Simulating Growth and Root-shoot Partitioning in Prairie Grasses Under Elevated Atmospheric CO2and Water Stress

We constructed a model simulating growth, shoot-root partitioning,plant nitrogen (N) concentration and total non-structural carbohydratesin perennial grasses. Carbon (C) allocation was based on theconcept of a functional balance between root and shoot growth,which responded to variable plant C and N supplies. Interactionsbetween the plant and environment were made explicit by wayof variables for soil water and soil inorganic N. The modelwas fitted to data on the growth of two species of perennialgrass subjected to elevated atmospheric CO₂and water stresstreatments. The model exhibited complex feedbacks between plantand environment, and the indirect effects of CO₂and water treatmentson soil water and soil inorganic N supplies were important ininterpreting observed plant responses. Growth was surprisinglyinsensitive to shoot-root partitioning in the model, apparentlybecause of the limited soil N supply, which weakened the expectedpositive relationship between root growth and total N uptake.Alternative models for the regulation of allocation betweenshoots and roots were objectively compared by using optimizationto find the least squares fit of each model to the data. Regulationby various combinations of C and N uptake rates, C and N substrateconcentrations, and shoot and root biomass gave nearly equivalentfits to the data, apparently because these variables were correlatedwith each other. A partitioning function that maximized growthpredicted too high a root to shoot ratio, suggesting that partitioningdid not serve to maximize growth under the conditions of theexperiment.Copyright 1998 Annals of Botany Company plant growth model, optimization, nitrogen, non-structural carbohydrates, carbon partitioning, elevated CO₂, water stress,Pascopyrum smithii,Bouteloua gracilis, photosynthetic pathway, maximal growth     

Simulating Grass-Legume Dynamics: a Phenomenological Submodel

A simple recipe for modelling the dynamics of the legume componentof a grass-legume pasture simulator is proposed, avoiding someof the difficulties associated with representing the pasturesubmodel as autonomous but interacting grass and legume components.A target legume content of the sward is assumed to depend onthe carbon[ratio]nitrogen ratio in the plant substrate carbonand nitrogen pools (these represent the labile and easily mobilizableC and N pools in the plant). The rate at which the actual legumecontent approaches the target content is proportional to thegross specific growth rate of the pasture. The canopy extinctioncoefficient and the dinitrogen fixation rate for the pastureare adjusted according to the changing legume content. The methodhas been incorporated into a generic single-species grasslandsimulator, the Hurley Pasture Model. Seasonal changes in legumefraction and associated variables are simulated. Next, the responseof the legume fraction to step changes in nitrogen fertilizerapplication, carbon dioxide concentration, rainfall and temperatureare predicted for a grazed pasture. Yield from frequent harvestingis also examined for four treatments: ambient and elevated carbondioxide x low and high nitrogen fertilizer application. Qualitatively,the simulations agree well with experimental findings. Thisindicates that some important aspects of grass-legume competitioncould operate primarily through the pasture carbon[ratio]nitrogensubstrate ratio. This ratio may determine those characteristicsof morphology, growth and function that largely define the differentialsuccess of the two components of grass-legume swards; however,causation would not be proved (as far as this is scientificallypossible) without a detailed mechanistic model. The approachmay be useful for the investigation of management and climate-changeproblems in grassland. Copyright 2001 Annals of Botany Company Grass, legume, model, grassland, ecosystem, simulator     

The Influence of the Rht1 and Rht2 Alleles on the Growth of Wheat Stems and Ears

A field experiment was carried out with a set of near-isogenicspring wheat lines (cv. Triple Dirk) to determine the influenceof the Rht₁ and Rht₂ alleles on the partitioning of dry matterbetween the developing stem and the ear. Each line was sampledtwice weekly and dissected into its component above-ground parts.The rate of change of the dry mass of the individual plant organswas expressed as a proportion of the rate of change of the totalplant dry mass. This ratio was used to assess the relative sinkstrengths of the stem and ear during crop growth. The Rht₁ andRht₂ alleles reduced plant height, but increased grain yield.The greater yield was achieved through a greater grain numberper ear in the Rhtl line, a greater ear number per plant inthe Rht₂ line, and a greater allocation of assimilate to thedeveloping ear than to the developing stem in both Rht lines,particularly at the time of maximum stem growth (17 d beforeanthesis). From the earliest stages of detectable ear growthuntil anthesis, the ear masses per unit area of the Rht₁ andRht₂ lines exceeded that of Triple Dirk (Rht). It was not possibleto determine whether the Rht₁ and Rht₂ alleles were directlyresponsible for increasing grain number per ear and ear numberper plant, respectively, since the increase in these componentsof yield could equally be explained by a greater partitioningof assimilate to developing ears and tillers caused simply bya reduction in plant height. Triticum aestivum L., wheat Rht genes, stem and ear development, dry matter partitioning, allocation ratio     

