variation in yield

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Variation in yield, N uptake and N use efficiency of

medium and late duration rice varieties

D I L L I P K U M A R S W A I N 1*, B U R L A C H A N D R A B H A S K A R 2, P R A M I L A K R I S H N A N 2,K U R K U R I S R I N I V A S A R A O 2, S A N G R A M K E S H A R I N A Y A K 2

A N D R A B I N D R A N A T H D A S H 2

1 Environment and Sustainable Development, United Nations University, 5370 Jingumae, 5-Chome, Shibuya-Ku,Tokyo-1508925, Japan

2 Central Rice Research Institute, Cuttack 753 006, India

(Revised MS received 18 October 2005)

S U M M A R Y

Field experiments were conducted at the village Kasiadihi, Dhenkanal district, Orissa, India duringwet seasons 2001, 2002 and 2003 under non water-stressed conditions (025 cm standing water) toassess variability in N uptake and utilization by medium and late duration rice varieties. The N rateswere 0, 40, 80 and 120 kg N/ha applied as urea in four equal splits at transplanting, active tillerinitiation, panicle initiation and flowering stages. The grain yield response was up to 80 kg N/ha. Theoptimum grain yield attainable by the efficient medium duration varieties was 4.5 t/ha. The N efficientlate duration varieties produced optimum grain yield of 5.8 t/ha. The relationship for total dry matterand grain yield production between N fertilized (40, 80 and 120 kg N/ha) and non-fertilizedtreatments were all significant, suggesting cultivar selection under optimum N fertilized conditions.The difference in optimum yield of the medium and late duration varieties was due to the differences

in the amount of N uptake and its use efficiency by the plant for grain production. There wasa curvilinear relationship between grain yield and N use efficiency for grain production. Therelationship between N use efficiency for grain production and N contents of leaf, stem and grain atmaturity was quadratic. The optimum plant N use efficiency of medium duration varieties was 49 kggrain/kg N uptake, achieved with leaf, stem and grain N contents of 10, 8 and 14 g/kg, respectively, atmaturity. For late duration varieties, the optimum plant N use efficiency was 68 kg grain/kg N uptakeand it was maintained with leaf and stem N content of 4 .0 g/kg each and grain N content of 9.0 g/kgat maturity. The N content in plant organs could be the selection guide used to obtain efficient ricevarieties.

I N T R O D U C T I O N

Nitrogen is one of the key nutrients that limitcrop growth of cereals in many production systems.Nitrogen deficiency is one of the most seriousnutritional disorders in lowland rice-producing areasof the world. The proportion of the applied Naccounted for by the above-ground N accumulationin rice was 0.300.40 (Cassman et al. 1993). Ammoniavolatilization, nitrification-denitrification, surface runoff, and leaching account for major N loss fromthe soil, which is associated with low N recoveryefficiency in lowland rice (Craswell & Vlek

1979). Cereals also release N from plant tissues,

predominantly as ammonia following anthesis(Harper et al. 1987; Francis et al. 1993).

Overuse of fertilizer and the occurrence of othermacronutrient deficiencies can give rise to lower Nuse efficiency (Von Uexkull 1993). Application ofsulphur and N prevents nitrate leaching and enhancesN use efficiency in grassland soils (Brown et al. 2000).By including cowpea in a ricewheat rotation in theproductive Indo-Gangetic plains of India, it mightbe possible to reduce the optimum fertilizer N dosedue to increased N availability and greater N useefficiency (Yadav et al . 2003). However, genetic

selection to improve N use efficiency has so farbeen unsuccessful. Nevertheless, there are significantdifferences in N use efficiency between rice varieties.

* To whom all correspondence should be addressed.E-mail: [email protected]

Journal of Agricultural Science, Page 1 of 15. f 2006 Cambridge University Press 1doi:10.1017/S0021859605005745 Printed in the United Kingdom


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Sta. Cruz & Wada (1994) evaluated several varietiesand concluded that growth duration, native soilfertility and cultural practices affect the pattern of Nabsorption, which in turn alters the amount of N inplants leading to profound effects on N use efficiency.

Rice varieties may respond differently to N appli-

cation (Singh et al. 1998). Cultivars selected underhigh N fertilizer application may not be suitable forsoils with low N status. Even after the application ofhigh rates of fertilizer N to rice, expected yield levelsmight not be obtained. If plant N status can beincreased without lodging or increasing the incidenceof disease, a significant increase in yield requiresincreased sink capacity, maintenance of high leaf Ncontent and a longer grain filling duration (Aggarwalet al. 1997).

