Radiation-use efficiency of irrigated biomass sorghumin a Mediterranean environment

Michele RinaldiAB and Pasquale GarofaloA

ACRAndashUnitagrave di Ricerca per lo studio dei Sistemi Colturali degli Ambienti caldo-aridi Bari ItalyBCorresponding author Email michelerinaldientecrait

Abstract Mathematical crop simulationmodels are useful tools in predicting the potential yield of field crops in a specificenvironment The main driving parameter used to estimate biomass accumulation in most of these models is radiation-useefficiency (RUE) Biomass sorghum (Sorghum bicolorLMoench) is a crop that can be used for energy production (thermaland bioethanol chains) and a knowledge of its RUE in different water supply conditions can help to improve modelsimulations and evaluate crop diffusion

A 3-year field experiment was carried out in Southern Italy where sorghum was submitted to four irrigated regimesbased on actual crop evapotranspiration (ETc) In the first year ETc was measured with weighted lysimeters while in theother 2 years it was estimated by means of estimated crop coefficient (Kc) and the reference evapotranspiration ET0

The RUE calculated as the slope of the first-order equation between dry biomass and intercepted photosyntheticallyactive radiation along a crop cycle showed an average of 291 054 gMJndash1 even if the RUE proved to be closelycorrelatedwith cropwater consumption The latter ranged between 891 and 454mmand the RUE increased 42mgMJndash1 permm of water used A high crop interception of solar radiation was observed in sorghum reaching its maximum efficiency40 days after sowing

To obtain high yielding yield biomass sorghum requires a large supply of water as confirmed by the Kc calculatedduring the crop cycle which resulted higher (especially in the development and middle stages) when compared with thosereported in the FAO 56 Paper The obtained RUE values also confirmed a high efficiency in biomass production of thiscrop allowing for the introduction of biomass sorghum in the cropping systems of Mediterranean environments as analternative crop for energy purposes but with adequate irrigation water supply

Additional keywords canopy interception Sorghum bicolor L Moench radiation-use efficiency water use

Received 12 April 2011 accepted 27 September 2011 published online 6 December 2011

Introduction

Energy crops are gaining increasing importance as a consequenceof rising fossil fuel prices depleted oil reserves and an increasein the lsquogreenhouse effectrsquo associated with the use of traditionalenergy sources In order to evaluate profitability and to extendenergy crop cultivation it is important to verify the most suitableconditions and the maximum attainable crop yield in a givencondition Crop production systems include components suchas the weather soils crop varieties and management cropsimulation models mathematically describe the growth anddevelopment of crops interacting with the environment andhuman management of the production process (Matthews andStephens 2002) An important group of simulation models(among others CERES Ritchie et al 1985 Jones and Kiniry1986 Jones et al 2003 EPIC Jones et al 1991 and STICSBrisson et al 2003) have been created with a crop growthmodule that uses radiation-use efficiency (RUE) as the maindriving variable as indicated by Monteith (1977) The lowerhierarchical processes which express the intermediary stepsnecessary to achieve biomass accumulation are synthetically

incorporated into the RUE which reduces the complexity andnumber of input variables

Plant biomass production is a consequence of two mainprocesses occurring at two levels The first at canopy level isrelated to the interception of photosynthetically active radiation(PAR) the radiant energy in the 400ndash700-nm waveband Thesecond is at chloroplast level where the intercepted radiation isused by the plant to drive both CO2 assimilation and water andtranspiration processes These reduce gaseous CO2 to thecarbohydrate medium (CH2O) convert the latter into biomassand affect its translocation and accumulation (eg Gallagherand Biscoe 1978 Gosse et al 1986 Hamdi et al 1987) Allthese processes can be summarised as RUE This efficiencyvaries between C4 and C3 plants for example the maximumconversion efficiency for solar energy to biomass for C4

photosynthesis (eg in sorghum) is 6 compared with 46for C3 photosynthesis at 308C and todayrsquos 380-ppm atmosphericCO2 concentration (Zhu et al 2008)

PAR interception and energy conversion into biomasshave often been considered crop-specific parameters but they

Journal compilation CSIRO 2011 wwwpublishcsiroaujournalscp

CSIRO PUBLISHING

Crop amp Pasture Science 2011 62 830ndash839httpdxdoiorg101071CP11091

can also depend on other environmental factors such astemperature and water availability and are directly correlatedwith canopy development (Biscoe and Gallagher 1977 Foaleet al 1984 Sinclair and Horie 1989 Rosenthal and Gerik 1991Rosenthal et al 1993)

Sorghum (Sorghum bicolor L Moench) is grown in manyparts of the world for a variety of purposes for grain forageproduction sugar yield and fibre Some of the forage types mayprove suitable for biomass and consequently energy productionSweet sorghum and biomass sorghum are characterised by anotable height (up to 3m) with a large stem for sweet sorghumand a fine stem for the biomass crop this latter is characterisedby elevated sucker growth and a large amount of lignin andcellulose in the internodes marrow whereas in sweet sorghum anelevated amount of non-structured carbohydrates such asfructose and glucose are accumulated during the growth stage(Lingle 1987) Grain sorghum has different habits comparedwith the other two kinds of sorghum mentioned above it isshorter (1ndash2m) than biomass sorghum and has a morevigorous stem No bibliographic references were found forbiomass sorghum grown under Mediterranean conditionsconsequently each comparison between our data and that ofother authors can be done only for sweet or grain sorghum

Some authors report the RUE as a stable parameter for manycrops (Hughes et al 1987Monteith 1990) but variability inRUEwas also pointed out by other authors (Sinclair and Muchow1999) who reported that this parameter can be influenced byvapour pressure deficit (VPD) (Stockle and Kiniry 1990) andunderlined that in sorghum the RUE can oscillate from 38 to21 gMJndash1 ranging from 211 to 089 kPA in terms of VPDtemperature (Hammer and Vanderlip 1989) and water status(Ong and Monteith 1985) For this reason it is importantto estimate the RUE coefficient not only in non-limitingconditions but also in different water situations so as toguarantee an accurate biomass prediction and consequentlyto be able to assess the potential of biomass sorghum inMediterranean environments

