrogen equalsthe minimum value for leaf photosynthesis, is calculated analyticallyas a function of canopy leaf-nitrogen content. At each time-stepof crop growth modelling, LAI_Nis compared to an independentcalculation of the non-nitrogen-limited LAI assuming no leafdeath during that time-step (LAI_NLD). In early stages, LAI_Nishigher than LAI_NLD; but with the advancement of crop growth,LAI_Nwill become smaller than LAI_NLD. The difference betweenLAI_NLDand LAI_N, whenever LAI_Nis smaller than LAI_NLD, gives theestimate of leaf area senesced at the time-step; the senescedleaf area divided by specific leaf area (SLA) gives the estimateof senesced leaf mass. The method was incorporated into twocrop models and the models adequately accounted for the LAIobserved in field experiments for rice and barley. The novelfeatures of the approach are that: (1) it suggests a coherent,biologically reasonable picture of leaf senescence based onthe link with photosynthesis and leaf nitrogen content; (2)it avoids the use of empirical leaf-turnover coefficients; (3)it avoids over-sensitivity of LAI prediction to SLA; and (4)it is presumably of sufficient generality as to be applicableto plant types other than crops. The method can be applied tomodels where leaf-nitrogen is used as an input variable or issimulated explicitly. Copyright 2000 Annals of Botany Company Leaf area index, leaf senescence, canopy nitrogen, modelling     

Nitrogen Deficiency in Small Closed Communities of S24 Ryegrass. I. Photosynthesis, Respiration, Dry Matter Production and Partition

Small communities of S24 ryegrass were grown under supplementarylights in a glasshouse at 20°C, and abundantly suppliedwith a complete nutrient solution containing 300 p.p.m. of nitrogen,until they had a leaf area index of 5 and fully interceptedthe light. Half were then given a solution containing only 3p.p.m. of nitrogen (LN) while the rest were kept at 300 p.p.m.(HN). The LN plants had a rate of single leaf photosynthesis lowerthan that of the HN plants at all but the lowest light intensities(33 per cent lower at the saturating irradiance of 170 W m^–2).Similarly, the LN communities had rates of canopy gross photosynthesis(P_sc) markedly lower than those of the HN communities. A comparisonof the observed rates of P_sc with those predicted by a mathematicalmodel of canopy photosynthesis indicated that it was the effectof nitrogen on single leaf photosynthesis, rather than differencesbetween the communities in leaf area, which led to the observeddifferences in P_sc. The superiority of the HN communities in terms of P_sc was partlyoffset by a higher rate of respiration so that they only exceededthe LN communities in terms of canopy net photosynthesis atirradiances in excess of 180 W m_–2, and produced only15 per cent more total dry matter. Nevertheless, the HN plantsdirected less of that dry matter into root and more into topsso that they came to possess twice the weight of live laminae,and the HN communities twice the leaf area, of their nitrogendeficient counterparts. Lolium perenne, S24 ryegrass, photosynthesis, respiration, dry matter production and partition, nitrogen dekieacy     

Evaluation of a Dynamic Simulation Model for Tomato Crop Growth and Development

A dynamic simulation model for tomato crop growth and development,TOMSIM, is evaluated. Potential crop growth and daily crop grossassimilation rate (P_gc,d) is computed by integration of leafassimilation rates over total crop leaf area throughout theday. Crop growth results fromP_gc,dminus maintenance respirationrate (R_m), multiplied by the conversion efficiency. Dry matterdistribution is simulated, based on the sink strength of theplant organs, which is quantified by their potential growthrate. Within the plant, individual fruit trusses and vegetativeunits (three leaves and stem internodes between two trusses)are distinguished. Sink strength of a truss or a vegetativeunit is described as a function of its developmental stage.In this paper, emphasis is on the interactions between the twosubmodels of, respectively, dry matter production and dry matterdistribution. Sensitivity analysis showed that global radiation,CO₂concentration, specific leaf area (SLA) and the developmentalstage of a vegetative unit at leaf pruning had a large influenceon crop growth rate, whereas temperature, number of fruits pertruss, sink strength of a vegetative unit and plant densitywere less important. Leaf area index (LAI) was very sensitiveto SLA and the developmental stage of a vegetative unit at leafpruning. Temperature did not influence the simulated R_m, asincreased respiration rate per unit of biomass at higher temperatureswas compensated by a decrease in biomass. The model was validatedfor four glasshouse experiments with plant density and fruitpruning treatments, and on data from two commercially growncrops. In general, measured and simulated crop growth ratesfrom 1 month after planting onwards agreed reasonably well,average overestimation being 12%. However, crop growth ratesin the first month after planting were overestimated by 52%on average. Final crop dry mass was overestimated by 0–31%,due to inaccurate simulation of LAI, resulting partly from inaccurateSLA prediction, which is especially important at low plant densityand in a young crop.Copyright 1999 Annals of Botany Company Crop growth, dry matter production, glasshouse, leaf area,Lycopersicon esculentum, partitioning, simulation model, tomato, TOMSIM.     