Rice varieties differ in their ability to extract soiland fertilizer N and in its distribution to different

plant organs. Understanding N uptake and assimi-lation is necessary in any attempt to optimize theefficiency of absorbed N for grain production.Senanayake et al. (1996) showed that vigorous bio-mass accumulation could lead to dilution of plantnitrogen content up to the panicle initiation stage,which could lead to inefficient use of N for spikeletformation. It is important to increase the efficiency ofsoil and fertilizer N by using nutrient efficient var-ieties. It is hypothesized that the N use efficiency ofthe rice plant can be optimized by critical leaf, stemand grain N content of rice varieties, which improves

the efficiency of grain production. The first objectiveof the present investigation was to assess variability ingrain yield, N uptake and N use efficiency for grainproduction of medium and late duration ricevarieties. The second objective was to ascertain thecritical N content in rice plant parts for optimizingthe efficiency of absorbed N for grain production. Theresults could contribute to an improvement in Nfertilizer management, and might also prove usefulfor the selection and breeding of efficient ricevarieties.

M A T E R I A L S A N D M E T H O D S

Field experiment

Field experiments were conducted in a field atKasiadihi village, Denkanal district, Orissa, India in2001, 2002 and 2003, to study variation in N uptakeand utilization by medium and late duration ricevarieties. The site is a rain-fed area spread over 6 km2

in the 20x40kN latitude and 85x38kE longitude at300 m asl. Medium and late duration rice varieties of115150 days maturity are usually grown at this site,without water stress. The climate is hot and humid.

The weekly mean maximum temperature ranges from26 xC in December to 41 xC in April. The weeklymean minimum temperature ranges from 12 xC in

December to 26 xC in April. The soil is acidic, pH 5.8,and the texture is sandy clay loam with 15% clay,72% sand and 13% silt by volume. The bulk densityof the soil is 1.45 g/ml. The organic C and total Ncontents are 4.7 g/kg and 0.7 g/kg, respectively. Therewere no deficiency or toxicity symptoms of any

nutrients other than N. The average annual rainfall ofthe district is 1421 mm. The proportion of the totalrainfall received during peak monsoon months (Juneto September) is 0.81. The average number of rainydays is 73 in a year. The relative humidity during thewet season is always above 90%.

Two experiments were conducted in the wet season(June to December) of 2001 under non water-stressedconditions (025 cm standing water). They were sideby side in two separate blocks for medium duration(115130 days maturity) and late duration (135150days maturity) rice varieties. Before the start of the

main experiments in the wet season, a uniformity trialwas conducted with rice variety IR 36 with N appliedat 20 kg N/ha in the previous dry seasons (January toMay) to reduce soil fertility heterogeneity throughoutthe field.

Altogether, 12 popular released rice varietiesfrom different states of India were included in theinvestigation, of which five varieties were of mediumduration and seven of late duration. The character-istics of these rice varieties are given in Table 1. Theexperiments were laid out in a split plot design withtwo replications, where varieties were in the main

plot and N rates were in subplots. The subplots were6r4 m. The N rates were 0, 40, 80 and 120 kg N/ha,applied as urea in four equal splits at Zadoksgrowth stage GS14 (transplanting), GS22 (activetiller initiation), GS37 (panicle initiation) and GS65(flowering stage) of the crop (Zadoks et al. 1974).Phosphorus and potassium were applied uniformly atthe rate of 60 kg P2O5/ha and 60 kg K2O/ha to thepuddled soil at transplanting. The plots were trans-planted with 30-day-old rice seedlings in the thirdweek of July each year with two seedlings/hill. Theinter-plant spacing was 15 cm and inter-row spacingwas 20 cm. Plots remained flooded with rainwater

from 025 cm throughout the experiment. The sub-plot bund height was 40 cm whereas the main bundheight was 50 cm so that rainwater did not overflowto adjacent plots. Polythene lining 30 cm deep wasprovided inside the subplots along the bund toprevent the flow of water and dissolved nutrientsthrough seepage. The same experimental site wasreused in successive years (2002 and 2003). All thesubplots in each block were broken to make into oneplot before the uniformity trials in the dry seasons of2002 and 2003. After the uniformity trials, freshexperimental layouts were made in the wet seasons

of 2002 and 2003 in each block. Even though theexperiments were repeated in the same design in twowet seasons of 2002 and 2003, the main plot and

2 D. K. S W A I N E T A L.


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subplot treatments were re-randomized each year.In total, there were six experiments in 3 years.

Plant sampling and sample analysis

Above-ground plant samples were collected at

physiological maturity. For this purpose, 10 hills ofone side of the plot and another 10 hills of theopposite side of the same plot were sampled afterdiscarding the border two rows in each side of theplot (Thiyagarajan et al. 1995). These plant sampleswere uprooted; root portions were rejected and theremainder separated into leaves, stems and grains. Allthese plant parts were oven-dried at 65 xC to constantweight. The sum of the weights of these plant organswas taken as the total aerial dry matter production(DM). The grain yield and harvest index (HI) weredetermined at maturity from the harvest of 10 m2 area

(Yoshida et al. 1976). The grain weight was adjustedto 14% moisture content. Suitable samples of leaves,stems and grains were powdered and digested withsalt mixture and concentrated sulphuric acid in theelectronic micro-block digestion system. The digestedmaterials were analysed for N content by the micro-Kjeldahl distillation method (Yoshida et al. 1976) inthe electronic automatic distillation system. Fromthe N contents of the plant parts, the N uptake byplant organs was computed. The total N uptake byrice was the sum of the N uptake by stems, leaves andgrains.

