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In the absence of drought and nutrient short-ages, the growth and development of cropsare ultimately controlled by the interactionof plant systems with specific elements ofthe solar spectrum. This chapter describesthe influence of solar energy on potentialgrowth and development in crops. InChapter 3 we consider how the availabilityand demand for water often set limits onactualcrop performance.

2.1 The solar spectrum and plantprocesses

Although the sun radiates energy across thewhole electromagnetic spectrum, the earthsatmosphere is transparent to all the visibleradiation but only to part of the infrared andultraviolet radiation that is emitted from thesolar disk. All forms of electromagnetic radi-ation have wave characteristics and travel atthe same speed (3 108 m s1) but the vari-

ous types of radiation differ in wavelength.Radiation can be considered in terms of dis-crete particles or quanta each of which hasan energy content that is inversely propor-

23

2

Solar Radiation

CAB International2002. Principles of Tropical AgronomyS.N Azam-Ali and G.R. Squire

1

Fig. 2.1. The solar spectrum indicating the wavelength characteristics of photosynthetically activeradiation (PAR) (0.40.7 m), daylength (0.660.73 m) and temperature (3100 m) effects on the

growth and development of crops.

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tional to its wavelength, i.e. the shorter thewavelength, the greater is the energy con-tent. There are two major types of radiation:shortwave radiation within the waveband

0.253.0 m and longwave radiation withinthe waveband 3100 m.There are three regions of the solar spec-

trum that have significance for plantprocesses. These are: radiation within thevisible range (0.40.7 m), often referredto as light; the red (0.66 m) to far-red(0.73 m) range which signal changes indaylength, and longwave or terrestrialradiation which determines temperature(3100 m). Figure 2.1 illustrates the major

regions of the solar spectrum in terms oftheir influence on plant processes.

2.1.1 Light and photosynthesis

Radiation within the visible range, i.e. light(0.40.7 m) is termed photosyntheticallyactive radiation (PAR). The energy con-tained within this waveband is the onlyradiation that can be actively used for plant

growth by driving pigment-based systems inthe process of photosynthesis. A quantum oflight is called a photon and the process ofphotosynthesis is powered by photons ofradiation which are absorbed by pigmentmolecules, such as chlorophyll, in order toconvert atmospheric carbon dioxide (CO2)to carbohydrate. The energy content of PARvaries with time and atmospheric condi-tions but is typically close to 50% of the totalenergy within the solar spectrum (Monteithand Unsworth, 1990).

Photosynthesis

Green plants must capture and use externalresources, principally light, CO2, water andnutrients, to produce dry matter via photo-synthesis. By this process, plants synthesizeorganic compounds from inorganic materi-als in the presence of sunlight. The majorchemical pathway in photosynthesis is theconversion of atmospheric CO2 and water to

carbohydrates and oxygen as shown inEquation 2.1:

CO2 + H2O CH2O + O2 (2.1)

By the input of solar radiation, two energy-poor compounds (carbon dioxide and water)are converted into two energy-rich com-pounds (carbohydrate and oxygen). The car-

bohydrates formed via photosynthesis pos-sess more energy than the materials fromwhich they are composed and we can con-sider photosynthesis as a process whichreduces atmospheric CO2 and convertslight energy into the chemical energy ofplant tissues. So, it is not surprising that aclose link exists between the photosynthet-ic rate of individual leaves and the amountof light that they absorb.

Photosynthesis reduces CO2 to carbo-

hydrate via two carboxylation pathways: theCalvin cycle and the Hatch-Slack pathway.In C3 crops, the Calvin cycle predominatesand the initial fixation product is a three-carbon compound. In C4 crops, the Hatch-Slack pathway predominates and a four-car-bon compound is the initial product. Here,CO2 is refixed by the Calvin cycle and littleor no carbon is lost through photo-respiration (see below). The C3 speciesinclude all the temperate crops, as well as

tropical legumes, root crops and trees,whereas C4 crops include most tropical cere-als and grasses, such as maize, sorghum,millet and sugarcane.

Respiration

The high-energy compounds produced byphotosynthesis are broken down (or decar-boxylated) by two pathways:photorespira-tion and dark respiration. The process ofphotorespiration is induced in C

3plants by

the presence of oxygen. Photorespirationacts on the CO2 initially fixed by photosyn-thesis and its rate is therefore closely linkedto the CO2 fixation rate. The importance ofphotorespiration increases with tempera-ture, resulting in a reduction in the initialefficiency of light use of individual leaves.There is no photorespiration in C4 plants.However, in these crops, the initial light useefficiency at low temperatures may be lessthan that of C

3

plants because of the energycosts involved in suppressing photorespira-tion.