The Relationship Between Photosynthesis and Tuber Growth in Solanum tuberosum L.

Measurements of the rate of photosynthesis of plants of Solanumtuberosum L. var. King Edward were made, using ¹⁴CO₂, at weeklyintervals throughout their growth in a controlled environment.Leaf area and dry weight of sections of the plant were alsodetermined. The results are discussed in relation to existingtheories that photosynthesis can be limited by carbohydrateaccumulation in the leaves, and stimulated by the initiationof tubers.     

The Contribution of Leaves from Different Levels within a Tomato Crop to Canopy Net Photosynthesis: An Experimental Examination of Two Canopy Models

The rates of net photosynthesis per unit ground area by a closedcanopy of tomato plants were measured over a range of naturallight flux densities. The canopy, of leaf area index 8.6, wasdivided into three horizontal layers of equal depth. On successivedays the canopy was progressively defoliated in layers fromthe ground upwards, allowing the photosynthetic contributionfrom individual leaf layers to be determined. The uppermostlayer, 23% of the total leaf area, assimilated 66% of the netCO₂ fixed by the canopy and accounted for a similar percentageof the total leaf respiration. Net photosynthesis versus light response curves for individualleaves from different positions within the canopy were alsoobtained. Leaf conductances to CO₂ transfer and the dark respirationrates of leaves from the uppermost leaf layer were approximatelyten times those from the lowest layer. The canopy data were analysed using a simple model which assumedthat the canopy was composed of leaves with identical photosyntheticand respiratory characteristics. The model fitted the data andallowed the characteristics of an ‘idealized’ leafto be estimated. The estimated values of the leaf light utilizationefficiency, ,and the leaf conductance CO₂ transfer, , were similarto values directly determined for individual leaves in the uppermostleaf layer and the estimated rate of leaf dark respiration,R_d, corresponded to measured rates for leaves much lower inthe canopy. The simple model may be used to examine gross effectsof crop environment on the leaf photosynthetic characteristicof an ‘idealized’ leaf, but cannot be used to predictaccurately canopy net photosynthesis from the photosyntheticand respiratory characteristics of any single real leaf. A moredetailed model, developed to allow explicitly for the observedvariation in and R_d within the canopy is appropriate for thispurpose.     

Seasonal Changes in the Physiology of S24 Perennial Ryegrass (Lolium perenne L.): 4. Comparison of the Carbon Balance of the Reproductive Crop in Spring and the Vegetative Crop in Autumn

Measurements of CO₂ exchange were used to construct a detailedaccount of the carbon economy of established simulated swardsof perennial ryegrass during 10 week periods in spring and autumn.Changes in sward dry weight estimated from gas exchange measurementsclosely matched observed changes in dry weight. In spring, light energy increased, the photosynthetic potentialof the canopy increased, and together these factors led to apattern of increasing photosynthetic uptake. In autumn, decreasinglight energy and decreasing canopy photosynthetic potentialled to decreasing photosynthetic uptake. During the periodsinvestigated, the changes in light energy receipt played themajor role in determining the pattern of photosynthetic uptake. A simple model of crop growth was used to illustrate the effectof such characteristic seasonal differences in the pattern ofphotosynthetic uptake on the subsequent loss of carbon duringrespiration and tissue death, and consequently on the productionof live tissue. The model describes how a reproductive cropin spring may accumulate more living dry matter than a vegetativecrop in autumn from the same total gross photosynthetic uptakeof carbon. Lolium perenne L., ryegrass, carbon economy, photosynthesis, respiration     