Derived parameters

The following variety specific parameters were com-puted (Singh et al. 1998; Inthapanya et al. 2000 b).Nitrogen harvest index (NHI)=grain N uptake/totalN uptake; Physiological N use efficiency (PE)=DM/total N uptake; Nitrogen use efficiency for grainproduction (NUE)=grain yield/total N uptake;Apparent recovery of applied N (AR)=(N uptake intreated plot N uptake in control plot)/amount of Napplied; Agronomic N use efficiency (ANUE)=(grain yield in N treated plot grain yield in control

plot)/amount of N appliedThe above parameters are related as follows.

Grain yield=Total N uptakerPErHI

=Total N uptakerNUE

PE=1000={(X) N content in leaf

+(Y) N content in stem

+(HI) N content in grain}

NUE=1000(HI)={(X) N content in leaf

+(Y) N content in stem

+(HI) N content in grain}

where X+Y+HI=1; X is the leaf fraction; Y is thestem fraction.

Statistical analysis

A combined analysis of variance for 3 years datawith two replications was carried out to evaluate thecontribution of year, variety, N rates, and varietyrNrates interaction for late and medium durationvarieties (Gomez & Gomez 1984). From the 3 yearspooled data with two replications, analyses of vari-ance was carried out on grain yield, DM, N content,N uptake and N use efficiency parameters, HI andNHI to evaluate the effect of varieties, N rates andtheir interactions. The interaction effects werepresented wherever they were significant. Linearcorrelations were used to examine the relationshipbetween non-fertilized and N fertilized treatments

for grain yield and biomass production. Linearcorrelations between grain yield and N uptake,ANUE, AR, NUE, HI and NHI were carried outseparately for medium and late duration varieties.The relationship between NUE and N content in leaf,stem and grain was examined to find out optimum Ncontents for grain production. Path coefficientanalysis was used to determine the direct and indirectcontributions of different components to NUE.Multiple regression analysis was used to estimatethe relationship between the NUE and thecomponents. For this purpose, the values of all 3

years and all treatments with two replications weretaken separately for medium and late durationvarieties.

R E S U L T S

Genotype variability in yield parameters

In the medium duration group, the effect of varietywas significant (P


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significant for N content, N uptake and NUE of bothmedium and late duration varieties.

Dry matter production and grain yield

The effect of variety and nitrogen interaction forDM and grain yield production is presented inTable 2. Under unfertilized conditions, the DM andgrain yield production of the medium durationvarieties was 6.48.5 t/ha and 3.33.8 t/ha, respect-ively. For late duration varieties, the DM productionwas higher in comparison with medium durationvarieties under non-fertilized conditions and was

7.910.4 t/ha, but the grain yield was 1.84.8 t/ha,respectively. There was a response to N up to80 kg N/ha.

A comparison of the response of rice varietiesto fertilizer N for grain yield and DM productionindicated considerable differences between the twomaturity duration groups. At 40 kg N/ha application,the N response of late duration varieties for grain yieldand DM production was 32 and 35% greater thanmedium duration varieties (12 kg grain and 27 kgDM/kg fertilizer N applied), respectively. IncreasingN application from 40 to 80 kg/ha increased the N

response of medium duration varieties to 15 kggrain/kg fertilizer N applied and this was comparableto that of late duration varieties. In a similar N

application range, the N response of late durationvarieties for DM production was 30% greater thanthat of medium duration varieties. Late durationvarieties responded poorly, and medium durationvarieties showed no positive response, to an increasein applied N from 80 to 120 kg/ha for both grainyield and DM production. The resultant effect wasa decline in grain yield of medium durationvarieties and marginal increases in grain yield oflate duration varieties with N applications above80 kg N/ha.

The grain yields of the medium duration varietieswere 4.25.0 t/ha at 80 kg N/ha. Of all the medium

duration varieties, the variety Khitish produced thelowest DM and grain yield at all N rates. The ricevariety Mahamaya produced the highest grainyield, whereas Lalat produced the highest DM at allN rates. The late duration varieties Savitri andRanjit produced higher grain yields (4.6 and 4.8 t/ha,respectively) than other varieties under non-fertilizedconditions. The varieties Swarna, Sashi and Rajshreeproduced grain yields of 3.13.5 t/ha. The grain yieldproduction of the variety Mahsuri and Madhuriwas very low (1.82.8 t/ha) in the similar situation.Swarna, Savitri and Ranjit responded better to

increased N application rates than other varieties.Their grain yields and DM were 5.46.2 t/ha and12.114.4 t/ha, respectively at 80 kg N/ha.