Irrespective of their photosynthetic sys-

24 Chapter 2

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tem, all green plants undergo the process ofdark respiration in which atmospheric oxy-gen is used by plants to convert carbohy-drates into CO2 and water, with the simul-

taneous liberation of energy. Plants use thisenergy to build more complex moleculesfrom the initial products of photosynthesis.Respiration therefore results in the loss oforganic matter and oxygen from plants anda release of CO2 to the atmosphere.Respiration is an important part of the car-bon budget of crops because it is responsi-ble for the loss of CO2 from plant cells. It canbe considered at two levels; that whichoccurs as a result of the growth of crops and

that which is required for their mainte-nance. It is generally assumed that, at anygiven temperature, respiration continues inthe light at a comparable rate to that in thedark and, moreover, that during the life of acrop, the relative contributions of thegrowth and maintenance components of res-piration change with the age and weight ofthe crop. However, there have been very fewmeasurements of respiration in the field andit is uncertain how total respiration is divid-

ed between growth and maintenanceprocesses.

Carbon dioxide exchange

At any time, the net photosynthetic rate ofa green plant depends on the distribution ofquanta over individual elements of crop

foliage, the relation between photosynthet-ic rate and irradiance for each element offoliage and on the corresponding rates ofCO2 loss. When light is not limiting, photo-synthesis is controlled by the rate at whichCO2 from the atmosphere is reduced to car-bohydrate. In very weak light, the relationfor both C3 and C4 plant systems is almostlinear because photosynthetic rate is limit-ed almost exclusively by the absorption oflight quanta. This initial slope, or quantum

efficiency is a measure of the amount of CO2absorbed per unit increase in irradiance.Typically, 810 photons are required to fixone molecule of CO2 to carbohydrate. Afterthis initial linear phase, the photosyntheticrate of C3 species in strong light approachesa plateau at a saturating irradiance with amaximum value that declines with leaf age.In contrast, C4 species show less evidence oflight saturation and, therefore, no markedplateau in photosynthetic rate at high irra-

diances. The apparent photosyntheticadvantage of C4 crops over C3 crops can thusbe ascribed both to the absence of photores-piration and to greater photosynthetic ratesin strong light. Figure 2.2 illustrates the typ-ical light response curves for a C3 and a C4leaf.

Irrespective of species, the net photo-synthetic rate, P(mol m2 s1) of any greenplant is the difference between the rate atwhich CO2 is reduced to carbohydrate and

the rate at which carbohydrate and othercompounds are oxidized to provide theenergy necessary for metabolic processes(Monteith, 1981). The net flux of CO2 into aleaf can be expressed as

where ca (g of CO2 per m3 of air) is the con-

centration of CO2 in the ambient air, ci is theconcentration of CO

2in the air spaces with-

in the leaf, ra (s m1) is the leaf boundary

layer resistance, i.e. the resistance providedby a thin layer of still air at the leaf surface

Solar Radiation 25

Fig. 2.2. The typical relation between

photosynthetic rate and irradiance for C3 and C4

species. The dashed line indicates the maximumphotosynthetic rate (g CO2 m

2 h1) or saturating

irradiance (W m2) for C3 species; C4 species

show less evidence of light saturation.

P

c c

r r=

+

( )

( )a i

a s

(2.2)

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to gas transport and rs (s m1) is the leaf

stomatal resistance which depends on thedistribution and degree of opening of stom-atal pores on the upper and lower surfaces

of the leaf. Figure 2.3a and b graphicallyillustrates the pathway and resistances tothe entry of CO2 into a C3 and a C4 leaf andthe concomitant loss of water vapour andoxygen through the same apertures (seeChapter 3).

2.1.2 Daylength and morphogenesis

The earth spins around its axis once every24 hours and at the equator all days of theyear are of equal length divided into equalperiods of dark and light. Strictly, thedaylength at the equator is 12 hours 7 min-utes rather than exactly 12 hours. (The extra7 minutes accounts for the time taken for theupper half of the sun to disappear under thehorizon at sunset and to fully appear at sun-rise.) With increasing distance from the

26 Chapter 2

Fig. 2.3. A schematic cross-section of (a) C3 leaf and (b) C4 leaf showing the pathways for carbon

dioxide entry and water vapour loss.