However as with the results of previous research it isimportant to take into account variations in RUE in line withchanges in the water supply For instance in a Mediterraneanenvironment values of RUE in sweet sorghum rangingfrom 34 to 47 g respectively of aboveground dry matter(ADM) MJndash1 of intercepted PAR were found by Mastrorilliet al (1995) and Perniola et al (1996) in well watered cropconditions A RUE value of 36 gMJndash1 was found by Varlet-Grancher et al (1992) in Northern Europe (France) Dercas andLiakatas (2007) confirmed that RUE is closely related to cropwater status with values of 355 gMJndash1 for non-water stressedcrops and 130 gMJndash1 for stressed crops

UnderMediterranean environments summer temperatures donot limit the potential productivity of sorghum in terms ofbiomass but the limiting factor is water In order to improvethe accuracy of RUE estimates various components used tocalculate RUE such as total dry biomass leaf area index(LAI) canopy development radiation extinction efficiency andconversion efficiency should be determined in situ and underdifferent water availability conditions

The aim of this study was to determine the RUE of biomasssorghum over different water regimes in a Mediterranean

environment and to evaluate the relationship between RUEand crop water use (WU)

Materials and methodsExperimental site

The field experiment was carried out over the 3-year period2008ndash10 in Foggia (lat 41880700N long 158 830500E alt 90masl) Southern Italy

The soil is a vertisol of alluvial origin Typic Calcixeret(USDA 2010) silty-clay with the following characteristicsorganic matter 21 total N 0122 NaHCO3

ndash extractableP 41 ppm NH4O Ac-extractable K2O 1598 ppm pH (water)83 field capacity water content 0396m3mndash3 permanentwilting point water content 0195m3mndash3 available soil water202mmmndash1

The climate is lsquoaccentuated thermo-Mediterraneanrsquo(UNESCO-FAO classification) with temperatures below 08Cin the winter and above 408C in the summer Annual rainfall(average 550mm) is mostly concentrated during the wintermonths and class lsquoA panrsquo evaporation exceeds 10mmdayndash1 insummer Daily meteorological data ndash temperatures humidityrainfall wind velocity and solar radiation ndash were recorded bythe local meteorological station

Field experiment

Biomass sorghum (cv Biomass 133 Syngenta) was sown atthe beginning of May in rows 05m apart and 008m betweenseeds in each row (250 000 seeds per hectare) The crop washarvested before heading (when the maximum dry matter yieldhad been achieved) on average in the middle of August

In the first year the crop evapotranspiration (ETc in mm)was measured by means of two weighted lysimeters Theweighing lysimeters were located in the middle of a 100100-m field to reduce the fetch influence The lysimeter tankwas made of steel and was of a square shape with an area of 4m2

(2 2m) and a depth of 15m it was fully inserted into thesoil without making contact with the surrounding soil Soil wasplaced in the tank in 2001 according to surrounding soilstratigraphy In the bottom of the tank there is a drainagesystem to collect and measure drained water The weightmeasurement were recorded manually every day at 700 amby a balance with a resolution of 100 g and automatically with aload cell that collected the data every 30min storing them on adata logger

Daily weight data were collected and no drainage water wasobserved at the bottom of the lysimeters Runoff was consideredequal to zero because of the flat lying of the land Daily cropmeasured ETc (in mm) was calculated as

ETc frac14 ethWLi WLi1THORN4

I Ri1 eth1THORN

where WLi and WLindash1 are the lysimeter weights in kg at day iand indash1 respectively I is the irrigation amount in mm and R isthe rain in mm The average values of two lysimeters were used

During the field experiment weather data were measured bya standard meteorological station located in the experimentalfarm Maximum and minimum temperatures global solarradiation (Rg) precipitation wind speed and relative

Radiation-use efficiency of irrigated biomass sorghum Crop amp Pasture Science 831

maximum and minimum air humidity were collected on a dailybasis

The irrigation schedule was set as a function of ETc andmeasured by means of lysimeters every time ETc reached60mm irrigation started according to the following regimes

ndash I_125= 125 ETc with each irrigation of 75mmndash I_100= 100 ETc with each irrigation of 60mmndash I_75= 75 ETc with each irrigation of 45mmndash I_50= 50 ETc with each irrigation of 30mm

The reference evapotranspiration (ET0 in mm) was calculatedusing the FAOndashPenmanndashMonteith method (Allen et al 1998)The crop coefficients (Kc) were calculated daily as the ratiobetween ETc and ET0 and successively averaged for initialdevelopment and middle crop stages

In the second year (2009) and third year (2010) the irrigationregime schedule was based on ETc estimated as follows usingthe Kc values derived from the first year of the experiment

ETc frac14 ET0 Kc eth2THORNEach time the cumulated ETc reached 60mm (subtracting

rainfall) irrigation started in the sameway as in the four irrigationtreatments described above for the first year

To ensure uniform water distribution a drip irrigation systemwas used with one line for each plant row and drippers with a4-L hndash1 flow Water flow meters were placed at the head ofeach plot to measure accurately the amount of irrigation watersupplied A pre-sowing fertilisation was applied with 72 kg handash1

of N and 87 kg handash1 P2O5 as diammonium phosphateThe experimental treatments were arranged in a completely

randomised block design with four replications and elementaryplots of 80-m2 size

The main crop phenological phases were recorded in all theplots and expressed as degree days (GDD) considering a basetemperature (Tb) of +88C and an optimal temperature (Topt) forthe growth of 258C (Alagarswamy and Ritchie 1991 Hammeret al 1993) From these values we calculated the different pathfor the lsquoTlimrsquo factor which describes the effect of daily averagetemperature Tm on radiation-dependent biomass accumulationas reported by Monteith (1977) In particular

Tlim frac14 0 when Tm lt Tb

Tlim frac14 1 when Tm gt Topt

Tlim frac14 Tm Tb

Topt Tbwhen Tb Tm Topt

Growth analysis was carried out from June to August at fivesampling dates aboveground plant dry matter separated intostems green anddead leaveswasmeasuredby taking a 05-linear-m sample from each plot which was dried at 808C until theweight was constant The last sampling was in correspondence ofplant harvest LAI ndash the destructive method ndash was determinedmeasuring green leaf area with Delta T Devices (DecagonDevices Inc Pullman WA USA)

Gravimetric soil water measurements were carried out atdepths of 02 04 06 and 08m at sowing harvest andgrowth analysis sampling dates Seasonal WU was estimatedaccording to the following water balance equation

WU frac14 DSWC thorn Rthorn I eth3THORNwhere DSWC is the variation between seeding and harvestdates of the volumetric soil water content in the 0ndash08-mdepth layer R is the rainfall and I the irrigations all expressedin mm