Table 2. Grain yield and total dry matter production at maturity of medium and late duration rice varieties at0 (N0), 40 (N40), 80 (N80) or 120 (N120) kg N/ha

Varieties

Grain yield (t/ha) Total dry matter (t/ha)

N0 N40 N80 N120 N0 N40 N80 N120

Medium durationIR-36 3.3 4.1 4.5 4.0 7.0 8.0 9.4 9.4Mahamaya 3.8 4.3 5.0 4.9 7.7 9.3 10.2 10.6Kranti 3.5 3.8 4.6 4.3 7.3 8.4 9.1 8.9Lalat 3.6 4.1 4.4 4.9 8.5 9.3 10.4 11.0Khitish 3.3 3.6 4.2 3.4 6.4 7.2 8.5 7.8

S.E.M.Variety (V) (D.F.=12) 0.10 0.22Nitrogen (N) (D.F.=45) 0.06 0.13VrN (D.F.=45) 0.13 0.30

Late durationMahsuri 2.8 3.5 3.8 3.4 9.4 10.3 11.1 11.9Madhuri 1.8 2.0 2.6 2.3 7.9 8.6 9.5 10.1Savitri 4.6 5.2 6.0 6.9 10.4 12.7 14.4 15.4Rajshree 3.1 3.7 4.2 4.3 8.5 10.1 11.3 11.7Swarna 3.5 4.5 5.4 5.9 8.1 9.768 12.1 14.0Sashi 3.1 3.6 4.1 4.2 9.1 10.9 12.2 12.8Ranjit 4.8 5.6 6.2 6.3 10.2 11.3 13.2 14.3


N use efficiency of rice 5


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N content and uptake by plant organs

The leaf, stem and grain N content and uptake by the

medium and late duration varieties increased withincreasing N rate (Table 3). For all varieties in themedium and late groups, the stem N content was thelowest, followed by leaf and grain. The N uptake byleaf was less than that of stem and grain.

The mean leaf, stem and grain N content ofmedium duration varieties across the N rateswas 6.47.8 g/kg, 5.26.2 g/kg, and 11.011.9 g/kg,respectively. The N uptake of leaf, stem and grain waslowest in Khitish and highest in Mahamaya andLalat. The variations in the N content and N uptakeby plant organs were very large in late duration

varieties. The mean N contents in leaf, stem and grainwere 5.58.4 g/kg, 4.26.9 g/kg and 10.615.3 g/kg,respectively. Leaf, stem and grain N contents were

relatively higher in the varieties Madhuri, Mashuri,Rajshree and Sashi than in Swarna, Savitri and

Ranjit. Similar trends were observed in these varietiesfor leaf and stem N uptake, but the opposite trendwas found for grain N uptake.

Total N uptake and plant N use efficiency forgrain production

The total N uptake by medium or late durationrice varieties increased with the increasing N rates,whereas the NUE decreased (Table 4). At each Nrate, the N uptake by medium duration varieties waslower and NUE was higher as compared with lateduration varieties.

Of the medium duration varieties, Khitish had thelowest N uptake. Under non-fertilized conditions,the N uptake by medium duration rice varieties was

Table 3. Nitrogen content and N uptake by leaf, stem and grain of medium and late duration rice varieties atmaturity as affected by different N applications

Treatments

Leaf Stem Grain

N content(g/kg)

N uptake(kg/ha)

N content(g/kg)

N uptake(kg/ha)

N content(g/kg)

N uptake(kg/ha)

Medium durationVarieties

IR-36 6.4 6.4 5.2 15.0 11.2 44.7Mahamaya 7.8 7.8 6.0 19.5 11.0 49.8Kranti 7.2 6.9 5.6 15.4 11.9 48.5Lalat 7.0 7.7 5.5 19.6 11.6 49.8Khitish 6.9 5.4 6.2 13.9 11.2 40.3

N rates (kg/ha)0 6.1 4.7 4.8 11.4 9.7 33.840 6.5 5.8 5.2 14.6 10.6 41.780 7.5 8.0 6.0 18.8 12.1 55.1

120 8.2 9.0 6.8 21.9 13.0 56.0S.E.M.