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equator, the difference between the shortestand longest day of the year increases. At lowlatitudes the increase is about 7 minutes perdegree but at higher latitudes the difference

is larger. For example, between 50 and 60it is about 28 minutes per degree of latitude.Changes in visible radiation in the red

(0.66 m) to far-red (0.73 m) range can haveprofound effects on many organisms. Interms of plant behaviour, changes in red :far-red can alter the pattern of growth anddevelopment of some plant species. Thisphenomenon, known as photomorphogene-sis, is responsible for the flowering behav-iour of many species. More precisely, it is

changes in the ratio of red to far-red radia-tion that influence the reproductive behav-iour of plants. Above a solar elevation ofabout 10 the ratio remains constant at about1:1. However, at sunrise and sunset the ratiochanges rapidly. Periodic changes in theratio from winter to summer are responsiblefor daylength effects in plant species. Theperception of changes in daylength can beconsidered as an informational resource incontrast with PAR which can be considered

as a growth resource.In some crops, an information signal isrequired for them to switch from vegetativeto reproductive activity. When this signal isprovided by changes in daylength the phe-nomenon is known as photoperiodism.Thus, crop species can be defined as pho-toperiod-sensitive or photoperiod-insensi-tive. Species that are sensitive to changes indaylength can be further sub-divided intothose which flower in response to short days(short-day plants) and those whichrespond to long days (long-day plants).Species that are unaffected by daylength areknown as day-neutral plants. Withinspecies that are predominantly short-day orlong-day, there exist varieties that may alsobe day neutral.

2.1.3 Temperature and plant development

Both the longer wavelengths of earthwardsolar radiation and long-wave terrestrialradiation, i.e. reflected radiation from theground, are insufficiently energetic to drive

pigment systems such as photosynthesis.However, the balance between incomingand outgoing radiation is primarily respon-sible for the temperature of plants and soils

which strongly controls the developmentrates of plants.

Germination

A good example of the dominance of tem-perature on developmental events is the ger-mination of seeds. For germination to occur,seeds must be viable, non-dormant and ade-quately supplied with water and oxygen.For most of the major crop plants so farexamined, the initiation of germination in

each species requires a minimum base tem-perature (Tb). Thereafter, germination rateincreases linearly with temperature to anoptimum value (To), above which the ratedeclines linearly to a maximum temperature(Tm). In some species, this optimum tem-perature may cover a range of severaldegrees rather than a single temperature.The typical relationship between germina-tion and temperature is illustrated in Fig.2.4. When the temperature, T, is above Tb

and below To the relation between Tand 1/tis given by

(2.3)

Solar Radiation 27

1

1t

T T=

( )b

Fig. 2.4. The typical relation between germination

rate (expressed as the reciprocal of time, t) and

temperature for viable seeds (open symbols). Thesolid line without data points indicates the same

germination sequence expressed in days.

(Adapted from Squire, 1990.)

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and when Tis above To and below Tm therelation is given by

(2.4)

The constants 1 and 2 represent thethermal duration for the developmentalprocess of germination to occur.

Thermal time

Because the quantity is the product oftemperature and time it is measured indegree-days (Cd) and is called thermal time.In practice, the value of Tb is usually simi-lar for different developmental processes

within a species and thus the thermal timefor any process can be defined as,

(2.5)

where n is the number of days experiencedby a plant at a mean daily temperature, T,above Tb. Thus, on a day when the temper-ature remains below T0, the thermal timeaccumulated by a crop is simply the differ-

ence between the mean daily temperatureand the value of Tb.

Thermal time is an extremely usefulconcept because it allows the developmentof crops at different locations and in differ-ent seasons to be compared. This is particu-larly true for determinate crops, which havea clearly defined point at which seeds arephysiologically mature. Often, the samecrop variety is grown at a number of sitesand so requires different amounts of chrono-

logicaltime, but similar amounts of thermaltime, to reach maturity. Thus, although thechronological duration for any particulardevelopmental process is shortest at theoptimum temperature and lengthens at tem-peratures below (or above) this value, thethermal duration for the same processremains similar at each location.

Where the switch to reproductivegrowth does not require a specific daylength(i.e. day-neutral or daylength insensitive

plant types), the date of flowering in tropi-cal crops can be estimated from a knowledgeof thermal time.

