5.4 Plantdata
H.vanKeulen
In the foregoing chapters it was shown that a quantitative assessment of
plant growth and crop productivity can be obtained in different production
situations, on the basis of knowledge about basic properties of plants and
soils. In this section some of the required plant data will be summarized,
methodstoobtainthemoutlined andindicativevaluesprovided for first estimates.
5.4.1 Photosyntheticcapacity
Asoutlined inSection2.1,grossC0 2 assimilation of acanopy, Fgc,maybe
calculated from theleaf areaindex and measured orestimated values forFg,
thediffusion—limitedmaximumgrossassimilationrateofasingleleafathigh
light intensity. Thelattercharacteristic maybeexperimentally determinedby
measuring the C0 2 absorption by a leaf of known surface area at high light
intensities.Thisismost conveniently donebyenclosing theleaf inachamber
throughwhichairispassedataknownflow rate.Theconcentrationof C0 2is
measuredattheentrancetothechamberandattheoutlet, sothattheamount
of C0 2 absorbed can be calculated. As assimilation and respiration proceed
concurrently, the measured value represents the net assimilation rate, thatis
thedifference betweenassimilationandrespiration.ThustoobtainFg,asused
byGoudriaan&vanLaar(1978a),themeasuredvaluemustbeaugmentedby
thevalueof thedarkrespiration(Subsection5.4.2)implicitlyassumingthatit
proceeds atthesamerateinthelight. Suchexperiments arenormallycarried
outundercontrolledconditions,withthedisadvantagethatitisoften difficult
toobtainsufficiently highlightintensitiesfromartificial lightsourcestoreach
lightsaturation, especially forplantsof theC4type.Although thevalueofFg
varieswithsuchcharacteristics asageof theleaf, itsnitrogencontent andits
positioninthecanopy(especiallyitspositionwithrespecttothesun),characteristic values of maximum C0 2 assimilation range from 15-50 kgha"1h"1
forplantsoftheC3typeand3 0 -90kgha"1h"1forC4species.Ifthevalueof
Fgisunknownforaspecies,40kgha"1h"1foraC3speciesand70kgha"1 h"1
foraC4speciesisgenerallyareasonableguess.
Listsof C4species havebeenpublished byDownton(1975)andRaghavendra& Das(1978).WhetheraspecieshastheC3ortheC4type photosynthetic
pathwaycanbededuced fromanatomical features. InC4typeplants,mesophyllcells arearranged radiallyaroundchlorenchymatic bundlesheaths,whereasinC3typeplantsmesophyllcellsarelaminar.Thesedifferencesareclearly
235
visibleunderamicroscope.
An alternative method is based on the fact that the C0 2 compensation
point, that isthevalueof theC0 2 concentration intheoutsideair, wherenet
C0 2assimilationbecomeszero,isabout 10ppmvforC4speciesand 120ppmv
for C3species.Whenanunknownspeciesisgrowninanairtightenvironment
in the presence of a C4 species, it will cease functioning long before theC4
speciesif ithappenstobelongtotheC3 group. Itwillholdout approximately
equally long if it is a C4 species. In the closed system, the photosynthetic
process will lead to gradually declining concentrations of C0 2 . When these
havefallenbelow 120ppmv,respirationoftheC3speciesexceedsassimilation,
whereastheC4speciescontinuetoassimilateatapositiverateuntill about 10
ppmv.
5.4.2 Respiratorylosses
Measurement of respiration isinprincipleidentical tothat of assimilation,
i.e. the C0 2 exchange rate is determined in the absence of an energy source
('darkrespiration').Whenplantsthathavebeeninthelight forsometimeare
transferred to thedark and C0 2 exchange is measured, thegeneral pictureis
thatat first therateof C0 2 evolution ishighbutgraduallydeclineswithtime
and finally reaches a more or less stable value. In analyzing such data it is
often assumed that the stable value represents maintenance respiration, and
thattheadditional component inthebeginningresultsfromtheconversionof
assimilatesinstructuralplantmaterial,i.e.growthrespiration.