Variety (D.F.=12) 0.70 0.68 0.17 0.65 0.25 1.72Nitrogen (D.F.=45) 0.20 0.20 0.14 0.50 0.18 1.01

Late durationVarieties

Mahsuri 7.7 12.3 5.8 28.4 12.6 43.0Madhuri 8.4 11.8 6.9 26.9 15.3 33.4Savitri 5.5 7.8 4.2 23.7 11.2 63.8Rajshree 6.8 9.9 5.9 26.3 13.4 51.4Swarna 6.0 7.2 5.5 23.5 13.4 65.5Sashi 7.5 10.4 5.9 28.6 13.2 50.1Ranjit 5.8 8.3 5.0 25.9 10.6 61.3

N rates (kg/ha)0 5.9 6.7 4.8 18.6 10.8 35.140 6.2 8.0 5.2 23.4 12.7 49.380 7.1 10.8 5.8 27.8 13.7 61.7120 8.1 13.1 6.7 35.1 14.1 64.4

S.E.M.Variety (D.F.=18) 0.20 0.33 0.20 1.04 0.50 1.88Nitrogen (D.F.=63) 0.20 0.28 0.10 0.47 0.20 1.14



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4554 kg N/ha. The N uptake rate was faster up to80 kg N/ha. At 80120 kg N/ha, the N uptake by themedium duration varieties was 74101 kg N/ha.The N uptake by rice varieties Lalat, Mahamayaand IR 36 increased beyond 80 kg N/ha. At a highrate of N application (80 kg N/ha) the N uptake byMahamaya was higher than Lalat, though there wasno difference in N uptake between these varietiesunder non-fertilized conditions.

The increase in N uptake by late duration ricevarieties was faster up to 80 kg N/ha, after which itwas reduced by 40% in several varieties. Of all thevarieties, Madhuri showed the lowest N uptake at all

N application rates. Under non-fertilized conditions,the N uptake by all late duration varieties exceptMadhuri was 6062 kg N/ha. The N uptake by lateduration varieties at 80120 kg N/ha was 83129 kgN/ha. At higher rates of N applications (80 or 120 kgN/ha) the N uptake by Savitri, Swarna and Ranjitwas significantly greater than in other varieties.

There appeared to be less variation in NUE ofthe medium duration varieties at all N rates. Theaverage NUE of medium duration varieties was 70 kggrain/kg N uptake under non-fertilized conditions,reducing to 64, 56 and 49 kg grain/kg N uptake at

40, 80 and 120 kg N/ha. Variations in NUE of thelate duration varieties were very wide. The NUE oflate duration varieties was 3476, 3066, 3158 and

2461 kg grain/kg N uptake at 0, 40, 80 and120 kg N/ha. Of all the varieties, the NUE of Ranjit,Savitri and Swarna was greater than for othervarieties.

Correlations among parameters

There were significant correlations of grain yield withAR (Fig. 1 a) and with total N uptake (Fig. 1 d) ofmedium duration varieties described by the equations1 a and b.

y=304+360 AR (R2=0464, D:F:=88) (Eqn 1a)

y=186+0

03 NU (R2=0

719, D:F:=118) (Eqn 1b)

The correlations between grain yield and HI (Fig. 1 c)and NHI (Fig. 1 b) were not significant. However,correlations of grain yield with AR (Fig. 2 a), withNHI (Fig. 2 b), with HI (Fig. 2 c) and with N uptake(Fig 2 d) of late duration varieties were linear andsignificant:

y=281+342 AR (R2=0171, D:F:=124) (Eqn 2a)

y=212+107 NHI (R2=0601, D:F:=166)

(Eqn 2b)

y=042+1197 HI (R2=0649, D:F:=166) (Eqn 2c)

Table 4. Total N uptake at maturity and N use efficiency for grain production of medium and late duration ricevarieties at 0 (N0), 40 (N40), 80 (N80) or 120 (N120) kg N/ha

Varieties

Total N uptake (kg/ha) N use efficiency (kg grain/kg N uptake)

N0 N40 N80 N120 N0 N40 N80 N120

Medium durationIR-36 45 61 75 84 73 68 60 47Mahamaya 53 67 91 98 71 64 56 51Kranti 51 62 85 85 68 61 54 50Lalat 54 68 85 101 66 60 52 48Khitish 45 53 74 66 72 67 57 51


Late durationMahsuri 61 77 96 102 45 44 40 34

Madhuri 53 65 83 95 34 30 31 24Savitri 62 90 110 113 74 58 55 61Rajshree 62 81 98 114 49 45 43 37Swarna 61 82 111 122 58 54 49 49Sashi 61 84 97 115 51 43 42 36Ranjit 63 85 106 129 76 66 58 49




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y=064+004 NU (R2=0423, D:F:=166) (Eqn 2d)

where y=grain yield (t/ha), AR=apparent Nrecovery (kg/kg), HI=harvest index (kg/kg), NHI=

nitrogen harvest index (kg/kg), and NU=total N

uptake (kg/ha). Comparison with Eqns 1 a and 2 ashows that there is no large variation in the rate ofgrain yield change relative to AR by the medium and

2

3

4

5

6

03 04 05 06 07

Harvest index

(c)

(a)

2

4

6

8

00 02 04 06 08

Apparent N recovery

Grai

nyield(t/ha)

Grainyield(t/ha)

Grai

nyield(t/ha)

Grainyield(t/ha)

(b)

2

3

4

5

6

05 06 07 08

N harvest index

(d)

2

3

4

5

6

20 40 60 80 100 120

Total N uptake (kg/ha)

Fig. 1. Relationship between grain yield and (a) apparent N recovery, (b) N harvest index, (c) harvest index and (d) total Nuptake of medium duration rice varieties at 0 (#), 40 ($), 80 (%) or 120 (&) kg N/ha.