The concept of thermal time remainsvalid over a wide range of environmentalconditions and is largely unaffected bychanges in solar radiation. However, ther-

mal time is often sensitive to water deficits.

Developmental windows for canopy

expansion

The prevailing temperature and photoperi-od set the potential duration of each devel-opmental phase of a crop. The amount ofgrowth that occurs within each develop-mental phase is set by the amount of solarradiation intercepted by plant canopies.

The rate at which leaves are produced

and expand early in the season determinesthe amount of radiation that a plant canintercept. Generally, temperature andphotoperiod determine the number of leaveson a plant, while temperature, water statusand nutrients dominate the extension ratesof individual leaves. So, to maximize thesize of its intercepting surface, a plant mustexpand individual leaves as rapidly as pos-sible within the developmental period set bythermal time.

To expand their leaves and synthesizeorganic compounds, plants must take upinorganic nutrients from the soil. Althoughmany elements are required for optimalplant growth, most of these are required insuch small amounts that the supply from theseed or from natural sources is often ade-quate (Van Keulen and Wolf, 1989).However, the macro-elements nitrogen,phosphorus and potassium are usuallyneeded in such large quantities that they

have to be provided as fertilizers to supple-ment the natural supply. This is especiallytrue where management practices aim atvery high yields. Nitrogen is the elementthat is needed in the largest quantities, andmajor deficiencies in plant nitrogen candirectly affect transpiration, carbon assimi-lation and partitioning to various organs.However, it is the crucial role of nitrogen inthe expansion of leaves that perhaps moststrongly determines the productivity of a

plant.Because nitrogen is needed in the plant

to synthesize new tissue, the total demand

28 Chapter 2

1

2t

T T=

( )m

=

=

=

( )bT Ti

i n

1

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for nitrogen increases with increasing plantweight. However, as more structural carbo-hydrate is accumulated, the ratio of nitrogento total biomass in each of the plant parts

falls, even when nitrogen is available in sur-plus (Van Keulen and Wolf, 1989). The tim-ing and method of application and rate ofuptake by plant roots is critical to the pro-portion of applied nitrogen that is utilizedby individual plants (see Chapter 7).

2.2 The fate of radiation in cropsystems

In this discussion, we have seen the criticalrole that solar radiation plays in the behav-iour of crops. It is therefore not surprisingthat most management decisions, e.g. sow-ing date, planting density, fertilizer applica-tion and irrigation, aim to modify the shapeand extent of crop canopies and the way inwhich they intercept radiation. We now con-sider the efficiency with which crops con-vert solar energy into the chemical energy ofplant products.

The productivity of a plant system canbe defined in terms of the efficiencies withwhich the solar resource is captured andconverted into biomass (Monteith, 1972;Azam-Ali et al., 1994) i.e. the overall effi-ciency, , of the system can be described as

= gapisqrH (2.6)

where, g is a geometrical ratio, i.e. the ratioof solar energy flux to solar constant. Thisfigure represents the proportion of energyreceived at any particular latitude relative tothe irradiance (typically 1373 W m2 or 1.37kJ m2 s1) received on a perpendicular sur-face located at the mean distance of the earthfrom the sun. The value a represents theatmospheric transparency, i.e. the propor-tion of radiation that passes through theatmosphere, p is the spectral ratio, i.e. PARas a fraction of total radiation, i is the inter-ception efficiency of foliage, s is the con-version efficiency of intercepted radiation,

q is the photochemical efficiency, i.e. theenergy value of carbohydrates, proteins andlipids, r is the respiration efficiency and H

is the yield efficiency, i.e. the proportion ofcrop weight that is reproductive or econom-ic yield.

2.2.1 g, aand p

These all influence the amount of photo-synthetically useful radiation that isreceived above a field crop. The value of gis determined by the geometry of the earthssurface in relation to the sun, which in turndepends on latitude and season. The annu-al average value of g decreases from about0.3 in the tropics to 0.2 in temperate lati-

tudes (Monteith, 1972).The value of solar radiation that passesthrough the atmosphere (a) depends onhow much is absorbed and scattered bygases, clouds and aerosols containing dust,smoke and biological material, such asspores and insects. When solar radiationcollides with small gas molecules of theatmosphere, the resulting diffusion isknown as Rayleigh scattering. This diffu-sion is inversely proportional to the fourth