Mineral Nutrition and Plant Growth Response to Climate Change

The limiting factor concept has often been used to describeplant growth responses to altered availability of resources.However, even preliminary experiments, where atmospheric CO₂concentrations and solution mineral concentrations were varied,demonstrated that a more complex concept was required to interpretthe potential effects of climate change and mineral availabilityon plant growth. It is proposed that these resources for plantgrowth may be better viewed as simultaneously limiting. Further,in considering the limitation in plant growth to mineral nutritionit is important to consider both the solution concentrationand the total amount of the individual minerals available tothe plant. Sustaining a positive response to increased CO₂ concentration,for example, requires an increase in plant uptake of the totalamount of minerals. Consequently, it is very difficult to predictthe plant growth response to climate change because of the largeuncertainty about mineral availability. On the one hand, increasedCO₂ concentrations should stimulate nitrogen fixation by bothfree-living organisms and symbiotic systems, and improve soilproperties for mineral availability as a result of increasedorganic matter deposition in the soil. On the other hand, increasedtemperature and altered rainfall patterns may result in increasedlosses of soil minerals. Even the direction in the net changein available soil minerals is unclear. Realistic evaluationsof the effects of climate change on plant growth will be challengedto contend with the large uncertainty and complexities in understandingmineral availability and plant mineral nutrition.     

The Relationship between Stem Diameter and Water Potentials in Stems of Young Cabbage Plants

Continuous measurements were made of stem shrinkage, stem waterpotential (₃) and transpiration rate (T) in young, pot-growncabbage plants subjected to cycling evaporative demands. Sequencesof increasing evaporative demands induced increases in T anddecreases in both ₃ and stem diameter and conversely, wheneverevaporative demand decreased, T declined and ₃ and stem diameterrose. Over short periods, stem water potentials and stem shrinkagewere virtually parallel even when rapid oscillations were induced.Over longer periods the effects of growth were important comparedwith those of water stress on stem diameter when the moisturecontent of the soil was high. Growth, however, ceased when theplant was subjected to relatively mild water stress (₃ = –0.4MPa). Stem diameters, after correction for growth, were linearlyrelated to plant water potential. The results suggest that stemshrinkage and only a few calibration measurements might be usedto provide continuous estimates of water potentials in fieldcrops. Key words: Cabbage, Stem diameter, Stem water potential     

A comparison between methods used to control nutrient supply

Experimental methods to supply nutrients to culture solutionsin order quantitatively to control plant nutrition are compared.In experiments with tomato and birch plants, for which the dataare available in databases (Ingestad et al., 1994a, b), thenutrients were supplied at constant relative addition rates(R_A over sufficiently long periods of time to achieve acclimatedplants and reliable measurements of plant responses. The plantswere maintained under steady-state conditions, i.e. the internalnutrient concentrations (c₁) remained constant, as a resultof a numerical equality between the relative uptake rate (R_U)and the relative growth rate (R_G). These results are comparedto experiments with pea plants (Macduff et al., 1993). In oneseries (a), R_A was applied, but without strict control of internalsteady-state, and in the other series (b), the external concentration(c_e) was maintained constant. With limiting nitrogen, in bothseries, there was a substantial deviation from equality betweenR_U and R_G. In (a), c_I changed during the experimental periodand the purpose of the R_A approach was lost. In (b), a constantc_e had little effect on nitrogen uptake and plant growth. Atthe three highest concentrations, steady-states were obtainedat non-limiting uptake rates. At the lowest concentration, theuptake rate of nitrogen was about the same, but there was adecrease of R_a, which apparently was not caused by reduced uptake.Clear-cut relationships can not therefore be established betweentreatment variables and plant responses and the conclusionsreached by Macduff et al. (1993) have little support in theirexperimental results. This indicates an urgent need to updateboth theories and experimental methods together: in particular,it is important to identify the system under investigation andto distinguish between control of the medium and control ofthe plant. Key words: Experimental control, external nutrient concentration, non-limiting and limiting nutrient supply, relative addition rate, relative uptake rate, relative growth rate, steady-state     

