Modelling partitioning between structure and storage in sugar beet: Effects of drought and soil nitrogen

In the UK sugar beet is grown on contrasting soils that vary both in their nutritional status and water-holding capacities. Water shortage and low nitrogen reduce canopy growth and dry matter production, which is compensated in part by an increase in the fraction of assimilates partitioned to storage. Conversely, high nitrogen and ample water encourage growth of the canopy, increase assimilation of carbon dioxide but reduce the proportion of assimilates stored as sugar. This paper sets out to examine simple relationships between sugar yield, total dry matter and soil nitrogen in rain-fed and irrigated sugar beet crops (Beta vulgaris L.) from 46 field experiments spanning 12 years and a range of soil types, in order to improve prediction of sugar yields. Two partitioning functions were fitted to the data. The first represents a useful alternative formulation of the allometric growth function that overcomes some of the difficulties in the interpretation of the parameters. This model adequately described the seasonal progress of sugar yield (Y) in relation to total dry matter (W), but it was difficult to postulate biological mechanisms as to how the parameters should vary in relation to varying soil nitrogen or to drought. The second partitioning function, given by Y = W − (1/k) log(1 + kW), also described the data well, but had the more useful parameter, k, the decay rate of the fraction of assimilates partitioned to structural matter. This was shown to be greater in crops which had experienced significant drought and was inversely proportional to the amount of nitrogen taken up by the crops. Relationships between k and amounts of nitrogen fertilizer applied and/or amounts of residual nitrogen in the soil at sowing, however, were more variable. These could be improved by additionally taking account of soil type and rainfall following nitrogen fertilizer application in late spring. The models are a useful extension to yield forecasting models because they provide a simple means of estimating sugar yield from total dry matter in relation to factors that affect partitioning of assimilates such as drought and soil nitrogen availability. This revised version was published online in June 2006 with corrections to the Cover Date.     

Modelling Asymmetrical Growth Curves that Rise and then Fall: Applications to Foliage Dynamics of Sugar Beet (Beta vulgaris L.)

This paper discusses the derivation and fitting of three empiricalmodels with turning points for describing the growth of plantcomponents, such as shoot weight, leaf area and root length,that typically rise and then fall during the course of the growingseason. The models (Models I, II and III) have analytical solutionsand may be viewed as extensions of the Gompertz, Richards andChanter growth equations. They differ by having an additionalparameter which, following a sigmoidal rise of the dependentvariable, determines subsequent net rate of decline. The modelswere fitted to sequential measures of foliage cover of sugarbeet crops grown in the UK during 1980–1991. It was importantthat this could be done with relative ease using standard statisticalprocedures. Partial linear transformations of two of the models,with one non-linear parameter remaining, are described; thesewere useful for estimating initial values for the parameters.All three models described the data well, although the fittingof Model II invariably failed to converge. For Models I andIII common and separate parameters, amongst years, were estimatedrelating to date of emergence, initial relative growth rate,maximum cover attained and rate of late season decline of foliagecover. The reduction in the residual mean square on fittingseparate, rather than common, parameters was usually significant.The models accommodate several biological processes that yieldsimilar shapes. This is demonstrated for Model I, in relationto its formulation and to effects of small perturbations inthe values of the parameters on the shape of the curves. ModelI, the simplest of the three models tested, has good fittingproperties, and in practice was best suited to describing foliagecover dynamics of sugar beet. Beta vulgaris L.; sugar beet; foliage cover; senescence; models; parameter estimation; growth functions     

Genotypic Variation for Drought Tolerance in Beta vulgaris

Insufficient soil moisture during summer months is now the majorcause of sugar beet yield losses in the UK. However, selectionfor increased drought tolerance has not been a breeding priorityuntil recently. Genetic variation for drought tolerance is anessential prerequisite for the development of more stress-tolerantvarieties, but commercial sugar beet varieties seem to havesimilar yield responses to drought. The objective of this studywas to assess the degree of genotypic variation for droughttolerance within a wide range of sugar beet germplasm and genebankaccessions within Beta. Thirty sugar beet genotypes were screenedunder field drought conditions, and putative drought tolerantand sensitive lines (in terms of yield reduction in polythene-coveredvs. irrigated plots) were identified. Significant genotype xwater treatment interactions were found for dry matter yieldand relative leaf expansion rate. Genotypic differences fordrought susceptibility index were also significant. Differentialsensitivity of seedling shoot growth to water deficit was examinedby comparing 350 genebank accessions in a simple growth chamberscreen. Methods of data management were devised to highlightlines for entry into subsequent field tests. The results ofthe field and seedling screens indicate that there is variationfor tolerance to water deficits within sugar beet and relatedtypes, and that there are lines that show greater drought tolerancethan selected commercial varieties. Divergent lines showingcontrasting behaviour should aid in the identification of keymorpho-physiological traits that confer drought tolerance.     