An Analysis of the Photosynthesis and Productivity of Vegetative Crops in the United Kingdom

The rate of total dry matter production of a vegetative crop,under optimal water and nutrient regimens is related to someleaf and canopy photosynthetic characteristics. Three leaf photosyntheticcharacteristics are examined in detail: the light utilizationefficiency at normal ambient CO₂ and O₂ concentrations, a, therate of light saturated photosynthesis per unit leaf area, F_max,and the ratio of the rates of photorespira tion and gross photosynthesis.The genetic variability in each of these characteristics issought from published data on a wide range of C₃ and C₄ planttypes. Within C₃ and C₄ plant types there are significant genetic differencesonly in F_max,, although differences exist between C₃ and C₄plants in the other two characteristics. The effects of thesedifferences on the rate total dry matter production are estimated,and it is concluded that there is no compelling evidence toindicate that improvements in total dry matter production rates,in the U.K., are likely to result from genetic manipulationof these characteristics in the existing range of plant material.     

Analysis of Growth, Photosynthesis and Light Interception for Single Plants and Stands

The two traditionally distinct treatments of growth analysis,using either relative growth rate (RGR) or crop growth rate(CGR), can be integrated in a single treatment in which RGRis a component of CGR. CGR can alternatively be analysed asthe product of incident light receipt, efficiency of light interception(as determined by leaf area index and extinction coefficient)and efficiency of use of intercepted light in dry-matter productionor in canopy net photosynthesis. Further, the net assimilationrate or net photosynthetic rate can be resolved into two componentswhich quantify the dependence of light interception on leafarea and of CO₂ assimilation on intercepted light. These relationsprovide increased flexibility in the analysis of assimilationand growth in terms of light interception and the structureof plants and stands. The usefulness of growth analysis in elucidatingphysiological mechanisms is discussed in relation to the useof more complex mechanistic models. Crop growth rate, light interception, growth analysis, leaf area index     

Validation of a Photosynthesis Model through the Use of the CO2Balance of a Greenhouse Tomato Canopy

Several leaf photosynthesis models were developed from wellcontrolled experiments in growth chambers. However, only a fewhave been validated under greenhouse conditions for their quantitativeand qualitative adequacy. In this paper, rates of net photosynthesisfor a tomato crop (Lycopersicon esculentum Mill) were measuredin a semi-commercial greenhouse (615 m³) for a significant timeperiod. Concomitant measurements of climatic conditions andLAI were used for simulation of net photosynthesis using theTOMGRO model which integrates Acock's model for photosynthesiscalculations. From simulations and from sensitivity analysis,the prediction of net photosynthesis appeared to be very sensitiveto the quantum use efficiency. The Acock model with originalparameters underestimated the net photosynthesis rate, but anincrease in the quantum use efficiency by 10% gave a good fit.In an effort to generalize the validity of the model, a residualanalysis was performed and showed a systematic bias relatedto light intensity intercepted by the canopy. The Marquardtalgorithm was used to adjust our data to the model but did noteliminate residual heterogeneity of variance with new parametervalues. On the basis of collected data, the criteria of goodnessof fit used showed that the photosynthesis model is inadequatein describing the CO₂-balance of the greenhouse agrosystem.However, it was determined that it could be used as a submodelwithin a more complex model for predicting growth and development.Copyright 1999 Annals of Botany Company Greenhouse, CO₂-balance, photosynthesis, TOMGRO model, Acock's model, residuals, tomato Lycopersicon esculentum Mill.     