Nitrogen Use Efficiency in Growth of Polygonum cuspidatum Sieb. et Zucc

The growth of Polygonum cuspidatum in sand culture was analysedunder varying nutrient conditions. Nitrogen availability influencednitrogen uptake of plants through the uptake rate per unit rootweight rather than the amount of root. In turn, the differentamounts of nitrogen taken up affected plant growth through theireffects on the rate of leaf expansion. Net assimilation rate (NAR) increased with nitrogen contentper unit leaf area (C), but further increase in leaf nitrogencaused diminishing returns of NAR Optimal nitrogen content perunit leaf area (C_opt) to maximize dry-matter production of aleaf could be determined by drawing a tangent from the onginto a curvilinear relation between NAR and C. This optimal contentdivides a nitrogen-limiting range (C < C_opt) from a carbon-limitingone (C> C_opt) along the axis of nitrogen content. Under nitrogenlimitation, efficiency of nitrogen use in dry-matter productioncould increase if the plant had a larger carbon sink. This givesa qualitative explanation to reduced shoot-to-root ratio underlimited availability of nitrogen. Polygonum cuspidatum Sieb. et Zucc, Japanese knotweed, carbon sink, growth analysis, leaf nitrogen, net assimilation rate, nitrogen use efficiency     

Dwarfism Caused by Floral-Bud Removal in Petunia and its Reversal by Gibberellin

The effect of floral-bud removal at different stages of developmenton the plant height and on the total number of buds of Petuniawas studied. Continuous removal of all the floral buds 2 d beforeanthesis caused a marked decrease in plant height and also increasedthe total number of floral buds formed thereafter. At otherstages of floral bud development, bud removal had a lesser effecton both phenomena. Moreover, the plants did not respond to budremoval at anthesis. GA₃ at 25 ppm applied to plants from which the buds had beenremoved, promoted stem elongation. The most pronounced effectwas on plants from which the buds were removed 2 d before anthesis,but it had no effect on plants from which the buds were removedat anthesis stage. The possible involvement of endogenous growth hormones in theresponse of Petunia plants to floral-bud removal and to applicationof GA₃ is discussed. Bud removal, bud number, dwarfness, GA₃, Petunia, plant height     

Compensatory Roles of Nitrogen Uptake and Photosynthetic N-use Efficiency in Determining Plant Growth Response to Elevated CO2: Evaluation Using a Functional Balance Model

We used a modified functional balance (FB) model to predictgrowth response of Helianthus annuus L. to elevated CO₂. Modelpredictions were evaluated against measurements obtained twiceduring the experiment. There was a good agreement between modelpredictions of relative growth rate (RGR) responses to elevatedCO₂and observations, particularly at the second harvest. Themodel was then used to compare the relative effects of biomassallocation to roots, nitrogen (N) uptake and photosyntheticN-use efficiency (PNUE) in determining plant growth responseto elevated CO₂. The model predicted that a rather substantialincrease in biomass allocation to root growth had little effecton whole plant growth response to elevated CO₂, suggesting thatplasticity in root allocation is relatively unimportant in determininggrowth response. Average N uptake rate at elevated comparedto ambient CO₂was decreased by 21–29%. In contrast, elevatedCO₂increased PNUE by approx. 50% due to a corresponding risein the CO₂-saturation factor for carboxylation at elevated CO₂.The model predicted that the decreased N uptake rate at elevatedCO₂lowered RGR modestly, but this effect was counterbalancedby an increase in PNUE resulting in a positive CO₂effect ongrowth. Increased PNUE may also explain why in many experimentselevated CO₂enhances biomass accumulation despite a significantdrop in tissue nitrogen concentration. The formulation of theFB model as presented here successfully predicted plant growthresponses to elevated CO₂. It also proved effective in resolvingwhich plant properties had the greatest leverage on such responses.Copyright 2000 Annals of Botany Company Elevated CO₂, functional balance model, Helianthus annuus L., N uptake, photosynthetic nitrogen use efficiency, root:shoot ratio     