Respiration Modelling and Hypothesis Testing with a Dynamic Model of Sugar Beet Growth

Respiration was predicted quantitatively during sugar-beet growthsimulations by assuming an intimate coupling to growth and maintenanceprocesses. Changes in the growth and maintenance respiratorycoefficients for successive simulations expressed alternativehypotheses regarding the nature of that coupling. Large differencesin yield, partitioning patterns, and the relative importanceof the growth and maintenance components were predicted in responseto changes in respiratory coefficients within the range consideredphysiologically realistic. Beta vulgaris L, sugar beet, respiration, growth yield, mathematical modelling     

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     

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     

Modelling the Seasonal Nitrogen Partitioning in Young Sycamore (Acer pseudoplatanus) Trees in Relation to Nitrogen Supply

A model of nitrogen partitioning during the seasonal growthof sycamore (Acer pseudoplatanus) seedlings was developed andtested against data from trees grown with two contrasting levelsof nitrogen supply. The model considered each tissue type (roots,trunk, stems and leaves) as sources and sinks for nitrogen andused flow equations to simulate the dynamics of nitrogen partitioningduring a growing season, with increases in tissue dry matteras driving force variables. Withdrawal of nitrogen from leavesduring senescence was allocated back to other tissues assuminga linear decrease in leaf mass. The model was fitted to data from trees grown in sand culturewith 6·0 molN m^-3 (high N) supplied with the irrigation.Model parameters thus determined were used to predict nitrogenpartitioning in trees grown with 1·0 molN m^-3 (low N)in the same year, and for trees from both treatments given eitherhigh or low N during a second year. The model accurately predictedthe nitrogen content of roots and leaves and gave small errorsin the amount of nitrogen partitioned to stems. In contrast,the nitrogen content of the trunks were over-estimated due toa failure to simulate the decreased in nitrogen content foundat the start of the growing season. The ability of the modelto simulate nitrogen partitioning by changes in tissue dry matterin trees of varying size and nitrogen status is discussed andpossible modifications to model partitioning of trunk nitrogenmore accurately suggested.Copyright 1993, 1999 Academic Press Modelling, nitrogen partitioning, ¹⁵N supply, Acer pseudoplatanus (sycamore), young seedlings     

Crop response to magnesium fertilization as affected by nitrogen supply

Background

Crop yield depends in large part on the availability and accessibility of nitrogen in the soil. For optimal yield, the soil nitrogen must be available at critical periods of crop development, and in a form that is accessible for plant uptake and use. Ancillary crop nutrients can alter the plant’s ability to access and utilize nitrogen. Therefore, crop fertilization with magnesium should focus on its effect on nitrogen management. This conceptual review aims to assess the present state of knowledge regarding the importance of magnesium in fulfilling both objectives.

Scope

The response to fertilizer magnesium of high-yielding wheat, maize, sugar beet and potato crops was evaluated using published and unpublished data on yield, yield components and nitrogen uptake. A simple, stepwise regression and path analysis was applied to explain the effect of fertilizer magnesium on yield and yield components.

Conclusions

The effect of soil or foliar applied magnesium on yield of crops was inconsistent due to (i) weather experienced during the growth season, (ii) rates of applied fertilizer nitrogen, and (iii) the (natural background levels of?) magnesium available in the soil. The yield increase due to magnesium application was related to the extra supply of nitrogen. In cereals, magnesium application resulted in a higher number of ears and/or thousand grain weight (TGW), stressing the magnesium-sensitive stages of yield formation. The increase of sugar beet yield was most pronounced in dry years. The main conclusion gleaned from the review underlines a positive effect of magnesium on nitrogen uptake efficiency. The optimal yield forming effect of fertilizer magnesium can generally occur under conditions of relatively low nitrogen supply (soil + fertilizer nitrogen), but high supply of magnesium. This phenomenon can best be described as “magnesium-induced nitrogen uptake”.     