The PAR was estimated using the following equation

PAR frac14 Rg 048 eth4THORNRg was measured with a thermophile pyranometer

(305ndash2800-nm wavelength range)The intercepted PAR (iPAR) was estimated with the formula

iPAR frac14 PAR IE eth5THORNwhere IE is the interception efficiency of the canopy calculatedwith Beerrsquos law as

IE frac14 1 eethk LAId cf THORN eth6THORNwhere k is the light extinction coefficient calculated as theslope of fitted regression between the natural logarithm ofdiffuse non-intercepted sky radiation and LAI both measuredwith a LI-COR 2000 portable area meter (LI-COR BiosciencesLincoln NE USA) For each plot the data were derived bythe average of six measurements carried out below the plantcanopy during the 12 00 to 02 00 pm daytime at eachgrowing sample LAId is the green leaf area and Cf is theclumping factor (Nilson 1971 Lang 1986 1987) calculatedwith the following equation where LAId is the green LAImeasured with the destructive method (Delta T Deviceequipment) as

Cf frac14 075thorn eth025THORN eth1 eeth035 LAIdTHORNTHORN eth7THORNRUE (g MJndash1 of iPAR) was calculated as the slope of first-

order equation between ADM and cumulated iPAR at eachsampling date The values of y-axis intercepts proved not to bedifferent from zero moreover the statistical test confirmed thatintercepts were equal to zero and thus the regression lines wereforced to pass from the axis origin (Charles-Edwards 1982)

RUE frac14 ADMXdfrac14 harvest

dfrac14 sowing

iPAReth8THORN

ANOVA was carried out separately for each year using alsquorandomised blockrsquo design model and the Least SignificantDifference was used to compare mean values A globalANOVA considering the lsquoyearrsquo as a lsquorandomrsquo effect was alsocarried out

Results and discussion

Climatic behaviour

Table 1 shows the meteorological data recorded from May toAugust in the 3 years of experiments compared with the long-term averages (over 55 years) while the patterns of daily airtemperatures recorded during the three crop growth cycles aredisplayed in Fig 1 In the first part of the growing cycle2009 recorded mean temperature (Tmean) greater than 2008

832 Crop amp Pasture Science M Rinaldi and P Garofalo

and 2010 whereas in the middle part of the growing season2009 and 2010were very similar Finally 2008was characterisedby a middle part of crop growth cycle (from 150 to 190Julian days) split in two phases the first one where Tmean wasalways lower than 2009 and 2010 with a gap up to 158C and thesecond phase when this gap was reversed Similarly in the lastpart of growing cycle 2008 was characterised by higher Tmean(+68C) than the other 2 experimental years Warm conditions

were observed at the end of the growing cycle with a remarkableheat wave at the middle July 2009 with maximum dailytemperatures over 408C

For Rg except August in 2009 the values exceeded therecorded ones in 2008 and 2010 of ~30MJmndash2 in July in2010 a Rg greater of 27 and 40MJmndash2 in June compared with2009 and 2008 respectively was observed Finally in 2009 Rg

was greater of ~3 and even 5MJmndash2 per day in May comparedwith 2008 and 2010 but with no effect because the cropemergence occurred at the end of month (sowing dates 9 12and 4 May emergence dates 20 25 and 13 May in 2008 2009and 2010 respectively) In the third year of the experimentvalues of solar radiation and ET0 were lower than theother 2 years

In the second year cumulated rainfall for the whole cropcycle was similar to the other 2 years but from 1 January to thesowing date cumulated rainfall in 2009 was 418mm larger thanthe other 2 years (168mm in 2008 and 248 in 2010) This largedifference in rainfall resulted in a greater water availability forthe second rather than in the first and third year of the experimentalso in the deeper soil layers

Comparable averages were observed in the 3 years withregard to daily ET0 and these were similar to long-termvalues In 2008 the ratio between the evapotranspirationmeasured by weighted lysimeter (ETc mm) and the ET0 (mm)calculated by the PenmanndashMonteith formula allowed us tocalculate the Kc (Allen et al 1998) for biomass sorghum(Fig 2) The calculated Kc were higher than those suggestedby FAO especially for the middle stage (149) even if theFAO-reported Kc refer to sweet (120) and grain sorghum (110)

Air temperature did not influence negatively the potentialcrop growth in fact Fig 3 shows that during the activegrowth phase (from 1000 to 1600 GDD) Tlim in 2008 nearto1100 GDD was for some days under the optimal value andin 2010 from 1300 to 1500 GDD Tlim reached value lowerto 09 However these values never reached very criticalthreshold thus confirming that the local environment issuitable for this crop considering its thermal requirements

Table 1 Meteorological data (monthly averages) recorded in Foggia(Italy) in 2008 2009 and 2010 compared with long-term period

(1952ndash2007)

Year May June July August

Daily Tmax (8C)2008 252 301 326 3402009 272 290 327 3372010 242 292 326 3331952ndash2007 250 294 319 313

Daily Tmin (8C)2008 108 158 188 1952009 126 156 188 1962010 123 159 192 1931952ndash2007 115 156 185 188

Rg (MJ mndash2 monthndash1)2008 750 757 851 7892009 847 770 883 7732010 693 797 853 7861952ndash2007 744 813 836 715

Rain (mm monthndash1)2008 302 410 38 042009 194 258 132 142010 326 262 234 001952ndash2007 337 335 204 326

ET0 (mm dayndash1)2008 44 52 60 562009 49 51 60 542010 41 54 60 551952ndash2007 49 57 60 58

10

20

30

40

120 140 160 180 200 220 240

Julian day

Tem

p (

degC)

LT 2008 2009 2010

Fig 1 Daily average temperatures in the 3 years of the experiment duringbiomass sorghumcropcycle comparedwith long-term(1952ndash2007) averages

0

04

08

12

16

20

0 25 50 75 100

Days after sowing

Fig 2 Crop coefficients (Kc) calculated in 2008 as ratio between ETc(from weighing lysimeters) and ET0 (3-day averages points) fitted Kc

values (full line) and FAO Kc (dashed line) for biomass sorghum

Radiation-use efficiency of irrigated biomass sorghum Crop amp Pasture Science 833

