Modelling the Components of Plant Respiration: Representation and Realism

This paper outlines the different ways in which plant respirationis modelled, with reference to the principles set out in Cannelland Thornley (Annals of Botany85: 55–67, 2000), firstin whole-plant ‘toy’ models, then within ecosystemor crop models using the growth-maintenance paradigm, and finallyrepresenting many component processes within the Hurley Pasture(HPM) and Edinburgh Forest Models (EFM), both of which separateC and N substrates from structure. Whole-plant models can beformulated so that either maintenance or growth respirationtake priority for assimilates, or so that growth respirationis the difference between total respiration and maintenanceassociated with the resynthesis of degraded tissues. All threeschemes can be converted to dynamic models which give similar,reasonable predictions of plant growth and respiration, butall have limiting assumptions and scope. Ecosystem and cropmodels which use the growth-maintenance respiration paradigmwithout separating substrates from structure, implicitly assumethat maintenance respiration is a fixed cost, uncoupled to assimilatesupply, and use fixed rate coefficients chosen from a rangeof measured values. Separation of substrates in the HPM andEFM enables estimates to be made of respiration associated withlocal growth, phloem loading, ammonium and nitrate N uptake,nitrate reduction, N₂fixation and other mineral ion uptake,leaving a ‘residual maintenance’ term. The lattercan be explicitly related to C substrate supply. Simulated changesin grassland respiration over a season and forest respirationover a rotation show that the ratio of total respiration togross canopy photosynthesis varies within the expected limitedrange, that residual maintenance accounts for 46–48% oftotal respiration, growth 36–42%, phloem loading 10–12%and the other components for the small remainder, with the ratiosbetween components varying during a season or forest rotation.It is concluded that the growth-maintenance approach to respiration,extended to represent many of the component processes, has considerablemerit. It can be connected to reality at many points, it givesmore information, it can be examined at the level of assumptionsas well as at the level of predictions, and it is open to modificationas more knowledge emerges. However, currently, there are stillparameters that require adjustment so that the predictions ofthe model are acceptable. Copyright 2000 Annals of Botany Company Respiration, photosynthesis, growth, maintenance, substrate, N uptake, mineral uptake, phloem loading, model.     

Pattern of Respiration of a Perennial Ryegrass Crop in the Field

‘Dark’ respiratory losses of CO₂ were measured ona one year old sward of S24 perennial ryegrass (Lolium perenneL.) at intervals during a 74 day reproductive growth period,between April and June, and a 21 day vegetative growth period,in July and August. Part of the sward was shaded for one weekbefore measure ments commenced. Measurements of ‘dark’respiration continued for 46 hand it was possible to distinguishtwo components which are designated ‘maintenance’and ‘synthetic’ ‘Maintenance’ respiration was taken to be the meanrate of CO₂ efflux after 40–46 h darkness. When calculatedon a plant d. wt basis at 15°C it ranged between 6 to 32mgCO₂ g^-1 day^-1 during reproductive growth and 10–14 mgCO₂ g^-1 day^-1 during vegetative growth. During reproductivegrowth, sward protein content ranged between 7–23 percent and when maintenance respiration was recalculated on thebasis of protein content it changed relatively little throughoutthe growth period (90–140 mg CO₂ g pro tein^-1 day^-1);the value for vegetative growth ranged between 70–100mgCO₂ g protein-day^-1. Total ‘synthetic’ CO₂ flux was determined duringreproductive growth and a rate of ‘synthetic’ CO₂flux was determined during both reproductive and vegetativegrowth. Between 15 and 35 per cent of the CO₂ fixed in the previousphotoperiod was lost in ‘synthetic’ respirationof above-ground material in reproductive swards. Previous shadingincreased the proportion of ‘synthetic’ CO₂ lossfrom above ground. The rate of ‘synthetic’ CO₂ outputduring the first hours of darkness increased with amount ofCO₂ fixed in the previous photoperiod, although it was not proportionalto it. There is some evidence that assimilate is ‘carried-over’from one photoperiod to the next.     