Modelling diverse root density dynamics and deep nitrogen uptake—A simple approach

We present a 2-D model for simulation of root density and plant nitrogen (N) uptake for crops grown in agricultural systems, based on a modification of the root density equation originally proposed by Gerwitz and Page in J Appl Ecol 11:773–781, (1974). A root system form parameter was introduced to describe the distribution of root length vertically and horizontally in the soil profile. The form parameter can vary from 0 where root density is evenly distributed through the soil profile, to 8 where practically all roots are found near the surface. The root model has other components describing root features, such as specific root length and plant N uptake kinetics. The same approach is used to distribute root length horizontally, allowing simulation of root growth and plant N uptake in row crops. The rooting depth penetration rate and depth distribution of root density were found to be the most important parameters controlling crop N uptake from deeper soil layers. The validity of the root distribution model was tested with field data for white cabbage, red beet, and leek. The model was able to simulate very different root distributions, but it was not able to simulate increasing root density with depth as seen in the experimental results for white cabbage. The model was able to simulate N depletion in different soil layers in two field studies. One included vegetable crops with very different rooting depths and the other compared effects of spring wheat and winter wheat. In both experiments variation in spring soil N availability and depth distribution was varied by the use of cover crops. This shows the model sensitivity to the form parameter value and the ability of the model to reproduce N depletion in soil layers. This work shows that the relatively simple root model developed, driven by degree days and simulated crop growth, can be used to simulate crop soil N uptake and depletion appropriately in low N input crop production systems, with a requirement of few measured parameters.     

A Model to Simulate the Final Number of Reproductive Nodes in Pea (Pisum sativumL.)

The final number of reproductive nodes (TRN) is highly variablein pea under field conditions and can limit yield. However,the determinants of this variability are unknown. This is aproblem for crop managers and for many crop simulation models,in which the assimilate production and partitioning modulesgenerally depend on a phenological module including simulationof TRN. Previous studies in growth cabinets have linked theend of flowering to the presence of growing pods near the apex.We investigated the effects of the position of reproductiveorgans on the stem on the cessation of leaf emergence by analyticalexperiments involving pod removal. We then analysed whetherdevelopmental characteristics, obtained in the field for variousgenotypes, locations, sowing dates, plant nitrogen status, plantwater status and plant densities, could account for the observedvariation in the number of reproductive nodes. On the basisof these results, we constructed a simple model simulating TRNfrom three developmental parameters. The model was calibratedon cultivar ‘Solara’, evaluated for a wide rangeof situations and extrapolated to many genotypes, and was foundto have high predictive value.Copyright 1998 Annals of BotanyCompany Pisum sativumL., pea, number of reproductive nodes, model, genotype, N nutrition, pod removal, plant density, development, flowering, apical senescence     

Using a crop model to account for the effects of local factors on the LCA of sugar beet ethanol in Picardy region, France

Purpose

The results of published Life Cycle Assessments (LCAs) of biofuels are characterized by a large variability, arising from the diversity of both biofuel chains and the methodologies used to estimate inventory data. Here, we suggest that the best option to maximize the accuracy of biofuel LCA is to produce local results taking into account the local soil, climatic and agricultural management factors.

Methods

We focused on a case study involving the production of first-generation ethanol from sugar beet in the Picardy region in Northern France. To account for local factors, we first defined three climatic patterns according to rainfall from a 20-year series of weather data. We subsequently defined two crop rotations with sugar beet as a break crop, corresponding to current practice and an optimized management scenario, respectively. The six combinations of climate types and rotations were run with the process-based model CERES-EGC to estimate crop yields and environmental emissions. We completed the data inventory and compiled the impact assessments using Simapro v.7.1 and Ecoinvent database v2.0.

Results

Overall, sugar beet ethanol had lower impacts than gasoline for the abiotic depletion, global warming, ozone layer depletion and photochemical oxidation categories. In particular, it emitted between 28 % and 42 % less greenhouse gases than gasoline. Conversely, sugar beet ethanol had higher impacts than gasoline for acidification and eutrophication due to losses of reactive nitrogen in the arable field. Thus, LCA results were highly sensitive to changes in local conditions and management factors. As a result, an average impact figures for a given biofuel chain at regional or national scales may only be indicative within a large uncertainty band.

Conclusions

Although the crop model made it possible to take local factors into account in the life-cycle inventory, best management practices that achieved high yields while reducing environmental impacts could not be identified. Further modelling developments are necessary to better account for the effects of management practices, in particular regarding the benefits of fertiliser incorporation into the topsoil in terms of nitrogen losses abatement. Supplementary data and modelling developments also are needed to better estimate the emissions of pesticides and heavy metals in the field.     