Trade Winds Reduce Growth and Influence Gas Exchange Patterns in Papaya Seedlings

Four experiments were conducted to determine the effect of tradewinds in Guam, USA, on growth and gas exchange of three papaya(Carica papaya L.) cultivars. ‘Known You 1’, ‘Sunrise’,and ‘Tainung 2’ papaya seedlings at two differentstages of development were exposed to 0, 36 or 100% ambientwind. Wind exposure reduced stem height and leaf or stem dryweight in most cases, but had little influence on root growth.Net CO₂assimilation (A_CO2) at midday was lower for seedlingsexposed to wind than for those protected from wind. Dark respirationof exposed seedlings increased as much as 120% above that ofthe protected seedlings during the night. Wind exposure decreasedwhole plant evapotranspiration by up to 36% during the photoperiod,but increased evapotranspiration by 58–87% during thenocturnal period. Responses to wind exposure were similar amongcultivars, except that growth of ‘Tainung 2’ seedlingswas less affected by wind than that of the other cultivars.Seedlings that were exposed to the various wind treatments fromgermination onwards were less influenced by wind exposure thanwere seedlings that were grown in a protected nursery beforebeing exposed to the various wind treatments. These data indicatethat: (1) ambient trade winds in Guam are strong enough to decreasethe growth of papaya seedlings; (2) plant age influences theresponse; (3) stem and leaf growth are more influenced thanroot growth; and (4) decreasedA_CO2 and increased dark respirationmay be partly responsible for growth reduction. Copyright 2001Annals of Botany Company Carica papaya, gas exchange, wind     

Effects of CO2 enrichment and intraspecific competition on biomass partitioning, nitrogen content and microbial biomass carbon in soil of perennial ryegrass and white clover

Seedlings of perennial ryegrass (Lolium perenne L. cv. Parcour)and white clover (Trifolium repens L. cv. Karina) grown at fivedifferent plant densities were exposed to ambient (390 ppm)and elevated (690 ppm) CO₂ concentrations. After 43 d the effectsof CO₂ enrichment and plant density on growth of shoot and root,nitrogen concentration of tissue, and microbial biomass carbon(C_mic) in soil were determined. CO₂ enrichment of Lolium perenneincreased shoot growth on average by 17% independent of plantdensity, while effects on root biomass ranged between -4% and+ 107% due to an interaction with plant density. Since tilernumber per plant was unaffected by elevated CO₂, the small responseof shoot growth to CO₂ enrichment was atributed to low sinkstrength. A significant correlation between nitrogen concentrationof total plant biomass and root fraction of total plant drymatter, which was not changed by CO₂ enrichment, indicates thatnitrogen status of the plant controls biomass partitioning andthe effect of CO₂ enrichment on root growth. Effects of elevatedCO₂ and plant density on shoot and root growth of Trifoliumrepens were not significantly interacting and mean CO₂-relatedincrease amounted to 29% and 66%, respectively. However, growthenhancement due to elevated CO₂ was strongest when leaf areaindex was lowest. Total amounts of nitrogen in shoots and rootswere bigger at 690 ppm than at 390 ppm CO₂. There was a significantincrease in C_mic in experiments with both species whereas plantdensity had no substantial effect. Key words: CO₂ enrichment, intraspecific competition, biomass partitioning, Lolium perenne, Trifolium repens, grassland     

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.     

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     

Shoot and Root Activities During Steady-state Plant Growth

A simple model for steady-state plant growth is described. Thegrowth constant, measured during steady-state exponential growth,is related to the specific activities of the shoot and the root,enabling the effects of certain environmental variables (light,carbon dioxide and nitrogen) on the growth constant to be examined.The model is used to interpret data on the growth kinetics ofwheat (Macdowall, 1972a, b, c).     