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     

Defoliation in White Clover: Nodule Metabolism, Nodule Growth and Maintenance, and Nitrogenase Functioning During Growth and Regrowth

In two experiments, the functioning and metabolism of nodulesof white clover, following a defoliation which removed abouthalf the shoot tissue, were compared with those of undefoliatedplants. In one experiment, the specific respiration rates of nodulesfrom undefoliated plants varied between 1160 and 1830 µmolCO₂ g^–1h^–1, of which nodule ‘growth and maintenance’accounted for 22 ± 2 per cent, or 27 ± 3.6 percent, according to method of calculation. Defoliation reducedspecific nodule respiration and nodule ‘growth and maintenance’respiration by 60–70 per cent, and rate of N₂ fixationby a similar proportion. The original rate of nodule metabolismwas re-established after about 5 d of regrowth; during regrowthnodule respiration was quantitatively related to rate of N₂,fixation: 9.1 µmol CO₂ µmol^–1N₂. With the possible exception of nodules examined 24 h after defoliation,the efficiency of energy utilization in nitrogenase functioningin both experiments was the same in defoliated and undefoliatedplants: 2.0±0.1 µmol CO₂ µmol^–1 C₂H₄;similarly, there was no change in the efficiency of nitrogenasefunctioning as rate of N₂ fixation increased with plant growthfrom 1 to 22 µmol N₂ per plant h^–1. Exposure of nodulated white clover root systems to a 10 percent acetylene gas mixture resulted in a sharp peak in rateof ethylene production after 1.5–2.5 min; subsequently,rate of ethylene production declined rapidly before stabilisingafter 0.5–1 h at a rate about 50 per cent of that initiallyobserved. Regression of ‘peak’ rate of ethyleneproduction on rate of N₂ fixation indicated a value of 2.9 µmolC₂H₄ µmol^–1 N₂, for rates of N₂ fixation between1 and 22 µmol N₂ per plant h^–1. The relationshipsbetween nitrogenase respiration, acetylene reduction rates andN₂ fixation rates are discussed. Trifolium repens, white clover, defoliation, nodule respiration, N₂, fixation, nitrogenase     

Growth and Assimilate Conversion of Cotton Bolls (Gossypium hirsutum L.) 1. Growth of Fruits: and Substrate Demand

A generalized growth pattern of cotton bolls and their componentsis derived from data available in the literature. This patternis used to calculate substrate requirements for dry matter accumulationand results in an estimated consumption of 138.5 g of (CH₂O)and 15.4 g of amino acids per 100 g of mature boll dry matteromitting maintenance respiration. For maintenance respirationat 12 h day and 12 h night temperature of 30 and 20 °C respectivelyand a boll maturation period of 50 days, 26.9 g CH₂O per 100g of mature bolls is found. The rate is considerably higherin the earlier phases of boll development when primarily ‘structuralgrowth’ occurs compared with later phases when ‘storagegrowth’ prevails.     

The Sensitivity of Net Photosynthesis in Several Plant Species to Short-term Fumigation with Sulphur Dioxide

The effects of exposure of up to 2 h with sulphur dioxide ona range of plant species was observed by measuring changes inthe rate of net photosynthesis under closely controlled environmentalconditions. Ryegrass, Lolium perenne ‘S23’ was thespecies most sensitive to SO₂; significant inhibition was detectedat 200 nl l^–1. Fumigations at 300 nl l^–1 also inhibitedphotosynthesis in field bean (Vicia faba cv. ‘Three FoldWhite’ and ‘Blaze’) and in barley (Hordeumvulgare cv. ‘Sonja’). No effect was detected inwheat (Triticum aestivum cv. ‘Virtue’) at concentrationsup to 600 nl l^–1 SO₂, or in oil-seed rape (Brassica napuscv. ‘Rafal’) except at 800 nl l^–1 SO₂). Recoverycommenced immediately after the fumigation was terminated andwas complete within 2 h when inhibition had not exceeded 20%during the SO₂ treatment. Key words: Sulphur dioxide, short-term fumigation, photosynthesis     

The Influence of the Plant Growth-regulator, 2-Methyl-4-Chlorophenoxyacetic Acid, on the Metabolism of Carbohydrate, Nitrogen and Minerals in Solanum lycopersicum (Tomato)