(a)

0

2

4

6

8

00 02 04 06 08 10

Apparent N recovery

(c)

0

2

4

6

8

00 01 02 03 04 05 06 07

Harvest index

(b)

0

2

4

6

8

02 03 04 05 06 07 08 09

N harvest index

(d)

0

2

4

6

8

20 40 60 80 100 120 140 160

Grainyield(t/ha)

Grainyield(t/ha)

Grainyield

(t/ha)

Grainyield(t/ha)

Total N uptake (kg/ha)

Fig. 2. Relationship between grain yield and (a) apparent N recovery, (b) N harvest index, (c) harvest index and (d) total Nuptake of late duration rice varieties at 0 (#), 40 ($), 80 (%) or 120 (&) kg N/ha.



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late duration rice varieties, as the slopes and inter-cepts of the equations were very similar. Nevertheless,there was a difference in the rate of grain yield changeto N uptake between medium and late durationvarieties. The N uptake efficiency, as indicated by theslope of Eqn 1 b for medium duration varieties is lessthan the slope of Eqn 2 d for late duration varieties.The rates of increase in grain yield for unit kg ad-ditional N uptake were 31 and 40 kg for mediumand late duration varieties, respectively. There was aprofound effect of NHI and HI on the grain yield oflate duration varieties (Eqns 2 b and 2 c). The slopesof the equations indicated 1.1 and 1.2 t/ha increase

in grain yield with 0.1 unit increase in NHI and HI,respectively.

The grain yield of medium duration varietieswas negatively and linearly correlated to PE(Fig. 3 a) as

y=630+0017 P E (R2=0249, D:F:=118) (Eqn 3)

The grain yield of late duration varieties was notcorrelated with PE (Fig. 3 c). There were significantquadratic relationships of grain yield with NUE inmedium (Fig. 3 b) and late (Fig. 3 d) duration ricevarieties:

y=123+0

12 NUEx0

0012 NUE2

(R2=0159, D:F:=118) (Eqn 4a)

y=x297+024 NUEx00017 NUE2

(R2=0478, D:F:=166) (Eqn 4b)

where y=grain yield (t/ha), PE=physiological N useefficiency (kg DM/kg N uptake), and NUE=N useefficiency for grain production (kg grain/kg N up-take). The correlations of NUE with leaf, stem andgrain N content at maturity for medium durationvarieties (Fig. 4 a, b and c) and late duration varieties(Fig. 5 a, b and c) were highly significant. For mediumduration varieties the curvilinear relationships weredescribed by the following quadratic equations:

y=739x0

76 A1x0

16 A

2

1 (R2

=031, D:F:=118)

(Eqn 5a)

y=1163x135 B1+061 B2

1(R2=0537, D:F:=118)

(Eqn 5b)

y=2171x225 C1+075 C2

1(R2=0824, D:F:=118)

(Eqn 5c)

Similarly, the curvilinear relationship for lateduration varieties is described by the followingquadratic equations:

y=1019x10

2 A2+032 A

2

2(R2=0

48, D:F:=166)

(Eqn 6a)

(a)

2

3

4

5

6

70 90 110 130 150 170 190

Physiological N use efficiency

(kg total dry matter/kg total N uptake)

Grainyield(t/ha)

(c)

0

2

4

6

8

75 100 125 150 175 200 225

Physiological N use efficiency

(kg total dry matter/kg total N uptake)

Grainyield(t/ha)

(b)

1

2

3

4

5

6

7

40 50 60 70 80 90

N use efficiency for grain production

(kg grain/kg total N uptake)

Grainyield(t/ha)

(d)

0

2

4

6

8

10 20 30 40 50 60 70 80 90

N use efficiency for grain production

(kg grain/kg total N uptake)

Grain

yield(t/ha)

Fig. 3. Relationship between grain yield and (a) physiological N use efficiency, (b) N use efficiency for grain production ofmedium duration rice varieties, (c) physiological N use efficiency and (d) N use efficiency for grain production of lateduration rice varieties at 0 (#), 40 ($), 80 (%) or 120 (&) kg N/ha.