power of the wavelength in other words,short wavelengths are more scattered thanlong wavelengths. The mean global trans-mission caused by Rayleigh scattering isabout 0.91 for direct-beam solar radiation(Bonhomme, 1993). Where diffusion occursdue to aerosols in the atmosphere, thedegree of scattering depends on the particlesizes within the aerosols. Because watervapour contains droplets that are larger thanthe wavelength of the solar radiation, cloudsscatter the radiation uniformly for all wave-lengths. The result of these various process-es is that, where the turbidity of the atmos-phere is small (i.e. the atmosphere istransparent), the diffused radiation containsa greater proportion of short wavelengthsand, because of Rayleigh scattering, the skyis blue. In contrast, when the sky is cloudy,the diffused radiation has a spectral compo-sition that is similar to direct beam radia-tion. Bonhomme (1993) provides a calcula-tion of mean global solar radiation, R

s, that

is composed of a direct-beam (Rb) and a dif-fuse (Rd) component. His calculated averageof 11 MJ m2 for total daily extraterrestrial

Solar Radiation 29

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radiation for the whole of the earths surfaceis distributed as follows (in MJ m2):

0.9 scattered by molecules and aerosols backto space

0.7 absorbed by the gaseous components ofthe atmosphere

2.9 scattered by clouds towards space

1.2 absorbed by clouds

5.3 reaches the ground, i.e. Rs which is par-titioned into Rb = 3.3 and Rd = 2.0.

The ratio of PAR to direct-beam solar radia-tion is approximately 0.45 (Monteith, 1965)and that for diffuse solar radiation is about

0.60 (Sinclair and Muchow, 1999). When thedirect and diffuse components are com-bined, Monteith (1972) estimates that avalue of 0.50 for p is appropriate for thetropics.

Although they each have importantinfluences on the availability of solar radia-tion to crop canopies across the globe, andtherefore the rate and duration of cropgrowth, in practice there is nothing that canbe done by an individual grower to alter g,

a or p at any particular location. In practi-cal terms, there are only two processes thatcan directly influence the performance of afield crop in relation to solar energy.Essentially, the behaviour of solar radiationin crop stands can be described in terms oftheir capture and conversion systems.

2.2.2 Capture of resource

For a crop to produce dry matter, its leavesmust intercept radiation and absorb CO2.The size and duration of crop foliage deter-mine the rate and duration of dry matteraccumulation. The size of the interceptingsurface depends on the green leaf area index(L) of a crop which can be expressed as theproduct of the number of plants per unit ofground area, the number of leaves per plantand the mean area of leaves per plant.

The amount of light that penetrates thecanopy and strikes the ground depends bothon L and on the angular arrangement ofindividual leaves. To describe the pattern of

light penetration through a crop canopy, itis convenient to imagine a crop as consist-

ing of a number of horizontal layers each ofL = 1 (see Fig. 2.5). If radiation is measuredat a number of levels down the crop profile,each corresponding to unit L, then the meas-ured irradiance at any level is a function ofthe angular arrangement of leaves above thatlevel. The relationship for the extinction oflight down a crop canopy is often describedby the MonsiSaeki (1953) equation:

(2.7)

where I0 is the irradiance above the cropcanopy, Iis the irradiance at a level withinthe canopy below a leaf area index of Landk is an extinction coefficient for radiation.However, it should be noted that theMonsiSaeki equation assumes that thecanopy is a homogeneous medium whoseleaves are randomly distributed (such thatthere is no effect of row structure or clump-ing on the pattern of light transmissionthrough the canopy).

In these (ideal) circumstances, lighttransmission obeys Beers law of exponen-tial decay. Strictly, for attenuation to be

30 Chapter 2

Fig. 2.5. The exponential decay of radiation

through a crop stand. In this illustration, the

canopy is divided into three layers of uniform leaf

area index (L = 1). The radiation, I, received at

any level in the canopy is expressed as I/Io = ekL,

where Io is irradiance and kis the extinction

coefficient for the species.

I

Ie kL

o

=

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exponential, the leaves should be black, i.e.opaque to radiation. However, Goudriaan

(1977) has shown Beers law to be a goodapproximation in many real canopies and,in these circumstances, a plot of ln(I/I0)against Lgives a straight line with a gradi-ent kwhich depends on the architecture ofthe crop. Table 2.1 presents some reportedranges in the value of kfor a number of majorcrops. Crops with narrow, erect leaves tendto have lower values of k than crops with

more horizontally displayed leaf arrange-ments.