The effects of the foliar application of phytocidal concentrationsof 2-methyl-4-chlorophenoxyacetic acid (MCPA) on change in totaldry weight, and in ‘available carbohydrate’ (starch,‘total’ and ‘reducing’ sugars), totalnitrogen, phosphorus, potassium, calcium, and magnesium of ‘tops’and roots of tomato plants have been followed over a periodof 14 days following spraying. There were two main treatments—‘nutrient’(nutrient supply to roots continued after spraying) and ‘water’(distilled water only supplied to roots after spraying) and‘water’ (distilled water only supplied to rootsafter spraying)—the sub-treatments consisting of ‘MCPA’versus ‘no-MCPA’ for each of the main treatments.Twelve different times of sampling were used. In analysing the present data, the quantity ‘residualdry weight’ (total dry weight less ‘available carbohydrate’),which was originally introduced by Mason and Maskell as a basisof reference for analyses of plant organs in short-period experimentsnot involving appreciable growth, has been used as an estimateof the permanent structure of plant growth. This new use ofthe ‘residual dry weight’ basis has brought outimportant features which were obscured when the data were leftin their primary form (as percentages of total dry weight oramounts per plant). Growth, as measured by increase in ‘residual dry weight’,was greatly inhibited by 2-methyl-4-chlorophenoxyacetic acidshortly after spraying, in both the presence and the absenceof nutrient. In the presence of 2-methyl-4-chlorophenoxyacetic acid, netassimilation rate (estimated as rate of increase in total dryweight per gram ‘residual dry weight’ of the ‘tops’)was greatly diminished while uptake of total nitrogen and ofP₂O₅ (estimated as increase in total nitrogen or of P₂O₅ ofthe whole plant per day per 1 g. ‘residual dry weight’of the roots) appeared to undergo a similar but much smallerdiminution. It seemed probable, however, that in the presenceof MCPA a larger proportion of the carbohydrate actually formedwas utilized for synthesis of aminoacids and protein. In the plant as a whole there was no evidence of actual depletionof ‘available carbohydrate’ as a result of MCPAtreatment, this fraction showing a steady increase in all treatmentsthroughout the experiment. The rate of increase was, however,much reduced by MCPA treatment. The ‘tops’ presentedmuch the same picture as the whole plant, but for the rootsthe situation was quite different. While the roots of the ‘no-MCPA’plants and also of the ‘MCPA-water’ plants showeda steady increase in available carbohydrate, those of the ‘MCPA-nutrient’plants rose only very slightly (from the initial value of 8mg. per plant to about 10 mg.) during the first 2 days, andthen in the next 2 days declined to a value (about 6 mg.) belowthe initial and remained at this low level for the rest of theexperiment. It is suggested that the phytocidal effect of 2-methyl-4-chlorophenoxyaceticacid in the presence of nutrient may be due to depletion ofthe ‘available carbohydrate’ supplies in the roots,which is shown to be brought about, in part, by reduced transportfrom the tops, and partly by the relatively greater utilizationof the carbohydrate present. These results offer an explanationfor the facts that plants showing vigorous growth are more easilykilled by MCPA and that perennial plants, particularly thosewith storage tissues in their roots, are more resistant. Further,they suggest the useful practical application that MCPA treatmentshould be given when the carbohydrate reserves of the rootsare at a minimum. For perennial plants, conditions might beexpected to be optimal for the application of MCPA in late spring,at a time when the first ‘flush’ of growth is slowingdown and before any appreciable new reserves of carbohydratehave been accumulated. It was also shown that 2-methyl-4-chlorophenoxyacetic acid preventedthe net synthesis of starch, but still permitted an appreciablenet formation of sucrose. 2-methyl-4-chlorophenoxyacetic acid appeared to have no effecton the uptake of potassium, calcium, or of magnesium. The lackof effect on potassium is contrasted with the previous observationby Rhodes, Templeman, and Thruston (1950) that sub-lethal concentrationsof MCPA, applied over a relatively long period to the rootsof tomato plants, specifically depressed the uptake of potassium.     