Themagnitude of therespiration losses depends primarily onthechemical
compositionofthestructuralmaterialoftheplant(Section2.1).Bothmaintenance respiration andgrowth respiration arehigher, whentheprotein(nitrogen)concentrationofthematerialishigher.
Valuesof themaintenancerequirementsmaybeestimated fromtheprotein
concentration andtheashconcentration of thematerial, byassuming thatat
20 °C the proteins require about 0.035 kg CH20 per kg per day for maintenance and the minerals about 0.07. In a situation where each of these two
components makes up about one tenth of the total weight of the standing
crop, the maintenance requirement will thus be 0.0105 kg CH20 per kgdry
weightperday. *
Exercise75
Calculate the carbohydrate requirement for maintenance respiration in kg
ha -1 d""1, foracropwithadryweightof 10000kgha"1,anitrogenconcentration of 0.025 kg kg"1 and an ash concentration of 0.01 kg kg"1 (N.B. the
nitrogenconcentrationofproteinsis0.16kgkg"1).
236
In most cases, however, such detailed analyses cannot be performed, because insufficient data areavailable. Moreover, inthehierarchical approach
presentedhere,thenitrogenconcentration of thematerial isnot knownwhen
the respiratory losses have to be calculated (Sections 2.3 and 3.4). In that
situationapproximatevaluesmaybeapplied,asillustratedinTable3.
The effect of growth respiration may be expressed bythe conversion efficiency, explained in Section 2.1. Thus, if the conversion efficiency is 0.7,
growth respiration amounts to a fraction 0.3 of the weight of the primary
photosynthates. Thusagain, highproportions of proteins, butalso fats, lead
tohighlossesingrowthrespiration.
Exercise76
Calculate the rateof increase indrymatter in kgha"1 d"1 for acropwhich,
aftermaintenance,hasacarbohydratesurplusof400kgha"1 d"1:
- if cassava tubers are filled (composition: proteins 0.075 kg kg"1; starch
0.925kgkg"1)
- ifmaizegrainsarefilled (composition:proteins0.125 kgkg'1;starch0.875
kgkg"1)
- ifsoybeanseedsarefilled (composition:proteins0.4 kgkg"1;starch0.4kg
kg"1,lipids0.2kgkg"1)
Whatarethecarbohydratelossesduetogrowthrespirationinthesecases?
Ifnoexactdataareavailableforspecificsituations,theconversion efficiencies as tabulated inTable 3maybe applied for thevarious groups of crops.
Chemicalcompositionof storageorgansofvariouscropsaregivenbySinclair
&deWit(1975)andPenningdeVriesetal.(1983).
5-4.3 Phenology
One of the most important crop characteristics that has to be taken into
account in establishing production potentials is phenological development
(Section 2.2). It has been explained that various methods exist to describe
phenological development asa function of temperature and daylength. The
most simple method, which, however, yields good results in many cases is
referredtoastheThermalUnitapproach. Inthatapproachitisassumedthat
for each phenological period, the rate of development can bedescribed asa
linearfunction of temperature.Therateof development duringsuchaperiod
isthendefinedastheinverseofthedurationofthatperiod,thus:
Rv = 1/Nd = bt(T. - To)
(94)
237
where
Rv
Nd
bt
Ta
T0
israteofdevelopment(d"l)
islengthofaphenologicalperiod(d)
isaconstant(d" l0 C _I )
isaveragedailyairtemperature(°C)
is apparent threshold temperature or base temperature (°C), below
whichphenologicaldevelopmentcomestoastandstill
Exercise77
Calculate therateof development for thewheat cropof the followingexample.Nhmdenotesthenumberofdaysfromheadingtomaturity.
Location
T.
Harrow
Ottawa
Normandin
SwiftCurrent
Lacomoe
Beaverlodge
FortVermilion
FortSimpson
21.1
20.6
15.0
19.1
14.7
13.9
15.3
16.4
N
hm
31
29
54
41
59
54
48
38
Makeagraphof theresults, temperature onthehorizontal axisanddevelopment rate on the vertical. Draw an eye-fitted straight line and estimate the
values of T0 and bt. How many days will it take this wheat variety from
headingtomaturityatanaverageairtemperatureof 12.5°C?