The different climatic behaviour among years especially forthe rainfall before sowing and consequently the soil wateravailability results in a different water restocking withirrigation and water used by the crop In 2008 irrigation wateramount oscillated between 280 and 550mm for I_50 and I_125respectively with values very similar to that supplied in 2010(226mm for I_50 and 565 for I_125) In 2009 soil waterrestocking was lower compared with the other years in factconsidering the two extreme treatments the water amount was365 and 185mm Also the water used by the crop was similar for2008 and 2010 with average values considering all irrigationtreatments equal to 650 and 638 respectively while in 2009was equal to 739mm

Crop growth and canopy radiation interception

Figure 4 shows the behaviours of LAI for the 3 years and foreach irrigation treatment In the first year maximumvalue of LAIwas recorded at second sampling (884 GDD) with averagevalues for the I_100 I_75 and I_50 treatments equal to67m2mndash2 on average and 82m2mndash2 for the I_125 treatmenteven if the latter was not different from the other treatments Atharvest no statistical significant difference was observedamong treatments the I_125 (6m2mndash2) was the highest thelowest I_75 and I_50 treatments (39m2mndash2) and theintermediate was the I_100 (53m2mndash2) treatment In 2009the I_125 I_100 and I_75 treatments reached the maximumLAI (84 81 and 67m2mndash2

respectively) at 1042 GDD Inthe I_50 treatment reduction in canopy expansion due tounfavourable water supply condition determined reduction interms of LAI with a value equal to 54m2mndash2 after 988 GDD Inthis year differences among treatments were observed 50 daysafter sowing with LAI in I_125 and I_100 always greater thanI_75 and I_50 At harvest in fact LAI values equal to 59m2mndash2on average were recorded for the I_125 and I_100 treatmentsand 42m2mndash2 on average for I_75 and I_50 In 2010 high valuesof maximum LAI reached in the last part of the growing cyclein fact between 1250 and 1500 GDD LAI values were equal to107 and93m2mndash2 in I_125 and I_100 and89 and77m2mndash2 inI_75 and I_50 respectively on average of the two last samplingsReduction of LAI in sweet sorghum as a consequence of

reduction in water supply is reported by Dercas and Liakatas(1999) who observed that by halving thewater regime the peak ofLAI was reduced by ~33

Figure 5 shows the ADM (g mndash2) recorded in the 3 years andfor all treatments During crop growing cycles in the first yearno differences were found except for the last sampling whereasthe I_125 and I_100 treatments furnished 2709 g of dry mattermndash2 on average for ADM greater of 35 if compared withthe average values obtained by I_75 and I_50 (2007 g mndash2) In2009 instead it is possible to notice differences in biomass

00

02

04

06

08

10

600 900 1200 1500 1800

Growing degree days

Tlim

0

1500

3000

4500

6000

AD

M (

g m

ndash2)

Fig 3 Behaviours of Tlim in 2008 (dashed line) 2009 (full line) 2010(coarse line) and biomass sorghum aboveground dry matter accumulation(ADM) during 2008 (triangle) 2009 (square) and 2010 (circle) for I_100irrigation regime

0

3

6

9

12

15

LAI (

m2

mndash2

)

0

3

6

9

12

15

0

3

6

9

12

15

0 500 1000 1500 2000

Growing degree days

Fig 4 Green leaf area index (LAI) recorded during the 3 years of theexperiment (2008 top 2009 middle 2010 bottom) For treatments I_125circle I_100 triangle I_75 rhombus I_50 square Vertical bars indicatedifferences statistically significant for each sampling (lsd test Pgt 005)

834 Crop amp Pasture Science M Rinaldi and P Garofalo

accumulation after the first irrigation In fact I_50 treatmentfurnished the lowest value during all crop cycles I_125 andI_100 gave the greatest values during all crop cycles I_75treatment had intermediate values At last sampling (harvest)the ADM produced by the four irrigation treatments were 40973427 2686 and 2322 gmndash2 for I_125 I_100 I_75 and I_50respectively In 2010 the ADM did not differ among irrigationtreatments during the crop cycle only at harvest differencesin ADM were observed being different I_125 and I_100(2970 gmndash2 on average) from I_75 and I_50 (2514 gmndash2 onaverage) treatments

The close dependence of cumulated biomass with the cropWU was reported by several authors for example for sweetsorghumADMvaried from3100 to 1700 gmndash2 inGreece (Dercasand Liakatas 2007) with 680mm and 450mm of cumulative

evapotranspiration respectively and from 3250 to 3170 gmndash2 ina similar environment with a cumulative evapotranspirationequal to 580 and 526mm For grain sorghum Farah et al(1997) found values of ADM oscillating between 3050 and2210 gmndash2 passing from 627 to 498mm of water supplied inSudan lowest ADM was obtained by Farregrave and Faci (2006) inNorthern Spain with values of 1838 gmndash2 for 588mm ofevapotranspiration and 522 gmndash2 for 274mm of water used bysorghum Habyarimana et al (2004) reported that ADM inbiomass sorghum can oscillate among 2900 and 2000 gmndash2 inrain-fed conditions and from 5100 to 3500 gmndash2 in well wateredconditions

From Figs 3 and 5 it can be globally observed the ADMaccumulation followed the same path in the first and third yearbut in the second year the crop delayed the flowering stage thatwas fixed as the time for harvest this condition allowed in2009 for an increase in the dry matter accumulation and sodifferences in biomass harvest was obtained among yearsProbably the mean temperatures recorded during the lastcrop growing phase as reported in Fig 1 higher in 2008 and2010 than in 2009 determined an acceleration in plantdevelopment reducing in this way the crop length cycle indetriment to biomass accumulation For this reason at harvestwe observed an average value of 32 of dry matter contentrecorded in 2009 higher than in 2008 and 2010 with the highestvalues for well watered regimes (2009 v 2008 and 2010on average 4097 v 2992 gmndash2 and 3427 v 2739 gmndash2 forI_125 and I_100 treatments respectively) and the lowest inthe other two treatments (2686 v 2513 gmndash2 and 2322 v2072 gmndash2 for I_75 and I_50 treatments respectively)

Figure 6 shows the experimental results for canopy andradiation interception for biomass sorghum The value of k inEqn 6 was estimated considering the LAI and non-interceptedlight fraction in the first year of the experiment for all waterregime treatments andwas equal to ndash075 (R2 = 079 n= 40) Thek value obtained in this experiment is slightly higher thanthose reported (k= ndash057) by Curt et al (1998) in central Spain(k= ndash062) by Perniola et al (1996) and Mastrorilli et al (1995)(k= ndash060) in southern Italy and by Dercas and Liakatas (2007)in central Greece Values of k greater than those found in thiswork for sorghum were reported for other crops like maize