Effects of increasing weed-beet density on sugar-beet yield and quality

Weed beets are an increasing problem in many sugar-beet crops in many countries. At present about one sugar-beet field in four in England is infested with weed-beet seed. Control in other crops can be achieved using selective herbicides but in sugar beet the weed beets, many of which are of annual habit, are not easily controlled and often compete with the crop. Experiments were done to quantify the yield loss caused by weed beet in sugar-beet crops. Transects were laid out across three fields in 1985 and 1986 and plots located thereon to include the range of weed-beet densities found in the field. Weed beet did not affect the concentration of sugar (sucrose), potassium, sodium, α amino nitrogen or invert sugar in the crop beets. Root and sugar yields were progressively reduced by increasing densities of weed beet. A rectangular hyperbola described the data slightly better than an asymptotic model. There was no indication of a threshold density of weed beet below which there was no yield loss, which averaged 11.7% for each weed beet plant/m². This corresponds to an average 0.6% sugar yield loss for each 1% of bolted weed beet in the root crop up to 100%, which is similar to the reported losses resulting from bolters in the root crop.     

ORCHIDEE‐STICS,a process‐based model of sugarcane biomass production: calibration of model parameters governing phenology

Agro‐Land Surface Models (agro‐LSM) combine detailed crop models and large‐scale vegetation models (DGVMs) to model the spatial and temporal distribution of energy, water, and carbon fluxes within the soil–vegetation–atmosphere continuum worldwide. In this study, we identify and optimize parameters controlling leaf area index (LAI) in the agro‐LSM ORCHIDEE‐STICS developed for sugarcane. Using the Morris method to identify the key parameters impacting LAI, at eight different sugarcane field trial sites, in Australia and La Reunion island, we determined that the three most important parameters for simulating LAI are (i) the maximum predefined rate of LAI increase during the early crop development phase, a parameter that defines a plant density threshold below which individual plants do not compete for growing their LAI, and a parameter defining a threshold for nitrogen stress on LAI. A multisite calibration of these three parameters is performed using three different scoring functions. The impact of the choice of a particular scoring function on the optimized parameter values is investigated by testing scoring functions defined from the model‐data RMSE, the figure of merit and a Bayesian quadratic model‐data misfit function. The robustness of the calibration is evaluated for each of the three scoring functions with a systematic cross‐validation method to find the most satisfactory one. Our results show that the figure of merit scoring function is the most robust metric for establishing the best parameter values controlling the LAI. The multisite average figure of merit scoring function is improved from 67% of agreement to 79%. The residual error in LAI simulation after the calibration is discussed.     

Modelling Nitrogen Content and Distribution in Cauliflower (Brassica oleracea L.botrytis )

A model of nitrogen uptake and distribution is presented whichdescribes these processes in relation to the amount of availablesoil nitrate and the rate of plant growth. Nitrogen uptake iseither sink or source limited. Sink limitation is based on maximumN-concentrations of plant compartments. The N-uptake model iscombined with a photosynthesis model based on the productivity-nitrogenrelationship at the single-leaf level. The model is parameterizedusing cauliflower as an example crop. Applied to an independentdata set, the combined model was able to predict leaf, stemand inflorescence nitrogen concentrations with correlation coefficientsbetween predicted and simulated values of 0.89, 0.66 and 0.86,respectively. The influence of nitrogen supply and light intensityon leaf nitrate-N could also be predicted with good accuracy(r² = 0.87). Dry matter production based on the productivity-Nrelationship and the partitioning into leaf, stem and inflorescencewas also reproduced satisfactorily (r² = 0.91, 0.93 and 0.92,respectively). Copyright 2000 Annals of Botany Company Brassica oleracea L. botrytis, cauliflower, nitrogen, nitrate, nitrogen supply, nitrogen uptake, nitrogen distribution, model     

A Single Equation to Quantify the Hierarchy in Plant Size Induced by Competition Within Monocultures

The following empirical model: Ra(i) = r(1+ln(w(i)/wm)Kn)(1–(w(i)/W))(1–(y/Y)) which is based on the logistic growth equation, is developedto describe the growth of differently sized individuals withinplant communities. The model is tested against extensive setsof carrot (Daucus carota L.) and red beet (Beta vulgaris L.)data and is shown to fit well. The model was used to predictindividual plant weights in independent data. The agreementsbetween observed and predicted weights were often close butsome systematic deviations did occur. Thus, a single equationdescribed most of the complex interactions that occurred withinmonocultures of annual crop plants. Carrot, Daucus carota L., red beet, Beta vulgaris L., model, growth, variation     

Nitrogen Uptake and Plant Growth II. An Empirical Model of Vegetative Growth and Partitioning