ApplicationoftheThermalUnitconceptleadstothenotionthataconstant
temperaturesum,countedabovethebasetemperature,hastobeaccumulated
during a certain phenological period. That period can be between any two
phenological eventsthatareimportant from thepoint of viewof production.
For modelling purposes, ingeneral two phases aredistinguished, one before
themomentthatgrowthofthestorageorgansstarts,oneafterthat. Incereals
thatisthemomentof anthesis,inrootandtubercropsitisthemomentofthe
startofbulking.Forbothperiodsascalefrom0to 1 isthenassigned.Inmost
cases it is convenient to start the pre-storage organ phase from emergence,
because the temperature relations for germination may be quite different.
Inpractice, therelation betweentemperature anddevelopment ratecanbe
determined experimentally by observing the dates of emergence and that of
thestartofthestorageorgangrowthandmeasuringtheambienttemperatures
238
duringthatperiod. If onlydatafor oneyearareavailable, basetemperatures
asgiveninTable7(Section2.2) maybeusedtoconstruct thelinearrelation.
Foramoreaccuratedescription, it willbenecessary, asarule,to performat
least two experiments under different temperature conditions to be able to
derivethebasetemperature. Suchexperimentscaneasilybecarriedoutunder
controlled conditions, i.e. in climate rooms or greenhouses. For the period
after the start of storage organ growth, experimental determination maybe
moredifficult, becausethemomentof maturityisoften notclearlyrecognizable. Fordifferent crops, different criteria have been specified (for maize for
instanceformationoftheblacklayeratthebaseofthekernel).Onthebasisof
these criteria, the maturity point may then be estimated and an identical
procedureapplied.Asarule,theslopeof thedevelopment rate- temperature
relationisdifferent fortheperiodbeforethestorageorganstartsgrowingand
theperiodthereafter.
Exercise78
Calculate the rate of development for the period emergence to anthesis for
twospringwheatcultivars.Taisaverageairtemperaturein°C;Nacisduration
oftheperiodindays.
cv.Timgalen
*a
12.5
14.1
17.0
^ac
93
72
61
cv.Orca
T
11.6
13.2
17.0
N
95
78
69
Dothesamefor theperiodofanthesisto maturity.
cv.Timgalen
Tx
a
19.5
20.5
22.1
cv. Orca
N
X1
am
49
48
41
Tx
N
a
17.6
17.6
16.9
43
40
39
Plot thecalculated data asin Exercise 77for thetwo different periods in two
separategraphs.Draweye-fitted curves.What doyounoticeabout theslope
ofthetwolines?
239
For modelling purposes the relation between temperature and development
ratewillthushavetobeestablished foratleasteachgroupof cultivars(short
durationvs.longduration)withinaspecies.
5.4.4 Drymatterdistribution
Thedistribution of thedrymatterformed betweenthevariousplantorgans
is a major determinant for both total dry matter production and economic
yield. Ifintheearlystagesof cropdevelopment alargeproportionof thedry
matter formed isinvested in theroot system, leaf area development isrelativelyslow, hencelightinterceptionwillbeincompleteforalongperiodoftime
andtotalproductionwillremainlow. Inthesameway,economicyieldmaybe
affected if during the period of storage- organ growth other organs of the
plant still claim asubstantial proportion of theavailable assimilates orif the
green area rapidly declines because of translocation of essential elements to
the developing storage organs, as may be the case under nitrogen shortage.
Since the basic processes governing dry matter partitioning arepoorlyunderstood, the subject istreated inanempirical way. Thesimplest method of
analyzingdistributionpatternswouldbetodescribetheincreaseindryweight
ofthevariousplantorgansinrelationtothetotalincreaseindryweightofthe
plant, i.e. to its age. However, asindicated in Section 2.2, theinitiation and
development of plant organsisgoverned byphenological development of the
plant, which is a function of environmental conditions, such astemperature
anddaylength, ratherthanof age.Thepartitioning patterninplantsistherefore relatedtothephenological stageof thecrop.Thisstartingpointseverely
limits the availability of suitable experimental data for the determination of
partitioning patterns, because thenumber of reports inwhich not only sufficiently detailed cropdataarereported, but also environmental conditions, is
verylow.