If the radiation transmitted through thecanopy follows the MonsiSaeki equation

and the extinction coefficient is known, thefraction (f) of radiation intercepted by acrop can be calculated from a knowledge ofL, i.e.

(2.8)

In many tropical systems, crops rarely, ifever, cover the ground completely. This canbe because crops are deliberately sown indistinct clumps or rows to optimize the useof available water rather than light or

because the hostile soil environment pre-vents uniform and complete crop establish-ment. In these circumstances, the Beers lawanalogy of randomly distributed leaves andthe corresponding MonsiSaeki equationfails. Figure 2.6 illustrates the type of radia-tion distribution in stands where crops arearranged in distinct rows compared with amore random arrangement. We return to the

Solar Radiation 31

Table 2.1. The range of some reported values for

the extinction coefficient (k) for ten major crops of

the world. (From Azam-Ali et al., 1994.)

Wheat 0.400.70

Rice 0.430.86Maize 0.560.78

Barley 0.480.69

Sorghum 0.430.60

Millet 0.300.59

Cassava 0.58

Soybean 0.450.96

Groundnut 0.400.66

f kL= 1 exp( ).

Fig. 2.6. A diagrammatic illustration of the pattern of radiation transmitted through a canopy with (a)

randomly distributed leaves (b) bimodal structure.

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implications of non-random canopy struc-ture in Chapters 7 and 8.

The role of nitrogen in light capture

Competition for light between individualplants within a crop canopy reduces thenitrogen concentration (N%) of individualplants (Lemaire and Gastal, 1997). Severalauthors have shown that the reduction ofN% through the vertical profile of severalspecies follows the shape of the attenuationprofile for light (Field, 1983; Charles-Edwards et al., 1987; Pons et al., 1993).Figure 2.7 shows that when leaf N contentis expressed on a leaf area basis, a close rela-

tion is often found between the irradiance(Iz) at any depth within the canopy and theN% per unit leaf area (Nz) (Hirose et al.,1988; Lemaire et al., 1991; Lemaire andGastal, 1997). There appear to be twoaspects to the decline in N% with depth inthe canopy. First, there is the decrease inN% with light attenuation as shown in Fig.2.7 to a light extinction level correspondingto the compensation point for net photo-synthesis. Second, there is a rapid decrease

beyond this point, when the carbon balanceof each leaf is negative and protein mainte-nance becomes impossible, leading to leafsenescence and abscission. Measurements

of N content per unit leaf area on yellowleaves collected immediately after abscis-sion indicate that up to 80% of the originalleaf N content is recycled to the remainderof the plant (Lemaire et al., 1991). Any nitro-gen remaining in the dead leaf is assumedto be structural N, whilst the remobilizedfraction corresponds to the metabolic pool(Lemaire and Gastal, 1997).

2.2.3 Conversion of resource

Conversion efficiency of solar radiation (s)

To calculate the productivity of a stand, thephotosynthetic rate of individual leavesmust be integrated with respect to space andtime. The rate of leaf photosynthesis is lim-ited by the rate at which it can assimilateCO2 from the atmosphere and reduce it tocarbohydrate. The theoretical minimum

energy required to reduce a CO2 molecule isabout 9.5 quanta of PAR (Penning de Vrieset al., 1989). However, the lowest valueobserved in C3 plants, in the absence ofphotorespiration, is about 13 quanta permolecule (Farquhar and von Caemmerer,1982). (The difference is largely becausesome light is absorbed by non-photosyn-thetic pigments.) Nevertheless, leaf photo-synthesis is initially limited by the amountof radiant energy incident on the leaf sur-face. As the incident energy increases, theconcentration of CO2 in the air around theleaves limits the rate of photosynthesis. Thespatial distribution of leaves within a cropcanopy and the extent to which individualleaves are shaded, therefore, markedly affectwhether canopy photosynthesis is light orCO2 limited.

For uniform crop stands, the spatialintegration of photosynthesis can again beapproximated by the MonsiSaeki model,and the irradiance that each leaf interceptscan be modelled in terms of the mean ori-entation of leaves in relation to incidentradiation. Individual leaf photosynthesis

32 Chapter 2

Fig. 2.7. The relationship between relative

irradiance at depth z within a canopy (Iz/Io) and

the relative nitrogen content per unit leaf area

(Nz/No) for a lucerne stand. Io and No representthe irradiance and leaf content at the top of the

canopy. (Taken from Lemaire et al., 1991;

Lemaire and Gastal, 1997.)