Defoliation in White Clover: Regrowth, Photosynthesis and N2 Fixation

Single plants of white clover, grown in a controlled environmentand dependent for nitrogen on fixation in their root nodules,were defoliated once by removing approximately half their shoottissue. Their regrowth was compared with the growth of comparableundefoliated plants. Two similar experiments were carried out:in the first, plants were defoliated at 2.5 g, and in the secondat 1.2 g total plant d. wt. Defoliation reduced rate of N₂ fixation by > 70 per cent,rate of photosynthesis by 83–96 per cent, and rate ofplant respiration by 30–40 per cent. Nodule weights initiallydeclined following defoliation as a result of loss of carbohydratesand other unidentified components. No immediate shedding ofnodules was observed but nodules on the most severely defoliatedplants exhibited accelerated senescence. The original rates of N₂ fixation were re-attained after 5–6or 9 d regrowth, with increase in plant size at defoliation.In general, the rate of recovery of N₂ fixation was relatedto the re-establishment and increase of the plant's photosyntheticcapacity. Throughout the growth of both defoliated and undefoliatedplants nodule respiration (metabolism) accounted for at least23 ± 2 per cent of gross photosynthesis. The unit ‘cost’of fixing N₂ in root nodules, in terms of photosynthate, appearedto be unaffected by defoliation, except perhaps for plants veryrecently defoliated. Similarly, the percentage nitrogen contentsof shoot, root and nodules of defoliated plants became adaptedwithin a few days to those characteristic of undefoliated plants. Trifolium repens, white clover, N₂ fixation, defoliation, photosynthesis, respiration     

Algal 14C and total carbon metabolisms. 1. Models to account for the physiological processes of respiration and recycling

A consistent set of equations has been written to describe thenet rate of algal ¹⁴CO₂ uptake (and where appropriate respirationand photosynthesis) which take into account separately complicationsdue to respiration of the labelled photosynthetic products andthe recycling of respiratory CO₂. Written specifically intothe equations is the concept of ‘new’ and ‘old’carbon, the coefficient q is used in the respiration model toallow for the differential respiration of organic material fromthe ‘new’ and ‘old’ carbon pools. Analyticalintegrals have been found for respiration and recycling models,and the behaviour of the models studied over periods of 12 h(i.e. up to 70% of the intrinsic generation time). The rateconstant for respiration has a greater effect on the behaviourof the recycling than the respiration model. Over short timecourses (up to 30% of the intrinsic generation time), the effectsof respiration and recycling on net ¹⁴CO₂ uptake are quite distinct,especially at high P/R ratios, and not complicated by assumptionsover the value of q. Although the value of q will have a time-dependentsecondary effect on the modelled total carbon-specific respirationrate, this was found not to give rise to major problems of interpretation.Beyond 50% of the intrinsic generation time, the separate treatmentof respiration and recycling in the models becomes less satisfactory.It was concluded that the present equations, which are not constrainedby mass balance considerations, would not be appropriate fora model that combines the two processes. The pattern of recyclingat low P/R values is identified as one of the major uncertaintiesin producing models of ¹⁴C uptake. The effect of the releaseof dissolved organic material can be anticipated in a generalway. The models have been used to define an experimental strategyto establish the separate effects of respiration and recyclingon the time course of net ¹⁴C uptake. The initial rates givethe clearest resolution of the two processes and it would appearthat with photosynthetic rates in the region of 1 day^–1,incubation periods up to 3–6 h would be suitable to determinethe importance of recycling in controlling net ¹⁴C uptake. Withthe present models, only in the absence of recycling could theeffect of respiration be studied and the value of q established.     