If, however, all of thesedataareavailable, ananalysis maybecarriedout,
yieldinginformation thatcanbeusedforextrapolationandprediction.Thisis
illustrated with an experiment on bunded rice, reported by Erdman(1972).
Thebasicdataof theexperimentaregiveninTable57.Thesedataareusedto
calculate the derived data as given in Table 58: for each harvest date the
development stage of thevegetation isdetermined astheratio betweenaccumulated temperature sumandthetemperature sumrequired fromtransplanting to anthesis (1950 d °C for this variety). As the distribution factors are
calculated for periods between two consecutive harvests, the value of the
developmentstagehalfway betweenthesetwodatesisenteredinTable58.For
eachperiod, theincrease indryweight of thevarious plant organsiscalculated as the difference between the weight at"two consecutive harvests. From
these partial values, increase in total dry weight is obtained and from these
numbersthefraction allocatedtoeachof theorgansineachperiodiscalculated.
240
Table 57. Basic data on dry matter distribution from a field experiment with bunded
rice.
Time
(daysafter
transplanting)
T.
(°C)
dryweightof plantorgans(gplant "*)
roots
leafblades stems
sheaths
+ ear
structures
total
26.0
11
•
0.15
0.26
0.24
0.65
0.26
0.94
0.84
2.07
0.34
2.19
2.23
4.76
0.81
3.98
4.48
9.27
1.93
6.06
9.37
17.36
2.81
7.82
12.76
23.39
2.99
9.76
21.46
34.21
3.17
10.77
26.83
40.77
3.36
11.85
21.48
36.69
3.37
9.11
21.94
34.42
26.3
18
26.3
25
26.3
32
26.3
39
26.2
46
26.2
53
26.2
60
26.2
67
26.3
74
26.3
81
26.3
88
Theaccuracyof theindividual observations isnot high.Evenwithareasonable number of replicates, the variance in dry matter yield is high, so the
increase in dry matter of individual organs is even less accurate. Ingeneral,
therefore, observations of several experiments havetobecombinedtoobtain
auseful relation. InFigure54,thedataof Table58aresummarizedbyeye fitted curves, illustrating the variability in the data. However, for the time
beingitisnecessarytorelyonthistypeofanalyses.
After anthesis, only the grains increase in dry weight, the other organs
eitherremainingconstant ordecreasing inweight, whentranslocation ofsubstances, nitrogenous compounds aswell ascarbohydrates, to the developing
graintakesplace.
AsexplainedinSection3.4,thistranslocationprocessistakenintoaccount
in the calculations by assuming partitioning of part of the assimilates to the
241
Table58.Deriveddataondrymatterdistributionfromafieldexperimentwithbunded
rice.
Period
Rd
(days after
transplantirig)
leaf blades
0-11
11-18
18-25
25-32
32-39
39-46
46-53
53-60
60-67
67-74*
gplant-1
0.26
0.68
1.25
1.79
2.08
1.76
1.94
0.99
1.08
negative
Increase indrywei ight
0.075
0.195
0.29
0.385
0.475
0.57
0.665
0.76
0.855
0.955
fraction
0.40
0.48
0.46
0.40
0.26
0.29
0.18
0.15
n.a.**
n.a.
roots
'stems'
gplant"1
0.24
0.60
1.39
2.25
4.89
3.39
8.70
5.37
negative
negative
fraction
0.37
0.42
0.52
0.50
0.60
0.56
0.80
0.82
n.a.
n.a.
gplant" "!
0.15
0.14
0.05
0.47
1.12
0.88
0.18
0.18
0.18
0.01
fraction
0.23
0.10
0.02
0.10
0.14
0.15
0.02
0.03
* AnthesisatDay74
**Notapplicable
grainbeforeanthesis.