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y=1071x139 B2+056 B2

2(R2=0538, D:F:=166)

(Eqn 6b)

y=1360x102C2+024 C2

2(R2=0521, D:F:=166)

(Eqn 6c)

where y=N use efficiency for grain production

(kg grain/kg N uptake); A1, B1 and C1 are the leaf,stem, and grain N content (g/kg) respectively formedium duration varieties, and A2, B2 and C2 are the

leaf, stem, and grain N content (g/kg), respectively,for late duration varieties.

D I S C U S S I O N

Dry matter and grain yield production of rice varieties

It was not surprising that the grain yield of rice

varieties at various N rates reflected the yield atcontrol. The correlations between non-fertilized andN fertilized treatments for DM and grain yield of

(a)

(b)

(c)

40

50

60

70

80

90

2 4 6 8 10 12

Leaf N content (g/kg)

Nu

seefficiencyfo

rgrainproduction

(kggrain/kgtotalNu

ptake)

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seefficiencyforgrain

production

(kggrain/kgtotalNuptake)

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uction

(kggrain/kgtotalNu

ptake)

40

50

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80

90

40

50

60

70

80

90

2 4 6 8 10

Stem N content (g/kg)

6 8 10 12 14 16

Grain N content (g/kg)

Fig. 4. Relationship between N use efficiency for grain pro-duction and (a) leaf, (b) stem and (c) grain N content ofmedium duration rice varieties at 0 (#), 40 ($), 80 (%) or 120(&) kg N/ha.

(a)

(b)

(c)

0

20

40

60

80

100

0

20

40

60

80

100

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20

40

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0 2 4 6 8 10 12 14 16

Leaf N content (g/kg)

Nu

seefficiencyfo

rgrainproduction

(kggrain/kgtotalNu

ptake)

Nu

seefficiencyforgrainproduction

(kggrain/kgtotalN

uptake)

Nu

seefficiencyforgrainpro

duction

(kggrain/kgtotalNu

ptake)

0 2 4 6 8 10 12

Stem N content (g/kg)

0 5 10 15 20 25

Grain N content (g/kg)

Fig. 5. Relationship between N use efficiency for grain pro-

duction and (a) leaf, (b) stem and (c) grain N content of lateduration rice varieties at 0 (#), 40 ($), 80 (%) or 120 (&)kg N/ha.



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differences in AR or ANUE at 40 and 80 kg N/hawere not significant. The N response of both mediumand late duration varieties in the wet season remainedhigh up to 80 kg N/ha, beyond which N applicationhad no further effect on grain yield production.De Datta & Malabuyoc (1976) obtained N responses

up to 8090 kg N/ha during the wet season in thetropics. This indicated that the full efficiency of themedium and late duration varieties was expressed at80 kg N/ha. The increase in N uptake and DM ofmany medium and late duration varieties beyond80 kg N/ha did not result in a significant increasein yield.

The late duration varieties had well-developed rootsystems (able to absorb sub-soil N) and a longervegetative growth period, both conducive to higheruptake of N (Vergara et al. 1966). As a result, thetotal N uptake by the late duration rice varieties was

higher than the medium duration varieties. High Napplications increased N uptake, but if the uptakeamount exceeded an optimum value, the yielddecreased. In contrast, the medium duration varietieswere of panicle number type, which absorb moresurface N because of their shallow rooting system.Large N uptake resulted in a very high leafarea index, which enhanced carbohydrate loss viadark respiration (Shi & Akita 1993). The limitedvegetative growth period of medium durationvarieties prevented excessive production of vegetativebiomass.

In general, the mean grain yield of all the mediumduration varieties was 4.5 t/ha at 80 kg N/ha. Butlarge differences were observed in N uptake by thesevarieties at 80 kg N/ha. Khitish was not efficientbecause it exhibited the least ANUE, AR, N uptakeand grain yield in both fertilized and non-fertilizedconditions. Kranti exhibited poor yield at all N ratesexcept 80 kg N/ha, which resulted in low ANUE andAR compared to IR 36, Mahamaya and Lalat, theefficient varieties. The late duration rice varietieswere also distinguished into two groups, efficient andnon-efficient. The grain yields of these varietieswere 2.66.2 t/ha with a mean yield of 4.6 t/ha at

80 kg N/ha. The varieties Ranjit, Savitri and Swarnawere efficient and produced higher grain yield thanthe others, with an average of 5.8 t/ha. The DM, Nuptake, HI and NHI of these three varieties werealso high. Mahsuri, Rajshree, Madhuri and Sashiwere not efficient. Non-efficient rice varieties havepoor translocation from source to sink, resulting inlow yield and poor HI (Ladha et al. 1998). The HI andNHI of medium duration varieties were 0.460.51and 0.650.68, respectively, when averaged over theN rates. The efficient late duration varieties Ranjit,Savitri and Swarn had HI and NHI of 0.450.48

and 0.650.69, respectively. Dingkuhn et al. (1991)reported NHI of 0.600.72 for three IRRI semi dwarfrice varieties differing in growth duration. Guindo

et al. (1994) used N15 fertilizers and concluded thatNHI of two lowland rice cultivars was 0.580.62.The non-efficient late duration varieties have poor HI(0.270.37) and poor NHI (0.450.56). Differencesin NUE could be due in part to variety differencesin apical development (despite external growth

stages being similar) leading to different internalN concentrations in the developing ear. This hasimplications for spikelet abortion (Senanayake et al.1994).