0

2000

4000

6000

0

2000

4000

6000

AD

M (g

mndash

2 )

0

2000

4000

6000

0 500 1000 1500 2000

Growing degree days

Fig 5 Aboveground biomass (ADM) recorded during the 3 years of theexperiment (2008 top 2009 middle 2010 bottom) For treatments I_125circle I_100 triangle I_75 rhombus I_50 square Vertical bars indicatedifferences statistically significant for each sampling (lsd test Pgt 005)

k = ndash07524

R2 = 079

ndash6

ndash5

ndash4

ndash3

ndash2

ndash1

0

0 3 6 9 12

LAI (m2 mndash2)

ln (

F)

Fig 6Linear regressionbetweennatural logarithmof not intercepted light (F)and leaf area index (LAI) in 2008

Radiation-use efficiency of irrigated biomass sorghum Crop amp Pasture Science 835

(k= ndash10 Hatfield and Carlson 1979) soybean (k= ndash083Willcott et al 1984) and for sunflower (k= ndash089 Bange et al1997)

The IE in this experiment is higher than that reported byCurt et al (1998) and an IE of 90 is reached later when LAI is~4m2mndash2 Biomass sorghum a crop characterised by a very fastgrowth reached an elevated value of IE (09) 40 days afterplanting when LAI was ~3m2mndash2 This is in line with valuesreported for other crops (eg for wheat Hipps et al 1983sunflower Sadras and Trapani 1999 soybean Wells 1991)whose efficiency in interception of more than 90 ofincoming energy is reached with values of LAI between 3 and5m2mndash2 This aspect is important in energy use and itmeans that sorghum maintains a high efficiency in radiationinterception for more than 60 of its crop cycle

Radiation-use efficiency

In Table 2 statistics for the linear regression between sorghumADM and cumulated iPAR (intercept forced to 0) at differentsampling times are reported The ANOVA of RUE valuesshowed no significant effect for lsquoyear irrigationrsquo interactionbut highly significant effects for lsquoyearrsquo and lsquoirrigationrsquo sources ofvariation

In 2008 and 2009 the response of RUE to differentirrigation regimes was similar in fact for both years thestatistical analysis separated I_125 and I_100 from I_75 andI_50 with a value of RUE equal to 300 gMJndash1 in the first year(average of I_125 and I_100) and 355 gMJndash1 in the second one(average of I_125 and I_100) In the third experimental yearI_125 had the greatest RUE value (325 gMJndash1) and I_50 thelowest (257 gMJndash1) while I_100 and I_75 showed intermediate

ones Finally the average RUE values considering allirrigation treatments were 274 021 gMJndash1 in 2008 293080 in 2009 and 298 029 in 2010 (Fig 7) with no statisticaldifferences among years This result is also shown in Table 3where a comparison for LAI and iPAR for the different waterregimes within years and among years during all crop cycles ismade In 2008 and 2009 the I_50 was statistically lower thanthe other irrigation treatments whereas in 2010 no differenceswas recorded but no difference statistically significant appearseither for the LAI or for iPAR among years

In previous research the influence of crop WU on RUE wasalso observed 47 gMJndash1 was reported by Perniola et al (1996)with a water consumption of 870mm Mastrorilli et al (1995)measured 34 gMJndash1 in a similar environment and with a crop

Table 2 Coefficient of determination (R2) slope (a or RUE g MJndash1)standard error for the slope (se) confidence limit and significanceprobability (P) for the linear regression (intercept forced to 0)

between aboveground dry biomass (g mndash2) and iPAR (MJ mndash2)Between treatments average values followed by the same letter are notsignificantly different considering the four water regimes for each year

separately and the averages of the 3 years (lsd lsquotestrsquo at Pgt 005)

Year Waterregimes

R2 a se Confidencelimit

P

RUE gt95 lt95

2008 I_125 086 307a 016 270 344 lt0001I_100 087 292a 012 265 318 lt0001I_75 078 259b 015 226 293 lt0001I_50 075 227b 013 198 257 lt0001

Average 077 274a 008 256 290 lt00012009 I_125 087 381a 019 340 421 lt0001

I_100 088 329a 015 296 362 lt0001I_75 078 267b 018 227 309 lt0001I_50 077 189b 012 162 215 lt0001

Average 069 293a 012 269 318 lt00012010 I_125 076 325a 021 370 280 lt0001

I_100 083 314ab 017 278 351 lt0001I_75 077 296b 020 253 339 lt0001I_50 080 257c 014 226 289 lt0001

Average 077 299a 010 279 317 lt0001

2008ndash10 average ndash 073 291 006 279 303 lt0001

RUE 2009 = 293 R2 = 089

RUE 2008 = 274 R2 = 094

RUE 2010 = 299 R2 = 085

0

1000

2000

3000

4000

0 300 600 900 1200

iPAR (MJ mndash2)

AD

M (

g m

ndash2)

Fig 7 Linear regressions between aboveground dry matter (ADM) andintercepted photosynthetic active solar radiation (iPAR) for allexperimental years (dashed line for 2008 black line for 2009 and grey linefor 2010) considering all irrigation regimes of biomass sorghum (triangle for2008 square for 2009 and circle for 2010)

Table 3 Average values of leaf area index (LAI) and interceptedphotosynthetic active solar radiation (iPAR) recorded during crop

cycle for the 3 yearsFor each year values followed by the same letter are not significant (ns) at

Pgt 005 (lsd test)

Waterregimes

LAI(m2mndash2)

iPAR(MJmndash2)

LAI 2008 v2009 v 2010

iPAR 2008 v2009 v 2010

(m2mndash2) (MJmndash2)

2008I_125 608a 613 ns nsI_100 576a 597 ns nsI_75 502a 582 ns nsI_50 466b 563 ns ns

2009I_125 624a 623 ns nsI_100 552a 626 ns nsI_75 459b 602 ns nsI_50 379b 600 ns ns

2010I_125 689a 650 ns nsI_100 639a 622 ns nsI_75 563a 625 ns nsI_50 521a 635 ns ns