Use of the Expolinear Growth Model to Analyse the Growth of Faba bean, Peas and Lentils at Three Densities: Fitting the Model

The expolinear equation for crop growth (Goudriaan and MonteithAnnalsof Botany66: 695–701, 1990) was fitted to measurementsof above ground dry weight made on two cultivars of each ofthree species, faba bean (Vicia fabaL.), peas (Pisum sativumL.)and lentils (Lens culinarsMedic.), each grown at three densitiesat the University of Reading, UK in 1992 and 1993. The expolinearequation fitted the data well but required frequent samplingto obtain good estimates of the parameters. The equation hasthree parameters,R_mthe maximum relative growth rate,C_ma maximumcrop growth rate, andt_bthe time at which the crop effectivelyreaches a linear phase of growth.R_mdid not differ between densities,cultivars or species but differed between years.C_mincreasedwith increased density and was lower for lentils than for fababeans or peas.t_bdecreased with increased density for faba beanbut not for the other species. Incorporating an extinction coefficientfor solar radiation and the maximum fraction of radiation interceptedenabled reasonably accurate time courses of leaf area indexto be derived, as suggested by Goudriaan (1994. In: MontiethJL, Scott RK, Unsworth MH, eds.Resource capture by crops. Nottingham:Nottingham University Press, 99–110).Copyright 1998 Annalsof Botany Company Expolinear equation, grain legumes, crop growth rate, crop density, relative growth rate, growth modelling, faba bean,Vicia fabaL., peas,Pisum sativumL., lentils,Lens culinarsMedic.     

The Growth and Development of Simulated Swards of Perennial Ryegrass: II. Carbon Assimilation and Respiration in a Seedling Sward

The rates of net photosynthesis (P_n,c) in the light (85 W m^–2visible), and respiration in the dark, of a simulated swardof S24 ryegrass were measured for 12 weeks during its developmentfrom a collection of two-leaved seedlings to a closed canopywith an LAI of 23 (15 of green leaf laminae). By the sixth week light interception was complete (LAI = 10.6)and P_n,c had risen to 24 mg CO₂ dm^–2 h^–1, similarto rates recorded in the field. Photosynthetic functions (lightresponse curves) showed that the swards remained unsaturatedup to energy receipts of almost 400 W m^–2, whereas singleleaves were light saturated at about 130 W m^–2. Earlyin the development of the sward LAI had a greater effect onP_n,c than radiation receipt, later the reverse was true. Thegrowth habit of the sward ranged from moderately erect (an Svalue of 0.72) to moderately prostrate (‘S’ = 0.37),while the ability of the two youngest fully expanded leaveson a tiller to make use of light in photosynthesis declinedas the sward increased in density from values of A max of 20to 5 mg CO₂ dm^–2 h^–1. By varying the values of Sand A max fed into a model of canopy photosynthesis, withinthe above limits, it was demonstrated that, in practice, A maxis a greater determinant of canopy photosynthesis than S, exceptat low LAI where a prostrate sward has a marked advantage overan erect one. The rate of dark respiration rose as the swards increased inweight, although not in proportion to it, until the ninth weekwhen a ceiling yield of live plant tissue was reached. Respiratorylosses from the sward came almost equally from a component associatedwith maintenance (R_m) and one associated with growth (R_g). Therate of Rm was estimated to be about 0.014 g day^–1 pergram of plant tissue, and that of Ra about 0.25 g per gram ofnew tissue produced—both close to theoretical values.The measured dry matter production curve of the swards was comparedwith that estimated from the gas analysis data. Similarly therates of gross photosynthesis estimated from the gas analysisdata were compared with the predictions of the mathematicalmodel. In both cases the fit was reasonably good. A balancesheet was drawn up; of every 100 units of carbon fixed, 45 werelost in respiration and 16 as dead leaf, 5 ended up in the rootand 6 in the stubble; only 28 remained as harvestable live leaftissue.     