The grain weight calculated with the model is on a dry weight basis. For
comparison with field data, the air dry moisture content of the grains has to
be taken into account. Normally grain yields are reported at 1 2 - 14% moisfraction to organs
0.80 _ stems*sheaths*ear structures
0.60
O ^
0.40
^
leaf blades
>^
0.20
roots „ ^N^s*-«. *
I
0.2
*
*
N.
i *
" i — X
0.4
0.6
0.8
1.0
development stage
I
Figure 54. Partitioning pattern of total dry matter increase between various organs of
bunded rice, as a function of development stage.
242
turecontent, hencethecalculateddrymattermustbemultipliedbyafactorof
1.13. Forcropsotherthangraincropsspecial moisturecontents mayhaveto
betakenintoaccount. Thisemphasizes, however, that forexperimentalwork
tooanaccuratedefinition ofthereporteddataisnecessary.
For rice the situation is somewhat more complex: in most cases yield is
expressed in rough rice (paddy) at a moisture content of 14%. Rough rice,
however, consistsof thetruefruit (brownrice)andthehull,whichconsistsof
leaf- likestructures.Brownriceislargelytheendospermandtheembryo,but
stillcontains several thinlayersof botanically different tissuesthatareremovedduringmilling,afterwhichwhiterice,milledriceorpolishedriceisobtained.Inthemodel,brownriceistheresultofthecalculations.Toconvertfrom
white rice to brown rice a conversion factor of 1.25 is generally used, from
brownricetopaddyafactorof 1.2.
If the growing conditions are suboptimal, i.e. when shortage of water or
nutrientsoccurs,theplantwillgenerallyrespondwithachangeinitsdistributionpattern. Inbothcases,themostcommonresponseisanincreasedinvestment in the root system, because that part of the plant is responsible for its
supplywithwaterandnutrients.Thisphenomenon isoften referred to asthe
functional balance (Brouwer, 1963). A quantitative treatment of thisphenomenonisdifficult withinthescopeofthepresentapproach,moreoverthedata
baseisnarrow.
5.4.5 Stomatalbehaviour
Stomatal aperture in plants can change in response to internal orexternal
conditions, so that plants canreact to theenvironment. Onesuch reactionis
thatof closureof thestomataunderconditions of waterstress,which effectively reduces transpiration. However, even under optimum moisture supply,
stomatal opening may vary. One mechanism governing stomatal aperture is
theresponse to light, i.e. stomata open inthelight andclose inthedark, the
transition zone being rather narrow. However, in certain plants, stomatal
aperture is regulated in such away, that theconcentration of C0 2 insidethe
stomatal cavity ismaintained within narrowlimits.Thismayeitherbeaconstant value, which isaround 120ppmv for plants withthe C4type of photosynthesis andaround 210ppmv for plants withtheC3type. Analternativeis
thattheinternal concentration isadjusted insuchawaythat aconstant ratio
of about 0.7 for C3speciesandabout0.4 for C4speciesbetweenexternaland
internal C0 2 concentration is maintained (Goudriaan &van Laar, 1978b).
Under present conditions thereis hardlyanydifference between thetwomechanismsinthefield, becauseatanexternalvalueof340ppmvaboutthesame
internalvaluesresultforbothsituations.
The type of stomatal control is of prime influence on water use efficiency
(Section 3.2). The ratio of the amount of C0 2 fixed to the amount of water
lost isabout twiceashigh forplantswithC0 2 inducedstomatal regulationas
243
for plants without that mechanism. It is therefore important to know what
typeof regulation operatesinagivencrop.Unfortunately, that isdifficult to
predict,becausenotonlydifferences existbetweendifferent species,butapparentlythesamespecies mayreact indifferent ways, depending onexternalor
internal conditions. The only way to determine experimentally the type of
regulating mechanism is by measuring concurrently C0 2 assimilation and
transpiration atarangeof external C0 2 concentrations. Suchdatapermitthe
construction of the type of graphs, schematically presented in Figure 55.
Increased C0 2 concentrations in the external air lead to stomatal closure in
regulating plants, hence decreased transpiration and aconstant assimilation.
In non- regulating plants assimilation increases, but transpiration is hardly
transpiration /assimilation
transpiration
non-regulating
assimilation
regulating
external C 0 2 Concentration
Figure 55. Schematic representation of transpiration and assimilation as a function of
external CO: concentration for plants with and without CO> induced stomatal regulation.