836 Crop amp Pasture Science M Rinaldi and P Garofalo

WU of 550mm Varlet-Grancher et al (1992) found in France36 gMJndash1 and Dercas and Liakatas (2007) reported RUEvalues between 355 gMJndash1 for non-water stressed crops and130 gMJndash1 in stressed crops with 657mm and 421mm of waterplant use respectively

This confirms that RUE is significantly dependent on cropwater consumption and that it cannot be considered a stablecrop parameter at least in the case of biomass sorghum Infact Dercas and Liakatas (2007) found a strong linearcorrelation between RUE and crop WU with an improvementof 30mgMJndash1 per mm of water used in water consumption inthe range 400ndash700mm From this experiment we can confirmand determine the relationship between RUE andWU at a linearrate of 42mg MJndash1mmndash1 (R2 = 060 Fig 8) of crop water usedin the range between 520 and 860mm

In general the overall average of 291 054 gMJndash1 isin agreement with the values reported by Monteith (1977)for C4 crops The RUE calculated in our experiment rangedfrom 179 for I_50 in 2008 to 381 gMJndash1 for I_125 in 2009this includes almost all the values obtained by the authorscited above

The sorghum capability of energy production is pointed outby Gerbens-Leenes et al (2009) who have carried out a studyonwater footprint or the conversion into different types of energyfor the water used by different energy crops In fact sorghumrequired ~180m3 of water to produce 1 GJ of electric energy viacombustion So sorghum could suffer the competition ofother energy crops such as sugar beet (46m3GJndash1) or corn(50m3GJndash1) But the same authors reported as considering theintrinsic energy source of biomass (MJ of biofuel per kg of freshweight crop) produced by the crop sorghum shows a similarvalue if compared with corn and 4 times greater compared withsugar beet (10 v 26MJ kgndash1)

The water supplied in the 3 years followed the croprequirement and this standardised the water treatmentsamong years consequently the differences in RUE emergedamong the water treatments within the year and this confirms theinfluence of water availability on RUE performance Thereduction of RUE as a consequence of a reduction of waterconsumption was observed by other authors in differentcrops such as rice (Boonjung and Fukai 1996) barley (Legget al 1979) and chickpeas (Thomas and Fukai 1995) Singh and

Singh (1995) reported how sorghum reduces stomatalconductance by ~18 and LAI by 20 when wateravailability is at 60 of optimal soil moisture conditionsThese authors also reported a reduction in net photosynthesisfor sorghum that follows the reduction in stomatal conductanceStudies on sunflowers (Takami et al 1981 1982) confirmed thatone of the effects of soil water reduction on plant developmentis the decrement in terms of leaf area expansion as a result of adecline in the expansion rate but not the duration of expansionAs shown in Table 3 average values were recorded duringcrop growth for each year that were statistically different inLAI among the water regimes however as explained abovethe light IE of sorghum reached its maximum level with a LAIvalue of 3m2mndash2 This explains why there are no statisticallyremarkable differences for iPAR In all probability consideringthe same level of intercepted radiation plants with a differentwater status have different stomata process developmentand thus different behaviour in net photosynthesis Theseconsiderations were supported by Cechin (1998) whichunderlined that the net photosynthesis in sorghum declined asconsequence of water stress via stomatal limitation

Other effects of water stress reported in literature are areduction in intercellular CO2 concentration with a consequentreduction in the net photosynthesis observed in sorghum (Kriegand Hutmacher 1986) an increase of stomata resistance in cottonand millet (Troughton 1969 Ludlow and Ng 1976) and stomatalclosure caused by the abscisic acid produced by plants (Davieset al 1994 Davies and Gowing 1999) All these effects of wateravailability can sufficiently explain the RUE level and itsvariations

Conclusions

From this study we obtained RUE values for biomass sorghumthat were not previously reported in literature for this specificcrop The observed values resulted higher than the reportedones for grain and sweet sorghum probably for the prevalenceof young and more efficient green leaves in the biomasssorghum usually harvested before heading The RUE valuessuggested a high yield potential for this crop in well wateredconditions and in a Mediterranean environment

The RUE proved to be significantly dependent on crop waterconsumption and it cannot be considered a stable cropparameter at least in the case of biomass sorghum Areduction equal to 15 in RUE was observed with a decreasein water supply of 25 of well watered conditions comparingI_100 v I_75

The observed water consumption is high when comparedwith soybeans (344mm) or sunflowers (400mm) (Mastrorilliet al 1995) ranging between 566 and 891mm in this experimentwith deficit irrigation despite sorghum being a drought-resistantspecies it can produce as consequence reduction in canopydevelopment biomass accumulation and RUE valuesConsequently in order to fully exploit the potential ofsorghum in Mediterranean environment it is necessary toensure an adequate supply of water during the entire cropgrowth cycle Indeed considering the amount of rainfallduring the crop cycle (a long-term average of 133mm) and asoil moisture at sowing of ~30 in volume at least 500mm of

RUE = 00042WUE R2 = 060

0

1

2

3

4

5

0 200 400 600 800 1000

Water consumption (mm)

RU

E (g

MJndash

1 )

Fig 8 Linear regression between radiation-use efficiency (RUE) andwater used by biomass sorghum in the 3 experimental years consideringall irrigation regimes

Radiation-use efficiency of irrigated biomass sorghum Crop amp Pasture Science 837

irrigation water is necessary to obtain a satisfactory amount ofbiomass from sorghum for energy purposes

This high water requirement could discourage growers fromcultivating sorghum as a biomass crop in favour of moreprofitable irrigated crops however if we consider the newecological functions of agriculture like providing asustainable environmental use of natural resources protectingthe environment by reducing agronomical input (for examplesorghum is in competition with weed plants so allowing for aconsistent reduction in terms of chemical weed controlpractice) and using alternative sources of energy this cropcan be seen as playing an important role in agro-ecosystemsecology

Acknowledgements

This work has been supported by the Italian Ministry of Agriculture andForestry Policies under contract no 209739305 (AQUATER Project)

References

Alagarswamy G Ritchie JT (1991) Phasic development in CERES-sorghummodel In lsquoPredicting crop phenologyrsquo (Ed T Hodges) pp 143ndash152(CRC Press Boca Raton FL)

Allen RG Pereira LS Raes D Smith M (1998) lsquoCrop evapotranspirationGuidelines for computing crop water requirementsrsquo Irrigation andDrainage Paper No 56 pp 301 (FAO Rome)