An empirical model of vegetative plant growth is presented.The model is based on experimental data on Polygonum cuspidatum,which showed (1) that the partitioning of dry matter and nitrogenamong organs was linearly related to the nitrogen concentrationof the whole plant and (2) that leaf thickness was negativelycorrelated with leaf nitrogen concentration. The model properlydescribes the behaviour of plants. Steady-state solutions ofthe model give the relative growth rate, specific leaf weight,and partitioning of dry matter and nitrogen among organs withthe net assimilation rate and the specific absorption rate asenvironmental variables. The effect of nitrogen removal on drymatter and nitrogen partitioning was examined as non-steady-statedynamic solutions of the model. The model predicted not onlyreduced leaf growth and enhanced root growth but also a fluxof nitrogen from the leaf to the root, which agreed with theexperimental results. Mathematical model, partitioning of dry matter and nitrogen, plant nitrogen, relative growth rate, shoot: root ratio, specific leaf weight     

Optimization of Tissue Nitrogen and Root-Shoot Allocation

A model is explored which describes the joint optimization oftissue nitrogen and root-shoot allocation in response to variationin nitrogen availability in the environment. The model plantis composed of root and shoot biomass and tissue nitrogen, andrelative growth rate is the parameter maximized. A 'source' (uptake limited) model of joint tissue nitrogen androot-shoot allocation is reviewed (Hilbert, 1990) that optimallybalances the uptake of carbon and nitrogen. Modifications includingroot respiratory costs and fixed root tissue nitrogen are explored.Then the role of tissue nitrogen in regulating 'sink' strengthis combined with the balanced source model, and modified byassuming separate tissue nitrogen involved in source vs. sink.Generally, the results indicate that as available nitrogen increases,optimal root allocation declines and tissue nitrogen increases.These results appear to be robust for more complicated versionsof the model provided that various internal nitrogen compartments'compete' for the same nitrogen.Copyright 1993, 1999 AcademicPress Plants growth, RGR, leaf nitrogen, nitrogen productivity, allocation, partitioning, root-shoot ratio     

The effect of soil fumigation with D-D on the yields of sugar beet and other crops

An experiment in a field where sugar beet in 1965 had suffered from Docking disorder caused by Longidorus attenuatus tested the effect of fumigating the soil with 3741/ha D-D and two amounts of nitrogen fertilizer on different crop sequences between 1966 and 1969. Although severe Docking disorder did not recur in sugar beet, fumigation increased yield in each of the three following years. Yield of barley was increased for 4 yr and of wheat, potatoes and ryegrass for 1 or 2 yr after treatment. All plant parasitic nematodes were controlled by the first fumigation and the numbers of those in unfumigated plots 3 yr after treatment. Fumigation also largely prevented infection of sugar beet by the fungus Helicobasidium purpureum.     

Growth of Lettuce, Onion and Red Beet. 2. Growth Modelling

Data from a field experiment carried out on growth of lettuce,onion, and red beet were used: (a) to fit logistic, Gompertz,expolinear and ‘Scaife and Jones’ (Journal of AgriculturalScience, Cambridge86 : 83–91, 1976) functions using time,day-degrees and effective day-degrees; and (b) to test a mechanistically-basedmodel that combines the effects of potentially limiting variables,such as temperature and light, and allows for plant zone areain light interception (Aikman and Benjamin,Annals of Botany73 : 185–194, 1994). The use of day-degrees and effective day-degrees instead oftime, in general, improved the fit and gave a better estimateof growth parameters. The best fit was obtained by the Gompertzfunction for lettuce, and by the expolinear function for redbeet and for onion. The expolinear function seemed the mostreliable function in estimating the early relative growth ratewhich is the crucial value in all the mechanistic models. Thezone area model showed very good simulations for lettuce andred beet, but it requires a modification for canopy senescencein onion. Lettuce; Lactuca sativa L. var.crispa ; onion; Allium cepa L.; red beet; Beta vulgaris L. var.conditiva ; growth modelling; logistic; expolinear; Gompertz; zone area; time; day-degrees; effective day-degrees     

An Analysis of Spatial Variation in the Nitrogen Content of Leaves from Different Horizons Within a Canopy

An analysis of spatial variation in the specific nitrogen contentof leaves from different positions within a canopy is developed.It is used to examine data obtained for contrasting crop species;the legume crop guar (Cyamopsis tetragonoloba), and crops ofa forage sorghum. The analysis distinguishes two componentsof the leaf nitrogen content: one associated with the metabolicapparatus within the leaf tissues and the other with structuralelements of the leaf. The analysis allows the spatial variationin specific leaf nitrogen content to be quantitatively described. Leaf nitrogen content, crop growth, light interception