Many non-efficient late duration varieties had highDM even under non-fertilized condition and theywere very tall (131142 cm) and susceptible to lodgingin high rainfall areas of the tropics resulting in poorHI. Grain yield in cereals is related to biological yieldand HI (Donald & Hamblin 1976). Grain yield oreconomic yield could be increased either by increasingDM or by increasing HI. Grain yield increases in

cereals such as rice could be accomplished with anincrease in total biological yield (Rahman 1984) asdemonstrated in barley by Naylor et al. (1998) andthis might be possible through breeding for improvedsemi-dwarf N responsive rice varieties of mediumduration. Roberts et al. (1993) observed that thehighest HI exhibited by California lowland ricecultivars under direct seeding was 0.59. This showedthat the HI of medium or late duration varieties couldbe improved through breeding or crop managementmethods for yield increase.

The significant correlation between grain yield, N

uptake and N use efficiency for grain productionindicated that the grain yield of medium durationvarieties was mostly dependent on total N uptake ofthe crop. However, the grain yield of late durationvarieties was governed by both total N uptake and theutilization efficiency of the variety. The correlationcoefficient for the relationship between grain yieldand the HI and the NHI of late duration varieties washigher than that for the N uptake and AR. Thisshowed that the plant N use efficiency of late durationvarieties strongly contributed to differences in grainyield. The average grain yield of efficient mediumduration varieties was lower than that of the efficient

late duration varieties. This yield difference wasclosely associated with lower N uptake by themedium duration varieties (Cassman et al. 1993).The present results show that the rate of increase ingrain yield for 1 kg additional N uptake of lateduration varieties was 29% higher than that ofmedium duration varieties.

Plant N use efficiency and N content for optimumgrain production

Optimum grain yield could be associated with

optimum PE and NUE of the rice plant becausethe higher N application (120 kg N/ha) did notensure higher grain yield. It might be inferred that



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improvement in grain yield by late duration varietiesthrough PE could be difficult as the correlationbetween grain yield and PE was not significant. Theyield of medium duration varieties was influencedmainly by total N uptake, followed by PE and NUE.

Using Eqns 4a and b, the optimum levels of NUE formedium and late duration varieties were computed tobe 49 and 68 kg grain/kg N uptake, respectively. Theoptimum grain yields of 4.5 t/ha by medium durationvarieties and 5.8 t/ha by late duration varieties areattained with these optimum values of NUE, whichcould serve as a guideline for developing new ricevarieties.

Yoshida (1981) reported a quadratic relationshipbetween photosynthetic rate and N content per leafarea. The indica rice varieties showed less promotionin their photosynthetic rate by the high N supply thandid japonica varieties. At high N applications, some

rice varieties have a lower proportion of spikeletsfilled. The probability of lodging or culm bending ishigher at high levels of N and disrupts the movementof assimilates and absorbed nutrients. High N con-tent and uptake in plant organs due to supra-optimalN application or soil extraction can reduce the utiliz-ation efficiency for maximum grain production. Theleaf, stem and grain N contents at maturity of 4.0,4.0 and 9.0 g/kg (from Eqns 6 a, b and c respectively),coincide with the optimum NUE (68 kg grain/kg Nuptake) of late duration varieties. The N contentsin plant organs for optimum N use efficiency (49 kg

grain/kg N uptake) of medium duration varieties(derived from Eqns 5 a, b and c) appeared to be muchhigher than the corresponding values in late duration

varieties and these N contents were 10 g/kg, 8 g/kgand 14 g/kg for leaf, stem and grain, respectively, atmaturity. Tirol-Padre et al. (1996) identified geno-types possessing promising traits for improvedN uptake and utilization efficiency among 180 ricegenotypes of different growth duration from 100 to

135 days without the addition of N fertilizer. Theyindicated genotypic variation in stem N contents andsuggested low stem N content for yield improvement.Significant genotypic variation in nutrient useefficiency also contributed to genotypic variation forgrain yield (Inthapanya et al. 2000 b). A larger effectof variety and smaller effect of variety by locationinteraction was observed for N use efficiency.

NUE was significantly and negatively correlated(P


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be effective based on N use efficiency alone. Hence, aselection of rice varieties for grain yield and NUEshould be based on the N content of grain, stemand leaf.

Acknowledgement is due to the research projectwhich was financed by the World Bank fundedNational Agricultural Technology Project (NATPRRPS 25) from May 2000 to December 2003.

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