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Australia, and Muchow (1992) showed asimilar effect in kenaf where the value of sfor a crop supplied with 240 kg ha1

declined from 1.20 g MJ

1

to 0.94 g MJ

1

ina crop which was not supplied with nitro-gen. Squire (1990) described a similar reduc-tion in s in relation to the leaf potassiumcontent of oil palm fronds growing inMalaysia. He also noted that for the sorghum,maize and oil palm examples given above,the decline in s was greater than the reduc-tion in the fraction (f) of radiation intercept-ed by the crops and that measured changesin f and s are seldom consistent betweensites, even for one genotype.

The sensitivity of s to saturationdeficit, even under non-stressed conditionshas previously been demonstrated for fieldcrops of sorghum and maize (Stockle andKiniry, 1990) and for sorghum growingin controlled-environment glasshouses(Hamdi et al., 1987). Stockle and Kiniry(1990) reported that the average daily meanvalue of D accounted for 76% of the vari-ability in s for sorghum and 50% of the vari-ability in

sfor maize for crops that were

reported as having no water, nutrient or lowtemperature stresses. The three types ofdeparture from the linear relation between

dry matter and radiation are illustrated inFig. 2.8. Table 2.2 summarizes some of thereported ranges in the value of s for variouscrops under non-limiting and apparently

limiting conditions. Sinclair and Muchow(1999) give a more complete review of radi-ation conversion efficiency.

Photochemical efficiency (q)

The process of plant growth consists of theconversion of glucose to other organic com-pounds, the translocation of the glucose tothe site of growth and, in legumes, the costof nitrogen fixation. In terms of relative ener-gy contents, plant tissue is composed of

varying proportions of five distinct bio-chemical groups; nitrogenous compounds(especially proteins), carbohydrates (cellu-lose, hemicellulose, starch), lipids (fats,fatty acids, oils), lignin and organic acids.The weight ratio of these biochemical prod-ucts to substrate varies between 0.35 and1.0 g g1 (Penning de Vries et al., 1989). Thisis because the synthesis of products that are

34 Chapter 2

Fig. 2.8. Schematic examples of how the relation

between crop dry weight and accumulated

intercepted radiation can break down: (1) late inthe season during reproductive growth; (2) at any

stage in the season when stress limits dry matter

production, e.g. drought, and (3) throughout the

season when stress limits dry matter production,

e.g. nutrient shortage.

Table 2.2. Values of the conversion coefficient

(g MJ1) for intercepted radiation (total solar) in

some major crops of the world. (From Azam-Ali

et al., 1994.)

Crop s s'

C4 species

Maize 2.43 1.30

Sorghum 2.69 1.20

Millet 2.62 0.57

C3

species

Wheat 1.50 0.80

Rice 2.05 1.00

Barley 1.20 1.10

Potato 1.84 1.00

Cassava 0.90

Sweet potato 1.55

Soybean 1.30 0.23

Groundnut 1.47 0.47

The maximum value (s) was derived fromexperiments under apparently optimal conditions.

The constrained value (s') was derived from

experiments where shortage of water and/ornutrients appear to have reduced the conversion

efficiency but individual plants have continued to

grow.

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rich in lipids and proteins requires consid-erably more energy per unit dry weight than

does the synthesis of carbohydrates. Table2.3 summarizes the energy content of thedifferent plant compounds in relation totheir biochemical composition. From this, itcan be seen that the lipid fraction of seedscontains about 2.19 times as much energyas an equivalent weight of carbohydrate.Therefore, calculations of s based on dryweight need to be adjusted to account for theenergy equivalents of the lipid and proteincomponents. Clearly, the unadjusted value

of s must decrease during the grain fillingof crops which produce seeds that are highin oil or protein (Muchow et al., 1982; Kiniryet al., 1989).

Respiration efficiency (r)

One method of quantifying the significanceof respiration to the total carbon balance ofa crop is to express growth efficiencyas theratio of the increase in biomass to the sum of

the increase in biomass and respiration overthe same period. Amthor (1989) estimatedthat the growth efficiency of several fieldcrops ranged between about 0.5 and 0.6, i.e.about half the carbon assimilated in photo-synthesis is eventually lost in respiration.