Studies on the Relation Between Net Photosynthesis and Nitrogenase-linked Respiration in Subterranean Clover

The relation between the rate of nitrogenase-linked respirationand net photosynthesis, and the effect of defoliation on thisrelation, was studied in plants of subterranean clover (Trifoliumsubterraneum L. cv. Seaton Park). Nitrogenase-linked respirationwas estimated as the difference between the rate of nodulatedroot respiration at 21% O₂ and at 3% O₂. The level to which the rate of nitrogenase-linked respirationfell several hours after defoliation was directly proportionalto the decline in the rate of net photosynthesis. Approximately9% of net photosynthesis was always expended in nitrogenaseactivity, irrespective of whether or not the plants were defoliated.This proportion was maintained during the first 3 d of regrowth. To determine whether the decline in nitrogenase-linked respirationafter defoliation was due solely to the decline in the rateof photosynthesis, a further experiment was conducted in whichthe pre-defoliation rate of net photosynthesis was restoredimmediately (with supplementary light) or within 5 min (supplementarylight and CO₂) after defoliation. Restoring the rate of netphotosynthesis did not prevent the post-defoliation declinein nitrogenase-linked respiration. However, when photosynthesiswas reduced to zero by the imposition of darkness, and the rateof nitrogenase-linked respiration allowed to decline to a steadyrate after 3 h, a rapid recovery in the rate of nodulated rootrespiration began within 2 h of returning the plants to thelight. It was hypothesized that a ‘shoot factor’,which was affected by defoliation, could override the apparentrelation between nitrogenase-linked respiration and the rateof current photosynthesis. Key words: Defoliation, N₂ fixation, photosynthesis, nitrogenase-linked respiration, subterranean clover     

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.     

CAM Photosynthesis under Drought Conditions in Kalanchoe blossfeldiana Grown with Nitrate or Ammonium as the Sole Nitrogen Source

K. blossfeldiana Poelln. cv. Hikan was grown in vermiculite,supplied daily with nutrient solution containing 1 mM (or 10mM) nitrate or ammonium as the sole nitrogen source. The nitrate-grownplants had more activity of CAM (Crassulacean acid metabolism)photosynthesis (nocturnal CO₂ uptake in the shoot and nocturnalincreases of titratable acidity and malate content in the leaves)than the ammonium-grown plants. Interruption of the solutionsupply for 5 or more days (drought conditions) increased theactivity of CAM photosynthesis in nitrate- or ammonium-grownplants, and the diurnal CO₂ uptake pattern in the nitrate-grownplants shifted from ‘weak-CAM’ to ‘full-CAM’.The difference in the activity of CAM photosynthesis betweennitrate- and ammonium-grown plants increased under the droughtconditions. When the solution was resupplied, the activity ofCAM photosynthesis rapidly decreased to the levels before theinterruption. The physiological mechanism and ecological significanceof the effect of the nitrogen source on CAM photosynthesis arediscussed (Received January 5, 1988; Accepted April 13, 1988)     

Temperate Grassland Responses to Climate Change: an Analysis using the Hurley Pasture Model

The Hurley Pasture Model is process-based and couples the carbon,nitrogen and water cycles in the soil-grass-animal system. Itwas used to examine the responses of grasslands in southern,lowland and northern, upland climates in Britain. Short-termresponse to step-wise increases in CO₂concentration (350 to700 µmol mol^-1) and temperature (5 °C) were contrastedwith long-term equilibrium (the term ‘equilibrium’is equivalent to ‘steady state’ throughout thispaper) responses and with responses to gradually increasing[CO₂] and temperature. Equilibrium responses to a range of climatevariables were also examined. Three conclusions were drawn regarding the interpretation ofexperiments: (1) initial ecosystem responses to step-wise changescan be different in both magnitude and sign to equilibrium responses,and this can continue for many years; (2) grazing can drasticallyalter the magnitude and sign of the response of grasslands toclimate change, especially rising temperatures; and (3) effectsof changes in climate, especially temperature and rainfall,are likely to be highly site-specific. It was concluded thatexperiments should try to lessen uncertainty about processeswithin models rather than try to predict ecosystem responsesdirectly. Three conclusions were also drawn about the operation of grasslandsas carbon sinks: (1) increasing [CO₂] alone will produce a carbonsink, as long as it continues to accelerate photosynthesis andincrease net primary productivity; (2) by contrast, increasingtemperatures alone are likely to produce a carbon source, becausesoil respiration is accelerated more than net primary productivity,even when assuming the same temperature function for most soiland plant biochemical processes; and (3) the net effect of projectedincreases in [CO₂] and temperature is likely to be a carbonsink of 5–15 g C m^-2yr^-1in humid, temperate grasslandsfor several decades, which is consistent with the magnitudeof the hypothesized current global terrestrial carbon sink. Grassland; climate change; carbon dioxide; temperature; ecosystem; model; carbon sink     