Growth and Biomass Allocation, with Varying Nitrogen Availability, of Near-isogenic Pea Lines with Differing Foliage Structure

The single-gene mutation afila in pea (Pisum sativum L.) resultsin the replacement of proximal leaflets with branched tendrils,thereby reducing leaf area. This study investigated whethertheafila line could adjust biomass partitioning when exposedto varying nutrient regimes, to compensate for reduced leafarea, compared with wild-type plants. Wild-type and afila near-isogeniclines were grown in solution culture with nitrate-N added toinitially N-starved seedlings at relative addition rates (R_N)of 0.06, 0.12, 0.15 and 0.50 d^-1. The relative growth rate (R_W)of the whole plants closely matched R_Nat 0.06 and 0.12 d^-1,but higher R_Nresulted in a slightly higher growth rate. At agiven R_N, the wild-type line had lower plant nitrogen statusthan the afila line. R_Wof the roots of the afila line was lessthan R_Wof the roots of the wild-type at the three higher ratesof N supply despite a greater accumulation of N in the rootsof the afila plants. Consequently, plant nitrogen productivity(growth rate per unit nitrogen) was lower for afila. Dry matterallocation was strongly influenced by nitrogen status, but nodifferences in shoot–root dry matter allocation were foundbetween wild-type and afila with the same plant N status. Theseresults imply that decreased leaf area as a result of the single-genemutation afila affects dry matter allocation, but only accordingto its effect on the nitrogen status. Copyright 2000 Annalsof Botany Company Pisum sativum, pea, nitrogen limitation, growth, shoot–root allocation, relative growth rate, nitrogen productivity, isolines     

A Model for Growth and Self-thinning in Even-aged Monocultures of Plants

A theoretical model is derived from simple postulates to describethe rates of growth and mortality of plants in populations ofdifferent densities. The growth rate is described by a modificationof the logistic growth differential equation in which the increasein weight of an individual plant depends on its area, s_i ratherthan on its weight. The effective area for growth of a plantis reduced by an empirical function, f(s_i) with two terms: oneterm expresses the constraint imposed upon the increasing totalarea of plants by the limited physical area of the plot; theother term allows for a competitive advantage or disadvantagefor plants of varying sizes. Depending on the value of the parametercontrolling the relative competitive advantage term, intrinsicvariability between plants can be amplified or suppressed. Anindividual plant dies if the f(s_i) results in a negative growthrate for that plant. Computer simulations of the growth andsurvival of plants at different population densities were run.The results exhibit characteristics that appear realistic uponcomparison with published data: a survival of the fittest occurringduring thinning; a line of slope close to –3/2 boundingthe graphs of log weight versus log density; and the occurrenceof bimodality, associated with subsequent mortality, on frequencydistribution of log weight. computer logistic model, growth differential equation, density-effect, competition, mortality, self-thinning     

The effect of stem and cob borers on maize subjected to different nitrogen treatments

The influence of four nitrogen levels (0, 60, 90 and 120 kg N/ha) on growth of maize and development of lepidopterous pests was investigatdd in a field trial. Nitrogen had a positive effect on both plant growth variables (plant height, stem diameter and yield), and development and survival ofSesamia calamistis andEldana saccharina, and thereby increased the incidence of dead hearts and stem tunneling. However, the percent yield loss due to artificial infestation decreased with increasing N application rate from 20% to 11% in the in the 0kg/ha and 120kg/ha treatment, respectively. Using a multiple regression analysis, plant height, plant diameter and stem tunneling were found to be the most important variables explaining the variability in maize yield.     

A Model for Plant and Crop Growth, Allowing for Competition for Light by the use of Potential and Restricted Projected Crown Zone Areas