Accurate measurements of respirationare difficult because the photosynthetic rate,temperature, size and age of a crop varythroughout its life and affect the relativeamounts of photosynthesis and respiration.Therefore, the exact influence of respirationon s is extremely difficult to assess.Nevertheless, in theory the value of sshould decrease as crop dry weight increas-

es during the season because of an increasein maintenance respiration (McCree and

Silsbury, 1978). In temperate regions, theseasonal increase in temperature shouldalso cause a reduction in s. However, Squire(1990) has summarized evidence to showthat s varies little with plant mass whilstKiniry et al. (1989), working with fivespecies in the USA, showed no effects ofchanging temperatures on the value of s.They concluded that energy loss due torespiration appeared to be proportional toplant growth. Recent evidence (Albrizio,

2001) demonstrated that the rate of darkrespiration in chickpea, wheat and sorghumstands was linearly related to assimilationrate irrespective of plant age or temperature.

Yield efficiency (H)

For many determinate crops, the reproduc-tive weight of individual plants, g, is close-ly related to the total dry weight, w, of eachplant above a minimum weight, wo, i.e. the

minimum amount of vegetative infrastruc-ture necessary before reproductive growthcan commence. Provided that total plantweight is much greater than wo, as it is formany well tended crops, the ratio of repro-ductive (or economic) yield to total dryweight, i.e. the harvest index, H, remainsconservative. Therefore,

g= H(w w0) (2.11)

In these circumstances, the total reproduc-

tive yield, G, per unit ground area is givenby

G= Pg = H(W Pw0) (2.12)

Solar Radiation 35

Table 2.3. Some characteristics of the five major groups of plant components and minerals. (From

Penning de Vries et al., 1989.)

Heat of

combustion Nitrogen content Carbon content

(kJ g1) (g g1) (g g1)

Carbohydrates 17.3 0.00 0.45

Proteins 22.7 0.15 0.53

Lipids 37.7 0.00 0.77

Lignins 29.9 0.00 0.69

Organic acids 13.9 0.00 0.38

Minerals (K, Ca, P, S) 0.0 0.00 0.00

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where Pis the plant population and Wis thetotal dry weight per unit area of ground.

However, the allocation of assimilate tothe reproductive or economic componentsis not always conservative (see Table 2.4)and estimates of yield based on such anassumption may be very wrong. Again, this

is particularly the case for crops which aregrown in marginal areas, often on a storedwater profile. Here, the vegetative phasemay continue more-or-less as normal whilst

there is adequate water but drought willbecome increasingly important during grainfilling. This will lead to premature senes-cence of leaves and a reduction in cropphotosynthetic potential. The net effect willbe a crop with a reasonable vegetative growthbut poor final yield, i.e. a lower value of H.

2.3 Conclusions

In this chapter, we have seen how a knowl-edge of intercepted radiation and the con-version efficiency of that radiation providea useful basis to analyse crop growth andyield. The importance of interception andconversion of solar radiation is obvioussince it is light that determines the potentialrate of photosynthesis. Of course, CO2 is anessential substrate for photosynthesis but, inpractice, this cannot be manipulated in thefield and changes little between sites or sea-

sons. The main drawback to using the cap-ture and conversion of solar radiation as asimple and apparently effective scheme forcalculating crop growth is that for much ifnot all of the life of many tropical crops, theremaining two resources, water and nutri-ents, constrain growth below the potentialset by the interception and conversion oflight.

36 Chapter 2

Table 2.4. Reported values of harvest indicesa for

some major crops of the world. (From Azam-Ali et

al., 1994.)

Crop H H'

Wheatb 0.50 0.22

Rice 0.50 0.25

Maize 0.45 0.30

Barley 0.52 0.35

Sorghum 0.40 0.20

Oat 0.50 0.23

Millet 0.45 0.20

Rye 0.29

Potatoc 0.65 0.55

Cassavad 0.77 0.30

Sweet potatoe 0.74 0.50

Soybean 0.59 0.25

Groundnut 0.48 0.16Sugarcanef 0.30 0.20

a The maximum harvest index (H) is presented for

crops growing under apparently optimal condi-

tions. The comparable constrained harvest index

(H') is shown for crops whose growth appears to

have been limited by adverse environmental fac-

tors; b refers to bread and durum wheat; c fresh

tuber at 32.5% dry weight; d fresh tuber at 35.0%

dry weight; e fresh tuber at 30.0% dry weight;f sugar at 1012% of fresh cane.