Bange MP Hammer GL Rickert KG (1997) Effect of specific leaf nitrogenon radiation use efficiency and growth of sunflower Crop Science 371208ndash1214 doi102135cropsci19970011183X003700040030x

Biscoe PV Gallagher JN (1977) Weather dry matter production and yieldIn lsquoEnvironmental effects on crop physiologyrsquo (Eds JJ LandsbergCV Cutting) pp 75ndash100 (Academic Press London)

Boonjung H Fukai S (1996) Effects of soil water deficit at different growthstages on rice growth and yield under upland conditions 1Growth duringdrought Field Crops Research 48 37ndash45 doi1010160378-4290(96)00038-X

Brisson N Gary C Justes E Roche R Mary B Ripoche D Zimmer DSierra J Bertuzzi P Burger P Bussiere F Cabidoche YM Cellier PDebaeke P Gaudillere JP Henault C Maraux F Seguin B Sinoquet H(2003) An overview of the crop model STICS European Journal ofAgronomy 18 309ndash332 doi101016S1161-0301(02)00110-7

Cechin I (1998) Photosynthesis and chlorophyll fluorescence in two hybridsof sorghum under different nitrogen and water regimes Photosynthetica35 233ndash240 doi101023A1006910823378

Charles-Edwards DA (1982) lsquoPhysiological determinations of cropgrowthrsquo 158 pp (Academic Press Sydney)

Curt MD Fernandez J Martinez M (1998) Productivity and radiation useefficiency of sweet sorghum [Sorghum bicolor (L)Moench] cvKeller inCentral SpainBiomass and Bioenergy 14 169ndash178 doi101016S0961-9534(97)10025-3

DaviesWJ Gowing DJG (1999) Plant response to small perturbations in soilwater status In lsquoPhysiological plant ecologyrsquo (Ed JDScholes) pp 67ndash89(Blackwell Science Oxford UK)

Davies WJ Tardieu F Trejo CL (1994) How do chemical signals work inplants that are grown in drying soil Plant Physiology 107 309ndash314

Dercas N Liakatas A (1999) Sorghum water loss in relation to irrigation inrelation to irrigation treatmentWaterResourcesManagement 13 39ndash57

Dercas N Liakatas A (2007) Water and radiation effect on sweetsorghum productivity Water Resources Management 21 1585ndash1600doi101007s11269-006-9115-2

FarahSMSalihAATahaAMAli ZIAli IA (1997)Grain sorghum responseto supplementary irrigations under post-rainy season conditionAgricultural Water Management 33 31ndash41 doi101016S0378-3774(96)01283-8

Farregrave I Faci JM (2006) Comparative response of maize (Zea mays L) andsorghum (Sorghum bicolor L Moench) to deficit irrigation in aMediterranean environment Agricultural Water Management 83135ndash143 doi101016jagwat200511001

Foale MA Wilson GL Coates DB Haydock KP (1984) Growth andproductivity of irrigated Sorghum bicolor (L Moench) in NorthernAustralia II Low solar altitude as a possible seasonal constraint toproductivity in the tropical dry season Australian Journal ofAgricultural Research 35 229ndash238

Gallagher JN Biscoe PV (1978) Radiation absorption growth and yield ofcereals The Journal of Agricultural Science 91 47ndash60 doi101017S0021859600056616

Gerbens-Leenes W Hoekstra AY Van der Meer TH (2009) The waterfootprint of bioenergy Proceedings of the National Academy ofSciences of the United States of America 106 10 219ndash10 223doi101073pnas0812619106

Gosse G Varlet-Grancher C BonhommeR ChartierM Allirand J LermaireG (1986) Production maximale de matiere seche et rayonnement solaireinterceptegrave par un couvert vegetal Agronomie 6 47ndash56 doi101051agro19860103

Habyarimana E Laureti D De Ninno M Lorenzoni C (2004) Performancesof biomass sorghum [Sorghum bicolor (L) Moench] under differentwater regimes in Mediterranean region Industrial Crops and Products20 23ndash28 doi101016jindcrop200312019

Hamdi QA Harris D Clarck JA (1987) Saturation deficit canopy formationand function in Sorghumbicolor (L) Journal of Experimental Botany381272ndash1283 doi101093jxb3881272

Hammer GL Vanderlip LR (1989) Genotype-by-environment interactionin grain sorghum I Effects of temperature on radiation use efficiencyCrop Science 29 370ndash376 doi102135cropsci19890011183X002900020028x

Hammer GL Carberry PS Muchow RC (1993) Modelling genotypic andenvironmental control of leaf area dynamics in grain sorghum Wholeplant level Field Crops Research 33 293ndash310 doi1010160378-4290(93)90087-4

Hatfield JL CarlsonRE (1979) Light quality distributions and spectral albedoof three maize canopies Agricultural Meteorology 20 215ndash226doi1010160002-1571(79)90022-0

Hipps LE Asrar G Kanemasu ET (1983) Assessing the interception ofphotosynthetically-active radiation in winter wheat AgriculturalMeteorology 28 253ndash259 doi1010160002-1571(83)90030-4

Hughes G Keatinge JDH Copper PJM Dee NF (1987) Solar radiationinterception and utilization by chickpea crops in northern SyriaJournal of Agricultural Science Cambridge 108 419ndash424 doi101017S0021859600079454

Jones CA Kiniry JR (1986) lsquoCERES-Maize a simulation model of maizegrowth and developmentrsquo (Texas AampM University Press CollegeStation TX)

Jones CA Dyke PT Williams JR Kiniry JR Benson CA Griggs RH(1991) EPIC an operational model for evaluation of agriculturalsustainability Agricultural Systems 37 341ndash350 doi1010160308-521X(91)90057-H

Jones JW Hoogenboom G Porter CH Boote KJ Batchelor WD Hunt LAWilkens PW Singh U Gijsman AJ Ritchie JT (2003) The DSSATcropping system model European Journal of Agronomy 18 235ndash265doi101016S1161-0301(02)00107-7

Krieg DR Hutmacher RB (1986) Photosynthetic rate control in sorghumstomatal and non-stomatal factors Crop Science 26 112ndash117doi102135cropsci19860011183X002600010027x

838 Crop amp Pasture Science M Rinaldi and P Garofalo

Lang ARG (1986) Leaf area and average leaf angle from transmittance ofdirect sunlight Australian Journal of Botany 34 349ndash355 doi101071BT9860349