ERRATA

Page 806, Preparation of Mitochondrial Fraction, line 4: The following should be inserted between ‘centrifugedat’ and ‘20 000 g for’: 3000 for 10 mm. Thesupernatant was centrifuged at The following corrections are required: Page 104, line 20: ‘2-hydroxylation’ should read ‘2-ß3-hydroxylation’ Page 106, line 11: ‘of Ga₈’ should read ‘to GA₈’ Page 113, last line:‘length 50 µm’ shouldread ‘length 150 µm’ Formula 15 should read: Formula 17 should read: y(0)– y^* = ß₁V₁+ß₂V₂ page 118: Formula 18 should read: Formula 23 should read: Formula 24 should read:      

ERRATA

On page 235, Table I: Equation (1) for Node 4 should read ‘A/A_c=0·840+0·0006A_c’;Equation (2) for Node 4 should read ‘A=0·89A_c’and Equation (2) for Node 5–10 should read ‘A=0·813A_c’.     

Respiratory Efflux in Relation to Temperature of Simulated Swards of Perennial Ryegrass with Contrasting Soluble Carbohydrate Contents

Fully light-intercepting simulated swards of S24 perennial ryegrasswere exposed to contrasting environmental conditions in a growthroom for 4 days. Half experienced 20 h days of 120 Wm^–2(400–700. nm) and 5 °C, and came to have a WSC (watersoluble carbohydrate) content of 235 mg g^–1 and half 4h days of 20 Wm^–2 and 25 °C leading to a WSC of 25mg g^–1. Their rates of CO₂ efflux were monitored at anumber of temperatures during an 8 h dark period; half experiencedincreasing (5–30 °C) and half decreasing (30–5°C) temperatures. The ‘high’ WSC swards hadrespiration rates of 3.7 mg CO₂ g^–1 (d. wt) h^–1at 15 °C, and the ‘low’ swards 0.8 mg CO₂ g^–1h^–1. The order in which the temperatures were experiencedwas immaterial. Even the ‘low’ WSC swards showedno evidence of a respiratory decline during the dark periodthat could be attributed to substrate shortage. The relationshipbetween temperature and CO₂ efflux was best represented by logisticcurves. Even so, a Q₁₀ of 2 fitted the data reasonably well,at least up to 20 °C, and has practical advantages wheninterpolating estimated between measured values of respirationin the construction of a carbon balance sheet. Lohum perenne L., ryegrass, respiration, temperature, Q₁₀, soluble carbohydrate content, simulated sward     

Uptake and Metabolism of Sugars by Suspension cultured Catharanthus roseus Cells

p. 186, right column line 11 ‘27.2 KBq’ change to‘37.2 MBq’ p. 187, left column line 8 ‘27.2 KBq’ change to‘37.2 MBq’ line 10 ‘13.6 KBq’ change to ‘18.6 MBq’ line 11 ‘13.6 KBq’ change to ‘18.6 MBq’ line 21 ‘89%’ change to ‘80%’ right column line 24 ‘CLC-NH₄’ change to ‘CLC-NH₂’ P. 189, Table 1 appeared incorrectly: it should appear as indicated.     

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.     

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.     

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Effects of coupled solute and water flow in plant roots withspecial reference to Brouwer's experiment. Edwin L. Fiscus. p. 71 Abstract: Line 3 delete ‘interval’ insert‘internal’. p. 73 Materials and Methods: line 6: delete ‘diversion’ insert ‘division’ line 9 equation should read J_v=L_p P–RT(C⁰–C¹). 74 Last line of figure legend: 10^–1 should read 10^–11. 75 Line 11: delete ‘seems’ insert ‘seem’. le 1 column heading—10⁶ should read 10¹¹. 77 delete ‘...membrane in series of...’ insert ‘membranein series or...’ Delete final paragraph.