244
affected asthestomataremainopen.Themostcommontypeseemstobethat
inwhichaconstant ratiobetweenexternalandinternalconcentrationismaintained. Theabsence of C0 2 induced regulation ismost likely associated with
maintenanceof optimummoisturesupplythroughout theplant'slifecycle.In
situations where noinformation on stomatal behaviour isavailable,assumptionoftheconstantratioismostrealistic.
5.4.6 Nutrientrequirements
InSection4.1 itwasshown for maizeandricethatunderconditionswhere
yield is determined byavailability of aspecific nutrient element, theconcentration of that element at a given development stage of the crop reaches a
species- characteristic minimum value. The determination of total nutrient
requirements as proposed in this approach, require the minimum element
concentrations atmaturity. Suchdatacanbedetermined forspecific cropsin
pot experiments, because thesevalues areindependent of thegrowing conditions. Asafirstapproximation, values havebeencollected from theliteraturepertaining to groups of crops. Thedeterminant factor for eachgroupis
thechemical composition of thematerial formed. Thedatausedinthemodel
structurearesummarizedinTable59.
5.4.7 Someadditionaldata
For initialization of the calculations for Production Situation 1or2(Sections 2.3 and 3.4), the values of above-ground dry weight and root dry
weight are necessary. It is possible to determine these values experimentally
foraspecific trial. Howeveramoregeneralapproachistolook attheconversion of the substrate contained in the seed into structural plant material.
Experiments with a number of species have shown that about half of that
energyislostinrespiration, whereastheremainderisequallydividedbetween
shoot androot (Penning deVriesetal., 1979).Thusboth root andshootdry
weight at emergence may be estimated as about one fourth of the seedrate,
Table59.Minimumconcentrations(kgkg"1)ofthemacro-elementsineconomicproductsandcropresiduesforanumberofcropgroups
Cropgroup
Economicproduct
grains
oilseeds
rootcorps
tubercrops
N
0.01
0.0155
0.008
0.0045
P
0.0011
0.0045
0.0013
0.0005
Cropresidues
K
0.003
0.0055
0.012
0.005
N
0.004
0.0034
0.012
0.015
P
0.0005
0.0007
0.0011
0.0019
K
0.008
0.008
0.0033
0.005
245
Table60.Indicativespecificleafareaofmajorcrops(m2kg"1).
CropSpecificleafarea(S.L.A.)
Barley
Cassava
Chickpea
Chillie
Cotton
Cowpea
Grasspea
Groundnut
Jute,capsularis
Jute,olitorius
Kenaf
Lentil
Maize
Mungbean
Mustard(black)
Onion
Rapeseed
Rice
Sesame
Sorghum
Soybean
Sugarcane
Sweetpotato
Tobacco
Wheat
25
22
13
27
20
25
13
28
31
28
25
33
18
30
23
25
23
25
23
20
26
10
22
16
20
providedthatgerminationisalmostcomplete.
The development of leaf area, important for interception of irradiance,
depends on the one hand on the amount of dry matter invested in the leaf
blades(Subsection 5.4.4), andontheotherhandontheareathateachunitof
leaf blade dry matter produces. In Sections 2.3 and 3.4 this parameter was
introduced as the specific leaf area.-It follows directly from simultaneous
measurement of dry weight and area of the leaf blades. Special leaf area
meters areavailable that electronically scan theareasof theleaves. A widely
usedmethodthatrequiresnosophisticatedequipmentistomeasureleafwidth
and leaf length and establish a relation between the product of the two and
actual leaf area, depending on leaf shape. Forsituations where experimental
data on specific leaf area are not available, indicative values for different
cropshavebeensummarizedinTable60.
Ingeneral,itmaybeconcludedthatforthetypeofcalculationsatwhichthe
present approach isaiming, and theirdegree of accuracy, theavailable plant
246
data are of sufficient quality. It means that these estimations can be made
withouthavingtogointodetailedplantphysiologicalresearch.
247