Lang ARG (1987) Simplified estimate of leaf area index from transmittanceof the sunrsquos beam Agricultural and Forest Meteorology 41 179ndash186doi1010160168-1923(87)90078-5

Legg BJ Day W Lawlor DW Parkinsins KJ (1979) The effects of droughton barley growth Journal of Agricultural Science Cambridge 92703ndash716 doi101017S0021859600053958

Lingle SE (1987) Sucrose metabolism in the primary culm of sweetsorghum during development Crop Science 27 1214ndash1219doi102135cropsci19870011183X002700060025x

Ludlow MM Ng TT (1976) Effects of water deficit on carbon dioxideexchange and leaf elongation rate of Panicum maximum vartrichoglume Australian Journal of Plant Physiology 3 401ndash413doi101071PP9760401

Mastrorilli M Katerji N Rana G Steduto P (1995) Sweet sorghum inMediterranean climate radiation use and biomass water useefficiencies Industrial Crops and Products 3 253ndash260 doi1010160926-6690(94)00002-G

Matthews RB Stephens W (Eds) (2002) lsquoCropndashsoil simulation modelsapplications in developing countriesrsquo pp 277 (CABI PublishingWallingford UK)

Monteith JL (1977) Climate and the efficiency of crop production in BritainPhilosophical Transactions of the Royal Society of London B 281277ndash294 doi101098rstb19770140

Monteith JL (1990) Steps in climatology In lsquoProceedings InternationalConference on Dryland Farming ndash Challenges in Dryland AgricoltureA Global Prospectiversquo AmarilloBushland TX (Eds PW Unger WRJordan TV Sneed RW Jensen) pp 273ndash282 (Texas AgriculturalExperiment Station Bushland TX)

Nilson T (1971) A theoretical analysis of the frequency of gaps in plantstands Agricultural Meteorology 8 25ndash38 doi1010160002-1571(71)90092-6

Ong CK Monteith JL (1985) Response of pearl millet to light andtemperature Field Crops Research 11 141ndash160 doi1010160378-4290(85)90098-X

Perniola M Tartaglia G Tarantino E (1996) Radiation use efficiency ofsweet sorghum and kenaf under field condition In lsquo9th EuropeanBioenergy Conferencersquo 24ndash27 June 1996 Copenhagen Denmark(abstr) p 156 (PergamonElsevier Publishers)

Ritchie JT Godwin DC Otter-Nacke S (1985) lsquoCERES-Wheat a simulationmodel of wheat growth and developmentrsquo (Texas AampM UniversityPress College Station TX)

Rosenthal WD Gerik TJ (1991) Radiation use efficiency among cottoncultivars Agronomy Journal 83 655ndash658 doi102134agronj199100021962008300040001x

Rosenthal WD Gerik TJ Wade LJ (1993) Radiation use efficiency amonggrain sorghum cultivars and plant densities Agronomy Journal 85703ndash705 doi102134agronj199300021962008500030034x

Sadras VO Trapani N (1999) Leaf expansion and phenologicaldevelopment key determinants of sunflower plasticity growth andyield In lsquoCrop yield physiology and processes Physiological controlof growth and yield infield cropsrsquo pp 205ndash233 (SpringerVerlag Berlin)

Sinclair TR Horie T (1989) Leaf nitrogen photosynthesis and crop radiationuse efficiency a review Crop Science 29 90ndash98 doi102135cropsci19890011183X002900010023x

Sinclair TR Muchow RC (1999) Radiation use efficiency Advances inAgronomy 65 215ndash265 doi101016S0065-2113(08)60914-1

Singh BR Singh DP (1995) Agronomic and physiological response ofsorghum maize and pearl millet to irrigation Field Crops Research42 57ndash67 doi1010160378-4290(95)00025-L

Stockle CO Kiniry JR (1990) Variability in crop radiation-use efficiencyassociated with vapour pressure deficit Field Crops Research 25171ndash181 doi1010160378-4290(90)90001-R

Takami S Rawson HM Turner NC (1982) Leaf expansion of four sunflower(Helianthus annuus L) cultivars in relation to water deficits 2 Patternsduring plant development Plant Cell amp Environment 5 279ndash286

Takami S Turner NC Rawson HM (1981) Leaf expansion of four sunflower(Helianthus annuus L) cultivars in relation to water deficits 1 Patternsduring plant development Plant Cell amp Environment 4 399ndash407doi101111j1365-30401981tb02118x

Thomas S Fukai S (1995) Growth and yield response of barley andchickpea to water stress under three environments in SoutheastQueensland 1 Light interception crop growth and grain yieldAustralian Journal of Agricultural Research 46 17ndash33 doi101071AR9950017

Troughton JH (1969) Plant water stress and carbon dioxide exchange ofcotton leaves Australian Journal of Biological Sciences 22 289ndash302

USDA (2010) lsquoSoil Survey Staff Keys to Soil Taxonomyrsquo 11th edn (USDA-Natural Resources Conservation Service Washington DC) Available atftpftp-fcscegovusdagovNSSCSoil_Taxonomykeys2010_Keys_to_Soil_Taxonomypdf (accessed November 2011)

Varlet-Grancher C ChartierM Lemaire G Grosse G BonhommeR Cruz PCastal F Lenoble S (1992) Productivity of sweet sorghum compared toSudan-grass and sorghum Sudan-grass hybrids radiation interceptionand biomass accumulation under non limiting water and nitrogencondition In lsquoProceedings 6th EC Conference Biomass for EnergyIndustry and Environmentrsquo (Eds G Grassi A Collina H Zibetta)pp 265ndash267 (Elsevier Applied Science Oxford)

Wells R (1991) Soybean growth responses to plant density relationshipamong canopy photosynthesis leaf area and light interception CropScience 31 755ndash761 doi102135cropsci19910011183X003100030044x

Willcott J Herbert SJ Zhi-yi L (1984) Leaf area display and light interceptionin short season soybeansField Crops Research 9 173ndash182 doi1010160378-4290(84)90023-6

Zhu X-G Long SP Ort DR (2008) What is the maximum efficiency withwhich photosynthesis can convert solar energy into biomass CurrentOpinion in Biotechnology 19 153ndash159 doi101016jcopbio200802004

Radiation-use efficiency of irrigated biomass sorghum Crop amp Pasture Science 839

wwwpublishcsiroaujournalscp