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 Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 1. Grown as Simulated Swards During a Regrowth Period

Simulated swards of each of two selection lines of Lolium perennecv. S23 with ‘fast’ and ‘slow’ ratesof ‘mature tissue’ respiration were establishedin growth rooms at 20/15 °C day/night temperatures and studiedover four successive regrowth periods of 46, 30, 26 and 53 daysduration. The ‘slow’ line outyielded the ‘fast’,both in harvestable shoot (above a 5 cm cut) and in root andstubble. Its advantage increased over successive regrowth periodsto 23 per cent (total biomass). Gas analysis measurements onthe entire communities (including roots), during the final regrowthperiod, showed that the ‘slow’ line had a 22–34per cent lower rate of dark respiration per unit dry weight.This enabled it to maintain its greater mass of tissue for thesame cost in terms of CO₂ efflux per unit ground area. Halfthe extra dry weight produced by the ‘slow’ line,relative to the ‘fast’, could be attributed to itsmore economic use of carbon. The rest could be traced to a 25per cent greater tiller number which enabled the ‘slow’line to expand leaf area faster (though not at a greater rateper tiller), intercept more light and fix more carbon, earlyin the regrowth period. Lolium perenne L., ryegrass, respiration, maintenance respiration, tiller production, simulated swards, canopy photosynthesis, carbon economy     

Growth Response of a Chrysanthemum Crop to the Environment. I. Experimental Techniques

The construction and mode of operation of six daylit assimilationchambers and the methods used to measure canopy photosynthesisand radiation interception are described. The chambers havea cuboid plant space, with sides 1.2 m long, in which temperaturecan be controlled from ambient to 30 °C with a temporalvariation of ±0.5 °C. Conductimetric controllersmaintain CO₂ concentration in the chambers within ±5per cent of the desired values. The amount of CO₂ injected andother variables needed to relate mean CO₂ assimilation ratesto mean radiation flux density over successive 10 min intervalsthroughout the day are recorded on punched tape for subsequentcomputer processing. The chambers have limitations in the number,range and variability of the environmental factors controlledbut they cost approximately one tenth as much as commercialdaylit cabinets and provide adequate, reproducible data formodelling many aspects of crop growth.     

Growth of Lettuce, Onion, and Red Beet. 1. Growth Analysis, Light Interception, and Radiation Use Efficiency

A field experiment was carried out to analyse the growth oflettuce, onion and red beet in terms of: (a) canopy architecture,radiation interception and absorption; (b) efficiency of conversionof absorbed radiation into biomass; and (c) dry matter partitioning.Growth analysis, total solar radiation interception, PAR interceptionand absorption by the crop canopy, ground cover, maintenancerespiration of onion bulbs and red beet storage roots were measured.Models for different leaf angle distribution and ground coverwere used to simulate light transmission by the crop canopy. The three crops are shown to have contrasting growth patternsfrom both a morphological and a physiological point of view.Lettuce showed very high light interception and growth afterthe early growth stages but, throughout the growth cycle, thisleafy crop showed the lowest radiation use efficiency due tothe respirational cost of the high leaf area. Onion showed alower early relative growth rate than lettuce and red beet.This was due partly to the low light interception per unit leafarea in the later stages of growth and partly to the low initialradiation use efficiency compared with the other two crops.On the other hand, thanks to more uniform distribution of theradiation inside the canopy, to the earlier termination of leafdevelopment and to the very low level of bulb respiration, onionshowed high radiation use efficiency and was able to producea large amount of dry matter. Red beet leaf posture and canopystructure resulted in high light interception and absorption.Its radiation use efficiency was lower than that of onion, partlyperhaps because of the more adverse distribution of the interceptedradiation fluxes within the canopy and partly because of thehigh respiration cost of a continuous dry-matter allocationto the leaves. However, this crop can accumulate a very largeamount of dry matter as leaf blade development and storage rootgrowth can both continue almost indefinitely, providing continuouslyavailable sinks. Ground cover gave a good estimate of the PAR interception onlyat low values of light interception but, in general, it underestimatedPAR interception in all three crops. Ratios between attenuationcoefficients established by considering PAR or total solar radiationand LAI or ground cover were calculated. Lettuce,Lactuca sativa L. var.crispa ; onion,Allium cepa L.; red beet; Beta vulgaris L. var.conditiva ; growth analysis; light interception and absorption; canopy architecture; ground cover; radiation use efficiency; maintenance respiration rate; dry matter distribution