Relations for competition for light are developed and used ina plant growth model applicable to the isolated plant, to plantsin even-aged monoculture and to plants in mixed-aged monoculture.In an isolated plant, it is assumed that a leaf area, proportionalto the plant mass, is contained within a crown whose projectedzone area is proportional to plant mass to the 2/3 power. Self-shadingprogressively reduces the specific growth rate. If light werethe sole limiting resource and were constant, one can derivea growth equation, dw/dt = rw[1 - exp (-KW^1/3)]KW^1/3, which,integrated, gives w^1/3 = K^-1 ln {1 + [exp (KW^1/3₀)-1] exp (rt/3)}.It approximates, initially, to a particular case of the Richards(1959) empirical growth equation. In even-aged evenly-spaced monocrops competing only for light,it is assumed that the zone areas merge at canopy closure, andgrowth then follows the expolinear equation of Goudriaan andMonteith (1990), giving a continuous function based on groundcover. For mixed-aged monocrops, we assume a phase of canopyclosure that affects the younger plants earlier than the olderones. Under varying environmental conditions in the field, plant growthmay be affected by other factors in addition, e.g. temperature.In the growth conductance model of Aikman and Scaife (1993),the shading expressions are applied to the light-dependence. Data from two sowings of cabbage and carrot in even-aged andmixed-aged monocrops were used to test the model. The parametervalues derived from the even-aged monocultures predict the growthrates in the mixed-aged monocultures better than models whichassume uniform canopies.Copyright 1994, 1999 Academic Press Growth, model, monocrop, even-aged, mixed-aged, PAR, density, competition, light, shading, zone area, ground cover, temperature, carbon dioxide, expolinear, carrot, Daucus carota L., cabbage, Brassica oleracea L     

Influence of nitrogen on growth and photosynthesis of a C3 cereal, Oryza sativa, infected with the root hemiparasite Striga hermonthica

Striga hermonthica is a root hemiparasitic angiosperm nativeto the African semi-arid tropics. It is a major weed of C₄ cerealsbut locally it is also an important weed of the C₃ plant, rice[Oryza sativa). Infected rice plants produced 17% and 42% ofthe total biomass of uninfected plants when grown at two differentammonium nitrate concentrations, 1 and 3 mol m^–3, respectively.S. hermonthica prevented grain production at both concentrationsof nitrogen. At the lower concentration no heads were produced.At the higher concentration head weight was only 6% of uninfectedcontrols. S. hermonthica also altered the partitioning of drymatter between plant parts, such that shoot growth was reducedto a greater extent than root growth. As a consequence the root-to-shootratio of infected plants was approximately five times greaterthan that of uninfected control plants. Light saturated ratesof photosynthesis In infected plants were 56% and 70% of thoseof uninfected controls, at low and high nitrogen, respectively.Infection also led to lower values of stomatal conductance althoughthe substom-atal CO₂ concentration was unaffected. Analysisof the response of photosynthesis to substomatal CO₂ concentration(A/C_I curves) demonstrated that lower rates of photosynthesiscould not be solely attributed to lower stomatal conductances.Lower initial slopes and asymptotic rates suggest that bothcarboxylation and processes controlling regeneration of ribulose-1,5-bisphosphate are reduced by infection. The data are discussedwith respect to the influence of S. hermonthica on the growthand photosynthesis of C₄ hosts, where in contrast to the situationwith rice, nitrogen feeding results in a marked alleviationof the effects of the parasite on the host. Key words: Rice, Striga, growth, photosynthesis, nitrogen     

Nitrogen Productivity Depends on Photosynthetic Nitrogen Use Efficiency and on Nitrogen Allocation Within the Plant

The concept of plant nitrogen productivity was introduced atthe end of the 1970s to interpret the dependency of plant growthon internal nitrogen. It is defined as the increase in plantdry matter per unit time and per unit plant nitrogen content.Recently, plant nitrogen productivity has been expressed asthe product of two terms: the leaf nitrogen ratio, which isthe proportion of the plant's nitrogen present in the leaves,and the leaf nitrogen productivity, which is defined as theincrease in plant dry matter per unit time and leaf nitrogencontent. In the present paper we use two data sets obtainedfrom C₃ herbaceous species to evaluate the relative importanceof variation in leaf nitrogen ratio and leaf nitrogen productivityin determining interspecific variation in plant nitrogen productivity.Further, we analyse to what extent leaf and plant nitrogen productivitiesdepend on photosynthetic nitrogen use efficiency. Results showthat in all cases, photosynthetic nitrogen use efficiency isa major determinant of both plant and leaf nitrogen productivities.A positive relationship between leaf nitrogen ratio and plantnitrogen productivity was found only when comparisons were madeover broad taxonomic groups.Copyright 1995, 1999 Academic Press Interspecific variation, leaf nitrogen ratio, nitrogen productivity, photosynthetic nitrogen use efficiency